Fulcrum Therapeutics (FULC) 10-Q 14 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 08, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Title of 12(b) Security	Common stock, par value $0.001 per share
Trading Symbol	FULC
Security Exchange Name	NASDAQ
Entity Registrant Name	FULCRUM THERAPEUTICS, INC.
Entity Central Index Key	0001680581
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		23,340,544
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity File Number	001-38978
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4839948
Entity Address, Address Line One	26 Landsdowne Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	651-8851
Document Quarterly Report	true
Document Transition Report	false

Consolidated Balance Sheets (Un

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 101,597	$ 72,797
Prepaid expenses and other current assets	3,625	1,298
Total current assets	105,222	74,095
Property and equipment, net	9,583	10,546
Restricted cash	1,092	1,092
Other assets	59	38
Total assets	115,956	85,771
Current liabilities:
Accounts payable	4,533	1,263
Accrued expenses and other current liabilities	3,583	2,497
Deferred lease incentive, current portion	469	469
Total current liabilities	8,585	4,229
Deferred rent, excluding current portion	1,527	1,402
Deferred lease incentive, excluding current portion	3,638	3,990
Other liabilities, excluding current portion	78	150
Total liabilities	13,828	9,771
Commitments and contingencies (Note 10)
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 200,000,000 and 135,000,000 shares authorized as of September 30, 2019 and December 31, 2018, respectively; 23,340,419 and 2,791,764 shares issued as of September 30, 2019 and December 31, 2018, respectively; 22,557,871 and 1,587,953 shares outstanding as of September 30, 2019 and December 31, 2018, respectively	23	2
Additional paid-in capital	236,783	4,452
Accumulated deficit	(134,678)	(68,124)
Total stockholders’ equity (deficit)	102,128	(63,670)
Total liabilities, convertible preferred stock, and stockholders’ equity (deficit)	$ 115,956	85,771
Series A Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		59,909
Series B Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		$ 79,761

Consolidated Balance Sheets (_2

Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	135,000,000
Common stock, shares issued	23,340,419	2,791,764
Common stock, shares outstanding	22,557,871	1,587,953
Treasury stock, shares	0	67,024
Series A Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, share authorized	0	60,000,000
Convertible preferred stock, share issued	0	60,000,000
Convertible preferred stock, share outstanding	0	60,000,000
Series B Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, share authorized	0	40,000,000
Convertible preferred stock, share issued	0	40,000,000
Convertible preferred stock, share outstanding	0	40,000,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 13,496	$ 6,963	$ 58,985	$ 18,324
General and administrative	3,510	2,125	8,742	5,923
Total operating expenses	17,006	9,088	67,727	24,247
Loss from operations	(17,006)	(9,088)	(67,727)	(24,247)
Other income, net:
Interest income, net	457	138	1,151	133
Other income	7	7	22	385
Net loss and comprehensive loss	(16,542)	(8,943)	(66,554)	(23,729)
Cumulative convertible preferred stock dividends	(796)	(1,858)	(7,128)	(3,736)
Net loss attributable to common stockholders	$ (17,338)	$ (10,801)	$ (73,682)	$ (27,465)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.97)	$ (8.08)	$ (10.33)	$ (23.28)
Weighted average number of common shares used in net loss per share attributable to common stockholders, basic and diluted	17,785	1,337	7,133	1,180

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Deficit
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2017		34,666,666
Convertible preferred stock, Beginning balance at Dec. 31, 2017		$ 34,587
Beginning balance at Dec. 31, 2017	$ (33,265)			$ 1		$ 2,270	$ (35,536)
Beginning balance, Shares at Dec. 31, 2017				972,266	8,036
Issuance of convertible preferred stock		$ 12,659
Issuance of convertible preferred stock, Shares		12,666,667
Issuance of common stock	4					4
Issuance of common stock, Shares				125,123
Repurchase of unvested restricted stock awards					12,599
Retirement of treasury shares					(20,635)
Stock-based compensation expense	429					429
Net loss	(6,940)						(6,940)
Convertible preferred stock, Ending balance, Shares at Mar. 31, 2018		47,333,333
Convertible preferred stock, Ending balance at Mar. 31, 2018		$ 47,246
Ending balance at Mar. 31, 2018	(39,772)			$ 1		2,703	(42,476)
Ending balance, Shares at Mar. 31, 2018				1,097,389
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2017		34,666,666
Convertible preferred stock, Beginning balance at Dec. 31, 2017		$ 34,587
Beginning balance at Dec. 31, 2017	(33,265)			$ 1		2,270	(35,536)
Beginning balance, Shares at Dec. 31, 2017				972,266	8,036
Net loss	(23,729)
Convertible preferred stock, Ending balance, Shares at Sep. 30, 2018		60,000,000	40,000,000
Convertible preferred stock, Ending balance at Sep. 30, 2018		$ 59,909	$ 79,761
Ending balance at Sep. 30, 2018	(55,580)			$ 1		3,684	(59,265)
Ending balance, Shares at Sep. 30, 2018				1,425,501
Convertible preferred stock, Beginning balance, Shares at Mar. 31, 2018		47,333,333
Convertible preferred stock, Beginning balance at Mar. 31, 2018		$ 47,246
Beginning balance at Mar. 31, 2018	(39,772)			$ 1		2,703	(42,476)
Beginning balance, Shares at Mar. 31, 2018				1,097,389
Issuance of convertible preferred stock		$ 12,663
Issuance of convertible preferred stock, Shares		12,666,667
Issuance of common stock	8					8
Issuance of common stock, Shares				187,421
Stock-based compensation expense	462					462
Net loss	(7,846)						(7,846)
Convertible preferred stock, Ending balance, Shares at Jun. 30, 2018		60,000,000
Convertible preferred stock, Ending balance at Jun. 30, 2018		$ 59,909
Ending balance at Jun. 30, 2018	(47,148)			$ 1		3,173	(50,322)
Ending balance, Shares at Jun. 30, 2018				1,284,810
Issuance of convertible preferred stock			$ 79,761
Issuance of convertible preferred stock, Shares			40,000,000
Issuance of common stock	5					5
Issuance of common stock, Shares				140,691
Stock-based compensation expense	506					506
Net loss	(8,943)						(8,943)
Convertible preferred stock, Ending balance, Shares at Sep. 30, 2018		60,000,000	40,000,000
Convertible preferred stock, Ending balance at Sep. 30, 2018		$ 59,909	$ 79,761
Ending balance at Sep. 30, 2018	(55,580)			$ 1		3,684	(59,265)
Ending balance, Shares at Sep. 30, 2018				1,425,501
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2018		60,000,000	40,000,000
Convertible preferred stock, Beginning balance at Dec. 31, 2018		$ 59,909	$ 79,761
Beginning balance at Dec. 31, 2018	(63,670)			$ 2		4,452	(68,124)
Beginning balance, Shares at Dec. 31, 2018				1,587,953	67,024
Issuance of convertible preferred stock			$ 25,466
Issuance of convertible preferred stock, Shares			12,500,000
Issuance of common stock	5					5
Issuance of common stock, Shares				134,013
Repurchase of unvested restricted stock awards					43,922
Retirement of treasury shares					(110,946)
Stock-based compensation expense	821					821
Net loss	(36,843)						(36,843)
Convertible preferred stock, Ending balance, Shares at Mar. 31, 2019		60,000,000	52,500,000
Convertible preferred stock, Ending balance at Mar. 31, 2019		$ 59,909	$ 105,227
Ending balance at Mar. 31, 2019	(99,687)			$ 2		5,278	(104,967)
Ending balance, Shares at Mar. 31, 2019				1,721,966
Convertible preferred stock, Beginning balance, Shares at Dec. 31, 2018		60,000,000	40,000,000
Convertible preferred stock, Beginning balance at Dec. 31, 2018		$ 59,909	$ 79,761
Beginning balance at Dec. 31, 2018	(63,670)			$ 2		4,452	(68,124)
Beginning balance, Shares at Dec. 31, 2018				1,587,953	67,024
Net loss	(66,554)
Convertible preferred stock, Ending balance, Shares at Sep. 30, 2019		0	0
Ending balance at Sep. 30, 2019	102,128			$ 23		236,783	(134,678)
Ending balance, Shares at Sep. 30, 2019				22,557,871
Convertible preferred stock, Beginning balance, Shares at Mar. 31, 2019		60,000,000	52,500,000
Convertible preferred stock, Beginning balance at Mar. 31, 2019		$ 59,909	$ 105,227
Beginning balance at Mar. 31, 2019	(99,687)			$ 2		5,278	(104,967)
Beginning balance, Shares at Mar. 31, 2019				1,721,966
Issuance of common stock	225					225
Issuance of common stock, Shares				148,320
Repurchase of unvested restricted stock awards					7,451
Retirement of treasury shares					(6,019)
Stock-based compensation expense	950					950
Net loss	(13,169)						(13,169)
Convertible preferred stock, Ending balance, Shares at Jun. 30, 2019		60,000,000	52,500,000
Convertible preferred stock, Ending balance at Jun. 30, 2019		$ 59,909	$ 105,227
Ending balance at Jun. 30, 2019	(111,681)			$ 2		6,453	(118,136)
Ending balance, Shares at Jun. 30, 2019				1,870,286	1,432
Issuance of common stock	35					35
Issuance of common stock, Shares				116,167
Conversion of convertible preferred stock into common stock	165,136			$ 16		165,120
Conversion of convertible preferred stock into common stock, Shares		(60,000,000)	(52,500,000)
Conversion of convertible preferred stock into common stock		$ (59,909)	$ (105,227)
Conversion of convertible preferred stock into common stock, Shares				16,071,418
Initial public offering net of underwriting discounts, commissions and offering costs	64,001			$ 5		63,996
Initial public offering net of underwriting discounts, commissions and offering costs, Shares				4,500,000
Repurchase of unvested restricted stock awards					4,951
Retirement of treasury shares					(6,383)
Stock-based compensation expense	1,179					1,179
Net loss	(16,542)						(16,542)
Convertible preferred stock, Ending balance, Shares at Sep. 30, 2019		0	0
Ending balance at Sep. 30, 2019	$ 102,128			$ 23		$ 236,783	$ (134,678)
Ending balance, Shares at Sep. 30, 2019				22,557,871

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Series A Convertible Preferred Stock
Shares issued, price per share			$ 1	$ 1
Convertible preferred stock, Issuance costs			$ 5	$ 7
Series B Convertible Preferred Stock
Shares issued, price per share		$ 2
Convertible preferred stock, Issuance costs	$ 34	$ 239

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net loss	$ (66,554)	$ (23,729)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	1,534	836
Stock-based compensation expense	2,950	1,397
In-process research and development expenses	25,591
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,714)	(29)
Other assets	(20)	20
Accounts payable	2,849	132
Accrued expenses and other liabilities	1,237	1,062
Deferred rent and deferred lease incentive	139	4,152
Net cash used in operating activities	(34,988)	(16,159)
Investing activities
Purchases of property and equipment	(745)	(5,515)
Transaction costs associated with asset acquisition	(91)
Net cash used in investing activities	(836)	(5,515)
Financing activities
Proceeds from initial public offering of common stock, net of underwriting discounts and commissions	64,443
Principal payments on capital lease obligations	(34)	(59)
Proceeds from issuance of common stock under benefit plans	249	18
Net cash provided by financing activities	64,624	105,115
Net increase in cash, cash equivalents and restricted cash	28,800	83,441
Cash, cash equivalents, and restricted cash, beginning of period	73,889	407
Cash, cash equivalents, and restricted cash, end of period	102,689	83,848
Supplemental cash flow information
Cash paid for interest	6	8
Non-cash investing and financing activities:
Acquisition of in process research and development through issuance of stock	25,500
Conversion of convertible preferred stock into common stock	165,136
Property and equipment purchases unpaid at end of period		2,976
Public offering costs unpaid at end of period	442	73
Cash and cash equivalents	101,597	82,756
Restricted cash	1,092	1,092
Series A Convertible Preferred Stock
Financing activities
Proceeds from issuance of stock, net of issuance costs		25,322
Series B Convertible Preferred Stock
Financing activities
Proceeds from issuance of stock, net of issuance costs	$ (34)	$ 79,834

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Fulcrum Therapeutics, Inc. (the “Company” or “Fulcrum”) was incorporated in Delaware on August 18, 2015. The Company is focused on improving the lives of patients with genetically defined diseases in areas of high unmet medical need, with an initial focus on rare diseases. The Company is subject to a number of risks similar to other companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, dependence on key personnel, protection of proprietary technology, reliance on third party organizations, risks of obtaining regulatory approval for any product candidate that it may develop, development by competitors of technological innovations, compliance with government regulations, and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). The accompanying consolidated financial statements and footnotes to the financial statements have been prepared on the same basis as the most recently audited annual financial statements and, in the opinion of management, reflect all normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of September 30, 2019 and the results of its operations and its cash flows for the three and nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s final prospectus for its initial public offering (“IPO”) filed pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Securities Act”), with the Securities and Exchange Commission (the “SEC”) on July 18, 2019. Initial Public Offering On July 22, 2019, the Company completed an IPO of its common stock and issued and sold 4,500,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $64.0 million after deducting underwriting discounts and commissions and estimated offering expenses. Upon the closing of the IPO, all 112,500,000 shares of outstanding preferred stock automatically converted into 16,071,418 shares of common stock. In July 2019, the Company effected a one-for-seven reverse stock split of the Company’s issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each of the Company’s outstanding series of preferred stock. All share and per share amounts in the accompanying consolidated financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Liquidity The Company has incurred recurring losses and negative cash flows from operations since inception and has primarily funded its operations with proceeds from the IPO and issuances of convertible notes and convertible preferred stock. As of September 30, 2019, the Company had an accumulated deficit of $134.7 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to expand its research and development efforts. The Company believes that its cash and cash equivalents as of September 30, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of filing this Quarterly Report on Form 10-Q. The Company will need additional funding to support its planned operating activities. If the Company is unable to obtain additional funding, it would be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion, or commercialization efforts, which could adversely affect its business prospects.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, Fulcrum Therapeutics Securities Corp., which is a Massachusetts subsidiary created to buy, sell, and hold securities. All intercompany transactions and balances have been eliminated. Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the accompanying consolidated financial statements are described in the Company’s audited consolidated financial statements for the year ended December 31, 2018 included in the Company’s final prospectus for its IPO filed pursuant to Rule 424(b)(4) under the Securities Act with the SEC on July 18, 2019 Use of Estimates The preparation of financial statements in accordance with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements, and the reported amount of expenses during the reported periods. Estimates inherent in the preparation of these consolidated financial statements include, but are not limited to, estimates related to accrued expenses, stock-based compensation expense, the fair value of the common stock (for periods prior to the completion of the IPO of the Company’s common stock) and convertible preferred stock, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions. Deferred Issuance Costs The Company capitalizes certain legal, professional, accounting and other third-party fees that are directly associated with in-process equity issuances as deferred issuance costs until such equity issuances are consummated. After consummation of the equity issuance, these costs are recorded as a reduction in the capitalized amount associated with the equity issuance. Should the planned financing be abandoned, the deferred issuance costs will be expensed immediately as a charge to operating expenses in the consolidated statement of operations and comprehensive loss. Research and Development Expenses Research and development expenses include costs directly attributable to the conduct of research and development programs, including personnel-related expenses such as salaries, payroll taxes, benefits, and stock-based compensation expense, manufacturing and external costs related to outside vendors engaged to conduct both preclinical studies and clinical trials, laboratory supplies, depreciation on and maintenance of research equipment, and the allocable portions of facility costs, such as rent, utilities, repairs and maintenance, depreciation, and general support services. Expenditures relating to research and development are expensed in the period incurred. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Research and development expenses also include in-process research and development (“IPR&D”) expenses, which relate to IPR&D acquired as part of an asset acquisition for which there is no alternative future use. Research and development with no alternative future use is expensed as incurred. Off-Balance Sheet Risk and Concentrations of Credit Risk The Company has no significant off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and restricted cash. The Company’s cash, cash equivalents, and restricted cash are deposited in accounts at large financial institutions. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash, cash equivalents and restricted cash are held. The Company maintains its cash equivalents in money market funds that invest in U.S. Treasury securities. Recent Accounting Pronouncements—To Be Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at September 30, 2019 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 101,597 $ 101,597 $ — $ — Total $ 101,597 $ 101,597 $ — $ — Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 72,797 $ 72,797 $ — $ — Total $ 72,797 $ 72,797 $ — $ — There have been no transfers between fair value levels during the three or nine months ended September 30, 2019 and 2018.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2019
Property Plant And Equipment [Abstract]
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): September 30, 2019 December 31, 2018 Lab equipment $ 5,634 $ 4,847 Furniture and fixtures 548 542 Computer equipment 512 517 Software 106 90 Leasehold improvements 6,210 6,174 Construction in process — 334 Total property and equipment 13,010 12,504 Less: accumulated depreciation (3,427 ) (1,958 ) Property and equipment, net $ 9,583 $ 10,546 Depreciation expense for the three months ended September 30, 2019 and 2018 was $0.5 million and $0.4 million, respectively. Depreciation expense for the nine months ended September 30, 2019 and 2018 was $1.5 million and $0.8 million, respectively.

Additional Balance Sheet Detail

Additional Balance Sheet Detail	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Additional Balance Sheet Detail	5. Additional Balance Sheet Detail Prepaid expenses and other current assets consisted of the following (in thousands): September 30, 2019 December 31, 2018 Prepaid expenses $ 2,976 $ 460 Prepaid sign-on bonuses subject to vesting provisions 185 99 Leasehold improvement allowance receivable — 366 Interest income receivable 159 135 Other 305 238 Total prepaid expenses and other current assets $ 3,625 $ 1,298 Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, 2019 December 31, 2018 External research and development $ 1,779 $ 437 Payroll and benefits 1,511 1,448 Professional services 168 254 Capital lease obligation, current portion 49 46 Restricted stock liability, current portion 15 18 Property and equipment purchases — 174 Other 61 120 Total accrued expenses and other current liabilities $ 3,583 $ 2,497

Preferred Stock

Preferred Stock	9 Months Ended
	Sep. 30, 2019
Temporary Equity Disclosure [Abstract]
Preferred Stock	6. Preferred Stock As of September 30, 2019, 5,000,000 shares of preferred stock were authorized. No shares of preferred stock were issued or outstanding as of September 30, 2019. As of December 31, 2018, 100,000,000 shares of convertible preferred stock were authorized, issued, and outstanding, of which 60,000,000 shares were designated as $0.001 par value Series A convertible preferred stock (the “Series A Preferred Stock”), and 40,000,000 shares were designated as $0.001 par value Series B convertible preferred stock (the “Series B Preferred Stock”, and together with the Series A Preferred Stock, the “Preferred Stock”). During the nine months ended September 30, 2019, the Company issued 12,500,000 shares of Series B Preferred Stock in connection with the GSK Agreement (Note 9). The rights, privileges, and preferences of the Series B Preferred Stock issued in connection with the GSK Agreement are consistent with the rights, privileges, and preferences of the Series B Preferred Stock issued during the year ended December 31, 2018. On July 5, 2019, the Company eliminated the gross proceeds threshold of $45.0 million for a firm-commitment underwritten public offering that would result in the automatic conversion of all outstanding shares of Preferred Stock into common stock. Upon the completion of the IPO on July 22, 2019, all 112,500,000 shares of outstanding Preferred Stock automatically converted into 16,071,418 shares of common stock. In addition, upon the completion of the IPO, the Company amended and restated its certificate of incorporation to authorize 5,000,000 shares of preferred stock, which shares of preferred stock are currently undesignated. No dividends have been declared since inception. Aggregate cumulative dividends associated with the Series A Preferred Stock and the Series B Preferred Stock as of December 31, 2018 were $7.2 million and $2.3 million, respectively.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Common Stock	7. Common Stock Common stockholders are entitled to receive dividends, as may be declared by the Company’s board of directors, if any, subject to the preferential dividend rights of any preferred stock then outstanding. No dividends have been declared or paid during the three and nine months ended September 30, 2019 and 2018. During the nine months ended September 30, 2019, the Company’s Certificate of Incorporation was amended to increase the number of authorized shares of common stock from 135,000,000 to 200,000,000 shares. As of September 30, 2019 and December 31, 2018 the Company has reserved for future issuance the following number of shares of common stock: September 30, 2019 December 31, 2018 Shares reserved for conversion of outstanding Series A Preferred Stock — 8,571,427 Shares reserved for conversion of outstanding Series B Preferred Stock — 5,714,277 Shares reserved for future issuance under the 2016 Stock Incentive Plan — 919,030 Shares reserved for exercises of outstanding stock options 1,981,457 507,891 Shares reserved for future issuance under the 2019 Stock Incentive Plan 2,398,391 — Shares reserved for future issuance under the 2019 Employee Stock Purchase Plan 252,142 — 4,631,990 15,712,625

Stock-based Compensation Expens

Stock-based Compensation Expense	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation Expense	8. Stock-based Compensation Expense 2016 Stock Incentive Plan In July 2016, the Company adopted the 2016 Stock Incentive Plan (the “2016 Plan”), which provided for the grant of restricted stock awards, restricted stock units, incentive stock options, non-statutory stock options, and other stock-based awards to the Company’s eligible employees, officers, directors, consultants, and advisors. The total number of shares of common stock that were authorized for issuance under the 2016 Plan as of December 31, 2018 was 3,209,285. As of the effective date of the 2019 Stock Incentive Plan (the “2019 Plan”), and as of September 30, 2019, no shares remained available for future issuance under the 2016 Plan. Any options or awards outstanding under the 2016 Plan remain outstanding and effective. 2019 Stock Incentive Plan On July 2, 2019, the Company’s stockholders approved the 2019 Plan, which became effective on July 17, 2019. The 2019 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to the Company’s officers, employees, directors, consultants and advisors. The number of shares initially reserved for issuance under the 2019 Plan is 2,017,142 shares, plus the shares of common stock remaining available for issuance under the 2016 Plan as of July 17, 2019. The number of shares reserved shall be annually increased on January 1, 2020 and each January 1 thereafter through January 1, 2029 by the least of (i) 2,000,000 shares, (ii) 4% of the number of shares of the Company’s common stock outstanding on the first day of the such year or (iii) an amount determined by the Company’s board of directors. As of September 30, 2019, there were 2,398,391 shares available for future issuance under the 2019 Plan. The shares of common stock underlying any awards that expire, terminate, or are otherwise surrendered, cancelled, forfeited or repurchased by the Company under the 2016 Plan or the 2019 Plan will be added back to the shares of common stock available for issuance under the 2019 Plan. As of July 17, 2019, no further awards will be made under the 2016 Plan. For financial reporting purposes, the Company performed common stock valuations with the assistance of a third-party specialist as of May 10, 2019, March 15, 2019, November 30, 2018, August 24, 2018, June 1, 2018, December 31, 2017, and December 31, 2016 to determine stock-based compensation expense for restricted stock awards and stock options. Upon completion of the IPO, the fair value of the common stock on the grant date was based on the closing price of the stock on the date of grant. Shares of common stock repurchased from employees and non-employees are shares held in the Company’s treasury (“Treasury Shares”). The board of directors may, at its discretion, authorize that the Treasury Shares be returned to the pool of authorized but unissued common stock. The following table summarizes the Company’s restricted stock activity under the 2019 Plan and 2016 Plan since December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2018 757,022 $ 3.07 Granted — — Vested (280,833 ) 3.04 Repurchased (56,324 ) 3.07 Unvested at September 30, 2019 419,865 $ 3.09 The following table summarizes the Company’s stock option activity under the 2019 Plan and 2016 Plan since December 31, 2018: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2018 507,891 $ 7.12 9.84 $ 367,427 Granted 1,516,294 9.95 Exercised (33,561 ) 7.50 Cancelled (9,167 ) 10.09 Outstanding at September 30, 2019 1,981,457 $ 9.27 9.38 $ 202,742 Exercisable at September 30, 2019 193,292 $ 7.96 9.23 $ 72,559 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock as of the balance sheet date for those options that had exercise prices lower than the fair value of the Company’s common stock. The weighted average grant date fair value of stock options granted during the three and nine months ended September 30, 2019 was $10.36 per share and $6.97 per share, respectively. The weighted average grant date fair value of stock options granted during the three and nine months ended September 30, 2018 was $3.49 per share and $3.47 per share, respectively. The aggregate intrinsic value of stock options exercised during the three and nine months ended September 30, 2019 was less than $0.1 million and $0.2 million, respectively. No stock options were exercised during the three and nine months ended September 30, 2018. The fair value of stock options granted during the three and nine months ended September 30, 2019 and 2018 under the 2019 Plan and the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions: Three Months Ended September 30, 2019 Three Months Ended September 30, 2018 Nine Months Ended September 30, 2019 Nine Months Ended September 30, 2018 Risk-free interest rate 1.7 % 2.8 % 2.4 % 2.8 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % Expected term (years) 6.0 6.0 6.0 5.9 Expected stock price volatility 80.1 % 84.3 % 81.0 % 83.9 % Grants Outside of Stock Incentive Plans The following table summarizes the Company’s restricted stock activity outside of the 2019 Plan and 2016 Plan since December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2018 446,789 $ 2.94 Granted — — Vested (84,106 ) 2.94 Repurchased — — Unvested at September 30, 2019 362,683 $ 2.94 The aggregate intrinsic value of all restricted stock awards that vested during the three and nine months ended September 30, 2019 was $0.6 million and $3.0 million, respectively. Stock-based Compensation Expense The total compensation cost recognized in the statements of operations associated with all stock-based compensation awards granted by the Company is as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 533 $ 282 $ 1,398 $ 808 General and administrative 646 224 1,552 589 Total stock-based compensation expense $ 1,179 $ 506 $ 2,950 $ 1,397 As of September 30, 2019, the Company had an aggregate of $13.1 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted average period of 3.32 years. 2019 Employee Stock Purchase Plan On July 2, 2019, the Company’s stockholders approved the 2019 Employee Stock Purchase Plan (the “ESPP”), which became effective on July 17, 2019. A total of 252,142 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock reserved under the ESPP shall be annually increased on January 1, 2020, and each January 1 thereafter through January 1, 2029, by the least of (i) 428,571 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the first day of each such year or (iii) an amount determined by the Company’s board of directors.

Asset Acquisition

Asset Acquisition	9 Months Ended
	Sep. 30, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Asset Acquisition	9. Asset Acquisition In February 2019, the Company entered into a right of reference and license agreement (the “GSK Agreement”) with subsidiaries of GlaxoSmithKline plc (collectively referred to as “GSK”), pursuant to which the Company has been granted an exclusive worldwide license to develop and commercialize losmapimod. Under the GSK Agreement, the Company also acquired reference rights to relevant regulatory and manufacturing documents and GSK’s existing supply of losmapimod drug substance and product. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The Company is obligated to use commercially reasonable efforts to develop and commercialize losmapimod at its sole cost. The Company is also responsible for costs related to the filing and maintenance of the licensed patent rights. Under the GSK Agreement, the Company issued 12,500,000 million shares of Series B Preferred Stock to GSK with an estimated fair value of $25.5 million, or $2.04 per share, which was determined with the assistance of a third-party specialist contemporaneously with the issuance of the Series B Preferred Stock to GSK. In addition, the Company may owe GSK up to $37.5 million in certain specified clinical and regulatory milestones, of which $2.5 million is due upon the initiation of a Phase 2 clinical trial, and up to $60.0 million in certain specified sales milestones. The Company has agreed to pay tiered royalties on annual net sales of losmapimod that range from mid single-digit percentages to a low double-digit, but less than teens, percentage. The royalties are payable on a product-by-product and country-by-country basis, and may be reduced in specified circumstances. The Company also incurred $0.1 million of direct expenses related to the transaction, which the Company included in the total consideration for the transaction. In August 2019, the $2.5 million milestone due upon the initiation of a Phase 2 clinical trial was achieved and became payable. The Company recorded the $2.5 million milestone due upon the initiation of a Phase 2 clinical trial as research and development expense in the Company’s consolidated statement of operations and comprehensive loss for the three and nine months ended September 30, 2019. The GSK Agreement may be terminated by either party for a material breach by the other, subject to notice and cure provisions. Unless earlier terminated, the GSK Agreement will continue in effect until the expiration of the Company’s royalty obligations, which expire on a country-by-country basis on the later of (i) ten years after the first commercial sale in the country or (ii) approval of a generic version of losmapimod by the applicable regulatory agency. The Company concluded the arrangement did not result in the acquisition of a business, as substantially all of the fair value of the gross assets acquired was concentrated in a single in-process research and development asset, losmapimod. In addition, the Company did not obtain any substantive processes in connection to the GSK Agreement and losmapimod was not generating revenue at the time the GSK Agreement was executed. Therefore, the Company accounted for the arrangement as an asset acquisition. The Company also concluded that the acquired assets do not have an alternative future use, and therefore the fair value attributable to the GSK Agreement of $25.6 million, inclusive of transaction costs, was recorded as in-process research and development expense (a component of research and development expenses) in the Company’s consolidated statement of operations and comprehensive loss for the nine months ended September 30, 2019, which is the period in which the Company obtained (i) the license to losmapimod, (ii) the right to reference relevant regulatory and manufacturing documents, and (iii) GSK’s existing supply of losmapimod drug substance and product. Additionally, the Company will recognize clinical and regulatory milestone payments when the underlying contingency is resolved and the consideration is paid or becomes payable. The milestone payments will be capitalized or expensed depending on the nature of the associated asset as of the date of recognition. The Company will record sales milestone payments and royalties as additional expense of the related product sales in the period in which the corresponding sales occur.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Operating Leases In May 2016, the Company entered into a sublease agreement for approximately 8,143 square feet of office and laboratory space at its prior corporate headquarters in Cambridge, Massachusetts. The sublease for this space terminated on June 30, 2018. Rent expense associated with this sublease for the nine months ended September 30, 2018 was $0.1 million. In November 2017, the Company entered into a lease agreement for its current corporate headquarters for approximately 28,731 square feet of office and laboratory space in Cambridge, Massachusetts. The lease has a total commitment of $25.1 million over the ten year term, and includes escalating rent payments. The lease agreement requires the Company to either pay a security deposit or maintain a letter of credit of $1.1 million. The Company maintains a letter of credit for this lease and has recorded the cash held to secure the letter of credit as restricted cash on the consolidated balance sheet as of September 30, 2019 and The future minimum lease payments associated with the lease for the Company’s current headquarters as of September 30, 2019, are as follows (in thousands): 2019 (1) $ 562 2020 2,285 2021 2,354 2022 2,424 2023 2,497 Thereafter 12,187 Total minimum lease payments $ 22,309 (1) Amounts are for the three months ending December 31, 2019. Other Agreements The Company has agreements with third parties in the normal course of business under which it can license certain developed technologies. If the Company exercises its rights to license the technologies it may be subject to additional fees and milestone payments. As of September 30, 2019, the Company has not exercised its rights to license such technologies. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters arising out of the relationship between such parties and the Company. In addition, the Company has entered into indemnification agreements with members of its board of directors and senior management that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations as of September 30, 2019 or December 31, 2018. Legal Proceedings The Company is not currently a party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses the costs related to its legal proceedings as they are incurred. No such costs have been incurred during the three or nine months ended September 30, 2019 and 2018.

Defined Contribution Plan

Defined Contribution Plan	9 Months Ended
	Sep. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	11. Defined Contribution Plan The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants the option to elect to defer a portion of their annual compensation on a pretax basis. As currently established, the Company is not required to make and has not made any contributions to the 401(k) Plan.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share	12. Net Loss per Share Net Loss per Share The following common stock equivalents were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Series A Preferred Stock — 8,571,427 — 8,571,427 Series B Preferred Stock — 5,714,277 — 5,714,277 Outstanding stock options 1,981,457 117,056 1,981,457 117,056 Unvested restricted stock awards 782,548 1,447,349 782,548 1,447,349 Total 2,764,005 15,850,109 2,764,005 15,850,109

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related-Party Transactions	13. Related-Party Transactions During the nine months ended September 30, 2019 and 2018, the Company paid fees to Third Rock Ventures, LLC (“TRV”), an affiliate of one of the Company’s principal stockholders, in exchange for consulting services. The Company recorded expenses related to such fees of less than $0.1 million during each of the nine months ended September 30, 2019 and 2018, and less than $0.1 million during the three months ended September 30, 2018. The Company did not record expenses related to such fees during the three months ended September 30, 2019. As of December 31, 2018, there was less than $0.1 million of amounts due to TRV for such services that were included in accounts payable and accrued expenses. As of September 30, 2019, there were no amounts due to TRV for such services that were included in accounts payable and accrued expenses. Additionally, consultants that provide services to the Company are employees of TRV. The Company has issued an aggregate of 142,284 shares of common stock to these consultants in exchange for their continuing consulting services. During the nine months ended September 30, 2018, the Company recorded other income of $0.4 million related to the sale of drug material to an entity affiliated with TRV.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, Fulcrum Therapeutics Securities Corp., which is a Massachusetts subsidiary created to buy, sell, and hold securities. All intercompany transactions and balances have been eliminated.
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the accompanying consolidated financial statements are described in the Company’s audited consolidated financial statements for the year ended December 31, 2018 included in the Company’s final prospectus for its IPO filed pursuant to Rule 424(b)(4) under the Securities Act with the SEC on July 18, 2019
Use of Estimates	Use of Estimates The preparation of financial statements in accordance with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements, and the reported amount of expenses during the reported periods. Estimates inherent in the preparation of these consolidated financial statements include, but are not limited to, estimates related to accrued expenses, stock-based compensation expense, the fair value of the common stock (for periods prior to the completion of the IPO of the Company’s common stock) and convertible preferred stock, and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions.
Deferred Issuance Costs	Deferred Issuance Costs The Company capitalizes certain legal, professional, accounting and other third-party fees that are directly associated with in-process equity issuances as deferred issuance costs until such equity issuances are consummated. After consummation of the equity issuance, these costs are recorded as a reduction in the capitalized amount associated with the equity issuance. Should the planned financing be abandoned, the deferred issuance costs will be expensed immediately as a charge to operating expenses in the consolidated statement of operations and comprehensive loss.
Research and Development Expense	Research and Development Expenses Research and development expenses include costs directly attributable to the conduct of research and development programs, including personnel-related expenses such as salaries, payroll taxes, benefits, and stock-based compensation expense, manufacturing and external costs related to outside vendors engaged to conduct both preclinical studies and clinical trials, laboratory supplies, depreciation on and maintenance of research equipment, and the allocable portions of facility costs, such as rent, utilities, repairs and maintenance, depreciation, and general support services. Expenditures relating to research and development are expensed in the period incurred. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Research and development expenses also include in-process research and development (“IPR&D”) expenses, which relate to IPR&D acquired as part of an asset acquisition for which there is no alternative future use. Research and development with no alternative future use is expensed as incurred.
Off-Balance Sheet Risk and Concentrations of Credit Risk	Off-Balance Sheet Risk and Concentrations of Credit Risk The Company has no significant off-balance sheet risk such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and restricted cash. The Company’s cash, cash equivalents, and restricted cash are deposited in accounts at large financial institutions. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash, cash equivalents and restricted cash are held. The Company maintains its cash equivalents in money market funds that invest in U.S. Treasury securities.
Recent Accounting Pronouncements-To Be Adopted	Recent Accounting Pronouncements—To Be Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Utilized	The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at September 30, 2019 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 101,597 $ 101,597 $ — $ — Total $ 101,597 $ 101,597 $ — $ — Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 72,797 $ 72,797 $ — $ — Total $ 72,797 $ 72,797 $ — $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): September 30, 2019 December 31, 2018 Lab equipment $ 5,634 $ 4,847 Furniture and fixtures 548 542 Computer equipment 512 517 Software 106 90 Leasehold improvements 6,210 6,174 Construction in process — 334 Total property and equipment 13,010 12,504 Less: accumulated depreciation (3,427 ) (1,958 ) Property and equipment, net $ 9,583 $ 10,546

Additional Balance Sheet Deta_2

Additional Balance Sheet Detail (Tables)	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): September 30, 2019 December 31, 2018 Prepaid expenses $ 2,976 $ 460 Prepaid sign-on bonuses subject to vesting provisions 185 99 Leasehold improvement allowance receivable — 366 Interest income receivable 159 135 Other 305 238 Total prepaid expenses and other current assets $ 3,625 $ 1,298
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, 2019 December 31, 2018 External research and development $ 1,779 $ 437 Payroll and benefits 1,511 1,448 Professional services 168 254 Capital lease obligation, current portion 49 46 Restricted stock liability, current portion 15 18 Property and equipment purchases — 174 Other 61 120 Total accrued expenses and other current liabilities $ 3,583 $ 2,497

Common Stock (Tables)

Common Stock (Tables)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock	As of September 30, 2019 and December 31, 2018 the Company has reserved for future issuance the following number of shares of common stock: September 30, 2019 December 31, 2018 Shares reserved for conversion of outstanding Series A Preferred Stock — 8,571,427 Shares reserved for conversion of outstanding Series B Preferred Stock — 5,714,277 Shares reserved for future issuance under the 2016 Stock Incentive Plan — 919,030 Shares reserved for exercises of outstanding stock options 1,981,457 507,891 Shares reserved for future issuance under the 2019 Stock Incentive Plan 2,398,391 — Shares reserved for future issuance under the 2019 Employee Stock Purchase Plan 252,142 — 4,631,990 15,712,625

Stock-based Compensation Expe_2

Stock-based Compensation Expense (Tables)	9 Months Ended
	Sep. 30, 2019
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity under the 2019 Plan and 2016 Plan since December 31, 2018: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2018 507,891 $ 7.12 9.84 $ 367,427 Granted 1,516,294 9.95 Exercised (33,561 ) 7.50 Cancelled (9,167 ) 10.09 Outstanding at September 30, 2019 1,981,457 $ 9.27 9.38 $ 202,742 Exercisable at September 30, 2019 193,292 $ 7.96 9.23 $ 72,559
Weighted Average Assumptions Used to Calculate Fair Value of Stock Option	The fair value of stock options granted during the three and nine months ended September 30, 2019 and 2018 under the 2019 Plan and the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions: Three Months Ended September 30, 2019 Three Months Ended September 30, 2018 Nine Months Ended September 30, 2019 Nine Months Ended September 30, 2018 Risk-free interest rate 1.7 % 2.8 % 2.4 % 2.8 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % Expected term (years) 6.0 6.0 6.0 5.9 Expected stock price volatility 80.1 % 84.3 % 81.0 % 83.9 %
Summary of Stock-Based Compensation Expense Recognized	The total compensation cost recognized in the statements of operations associated with all stock-based compensation awards granted by the Company is as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 533 $ 282 $ 1,398 $ 808 General and administrative 646 224 1,552 589 Total stock-based compensation expense $ 1,179 $ 506 $ 2,950 $ 1,397
2019 Plan and 2016 Plan
Summary of Restricted Stock Activity	The following table summarizes the Company’s restricted stock activity under the 2019 Plan and 2016 Plan since December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2018 757,022 $ 3.07 Granted — — Vested (280,833 ) 3.04 Repurchased (56,324 ) 3.07 Unvested at September 30, 2019 419,865 $ 3.09
Grants Outside of Stock Incentive Plans
Summary of Restricted Stock Activity	The following table summarizes the Company’s restricted stock activity outside of the 2019 Plan and 2016 Plan since December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Unvested at December 31, 2018 446,789 $ 2.94 Granted — — Vested (84,106 ) 2.94 Repurchased — — Unvested at September 30, 2019 362,683 $ 2.94

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	The future minimum lease payments associated with the lease for the Company’s current headquarters as of September 30, 2019, are as follows (in thousands): 2019 (1) $ 562 2020 2,285 2021 2,354 2022 2,424 2023 2,497 Thereafter 12,187 Total minimum lease payments $ 22,309 (1) Amounts are for the three months ending December 31, 2019.

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders	Net Loss per Share The following common stock equivalents were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Series A Preferred Stock — 8,571,427 — 8,571,427 Series B Preferred Stock — 5,714,277 — 5,714,277 Outstanding stock options 1,981,457 117,056 1,981,457 117,056 Unvested restricted stock awards 782,548 1,447,349 782,548 1,447,349 Total 2,764,005 15,850,109 2,764,005 15,850,109

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands	Jul. 22, 2019USD ($)$ / sharesshares	Jul. 05, 2019	Sep. 30, 2019USD ($)shares	Jun. 30, 2019shares	Mar. 31, 2019shares	Sep. 30, 2018shares	Jun. 30, 2018shares	Mar. 31, 2018shares	Sep. 30, 2019USD ($)shares	Dec. 31, 2018USD ($)shares
Nature Of Business And Basis Of Presentation [Line Items]
Net proceeds of common stock | $									$ 64,443
Reverse stock split		one-for-seven
Stock split		0.14285
Accumulated deficit | $			$ 134,678						$ 134,678	$ 68,124
Convertible Preferred Stock
Nature Of Business And Basis Of Presentation [Line Items]
Convertible preferred stock, share outstanding	112,500,000		112,500,000						112,500,000	100,000,000
Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock, Shares			116,167	148,320	134,013	140,691	187,421	125,123
IPO
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock, Shares	4,500,000
Shares issued, price per share | $ / shares	$ 16
Net proceeds of common stock | $	$ 64,000
IPO | Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Number of preferred stock converted into common stock	16,071,418

Summary of Fair Value Measureme

Summary of Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Utilized (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash equivalents:
Total	$ 101,597	$ 72,797
Level 1
Cash equivalents:
Total	101,597	72,797
Money Market Funds
Cash equivalents:
Total	101,597	72,797
Money Market Funds | Level 1
Cash equivalents:
Total	$ 101,597	$ 72,797

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value transfers between levels	$ 0	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 13,010	$ 12,504
Less: accumulated depreciation	(3,427)	(1,958)
Property and equipment, net	9,583	10,546
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,634	4,847
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	548	542
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	512	517
Software
Property Plant And Equipment [Line Items]
Total property and equipment	106	90
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 6,210	6,174
Construction in Process
Property Plant And Equipment [Line Items]
Total property and equipment		$ 334

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation expense	$ 500	$ 400	$ 1,534	$ 836

Additional Balance Sheet Deta_3

Additional Balance Sheet Detail - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Prepaid expenses	$ 2,976	$ 460
Prepaid sign-on bonuses subject to vesting provisions	185	99
Leasehold improvement allowance receivable		366
Interest income receivable	159	135
Other	305	238
Total prepaid expenses and other current assets	$ 3,625	$ 1,298

Additional Balance Sheet Deta_4

Additional Balance Sheet Detail - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
External research and development	$ 1,779	$ 437
Payroll and benefits	1,511	1,448
Professional services	168	254
Capital lease obligation, current portion	49	46
Restricted stock liability, current portion	15	18
Property and equipment purchases		174
Other	61	120
Total accrued expenses and other current liabilities	$ 3,583	$ 2,497

Preferred Stock - Additional In

Preferred Stock - Additional Information (Details) - USD ($)	Jul. 05, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018	Jul. 22, 2019	Jun. 30, 2019	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred stock, shares authorized						5,000,000	5,000,000	5,000,000
Preferred stock, shares issued						0	0
Preferred stock, shares outstanding						0	0
Gross proceeds threshold of automatic conversion of preferred stock eliminated	$ 45,000,000
IPO | Common Stock
Temporary Equity [Line Items]
Number of preferred stock converted into common stock								16,071,418
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, share authorized							100,000,000
Convertible preferred stock, share issued							100,000,000
Convertible preferred stock, share outstanding						112,500,000	100,000,000	112,500,000
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, share authorized						0	60,000,000
Convertible preferred stock, share issued						0	60,000,000
Convertible preferred stock, share outstanding		60,000,000	60,000,000	60,000,000	47,333,333	0	60,000,000		60,000,000	34,666,666
Convertible preferred stock, par value						$ 0.001	$ 0.001
Issuance of convertible preferred stock				12,666,667	12,666,667
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, share authorized						0	40,000,000
Convertible preferred stock, share issued						0	40,000,000
Convertible preferred stock, share outstanding		52,500,000	40,000,000			0	40,000,000		52,500,000
Convertible preferred stock, par value						$ 0.001	$ 0.001
Issuance of convertible preferred stock		12,500,000	40,000,000
Series B Preferred Stock
Temporary Equity [Line Items]
Temporary equity, dividends declared						$ 0
Aggregate cumulative dividends							$ 2,300,000
Series B Preferred Stock | GSK Agreement
Temporary Equity [Line Items]
Issuance of convertible preferred stock						12,500,000
Series A Preferred Stock
Temporary Equity [Line Items]
Temporary equity, dividends declared						$ 0
Aggregate cumulative dividends							$ 7,200,000

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Equity [Abstract]
Dividends declared or paid	$ 0	$ 0	$ 0	$ 0
Common stock, shares authorized	200,000,000		200,000,000		135,000,000

Common Stock - Schedule of Pote

Common Stock - Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Shares reserved for conversion of preferred stock and future issuance of common stock	4,631,990	15,712,625
Outstanding Stock Options
Class Of Stock [Line Items]
Shares reserved for future issuance	1,981,457	507,891
2016 Stock Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance		919,030
2019 Stock Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	2,398,391
2019 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	252,142
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Shares reserved for conversion of outstanding stock		8,571,427
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Shares reserved for conversion of outstanding stock		5,714,277

Stock-based Compensation Expe_3

Stock-based Compensation Expense - Additional Information (Details) - USD ($)	Jul. 02, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 17, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 13,100,000		$ 13,100,000
Unrecognized stock-based compensation expense, weighted average period expect to recognized				3 years 3 months 25 days
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance	252,142
Increase in number of shares reserved for issuance (shares)	428,571
Increase in number of shares reserved for issuance, percent of common stock outstanding	1.00%
2016 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares of common stock that authorized for issuance							3,209,285
Shares reserved for future issuance		0		0		0
2019 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance	2,017,142	2,398,391		2,398,391
Increase in number of shares reserved for issuance (shares)	2,000,000
Increase in number of shares reserved for issuance, percent of common stock outstanding	4.00%
2019 Plan and 2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value		$ 10.36	$ 3.49	$ 6.97	$ 3.47
Aggregate intrinsic value of stock options exercised			$ 0		$ 0
2019 Plan and 2016 Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of stock options exercised		$ 100,000		$ 200,000
Grants Outside of Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of restricted stock awards vested		$ 600,000		$ 3,000,000

Stock-based Compensation Expe_4

Stock-based Compensation Expense - Summary of Restricted Stock Activity under 2019 Plan and 2016 Plan (Details) - 2019 Plan and 2016 Plan - Restricted Stock	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance | shares	757,022
Number of Shares, Vested | shares	(280,833)
Number of Shares, Repurchased | shares	(56,324)
Number of Shares, Unvested, Ending balance | shares	419,865
Weighted Average Grant Date Fair Value, Unvested, Beginning balance | $ / shares	$ 3.07
Weighted Average Grant Date Fair Value, Vested | $ / shares	3.04
Weighted Average Grant Date Fair Value, Repurchased | $ / shares	3.07
Weighted Average Grant Date Fair Value, Unvested, Ending balance | $ / shares	$ 3.09

Stock-based Compensation Expe_5

Stock-based Compensation Expense - Summary of Stock Option Activity (Details) - 2019 Plan and 2016 Plan - Stock Option - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Outstanding, Beginning balance	507,891
Number of Shares, Granted	1,516,294
Number of Shares, Exercised	(33,561)
Number of Shares, Cancelled	(9,167)
Number of Shares, Outstanding, Ending balance	1,981,457	507,891
Number of Shares, Exercisable	193,292
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 7.12
Weighted Average Exercise Price, Granted	9.95
Weighted Average Exercise Price, Exercised	7.50
Weighted Average Exercise Price, Cancelled	10.09
Weighted Average Exercise Price, Outstanding, Ending balance	9.27	$ 7.12
Weighted Average Exercise Price, Exercisable	$ 7.96
Weighted Average Remaining Contractual Term (in years), Outstanding	9 years 4 months 17 days	9 years 10 months 2 days
Weighted Average Remaining Contractual Term (in years), Exercisable	9 years 2 months 23 days
Aggregate Intrinsic Value, Outstanding	$ 202,742	$ 367,427
Aggregate Intrinsic Value, Exercisable	$ 72,559

Stock-based Compensation Expe_6

Stock-based Compensation Expense - Summary of Weighted Average Assumptions Used to Calculate Fair Value of Stock Option (Details) - 2019 Plan and 2016 Plan	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	1.70%	2.80%	2.40%	2.80%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term (years)	6 years	6 years	6 years	5 years 10 months 24 days
Expected stock price volatility	80.10%	84.30%	81.00%	83.90%

Stock-based Compensation Expe_7

Stock-based Compensation Expense - Summary of Restricted Stock Activity outside of 2019 and 2016 Plan (Details) - Grants Outside of Stock Incentive Plans - Restricted Stock	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance | shares	446,789
Number of Shares, Vested | shares	(84,106)
Number of Shares, Unvested, Ending balance | shares	362,683
Weighted Average Grant Date Fair Value, Unvested, Beginning balance | $ / shares	$ 2.94
Weighted Average Grant Date Fair Value, Vested | $ / shares	2.94
Weighted Average Grant Date Fair Value, Unvested, Ending balance | $ / shares	$ 2.94

Stock-based Compensation Expe_8

Stock-based Compensation Expense - Summary of Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,179	$ 506	$ 2,950	$ 1,397
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	533	282	1,398	808
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 646	$ 224	$ 1,552	$ 589

Asset Acquisition - Additional

Asset Acquisition - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2019	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
In-process research and development expenses			$ 25,591,000
GSK Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones phase two clinical trial			2,500,000
Direct expenses related to the transaction			100,000
Specified clinical and regulatory milestones phase two clinical trial achieved	$ 2,500,000
GSK Agreement | Research and Development
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Clinical and regulatory milestone payables upon phase two clinical trial achieved		$ 2,500,000	2,500,000
GSK Agreement | Maximum
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones			37,500,000
Specified sales milestone			$ 60,000,000
Series B Convertible Preferred Stock
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Shares issued, price per share				$ 2
Series B Convertible Preferred Stock | GSK Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Shares issued for asset acquisition			12,500,000
Shares issued for asset acquisition, estimated fair value			$ 25,500,000
Shares issued, price per share		$ 2.04	$ 2.04

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Nov. 30, 2017USD ($)ft²	May 31, 2016ft²
Commitments And Contingencies [Line Items]
Total commitment	$ 22,309,000		$ 22,309,000
Indemnification Agreement
Commitments And Contingencies [Line Items]
Accrued liabilities for indemnification agreements	0		0		$ 0
Legal Proceedings
Commitments And Contingencies [Line Items]
Expenses incurred for legal proceedings	0	$ 0	0	$ 0
Cambridge, Massachusetts | Commitment for Office and Laboratory Space Lease of Prior Corporate Headquarters
Commitments And Contingencies [Line Items]
Square feet of office and laboratory space leased under operating lease | ft²							8,143
Rent expense				100,000
Cambridge, Massachusetts | Commitment for Office and Laboratory Space Lease of Current Corporate Headquarters
Commitments And Contingencies [Line Items]
Square feet of office and laboratory space leased under operating lease | ft²						28,731
Rent expense	$ 500,000	$ 500,000	$ 1,400,000	$ 1,400,000
Total commitment						$ 25,100,000
Operating lease term						10 years
Letters of credit						$ 1,100,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Sep. 30, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 562
2020	2,285
2021	2,354
2022	2,424
2023	2,497
Thereafter	12,187
Total minimum lease payments	$ 22,309

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details)	9 Months Ended
	Sep. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan name	401(k)

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	2,764,005	15,850,109	2,764,005	15,850,109
Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share		8,571,427		8,571,427
Series B Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share		5,714,277		5,714,277
Outstanding Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	1,981,457	117,056	1,981,457	117,056
Unvested Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	782,548	1,447,349	782,548	1,447,349

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details) - TRV - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Due to related party	$ 0		$ 0
Common stock shares issued in exchange for services			142,284
Other income related to sale of drug material				$ 400,000
Consulting Services
Related Party Transaction [Line Items]
Fees paid in exchange for consulting services	$ 0
Maximum
Related Party Transaction [Line Items]
Due to related party					$ 100,000
Maximum | Consulting Services
Related Party Transaction [Line Items]
Fees paid in exchange for consulting services		$ 100,000	$ 100,000	$ 100,000