0001564590-20-007350 | 10-K | XBRL Viewer | Apellis Pharmaceuticals INC

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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2019	Feb. 21, 2020	Jun. 30, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	APLS
Entity Registrant Name	APELLIS PHARMACEUTICALS, INC.
Entity Central Index Key	0001492422
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		75,321,471
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Public Float			$ 1,151,210,910
Entity Current Reporting Status	Yes
Entity File Number	001-38276
Entity Tax Identification Number	27-1537290
Entity Address, Address Line One	100 Fifth Avenue
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	617
Local Phone Number	977-5700
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	The registrant intends to file a definitive proxy statement pursuant to Regulation 14A in connection with its 2020 Annual Meeting of Stockholders. Portions of such proxy statement are incorporated by reference into Part III of this Annual Report on Form 10-K.

Consolidated Balance Sheets - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 351,985,085	$ 176,267,666
Prepaid assets	19,802,008	24,333,851
Other current assets	1,307,591	1,837,704
Total current assets	373,094,684	202,439,221
Non-current Assets:
Right-of-use assets	14,110,209
Property and equipment, net	1,654,999	977,918
Other assets	385,088	116,420
Total assets	389,244,980	203,533,559
Current liabilities:
Accounts payable	8,360,527	10,254,938
Accrued expenses	54,782,951	5,103,002
Current portion of long-term-debt		1,666,667
Current portion of operating lease liabilities	2,609,341
Total current liabilities	65,752,819	17,024,607
Long-term liabilities:
Convertible Senior Notes	142,566,851
Development derivative liability	134,839,000
Term loan facility		18,722,321
Promissory note		6,655,193
Operating lease liabilities	11,856,634
Other liabilities		158,783
Total liabilities	355,015,304	42,560,904
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and zero shares issued and outstanding at December 31, 2018 and 2019	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized and at December 31, 2018 and 2019; 56,279,307 and 63,938,003 shares issued and outstanding at December 31, 2018 and 2019, respectively	6,393	5,628
Additional Paid-in capital	615,849,518	437,855,681
Accumulated other comprehensive loss	(153,775)	(122,807)
Accumulated deficit	(581,472,460)	(276,765,847)
Total stockholders' equity	34,229,676	160,972,655
Total liabilities and stockholders' equity	$ 389,244,980	$ 203,533,559

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, issued	63,938,003	56,279,307
Common stock, outstanding	63,938,003	56,279,307

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 220,968,770	$ 105,285,576	$ 40,303,878
General and administrative	67,046,483	22,639,184	10,463,151
Operating loss	(288,015,253)	(127,924,760)	(50,767,029)
Loss on extinguishment of debt	(1,501,215)
Loss from remeasurement of development derivative liability	(14,839,000)
Interest expense	(5,284,610)	(2,512,956)	(374,749)
Interest income	5,108,779	2,960,771	277,834
Other expense, net	(175,314)	(25,249)	(142,150)
Net loss	(304,706,613)	(127,502,194)	(51,006,094)
Other comprehensive loss:
Foreign currency loss	(30,968)	(122,807)
Total other comprehensive loss	(30,968)	(122,807)
Comprehensive loss, net of tax	$ (304,737,581)	$ (127,625,001)	$ (51,006,094)
Net loss per common share, basic and diluted	$ (4.90)	$ (2.34)	$ (3.68)
Weighted-average number of common shares used in net loss per common share, basic and diluted	62,228,601	54,396,483	13,870,949

Consolidated Statements of Changes in Stockholder's Equity - USD ($)	Total	IPO [Member]	Series E Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member] Series E Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] IPO [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] IPO [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2016	$ 23,794,320			$ 92,054,926		$ 843		$ 29,996,110			$ (98,257,559)
Beginning balance, Shares at Dec. 31, 2016				56,347,420		8,428,366
Issuance of stock, net of issuance costs			$ 19,747,847		$ 19,747,847
Issuance of stock, net of issuance costs, shares					7,792,035
Issuance of common stock upon exercise of stock options	164,501					$ 5		164,496
Issuance of common stock upon exercise of stock options, shares						46,881
Issuance of warrants to an affiliate of stockholder	430,160							430,160
Issuance of common stock upon exercise of warrants	514,200					$ 9		514,191
Issuance of common stock upon exercise of warrants, shares						93,764
Conversion of convertible preferred stock to common stock				$ (111,802,773)		$ 3,007		111,799,766
Conversion of convertible preferred stock to common stock, shares				(64,139,455)		30,070,034
Issuance of common stock in initial public offering, net of issuance costs		$ 149,879,216					$ 1,169		$ 149,878,047
Issuance of common stock in initial public offering, net of issuance costs, shares							11,695,107
Share-based compensation expense	5,418,710							5,418,710
Net loss	(51,006,094)										(51,006,094)
Ending balance at Dec. 31, 2017	148,942,860					$ 5,033		298,201,480			(149,263,653)
Ending balance, Shares at Dec. 31, 2017						50,334,152
Issuance of stock, net of issuance costs	131,194,030					$ 550		131,193,480
Issuance of stock, net of issuance costs, shares						5,500,000
Issuance of common stock upon exercise of stock options	$ 727,510					$ 45		727,465
Issuance of common stock upon exercise of stock options, shares	468,263					445,155
Share-based compensation expense	$ 7,733,256							7,733,256
Net loss	(127,502,194)										(127,502,194)
Foreign currency loss	(122,807)									$ (122,807)
Ending balance at Dec. 31, 2018	160,972,655					$ 5,628		437,855,681		(122,807)	(276,765,847)
Ending balance, Shares at Dec. 31, 2018						56,279,307
Issuance of stock, net of issuance costs	109,581,108					$ 690		109,580,418
Issuance of stock, net of issuance costs, shares						6,900,000
Issuance of common stock upon exercise of stock options or warrants	$ 3,129,430					$ 75		3,129,355
Issuance of common stock upon exercise of stock options or warrants, Shares						758,696
Issuance of common stock upon exercise of stock options, shares	747,136
Recognition of equity component of convertible notes	$ 72,520,035							72,520,035
Purchase of capped call transactions and associated costs	(28,380,000)							(28,380,000)
Share-based compensation expense	21,144,029							21,144,029
Net loss	(304,706,613)										(304,706,613)
Foreign currency loss	(30,968)									(30,968)
Ending balance at Dec. 31, 2019	$ 34,229,676					$ 6,393		$ 615,849,518		$ (153,775)	$ (581,472,460)
Ending balance, Shares at Dec. 31, 2019						63,938,003

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
Consolidated Statements of Cash Flows - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net loss	$ (304,706,613)	$ (127,502,194)	$ (51,006,094)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	21,144,029	7,733,256	5,418,710
Loss on early extinguishment of debt	1,501,215
Loss from remeasurement of development derivative liability	14,839,000
Non-cash lease expense	355,766
Depreciation expense	239,683
Amortization of debt discounts	51,725
Amortization of discounts for promissory note		71,791	13,563
Amortization of term loan facility discounts	104,172	582,043	11,935
Amortization of discounts for convertible notes, net of financing costs	2,186,014
Changes in operating assets and liabilities:
Prepaid assets	4,531,039	(299,006)	(3,927,155)
Other current assets	287,353	(19,275,118)	50,443
Other assets	(322,789)	(309,361)	10,177
Accounts payable	(1,640,774)	(978,188)	(51,622)
Accrued expenses	50,453,518	6,587,469	932,299
Other liabilities	(158,783)	2,148,511	1,952,671
Net cash used in operating activities	(211,135,445)	(131,240,797)	(46,595,073)
Investing Activities
Purchase of property and equipment	(1,693,322)
Net cash used in investing activities	(1,693,322)
Financing Activities
Issuance of Series E convertible preferred stock, net of issuance costs			19,747,847
Deferred issuance costs		(18,180)
Proceeds from issuance of term loan, net of issuance costs			19,795,007
Proceeds from issuance of promissory note, net of issuance costs			6,569,840
Proceeds from issuance of common stock, net of issuance costs	109,581,108	131,194,030	150,062,959
Proceeds from development derivative liability	120,000,000
Payments for capped call transactions and associated costs	(28,380,000)
Proceeds from issuance of convertible notes, net of issuance costs	212,912,238
Proceeds from issuance of stock options and warrants			520,760
Proceeds from exercise of stock options and warrants	3,129,430	727,510	678,701
Repayment of promissory note	(7,000,000)
Repayment of term loan facility	(21,701,292)
Net cash provided by financing activities	388,541,484	131,903,360	197,375,114
Effect of exchange rate changes on cash and cash equivalents	4,702	(38,426)
Net increase in cash and cash equivalents	175,717,419	624,137	150,780,041
Cash and cash equivalents at beginning of period	176,267,666	175,643,529	24,863,488
Cash and cash equivalents at end of period	351,985,085	176,267,666	175,643,529
Supplemental Disclosure of Non-Cash Financing Activities
Conversion of convertible preferred stock to common stock			111,802,773
Cash paid for interest	987,377	$ 1,816,333	$ 134,167
Equity component of convertible notes	$ 72,520,035

Nature of Organization and Operations

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of Organization and Operations

1. Nature of Organization and Operations

Apellis Pharmaceuticals, Inc. (the “Company”) is a clinical-stage biopharmaceutical company focused on the development of novel therapeutic compounds to treat disease through the inhibition of the complement system, which is an integral component of the immune system, at the level of C3, the central protein in the complement cascade.

The Company was incorporated in September 2009 under the laws of the State of Delaware. The Company’s principal executive offices are located in Waltham, Massachusetts, effective January 1, 2020. Prior to that date, the Company’s principal offices were in Crestwood, Kentucky.

The Company’s operations since inception have been limited to organizing and staffing the Company, acquiring rights to product candidates, business planning, raising capital and developing its product candidates.

The Company is subject to risks common in the biotechnology industry including, but not limited to, raising additional capital, development by its competitors of new technological innovations, its ability to successfully complete preclinical and clinical development of product candidates and receive timely regulatory approval of products, market acceptance of the Company’s products, protection of proprietary technology, healthcare cost containment initiatives, and compliance with governmental regulations, including those of the U.S. Food and Drug Administration (“FDA”).

Convertible Notes Offering

On September 16, 2019, the Company completed a private offering of $220,000,000 aggregate principal amount of 3.5% convertible senior notes due 2026 (the “Convertible Notes”).

The net proceeds from the sale of the Convertible Notes were approximately $212,912,000 after deducting the initial purchasers’ discounts and commissions of $6,600,000 and offering expenses of $487,763 paid by the Company. The Company used $28,380,000 of the net proceeds from the sale of the Convertible Notes to pay the cost of the capped call transactions described below in Note 7.

On September 16, 2019, the Company entered into an indenture (the “Indenture”) with respect to the Convertible Notes with U.S. Bank National Association, as trustee (the “Trustee”). The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020. The Convertible Notes will mature on September 15, 2026, unless converted earlier, redeemed or repurchased in accordance with the terms of the Convertible Notes. See Note 7 – Long-term Debt for additional information.

Development Derivative Liability

On February 28, 2019, the Company entered into a development funding agreement with SFJ Pharmaceuticals Group (“SFJ”) under which SFJ agreed to provide funding to the Company to support the development of pegcetacoplan for the treatment of patients with paroxysmal nocturnal hemoglobinuria (“PNH”). Pursuant to the agreement, SFJ paid the Company $60,000,000 following the signing of the agreement, and agreed to pay the Company up to an additional $60,000,000 in the aggregate in three equal installments upon the achievement of specified development milestones with respect to the Company’s Phase 3 program for pegcetacoplan in PNH and subject to the Company having cash resources at the time sufficient to fund at least ten months of the Company’s operations. In addition, upon the mutual agreement of the Company and SFJ, at any time after the earlier of the date that the Company has reviewed the primary endpoint data from its PEGASUS Phase 3 trial of pegcetacoplan in patients with PNH and March 31, 2020, SFJ may fund an additional $50,000,000 of the Company’s development costs (the “Additional SFJ Funding”).

On June 7, 2019, the Company and SFJ amended the development funding agreement (the “SFJ amendment”). Under the SFJ amendment, SFJ agreed to make an additional $20,000,000 funding payment to the Company to support the development of pegcetacoplan for the treatment of patients with PNH. This additional $20,000,000 payment is in addition to and not part of the Additional SFJ Funding.

On June 27, 2019, the Company received $40,000,000 from SFJ, consisting of $20,000,000 as the first installment of the additional $60,000,000 upon the achievement of a milestone and the $20,000,000 payable under the SFJ amendment.

On September 23, 2019, the Company received $20,000,000 from SFJ as the second installment of the additional $60,000,000 upon the achievement of a milestone.

The Company met the remaining development milestone under the SFJ agreement upon the Company’s announcement of the primary endpoint data from its PEGASUS Phase 3 trial on January 7, 2020 and the final installment of $20,000,000 of the additional $60,000,000 was paid on January 29, 2020.

Follow-on Public Offering

On March 11, 2019, the Company issued and sold 6,900,000 shares of its common stock at a price per share of $17.00 in a follow-on public offering (“2019 follow-on offering”). The Company received net proceeds of approximately $109,581,000 after deducting underwriting discounts and commissions of approximately $7,038,000 and offering costs of $680,891.

On April 23, 2018, the Company issued and sold 5,500,000 shares of its common stock at a price per share of $25.50 in a follow-on public offering. The Company received net proceeds of $131,324,477 after deducting underwriting discounts and commissions of $8,415,000 and offering costs of $509,973.

Liquidity and Going Concern

The accompanying consolidated financial statements have been prepared on the basis of the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. Subsequent to December 31, 2019, on January 13, 2020, the Company issued and sold 10,925,000 shares of its common stock at a price per share to the public of $37.00 in a follow-on public offering, including an additional 1,425,000 shares of its common stock that were sold at the follow-on public offering price of $37.00 per share pursuant to the underwriters’ in full exercise of their option to purchase additional shares of common stock. The Company received net proceeds of approximately $381,374,000 from the sale of shares in the follow on offering after deducting underwriting discounts and commissions of approximately $22,233,000 and offering costs of approximately $618,000. As of February 27, 2020, the date of issuance of these consolidated financial statements, the Company believes that its cash and cash equivalents as of December 31, 2019 of $351,985,085, together with the net proceeds of its follow-on offering of approximately $381,374,000 in January 2020 and the $20,000,000 received from SFJ in January 2020, will be sufficient to fund its operations and capital expenditures for at least the next twelve months from the date of issuance of these consolidated financial statements. The Company’s future viability beyond that point is dependent on its ability to raise additional capital to finance its operations.

The Company is subject to risks common to other life science companies in the development stage including, but not limited to, uncertainty of product development and commercialization, lack of marketing and sales history, development by its competitors of new technological innovations, dependence on key personnel, market acceptance of products, product liability, protection of proprietary technology, ability to raise additional financing, and compliance with FDA and other government regulations. If the Company does not successfully commercialize any of its product candidates, it will be unable to generate recurring product revenue or achieve profitability. Management’s plans in order to meet its short-term and longer-term operating cash flow requirements include obtaining additional funding.

There are uncertainties associated with the Company’s ability to (1) obtain additional debt or equity financing on terms that are favorable to the Company, (2) enter into collaborative agreements with strategic partners, and (3) succeed in its future operations. If the Company is not able to obtain the required funding for its operations, or is not able to obtain funding on terms that are favorable to the Company, it could be forced to delay, reduce or eliminate its research and development programs or future commercialization efforts and its business could be materially harmed.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Apellis Australia Pty Ltd, Apellis Switzerland GmbH, Apellis Ireland Ltd., and Apellis MA Securities, Inc. All intercompany balances and transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”).

Reclassifications

Certain prior year amounts have been reclassified to conform to the 2019 presentation.

Offering Costs

Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s follow-on offerings and filing of a registration statements on Form S-3 in 2018 and 2019, and related to the Company’s offering of Convertible Notes in 2019. Costs were deferred until completion of the follow-on offerings and offering of Convertible Notes, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds. Deferred issuance costs related to the registration statement on Form S-3 that the Company filed in January 2020 have been capitalized and will be reclassified to additional paid-in capital as a reduction of the proceeds at the time when shares are issued. The Company does not consider these amounts material at December 31, 2019.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: share-based compensation expense, fair value of common stock and preferred stock, development derivative liability, accrued expenses, prepaid expenses and convertible debt.

Historically for all periods prior to the IPO, the Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the retrospective fair value of its common stock during all periods presented. The methodologies include a probability analysis including both a potential public trading scenario and potential sale scenario. In both scenarios, value is estimated using the guideline public company method. The sale scenario includes an adjustment for a market participant acquisition premium. Value is allocated among the preferred and common shares according to the rights associated with each type of security. Valuation methodologies include estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock and the associated fair value of stock options granted at each valuation date. Since the completion of the IPO, the Company’s board of directors determines the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of grant.

Fair Value of Financial Instruments

The Company is required to disclose information on the fair value of financial instruments and inputs that enable an assessment of the fair value. The three levels of the fair value hierarchy prioritize valuation inputs based upon the observable nature of those inputs as follows:

Level 1 – Quoted prices in active markets for identical assets or liabilities;

Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly;

Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability.

The Company’s financial instruments, in addition to those presented in Note 7, Long-term Debt and Note 9, Fair Value Measurements, include cash and cash equivalents, the Australian research and development credit, accounts payable and accrued liabilities. Management believes that the carrying amounts of cash and cash equivalents, the Australian research and development credit, accounts payable and accrued expenses approximate the fair value due to the short-term nature of those instruments.

Cash and Cash Equivalents

Cash and cash equivalents are defined as cash in banks and investment instruments having maturities of three months or less from their acquisition date. The carrying amounts reported in the consolidated balance sheets for cash and cash equivalents are valued at cost, which approximates the fair value. As of December 31, 2018 and 2019, the Company had assets consisting of cash and cash equivalents held in banks outside of the United States of approximately $121,600 and $1,598,300, respectively.

Foreign Currency

The financial position and results of operations of the Company's Australian and Swiss subsidiaries are measured using the foreign subsidiary's local currency. Revenues and expenses of the subsidiaries have been translated into U.S. dollars at average exchange rates prevailing during the period from January 1, 2019 through December 31, 2019. Assets and liabilities have been translated at the rates of exchange on the balance sheet date. The resulting translation gain and loss adjustments are recorded directly as a separate component of stockholders’ equity. Prior to August 1, 2018, when the functional currency of the Company’s Australian subsidiary was changed from the U.S. dollar to the Australian dollar, the transactions with the Australian subsidiary were recorded in U.S. dollars and were not translated.

Research and Development

Costs incurred in connection with research and development activities are expensed as incurred. Research and development expenses include (i) employee-related expenses, including salaries, benefits, travel and share-based compensation expense; (ii) external research and development expenses incurred under arrangements with third parties, such as contract research and contract manufacturing organizations, investigational sites and consultants, including share-based compensation expense for consultants; (iii) the cost of acquiring, developing and manufacturing clinical study materials; and (iv) costs associated with preclinical and clinical activities and regulatory operations.

The Company enters into consulting, research and other agreements with commercial entities, researchers, universities and others for the provision of goods and services. Such arrangements are generally cancellable upon reasonable notice and payment of costs incurred. Costs are considered incurred based on an evaluation of the progress to completion of specific tasks under each contract using information and data provided by the Company’s clinical sites and vendors. These costs consist of direct and indirect costs associated with specific projects, as well as fees paid to various entities that perform certain research on behalf of the Company.

Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management’s estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly.

Included in research and development is a refundable Australian research and development credit. The credit is recognized as a reduction of clinical trial costs over the periods necessary to match the benefit of the credit with the costs for which it is intended to compensate.

Patents

Costs incurred in connection with the application for and issuance of patents are expensed as incurred.

Income Taxes

The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of December 31, 2018 and 2019, the Company did not have any significant uncertain tax positions.

Share-based Compensation

The Company recognizes all share-based payments to employees, including grants of employee stock options, in the statements of operations and comprehensive loss based on their estimated fair values over the requisite service periods for each award. The Company remeasures the fair value of share-based awards to non-employees until a performance commitment is reached or counterparty performance is complete. The Company’s share-based compensation awards are comprised of stock options. The Company estimates the fair value of each stock option grant using the Monte Carlo simulation model (“Monte Carlo”) for grants made prior to June 30, 2015 and the Black-Scholes option pricing model (“Black-Scholes”) for grants made on or after July 1, 2015 through October 2017. After the Company’s IPO in November 2017, the Company’s board of directors determined the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of the grant.

Both Monte Carlo and Black-Scholes for option pricing require the input of the six minimum considerations detailed in ASC 718, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the trading of the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of comparable companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company believes the group selected has sufficient similar economic and industry characteristics. Before the IPO, the Company calculated the expected term for options granted to employees based on a quarterly weighted average probability of exit analysis considering milestones that the Company had achieved and each of the potential exit scenarios available to the Company at that time. After the IPO, the Company calculates the expected term for options granted to employees based on the “simplified” method set forth in SEC Staff Accounting Bulletin 107, because it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term due to the limited period of time its equity shares have been publicly traded. Under the simplified method, the weighted average vesting period is averaged with the contractual term of the option being valued. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

The Company’s share-based awards are subject to service based vesting conditions. Compensation expense related to awards to employees with service based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Consistent with the guidance in ASC 505-50, compensation expense related to awards to non-employees with service based vesting conditions is recognized on a straight-line basis based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term.

The Company recognizes forfeitures for employee and non-employee grants at the time they occur. Share-based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest.

Concentrations of Credit Risk

Cash and cash equivalents are the only financial instruments that potentially subject the Company to concentrations of credit risk. Cash and cash equivalents are held at financial institutions in the United States, Australia and Switzerland. The Company is exposed to credit risk in the event of default by the financial institution to the extent that cash and cash equivalent balances recorded in the balance sheets are in excess of the amounts that are insured by the Federal Deposit Insurance Corporation. The Company has not experienced any losses on its deposits since inception, and management believes that minimal credit risk exists with respect to these financial institutions.

Net Loss per Share

Basic net loss per common share is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, convertible notes and common stock options are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.

Comprehensive Loss

The Company’s components of comprehensive loss other than its net loss, are the foreign currency gains/losses recorded from the remeasurement of the long term intra-entity loan transaction to the Company’s wholly owned subsidiaries as well as the foreign currency gain/ loss from the translation of the Company’s wholly owned subsidiaries into U.S. dollars in 2018 and 2019. These components are a result of the change in the Australian subsidiary’s functional currency to Australian dollars effective August 1, 2018 and the creation of the Swiss subsidiary in 2019.

Recent Accounting Pronouncements

In May 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606). The standard will apply one comprehensive revenue recognition model across all contracts, entities and sectors. The core principle of the new standard is that revenue should be recognized to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 will replace most of the existing revenue recognition requirements in U.S. GAAP. The FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which deferred the effective date of the standard for one year. As a result, the new standard was effective for annual reporting periods beginning after December 15, 2017, for public companies, including interim periods within that reporting period. As the Company does not currently have revenue, the adoption of the new standard had no impact on its consolidated financial statements.

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments – Overall, amending several elements surrounding the recognition and measurement of financial instruments and requiring equity investments (except those accounted for under the equity method of accounting or those that result in consolidation) to be measured at fair value with changes in fair value recognized in net income. Effective January 1, 2018, the Company adopted this ASU on a prospective basis. The adoption did not have a material impact on its consolidated financial statements and related disclosures.

In February 2016, the FASB issued ASU 2016-02 Leases (Topic 842) which establishes a right-of-use ("ROU") model that requires a lessee to record a ROU asset and a lease liability on the consolidated balance sheet for all leases with terms longer than twelve months. Leases will be either classified as finance or operating, with classification affecting the pattern of expense recognition in the income statement. ASU 2016-02 also requires significant additional disclosures about the amount, timing and uncertainty of cash flows from leases. The Company adopted ASU 2016-02 effective January 1, 2019 using a modified retrospective method and will not restate comparative periods. As permitted under the guidance, the Company carried forward the assessment of whether contracts contain or are leases, the classification of leases and remaining terms. The Company recognized $5,511,865 of ROU assets and operating lease liabilities, primarily relating to office space leases, on the consolidated balance sheet upon adoption. During 2019, the Company leased additional office space resulting in an initial recognition value of $9,838,982 of ROU assets and operating lease liabilities.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement – Disclosure Framework, that modifies the disclosure requirements with respect to fair value measurements. Among the changes, entities will no longer be required to disclose the amount of and reasons for transfers between Levels 1 and 2 of the fair value hierarchy, but will be required to disclose the range and weighted average used to develop significant unobservable inputs for Level 3 fair value measurements. Effective January 1, 2018, the Company early adopted this ASU, which did not have a material impact on its consolidated financial statements and related disclosures.

Common Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Common Stock

3. Common Stock

The Company has reserved the following shares of common stock for future issuance:

	December 31,
	2017		2018		2019
Shares reserved under 2010 Equity Incentive Plan		5,817,844		5,013,153		—
Shares reserved under 2017 Equity Incentive Plan		1,682,596		4,032,390		6,319,001
Shares reserved under 2017 Employee Stock Purchase Plan		468,823		972,164		972,164
Common stock warrants		14,064		14,064		—
Total		7,983,327		10,031,771		7,291,165

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Convertible Preferred Stock

4. Convertible Preferred Stock

Authorized capital stock includes 10,000,000 shares of preferred stock at December 31, 2018 and 2019.

Series E Convertible Preferred Stock

In August 2017, the Company issued 7,792,035 shares of Series E convertible preferred stock at $2.571 per share for aggregate proceeds of $20,033,322, less issuance costs of $285,475.

Upon closing of the Company’s IPO on November 13, 2017, all shares of convertible preferred stock converted into shares of common stock.

Development Derivative Liability

12 Months Ended

Dec. 31, 2019

Research And Development [Abstract]

Development Derivative Liability

5. Development Derivative Liability

On February 28, 2019, the Company entered into the SFJ agreement under which SFJ agreed to provide funding to the Company to support the development of pegcetacoplan for the treatment of patients with PNH. Pursuant to the agreement, SFJ paid the Company $60,000,000 following the signing of the agreement, and agreed to pay the Company up to an additional $60,000,000 in the aggregate in three equal installments upon the achievement of specified development milestones with respect to the Company’s Phase 3 program for pegcetacoplan in PNH and subject to the Company having cash resources at the time sufficient to fund at least 10 months of the Company’s operations. In addition, upon the mutual agreement of the Company and SFJ, at any time after the earlier of the date that the Company has reviewed the primary endpoint data from its PEGASUS Phase 3 trial of pegcetacoplan in patients with PNH and March 31, 2020, SFJ may fund an additional $50,000,000 of the Company’s development costs.

On June 7, 2019, the Company and SFJ amended the development funding agreement. Under the SFJ amendment, SFJ agreed to make an additional $20,000,000 funding payment to the Company to support the development of pegcetacoplan for the treatment of patients with PNH. This additional $20,000,000 payment is in addition to and not part of the Additional SFJ Funding.

On September 23, 2019, the Company received $20,000,000 from SFJ as the second installment of the additional $60,000,000 upon the achievement of a milestone.

Under the SFJ agreement following regulatory approval by the FDA or EMA for the use of pegcetacoplan as a treatment for PNH the Company will be obligated to pay SFJ an initial payment of up to $5,000,000 (or a total of $10,000,000 if regulatory approval is granted by the FDA and the EMA) and then up to an additional $226,000,000 in the aggregate (or up to $452,000,000 if regulatory approval is granted by the FDA and the EMA) in six additional annual payments with the majority of the payments being made from the third anniversary to the sixth anniversary of regulatory approval. Such payments will be proportionately adjusted in the event that the actual funding from SFJ is lower or greater than $120,000,000 (including as a result of the payment of the Additional SFJ Funding but excluding the $20,000,000 funding payment made under the SFJ amendment). Additionally, the Company granted a

security interest in all of its assets, excluding intellectual property and license agreements to which it is a party. In connection with the grant of the security interest, the Company agreed to certain affirmative and negative covenants, including restrictions on its ability to pay dividends, incur additional debt or enter into licensing transactions with respect to its intellectual property, other than specified types of licenses.

The SFJ agreement is presented as a derivative liability on the consolidated balance sheet as of December 31, 2019. The liability was initially recorded at the value of the $60,000,000 aggregate cash received pursuant to the contractual terms, which was determined to have been fair value, measured as a level 3 derivative. During 2019, the Company received an additional $60,000,000 under the SFJ agreement and the Company met additional milestones in the agreement. The derivative liability was remeasured quarterly at fair value in 2019 and finally at December 31, 2019 as a level 3 derivative, with the total change in fair value of $14,839,000 recorded for the year ended December 31, 2019 in loss from remeasurement of development derivative liability on the consolidated income statement. The remeasurement of the development derivative liability at December 31, 2019 resulted in a remeasured fair value of $134,839,000 as of December 31, 2019 on the consolidated balance sheet.

The following table presents a rollforward of the liability:

	Development Derivative Liability
Balance at fair market value, January 1, 2019	$	—
Amounts received under the SFJ agreement and SFJ amendment		120,000,000
Loss recorded in loss from remeasurement of development derivative liability		14,839,000
Balance at fair market value, December 31, 2019	$	134,839,000

The derivative is valued using a scenario-based discounted cash flow method, whereby each scenario makes assumptions about the probability and timing of cash flows, and such cash flows are present valued using a risk-adjusted discount rate. The analysis is calibrated such that the value of the derivative as of the date of the SFJ agreement was consistent with an arm’s-length transaction. Key inputs to the level 3 fair value model include (i) the probability and timing of achieving stated development milestones to receive the next tranches of funding, (ii) the probability and timing of achieving FDA and EMA approval, (iii) SFJ’s cost of borrowing (8.0%), and (iv) the Company’s cost of borrowing (16.6%).

SFJ’s implied cost of borrowing was 8.0% and the Company’s implied cost of borrowing was 16.6% as of the reporting date. These implied costs of borrowing were determined assuming the SFJ agreement was initially executed with arm’s-length terms. If the SFJ agreement was instead not determined to be an arm’s-length transaction, then implied discount rates could differ.

Accrued Expenses and Prepaid Assets

12 Months Ended

Dec. 31, 2019

Accrued Expenses And Prepaid Assets [Abstract]

Accrued Expenses and Prepaid Assets

6. Accrued Expenses and Prepaid Assets

Accrued expenses are as follows:

	December 31,
	2018		2019
Accrued research and development	$	3,481,570	$	36,449,386
Accrued payroll liabilities		269,046		11,442,889
Other		1,352,386		6,890,676
Total	$	5,103,002	$	54,782,951

Prepaid assets include $23,053,511 and $18,102,049 of prepaid research and development costs as of December 31, 2018 and 2019, respectively.

Long-term Debt

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Long-term Debt

7. Long-term Debt

Convertible Senior Notes

On September 16, 2019, the Company completed a private offering of Convertible Notes with an aggregate principal amount of $220,000,000 issued pursuant to an indenture (the “Indenture”) with U.S. Bank National Association, as trustee (the “Trustee”).

The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020. The Convertible Notes will mature on September 15, 2026, unless converted earlier, redeemed or repurchased in accordance with the terms of the Convertible Notes.

The Convertible Notes are convertible into shares of the Company’s common stock at an initial conversion rate of 25.3405 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $39.4625 per share of common stock). The conversion rate is subject to customary anti-dilution adjustments. In addition, following certain events that occur prior to the maturity date or if the Company deliver a notice of redemption, the Company will increase the conversion rate for a holder who elects to convert its Convertible Notes in connection with such corporate event or a notice of redemption, as the case may be, in certain circumstances as provided in the indenture.

Prior to March 15, 2026, the Convertible Notes are convertible only upon the occurrence of certain events. On or after March 15, 2026 until the close of business on the second scheduled trading day immediately preceding the maturity date of the Convertible Notes, holders may convert the Convertible Notes at any time. Upon conversion of the Convertible Notes, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock or a combination of cash and shares of common stock, at the Company’s election.

Prior to September 20, 2023, the Company may not redeem the Convertible Notes. The Company may redeem for cash all or a portion of the Convertible Notes, at its option, on or after September 20, 2023 if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including the trading day immediately preceding the date on which the Company provides a notice of redemption, during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption. The redemption price will be equal to 100% of the principal amount of the Convertible Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date.

If the Company undergoes a “fundamental change,” as defined in the Indenture, prior to maturity, subject to certain conditions, holders may require the Company to repurchase for cash all or any portion of their Convertible Notes at a fundamental change repurchase price equal to 100% of the principal amount of the notes to be repurchased, plus any accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

Pursuant to Accounting Standards Codification (“ASC”) Subtopic 470-20 Convertible Debt, the Company used an effective interest rate of 10.5% to determine the liability component of the Convertible Notes. This resulted in the recognition of $145,065,803 as the liability component of the Convertible Notes and the recognition of the residual amount of $74,934,197 as the debt discount with a corresponding increase to additional paid in capital for the equity component of the Convertible Notes. Aggregate debt issuance costs of $7,087,763 were allocated to the liability and equity components in the amounts of $4,673,600 and $2,414,162, respectively.

Interest expense for the Convertible Notes was $4,399,093 from inception through December 31, 2019, which included amortization of the discount on the Convertible Notes of $2,089,368, accrued semi-annual coupon payable of $2,224,445 and amortization of debt issuance costs of $85,280. As of December 31, 2019, $4,588,320 of debt issuance costs was recorded on the consolidated balance sheet as a reduction to the carrying amount of the Convertible Notes.

The aggregate balance of the Convertible Notes, net of unamortized debt issuance costs, as of December 31, 2019 is $142,566,851.

Capped Call Transactions

On September 11, 2019, concurrently with the pricing of the Convertible Notes, the Company entered into capped call transactions with two counterparties. The capped call transactions are expected generally to reduce the potential dilution to the Company’s common stock upon any conversion of Convertible Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Convertible Notes, as the case may be, in the event that the market price per share of the Company’s common stock, as measured under the terms of the capped call transactions, is greater than the strike price of the capped call transactions, which is initially $39.4625 (the conversion price of the Convertible Notes) and is subject to anti-dilution adjustments substantially similar to those applicable to the conversion rate of such Convertible Notes. If, however, the market price per share of the Company’s common stock, as measured under the terms of the capped call transactions, exceeds the cap price of the capped call transactions, which is initially $63.14 per share, representing a premium of 100% above the last reported sale price of $31.57 per share of its common stock on The Nasdaq Global Select Market on September 11, 2019, there would nevertheless be dilution and/or there would not be an offset of such potential cash payments, in each case, to the extent that such market price exceeds the cap price of the capped call transactions.

Pursuant to ASC 815-40 Derivatives and Hedging, the Company determined that the capped call transactions should be classified as equity instruments and the capped call premium paid in the amount of $28,380,000 was recorded as a reduction to additional paid-in capital as of September 30, 2019.

Term Loan Facility

On October 20, 2017, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”) to provide for a $20,000,000 term loan facility (the “term loan facility”). Borrowings under the term loan facility accrued interest at a floating rate per annum equal to the WSJ prime rate plus 1.50%. Under the agreement, the Company was required to make monthly interest-only payments through November 1, 2019 and to make 24 equal monthly payments of principal, plus accrued interest, thereafter from November 1, 2019 through October 1, 2021, when all unpaid principal and interest would become due and payable.

On March 26, 2019, the Company voluntarily repaid all outstanding amounts due and owed, including applicable termination fees, under the term loan facility. The final payment of $21,802,403 totaled per diem interest of $101,111 and $21,701,292 for the outstanding balance of the term loan facility which included (i) a final payment equal to 8% of the original principal amount of the term loan facility of $1,600,000, and (ii) a prepayment fee contractually owed of $100,000 plus other fees of $1,292 which resulted in a total loss on extinguishment of debt of $1,208,132.

In connection with the Company’s entry into the term loan facility, the Company issued to SVB a warrant to purchase 14,064 shares of the Company’s common stock with an exercise price per share of $5.484. The warrant had a ten-year term and includes a put option pursuant to which, in the event of an acquisition, change in control or dissolution or winding up of the Company or the expiration of the warrant, SVB could require the Company to repurchase the warrant for a total aggregate purchase price of $250,000. The warrants were exercised in November 2019.

Related Party Promissory Note

On October 19, 2017, the Company issued and sold an unsecured promissory note in the principal amount of $7,000,000 to Golda Darty Partners S.A. (“GDP”), an affiliate of one of the Company’s stockholders. The promissory note accrued interest at a rate per annum of 8.0%, and was due and payable quarterly in arrears on the 19th day of each April, July, October and January. The promissory note had a maturity date of October 19, 2022 when the $7,000,000 would be due and payable in its entirety. The promissory note was contractually subordinated to the Company’s obligations to SFJ under the SFJ agreement.

On September 16, 2019, the Company voluntarily repaid all outstanding amounts due and owed under the promissory note, except for a small amount of interest subsequently repaid. The payment of $7,118,904 reflected per diem interest of $118,904 and $7,000,000 for the outstanding principal balance of the promissory note.

In connection with the issuance and sale of the above promissory note, the Company issued to GDP a warrant to purchase 93,764 shares of the Company’s common stock at a price per share of $5.484, which was exercised in whole in October 2017. The Company recorded the fair value of the warrant in the aggregate amount of $430,160 as a discount to the promissory note. This amount was being accreted as additional interest expense over the term of the promissory note. Upon the repayment of the promissory note, the Company recorded a loss on extinguishment of debt of $293,083 due to the remaining discount.

The contractual maturities of the Company’s long-term debt obligations due subsequent to December 31, 2019 consist of the $220,000,000 Convertible Senior Notes which mature in September 2026.

Leases

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Leases

8. Leases

On January 1, 2019, the Company adopted ASU 2016-02 Leases (Topic 842) using a modified retrospective method. The Company recognized $5,511,865 of lease assets and liabilities. There was no impact to retained earnings upon adoption of Topic 842. The underlying assets of the Company’s leases primarily relate to office space leases, but also include some equipment leases. The Company determines if an arrangement qualifies as a lease at its inception. During 2019, the Company leased additional office space resulting in an initial recognition value of $9,838,982 of lease assets and liabilities.

As a practical expedient permitted under Topic 842, the Company has elected to account for the lease and non-lease components as a single lease component for all leases of which it is the lessee. Lease payments, which may include lease and non-lease components, are included in the measurement of the Company’s lease liabilities to the extent that such payments are either fixed amounts or variable amounts that depend on a rate or index as stipulated in the lease contract. When the Company cannot readily determine the rate implicit in the lease, the Company determines its incremental borrowing rate by using the rate of interest that it would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment.

The Company enters into lease agreements with terms generally ranging from 2-7 years. Some of the Company’s lease agreements include Company options to extend the lease on a month to month basis or for set periods for up to five years. Many leases also include options to terminate the leases within one year or per other contractual terms. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases.

As of December 31, 2019, all leases were classified as operating lease assets and liabilities. Operating lease assets were $14,110,209 and operating lease liabilities were $14,465,975 at December 31, 2019. At December 31, 2019, the weighted average remaining lease term of operating leases was 5.01 years and the weighted average discount rate used to measure the outstanding operating lease liabilities leases was 8.19%.

For the year ended December 31, 2019, the total lease cost for operating lease expense was approximately $2,369,000.

Supplemental cash flow information related to operating leases for the year ended December 31, 2019 is as follows:

Operating cash flows from operating leases

2,013,273

Right of use assets obtained in exchange for

lease obligations

10,201,170

The maturity of the Company’s operating lease liabilities as of December 31, 2019 are as follows:

2020	$	3,679,786
2021		3,336,325
2022		3,269,202
2023		3,287,748
2024 and thereafter		4,106,743
Total future minimum lease payments		17,679,804
Less imputed interest		(3,213,829	)
Total operating lease liabilities	$	14,465,975

Rental expense under operating leases totaled $798,760 for the year ended December 31, 2018.

Under the previous lease accounting standard, ASC 840, Leases, future minimum rental payments for lease obligations with initial terms in excess of one year as of December 31, 2018 were as follows:

2019	$	1,251,720
2020		1,292,025
2021		975,750
2022		998,350
2023 and thereafter		2,341,219
Total	$	6,859,064

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

9. Fair Value Measurements

Level 1 – Quoted prices in active markets for identical assets or liabilities;

Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly;

Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability.

The following table presents the fair value of financial instruments recorded originally at amortized cost or fair value and not re-measured on a recurring basis:

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	92,355,167					$	92,355,167
Total Financial Assets	$	92,355,167	$	—	$	—	$	92,355,167
Financial Liabilities:
Term loan facility			$	18,722,321			$	18,722,321
Related party promissory note				7,401,203				7,401,203
Total Financial Liabilities	$	—	$	26,123,524	$	—	$	26,123,524

	December 31, 2019
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	281,314,069		-			$	281,314,069
Total Financial Assets	$	281,314,069	$	—	$	—	$	281,314,069
Financial Liabilities:
Convertible senior notes*					$	149,496,359		149,496,359
Total Financial Liabilities	$	—	$	—	$	149,496,359	$	149,496,359

*The convertible notes were measured and recognized at fair value on the consolidated balance sheet at inception.

As of December 31, 2018, the fair value of the Company's promissory note was $7,401,203, based on discounted cash flows, market-based expectations for interest rates, credit risk and the contractual terms of the debt instrument. The Company paid interest on the term loan facility at a variable interest rate and accordingly the carrying amount approximated fair value at December 31, 2018.

As of December 31, 2019, the fair value of debt component of the Convertible Notes was $149,496,359, based on the discounted cash flows for comparable straight debt instrument. The Convertible Notes accrue a semi-annual coupon at an annual rate of 3.5%, which was included in accrued expenses in the consolidated balance sheet at December 31, 2019.

The following table presents the fair value of financial instruments recorded at fair value at inception and remeasured on a recurring basis:

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Financial Liabilities:
Common stock warrant liability			$	158,783			$	158,783
Total Financial Liabilities	$	—	$	158,783	$	—	$	158,783

	December 31, 2019
	Level 1		Level 2		Level 3		Total
Financial Liabilities:
Development derivative liability						134,839,000		134,839,000
Total Financial Liabilities	$	—	$	—	$	134,839,000	$	134,839,000

The fair value of the SFJ agreement is presented as a derivative liability based on level 3 inputs. The derivative is valued using a scenario-based discounted cash flow method, whereby each scenario makes assumptions about the probability and timing of cash flows, and such cash flows are present valued using a risk-adjusted discount rate. The analysis is calibrated such that the value of the derivative as of the date of the SFJ agreement was consistent with an arm’s-length transaction. Key inputs to the level 3 fair value model include (i) the probability and timing of achieving stated development milestones to receive the next tranches of funding, (ii) the probability and timing of achieving FDA and EMA approval, (iii) SFJ’s cost of borrowing (8.0%), and (iv) the Company’s cost of borrowing (16.6%). A 10% change in the key inputs of i) the probability and timing of payment and ii) the discount rate utilized would result in a change in fair value of the development derivative liability of approximately 13.3% or $17,886,000.

The common stock warrant liability was valued based upon the underlying value of the Company’s common stock.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

License Agreements

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

License Agreements

10. License Agreements

In connection with its purchase of assets from Potentia in September 2014, the Company became party to a license agreement with the Trustees of the University of Pennsylvania (“Penn”) as a result of an agreement to purchase substantially all the assets of Potentia Pharmaceuticals, Inc, for an exclusive, worldwide license to specified patent rights. The Company is required to pay annual maintenance fees of $100,000 until the first sale of a licensed product. The Company is also required to make milestone payments aggregating up to $3,200,000 based upon the achievement of specified development and regulatory milestones and up to $5,000,000 based upon the achievement of specified annual sales milestones with respect to each licensed product, and to pay low single-digit royalties based on net sales of each licensed product and with minimum quarterly royalty thresholds. In addition, the Company is obligated to pay a specified portion of income it receives from sublicensees.

In addition, the Company is also party to a license agreement with Penn for an exclusive, worldwide license to specified patent rights for the development and commercialization of products in fields of use, as defined therein. The Company is required to pay annual maintenance fees of $100,000 until the first sale of a licensed product. The Company is required to make milestone payments aggregating up to $1,650,000, based upon the achievement of development and regulatory approval milestones, and up to $2,500,000, based upon the achievement of annual sales milestones with respect to each of the first two licensed products. The license agreement also requires the Company to pay low single digit royalties based on net sales of each licensed product, subject to minimum quarterly royalty thresholds. In addition, the Company is obligated to pay a specified portion of income it receives from sublicensees.

In 2018, the Company made payments of $1,025,000, net of a credit for the annual license maintenance payment, for the achievement of milestones under these agreements.

401(k) Profit Sharing Plan and Trust

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

401(k) Profit Sharing Plan and Trust

11. 401(k) Profit Sharing Plan and Trust

In July 2010, the Company adopted an employee profit-sharing plan (the “401(k) Plan”), qualified under Section 401(k) of the Internal Revenue Code (the “IRC”). All of the Company’s full-time employees who have attained the age of 21 are eligible to participate in the 401(k) Plan immediately upon employment. Pursuant to the 401(k) Plan, employees may elect to reduce their current compensation by up to the statutorily prescribed annual limit and have the amount of the reduction contributed to the 401(k) Plan. In 2017, 2018 and 2019 the Company recorded $123,624, $322,459 and $806,915 respectively, for employer contributions made to the 401(k) Plan.

Refundable Research and Development Credits and Income Taxes

12 Months Ended

Dec. 31, 2019

Text Block [Abstract]

Refundable Research and Development Credits and Income Taxes

12. Refundable Research and Development Credits and Income Taxes

The Company earns non-income related refundable Australian research and development credits that are settled and paid to the Company annually. The associated income from the credits are an offset to research and development expenses.

The components of loss from continuing operations before provision for income taxes are as follows:

	Year Ended December 31,
	2017			2018			2019
Domestic	$	(47,523,183	)	$	(122,900,285	)	$	(297,127,180	)
Foreign		(3,482,911	)		(4,601,909	)		(7,579,433	)
Total	$	(51,006,094	)	$	(127,502,194	)	$	(304,706,613	)

The Company’s income tax provision is computed based on the federal statutory rate and the average state statutory rates, net of the related federal benefit. For the years ended December 31, 2018 and 2019, there was no current or deferred income tax expense or benefit due to the Company’s net losses and increases in its deferred tax asset valuation allowance.

The Company’s effective income tax provision differs from the amount calculated using the statutory U.S. federal income tax rate, principally due to the following:

	Year Ended December
	2017						2018						2019
	Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes
Statutory U.S. federal income tax	$	(17,852,133	)		35.0	%	$	(26,775,461	)		21.0	%	$	(63,988,409	)		21.0	%
State income taxes, net of federal benefit		(1,634,924	)		3.3			(6,397,993	)		5.0			(16,423,857	)		5.4
Change in valuation allowances		11,176,291			(20.8	)		38,156,506			(29.9	)		90,300,408			(29.6	)
Re-measurement of deferred taxes		15,659,916			(31.9	)		—			—			—			—
Tax credits		(3,316,729	)		7.7			(6,149,021	)		4.8			(6,113,296	)		2.0
Change in state apportionment, Kentucky tax reform and other		—			—			873,508			(0.7	)		(16,861	)		0.0
Permanent and other		(4,032,421	)		6.7			292,461			(0.2	)		(3,757,985	)		1.2
Effective income tax provision	$	—			—	%	$	—			—	%	$	—			—	%

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities are as follows:

	December 31,
	2018			2019
Deferred tax assets:
Intangible assets	$	19,616		$	15,519
Share-based compensation		2,914,583			6,624,638
Deferred interest expense		135,585			—
Contribution carryforwards		36,759			41,409
Net operating loss carryforwards		64,296,165			102,623,330
Research and development credits		7,269,624			14,476,348
Orphan drug credits		7,165,354			9,380,475
Development derivative liability		—			35,874,491
Accruals		—			2,259,190
Other		—			11,250
Total deferred tax assets		81,837,686			171,306,650
Deferred tax liabilities:
Accrual to cash adjustment		(2,463,715	)		—
Unrealized gain		(12,094	)		—
Convertible debt		—			(11,479,891	)
481(a) adjustment		—			(2,891,490	)
Total deferred tax liabilities		(2,475,809	)		(14,371,381	)
Net deferred tax assets before allowance:		79,361,877			156,935,269
Less valuation allowance		(79,361,877	)		(156,935,269	)
Net deferred tax assets	$	—		$	—

At December 31, 2018, the Company had approximately $251,388,000 and $254,631,000 of Federal and state net operating loss carryforward, respectively. At December 31, 2019, the Company had approximately $392,878,000 and $358,345,000 of Federal and state net operating loss carryforwards, respectively. The Company also had $24,163,000 of federal research and development tax credit carryforwards as of December 31, 2019. The net operating loss and research and development tax credit carryforwards begin to expire in 2024.

The net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the IRC, respectively, as well as similar state provisions.

The Company’s net operating loss carryforwards (“NOLs”) and other deferred tax assets can generally be used to offset future taxable income and therefore reduce federal income tax obligations. However, the Company's ability to use its NOLs would be limited if there was an “ownership change” as defined by Section 382. This would occur if shareholders owning (or deemed to own under the tax rules) 5% or more of the Company's common stock increase their aggregate ownership of the common stock of the Company by more than 50 percentage points over a defined period of time. The Company had an ownership change on September 8, 2015. As a result, a portion of the NOL and tax credit carryforwards are subject to an annual utilization limitation.

The deferred tax asset includes approximately $99,955,000 of Federal and State NOLs and $24,163,000 of Federal and State research & development tax credit carryforwards. Of the $99,955,000 of Federal and State NOLs, approximately $10,716,000 is limited under Section 382. Of the $24,163,000 of Federal and State tax credit carryforwards, approximately $1,892,000 is limited under Section 383. Subsequent ownership changes may further affect the limitation in future years.

The Company’s estimate of the realizability of the deferred tax asset is dependent on its estimate of projected future levels of taxable income. In analyzing future taxable income levels, the Company considered all evidence currently available, both positive and negative. Based on its analysis, the Company recorded a valuation allowance for deferred tax assets, net of deferred tax liabilities, as of December 31, 2018 and 2019.

Tax Act Enactment

On December 22, 2017, the U.S. government enacted comprehensive federal tax legislation commonly referred to as the Tax Cuts and Jobs Act of 2017 (the "Tax Act"). The Tax Act significantly modifies the U.S. corporate income tax system by, among other things, reducing the federal income tax rate from 35% to 21% beginning in 2018, imposing a mandatory one-time deemed repatriation tax on accumulated foreign earnings and creating a territorial tax system that changes the manner in which future foreign earnings are subject to U.S. tax. The Tax Act also provides that undistributed and previously untaxed post-1986 foreign earnings will be deemed distributed in 2017 and be subject to tax at reduced effective rates (the “Transition Tax”). The Company has a net cumulative deficit in earnings and profits from its foreign subsidiaries and, consequently, will not be subject to the Transition Tax. The company re-measured certain net deferred tax assets and liabilities based on the tax rates at which they are expected to reverse in the future. The total impact to the gross deferred tax asset balance (before valuation allowance) upon enactment of the Tax Act is $15,659,916. As there is a valuation allowance recorded for all deferred tax assets as of December 31, 2017, there was no impact to the net deferred tax assets balance. The Company has completed its assessment of the U.S. Tax Reform Act and has determined no additional impacts during 2018.

The Company has generated research credits and orphan drug credits but has not conducted a detailed study to document its qualified activities. A detailed study could result in an adjustment to the Company’s research and development credit carryforwards; however, until such a study is completed and any adjustment is identified, no amounts are being presented as an uncertain tax position.

The Company does not have any unrecognized tax benefits during any periods presented and does not expect this to significantly change in the next twelve months.

There were no interest and penalties recorded in the statement of operations during any period and no amounts accrued for interest and penalties at December 31, 2018, or 2019.

The Company files income tax returns in the U.S. federal jurisdiction, and applicable state jurisdictions and in Australia’s and Switzerland’s federal jurisdiction. The Company’s earliest tax years open and subject to examination by taxing authorities are 2016, for the U.S. federal jurisdiction, and 2015, for U.S. states’ and Australia’s federal jurisdiction. Federal and state net operating losses are subject to review by taxing authorities in the year utilized.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

13. Commitments and Contingencies

The Company has entered into a leasing agreement for lab space in Watertown, Massachusetts with an estimated commencement date of June 2, 2020 (the “Lab Lease”). The Lab Lease has a term of 85 months and includes leasing 9,704 square feet of office space. The Lab Lease provides for initial monthly lease payments of $58,224 per month. The base rent payable over the lease period is $5,425,279.

The Company contracts to conduct research and development activities with third parties. Certain of these contracts commit the Company to pay future milestone payments up to $15,000,000 or to pay royalty fees ranging from 3-6% if any of the research results in regulatory approval or commercial revenue for a product. The scope of the services under the research and development contracts can be modified and the contracts cancelled by the Company upon written notice. In some instances, the contracts may be cancelled by the third party upon written notice. If the Company were to cancel these contracts as of December 31, 2019, the Company would be required to pay certain termination costs and other fees of approximately $1,789,000 that would be incurred in future periods.

As of December 31, 2019, the Company has non-cancellable purchase commitments for 2020 with certain manufacturing vendors in the amount of approximately $35,300,000.

The Company also has certain payment and other obligations under the SFJ agreement, which are discussed above in Note 5.

Indemnifications—In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend indemnified parties for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has not incurred any cost to defend lawsuits or settle claims related to these indemnification provisions.

Legal—During the normal course of business, the Company may be a party to legal claims that may not be covered by insurance. Management does not believe that any such claims would have a material impact on the Company’s consolidated financial statements.

Equity Incentive Plans

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans

14. Equity Incentive Plans

Share-based Compensation

The Company’s Board of Directors adopted, and its stockholders approved, an equity incentive plan in 2010 (as amended, the “2010 Plan”). The Board of Directors and stockholders amended the 2010 Plan in August 2017 to increase the number of shares of common stock reserved for issuance thereunder to 6,188,466. The 2010 Plan allowed for the grant of incentive stock options and non-qualified stock options to purchase common stock for employees, directors and consultants under terms and conditions established by the Board of Directors. Incentive stock options and nonqualified stock options were granted at exercise prices that were no less than 100% of the estimated fair value per share of the common stock on the date of grant. If an individual owns capital stock representing more than 10% of the voting shares, the price of each share was at least 110% of the fair value on the date of grant. The Board of Directors determined the fair value of common stock with the assistance of a third-party specialist. Options expire 10 years from the issuance date. Following the adoption of the 2017 Stock Incentive Plan, the Company no longer grants stock options or other awards under the 2010 Plan.

In October 2017, the Company’s Board of Directors adopted, and its stockholders approved, the 2017 Stock Incentive Plan (the “2017 Plan”), which became effective on November 8, 2017. The 2017 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, awards of restricted stock, restricted stock units and other stock-based awards. The number of shares of common stock reserved for issuance under the 2017 plan is the sum of (i) 1,359,587 shares of common stock, plus (ii) an additional number of shares of common stock equal to the sum of (a) the number of shares of common stock reserved for issuance under the 2010 equity incentive plan that remained available for future issuance immediately prior to the effectiveness of the 2017 Plan, which was 299,568 shares, and (b) the number of shares of common stock subject to outstanding awards under the 2010 equity incentive plan upon effectiveness of the 2017 plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by us at their original issuance price pursuant to a contractual repurchase right plus (iii) an annual increase, to be added the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors.

Additionally, during 2019 and thereafter, the Company has granted equity awards as equity inducement awards material to entry into employment with the Company to certain newly hired employees outside of the Company’s existing plans in accordance with Nasdaq listing rule 5635(c)(4).

Stock Options—Options granted generally vest over 48 months. Options granted to employees on or after December 5, 2013 generally vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial vesting commencement date (as defined) subject to the employee’s continuous service with the Company. Options granted before December 5, 2013 vest over four years in equal annual installments of 25% at each anniversary of the grant date.

Under the Executive Separation Benefits and Retention Plan and by resolutions adopted by the Compensation Committee in October 2019, the stock options granted to the Company’s executives and employees will become fully vested upon the occurrence of a change in control, as defined in the Executive Separation Benefits and Retention Plan, if such executive or employee is terminated without cause or resigns for good reason within 12 months after such change in control.

Effective January 22, 2016, the Board of Directors approved a modification in the exercise price of stock options to purchase 93,762 shares of common stock that were granted under the 2010 Plan to reduce the exercise price per share to $3.76 per share, which was the estimated fair market value of the common stock on the effective date of the repricing. Other stock options granted under the 2010 Plan were excluded from this repricing and will maintain their original exercise prices. The stock options that were repriced had been granted with an exercise price greater than the estimated fair market value in July and September 2015 (i.e., exercise prices ranging from $6.81 to $6.89 per share). Because the exercise prices of these stock options exceeded the estimated fair market value of the Company’s common stock on the modification date, the Board of Directors determined that the retentive value of these awards had substantially diminished from the time they had been granted. The Board of Directors determined that this repricing was in the best interests of the Company and its stockholders to provide a continued incentive for highly qualified employees and consultants with substantial experience in the Company’s business to remain employed during a critical period for the Company.

The following table summarizes the Company’s stock option activity:

				Weighted -		Weighted -		Weighted -
				Average		Average		Average
				Exercise		Grant Date		Contractual		Aggregate
				Price		Fair Value		Life		Intrinsic
	Shares			Per Share		Per Option		(in years)		Value
Outstanding, December 31, 2017		6,355,581			4.78		2.66		7.64		107,534,863
Granted		2,060,685			15.86		13.13		—		—
Exercised		(468,263	)		2.61		1.46		—		4,953,187
Forfeited		(450,269	)		6.58		4.27		—		—
Outstanding, December 31, 2018		7,497,734			8.09		5.52		7.31		45,706,169
Granted		4,365,520			21.15		17.31		—		—
Exercised		(747,136	)		3.85		2.81		—		15,659,119
Forfeited		(261,681	)		12.67		10.34		—		—
Outstanding, December 31, 2019		10,854,437			13.52		10.43		7.59		185,847,165
Options exercisable, December 31, 2019		4,808,730			6.94		4.62		5.82		113,868,890
Expected to vest, December 31, 2019		6,045,707			18.76		15.04		9.01		71,978,275

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the common stock as of December 31, 2019. Estimated fair values of the common stock at the time of the grants between May 12, 2010 and the Company’s IPO in November 2017 were between $1.71 and $14.00. Actual fair market values of the Company’s common stock from the time of the IPO through December 31, 2019 were between $11.47 and $32.18. The fair market value of options vested during the years ended December 31, 2018 and 2019 were $5,062,428 and $12,906,436, respectively.

Total share-based compensation expense during the years ended was as follows:

	Year Ended December 31,
	2017		2018		2019
Research and development	$	2,678,956	$	3,559,047	$	10,682,553
General and administrative		2,739,754		4,174,209		10,461,476
Total share-based compensation expense	$	5,418,710	$	7,733,256	$	21,144,029

At December 31, 2017, 2018 and 2019, unrecognized compensation expense related to unvested options, was $10,415,787, $27,266,206 and $79,291,036, respectively, which the Company expects to recognize over an estimated weighted-average period of 3.14, 2.99, and 3.24 years, respectively.

The assumptions used in the Monte Carlo and Black-Scholes models to estimate the grant date fair value are as follows:

	Year Ended December 31,
	2017	2018	2019
Risk-free interest rate	1.21 - 2.30%	2.68 - 2.99%	1.42 - 2.56%
Dividend yield	0%	0%	0%
Volatility	51.6 - 55.8%	87.0 - 110.9%	102.6 - 111.2%
Expected terms (years)	5.3 - 7.0	5.94 - 6.25	5.31 - 6.08

2017 Employee Stock Purchase Plan

In October 2017, the Company’s board of directors adopted and the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (“ESPP”), which became effective upon the IPO and provides participating employees with the opportunity to purchase up to an aggregate of 468,823 shares of common stock. The number of shares of common stock reserved for issuance under the 2017 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2018 and continuing until, and including, the fiscal year ending December 31, 2027, equal to the lowest of (i) 937,646 shares of common stock, (ii) 1.0% of the number of shares of common stock outstanding on the first day of the fiscal year and (iii) an amount determined by the board of directors. The board of directors initiated the first offering under the ESPP in October 2019.

Net Loss per Common Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss per Common Share

15. Net Loss per Common Share

The following table presents the calculation of basic and diluted net loss per common share:

	Year Ended December 31,
	2017			2018			2019
Numerator:
Net loss	$	(51,006,094	)	$	(127,502,194	)	$	(304,706,613	)
Denominator:
Weighted-average number of common shares used in net loss per common share -- basic and diluted		13,870,949			54,396,483			62,228,601
Net loss per common share -- basic and diluted	$	(3.68	)	$	(2.34	)	$	(4.90	)

Shares outstanding presented below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, as their effect is anti-dilutive:

	Year Ended December 31,
	2017		2018		2019
Common stock under option		6,355,581		7,497,734		10,854,437
Common stock warrants		14,064		14,064		—
Total		6,369,645		7,511,798		10,854,437

Related Party Transaction

12 Months Ended

Dec. 31, 2019

Related Party Transaction

16. Related Party Transaction

Effective as of May 1, 2018, the Company entered into a subscription license agreement and a services agreement with Revon Systems, Inc. (“Revon”). Under the subscription license agreement, Revon granted the Company an exclusive license to use the Revon software platform and applications for any purpose with respect to the Company's programs in age-related macular degeneration, hemolytic diseases and complement-dependent nephropathies for an annual license fee of $175,000 and an option to obtain a perpetual, exclusive license thereafter for $350,000. Under the services agreement, Revon provided development services with respect to the Revon software to the Company for $250,000 during the first year. Prior to the acquisition of Revon by an unrelated third party in July 2019, the Company paid the remainder of the annual license fee due for the second year, exercised the option for the perpetual license and discontinued the services agreement. For the year ended December 31, 2019, the Company paid Revon a total of $691,667.

Each of Cedric Francois, the Company’s chief executive officer, Pascal Deschatelets, the Company’s chief operating officer, and Alec Machiels, a member of the board of directors, was an affiliate of Revon. The Board approved the Revon agreements after review by a subcommittee of the disinterested members of the Board and determination by the full Board that the terms of the Revon agreements were fair, reasonable and in the best interests of the Company. The exercise of the option for the perpetual license was approved in accordance with the Company’s related party transaction policy.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

17. Selected Quarterly Financial Data (Unaudited)

The following interim financial information presents the Company’s 2018 and 2019 results of operations on a quarterly basis:

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2018			2018			2018			2018
Operating Loss	$	(21,438,145	)	$	(33,485,442	)	$	(35,804,581	)	$	(37,196,592	)
Net Loss		(21,736,304	)		(33,334,300	)		(35,972,115	)		(36,459,475	)
Net Loss per common share, basic and diluted (1)		(0.43	)		(0.61	)		(0.64	)		(0.65	)

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2019			2019			2019			2019
Operating Loss	$	(48,650,570	)	$	(63,476,438	)	$	(69,947,962	)	$	(105,940,283	)
Net Loss		(50,574,367	)		(71,090,164	)		(69,824,659	)		(113,217,423	)
Net Loss per common share, basic and diluted (1)		(0.87	)		(1.12	)		(1.10	)		(1.77	)

(1)

The sum of the four quarters of earnings per share for 2018 and 2019 may not add to the full year earnings per share amount due to rounding and/or the use of quarter-to-date weighted average shares to calculate the earnings per share amount in each respective quarter.

Subsequent Events

12 Months Ended

Dec. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

18. Subsequent Events

On January 13, 2020, the Company issued and sold 10,925,000 shares of its common stock at a price per share to the public of $37.00 in a follow-on public offering, including an additional 1,425,000 shares of its common stock that were sold at the follow-on public offering price of $37.00 per share pursuant to the underwriters’ in full exercise of their option to purchase additional shares of common stock. The Company received net proceeds of approximately $381,374,000 after deducting underwriting discounts and commissions of approximately $22,232,000 and offering costs of approximately $618,000 for these transactions.

The Company received the final $20,000,000 from the SFJ development funding agreement on January 29, 2020.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Apellis Australia Pty Ltd, Apellis Switzerland GmbH, Apellis Ireland Ltd., and Apellis MA Securities, Inc. All intercompany balances and transactions have been eliminated in consolidation. The consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”).
Reclassifications	Reclassifications Certain prior year amounts have been reclassified to conform to the 2019 presentation.
Offering Costs	Offering Costs Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s follow-on offerings and filing of a registration statements on Form S-3 in 2018 and 2019, and related to the Company’s offering of Convertible Notes in 2019. Costs were deferred until completion of the follow-on offerings and offering of Convertible Notes, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds. Deferred issuance costs related to the registration statement on Form S-3 that the Company filed in January 2020 have been capitalized and will be reclassified to additional paid-in capital as a reduction of the proceeds at the time when shares are issued. The Company does not consider these amounts material at December 31, 2019.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: share-based compensation expense, fair value of common stock and preferred stock, development derivative liability, accrued expenses, prepaid expenses and convertible debt. Historically for all periods prior to the IPO, the Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the retrospective fair value of its common stock during all periods presented. The methodologies include a probability analysis including both a potential public trading scenario and potential sale scenario. In both scenarios, value is estimated using the guideline public company method. The sale scenario includes an adjustment for a market participant acquisition premium. Value is allocated among the preferred and common shares according to the rights associated with each type of security. Valuation methodologies include estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock and the associated fair value of stock options granted at each valuation date. Since the completion of the IPO, the Company’s board of directors determines the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of grant.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company is required to disclose information on the fair value of financial instruments and inputs that enable an assessment of the fair value. The three levels of the fair value hierarchy prioritize valuation inputs based upon the observable nature of those inputs as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities; Level 2 – Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly; Level 3 – Unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. The Company’s financial instruments, in addition to those presented in Note 7, Long-term Debt and Note 9, Fair Value Measurements, include cash and cash equivalents, the Australian research and development credit, accounts payable and accrued liabilities. Management believes that the carrying amounts of cash and cash equivalents, the Australian research and development credit, accounts payable and accrued expenses approximate the fair value due to the short-term nature of those instruments.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents are defined as cash in banks and investment instruments having maturities of three months or less from their acquisition date. The carrying amounts reported in the consolidated balance sheets for cash and cash equivalents are valued at cost, which approximates the fair value. As of December 31, 2018 and 2019, the Company had assets consisting of cash and cash equivalents held in banks outside of the United States of approximately $121,600 and $1,598,300, respectively.
Foreign Currency	Foreign Currency The financial position and results of operations of the Company's Australian and Swiss subsidiaries are measured using the foreign subsidiary's local currency. Revenues and expenses of the subsidiaries have been translated into U.S. dollars at average exchange rates prevailing during the period from January 1, 2019 through December 31, 2019. Assets and liabilities have been translated at the rates of exchange on the balance sheet date. The resulting translation gain and loss adjustments are recorded directly as a separate component of stockholders’ equity. Prior to August 1, 2018, when the functional currency of the Company’s Australian subsidiary was changed from the U.S. dollar to the Australian dollar, the transactions with the Australian subsidiary were recorded in U.S. dollars and were not translated.
Research and Development	Research and Development Costs incurred in connection with research and development activities are expensed as incurred. Research and development expenses include (i) employee-related expenses, including salaries, benefits, travel and share-based compensation expense; (ii) external research and development expenses incurred under arrangements with third parties, such as contract research and contract manufacturing organizations, investigational sites and consultants, including share-based compensation expense for consultants; (iii) the cost of acquiring, developing and manufacturing clinical study materials; and (iv) costs associated with preclinical and clinical activities and regulatory operations. The Company enters into consulting, research and other agreements with commercial entities, researchers, universities and others for the provision of goods and services. Such arrangements are generally cancellable upon reasonable notice and payment of costs incurred. Costs are considered incurred based on an evaluation of the progress to completion of specific tasks under each contract using information and data provided by the Company’s clinical sites and vendors. These costs consist of direct and indirect costs associated with specific projects, as well as fees paid to various entities that perform certain research on behalf of the Company. Depending upon the timing of payments to the service providers, the Company recognizes prepaid expenses or accrued expenses related to these costs. These accrued or prepaid expenses are based on management’s estimates of the work performed under service agreements, milestones achieved and experience with similar contracts. The Company monitors each of these factors and adjusts estimates accordingly. Included in research and development is a refundable Australian research and development credit. The credit is recognized as a reduction of clinical trial costs over the periods necessary to match the benefit of the credit with the costs for which it is intended to compensate.
Patents	Patents Costs incurred in connection with the application for and issuance of patents are expensed as incurred.
Income Taxes	Income Taxes The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of December 31, 2018 and 2019, the Company did not have any significant uncertain tax positions.
Share-based Compensation	Share-based Compensation The Company recognizes all share-based payments to employees, including grants of employee stock options, in the statements of operations and comprehensive loss based on their estimated fair values over the requisite service periods for each award. The Company remeasures the fair value of share-based awards to non-employees until a performance commitment is reached or counterparty performance is complete. The Company’s share-based compensation awards are comprised of stock options. The Company estimates the fair value of each stock option grant using the Monte Carlo simulation model (“Monte Carlo”) for grants made prior to June 30, 2015 and the Black-Scholes option pricing model (“Black-Scholes”) for grants made on or after July 1, 2015 through October 2017. After the Company’s IPO in November 2017, the Company’s board of directors determined the fair value of underlying common stock based upon the closing price of the Company’s common stock as reported on the date of the grant. Both Monte Carlo and Black-Scholes for option pricing require the input of the six minimum considerations detailed in ASC 718, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the trading of the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of comparable companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company believes the group selected has sufficient similar economic and industry characteristics. Before the IPO, the Company calculated the expected term for options granted to employees based on a quarterly weighted average probability of exit analysis considering milestones that the Company had achieved and each of the potential exit scenarios available to the Company at that time. After the IPO, the Company calculates the expected term for options granted to employees based on the “simplified” method set forth in SEC Staff Accounting Bulletin 107, because it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term due to the limited period of time its equity shares have been publicly traded. Under the simplified method, the weighted average vesting period is averaged with the contractual term of the option being valued. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company’s share-based awards are subject to service based vesting conditions. Compensation expense related to awards to employees with service based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Consistent with the guidance in ASC 505-50, compensation expense related to awards to non-employees with service based vesting conditions is recognized on a straight-line basis based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. The Company recognizes forfeitures for employee and non-employee grants at the time they occur. Share-based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest.
Concentrations of Credit Risk	Concentrations of Credit Risk Cash and cash equivalents are the only financial instruments that potentially subject the Company to concentrations of credit risk. Cash and cash equivalents are held at financial institutions in the United States, Australia and Switzerland. The Company is exposed to credit risk in the event of default by the financial institution to the extent that cash and cash equivalent balances recorded in the balance sheets are in excess of the amounts that are insured by the Federal Deposit Insurance Corporation. The Company has not experienced any losses on its deposits since inception, and management believes that minimal credit risk exists with respect to these financial institutions.
Net Loss per Share	Net Loss per Share Basic net loss per common share is calculated by dividing net loss by the weighted-average shares outstanding during the period. For purposes of the diluted net loss per share calculation, convertible notes and common stock options are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.
Comprehensive Loss	Comprehensive Loss The Company’s components of comprehensive loss other than its net loss, are the foreign currency gains/losses recorded from the remeasurement of the long term intra-entity loan transaction to the Company’s wholly owned subsidiaries as well as the foreign currency gain/ loss from the translation of the Company’s wholly owned subsidiaries into U.S. dollars in 2018 and 2019. These components are a result of the change in the Australian subsidiary’s functional currency to Australian dollars effective August 1, 2018 and the creation of the Swiss subsidiary in 2019.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606). The standard will apply one comprehensive revenue recognition model across all contracts, entities and sectors. The core principle of the new standard is that revenue should be recognized to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 will replace most of the existing revenue recognition requirements in U.S. GAAP. The FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which deferred the effective date of the standard for one year. As a result, the new standard was effective for annual reporting periods beginning after December 15, 2017, for public companies, including interim periods within that reporting period. As the Company does not currently have revenue, the adoption of the new standard had no impact on its consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments – Overall, amending several elements surrounding the recognition and measurement of financial instruments and requiring equity investments (except those accounted for under the equity method of accounting or those that result in consolidation) to be measured at fair value with changes in fair value recognized in net income. Effective January 1, 2018, the Company adopted this ASU on a prospective basis. The adoption did not have a material impact on its consolidated financial statements and related disclosures. In February 2016, the FASB issued ASU 2016-02 Leases (Topic 842) which establishes a right-of-use ("ROU") model that requires a lessee to record a ROU asset and a lease liability on the consolidated balance sheet for all leases with terms longer than twelve months. Leases will be either classified as finance or operating, with classification affecting the pattern of expense recognition in the income statement. ASU 2016-02 also requires significant additional disclosures about the amount, timing and uncertainty of cash flows from leases. The Company adopted ASU 2016-02 effective January 1, 2019 using a modified retrospective method and will not restate comparative periods. As permitted under the guidance, the Company carried forward the assessment of whether contracts contain or are leases, the classification of leases and remaining terms. The Company recognized $5,511,865 of ROU assets and operating lease liabilities, primarily relating to office space leases, on the consolidated balance sheet upon adoption. During 2019, the Company leased additional office space resulting in an initial recognition value of $9,838,982 of ROU assets and operating lease liabilities. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement – Disclosure Framework, that modifies the disclosure requirements with respect to fair value measurements. Among the changes, entities will no longer be required to disclose the amount of and reasons for transfers between Levels 1 and 2 of the fair value hierarchy, but will be required to disclose the range and weighted average used to develop significant unobservable inputs for Level 3 fair value measurements. Effective January 1, 2018, the Company early adopted this ASU, which did not have a material impact on its consolidated financial statements and related disclosures.

Common Stock (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Summary of Shares of Common Stock Reserved for Future Issuance

The Company has reserved the following shares of common stock for future issuance:

	December 31,
	2017		2018		2019
Shares reserved under 2010 Equity Incentive Plan		5,817,844		5,013,153		—
Shares reserved under 2017 Equity Incentive Plan		1,682,596		4,032,390		6,319,001
Shares reserved under 2017 Employee Stock Purchase Plan		468,823		972,164		972,164
Common stock warrants		14,064		14,064		—
Total		7,983,327		10,031,771		7,291,165

Development Derivative Liability (Tables)

12 Months Ended

Dec. 31, 2019

Research And Development [Abstract]

Schedule of Development Derivative Liability

The following table presents a rollforward of the liability:

	Development Derivative Liability
Balance at fair market value, January 1, 2019	$	—
Amounts received under the SFJ agreement and SFJ amendment		120,000,000
Loss recorded in loss from remeasurement of development derivative liability		14,839,000
Balance at fair market value, December 31, 2019	$	134,839,000

Accrued Expenses and Prepaid Assets (Tables)

12 Months Ended

Dec. 31, 2019

Accrued Expenses And Prepaid Assets [Abstract]

Schedule of Accrued Expenses

Accrued expenses are as follows:

	December 31,
	2018		2019
Accrued research and development	$	3,481,570	$	36,449,386
Accrued payroll liabilities		269,046		11,442,889
Other		1,352,386		6,890,676
Total	$	5,103,002	$	54,782,951

Leases (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Supplemental Cash Flow Information Related to Operating Leases

Supplemental cash flow information related to operating leases for the year ended December 31, 2019 is as follows:

Operating cash flows from operating leases

2,013,273

Right of use assets obtained in exchange for

lease obligations

10,201,170

Maturity of Operating Lease Liabilities

The maturity of the Company’s operating lease liabilities as of December 31, 2019 are as follows:

2020	$	3,679,786
2021		3,336,325
2022		3,269,202
2023		3,287,748
2024 and thereafter		4,106,743
Total future minimum lease payments		17,679,804
Less imputed interest		(3,213,829	)
Total operating lease liabilities	$	14,465,975

Schedule of Future Minimum Rental Payments for Lease Obligations

future minimum rental payments for lease obligations with initial terms in excess of one year as of December 31, 2018 were as follows:

2019	$	1,251,720
2020		1,292,025
2021		975,750
2022		998,350
2023 and thereafter		2,341,219
Total	$	6,859,064

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value, Nonrecurring [Member]

Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized

The following table presents the fair value of financial instruments recorded originally at amortized cost or fair value and not re-measured on a recurring basis:

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	92,355,167					$	92,355,167
Total Financial Assets	$	92,355,167	$	—	$	—	$	92,355,167
Financial Liabilities:
Term loan facility			$	18,722,321			$	18,722,321
Related party promissory note				7,401,203				7,401,203
Total Financial Liabilities	$	—	$	26,123,524	$	—	$	26,123,524

	December 31, 2019
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	281,314,069		-			$	281,314,069
Total Financial Assets	$	281,314,069	$	—	$	—	$	281,314,069
Financial Liabilities:
Convertible senior notes*					$	149,496,359		149,496,359
Total Financial Liabilities	$	—	$	—	$	149,496,359	$	149,496,359

*The convertible notes were measured and recognized at fair value on the consolidated balance sheet at inception.

Fair Value, Recurring [Member]

Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized

The following table presents the fair value of financial instruments recorded at fair value at inception and remeasured on a recurring basis:

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Financial Liabilities:
Common stock warrant liability			$	158,783			$	158,783
Total Financial Liabilities	$	—	$	158,783	$	—	$	158,783

	December 31, 2019
	Level 1		Level 2		Level 3		Total
Financial Liabilities:
Development derivative liability						134,839,000		134,839,000
Total Financial Liabilities	$	—	$	—	$	134,839,000	$	134,839,000

Refundable Research and Development Credits and Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Refundable Research And Development Credits And Income Taxes [Abstract]

Components of Loss From Continuing Operation Before Provision for Income Taxes

The components of loss from continuing operations before provision for income taxes are as follows:

	Year Ended December 31,
	2017			2018			2019
Domestic	$	(47,523,183	)	$	(122,900,285	)	$	(297,127,180	)
Foreign		(3,482,911	)		(4,601,909	)		(7,579,433	)
Total	$	(51,006,094	)	$	(127,502,194	)	$	(304,706,613	)

Schedule of Effective Income Tax Rate Provision Amount Differs from Statutory U.S. Federal Income Tax Rate

The Company’s effective income tax provision differs from the amount calculated using the statutory U.S. federal income tax rate, principally due to the following:

	Year Ended December
	2017						2018						2019
	Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes			Amount			Percentage of income before income taxes
Statutory U.S. federal income tax	$	(17,852,133	)		35.0	%	$	(26,775,461	)		21.0	%	$	(63,988,409	)		21.0	%
State income taxes, net of federal benefit		(1,634,924	)		3.3			(6,397,993	)		5.0			(16,423,857	)		5.4
Change in valuation allowances		11,176,291			(20.8	)		38,156,506			(29.9	)		90,300,408			(29.6	)
Re-measurement of deferred taxes		15,659,916			(31.9	)		—			—			—			—
Tax credits		(3,316,729	)		7.7			(6,149,021	)		4.8			(6,113,296	)		2.0
Change in state apportionment, Kentucky tax reform and other		—			—			873,508			(0.7	)		(16,861	)		0.0
Permanent and other		(4,032,421	)		6.7			292,461			(0.2	)		(3,757,985	)		1.2
Effective income tax provision	$	—			—	%	$	—			—	%	$	—			—	%

Schedule of Deferred Tax Assets and Liabilities

Significant components of the Company’s deferred tax assets and liabilities are as follows:

	December 31,
	2018			2019
Deferred tax assets:
Intangible assets	$	19,616		$	15,519
Share-based compensation		2,914,583			6,624,638
Deferred interest expense		135,585			—
Contribution carryforwards		36,759			41,409
Net operating loss carryforwards		64,296,165			102,623,330
Research and development credits		7,269,624			14,476,348
Orphan drug credits		7,165,354			9,380,475
Development derivative liability		—			35,874,491
Accruals		—			2,259,190
Other		—			11,250
Total deferred tax assets		81,837,686			171,306,650
Deferred tax liabilities:
Accrual to cash adjustment		(2,463,715	)		—
Unrealized gain		(12,094	)		—
Convertible debt		—			(11,479,891	)
481(a) adjustment		—			(2,891,490	)
Total deferred tax liabilities		(2,475,809	)		(14,371,381	)
Net deferred tax assets before allowance:		79,361,877			156,935,269
Less valuation allowance		(79,361,877	)		(156,935,269	)
Net deferred tax assets	$	—		$	—

Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity:

				Weighted -		Weighted -		Weighted -
				Average		Average		Average
				Exercise		Grant Date		Contractual		Aggregate
				Price		Fair Value		Life		Intrinsic
	Shares			Per Share		Per Option		(in years)		Value
Outstanding, December 31, 2017		6,355,581			4.78		2.66		7.64		107,534,863
Granted		2,060,685			15.86		13.13		—		—
Exercised		(468,263	)		2.61		1.46		—		4,953,187
Forfeited		(450,269	)		6.58		4.27		—		—
Outstanding, December 31, 2018		7,497,734			8.09		5.52		7.31		45,706,169
Granted		4,365,520			21.15		17.31		—		—
Exercised		(747,136	)		3.85		2.81		—		15,659,119
Forfeited		(261,681	)		12.67		10.34		—		—
Outstanding, December 31, 2019		10,854,437			13.52		10.43		7.59		185,847,165
Options exercisable, December 31, 2019		4,808,730			6.94		4.62		5.82		113,868,890
Expected to vest, December 31, 2019		6,045,707			18.76		15.04		9.01		71,978,275

Summary of Share-based Compensation Expense

Total share-based compensation expense during the years ended was as follows:

	Year Ended December 31,
	2017		2018		2019
Research and development	$	2,678,956	$	3,559,047	$	10,682,553
General and administrative		2,739,754		4,174,209		10,461,476
Total share-based compensation expense	$	5,418,710	$	7,733,256	$	21,144,029

Summary of Assumption Used to Estimate the Grant Date Fair Value

The assumptions used in the Monte Carlo and Black-Scholes models to estimate the grant date fair value are as follows:

	Year Ended December 31,
	2017	2018	2019
Risk-free interest rate	1.21 - 2.30%	2.68 - 2.99%	1.42 - 2.56%
Dividend yield	0%	0%	0%
Volatility	51.6 - 55.8%	87.0 - 110.9%	102.6 - 111.2%
Expected terms (years)	5.3 - 7.0	5.94 - 6.25	5.31 - 6.08

Net Loss per Common Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss per Common Share

The following table presents the calculation of basic and diluted net loss per common share:

	Year Ended December 31,
	2017			2018			2019
Numerator:
Net loss	$	(51,006,094	)	$	(127,502,194	)	$	(304,706,613	)
Denominator:
Weighted-average number of common shares used in net loss per common share -- basic and diluted		13,870,949			54,396,483			62,228,601
Net loss per common share -- basic and diluted	$	(3.68	)	$	(2.34	)	$	(4.90	)

Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share

Shares outstanding presented below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, as their effect is anti-dilutive:

	Year Ended December 31,
	2017		2018		2019
Common stock under option		6,355,581		7,497,734		10,854,437
Common stock warrants		14,064		14,064		—
Total		6,369,645		7,511,798		10,854,437

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Summary of Interim Financial Information on Quarterly Basis

The following interim financial information presents the Company’s 2018 and 2019 results of operations on a quarterly basis:

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2018			2018			2018			2018
Operating Loss	$	(21,438,145	)	$	(33,485,442	)	$	(35,804,581	)	$	(37,196,592	)
Net Loss		(21,736,304	)		(33,334,300	)		(35,972,115	)		(36,459,475	)
Net Loss per common share, basic and diluted (1)		(0.43	)		(0.61	)		(0.64	)		(0.65	)

	Quarter Ended
	March 31,			June 30,			September 30,			December 31,
	2019			2019			2019			2019
Operating Loss	$	(48,650,570	)	$	(63,476,438	)	$	(69,947,962	)	$	(105,940,283	)
Net Loss		(50,574,367	)		(71,090,164	)		(69,824,659	)		(113,217,423	)
Net Loss per common share, basic and diluted (1)		(0.87	)		(1.12	)		(1.10	)		(1.77	)

(1)

Nature of Organization and Operations - Additional Information (Detail)										1 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Jan. 29, 2020 USD ($)	Jan. 13, 2020 USD ($) $ / shares shares	Sep. 23, 2019 USD ($)	Sep. 16, 2019 USD ($)	Jun. 27, 2019 USD ($)	Mar. 11, 2019 USD ($) $ / shares shares	Feb. 28, 2019 USD ($) Installment	Apr. 23, 2018 USD ($) $ / shares shares	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 07, 2019 USD ($)
Nature Of Organization And Operations [Line Items]
Proceeds from follow-on public offering after deducting underwriting discounts and commissions											$ 109,581,108	$ 131,194,030	$ 150,062,959
Cash and cash equivalents											351,985,085	$ 176,267,666
Subsequent Event [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares			10,925,000
Price of common stock \| $ / shares			$ 37.00
SFJ Agreement [Member]
Nature Of Organization And Operations [Line Items]
Proceeds from SFJ agreement						$ 20,000,000		$ 60,000,000			60,000,000
Additional funding amount upon achievement of development milestones				$ 20,000,000		40,000,000		$ 60,000,000
Number of milestone payments \| Installment								3
Development funding for minimum period of operating expense								10 months
Increase in additional funding for development costs														$ 20,000,000
Proceeds SFJ agreement upon achievement of milestone						$ 20,000,000
SFJ Agreement [Member] \| Subsequent Event [Member]
Nature Of Organization And Operations [Line Items]
Proceeds from SFJ agreement		$ 20,000,000								$ 20,000,000
Additional funding amount upon achievement of development milestones		$ 20,000,000
SFJ Agreement [Member] \| Scenario Forecast [Member]
Nature Of Organization And Operations [Line Items]
Additional funding for development costs	$ 50,000,000
Follow-on Public Offering [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares							6,900,000		5,500,000
Price of common stock \| $ / shares							$ 17.00		$ 25.50
Proceeds from follow-on public offering after deducting underwriting discounts and commissions							$ 109,581,000		$ 131,324,477
Underwriting discounts and commissions							7,038,000		8,415,000
Offering Cost							$ 680,891		$ 509,973
Follow-on Public Offering [Member] \| Subsequent Event [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares			10,925,000
Price of common stock \| $ / shares			$ 37.00
Proceeds from follow-on public offering after deducting underwriting discounts and commissions			$ 381,374,000
Underwriting discounts and commissions			22,232,000
Offering Cost			$ 618,000
Over Allotment Option [Member] \| Subsequent Event [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares			1,425,000
Price of common stock \| $ / shares			$ 37.00
Proceeds from follow-on public offering after deducting underwriting discounts and commissions			$ 381,374,000							$ 381,374,000
Underwriting discounts and commissions			22,233,000
Offering Cost			$ 618,000
Convertible Senior Notes Due 2026 [Member]
Nature Of Organization And Operations [Line Items]
Principal amount											220,000,000
Debt instrument, stated percentage					3.50%
Debt instrument maturity year					2026
Net proceeds from the sale of the notes											212,912,000
Payment of convertible debt discounts and commissions											6,600,000
Payment of convertible debt offering expenses											487,763
Payments of transactions cost											$ 28,380,000
Debt instrument, due and payment description											The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment											semiannually
Maturity date					Sep. 15, 2026
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member]
Nature Of Organization And Operations [Line Items]
Principal amount					$ 220,000,000

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017	Jan. 01, 2019 USD ($)
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Number of operating segment \| Segment	1
Cash and cash equivalents	$ 351,985,085	$ 176,267,666
Expected dividend payments	$ 0
Expected dividend yield	0.00%	0.00%	0.00%
ROU assets	$ 14,110,209			$ 5,511,865
Operating lease liabilities	14,465,975			5,511,865
Additional ROU asset				9,838,982
Additional operating lease liability				$ 9,838,982
Non-US [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 1,598,300	$ 121,600

Common Stock - Summary of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Class Of Stock [Line Items]
Common stock reserved for future issuance	10,031,771	7,983,327	7,291,165
Common stock warrants	14,064	14,064
2010 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	5,013,153	5,817,844
2017 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	4,032,390	1,682,596	6,319,001
2017 Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	972,164	468,823	972,164

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2019
Class Of Stock [Line Items]
Authorized capital stock		10,000,000	10,000,000
Convertible preferred stock issued and sold		0	0
Stock issuance costs		$ 18,180
Series E Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Convertible preferred stock issued and sold	7,792,035
Preferred stock, par value	$ 2.571
Gross proceeds from issuance and sale of convertible preferred stock	$ 20,033,322
Stock issuance costs	$ 285,475

Development Derivative Liability - Additional Information (Detail)						1 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Jan. 29, 2020 USD ($)	Sep. 23, 2019 USD ($)	Jun. 27, 2019 USD ($)	Feb. 28, 2019 USD ($) Installment	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 07, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Minimum payment of additional funding amount							$ 120,000,000
Loss from remeasurement of development derivative liability							14,839,000
Development derivative liability							$ 134,839,000
Level 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Implied cost of borrowing discount rates							16.60%
SFJ Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from SFJ agreement				$ 20,000,000	$ 60,000,000		$ 60,000,000
Additional funding amount upon achievement of development milestones			$ 20,000,000	40,000,000	$ 60,000,000
Number of milestone payments \| Installment					3
Development funding for minimum period of operating expense					10 months
Increase in additional funding for development costs								$ 20,000,000
Proceeds SFJ agreement upon achievement of milestone				$ 20,000,000
Number of additional annual payments \| Installment					6
Minimum payment of additional funding amount					$ 120,000,000
Increase in additional funding amount under amendment								$ 20,000,000
Loss from remeasurement of development derivative liability							14,839,000
Proceeds from collaborators of additional milestones							60,000,000
Development derivative liability							$ 134,839,000
SFJ Agreement [Member] \| Level 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Implied cost of borrowing discount rates							8.00%
SFJ Agreement [Member] \| Subsequent Event [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from SFJ agreement		$ 20,000,000				$ 20,000,000
Additional funding amount upon achievement of development milestones		$ 20,000,000
SFJ Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligated to pay initial payment					5,000,000
Aggregate amount of additional annual payments					226,000,000
SFJ Agreement [Member] \| Regulatory Approval Granted by FDA and EMA [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligated to pay initial payment					10,000,000
SFJ Agreement [Member] \| Regulatory Approval Granted by FDA and EMA [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount of additional annual payments					$ 452,000,000
SFJ Agreement [Member] \| Scenario Forecast [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional funding for development costs	$ 50,000,000

Development Derivative Liability - Schedule of Development Derivative Liability (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance at fair market value, January 1, 2019
Amounts received under the SFJ agreement and SFJ amendment	120,000,000
Loss recorded in loss from remeasurement of development derivative liability	14,839,000
Balance at fair market value, December 31, 2019	$ 134,839,000

Accrued Expenses and Prepaid Assets - Schedule of Accrued Expenses (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses And Prepaid Assets [Abstract]
Accrued research and development	$ 36,449,386	$ 3,481,570
Accrued payroll liabilities	11,442,889	269,046
Other	6,890,676	1,352,386
Total	$ 54,782,951	$ 5,103,002

Accrued Expenses and Prepaid Assets - Additional Information (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses And Prepaid Assets [Abstract]
Prepaid research and development	$ 18,102,049	$ 23,053,511

Long-term Debt - Additional Information (Detail)					4 Months Ended	9 Months Ended	12 Months Ended
Long-term Debt - Additional Information (Detail)	Sep. 16, 2019 USD ($)	Mar. 26, 2019 USD ($)	Oct. 20, 2017 USD ($) Installment $ / shares shares	Oct. 19, 2017 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) d $ / shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares	Sep. 11, 2019 USD ($) CounterParty $ / shares
Debt Instrument [Line Items]
Amortization of debt discounts							$ 51,725
Premium paid for capped call transactions						$ 28,380,000
Loss on extinguishment of debt							$ 1,501,215
Warrant to purchase common stock \| shares								14,064	14,064
Term Loan Facility [Member]
Debt Instrument [Line Items]
Principal amount		$ 21,701,292
Final payment due amount		21,802,403
Debt instrument per diem interest amount		$ 101,111
Final payment percentage equal to original principal amount		8.00%
Final payment towards original principal amount		$ 1,600,000
Prepayment fee		100,000
Other fees		1,292
Loss on extinguishment of debt		$ 1,208,132
Silicon Valley Bank [Member] \| Term Loan Facility [Member]
Debt Instrument [Line Items]
Warrant to purchase common stock \| shares			14,064
Exercise price per share \| $ / shares			$ 5.484
Warrant term			10 years
Aggregate purchase price			$ 250,000
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Debt instrument, due and payment description							the Company was required to make monthly interest-only payments through November 1, 2019 and to make 24 equal monthly payments of principal, plus accrued interest, thereafter from November 1, 2019 through October 1, 2021, when all unpaid principal and interest would become due and payable
Term loan facility			$ 20,000,000
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member] \| Convertible Preferred Stock [Member]
Debt Instrument [Line Items]
Debt instrument, number of equal monthly installments \| Installment			24
Loan and Security Agreement [Member] \| Silicon Valley Bank [Member] \| Prime Rate [Member]
Debt Instrument [Line Items]
Interest bear rate			1.50%
Capped Call Transactions [Member]
Debt Instrument [Line Items]
Number of counterparties \| CounterParty										2
Initial conversion price \| $ / shares										$ 39.4625
Initial cap price \| $ / shares										63.14
Sale price \| $ / shares										$ 31.57
Convertible Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Principal amount					$ 220,000,000		$ 220,000,000
Net proceeds from the sale of the notes							212,912,000
Payment of convertible debt discounts and commissions							6,600,000
Payment of convertible debt offering expenses							487,763
Payments of transactions cost							$ 28,380,000
Debt instrument, stated percentage	3.50%
Debt instrument, due and payment description							The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment							semiannually
Maturity date	Sep. 15, 2026
Redemption period, start date							Sep. 20, 2023
Threshold percentage of stock price trigger							130.00%
Threshold trading days \| d							20
Threshold consecutive trading days \| d							30
Redemption price, percentage							100.00%
Debt instrument effective interest rate										10.50%
Debt instrument convertible carrying amount of liability component										$ 145,065,803
Debt discount										74,934,197
Debt issuance costs										7,087,763
Debt interest expense					4,399,093
Amortization of debt discounts					2,089,368
Accrued semi annual coupon payable					2,224,445
Amortization of debt issuance costs					85,280
Debt issuance costs gross					4,588,320		$ 4,588,320
Long-term debt					$ 142,566,851		$ 142,566,851
Debt instrument, maturity							2026-09
Convertible Senior Notes Due 2026 [Member] \| Liability Component [Member]
Debt Instrument [Line Items]
Debt issuance costs										4,673,600
Convertible Senior Notes Due 2026 [Member] \| Equity Component [Member]
Debt Instrument [Line Items]
Debt issuance costs										$ 2,414,162
Convertible Senior Notes Due 2026 [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Terms of conversion							The Convertible Notes are convertible into shares of the Company’s common stock at an initial conversion rate of 25.3405 shares per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $39.4625 per share of common stock).
Conversion ratio							0.0253405
Conversion price \| $ / shares					$ 39.4625		$ 39.4625
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member]
Debt Instrument [Line Items]
Principal amount	$ 220,000,000
Unsecured Promissory Note [Member] \| Golda Darty Partners S A [Member]
Debt Instrument [Line Items]
Principal amount	7,000,000			$ 7,000,000
Debt instrument, due and payment description							The promissory note accrued interest at a rate per annum of 8.0%, and was due and payable quarterly in arrears on the 19th day of each April, July, October and January.
Maturity date				Oct. 19, 2022
Long-term debt				$ 7,000,000
Debt instrument per diem interest amount	118,904
Loss on extinguishment of debt				$ 293,083
Warrant to purchase common stock \| shares				93,764
Exercise price per share \| $ / shares				$ 5.484
Accrued interest rate				8.00%
Repayment of related party due amount including diem interest	$ 7,118,904
Fair value of warrant				$ 430,160

Leases - Additional Information (Detail) - USD ($)	12 Months Ended
Leases - Additional Information (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Lessee Lease Description [Line Items]
Operating lease asset	$ 14,110,209		$ 5,511,865
Operating lease liability	$ 14,465,975		5,511,865
Additional operating lease asset			9,838,982
Additional operating lease liability			9,838,982
Operating lease description	The Company enters into lease agreements with terms generally ranging from 2-7 years. Some of the Company’s lease agreements include Company options to extend the lease on a month to month basis or for set periods for up to five years. Many leases also include options to terminate the leases within one year or per other contractual terms. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases.
Operating lease, existence of option to extend	true
Operating lease maximum term of options to terminate lease	1 year
Weighted average remaining lease term of operating leases	5 years 3 days
Weighted average discount rate used to measure outstanding operating lease liabilities leases	8.19%
Operating lease expense	$ 2,369,000
Rental expense under operating leases		$ 798,760
Minimum [Member]
Lessee Lease Description [Line Items]
Operating lease, term of contract	2 years
Maximum [Member]
Lessee Lease Description [Line Items]
Operating lease, term of contract	7 years
Operating lease options to extend lease	5 years
ASU 2016-02 [Member]
Lessee Lease Description [Line Items]
Operating lease asset			5,511,865
Operating lease liability			5,511,865
Additional operating lease asset			9,838,982
Additional operating lease liability			$ 9,838,982

Leases - Supplemental Cash Flow Information Related to Operating Leases (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($)
Leases [Abstract]
Operating cash flows from operating leases	$ 2,013,273
Right of use assets obtained in exchange for lease obligations	$ 10,201,170

Leases - Maturity of Operating Lease Liabilities (Detail) - USD ($)	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
2020	$ 3,679,786
2021	3,336,325
2022	3,269,202
2023	3,287,748
2024 and thereafter	4,106,743
Total future minimum lease payments	17,679,804
Less imputed interest	(3,213,829)
Total operating lease liabilities	$ 14,465,975	$ 5,511,865

Leases - Schedule of Future Minimum Rental Payments for Lease Obligations (Detail)	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 1,251,720
2020	1,292,025
2021	975,750
2022	998,350
2023 and thereafter	2,341,219
Total	$ 6,859,064

Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Level 2 [Member] \| Related Party Promissory Note [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		$ 7,401,203
Fair Value, Nonrecurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 281,314,069	92,355,167
Total Financial Liabilities	149,496,359	26,123,524
Fair Value, Nonrecurring [Member] \| Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	281,314,069	92,355,167
Fair Value, Nonrecurring [Member] \| Term Loan Facility [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		18,722,321
Fair Value, Nonrecurring [Member] \| Related Party Promissory Note [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		7,401,203
Fair Value, Nonrecurring [Member] \| Convertible Senior Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	149,496,359
Fair Value, Nonrecurring [Member] \| Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	281,314,069	92,355,167
Fair Value, Nonrecurring [Member] \| Level 1 [Member] \| Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	281,314,069	92,355,167
Fair Value, Nonrecurring [Member] \| Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		26,123,524
Fair Value, Nonrecurring [Member] \| Level 2 [Member] \| Term Loan Facility [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		18,722,321
Fair Value, Nonrecurring [Member] \| Level 2 [Member] \| Related Party Promissory Note [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		$ 7,401,203
Fair Value, Nonrecurring [Member] \| Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	149,496,359
Fair Value, Nonrecurring [Member] \| Level 3 [Member] \| Convertible Senior Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 149,496,359

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Level 2 [Member] \| Promissory Note [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Long-term Debt, fair value		$ 7,401,203
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of cost of borrowing	16.60%
Derivative liability measurement input	0.10
Level 3 [Member] \| SFJ Agreement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of cost of borrowing	8.00%
Level 3 [Member] \| Convertible Senior Notes [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Convertible notes	$ 149,496,359
Convertible notes, interest rate	3.50%
Level 3 [Member] \| Development Derivative Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of derivative liability, percentage	13.30%
Change in fair value of derivative liability	$ 17,886,000

Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Details) - Fair Value, Recurring [Member] - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 134,839,000	$ 158,783
Common Stock Warrant Liability
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		158,783
Development Derivative Liability
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	134,839,000
Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		158,783
Level 2 [Member] \| Common Stock Warrant Liability
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities		$ 158,783
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	134,839,000
Level 3 [Member] \| Development Derivative Liability
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 134,839,000

License Agreements - Additional Information (Detail)	1 Months Ended	12 Months Ended
License Agreements - Additional Information (Detail)	Sep. 30, 2014 USD ($)	Dec. 31, 2019 USD ($) License	Dec. 31, 2018 USD ($)
University of Pennsylvania [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees	$ 100,000
University of Pennsylvania [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestones payments	3,200,000
Milestone payments based on annual sales milestones	$ 5,000,000
2010 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Annual maintenance fees		$ 100,000
Number of licensed products \| License		2
Annual milestone payment made			$ 1,025,000
2010 License Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate milestones payments		$ 1,650,000
Milestone payments based on annual sales milestones		$ 2,500,000

401(k) Profit Sharing Plan and Trust - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan, plan name	401(k) Plan
Minimum age of full-time employees eligible to participate in the plan	21 years
Employer contributions made to the 401(k) Plan	$ 806,915	$ 322,459	$ 123,624

Refundable Research and Development Credits and Income Taxes - Components of Loss From Continuing Operation Before Provision for Income Taxes (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments [Abstract]
Domestic	$ (297,127,180)	$ (122,900,285)	$ (47,523,183)
Foreign	(7,579,433)	(4,601,909)	(3,482,911)
Total	$ (304,706,613)	$ (127,502,194)	$ (51,006,094)

Refundable Research and Development Credits and Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 08, 2015
Refundable Research And Development Credits And Income Taxes [Line Items]
Current income tax expense or benefit	$ 0	$ 0
Deferred income tax expense or benefit	$ 0	0
Net operating loss carryforward expiration year	2024
Research and development tax credit carryforward expiration year	2024
Net operating loss carryforward period	3 years
Cumulative changes in ownership interest of significant stockholders	50.00%
Minimum increase in ownership percentage				5.00%
Net operating loss carryforwards	$ 102,623,330	$ 64,296,165
Statutory U.S. federal income tax, Percentage of income before income taxes	21.00%	21.00%	35.00%
Gross deferred tax asset balance			$ 15,659,916
Interest and penalties	$ 0	$ 0
Accrued for interest and penalties	$ 0	0
Income tax examination description	The Company’s earliest tax years open and subject to examination by taxing authorities are 2016, for the U.S. federal jurisdiction, and 2015, for U.S. states’ and Australia’s federal jurisdiction
Federal [Member]
Refundable Research And Development Credits And Income Taxes [Line Items]
Net operating loss carryforwards	$ 392,878,000	251,388,000
Research and development tax credit carryforwards	24,163,000
State [Member]
Refundable Research And Development Credits And Income Taxes [Line Items]
Net operating loss carryforwards	358,345,000	$ 254,631,000
Federal and State [Member]
Refundable Research And Development Credits And Income Taxes [Line Items]
Research and development tax credit carryforwards	24,163,000
Net operating loss carryforwards	99,955,000
Federal and State [Member] \| IRC Sec. 382 [Member]
Refundable Research And Development Credits And Income Taxes [Line Items]
Net operating loss carryforwards	10,716,000
Federal and State [Member] \| IRC Sec. 383 [Member]
Refundable Research And Development Credits And Income Taxes [Line Items]
Research and development tax credit carryforwards	$ 1,892,000

Refundable Research and Development Credits and Income Taxes - Schedule of Effective Income Tax Rate Provision Amount Differs from Statutory U.S. Federal Income Tax Rate (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Amount	$ (63,988,409)	$ (26,775,461)	$ (17,852,133)
State income taxes, net of federal benefit, Amount	(16,423,857)	(6,397,993)	(1,634,924)
Change in valuation allowances, Amount	90,300,408	38,156,506	11,176,291
Re-measurement of deferred taxes, Amount			15,659,916
Tax credits, Amount	(6,113,296)	(6,149,021)	(3,316,729)
Change in state apportionment, Kentucky tax reform and other, Amount	(16,861)	873,508
Permanent and other, Amount	$ (3,757,985)	$ 292,461	$ (4,032,421)

Refundable Research and Development Credits and Income Taxes - Schedule of Effective Income Tax Rate Provision Differs Amount from Statutory U.S. Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Percentage of income before income taxes	21.00%	21.00%	35.00%
State income taxes, net of federal benefit, Percentage of income before income taxes	5.40%	5.00%	3.30%
Change in valuation allowances, Percentage of income before income taxes	(29.60%)	(29.90%)	(20.80%)
Re-measurement of deferred taxes, Percentage of income before income taxes			(31.90%)
Tax credits, Percentage of income before income taxes	2.00%	4.80%	7.70%
Change in state apportionment, Kentucky tax reform and other, Percentage of income before income taxes	0.00%	(0.70%)
Permanent and other, Percentage of income before income taxes	1.20%	(0.20%)	6.70%

Refundable Research and Development Credits and Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Intangible assets	$ 15,519	$ 19,616
Share-based compensation	6,624,638	2,914,583
Deferred interest expense		135,585
Contribution carryforwards	41,409	36,759
Net operating loss carryforwards	102,623,330	64,296,165
Research and development credits	14,476,348	7,269,624
Orphan drug credits	9,380,475	7,165,354
Development derivative liability	35,874,491
Accruals	2,259,190
Other	11,250
Total deferred tax assets	171,306,650	81,837,686
Deferred tax liabilities:
Accrual to cash adjustment		(2,463,715)
Unrealized gain		(12,094)
Convertible debt	(11,479,891)
481(a) adjustment	(2,891,490)
Total deferred tax liabilities	(14,371,381)	(2,475,809)
Net deferred tax assets before allowance:	156,935,269	79,361,877
Less valuation allowance	$ (156,935,269)	$ (79,361,877)

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) ft²
Commitments And Contingencies [Line Items]
Base rent payable	$ 17,679,804
Incurred any cost to defend lawsuits or settle claims	0
Estimated termination costs and other fees	1,789,000
Non-cancellable purchase commitments	$ 35,300,000
Minimum [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	2 years
Payment of royalty fees	3.00%
Maximum [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	7 years
Future milestone payments	$ 15,000,000
Payment of royalty fees	6.00%
Watertown, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Operating lease, term of contract	85 months
Area of operating lease \| ft²	9,704
Initial monthly lease payments per month	$ 58,224
Base rent payable	$ 5,425,279
Lease commencement date	Jun. 02, 2020

Equity Incentive Plans - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Jan. 22, 2016	Oct. 31, 2017	Aug. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 30, 2017	May 12, 2010
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting period				48 months
Stock options vesting description				Options granted to employees on or after December 5, 2013 generally vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial vesting commencement date (as defined) subject to the employee’s continuous service with the Company.
Number of shares granted				4,365,520	2,060,685
Exercise price per share				$ 21.15	$ 15.86
Estimated fair values of common stock, per share							$ 14.00	$ 1.71
Actual fair values of common stock, per share				$ 32.18			$ 11.47
Fair market value of options vested				$ 12,906,436	$ 5,062,428
Unrecognized compensation expense				$ 79,291,036	$ 27,266,206	$ 10,415,787
Estimated weighted-average period to recognize				3 years 2 months 26 days	2 years 11 months 26 days	3 years 1 month 20 days
Common stock reserved for future issuance				7,291,165	10,031,771	7,983,327
Frist Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Second Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Third Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Fourth Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage				25.00%
Equity Incentive Plan 2010 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance			6,188,466
Percentage of voting shares			10.00%
Options expire from issuance date			10 years
Number of shares granted	93,762
Exercise price per share	$ 3.76
Exercise price range, lower range limit	6.81
Exercise price range, higher range limit	$ 6.89
Equity Incentive Plan 2010 [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of estimated fair value per share of common stock on the date of grant			100.00%
Equity Incentive Plan 2010 [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage price of each share of the fair value on date of grant			110.00%
2017 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		1,359,587
Percentage of voting shares		4.00%
Number of shares available for future grant		299,568
Shares issuable increased during the period		4,219,409
Common Stock Issuance Description				equal to the lowest of 4,219,409 shares of common stock, 4.0% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the board of directors.
Common stock reserved for future issuance				6,319,001	4,032,390	1,682,596
2017 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		468,823
Percentage of voting shares				1.00%
2017 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance				937,646

Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding, Beginning balance	7,497,734	6,355,581
Shares, Granted	4,365,520	2,060,685
Shares, Exercised	(747,136)	(468,263)
Shares, Forfeited	(261,681)	(450,269)
Shares Outstanding, Ending balance	10,854,437	7,497,734	6,355,581
Shares, Options exercisable	4,808,730
Shares, Expected to vest	6,045,707
Weighted -Average Exercise Price Per Share Outstanding, Beginning balance	$ 8.09	$ 4.78
Weighted -Average Exercise Price Per Share, Granted	21.15	15.86
Weighted -Average Exercise Price Per Share, Exercised	3.85	2.61
Weighted -Average Exercise Price Per Share, Forfeited	12.67	6.58
Weighted -Average Exercise Price Per Share, Outstanding, Ending balance	13.52	8.09	$ 4.78
Weighted -Average Exercise Price Per Share, Options exercisable	6.94
Weighted -Average Exercise Price Per Share, Expected to vest	18.76
Weighted - Average Grant Date Fair Value Per Option Outstanding, Beginning balance	5.52	2.66
Weighted - Average Grant Date Fair Value Per Option, Granted	17.31	13.13
Weighted - Average Grant Date Fair Value Per Option, Exercised	2.81	1.46
Weighted - Average Grant Date Fair Value Per Option, Forfeited	10.34	4.27
Weighted - Average Grant Date Fair Value Per Option Outstanding, Ending balance	10.43	$ 5.52	$ 2.66
Weighted - Average Grant Date Fair Value Per Option, Options exercisable	4.62
Weighted - Average Grant Date Fair Value Per Option, Expected to vest	$ 15.04
Weighted - Average Contractual Life Outstanding, Balance	7 years 7 months 2 days	7 years 3 months 21 days	7 years 7 months 20 days
Weighted - Average Contractual Life, Options exercisable	5 years 9 months 25 days
Weighted - Average Contractual Life, Expected to vest	9 years 3 days
Aggregate Intrinsic Value Outstanding, Beginning balance	$ 45,706,169	$ 107,534,863
Aggregate Intrinsic Value, Exercised	15,659,119	4,953,187
Aggregate Intrinsic Value, Outstanding, Ending balance	185,847,165	$ 45,706,169	$ 107,534,863
Aggregate Intrinsic Value, Options exercisable	113,868,890
Aggregate Intrinsic Value, Expected to vest	$ 71,978,275

Equity Incentive Plans - Summary of Share-based Compensation Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 21,144,029	$ 7,733,256	$ 5,418,710
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	10,682,553	3,559,047	2,678,956
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 10,461,476	$ 4,174,209	$ 2,739,754

Equity Incentive Plans - Summary of Assumption Used to Estimate the Grant Date Fair Value (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate, minimum	1.42%	2.68%	1.21%
Risk-free interest rate, maximum	2.56%	2.99%	2.30%
Dividend yield	0.00%	0.00%	0.00%
Volatility, minimum	102.60%	87.00%	51.60%
Volatility, maximum	111.20%	110.90%	55.80%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	5 years 3 months 21 days	5 years 11 months 8 days	5 years 3 months 18 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	6 years 29 days	6 years 3 months	7 years

Net Loss per Common Share - Summary of Basic and Diluted Net Loss per Common Share (Detail) - USD ($)

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Numerator:

Net loss

$ (113,217,423)

$ (69,824,659)

$ (71,090,164)

$ (50,574,367)

$ (36,459,475)

$ (35,972,115)

$ (33,334,300)

$ (21,736,304)

$ (304,706,613)

$ (127,502,194)

$ (51,006,094)

Denominator:

Weighted-average number of common shares used in net loss per common share, basic and diluted

62,228,601

54,396,483

13,870,949

Net loss per common share, basic and diluted

$ (1.77)

^[1]

$ (1.10)

^[1]

$ (1.12)

^[1]

$ (0.87)

^[1]

$ (0.65)

^[1]

$ (0.64)

^[1]

$ (0.61)

^[1]

$ (0.43)

^[1]

$ (4.90)

$ (2.34)

$ (3.68)

[1]	The sum of the four quarters of earnings per share for 2018 and 2019 may not add to the full year earnings per share amount due to rounding and/or the use of quarter-to-date weighted average shares to calculate the earnings per share amount in each respective quarter.

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Common Share - Summary of Shares Outstanding that were Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	10,854,437	7,511,798	6,369,645
Common Stock Under Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	10,854,437	7,497,734	6,355,581
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount		14,064	14,064

Related Party Transaction - Additional Information (Detail) - Revon [Member] - License Agreement and Services Agreement [Member] - USD ($)		12 Months Ended
	May 01, 2018	Dec. 31, 2019
Related Party Transaction [Line Items]
Annual license fee	$ 175,000
Exclusive license option, thereafter	350,000
Development service costs	$ 250,000
Related party transaction, amount paid		$ 691,667

Selected Quarterly Financial Data (Unaudited) - Summary of Interim Financial Information on Quarterly Basis (Detail) - USD ($)

3 Months Ended

12 Months Ended

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Operating Loss

$ (105,940,283)

$ (69,947,962)

$ (63,476,438)

$ (48,650,570)

$ (37,196,592)

$ (35,804,581)

$ (33,485,442)

$ (21,438,145)

$ (288,015,253)

$ (127,924,760)

$ (50,767,029)

Net loss

$ (113,217,423)

$ (69,824,659)

$ (71,090,164)

$ (50,574,367)

$ (36,459,475)

$ (35,972,115)

$ (33,334,300)

$ (21,736,304)

$ (304,706,613)

$ (127,502,194)

$ (51,006,094)

Net loss per common share, basic and diluted

$ (1.77)

^[1]

$ (1.10)

^[1]

$ (1.12)

^[1]

$ (0.87)

^[1]

$ (0.65)

^[1]

$ (0.64)

^[1]

$ (0.61)

^[1]

$ (0.43)

^[1]

$ (4.90)

$ (2.34)

$ (3.68)

[1]	The sum of the four quarters of earnings per share for 2018 and 2019 may not add to the full year earnings per share amount due to rounding and/or the use of quarter-to-date weighted average shares to calculate the earnings per share amount in each respective quarter.

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Events - Additional Information (Detail) - USD ($)							1 Months Ended	12 Months Ended
	Jan. 29, 2020	Jan. 13, 2020	Jun. 27, 2019	Mar. 11, 2019	Feb. 28, 2019	Apr. 23, 2018	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Proceeds from issuance of common stock, net of issuance costs								$ 109,581,108	$ 131,194,030	$ 150,062,959
Follow-on Public Offering [Member]
Subsequent Event [Line Items]
Issuance of common stock in follow-on offering, shares				6,900,000		5,500,000
Price of common stock				$ 17.00		$ 25.50
Proceeds from issuance of common stock, net of issuance costs				$ 109,581,000		$ 131,324,477
Underwriting discounts and commissions				7,038,000		8,415,000
Offering Cost				$ 680,891		$ 509,973
SFJ Agreement [Member]
Subsequent Event [Line Items]
Proceeds from SFJ agreement			$ 20,000,000		$ 60,000,000			$ 60,000,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Issuance of common stock in follow-on offering, shares		10,925,000
Price of common stock		$ 37.00
Subsequent Event [Member] \| Follow-on Public Offering [Member]
Subsequent Event [Line Items]
Issuance of common stock in follow-on offering, shares		10,925,000
Price of common stock		$ 37.00
Proceeds from issuance of common stock, net of issuance costs		$ 381,374,000
Underwriting discounts and commissions		22,232,000
Offering Cost		$ 618,000
Subsequent Event [Member] \| Over Allotment Option [Member]
Subsequent Event [Line Items]
Issuance of common stock in follow-on offering, shares		1,425,000
Price of common stock		$ 37.00
Proceeds from issuance of common stock, net of issuance costs		$ 381,374,000					$ 381,374,000
Underwriting discounts and commissions		22,233,000
Offering Cost		$ 618,000
Subsequent Event [Member] \| SFJ Agreement [Member]
Subsequent Event [Line Items]
Proceeds from SFJ agreement	$ 20,000,000						$ 20,000,000