Apellis Pharmaceuticals (APLS) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 19, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	APLS
Entity Registrant Name	Apellis Pharmaceuticals, Inc.
Entity Central Index Key	1,492,422
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		56,279,307
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Public Float			$ 912,252,858
Entity Current Reporting Status	Yes

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 176,267,666	$ 175,643,529
Refundable research and development credit	1,473,591	1,297,361
Prepaid assets	24,333,851	5,059,593
Other current assets	364,113	14,823
Total current assets	202,439,221	182,015,306
Other assets	1,094,338	116,150
Total assets	203,533,559	182,131,456
Current liabilities:
Accounts payable	10,254,938	3,663,253
Accrued expenses	5,103,002	2,890,705
Current portion of long-term debt	1,666,667
Total current liabilities	17,024,607	6,553,958
Long-term liabilities:
Term loan facility, net of current portion	18,722,321	19,806,944
Promissory note	6,655,193	6,583,402
Common stock warrant liability	158,783	244,292
Total liabilities	42,560,904	33,188,596
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and zero shares issued and outstanding at December 31, 2017 and 2018	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized and 50,334,152 shares issued and outstanding at December 31, 2017 and 200,000,000 shares authorized and 56,279,307 shares issued and outstanding at December 31, 2018	5,628	5,033
Additional paid in capital	437,855,681	298,201,480
Accumulated other comprehensive loss	(122,807)
Accumulated deficit	(276,765,847)	(149,263,653)
Total stockholders' equity	160,972,655	148,942,860
Total liabilities and stockholders' equity	$ 203,533,559	$ 182,131,456

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
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	56,279,307	50,334,152
Common stock, outstanding	56,279,307	50,334,152

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 105,285,576	$ 40,303,878	$ 22,978,599
General and administrative	22,639,184	10,463,151	4,303,743
Operating loss	(127,924,760)	(50,767,029)	(27,282,342)
Gain (loss) from remeasurement of fair value of warrants	85,509	(153,692)
Interest expense	(2,512,956)	(374,749)
Interest income	2,960,771	277,834	135,309
Other income (expense), net	(110,758)	11,542	22,396
Net loss	(127,502,194)	(51,006,094)	(27,124,637)
Other comprehensive loss:
Foreign currency loss	(122,807)
Total other comprehensive loss	(122,807)
Comprehensive loss, net of tax	$ (127,625,001)	$ (51,006,094)	$ (27,124,637)
Net loss per common share, basic and diluted	$ (2.34)	$ (3.68)	$ (3.22)
Weighted-average number of common shares used in net loss per common share, basic and diluted	54,396,483	13,870,949	8,428,366

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholder's Equity - USD ($)	Total	IPO [Member]	Series D Convertible Preferred Stock [Member]	Series E Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]Series D 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, 2015	$ 34,976,949				$ 77,191,906			$ 843		$ 28,917,122			$ (71,132,922)
Beginning balance, Shares at Dec. 31, 2015					49,633,007			8,428,366
Issuance of stock, net of issuance costs			$ 14,863,020			$ 14,863,020
Issuance of stock, net of issuance costs, shares						6,714,413
Share-based compensation expense	1,078,988									1,078,988
Net loss	(27,124,637)												(27,124,637)
Ending balance at Dec. 31, 2016	23,794,320				$ 92,054,926			$ 843		29,996,110			(98,257,559)
Ending 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							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

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net loss	$ (127,502,194)	$ (51,006,094)	$ (27,124,637)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	7,733,256	5,418,710	1,078,988
(Gain)/loss from remeasurement of fair value of warrants	(85,509)	153,692
Accretion of discounts for promissory note	71,791	13,563
Accretion of discounts for term loan facility	582,043	11,935
Changes in operating assets and liabilities:
Refundable research and development credit	(299,006)	50,443	408,934
Prepaid assets	(19,275,118)	(3,927,155)	(873,884)
Other current assets	(309,361)	10,177
Other assets	(978,188)	(51,622)	68,743
Accounts payable	6,587,469	932,299	69,779
Accrued expenses	2,234,020	1,798,979	368,999
Net cash used in operating activities	(131,240,797)	(46,595,073)	(26,003,078)
Financing Activities
Deferred issuance costs	(18,180)
Proceeds from issuance of term loan, net of issuance costs		19,795,007
Proceeds from issuance of promissory note payable to an affiliate of stockholder		6,569,840
Proceeds from issuance of common stock, net of issuance costs	131,194,030	150,062,959
Proceeds from the issuance of common stock warrants		520,760
Proceeds from exercise of stock options and warrants	727,510	678,701
Net cash provided by financing activities	131,903,360	197,375,114	14,863,020
Effect of exchange rate changes on cash and cash equivalents	(38,426)
Net increase (decrease) in cash and cash equivalents	624,137	150,780,041	(11,140,058)
Cash and cash equivalents at beginning of period	175,643,529	24,863,488	36,003,546
Cash and cash equivalents at end of period	176,267,666	175,643,529	24,863,488
Supplemental Disclosure of Non-Cash Financing Activities
Conversion of convertible preferred stock to common stock		111,802,773
Cash paid for interest	$ 1,816,333	134,167
Series D Convertible Preferred Stock [Member]
Financing Activities
Issuance of convertible preferred stock, net of issuance costs			$ 14,863,020
Series E Convertible Preferred Stock [Member]
Financing Activities
Issuance of convertible preferred stock, net of issuance costs		$ 19,747,847

Nature of Organization and Oper

Nature of Organization and Operations	12 Months Ended
	Dec. 31, 2018
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 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”). Follow-on Public Offering 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 $131.2 million after deducting underwriting discounts and commissions of $8.4 million and offering costs of $0.5 million. Initial Public Offering On November 13, 2017, the Company issued and sold 10,714,000 shares of its common stock at a price per share to the public of $14.00 in its initial public offering (“IPO”). The Company received net proceeds of $137.2 million after deducting underwriting discounts and commissions of $10.5 million and offering costs of $2.3 million. In addition, on December 13, 2017, the Company issued and sold an additional 981,107 shares of its common stock at the initial public offering price of $14.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, which resulted in net proceeds of approximately $12.8 million, after deducting underwriting discounts and commissions of $1.0 million. Liquidity and Going Concern The accompanying audited consolidated financial statements have been prepared on a going concern basis which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. As of February 26, 2019, the date of issuance of these audited consolidated financial statements, the Company believes that its cash and cash equivalents as of December 31, 2018 of $176.3 million will not be sufficient to fund its operations and capital expenditures for at least twelve months. The 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. The 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, raise substantial doubt about the Company’s ability to continue as a going concern. The consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or amounts of liabilities that might be necessary should the Company be unable to continue its operations. If the Company is not able to obtain the required funding in the near future, or is not able to obtain funding on terms that are favorable to the Company, it will have a material adverse effect on its operations and strategic development plan for future growth. If the Company cannot successfully raise additional funding and implement its strategic development plan, then its liquidity, financial condition and business prospects will be materially and adversely affected, and the Company may have to cease operations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
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 and Apellis Ireland Ltd. 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 2018 presentation. Offering Costs Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s IPO in 2017 and the follow-on offering and filing of a registration statement on Form S-3 in 2018. Costs were deferred until completion of the IPO and the follow-on offering, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds. Deferred issuance costs related to the S-3 registration in December 2018 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, 2018. 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, accrued expenses, prepaid expenses and income taxes. 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 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 6, Long-Term Debt, and Note 7, Fair Value Measurement, 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 their fair value. Foreign Currency Due to the growing volume of research contracts and intercompany loans that are now being exclusively denominated in local currency, effective August 1, 2018, the functional currency of the Company’s Australian subsidiary was changed from the U.S. dollar to the Australian dollar. The impact of the change in functional currency is not material to the audited consolidated financial statements for the year ended December 31, 2018. As a result of the change in functional currency, the financial position and results of operations of the Company's Australian subsidiary are measured using the foreign subsidiary's local currency. Revenues and expenses of the Australian subsidiary have been translated into U.S. dollars at average exchange rates prevailing during the period from August 1, 2018 through December 31, 2018. 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 . 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 Income taxes are recorded in accordance with Financial Accounting Standards Board (“FASB”) ASC Topic 740, Income Taxes 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, 2017 and 2018, the Company did not have any significant uncertain tax positions. Share-based Compensation The Company accounts for its share-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation Equity-Based Payments to Non-Employees 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 is in a very early stage of product development with no revenues, and the representative group of companies has similar characteristics. 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 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 accounts receivable are the only financial instruments that potentially subject the Company to concentrations of credit risk. The Company’s cash is maintained with accredited financial institutions and is insured by the Federal Deposit Insurance Corporation, except for the cash maintained by the Australian subsidiary in an Australian bank. The Company does not consider these amounts to be material. 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 preferred stock 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 gain/loss recorded from the remeasurement of the long term intra-entity loan transaction to the Company’s wholly owned subsidiary as well as the foreign currency gain/ loss from the translation of the Company’s wholly owned subsidiary into US dollars in 2018. These components are a result of the change in the subsidiary’s functional currency to Australian dollars effective August 1, 2018. Comprehensive loss is the same as net loss for the years ended December 31, 2017 and 2016. Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date In January 2016, the FASB issued Accounting Standards Update (“ASU”) No. 2016-01, Financial Instruments – Overall In February 2016, the FASB issued ASU 2016-02 Leases (Topic 842) - In August 2018, the FASB issued Accounting Standards Update (“ASU) No. 2018-13, Fair Value Measurement – Disclosure Framework

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Common Stock	3. Common Stock The Company has reserved the following shares of common stock for future issuance: December 31, 2016 2017 2018 Conversion of Series A convertible preferred stock 1,251,758 — — Conversion of Series B convertible preferred stock 2,982,962 — — Conversion of Series C convertible preferred stock 12,290,394 — — Conversion of Series D convertible preferred stock 9,891,866 — — Shares reserved under 2010 Equity Incentive Plan 4,781,264 5,817,844 5,013,153 Shares reserved under 2017 Equity Incentive Plan — 1,682,596 4,032,390 Shares reserved under 2017 Employee Stock Purchase Plan — 468,823 972,164 Common stock warrants — 14,064 14,064 Total 31,198,244 7,983,327 10,031,771

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Convertible Preferred Stock	4. Convertible Preferred Stock Authorized capital stock includes 10,000,000 shares of preferred stock at December 31, 2017 and 2018. Series D Convertible Preferred Stock In January 2016, the Company issued 6,714,413 shares of Series D convertible preferred stock at $2.234 per share for aggregate proceeds of $15,000,000, less issuance costs of $136,980. 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.

Accrued Expenses and Prepaid As

Accrued Expenses and Prepaid Assets	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Accrued Expenses and Prepaid Assets	5. Accrued Expenses and Prepaid Assets Accrued expenses are as follows: December 31, 2017 2018 Accrued research and development $ 2,463,808 $ 3,481,570 Accrued payroll liabilities 251,491 269,046 Other 175,406 1,352,386 Total $ 2,890,705 $ 5,103,002 Prepaid assets include $4.1 million and $23.1 million of prepaid research and development costs as of December 31, 2017 and 2018, respectively.

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Long-term Debt	6. Long-term Debt 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 $20.0 million term loan facility (the “term loan facility”). Borrowings under the term loan facility bear interest at a floating rate per annum equal to the WSJ prime rate plus 1.50%; provided, however, that in an event of default, as defined in the loan and security agreement, the interest rate applicable to borrowings under such agreement will be increased by 5.0%. Under the agreement, the Company is required to make monthly interest-only payments through November 1, 2019 and is required to make 24 equal monthly payments of principal, plus accrued interest, thereafter from November 1, 2019 through October 1, 2021, at which time all unpaid principal and interest becomes due and payable. The Company may voluntarily prepay all, but not less than all, of the outstanding principal at any time prior to the maturity date, subject to a prepayment fee, which ranges from 0% to 2% of the outstanding principal. A final payment of $1.6 million is due upon the earlier to occur of the maturity of the loan, the acceleration or prepayment of all outstanding principal or the termination of the term loan facility. This amount is being accreted as additional interest expense over the term of the term loan facility. Borrowings under the term loan facility are secured by a first priority lien on all of the Company’s assets, excluding intellectual property owned by the Company and its interest in its license from Penn. The Company has agreed to a negative pledge on its intellectual property. The term loan facility contains customary events of default and affirmative and negative covenants, including restrictions on the Company’s ability to pay dividends and incur additional debt, but does not contain any financial covenants. 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 has 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 may require the Company to repurchase the warrant for a total aggregate purchase price of $250,000. As the warrant may be put to the Company at SVB’s option based on the passage of time (upon the expiration of the warrant), the Company has accounted for the warrant as a liability pursuant to ASC 480. Related Party Promissory Note On October 19, 2017, the Company issued and sold an unsecured promissory note in the principal amount of $7.0 million to Golda Darty Partners S.A. (“GDP”), an affiliate of one of the Company’s stockholders. The promissory note bears interest at a rate per annum of 8.0%, and is due and payable quarterly in arrears on the 19th day of each April, July, October and January beginning on January 19, 2018. The promissory note has a maturity date of October 19, 2022. The promissory note is contractually subordinated to the term loan facility with SVB. 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 is being accreted as additional interest expense over the term of the promissory note. The contractual maturities of the Company’s long-term debt obligations under the SVB term loan facility and the GDP promissory note due subsequent to December 31, 2018 are as follows: 2019 $ 1,666,667 2020 10,000,000 2021 8,333,333 2022 7,000,000 2023 — $ 27,000,000

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	7. Fair Value Measurements The fair value of the following financial instruments are based on level 2 inputs. As of December 31, 2017 and 2018, the fair value of the Company's promissory note was approximately $7.7 million and $7.4 million, respectively,

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License Agreements	8. 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 2010 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. In 2018 we made payments of $1,025,000, net of a credit for the annual license maintenance payment, for the achievement of milestones under these agreements. 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.

401(k) Profit Sharing Plan and

401(k) Profit Sharing Plan and Trust	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
401(k) Profit Sharing Plan and Trust	9. 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 2016, 2017 and 2018, the Company recorded $50,867, $123,264 and $322,459 respectively, for employer contributions made to the 401(k) Plan.

Refundable Research Development

Refundable Research Development and Income Taxes	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Refundable Research Development And Income Taxes	10. Refundable Research Development 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 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, 2017 and 2018, 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 2016 2017 2018 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 $ (9,222,377 ) 34.0 % $ (17,852,133 ) 35.0 % $ (26,775,461 ) 21.0 % State income taxes, net of federal benefit (992,768 ) 3.5 (1,634,924 ) 3.3 (6,397,993 ) 5.0 Change in valuation allowances 12,923,538 (46.4 ) 11,176,291 (20.8 ) 38,156,506 (29.9 ) Re-measurement of deferred taxes — — 15,659,916 (31.9 ) — — Tax credits (2,933,836 ) 10.8 (3,316,729 ) 7.7 (6,149,021 ) 4.8 Change in state apportionment, Kentucky tax reform and other — — — — 873,508 (0.7 ) Permanent and other 225,443 (1.9 ) (4,032,421 ) 6.7 292,461 (0.2 ) 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, 2017 2018 Deferred tax assets: Accrual to cash adjustment $ 345,623 $ — Intangible assets 23,543 19,616 Share-based compensation 1,826,372 2,914,583 Deferred interest expense — 135,585 Contribution carryforwards 24,465 36,759 Net operating loss carryforwards 30,699,411 64,296,165 Research and development credits 3,357,291 7,269,624 Orphan drug credits 4,928,666 7,165,354 Total deferred tax assets 41,205,371 81,837,686 Deferred tax liabilities: Accrual to cash adjustment — (2,463,715 ) Unrealized gain — (12,094 ) Total deferred tax liabilities — (2,475,809 ) Net deferred tax assets before allowance: 41,205,371 79,361,877 Less valuation allowance (41,205,371 ) (79,361,877 ) Net deferred tax assets $ — $ — At December 31, 2017, the Company had approximately $118.8 million of both Federal and state net operating loss carryforwards. At December 31, 2018, the Company had approximately $251.4 million and $254.6 million of Federal and state net operating loss carryforwards, respectively. The Company also had $14.4 million of federal research and development tax credit carryforwards as of December 31, 2018. 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 $64.3 million of NOLs and $14.4 million of research & development tax credit carryforwards. Of the $64.3 million of NOLs, approximately $10.7 million is limited under Section 382. Of the $14.4 million in tax credit carryforwards, approximately $1.9 million 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, 2017 and 2018. 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 $15.7 million. 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 2017 The Company files income tax returns in the U.S. federal jurisdiction, and applicable state jurisdictions and in Australia’s federal jurisdiction. The Company’s earliest tax years open and subject to examination by taxing authorities are 2015, for the U.S. federal jurisdiction, and 2014, 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

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies The Company leases office space in Crestwood, Kentucky under the terms of an operating lease, as amended. The lease expires in July 2023. In April 2017, the Company entered into a lease for office space in Waltham, Massachusetts (the “Waltham Lease”). The Waltham Lease has a term of 60 months and includes leasing 6,126 square feet of office space. The Waltham Lease provides for initial monthly lease payments of $17,612 per month. The base rent payable over the lease period is $1,117,995. In July 2018, this lease was amended for payments starting January 2019. The Waltham lease as amended has a term of 78 months and includes leasing approximately 22,600 square feet of office space. The lease provides for initial monthly payments of $44,375 per month. The base rent payable over the lease period is $5,352,094. Lease expense for the years ended December 31, 2016, 2017, and 2018 was $117,222, $351,468 and $636,835, respectively. The Company leases certain medical equipment for use in clinical trials under the terms of an operating lease effective May 2018, whereby each piece of equipment will be rented for a minimum of 24 months upon installation at clinical sites. The agreement can be extended upon request. Lease expense for the year December 31, 2018 was $161,925. At December 31, 2018, the Company’s future minimum payments required under our leases are as follows: 2019 $ 1,251,720 2020 1,292,025 2021 975,750 2022 998,350 2023 and thereafter 2,341,219 $ 6,859,064 The Company contracts to conduct research and development activities with remaining contract costs of approximately $1.9 million at December 31, 2018 that will be incurred in future periods. 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. 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 never 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

Equity Incentive Plans	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity Incentive Plans	12. 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. 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. The Company’s options granted prior to February 2016 become fully vested upon the occurrence of a change in control, as defined in the 2010 Plan. Options granted to executives after February 2016 become vested and exercisable with respect to 50% of the then unvested options if such executive is terminated without cause or resigns for good reason within 12 months following a change of control. Options granted to Dr. Francois, Dr. Deschatelets and Mr. Sullivan will become fully vested upon the occurrence of a change in control. The balance of the Company’s options do not receive additional vesting upon a change of 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, 2016 3,831,237 2.84 1.75 7.20 243,000 Granted 2,940,444 6.94 3.61 — — Exercised (46,881 ) 3.51 2.25 — 852,818 Forfeited (369,219 ) 2.56 1.22 — — 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 Options exercisable, December 31, 2018 4,068,602 4.64 2.59 5.89 35,186,023 Expected to vest, December 31, 2018 3,429,132 12.17 8.99 8.99 10,520,167 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, 2018. 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 values of the Company’s stock from the time of the IPO through December 31, 2018 were between $13.99 and $26.73. Total share-based compensation expense recognized was as follows: Year Ended December 31, 2016 2017 2018 Research and development $ 377,776 $ 2,678,956 $ 3,559,047 General and administrative 701,212 2,739,754 4,174,209 Total share-based compensation expense $ 1,078,988 $ 5,418,710 $ 7,733,256 At December 31, 2016, 2017 and 2018, unrecognized compensation expense related to unvested options, net of estimated forfeitures, was $2,910,870, $10,415,787 and $27,266,206, respectively, which the Company expects to recognize over an estimated weighted-average period of 2.85, 3.14, and 2.99 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, 2016 2017 2018 Risk-free interest rate 1.21 - 1.60% 1.21 - 2.30% 2.68 - 2.99% Dividend yield 0% 0% 0% Volatility 52.0 - 78.1% 51.6 - 55.8% 87.0 - 110.9% Expected terms (years) 5.2 - 5.7 5.3 - 7.0 5.94 - 6.25 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 our common stock. The number of shares of our 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 our common stock, (ii) 1.0% of the number of shares of our common stock outstanding on the first day of the fiscal year and (iii) an amount determined by our board of directors. The board of directors has not initiated any offerings under the ESPP.

Net Loss per Common Share

Net Loss per Common Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Common Share	13. Net Loss per Common Share The following table presents the calculation of basic and diluted net loss per common share: Year Ended December 31, 2016 2017 2018 Numerator: Net loss $ (27,124,637 ) $ (51,006,094 ) $ (127,502,194 ) Denominator: Weighted-average number of common shares used in net loss per common share -- basic and diluted 8,428,366 13,870,949 54,396,483 Net loss per common share -- basic and diluted $ (3.22 ) $ (3.68 ) $ (2.34 ) 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, 2016 2017 2018 Convertible preferred stock 26,416,896 — — Common stock under option 3,831,237 6,355,581 7,497,734 Common stock warrants — 14,064 14,064 Total 30,248,133 6,369,645 7,511,798

Related Party Transaction

Related Party Transaction	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transaction	14. 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 will provide development services with respect to the Revon software to the Company for $250,000 during the first year. 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, is 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.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	15. Selected Quarterly Financial Data (Unaudited) The following interim financial information presents the Company’s 2017 and 2018 results of operations on a quarterly basis: Quarter Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Operating Loss $ (9,052,493 ) $ (12,132,403 ) $ (11,589,319 ) $ (17,992,814 ) Net Loss (9,050,773 ) (12,126,152 ) (11,573,273 ) (18,255,896 ) Net Loss per common share, basic and diluted (1) (1.07 ) (1.44 ) (1.37 ) (0.61 ) 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 ) (1) The sum of the four quarters of earnings per share for 2017 and 2018 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

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events In January 2019, the Company granted an aggregate of 70,320 options to purchase shares of common stock to members of the Board of Directors with an exercise price of $13.19. In February 2019, the Company granted an aggregate of 1,451,600 options to purchase shares of common stock to employees with an exercise price ranging from $13.72 to $13.85.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 and Apellis Ireland Ltd. 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 2018 presentation.
Offering Costs	Offering Costs Offering costs represent underwriting, legal, accounting and other direct costs related to the Company’s IPO in 2017 and the follow-on offering and filing of a registration statement on Form S-3 in 2018. Costs were deferred until completion of the IPO and the follow-on offering, at which time they were reclassified to additional paid-in capital as a reduction of the proceeds. Deferred issuance costs related to the S-3 registration in December 2018 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, 2018.
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, accrued expenses, prepaid expenses and income taxes. 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
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 6, Long-Term Debt, and Note 7, Fair Value Measurement, 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 their fair value.
Foreign Currency	Foreign Currency Due to the growing volume of research contracts and intercompany loans that are now being exclusively denominated in local currency, effective August 1, 2018, the functional currency of the Company’s Australian subsidiary was changed from the U.S. dollar to the Australian dollar. The impact of the change in functional currency is not material to the audited consolidated financial statements for the year ended December 31, 2018. As a result of the change in functional currency, the financial position and results of operations of the Company's Australian subsidiary are measured using the foreign subsidiary's local currency. Revenues and expenses of the Australian subsidiary have been translated into U.S. dollars at average exchange rates prevailing during the period from August 1, 2018 through December 31, 2018. 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 .
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 Income taxes are recorded in accordance with Financial Accounting Standards Board (“FASB”) ASC Topic 740, Income Taxes 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, 2017 and 2018, the Company did not have any significant uncertain tax positions.
Share-based Compensation	Share-based Compensation The Company accounts for its share-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation Equity-Based Payments to Non-Employees 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 is in a very early stage of product development with no revenues, and the representative group of companies has similar characteristics. 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 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 accounts receivable are the only financial instruments that potentially subject the Company to concentrations of credit risk. The Company’s cash is maintained with accredited financial institutions and is insured by the Federal Deposit Insurance Corporation, except for the cash maintained by the Australian subsidiary in an Australian bank. The Company does not consider these amounts to be material.
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 preferred stock 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 gain/loss recorded from the remeasurement of the long term intra-entity loan transaction to the Company’s wholly owned subsidiary as well as the foreign currency gain/ loss from the translation of the Company’s wholly owned subsidiary into US dollars in 2018. These components are a result of the change in the subsidiary’s functional currency to Australian dollars effective August 1, 2018. Comprehensive loss is the same as net loss for the years ended December 31, 2017 and 2016.
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) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date In January 2016, the FASB issued Accounting Standards Update (“ASU”) No. 2016-01, Financial Instruments – Overall In February 2016, the FASB issued ASU 2016-02 Leases (Topic 842) - In August 2018, the FASB issued Accounting Standards Update (“ASU) No. 2018-13, Fair Value Measurement – Disclosure Framework

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2016 2017 2018 Conversion of Series A convertible preferred stock 1,251,758 — — Conversion of Series B convertible preferred stock 2,982,962 — — Conversion of Series C convertible preferred stock 12,290,394 — — Conversion of Series D convertible preferred stock 9,891,866 — — Shares reserved under 2010 Equity Incentive Plan 4,781,264 5,817,844 5,013,153 Shares reserved under 2017 Equity Incentive Plan — 1,682,596 4,032,390 Shares reserved under 2017 Employee Stock Purchase Plan — 468,823 972,164 Common stock warrants — 14,064 14,064 Total 31,198,244 7,983,327 10,031,771

Accrued Expenses and Prepaid _2

Accrued Expenses and Prepaid Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Schedule of Accrued Expenses	Accrued expenses are as follows: December 31, 2017 2018 Accrued research and development $ 2,463,808 $ 3,481,570 Accrued payroll liabilities 251,491 269,046 Other 175,406 1,352,386 Total $ 2,890,705 $ 5,103,002

Long-term Debt (Tables)

Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Summary of Contractual Maturities of Long-term Debt Obligations	The contractual maturities of the Company’s long-term debt obligations under the SVB term loan facility and the GDP promissory note due subsequent to December 31, 2018 are as follows: 2019 $ 1,666,667 2020 10,000,000 2021 8,333,333 2022 7,000,000 2023 — $ 27,000,000

Refundable Research Developme_2

Refundable Research Development and Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Refundable Research Development And Income Taxes [Abstract]
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 2016 2017 2018 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 $ (9,222,377 ) 34.0 % $ (17,852,133 ) 35.0 % $ (26,775,461 ) 21.0 % State income taxes, net of federal benefit (992,768 ) 3.5 (1,634,924 ) 3.3 (6,397,993 ) 5.0 Change in valuation allowances 12,923,538 (46.4 ) 11,176,291 (20.8 ) 38,156,506 (29.9 ) Re-measurement of deferred taxes — — 15,659,916 (31.9 ) — — Tax credits (2,933,836 ) 10.8 (3,316,729 ) 7.7 (6,149,021 ) 4.8 Change in state apportionment, Kentucky tax reform and other — — — — 873,508 (0.7 ) Permanent and other 225,443 (1.9 ) (4,032,421 ) 6.7 292,461 (0.2 ) 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, 2017 2018 Deferred tax assets: Accrual to cash adjustment $ 345,623 $ — Intangible assets 23,543 19,616 Share-based compensation 1,826,372 2,914,583 Deferred interest expense — 135,585 Contribution carryforwards 24,465 36,759 Net operating loss carryforwards 30,699,411 64,296,165 Research and development credits 3,357,291 7,269,624 Orphan drug credits 4,928,666 7,165,354 Total deferred tax assets 41,205,371 81,837,686 Deferred tax liabilities: Accrual to cash adjustment — (2,463,715 ) Unrealized gain — (12,094 ) Total deferred tax liabilities — (2,475,809 ) Net deferred tax assets before allowance: 41,205,371 79,361,877 Less valuation allowance (41,205,371 ) (79,361,877 ) Net deferred tax assets $ — $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Payments Required Under the Leases	At December 31, 2018, the Company’s future minimum payments required under our leases are as follows: 2019 $ 1,251,720 2020 1,292,025 2021 975,750 2022 998,350 2023 and thereafter 2,341,219 $ 6,859,064

Equity Incentive Plans (Tables)

Equity Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2016 3,831,237 2.84 1.75 7.20 243,000 Granted 2,940,444 6.94 3.61 — — Exercised (46,881 ) 3.51 2.25 — 852,818 Forfeited (369,219 ) 2.56 1.22 — — 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 Options exercisable, December 31, 2018 4,068,602 4.64 2.59 5.89 35,186,023 Expected to vest, December 31, 2018 3,429,132 12.17 8.99 8.99 10,520,167
Summary of Share-based Compensation Expense Recognized	Total share-based compensation expense recognized was as follows: Year Ended December 31, 2016 2017 2018 Research and development $ 377,776 $ 2,678,956 $ 3,559,047 General and administrative 701,212 2,739,754 4,174,209 Total share-based compensation expense $ 1,078,988 $ 5,418,710 $ 7,733,256
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, 2016 2017 2018 Risk-free interest rate 1.21 - 1.60% 1.21 - 2.30% 2.68 - 2.99% Dividend yield 0% 0% 0% Volatility 52.0 - 78.1% 51.6 - 55.8% 87.0 - 110.9% Expected terms (years) 5.2 - 5.7 5.3 - 7.0 5.94 - 6.25

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2016 2017 2018 Numerator: Net loss $ (27,124,637 ) $ (51,006,094 ) $ (127,502,194 ) Denominator: Weighted-average number of common shares used in net loss per common share -- basic and diluted 8,428,366 13,870,949 54,396,483 Net loss per common share -- basic and diluted $ (3.22 ) $ (3.68 ) $ (2.34 )
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, 2016 2017 2018 Convertible preferred stock 26,416,896 — — Common stock under option 3,831,237 6,355,581 7,497,734 Common stock warrants — 14,064 14,064 Total 30,248,133 6,369,645 7,511,798

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Summary of Interim Financial Information on Quarterly Basis	The following interim financial information presents the Company’s 2017 and 2018 results of operations on a quarterly basis: Quarter Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Operating Loss $ (9,052,493 ) $ (12,132,403 ) $ (11,589,319 ) $ (17,992,814 ) Net Loss (9,050,773 ) (12,126,152 ) (11,573,273 ) (18,255,896 ) Net Loss per common share, basic and diluted (1) (1.07 ) (1.44 ) (1.37 ) (0.61 ) 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 ) (1) The sum of the four quarters of earnings per share for 2017 and 2018 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.

Nature of Organization and Op_2

Nature of Organization and Operations - Additional Information (Detail) - USD ($)	Apr. 23, 2018	Dec. 13, 2017	Nov. 13, 2017	Dec. 31, 2018	Dec. 31, 2017
Nature of Organization and Operations [Line Items]
Proceeds from follow-on public offering after deducting underwriting discounts and commissions				$ 131,194,030	$ 150,062,959
Cash and cash equivalents				$ 176,267,666	$ 175,643,529
Follow-on Public Offering [Member]
Nature of Organization and Operations [Line Items]
Number of additional shares issued and sold	5,500,000
Price of common stock	$ 25.50
Proceeds from follow-on public offering after deducting underwriting discounts and commissions	$ 131,200,000
Underwriting discounts and commissions	8,400,000
Offering Cost	$ 500,000
IPO [Member]
Nature of Organization and Operations [Line Items]
Number of additional shares issued and sold		981,107	10,714,000
Price of common stock		$ 14	$ 14
Underwriting discounts and commissions		$ 1,000,000	$ 10,500,000
Offering Cost			2,300,000
Proceeds from initial offering after deducting underwriting discounts and commissions		$ 12,800,000	$ 137,200,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Number of operating segment | Segment	1
Revenues on initial stage of product development	$ 0
Expected dividend payments	$ 0
Expected dividend yield	0.00%	0.00%	0.00%
ASU 2016-02 [Member] | Minimum [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Lease assets	$ 5,000,000
Lease liabilities	5,000,000
ASU 2016-02 [Member] | Maximum [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Lease assets	7,000,000
Lease liabilities	$ 7,000,000

Common Stock - Summary of Share

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, 2016
Class Of Stock [Line Items]
Common stock reserved for future issuance	10,031,771	7,983,327	31,198,244
Common stock warrants	14,064	14,064
Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance			1,251,758
Series B Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance			2,982,962
Series C Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance			12,290,394
Series D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance			9,891,866
2010 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	5,013,153	5,817,844	4,781,264
2017 Stock Incentive Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	4,032,390	1,682,596
2017 Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	972,164	468,823

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2017	Jan. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
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 D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Convertible preferred stock issued and sold		6,714,413
Preferred stock, par value		$ 2.234
Gross proceeds from issuance and sale of convertible preferred stock		$ 15,000,000
Stock issuance costs		$ 136,980
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

Accrued Expenses and Prepaid _3

Accrued Expenses and Prepaid Assets - Schedule of Accrued Expenses (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued research and development	$ 3,481,570	$ 2,463,808
Accrued payroll liabilities	269,046	251,491
Other	1,352,386	175,406
Total	$ 5,103,002	$ 2,890,705

Accrued Expenses and Prepaid _4

Accrued Expenses and Prepaid Assets - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Prepaid research and development	$ 23.1	$ 4.1

Long-term Debt - Additional Inf

Long-term Debt - Additional Information (Detail)	Oct. 20, 2017USD ($)Installment$ / sharesshares	Oct. 19, 2017USD ($)$ / sharesshares	Dec. 31, 2018shares	Dec. 31, 2017shares
Debt Instrument [Line Items]
Warrant to purchase common stock | shares			14,064	14,064
Unsecured Promissory Note | Golda Darty Partners S A [Member]
Debt Instrument [Line Items]
Debt instrument, due and payment description			The promissory note bears interest at a rate per annum of 8.0%, and is due and payable quarterly in arrears on the 19th day of each April, July, October and January beginning on January 19, 2018.
Warrant to purchase common stock | shares		93,764
Exercise price per share | $ / shares		$ 5.484
Principal amount		$ 7,000,000
Interest rate		8.00%
Maturity date		Oct. 19, 2022
Fair value of warrant		$ 430,160
Term Loan Facility [Member]
Debt Instrument [Line Items]
Final payment due amount	$ 1,600,000
Term Loan Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Prepayment fee, percentage	0.00%
Term Loan Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Prepayment fee, percentage	2.00%
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]
Term loan facility	$ 20,000,000
Increase in interest rate	5.00%
Debt instrument, due and payment description			The Company is required to make monthly interest-only payments through November 1, 2019 and is required to make 24 equal monthly payments of principal, plus accrued interest, thereafter from November 1, 2019 through October 1, 2021, at which time all unpaid principal and interest becomes due and payable
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%

Long-term Debt - Summary of Con

Long-term Debt - Summary of Contractual Maturities of Long-term Debt Obligations (Detail) - Silicon Valley Bank [Member] - Term Loan Facility [Member] - Golda Darty Partners S A [Member]	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
2,019	$ 1,666,667
2,020	10,000,000
2,021	8,333,333
2,022	7,000,000
Long-term debt	$ 27,000,000

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Level 2 [Member] | Promissory Note [Member]
Cash And Cash Equivalents [Line Items]
Long-term Debt	$ 7.4	$ 7.7

License Agreements - Additional

License Agreements - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Sep. 30, 2014USD ($)	Dec. 31, 2018USD ($)License
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 an_2

401(k) Profit Sharing Plan and Trust - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Minimum age of full-time employees eligilble to participate in the plan	21 years
Employer contributions made to the 401(k) Plan	$ 322,459	$ 123,264	$ 50,867

Refundable Research Developme_3

Refundable Research Development and Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 08, 2015
Refundable Research Development 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	2,024
Research and development tax credit carryforward expiration year	2,024
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	$ 64,296,165	$ 30,699,411
Statutory U.S. federal income tax, Percentage of income before income taxes	21.00%	35.00%	34.00%
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 2015, for the U.S. federal jurisdiction, and 2014, for U.S. states’ and Australia’s federal jurisdiction
IRC Sec. 382 [Member]
Refundable Research Development And Income Taxes [Line Items]
Net operating loss carryforwards	$ 10,700,000
IRC Sec. 383 [Member]
Refundable Research Development And Income Taxes [Line Items]
Research and development tax credit carryforwards	1,900,000
Federal [Member]
Refundable Research Development And Income Taxes [Line Items]
Net operating loss carryforwards	251,400,000	118,800,000
Research and development tax credit carryforwards	14,400,000
State [Member]
Refundable Research Development And Income Taxes [Line Items]
Net operating loss carryforwards	$ 254,600,000	$ 118,800,000

Refundable Research Developme_4

Refundable Research Development 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Amount	$ (26,775,461)	$ (17,852,133)	$ (9,222,377)
State income taxes, net of federal benefit, Amount	(6,397,993)	(1,634,924)	(992,768)
Change in valuation allowances, Amount	38,156,506	11,176,291	12,923,538
Re-measurement of deferred taxes, Amount		15,659,916
Tax credits, Amount	(6,149,021)	(3,316,729)	(2,933,836)
Change in state apportionment, Kentucky tax reform and other, Amount	873,508
Permanent and other, Amount	$ 292,461	$ (4,032,421)	$ 225,443

Refundable Research Developme_5

Refundable Research Development 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expense Benefit Continuing Operations [Abstract]
Statutory U.S. federal income tax, Percentage of income before income taxes	21.00%	35.00%	34.00%
State income taxes, net of federal benefit, Percentage of income before income taxes	5.00%	3.30%	3.50%
Change in valuation allowances, Percentage of income before income taxes	(29.90%)	(20.80%)	(46.40%)
Re-measurement of deferred taxes, Percentage of income before income taxes		(31.90%)
Tax credits, Percentage of income before income taxes	4.80%	7.70%	10.80%
Change in state apportionment, Kentucky tax reform and other, Percentage of income before income taxes	(0.70%)
Permanent and other, Percentage of income before income taxes	(0.20%)	6.70%	(1.90%)

Refundable Research Developme_6

Refundable Research Development and Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Accrual to cash adjustment		$ 345,623
Intangible assets	$ 19,616	23,543
Share-based compensation	2,914,583	1,826,372
Deferred interest expense	135,585
Contribution carryforwards	36,759	24,465
Net operating loss carryforwards	64,296,165	30,699,411
Research and development credits	7,269,624	3,357,291
Orphan drug credits	7,165,354	4,928,666
Total deferred tax assets	81,837,686	41,205,371
Deferred tax liabilities:
Accrual to cash adjustment	(2,463,715)
Unrealized gain	(12,094)
Total deferred tax liabilities	(2,475,809)
Net deferred tax assets before allowance:	79,361,877	41,205,371
Less valuation allowance	$ (79,361,877)	$ (41,205,371)

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Jul. 31, 2018USD ($)ft²	Apr. 30, 2017USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	May 31, 2018
Commitments And Contingencies [Line Items]
Base rent payable			$ 6,859,064
Remaining contract costs			1,900,000
Medical Equipment [Member]
Commitments And Contingencies [Line Items]
Lease expense			$ 161,925
Medical Equipment [Member] | Minimum [Member]
Commitments And Contingencies [Line Items]
Office space lease term						24 months
Crestwood, Kentucky [Member]
Commitments And Contingencies [Line Items]
Lease expiration date			Jul. 31, 2023
Waltham, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Office space lease term		60 months
Area of operating lease | ft²		6,126
Initial monthly lease payments per month		$ 17,612
Base rent payable		$ 1,117,995
Lease expense			$ 636,835	$ 351,468	$ 117,222
Waltham, Massachusetts [Member] | Lease Amendment [Member]
Commitments And Contingencies [Line Items]
Office space lease term	78 months
Area of operating lease | ft²	22,600
Initial monthly lease payments per month	$ 44,375
Base rent payable	$ 5,352,094

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Payments Required Under the Leases (Detail)	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 1,251,720
2,020	1,292,025
2,021	975,750
2,022	998,350
2023 and thereafter	2,341,219
Total	$ 6,859,064

Equity Incentive Plans - Additi

Equity Incentive Plans - Additional Information (Detail) - USD ($)	Jan. 22, 2016	Oct. 31, 2017	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	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				2,060,685	2,940,444
Exercise price per share				$ 15.86	$ 6.94
Estimated fair values of common stock, per share							$ 14	$ 1.71
Actual fair values of common stock, per share				$ 26.73			$ 13.99
Unrecognized compensation expense				$ 27,266,206	$ 10,415,787	$ 2,910,870
Estimated weighted-average period to recognize				2 years 11 months 26 days	3 years 1 month 20 days	2 years 10 months 6 days
Common stock reserved for future issuance				10,031,771	7,983,327	31,198,244
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
Precentage 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] | Executive [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vested and exercisable percentage				50.00%
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
Precentage 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				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
Precentage 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 - Summar

Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding, Beginning balance	6,355,581	3,831,237
Shares, Granted	2,060,685	2,940,444
Shares, Exercised	(468,263)	(46,881)
Shares, Forfeited	(450,269)	(369,219)
Shares Outstanding, Ending balance	7,497,734	6,355,581	3,831,237
Shares, Options exercisable	4,068,602
Shares, Expected to vest	3,429,132
Weighted -Average Exercise Price Per Share Outstanding, Beginning balance	$ 4.78	$ 2.84
Weighted -Average Exercise Price Per Share, Granted	15.86	6.94
Weighted -Average Exercise Price Per Share, Exercised	2.61	3.51
Weighted -Average Exercise Price Per Share, Forfeited	6.58	2.56
Weighted -Average Exercise Price Per Share, Outstanding, Ending balance	8.09	4.78	$ 2.84
Weighted -Average Exercise Price Per Share, Options exercisable	4.64
Weighted -Average Exercise Price Per Share, Expected to vest	12.17
Weighted - Average Grant Date Fair Value Per Option Outstanding, Beginning balance	2.66	1.75
Weighted - Average Grant Date Fair Value Per Option, Granted	13.13	3.61
Weighted - Average Grant Date Fair Value Per Option, Exercised	1.46	2.25
Weighted - Average Grant Date Fair Value Per Option, Forfeited	4.27	1.22
Weighted - Average Grant Date Fair Value Per Option Outstanding, Ending balance	5.52	$ 2.66	$ 1.75
Weighted - Average Grant Date Fair Value Per Option, Options exercisable	2.59
Weighted - Average Grant Date Fair Value Per Option, Expected to vest	$ 8.99
Weighted - Average Contractual Life Outstanding, Balance	7 years 3 months 21 days	7 years 7 months 20 days	7 years 2 months 12 days
Weighted - Average Contractual Life, Options exercisable	5 years 10 months 20 days
Weighted - Average Contractual Life, Expected to vest	8 years 11 months 26 days
Aggregate Intrinsic Value Outstanding, Beginning balance	$ 107,534,863	$ 243,000
Aggregate Intrinsic Value, Exercised	4,953,187	852,818
Aggregate Intrinsic Value, Outstanding, Ending balance	45,706,169	$ 107,534,863	$ 243,000
Aggregate Intrinsic Value, Options exercisable	35,186,023
Aggregate Intrinsic Value, Expected to vest	$ 10,520,167

Equity Incentive Plans - Summ_2

Equity Incentive Plans - Summary of Share-based Compensation Expense Recognized (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 7,733,256	$ 5,418,710	$ 1,078,988
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	3,559,047	2,678,956	377,776
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 4,174,209	$ 2,739,754	$ 701,212

Equity Incentive Plans - Summ_3

Equity Incentive Plans - Summary of Assumption Used to Estimate the Grant Date Fair Value (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate, minimum	2.68%	1.21%	1.21%
Risk-free interest rate, maximum	2.99%	2.30%	1.60%
Dividend yield	0.00%	0.00%	0.00%
Volatility, minimum	87.00%	51.60%	52.00%
Volatility, maximum	110.90%	55.80%	78.10%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	5 years 11 months 8 days	5 years 3 months 18 days	5 years 2 months 12 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected terms (years)	6 years 3 months	7 years	5 years 8 months 12 days

Net Loss per Common Share - Sum

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, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (36,459,475)		$ (35,972,115)		$ (33,334,300)		$ (21,736,304)		$ (18,255,896)		$ (11,573,273)		$ (12,126,152)		$ (9,050,773)		$ (127,502,194)	$ (51,006,094)	$ (27,124,637)
Denominator:
Weighted-average number of common shares used in net loss per common share, basic and diluted																	54,396,483	13,870,949	8,428,366
Net loss per common share, basic and diluted	$ (0.65)	[1]	$ (0.64)	[1]	$ (0.61)	[1]	$ (0.43)	[1]	$ (0.61)	[1]	$ (1.37)	[1]	$ (1.44)	[1]	$ (1.07)	[1]	$ (2.34)	$ (3.68)	$ (3.22)
[1]	The sum of the four quarters of earnings per share for 2017 and 2018 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.

Net Loss per Common Share - S_2

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	7,511,798	6,369,645	30,248,133
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount			26,416,896
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	7,497,734	6,355,581	3,831,237
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 - Add

Related Party Transaction - Additional Information (Detail) - Revon [Member] - License Agreement and Services Agreement [Member]	May 01, 2018USD ($)
Related Party Transaction [Line Items]
Annual license fee	$ 175,000
Exclusive license option, thereafter	350,000
Development service costs	$ 250,000

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Summary of Interim Financial Information on Quarterly Basis (Detail) - USD ($)	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating Loss	$ (37,196,592)		$ (35,804,581)		$ (33,485,442)		$ (21,438,145)		$ (17,992,814)		$ (11,589,319)		$ (12,132,403)		$ (9,052,493)		$ (127,924,760)	$ (50,767,029)	$ (27,282,342)
Net loss	$ (36,459,475)		$ (35,972,115)		$ (33,334,300)		$ (21,736,304)		$ (18,255,896)		$ (11,573,273)		$ (12,126,152)		$ (9,050,773)		$ (127,502,194)	$ (51,006,094)	$ (27,124,637)
Net loss per common share, basic and diluted	$ (0.65)	[1]	$ (0.64)	[1]	$ (0.61)	[1]	$ (0.43)	[1]	$ (0.61)	[1]	$ (1.37)	[1]	$ (1.44)	[1]	$ (1.07)	[1]	$ (2.34)	$ (3.68)	$ (3.22)
[1]	The sum of the four quarters of earnings per share for 2017 and 2018 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 - Additional

Subsequent Events - Additional Information (Detail) - $ / shares	1 Months Ended		12 Months Ended
	Feb. 28, 2019	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Number of shares granted			2,060,685	2,940,444
Exercise price per share			$ 15.86	$ 6.94
Subsequent Event [Member]
Subsequent Event [Line Items]
Number of shares granted	1,451,600
Exercise price range, lower range limit	$ 13.72
Exercise price range, higher range limit	$ 13.85
Board of Directors [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of shares granted		70,320
Exercise price per share		$ 13.19