Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Aug. 23, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
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		23,340,419
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 (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 49,628	$ 72,797
Prepaid expenses and other current assets	1,738	1,298
Total current assets	51,366	74,095
Property and equipment, net	10,005	10,546
Restricted cash	1,092	1,092
Deferred offering costs	2,064
Other assets	624	38
Total assets	65,151	85,771
Current liabilities:
Accounts payable	3,081	1,263
Accrued expenses and other current liabilities	2,793	2,497
Deferred lease incentive, current portion	469	469
Total current liabilities	6,343	4,229
Deferred rent, excluding current portion	1,496	1,402
Deferred lease incentive, excluding current portion	3,756	3,990
Other liabilities, excluding current portion	101	150
Total liabilities	11,696	9,771
Commitments and contingencies (Note 10)
Stockholders’ deficit:
Common stock, $0.001 par value; 147,500,000 and 135,000,000 shares authorized as of June 30, 2019 and December 31, 2018, respectively; 2,768,596 and 2,791,764 shares issued as of June 30, 2019 and December 31, 2018, respectively; 1,870,286 and 1,587,953 shares outstanding as of June 30, 2019 and December 31, 2018, respectively	2	2
Additional paid-in capital	6,453	4,452
Accumulated deficit	(118,136)	(68,124)
Total stockholders’ deficit	(111,681)	(63,670)
Total liabilities, convertible preferred stock, and stockholders’ deficit	65,151	85,771
Series A Convertible Preferred Stock
Current liabilities:
Convertible preferred stock	59,909	59,909
Series B Convertible Preferred Stock
Current liabilities:
Convertible preferred stock	$ 105,227	$ 79,761

Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	147,500,000	135,000,000
Common stock, shares issued	2,768,596	2,791,764
Common stock, shares outstanding	1,870,286	1,587,953
Treasury stock, shares	1,432	67,024
Series A Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, share authorized	60,000,000	60,000,000
Convertible preferred stock, share issued	60,000,000	60,000,000
Convertible preferred stock, share outstanding	60,000,000	60,000,000
Convertible preferred stock, liquidation preference	$ 69,567
Series B Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.001	$ 0.001
Convertible preferred stock, share authorized	52,500,000	40,000,000
Convertible preferred stock, share issued	52,500,000	40,000,000
Convertible preferred stock, share outstanding	52,500,000	40,000,000
Convertible preferred stock, liquidation preference	$ 111,214

Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 10,860	$ 5,788	$ 45,489	$ 11,361
General and administrative	2,634	2,061	5,232	3,798
Total operating expenses	13,494	7,849	50,721	15,159
Loss from operations	(13,494)	(7,849)	(50,721)	(15,159)
Other income, net:
Interest income (expense), net	317	(5)	694	(5)
Other income	8	8	15	378
Net loss and comprehensive loss	(13,169)	(7,846)	(50,012)	(14,786)
Cumulative convertible preferred stock dividends	(3,291)	(1,050)	(6,332)	(1,878)
Net loss attributable to common stockholders	$ (16,460)	$ (8,896)	$ (56,344)	$ (16,664)
Net loss per share attributable to common stockholders, basic and diluted	$ (9.21)	$ (7.50)	$ (32.85)	$ (15.14)
Weighted average number of common shares used in net loss per share attributable to common stockholders, basic and diluted	1,787	1,186	1,715	1,101

Consolidated Statements of Convertible Preferred Stock and Stockholders' 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	(14,786)
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
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
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	(50,012)
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
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

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating activities
Net loss			$ (50,012)	$ (14,786)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	$ 500	$ 200	1,018	377
Stock-based compensation expense	950	462	1,771	891
In-process research and development expenses			25,591
Changes in operating assets and liabilities:
Prepaid expenses and other current assets			(828)	(30)
Other assets			(586)	38
Accounts payable			451	503
Accrued expenses and other liabilities			399	16
Deferred rent and deferred lease incentive			225	952
Net cash used in operating activities			(21,971)	(12,039)
Investing activities
Purchases of property and equipment			(531)	(4,589)
Transaction costs associated with asset acquisition			(91)
Net cash used in investing activities			(622)	(4,589)
Financing activities
Proceeds from issuance of Series A convertible preferred stock, net of issuance costs				25,322
Issuance costs associated with issuance of Series B convertible preferred stock			(34)
Payment of initial public offering costs			(739)
Principal payments on capital lease obligations			(21)	(49)
Proceeds from issuance of common stock			218	17
Net cash (used in) provided by financing activities			(576)	25,290
Net (decrease) increase in cash, cash equivalents and restricted cash			(23,169)	8,662
Cash, cash equivalents, and restricted cash, beginning of period			73,889	407
Cash, cash equivalents, and restricted cash, end of period	50,720	9,069	50,720	9,069
Supplemental cash flow information
Cash paid for interest			4	5
Non-cash investing and financing activities:
Acquisition of in process research and development through issuance of stock			25,500
Property and equipment purchases unpaid at end of period			120	3,626
Deferred offering costs unpaid at end of period			1,325
Cash and cash equivalents	49,628	7,977	49,628	7,977
Restricted cash	$ 1,092	$ 1,092	$ 1,092	$ 1,092

Nature of the Business and Basis of Presentation

6 Months Ended

Jun. 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 June 30, 2019 and the results of its operations and its cash flows for the three and six months ended June 30, 2019 and 2018. The results for the three and six months ended June 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.

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 issuance of convertible notes and convertible preferred stock, and most recently, with proceeds from the IPO completed in July 2019 (Note 14). As of June 30, 2019, the Company had an accumulated deficit of $118.1 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 June 30, 2019, together with the net proceeds from the IPO of the Company’s common stock completed in July 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 Accounting Policies

6 Months Ended

Jun. 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. There have been no material changes in the Company’s significant accounting policies during the three months ended June 30, 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) (“ASU 2016-02”), as amended by various subsequently issued ASUs. The standard requires lessees to recognize an operating lease with a term greater than one year on their balance sheets as a right-of-use asset and corresponding lease liability, measured at the present value of the lease payments. Lessees are required to classify leases as either finance or operating leases. If the lease is effectively a financed-purchase by the lessee, it is classified as a financing lease, otherwise it is classified as an operating lease. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. In July 2018, the FASB also issued ASU 2018-11, Leases (Topic 842): Targeted Improvements (“ASU 2018-11”), which permits entities to continue applying legacy guidance in ASC 840, Leases, including its disclosure requirements, in the comparative periods presented in the year that the entity adopts the new leasing standard. The new standard will be effective for the Company on January 1, 2020. The Company will apply the transition method permitted by ASU 2018-11. The Company is currently evaluating the effect that adoption of the standard is expected to have on the Company’s consolidated financial statements and related disclosures. The Company expects to take advantage of certain available expedients by electing the transition package of practical expedients permitted within ASU 2016-02, which allows the Company to not reassess previous accounting conclusions around whether arrangements are, or contain, leases, the classification of leases, and the treatment of initial direct costs. The Company also expects to make an accounting policy election to exclude leases with an initial term of twelve months or less from the balance sheet.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). The standard requires that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, this standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The new standard will be effective for the Company on January 1, 2021. The Company is currently evaluating the potential impact that this standard may have on its consolidated financial position and results of operations.

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 (“ASU 2017-11”). Part I of the standard applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II of the standard replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. The new standard will be effective for the Company on January 1, 2020. The Company is currently evaluating the impact that the adoption of ASU 2017-11 may have on its consolidated financial statements.

Fair Value Measurements

6 Months Ended

Jun. 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 June 30, 2019
	Total		Level 1		Level 2		Level 3
Cash equivalents:
Money market funds	$	49,628	$	49,628	$	—	$	—
Total	$	49,628	$	49,628	$	—	$	—

	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 six months ended June 30, 2019 or 2018.

- Definition

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

+ References + Details

Property and Equipment, Net

6 Months Ended

Jun. 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):

	June 30, 2019			December 31, 2018
Lab equipment	$	5,557		$	4,847
Furniture and fixtures		548			542
Computer equipment		512			517
Software		90			90
Leasehold improvements		6,210			6,174
Construction in process		—			334
Total property and equipment		12,917			12,504
Less: accumulated depreciation		(2,912	)		(1,958	)
Property and equipment, net	$	10,005		$	10,546

Depreciation expense for the three months ended June 30, 2019 and 2018 was $0.5 million and $0.2 million, respectively. Depreciation expense for the six months ended June 30, 2019 and 2018 was $1.0 million and $0.4 million, respectively.

Additional Balance Sheet Detail

6 Months Ended

Jun. 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):

	June 30, 2019		December 31, 2018
Prepaid expenses	$	1,249	$	460
Prepaid sign-on bonuses subject to vesting provisions		182		99
Leasehold improvement allowance receivable		—		366
Interest income receivable		96		135
Other		211		238
Total prepaid expenses and other current assets	$	1,738	$	1,298

Accrued expenses and other current liabilities consisted of the following (in thousands):

	June 30, 2019		December 31, 2018
External research and development	$	1,572	$	437
Payroll and benefits		934		1,448
Professional services		166		254
Capital lease obligation, current portion		48		46
Restricted stock liability, current portion		15		18
Property and equipment purchases		—		174
Other		58		120
Total accrued expenses and other current liabilities	$	2,793	$	2,497

Common Stock

6 Months Ended

Jun. 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 the Preferred Stock. No dividends have been declared or paid during the three and six months ended June 30, 2019 or 2018.

During the six months ended June 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 147,500,000 shares. In July 2019, the Company’s Certificate of Incorporation was amended and restated to increase the number of authorized shares of common stock from 147,500,000 to 200,000,000 shares (Note 14).

The Company had reserved the following shares of common stock for the potential conversion of Preferred Stock and for the future issuance of common stock pursuant to the Company’s 2016 Stock Incentive Plan, as amended from time to time (the “2016 Plan”):

	June 30, 2019		December 31, 2018
Shares reserved for conversion of outstanding Series A Preferred Stock		8,571,427		8,571,427
Shares reserved for conversion of outstanding Series B Preferred Stock		7,499,991		5,714,277
Shares reserved for future issuance under the 2016 Stock Incentive Plan		280,071		919,030
		16,351,489		15,204,734

Stock-based Compensation Expense

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation Expense

8. Stock-based Compensation Expense

Grants Under the 2016 Stock Incentive Plan

In July 2016, the Company adopted the 2016 Plan, under which it may grant restricted stock awards, restricted stock units, incentive stock options, non-statutory stock options, and other stock-based awards to 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 June 30, 2019 and December 31, 2018 was 3,566,428 and 3,209,285, respectively.

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.

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 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		(198,056	)		3.04
Repurchased		(51,373	)		3.07
Unvested at June 30, 2019		507,593		$	3.08

The following table summarizes the Company’s stock option activity under the 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		1,114,491			8.12
Exercised		(28,205	)		7.81
Cancelled		(1,424	)		6.10
Outstanding at June 30, 2019		1,592,753		$	7.81		9.52	$	12,278,821
Exercisable at June 30, 2019		109,951		$	6.94		9.39	$	942,933

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 six months ended June 30, 2019 was $7.75 per share and $5.74 per share, respectively. The weighted average grant date fair value of stock options granted during each of the three and six months ended June 30, 2018 was $3.46 per share. The aggregate intrinsic value of stock options exercised during each of the three and six months ended June 30, 2019 was $0.2 million. No stock options were exercised during the three and six months ended June 30, 2018. The fair value of stock options granted during the three and six months ended June 30, 2019 and 2018 under the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions:

	Three Months Ended June 30, 2019			Three Months Ended June 30, 2018			Six Months Ended June 30, 2019			Six Months Ended June 30, 2018
Risk-free interest rate		2.3	%		2.8	%		2.6	%		2.8	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected term (years)		6.1			5.9			6.0			5.9
Expected stock price volatility		80.7	%		83.7	%		81.3	%		83.7	%

Grants Outside the 2016 Stock Incentive Plan

The following table summarizes the Company’s restricted stock activity outside of the 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		(56,072	)		2.94
Repurchased		—			—
Unvested at June 30, 2019		390,717		$	2.94

The aggregate intrinsic value of all restricted stock awards that vested during the three and six months ended June 30, 2019 was $1.0 million and $2.4 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	459	$	270	$	865	$	526
General and administrative		491		192		906		365
Total stock-based compensation expense	$	950	$	462	$	1,771	$	891

As of June 30, 2019, the Company had an aggregate of $10.2 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted average period of 3.11 years.

Asset Acquisition

6 Months Ended

Jun. 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 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 six months ended June 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

6 Months Ended

Jun. 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 six months ended June 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 December 31, 2018 and June 30, 2019. Rent expense associated with this lease for each of the three months ended June 30, 2019 and 2018 was approximately $0.5 million. Rent expense associated with this lease for each of the six months ended June 30, 2019 and 2018 was approximately $1.0 million.

The future minimum lease payments associated with the lease for the Company’s current headquarters as of June 30, 2019, are as follows (in thousands):

2019⁽¹⁾	$	1,125
2020		2,285
2021		2,354
2022		2,424
2023		2,497
Thereafter		12,187
Total minimum lease payments	$	22,872

(1)	Amounts are for the six 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 June 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 June 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 six months ended June 30, 2019 and 2018.

Net Loss per Share

6 Months Ended

Jun. 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Series A Preferred Stock		8,571,427		8,571,427		8,571,427		8,571,427
Series B Preferred Stock		7,499,991		—		7,499,991		—
Outstanding stock options		1,592,753		82,700		1,592,753		82,700
Unvested restricted stock awards		898,310		1,506,954		898,310		1,506,954
Total		18,562,481		10,161,081		18,562,481		10,161,081

Subsequent Events

6 Months Ended

Jun. 30, 2019

Subsequent Events [Abstract]

Subsequent Events

14. Subsequent Events

Reverse Stock Split and Other

On July 5, 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. Shares of common stock underlying outstanding stock options were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the appropriate securities agreements. Stockholders entitled to fractional shares as a result of the reverse stock split will receive a cash payment in lieu of receiving fractional shares.

On July 5, 2019, the Company eliminated the gross proceeds threshold of $45 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.

2019 Stock Incentive Plan

In anticipation of the Company’s IPO, on July 2, 2019, the Company’s stockholders approved the 2019 Stock Incentive Plan (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.

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.

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.

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 $63.9 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.

Changes to Authorized Common Stock and Preferred Stock

On July 22, 2019, the Company amended and restated its certificate of incorporation to authorize 200,000,000 shares of common stock and 10,000,000 shares of Preferred Stock, which shares of Preferred Stock are currently undesignated.

Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 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. There have been no material changes in the Company’s significant accounting policies during the three months ended June 30, 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) (“ASU 2016-02”), as amended by various subsequently issued ASUs. The standard requires lessees to recognize an operating lease with a term greater than one year on their balance sheets as a right-of-use asset and corresponding lease liability, measured at the present value of the lease payments. Lessees are required to classify leases as either finance or operating leases. If the lease is effectively a financed-purchase by the lessee, it is classified as a financing lease, otherwise it is classified as an operating lease. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. In July 2018, the FASB also issued ASU 2018-11, Leases (Topic 842): Targeted Improvements (“ASU 2018-11”), which permits entities to continue applying legacy guidance in ASC 840, Leases, including its disclosure requirements, in the comparative periods presented in the year that the entity adopts the new leasing standard. The new standard will be effective for the Company on January 1, 2020. The Company will apply the transition method permitted by ASU 2018-11. The Company is currently evaluating the effect that adoption of the standard is expected to have on the Company’s consolidated financial statements and related disclosures. The Company expects to take advantage of certain available expedients by electing the transition package of practical expedients permitted within ASU 2016-02, which allows the Company to not reassess previous accounting conclusions around whether arrangements are, or contain, leases, the classification of leases, and the treatment of initial direct costs. The Company also expects to make an accounting policy election to exclude leases with an initial term of twelve months or less from the balance sheet. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). The standard requires that credit losses be reported using an expected losses model rather than the incurred losses model that is currently used, and establishes additional disclosures related to credit risks. For available-for-sale debt securities with unrealized losses, this standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The new standard will be effective for the Company on January 1, 2021. The Company is currently evaluating the potential impact that this standard may have on its consolidated financial position and results of operations. 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 (“ASU 2017-11”). Part I of the standard applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II of the standard replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. The new standard will be effective for the Company on January 1, 2020. The Company is currently evaluating the impact that the adoption of ASU 2017-11 may have on its consolidated financial statements.

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Utilized

	Fair Value Measurements at June 30, 2019
	Total		Level 1		Level 2		Level 3
Cash equivalents:
Money market funds	$	49,628	$	49,628	$	—	$	—
Total	$	49,628	$	49,628	$	—	$	—

	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 (Tables)

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	June 30, 2019			December 31, 2018
Lab equipment	$	5,557		$	4,847
Furniture and fixtures		548			542
Computer equipment		512			517
Software		90			90
Leasehold improvements		6,210			6,174
Construction in process		—			334
Total property and equipment		12,917			12,504
Less: accumulated depreciation		(2,912	)		(1,958	)
Property and equipment, net	$	10,005		$	10,546

Additional Balance Sheet Detail (Tables)

6 Months Ended

Jun. 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):

	June 30, 2019		December 31, 2018
Prepaid expenses	$	1,249	$	460
Prepaid sign-on bonuses subject to vesting provisions		182		99
Leasehold improvement allowance receivable		—		366
Interest income receivable		96		135
Other		211		238
Total prepaid expenses and other current assets	$	1,738	$	1,298

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	June 30, 2019		December 31, 2018
External research and development	$	1,572	$	437
Payroll and benefits		934		1,448
Professional services		166		254
Capital lease obligation, current portion		48		46
Restricted stock liability, current portion		15		18
Property and equipment purchases		—		174
Other		58		120
Total accrued expenses and other current liabilities	$	2,793	$	2,497

Common Stock (Tables)

6 Months Ended

Jun. 30, 2019

Equity [Abstract]

Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock

	June 30, 2019		December 31, 2018
Shares reserved for conversion of outstanding Series A Preferred Stock		8,571,427		8,571,427
Shares reserved for conversion of outstanding Series B Preferred Stock		7,499,991		5,714,277
Shares reserved for future issuance under the 2016 Stock Incentive Plan		280,071		919,030
		16,351,489		15,204,734

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21506-112644

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21484-112644

Reference 14: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Stock-based Compensation Expense (Tables)

6 Months Ended

Jun. 30, 2019

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity under the 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		1,114,491			8.12
Exercised		(28,205	)		7.81
Cancelled		(1,424	)		6.10
Outstanding at June 30, 2019		1,592,753		$	7.81		9.52	$	12,278,821
Exercisable at June 30, 2019		109,951		$	6.94		9.39	$	942,933

Weighted Average Assumptions Used to Calculate Fair Value of Stock Option

The fair value of stock options granted during the three and six months ended June 30, 2019 and 2018 under the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions:

	Three Months Ended June 30, 2019			Three Months Ended June 30, 2018			Six Months Ended June 30, 2019			Six Months Ended June 30, 2018
Risk-free interest rate		2.3	%		2.8	%		2.6	%		2.8	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected term (years)		6.1			5.9			6.0			5.9
Expected stock price volatility		80.7	%		83.7	%		81.3	%		83.7	%

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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	459	$	270	$	865	$	526
General and administrative		491		192		906		365
Total stock-based compensation expense	$	950	$	462	$	1,771	$	891

Grants Under the 2016 Stock Incentive Plan

Summary of Restricted Stock Activity

The following table summarizes the Company’s restricted stock activity under the 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		(198,056	)		3.04
Repurchased		(51,373	)		3.07
Unvested at June 30, 2019		507,593		$	3.08

Grants Outside the 2016 Stock Incentive Plan

Summary of Restricted Stock Activity

The following table summarizes the Company’s restricted stock activity outside of the 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		(56,072	)		2.94
Repurchased		—			—
Unvested at June 30, 2019		390,717		$	2.94

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 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 June 30, 2019, are as follows (in thousands):

2019⁽¹⁾	$	1,125
2020		2,285
2021		2,354
2022		2,424
2023		2,497
Thereafter		12,187
Total minimum lease payments	$	22,872

(1)	Amounts are for the six months ending December 31, 2019.

Net Loss per Share (Tables)

6 Months Ended

Jun. 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

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Series A Preferred Stock		8,571,427		8,571,427		8,571,427		8,571,427
Series B Preferred Stock		7,499,991		—		7,499,991		—
Outstanding stock options		1,592,753		82,700		1,592,753		82,700
Unvested restricted stock awards		898,310		1,506,954		898,310		1,506,954
Total		18,562,481		10,161,081		18,562,481		10,161,081

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 12,917	$ 12,504
Less: accumulated depreciation	(2,912)	(1,958)
Property and equipment, net	10,005	10,546
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,557	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	90	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

Additional Balance Sheet Detail - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Prepaid expenses	$ 1,249	$ 460
Prepaid sign-on bonuses subject to vesting provisions	182	99
Leasehold improvement allowance receivable		366
Interest income receivable	96	135
Other	211	238
Total prepaid expenses and other current assets	$ 1,738	$ 1,298

Additional Balance Sheet Detail - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
External research and development	$ 1,572	$ 437
Payroll and benefits	934	1,448
Professional services	166	254
Capital lease obligation, current portion	48	46
Restricted stock liability, current portion	15	18
Property and equipment purchases		174
Other	58	120
Total accrued expenses and other current liabilities	$ 2,793	$ 2,497

Convertible Preferred Stock - Additional Information (Details) - USD ($)	3 Months Ended			6 Months Ended	12 Months Ended
	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, share authorized				112,500,000	100,000,000
Convertible preferred stock, share issued				112,500,000	100,000,000
Convertible preferred stock, share outstanding				112,500,000	100,000,000
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, share authorized				60,000,000	60,000,000
Convertible preferred stock, share issued				60,000,000	60,000,000
Convertible preferred stock, share outstanding	60,000,000	60,000,000	47,333,333	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				52,500,000	40,000,000
Convertible preferred stock, share issued				52,500,000	40,000,000
Convertible preferred stock, share outstanding	52,500,000			52,500,000	40,000,000
Convertible preferred stock, par value				$ 0.001	$ 0.001
Issuance of convertible preferred stock	12,500,000
Series B Preferred Stock
Temporary Equity [Line Items]
Temporary equity, dividends declared				$ 0
Aggregate cumulative dividends				$ 6,200,000	$ 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				$ 9,600,000	$ 7,200,000

Common Stock - Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock (Details) - shares	Jun. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Shares reserved for conversion of preferred stock and future issuance of common stock	16,351,489	15,204,734
2016 Stock Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	280,071	919,030
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Shares reserved for conversion of outstanding stock	8,571,427	8,571,427
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Shares reserved for conversion of outstanding stock	7,499,991	5,714,277

Stock-based Compensation Expense - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense	$ 10,200,000		$ 10,200,000
Unrecognized stock-based compensation expense, weighted average period expect to recognized			3 years 1 month 9 days
Grants Under the 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	3,566,428		3,566,428		3,209,285
Weighted average grant date fair value	$ 7.75	$ 3.46	$ 5.74	$ 3.46
Aggregate intrinsic value of stock options exercised	$ 200,000	$ 0	$ 200,000	$ 0
Grants Outside the 2016 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of restricted stock awards vested	$ 1,000,000		$ 2,400,000

Stock-based Compensation Expense - Summary of Restricted Stock Activity under 2016 Plan (Details) - Grants Under the 2016 Stock Incentive Plan - Restricted Stock	6 Months Ended
	Jun. 30, 2019 $ / shares shares
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	(198,056)
Number of Shares, Repurchased \| shares	(51,373)
Number of Shares, Unvested, Ending balance \| shares	507,593
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	$ 3.08

Stock-based Compensation Expense - Summary of Stock Option Activity (Details) - Grants Under the 2016 Stock Incentive Plan - Stock Option - USD ($)	6 Months Ended	12 Months Ended
	Jun. 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	1,114,491
Number of Shares, Exercised	(28,205)
Number of Shares, Cancelled	(1,424)
Number of Shares, Outstanding, Ending balance	1,592,753	507,891
Number of Shares, Exercisable	109,951
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 7.12
Weighted Average Exercise Price, Granted	8.12
Weighted Average Exercise Price, Exercised	7.81
Weighted Average Exercise Price, Cancelled	6.10
Weighted Average Exercise Price, Outstanding, Ending balance	7.81	$ 7.12
Weighted Average Exercise Price, Exercisable	$ 6.94
Weighted Average Remaining Contractual Term (in years), Outstanding	9 years 6 months 7 days	9 years 10 months 2 days
Weighted Average Remaining Contractual Term (in years), Exercisable	9 years 4 months 20 days
Aggregate Intrinsic Value, Outstanding	$ 12,278,821	$ 367,427
Aggregate Intrinsic Value, Exercisable	$ 942,933

Stock-based Compensation Expense - Summary of Weighted Average Assumptions Used to Calculate Fair Value of Stock Option (Details) - Grants Under the 2016 Stock Incentive Plan	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.30%	2.80%	2.60%	2.80%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term (years)	6 years 1 month 6 days	5 years 10 months 24 days	6 years	5 years 10 months 24 days
Expected stock price volatility	80.70%	83.70%	81.30%	83.70%

Stock-based Compensation Expense - Summary of Restricted Stock Activity outside of 2016 Plan (Details) - Grants Outside the 2016 Stock Incentive Plan - Restricted Stock	6 Months Ended
	Jun. 30, 2019 $ / shares shares
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	(56,072)
Number of Shares, Unvested, Ending balance \| shares	390,717
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

Asset Acquisition - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	6 Months Ended
	Aug. 31, 2019	Jun. 30, 2019
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
GSK Agreement \| Subsequent Event
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones phase two clinical trial achieved	$ 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 \| 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

Commitments and Contingencies - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2017 USD ($) ft²	May 31, 2016 ft²
Commitments And Contingencies [Line Items]
Total commitment	$ 22,872,000		$ 22,872,000
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	$ 1,000,000	$ 1,000,000
Total commitment						$ 25,100,000
Operating lease term						10 years
Letters of credit						$ 1,100,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 1,125
2020	2,285
2021	2,354
2022	2,424
2023	2,497
Thereafter	12,187
Total minimum lease payments	$ 22,872

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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	18,562,481	10,161,081	18,562,481	10,161,081
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	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	7,499,991		7,499,991
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	1,592,753	82,700	1,592,753	82,700
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	898,310	1,506,954	898,310	1,506,954

Related-Party Transactions - Additional Information (Details) - TRV - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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		$ 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

Subsequent Events - Additional Information (Detail) $ / shares in Units, $ in Millions				3 Months Ended
	Jul. 22, 2019 USD ($) $ / shares shares	Jul. 05, 2019 USD ($)	Jul. 02, 2019 shares	Jun. 30, 2019 shares	Mar. 31, 2019 shares	Jun. 30, 2018 shares	Mar. 31, 2018 shares	Dec. 31, 2018 shares
Subsequent Event [Line Items]
Common stock, shares authorized				147,500,000				135,000,000
Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock, share outstanding				112,500,000				100,000,000
Common Stock
Subsequent Event [Line Items]
Issuance of common stock, Shares				148,320	134,013	187,421	125,123
Subsequent Event
Subsequent Event [Line Items]
Reverse stock split		one-for-seven
Stock split		0.14285
Gross proceeds threshold of automatic conversion of preferred stock eliminated \| $		$ 45.0
Common stock, shares authorized	200,000,000
Preferred stock authorized	10,000,000
Subsequent Event \| 2019 Employee Stock Purchase Plan
Subsequent Event [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%
Subsequent Event \| IPO
Subsequent Event [Line Items]
Issuance of common stock, Shares	4,500,000
Shares issued, price per share \| $ / shares	$ 16.00
Net proceeds of common stock \| $	$ 63.9
Subsequent Event \| IPO \| Common Stock
Subsequent Event [Line Items]
Number of preferred stock converted into common stock	16,071,418
Subsequent Event \| 2019 Stock Incentive Plan
Subsequent Event [Line Items]
Shares reserved for future issuance			2,017,142
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%

Defined Contribution Plan - Additional Information (Details)	6 Months Ended
	Jun. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan name	401(k)

Nature of the Business and Basis of Presentation - Additional Information (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ 118,136	$ 68,124

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	Jun. 30, 2019	Dec. 31, 2018
Cash equivalents:
Total	$ 49,628	$ 72,797
Level 1
Cash equivalents:
Total	49,628	72,797
Money Market Funds
Cash equivalents:
Total	49,628	72,797
Money Market Funds \| Level 1
Cash equivalents:
Total	$ 49,628	$ 72,797

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