Fulcrum Therapeutics (FULC) 10-Q 13 May 24 2024 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2024	May 06, 2024
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
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		62,153,638
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	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 39,949	$ 25,563
Marketable securities	173,365	210,658
Unbilled accounts receivable	0	537
Prepaid expenses and other current assets	4,638	5,441
Total current assets	217,952	242,199
Property and equipment, net	4,765	5,216
Operating lease right-of-use assets	6,682	7,176
Restricted cash	1,201	1,092
Other assets	1,988	2,011
Total assets	232,588	257,694
Current liabilities:
Accounts payable	3,857	2,757
Accrued expenses and other current liabilities	4,805	8,726
Operating Lease Liability, current	2,032	2,192
Total current liabilities	10,694	13,675
Operating lease liability, excluding current portion	8,105	8,629
Other liabilities, excluding current portion	197	197
Total liabilities	18,996	22,501
Commitments and contingencies (Note 13)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued or outstanding	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized; 62,141,011 and 61,915,367 shares issued and outstanding as of March 31, 2024 and December 31, 2023, respectively	62	62
Additional paid-in capital	750,507	744,940
Accumulated other comprehensive loss	(434)	(136)
Accumulated deficit	(536,543)	(509,673)
Total stockholders’ equity	213,592	235,193
Total liabilities and stockholders’ equity	$ 232,588	$ 257,694

Consolidated Balance Sheets (_2

Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
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	200,000,000
Common stock, shares issued	62,141,011	61,915,367
Common stock, shares outstanding	62,141,011	61,915,367

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Income Statement [Abstract]
Collaboration revenue	$ 0	$ 295
Operating expenses:
Research and development	19,773	16,715
General and administrative	10,061	11,520
Total operating expenses	29,834	28,235
Loss from operations	(29,834)	(27,940)
Other income, net	2,964	3,161
Net loss	$ (26,870)	$ (24,779)
Net loss per share, basic	$ (0.43)	$ (0.41)
Net loss per share, diluted	$ (0.43)	$ (0.41)
Weighted-average common shares outstanding, basic	61,984	59,722
Weighted-average common shares outstanding, diluted	61,984	59,722
Comprehensive loss:
Net Income (Loss)	$ (26,870)	$ (24,779)
Other comprehensive gain (loss):
Unrealized (loss) gain on marketable securities	(298)	146
Total other comprehensive (loss) gain	(298)	146
Comprehensive loss	$ (27,168)	$ (24,633)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2022	$ 198,942	$ 52	$ 612,025	$ (797)	$ (412,338)
Beginning balance, Shares at Dec. 31, 2022		52,099,211
Issuance of common stock in connection with public offering, net of issuance costs	117,346	$ 10	117,336
Issuance of common stock in connection with public offering, net of issuance costs, Shares		9,615,384
Issuance of common stock under employee benefit plans	348		348
Issuance of common stock under employee benefit plans, Shares		38,903
Vesting of restricted stock awards, Shares		5,496
Stock-based compensation expense	4,253		4,253
Unrealized (loss) gain on marketable securities	146			146
Net Income (Loss)	(24,779)				(24,779)
Ending balance at Mar. 31, 2023	296,256	$ 62	733,962	(651)	(437,117)
Ending balance, Shares at Mar. 31, 2023		61,758,994
Beginning balance at Dec. 31, 2022	198,942	$ 52	612,025	(797)	(412,338)
Beginning balance, Shares at Dec. 31, 2022		52,099,211
Ending balance at Dec. 31, 2023	235,193	$ 62	744,940	(136)	(509,673)
Ending balance, Shares at Dec. 31, 2023		61,915,367
Issuance of common stock under employee benefit plans	1,651		1,651
Issuance of common stock under employee benefit plans, Shares		214,094
Vesting of restricted stock awards, Shares		11,550
Stock-based compensation expense	3,916		3,916
Unrealized (loss) gain on marketable securities	(298)			(298)
Net Income (Loss)	(26,870)				(26,870)
Ending balance at Mar. 31, 2024	$ 213,592	$ 62	$ 750,507	$ (434)	$ (536,543)
Ending balance, Shares at Mar. 31, 2024		62,141,011

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Operating activities
Net loss	$ (26,870)	$ (24,779)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	479	566
Stock-based compensation expense	3,916	4,253
Net (accretion of discounts) amortization of premiums on marketable securities	(1,161)	(1,366)
Changes in operating assets and liabilities:
Unbilled accounts receivable	537	130
Prepaid expenses and other current assets	803	(522)
Operating lease assets and liabilities	(190)	(164)
Other assets	23	74
Accounts payable	1,100	88
Accrued expenses and other liabilities	(3,921)	(2,199)
Deferred revenue	0	(197)
Net cash used in operating activities	(25,284)	(24,116)
Investing activities
Purchases of marketable securities	(55,816)	(126,504)
Maturities of marketable securities	93,972	32,734
Purchases of property and equipment	(28)	(171)
Net cash (used in) provided by investing activities	38,128	(93,941)
Financing activities
Proceeds from issuance of common stock in connection with follow-on offering, net of issuance costs	0	117,345
Proceeds from issuance of common stock under benefit plans, net	1,651	348
Net cash provided by financing activities	1,651	117,693
Net (decrease) increase in cash, cash equivalents and restricted cash	14,495	(364)
Cash, cash equivalents, and restricted cash, beginning of period	26,655	36,190
Cash, cash equivalents, and restricted cash, end of period	41,150	35,826
Supplemental cash flow information
Cash paid for operating lease liabilities	843	819
Non-cash investing and financing activities:
Property and equipment purchases unpaid at end of period	0	79
Cash and cash equivalents	39,949	34,734
Restricted cash	$ 1,201	$ 1,092

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pay vs Performance Disclosure
Net Income (Loss)	$ (26,870)	$ (24,779)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Mar. 31, 2024 shares
Trading Arrangements, by Individual
Material Terms of Trading Arrangement	On February 29, 2024 , Curtis Oltmans , our chief legal officer , adopted a trading plan for the potential sale of up to 5,192 shares of our common stock and for the potential exercise of vested stock options and the associated sale of up to 19,500 shares of our common stock. The trading plan is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act and is expected to remain in effect until May 30, 2025.
Curtis Oltmans
Trading Arrangements, by Individual
Name	Curtis Oltmans
Title	chief legal officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	February 29, 2024
Aggregate Available	19,500

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2024
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 developing small molecules to improve the lives of patients with genetically-defined rare diseases in areas of high unmet medical need. 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 generally accepted accounting principles 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 consolidated 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 March 31, 2024 and the results of its operations and its cash flows for the three months ended March 31, 2024 and 2023. The results for the three months ended March 31, 2024 are not necessarily indicative of results to be expected for the year ending December 31, 2024, 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, 2023 included in the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on February 27, 2024 (the “Annual Report on Form 10-K”). Sales of Common Stock In January 2023, the Company completed a public offering of its common stock and issued and sold 9,615,384 shares of common stock at a public offering price of $ 13.00 per share, resulting in net proceeds of $ 117.3 million after deducting underwriting discounts and commissions and offering expenses. 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 sale of shares of its capital stock and from upfront payments received from collaboration and license agreements. As of March 31, 2024, the Company had an accumulated deficit of $ 536.5 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 expects to finance its future cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. The Company expects that its cash, cash equivalents, and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the date of issuance of these financial statements. However, the Company has based this estimate on assumptions that may prove to be wrong, and its operating plan may change as a result of many factors currently unknown to it. As a result, the Company could deplete its capital resources sooner than it currently expects. If the Company is unable to raise additional funds through equity or debt financings when needed, it may be required to delay, limit, reduce or terminate development or future commercialization efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
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, 2023 included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2024. 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 revenue recognition, accrued expenses, stock-based compensation expense, 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. 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, marketable securities, 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, U.S. Treasury securities, and commercial paper . The Company’s marketable securities consist of U.S. Treasury securities, corporate bonds, and commercial paper, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment. The Company has not experienced any credit losses and does not believe it is exposed to any significant credit risk on these funds. Recent Accounting Pronouncements During the periods presented, the Company was not required to adopt any recently issued accounting standards. The Company does not expect any recently issued accounting standards to have a material impact on its financial statements.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2024
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 fair value hierarchy classification of such fair values as of March 31, 2024 and December 31, 2023 (in thousands): Fair Value Measurements at Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 30,013 $ 30,013 $ — $ — U.S. Treasury securities 9,936 — 9,936 — Marketable securities: U.S. Treasury securities 20,249 — 20,249 — Government agency securities 31,824 — 31,824 — Commercial paper 13,431 — 13,431 — Corporate bonds 107,861 — 107,861 — Total $ 213,314 $ 30,013 $ 183,301 $ — Fair Value Measurements at Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 25,563 $ 25,563 $ — $ — Marketable securities: U.S. Treasury securities 14,215 — 14,215 — Government agency securities 65,107 — 65,107 — Commercial paper 17,889 — 17,889 — Corporate bonds 113,447 — 113,447 — Total $ 236,221 $ 25,563 $ 210,658 $ — There were no transfers between fair value levels during the three months ended March 31, 2024 .

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	3 Months Ended
	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Cash Equivalents And Marketable Securities	4. Cash Equivalents and Marketable Securities Cash equivalents and marketable securities consisted of the following as of March 31, 2024 and December 31, 2023 (in thousands): Fair Value Measurements at Amortized Gross Gross Fair Value Cash equivalents: Money market funds $ 30,013 $ — $ — $ 30,013 U.S. Treasury securities 9,936 — — 9,936 Total cash equivalents 39,949 — — 39,949 Marketable securities: U.S. Treasury securities 20,266 1 ( 18 ) 20,249 Government agency securities 31,878 — ( 54 ) 31,824 Commercial paper 13,589 — ( 158 ) 13,431 Corporate bonds 108,066 7 ( 212 ) 107,861 Total marketable securities 173,799 8 ( 442 ) 173,365 Total cash equivalents and marketable securities $ 213,748 $ 8 $ ( 442 ) $ 213,314 Fair Value Measurements at Amortized Gross Gross Fair Value Cash equivalents: Money market funds $ 25,563 $ — $ — $ 25,563 Total cash equivalents 25,563 — — 25,563 Marketable securities: U.S. Treasury securities 14,229 10 ( 24 ) 14,215 Government agency securities 65,182 23 ( 98 ) 65,107 Commercial paper 17,891 8 ( 10 ) 17,889 Corporate bonds 113,492 90 ( 135 ) 113,447 Total marketable securities 210,794 131 ( 267 ) 210,658 Total cash equivalents and marketable securities $ 236,357 $ 131 $ ( 267 ) $ 236,221 There were no sales of marketable securities during the three months ended March 31, 2024. As of March 31, 2024 , the Company held 46 debt securities that were in an unrealized loss position for less than 12 months with an aggregate fair value of $ 125.7 million. As of March 31, 2024 , the Company held 13 debt securities that were in an unrealized loss position for greater than 12 months with an aggregate fair value of $ 31.3 million. As of March 31, 2024 , the aggregate fair value of marketable securities with a remaining contractual maturity of greater than one year was $ 31.4 million. The Company has the intent and ability to hold its debt securities until recovery. As a result, the Company did not record any charges for credit-related impairments for its marketable securities for the three months ended March 31, 2024 .

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2024
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): March 31, December 31, Lab equipment $ 9,645 $ 9,682 Furniture and fixtures 600 600 Computer equipment 393 393 Software 199 199 Leasehold improvements 7,102 7,102 Total property and equipment 17,939 17,976 Less: accumulated depreciation ( 13,174 ) ( 12,760 ) Property and equipment, net $ 4,765 $ 5,216 Depreciation expense for the three months ended March 31, 2024 and 2023 was $ 0.5 million and $ 0.6 million, respectively.

Additional Balance Sheet Detail

Additional Balance Sheet Detail	3 Months Ended
	Mar. 31, 2024
Balance Sheet Related Disclosures [Abstract]
Additional Balance Sheet Detail	6. Additional Balance Sheet Detail Prepaid expenses and other current assets consisted of the following (in thousands): March 31, December 31, Prepaid expenses $ 3,524 $ 3,658 Prepaid sign-on bonuses subject to vesting provisions 30 71 Interest income receivable 1,084 1,712 Total prepaid expenses and other current assets $ 4,638 $ 5,441 Accrued expenses and other current liabilities consisted of the following (in thousands): March 31, December 31, External research and development $ 2,325 $ 3,164 Payroll and benefits 1,615 4,712 Professional services 480 454 Other 385 396 Total accrued expenses and other current liabilities $ 4,805 $ 8,726

Preferred Stock

Preferred Stock	3 Months Ended
	Mar. 31, 2024
Temporary Equity Disclosure [Abstract]
Preferred Stock	7. Preferred Stock As of March 31, 2024 and December 31, 2023 , 5,000,000 shares of undesignated preferred stock were authorized. No shares of preferred stock were issued or outstanding as of March 31, 2024 and December 31, 2023. No dividends have been declared since inception.

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Common Stock	8. Common Stock As of March 31, 2024 and December 31, 2023 , 200,000,000 shares of common stock, $ 0.001 par value per share, were authorized. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders . Common stockholders are not entitled to receive dividends, unless declared by the Company’s board of directors, subject to the preferential dividend rights of any preferred stock then outstanding. No dividends have been declared or paid by the Company since its inception. As of March 31, 2024 and December 31, 2023, the Company has reserved for future issuance the following number of shares of common stock: March 31, December 31, Shares reserved for exercises of outstanding stock options 12,289,352 9,972,217 Shares reserved for vesting of restricted stock units 128,709 75,017 Shares reserved for future issuance under the 2019 Stock Incentive Plan 3,008,629 3,157,537 Shares reserved for future issuance under the 2019 Employee Stock Purchase Plan 1,774,696 1,346,125 Shares reserved for future issuance under the 2022 Inducement Stock Incentive Plan 390,314 837,877 17,591,700 15,388,773

Stock-based Compensation Expens

Stock-based Compensation Expense	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Stock-based Compensation Expense	9. 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. As of the effective date of the 2019 Stock Incentive Plan (the “2019 Plan”), and as of March 31, 2024 , no shares remained available for future issuance under the 2016 Plan. Any options or other 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, non-statutory 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 was 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 was annually increased on January 1, 2020 and will be increased 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 each such year or (iii) an amount determined by the Company’s board of directors. On January 1, 2024, the number of shares reserved for issuance under the 2019 Plan was increased by 2,000,000 shares. As of March 31, 2024, there were 3,008,629 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. 2022 Inducement Stock Incentive Plan In February 2022, the Company’s board of directors adopted the 2022 Inducement Stock Incentive Plan (the “Inducement Plan”), pursuant to which the Company may grant, subject to the terms of the Inducement Plan and Nasdaq rules, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units, and other stock-based awards. The Company initially reserved a total of 1,750,000 shares of common stock for the issuance of awards under the Inducement Plan. The number of shares reserved and available for issuance under the Inducement Plan can be increased at any time with the approval of the Company’s board of directors. The Inducement Plan permits the board of directors, a delegated committee of the board of directors, or a delegated officer of the Company to grant the stock-based awards available under the Inducement Plan to attract key employees for the growth of the Company. Effective March 8, 2023, the Company’s board of directors amended the Inducement Plan to increase the number of shares reserved for issuance by 2,000,000 shares. Effective May 18, 2023, the Company’s board of directors amended the Inducement Plan to increase the number of shares reserved for issuance by 1,400,000 shares. As of March 31, 2024, there were 390,314 shares available for future issuance under the Inducement Plan. Stock Options Stock options granted by the Company typically vest over a four-year period and have a ten year contractual term. Shares issued upon the exercise of stock options are issued from the Company’s pool of authorized but unissued common stock. In addition to stock options granted under the 2019 Plan and 2016 Plan, the Company has granted stock options as material inducements to employment in accordance with Nasdaq Listing Rule 5635(c)(4), which were granted outside of the 2019 Plan and 2016 Plan. The following table summarizes the Company’s stock option activity during the three months ended March 31, 2024: Number of Weighted Weighted Aggregate Outstanding at December 31, 2023 9,972,217 $ 8.44 8.49 $ 15,114,074 Granted 2,629,058 7.59 Exercised ( 214,094 ) 7.71 Cancelled ( 97,829 ) 9.22 Outstanding at March 31, 2024 12,289,352 $ 8.26 8.64 $ 35,107,034 Exercisable at March 31, 2024 3,365,888 $ 12.71 7.19 $ 2,509,520 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 in the three months ended March 31, 2024 was $ 6.23 per share. The total intrinsic value of stock options exercised during the three months ended March 31, 2024 was $ 0.8 million. The fair value of stock options granted during the three months ended March 31, 2024 and 2023 has been calculated on the date of grant using the following weighted average assumptions: Three Months Three Months Risk-free interest rate 4.1 % 3.8 % Expected dividend yield 0.0 % 0.0 % Expected term (years) 6.0 6.0 Expected stock price volatility 103.1 % 94.3 % Restricted Stock Units The Company has also granted restricted stock units. The shares of common stock underlying restricted stock units typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest. The following table summarizes the Company’s restricted stock unit activity during the three months ended March 31, 2024: Number of Weighted Unvested at December 31, 2023 75,017 $ 10.77 Granted 70,445 6.95 Vested ( 11,550 ) 12.53 Cancelled ( 5,203 ) 9.38 Unvested at March 31, 2024 128,709 $ 8.58 Stock-based Compensation Expense The total compensation cost recognized in the statements of operations and comprehensive loss associated with all stock-based compensation awards granted by the Company is as follows (in thousands) : Three Months Ended 2024 2023 General and administrative $ 2,587 $ 3,408 Research and development 1,329 845 Total stock-based compensation expense $ 3,916 $ 4,253 As of March 31, 2024 , the Company had an aggregate of $ 42.8 million of unrecognized stock-based compensation expense, which is expected to be recognized over a weighted average period of 2.99 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 initially reserved for issuance under the ESPP. In addition, the number of shares of common stock reserved under the ESPP was increased on January 1, 2020, and will be increased annually on 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. On January 1, 2024, the number of shares reserved for issuance under the 2019 ESPP was increased by 428,571 shares. As of March 31, 2024, there were 1,774,696 shares available for future issuance under the ESPP.

Collaboration and License Agree

Collaboration and License Agreements	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreement	10. Collaboration Agreement On July 20, 2020, the Company entered into the MyoKardia Collaboration Agreement, as amended, pursuant to which the Company granted to MyoKardia an exclusive worldwide license under certain intellectual property rights to research, develop, make, have made, use, have used, sell, have sold, offer for sale, have offered for sale, import, have imported, export, have exported, distribute, have distributed, market, have marketed, promote, have promoted, or otherwise exploit products directed against certain biological targets identified by the Company that are capable of modulating up to a certain number of genes of interest with relevance to certain genetically defined cardiomyopathies. Pursuant to a mutually agreed research plan, the Company will perform assay screening and related research activities to identify and validate up to a specified number of potential cardiomyopathy gene targets (“Identified Targets”) for further research, development, manufacture and commercialization by MyoKardia. The Company and MyoKardia will work together to determine how best to advance at each stage of the research activities under the research plan and to identify which of the Identified Targets, if any, meet the criteria set forth in the research plan (the “Cardiomyopathy Target Candidates”). Upon completion of the research plan, the parties will work together to prepare a final data package and MyoKardia may designate certain Cardiomyopathy Target Candidates for MyoKardia’s further exploitation under the MyoKardia Collaboration Agreement (the “Cardiomyopathy Targets”). If MyoKardia does not designate any Cardiomyopathy Targets during the designated period, then the MyoKardia Collaboration Agreement will automatically terminate. If MyoKardia designates one or more Cardiomyopathy Targets, then MyoKardia will be obligated to use commercially reasonable efforts to seek regulatory approval for and to commercialize one product directed against an Identified Target in certain specified countries. During the period in which the Company is performing the research activities pursuant to the research plan (the “Research Term”) and for a specified period beyond the Research Term if MyoKardia designates a Cardiomyopathy Target, the Company may only use the data generated from such research activities for MyoKardia in accordance with the MyoKardia Collaboration Agreement. During the Research Term and for a specified period thereafter, the Company may not research, develop, manufacture, commercialize, use, or otherwise exploit any compound or product (a) that is a Compound or Product under the MyoKardia Collaboration Agreement that is directed against the Cardiomyopathy Target Candidates for the treatment, prophylaxis, or diagnosis of any indication or (b) for the treatment of any genetically defined cardiomyopathies shown to be related to certain specified genes of interest that are modulated by the Cardiomyopathy Targets. Under the MyoKardia Collaboration Agreement, MyoKardia made a $ 10.0 million upfront payment and a $ 2.5 million payment as prepaid research funding to the Company in July 2020. MyoKardia will also reimburse the Company for the costs of the research activities not covered by the prepaid research funding, up to a maximum amount of total research funding (including the prepaid research funding). Upon the achievement of specified preclinical, development and sales milestones, the Company will be entitled to preclinical milestone payments, development milestone payments and sales milestone payments of up to $ 298.5 million in the aggregate per target for certain Identified Targets, and of up to $ 150.0 million in the aggregate per target for certain other Identified Targets. To date, the Company has achieved a $ 2.5 million specified preclinical milestone. MyoKardia will also pay the Company tiered royalties ranging from a mid single-digit percentage to a low double-digit percentage based on MyoKardia’s, and any of its affiliates’ and sublicensees’, annual worldwide net sales of products under the MyoKardia Collaboration Agreement directed against any Identified Target. The royalties are payable on a product-by-product basis during a specified royalty term, and may be reduced in specified circumstances. The MyoKardia Collaboration Agreement continues on a country-by-country and product-by-product basis until the last to expire royalty term for a product, at which time the MyoKardia Collaboration Agreement expires with respect to such product in such country. Either party has the right to terminate the MyoKardia Collaboration Agreement if the other party has materially breached in the performance of its obligations under the MyoKardia Collaboration Agreement and such breach has not been cured within the applicable cure period. MyoKardia also has the right to terminate the MyoKardia Collaboration Agreement for convenience in its entirety or on a target-by-target, product-by-product or molecule-by-molecule basis. Accounting Analysis Identification of the Contract The Company assessed the MyoKardia Collaboration Agreement and concluded that it represents a contract with a customer within the scope of ASC 606. Identification of the Promises and Performance Obligations The Company determined that the MyoKardia Collaboration Agreement contains the following promises: (i) an exclusive worldwide license under certain intellectual property rights, including rights to a specified number of potential cardiomyopathy gene targets identified by the Company for further research, development, manufacture and commercialization for the treatment, prophylaxis, or diagnosis of certain genetically defined cardiomyopathies that was conveyed at the inception of the arrangement (the “MyoKardia License”), (ii) research services to identify and validate potential biological targets (the “MyoKardia Research Services”), and (iii) participation in the joint steering committee (the “MyoKardia JSC”). The Company assessed the above promises and concluded that the MyoKardia License is not capable of being distinct from the MyoKardia Research Services given that the MyoKardia License has limited value without the performance of the MyoKardia Research Services and the MyoKardia Research Services can only be performed by the Company due to their specialized nature. Therefore, the Company has concluded that the MyoKardia License and the MyoKardia Research Services represent a single combined performance obligation. The Company also assessed the participation on the MyoKardia JSC and concluded that the promise is quantitatively and qualitatively immaterial in the context of the MyoKardia Collaboration Agreement. Accordingly, the Company has disregarded its participation on the MyoKardia JSC as a performance obligation. Determination of the Transaction Price The Company received a non-refundable upfront payment of $ 10.0 million, which the Company included in the transaction price. In December 2021, the Company achieved a $ 2.5 million specified preclinical milestone associated with the MyoKardia Collaboration Agreement, which was previously constrained due to the significant uncertainty regarding whether such preclinical milestone would be achieved. The Company included this amount in the transaction price as of December 31, 2021. Based on the continued uncertainty associated with the achievement of any of the remaining preclinical and development milestone payments that the Company is eligible to receive, the Company has constrained the variable consideration associated with those milestone payments and excluded them from the transaction price. As part of its evaluation of constraining the preclinical and development milestones, the Company considered numerous factors, including the fact that the achievement of the preclinical and development milestones are contingent upon the results of the underlying preclinical and development activities and are thus outside of the control of the Company. The Company also included in the transaction price the expected amount of costs to be reimbursed for the MyoKardia Research Services, which includes the $ 2.5 million prepaid research funding payment that the Company received in the third quarter of 2020. The Company reassesses the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjusts its estimate of the transaction price. There was no change in the amount of variable consideration constrained during the three months ended March 31, 2024. Any consideration related to sales milestone payments (including royalties) will be recognized when the related sales occur as these amounts have been determined to relate predominantly to the license granted to MyoKardia and therefore are recognized at the later of when the related sales occur or the performance obligation is satisfied. Allocation of the Transaction Price to Performance Obligations As noted above, the Company has identified a single performance obligation associated with the MyoKardia Collaboration Agreement. Therefore, the Company will allocate the entire amount of the transaction price to the identified single performance obligation. Recognition of Revenue The Company recognizes revenue related to the MyoKardia Collaboration Agreement over time as the MyoKardia Research Services are rendered. The Company has concluded that an input method is a representative depiction of the transfer of services under the MyoKardia Collaboration Agreement. The method of measuring progress towards the delivery of the services incorporates actual cumulative internal and external costs incurred relative to total internal and external costs expected to be incurred to satisfy the performance obligation. The period over which total costs are estimated reflects the Company’s estimate of the period over which it will perform the MyoKardia Research Services. Changes in estimates of total internal and external costs expected to be incurred are recognized in the period of change as a cumulative catch-up adjustment. The Company satisfied its obligation to perform research services as of December 31, 2023. During the three months ended March 31, 2024, the Company recognized no collaboration revenue associated with the MyoKardia Collaboration Agreement. During the three months ended March 31, 2023 , the Company recognized $ 0.3 million of collaboration revenue associated with the MyoKardia Collaboration Agreement, which includes $ 0.2 million of revenue recognized that was in deferred revenue as of December 31, 2022. As of March 31, 2024 and December 31, 2023 , the Company recorded no deferred revenue or accounts receivable associated with the MyoKardia Collaboration Agreement. As of March 31, 2024 , the Company had received $ 7.7 million of cost reimbursement payments under the MyoKardia Collaboration Agreement and $ 2.5 million associated with the achievement of a preclinical milestone. As of December 31, 2023, the Company had received $ 7.2 million of cost reimbursement payments under the MyoKardia Collaboration Agreement and $ 2.5 million associated with the achievement of a preclinical milestone. As of March 31, 2024, the Company recorded no unbilled accounts receivable related to reimbursable research and development costs under the MyoKardia Collaboration Agreement for activities performed during the three months ended March 31, 2024. As of December 31, 2023 , the Company recorded unbilled accounts receivable of $ 0.5 million related to reimbursable research and development costs under the MyoKardia Collaboration Agreement for activities performed during the three months ended December 31, 2023.

License Agreements

License Agreements	3 Months Ended
	Mar. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]
License Agreements	11. License Agreements GSK Agreement In February 2019, the Company entered into the right of reference and license agreement, as amended (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 shares of Series B Preferred Stock to GSK (all of which converted to common stock in connection with the Company’s 2019 initial public offering). In addition, the Company may owe GSK up to $ 37.5 million in certain specified clinical and regulatory milestones, including a $ 5.0 million milestone that was achieved during 2022 and a $ 2.5 million milestone that was achieved 2019, and up to $ 60.0 million in certain specified sales milestones. The Company 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 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 recognizes clinical and regulatory milestone payments when the underlying contingency is resolved and the consideration is paid or becomes payable. The milestone payments are capitalized or expensed depending on the nature of the associated asset as of the date of recognition. CAMP4 Agreement In July 2023, the Company entered into a license agreement (the “CAMP4 Agreement”) with CAMP4 Therapeutics Corporation (“CAMP4”) pursuant to which the Company received a worldwide exclusive license (including the right to sublicense) from CAMP4 to rights under its Diamond Blackfan Anemia (“DBA”) program, which includes certain small molecule compounds, composition of matter and method of use patent rights, and know-how for the Company to research, develop, manufacture, use, commercialize or otherwise exploit therapeutic products in any indication, including the grant of a sublicense under certain intellectual property rights that CAMP4 has licensed under an agreement with Children’s Medical Center Corporation (“CMCC”). The Company made an undisclosed upfront non-refundable, non-creditable payment to CAMP4. If the Company succeeds in developing and commercializing licensed products, CAMP4 will be eligible to receive (i) up to $ 35.0 million in development and regulatory milestone payments, and (ii) up to $ 35.0 million in sales milestone payments. CAMP4 is also eligible to receive royalties on worldwide net sales of licensed products ranging from mid-single digit to low-double digit, subject to potential reduction following loss of patent coverage, the launch of certain generic products or royalty stacking for licenses of third party intellectual property. The royalties will expire on a product-by-product and country-by-country basis upon the latest to occur of (i) the expiration of all valid patent claims covering the compounds in such country, (ii) the expiration of all regulatory exclusivities in such country, and (iii) 10 years following the first commercial s ale in such country. The Company is responsible for the costs associated with the development and regulatory approvals of licensed products. Unless earlier terminated in accordance with its terms, the license agreement continues on a country-by-country and licensed product-by-licensed product basis until the expiration of the royalty term in each country, at which time the license agreement expires with respect to such licensed product in such country and the Company will have a fully-paid up, royalty-free and perpetual license to the licensed patent rights and know-how with respect to such licensed product in such country. CAMP4 has the right to terminate the license agreement in the event of the Company’s non-payment (subject to cure periods and tolling for bona fide disputes). CAMP4 may also terminate the license agreement if the Company challenges certain patents sublicensed to the Company by CAMP4. Either party may terminate the license agreement in its entirety for the other party’s material breach if such other party fails to cure the breach. Either party may also terminate the agreement in its entirety upon certain insolvency events involving the other party. The Company has the right to terminate the license agreement with CAMP4 for any or no reason upon prior written notice to CAMP4. The Company recognizes development and regulatory milestone payments when the underlying contingency is resolved and the consideration is paid or becomes payable. The milestone payments are capitalized or expensed depending on the nature of the associated asset as of the date of recognition.

Leases

Leases	3 Months Ended
	Mar. 31, 2024
Leases [Abstract]
Leases, Operating Leases	12. Leases Operating Leases 26 Landsdowne Street In November 2017, the Company entered into a lease agreement for its current corporate headquarters comprising approximately 28,731 square feet of office and laboratory space at 26 Landsdowne Street in Cambridge, Massachusetts, commencing December 2017. The Company began to occupy and use the leased space for its intended purpose in June 2018. The lease ends on June 30, 2028. The Company has the option to extend the term of the lease for an additional five-year period, at the market rate, by giving the landlord written notice of its election to exercise the extension at least nine months prior to the original expiration of the lease term. The lease has a total commitment of $ 25.1 million over the ten year term, and includes escalating rent payments. The lease provides the Company with an allowance for normal leasehold improvements of $ 5.0 million. 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 March 31, 2024 and December 31, 2023. Operating lease expense and variable lease expense associated with this lease for the three months ended March 31, 2024 was approximately $ 0.5 million and $ 0.2 million, respectively. The future minimum lease payments associated with the 26 Landsdowne Street lease as of March 31, 2024, are as follows (in thousands): 2024(1) 1,939 2025 2,649 2026 2,729 2027 2,811 Thereafter 1,426 Total minimum lease payments 11,554 Less: imputed interest ( 1,417 ) Total lease liability $ 10,137 1. Amounts are for the nine months ending December 31, 2024. 125 Sidney Street In November 2021, the Company entered into a lease agreement comprising approximately 12,196 square feet of office space at 125 Sidney Street in Cambridge, Massachusetts, commencing November 2021. The Company began recognizing rent expense associated with this lease during November 2021. The lease ended on March 31, 2024. The Company had the option to extend the term of the lease for two successive one-year periods, at the market rate, by giving the landlord written notice of its election to exercise the extension at least nine months prior to the original expiration of the lease term. The lease had a total commitment of $ 1.7 million over the initial term, and included escalating rent payments. O perating lease expense associated with this lease for the three months ended March 31, 2024 was approximately $ 0.2 million. No variable lease expense was recorded associated with this lease for the three months ended March 31, 2024. There are no future minimum lease payments associated with this lease as of March 31, 2024 .

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies 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 no t accrued any liabilities related to such obligations as of March 31, 2024 or December 31, 2023. Legal Proceedings On April 28, 2023, a class action complaint was filed in the United States District Court for the District of New Jersey against the Company and current and former officers (the “Securities Action”). On May 19, 2023, the Securities Action was transferred to the United States District Court for the District of Massachusetts, captioned Celano v. Fulcrum Therapeutics, Inc., et al ., Case No. 1:23-cv-11125-IT. On July 31, 2023, the court appointed a lead plaintiff, who filed an amended complaint on September 29, 2023. The Securities Action alleges violations of Section 10(b) of the Securities Exchange Act of 1934, as amended, and Rule 10b-5 promulgated thereunder against all defendants and control person violations of Section 20(a) against the individuals, related to the Company’s February 2023 announcement that the U.S. Food and Drug Administration issued a clinical hold regarding the investigational new drug application for pociredir for the potential treatment of sickle cell disease. The Securities Action alleges that the defendants made misleading statements and omitted to disclose material information related to the clinical hold and seeks, among other things, compensatory damages in connection with an allegedly inflated stock price between March 3, 2022, and March 8, 2023, as well as attorneys’ fees and costs. On November 28, 2023, all defendants filed a motion to dismiss the Securities Action, which motion is currently pending. The Company intends to defend vigorously against this litigation. 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. Other than attorneys’ fees and costs related to the defense of the Securities Action, no such costs have been incurred during the three months ended March 31, 2024 .

Defined Contribution Plan

Defined Contribution Plan	3 Months Ended
	Mar. 31, 2024
Retirement Benefits [Abstract]
Defined Contribution Plan	14. 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 contributions to the 401(k) Plan. T he Company made $ 0.2 million in contributions to the 401(k) Plan for each of the three months ended March 31, 2024 and 2023 .

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss per Share	15. Net Loss per Share The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended 2024 2023 Outstanding stock options 12,289,352 6,876,679 Unvested restricted stock units 128,709 104,543 Total 12,418,061 6,981,222

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2024
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events Sanofi Collaboration and License Agreement On May 11, 2024, the Company entered into a collaboration and license agreement (the “Sanofi Agreement”) with Genzyme Corporation (“Sanofi”) pursuant to which the Company granted Sanofi an exclusive license under certain intellectual property rights to commercialize losmapimod, an oral small molecule for the treatment of facioscapulohumeral muscular dystrophy (“FSHD”) outside of the United States. Pursuant to a mutually agreed global development plan, the Company will continue to conduct the ongoing Phase 3 clinical trial for losmapimod for the treatment of FSHD. The Company and Sanofi will equally share global development costs. In addition to potential future activities conducted under a mutually agreed global development plan, Sanofi will also have the right to conduct certain development activities that are solely intended to support obtaining or maintaining regulatory approval outside of the United States. The Company will have the sole right to manufacture for its activities under the global development plan and for commercialization in the United States and, subject to the terms of a supply agreement, the Company will supply Sanofi’s clinical and commercial supply requirements of losmapimod until Sanofi elects to take over such manufacturing responsibilities. Per the terms of the Sanofi Agreement, Sanofi will make an upfront payment of $ 80.0 million to the Company. The Company is also eligible to receive up to an additional $ 975.0 million in specified regulatory and sales-based milestones, and Sanofi will pay the Company tiered royalties ranging from low-teens to mid-twenties based on Sanofi’s and any of its affiliates’ and sublicensees’ annual net sales of losmapimod outside the United States. The royalties are payable on a product-by-product basis during a specified royalty term, and may be reduced in specified circumstances. During the term of the Sanofi Agreement, the Company may not research, develop, manufacture, commercialize, use, or otherwise exploit any compound or product that binds or otherwise modulates p38a/b MAPK anywhere in the world, other than losmapimod in the United States. The Sanofi Agreement continues on a country-by-country and product-by-product basis until the last to expire royalty term for a product in a country, at which time the Sanofi Agreement expires with respect to such product in such country. Either party has the right to terminate the Sanofi Agreement if the other party has materially breached its obligations under the Sanofi Agreement and such breach has not been cured within the applicable cure period. Sanofi also has the right to terminate the Sanofi Agreement for convenience in its entirety or on a product-by-product or region-by-region (or country-by-country with respect to certain major markets) basis. The Company also has the right to terminate the Sanofi Agreement if Sanofi terminates all bona fide material development and commercialization activities for a specified period and such cessation is not the result of certain agreed upon reasons. The Company is currently evaluating the accounting treatment for the Sanofi Agreement.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
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, 2023 included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2024.
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 revenue recognition, accrued expenses, stock-based compensation expense, 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.
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, marketable securities, 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, U.S. Treasury securities, and commercial paper . The Company’s marketable securities consist of U.S. Treasury securities, corporate bonds, and commercial paper, and potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment. The Company has not experienced any credit losses and does not believe it is exposed to any significant credit risk on these funds.
Recent Accounting Pronouncements	Recent Accounting Pronouncements During the periods presented, the Company was not required to adopt any recently issued accounting standards. The Company does not expect any recently issued accounting standards to have a material impact on its financial statements.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Classification	The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the fair value hierarchy classification of such fair values as of March 31, 2024 and December 31, 2023 (in thousands): Fair Value Measurements at Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 30,013 $ 30,013 $ — $ — U.S. Treasury securities 9,936 — 9,936 — Marketable securities: U.S. Treasury securities 20,249 — 20,249 — Government agency securities 31,824 — 31,824 — Commercial paper 13,431 — 13,431 — Corporate bonds 107,861 — 107,861 — Total $ 213,314 $ 30,013 $ 183,301 $ — Fair Value Measurements at Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 25,563 $ 25,563 $ — $ — Marketable securities: U.S. Treasury securities 14,215 — 14,215 — Government agency securities 65,107 — 65,107 — Commercial paper 17,889 — 17,889 — Corporate bonds 113,447 — 113,447 — Total $ 236,221 $ 25,563 $ 210,658 $ —

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities	Cash equivalents and marketable securities consisted of the following as of March 31, 2024 and December 31, 2023 (in thousands): Fair Value Measurements at Amortized Gross Gross Fair Value Cash equivalents: Money market funds $ 30,013 $ — $ — $ 30,013 U.S. Treasury securities 9,936 — — 9,936 Total cash equivalents 39,949 — — 39,949 Marketable securities: U.S. Treasury securities 20,266 1 ( 18 ) 20,249 Government agency securities 31,878 — ( 54 ) 31,824 Commercial paper 13,589 — ( 158 ) 13,431 Corporate bonds 108,066 7 ( 212 ) 107,861 Total marketable securities 173,799 8 ( 442 ) 173,365 Total cash equivalents and marketable securities $ 213,748 $ 8 $ ( 442 ) $ 213,314 Fair Value Measurements at Amortized Gross Gross Fair Value Cash equivalents: Money market funds $ 25,563 $ — $ — $ 25,563 Total cash equivalents 25,563 — — 25,563 Marketable securities: U.S. Treasury securities 14,229 10 ( 24 ) 14,215 Government agency securities 65,182 23 ( 98 ) 65,107 Commercial paper 17,891 8 ( 10 ) 17,889 Corporate bonds 113,492 90 ( 135 ) 113,447 Total marketable securities 210,794 131 ( 267 ) 210,658 Total cash equivalents and marketable securities $ 236,357 $ 131 $ ( 267 ) $ 236,221

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2024
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): March 31, December 31, Lab equipment $ 9,645 $ 9,682 Furniture and fixtures 600 600 Computer equipment 393 393 Software 199 199 Leasehold improvements 7,102 7,102 Total property and equipment 17,939 17,976 Less: accumulated depreciation ( 13,174 ) ( 12,760 ) Property and equipment, net $ 4,765 $ 5,216

Additional Balance Sheet Deta_2

Additional Balance Sheet Detail (Tables)	3 Months Ended
	Mar. 31, 2024
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): March 31, December 31, Prepaid expenses $ 3,524 $ 3,658 Prepaid sign-on bonuses subject to vesting provisions 30 71 Interest income receivable 1,084 1,712 Total prepaid expenses and other current assets $ 4,638 $ 5,441
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): March 31, December 31, External research and development $ 2,325 $ 3,164 Payroll and benefits 1,615 4,712 Professional services 480 454 Other 385 396 Total accrued expenses and other current liabilities $ 4,805 $ 8,726

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock	As of March 31, 2024 and December 31, 2023, the Company has reserved for future issuance the following number of shares of common stock: March 31, December 31, Shares reserved for exercises of outstanding stock options 12,289,352 9,972,217 Shares reserved for vesting of restricted stock units 128,709 75,017 Shares reserved for future issuance under the 2019 Stock Incentive Plan 3,008,629 3,157,537 Shares reserved for future issuance under the 2019 Employee Stock Purchase Plan 1,774,696 1,346,125 Shares reserved for future issuance under the 2022 Inducement Stock Incentive Plan 390,314 837,877 17,591,700 15,388,773

Stock-based Compensation Expe_2

Stock-based Compensation Expense (Tables)	3 Months Ended
	Mar. 31, 2024
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity during the three months ended March 31, 2024: Number of Weighted Weighted Aggregate Outstanding at December 31, 2023 9,972,217 $ 8.44 8.49 $ 15,114,074 Granted 2,629,058 7.59 Exercised ( 214,094 ) 7.71 Cancelled ( 97,829 ) 9.22 Outstanding at March 31, 2024 12,289,352 $ 8.26 8.64 $ 35,107,034 Exercisable at March 31, 2024 3,365,888 $ 12.71 7.19 $ 2,509,520
Weighted Average Assumptions Used to Calculate Fair Value of Stock Option	The fair value of stock options granted during the three months ended March 31, 2024 and 2023 has been calculated on the date of grant using the following weighted average assumptions: Three Months Three Months Risk-free interest rate 4.1 % 3.8 % Expected dividend yield 0.0 % 0.0 % Expected term (years) 6.0 6.0 Expected stock price volatility 103.1 % 94.3 %
Summary of Stock-Based Compensation Expense Recognized	The total compensation cost recognized in the statements of operations and comprehensive loss associated with all stock-based compensation awards granted by the Company is as follows (in thousands) : Three Months Ended 2024 2023 General and administrative $ 2,587 $ 3,408 Research and development 1,329 845 Total stock-based compensation expense $ 3,916 $ 4,253
Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan
Summary of Restricted Stock Activity	The Company has also granted restricted stock units. The shares of common stock underlying restricted stock units typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest. The following table summarizes the Company’s restricted stock unit activity during the three months ended March 31, 2024: Number of Weighted Unvested at December 31, 2023 75,017 $ 10.77 Granted 70,445 6.95 Vested ( 11,550 ) 12.53 Cancelled ( 5,203 ) 9.38 Unvested at March 31, 2024 128,709 $ 8.58

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2024
26 Landsdowne Street
Related Party Transaction [Line Items]
Schedule of Future Minimum Lease Payments	The future minimum lease payments associated with the 26 Landsdowne Street lease as of March 31, 2024, are as follows (in thousands): 2024(1) 1,939 2025 2,649 2026 2,729 2027 2,811 Thereafter 1,426 Total minimum lease payments 11,554 Less: imputed interest ( 1,417 ) Total lease liability $ 10,137 1. Amounts are for the nine months ending December 31, 2024.

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended 2024 2023 Outstanding stock options 12,289,352 6,876,679 Unvested restricted stock units 128,709 104,543 Total 12,418,061 6,981,222

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock in connection with public offering, net of issuance costs, Shares	9,615,384
Share price	$ 13
Proceeds from issuance of common stock under benefit plans, net		$ 1,651	$ 348
Net proceeds of common stock	$ 117,300
Accumulated deficit		$ 536,543		$ 509,673

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Additional Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset	$ 6,682	$ 7,176
Operating Lease Liability, current	2,032	2,192
Operating Lease, Liability, Noncurrent	$ 8,105	$ 8,629

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of Financial Statement Line Item Affected by the Adoption of ASU 2016-02, as Amended (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 6,682	$ 7,176
Operating Lease Liability, current	2,032	2,192
Accrued expenses and other current liabilities	4,805	8,726
Operating lease liability, net of current portion	$ 8,105	$ 8,629

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Utilized (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 39,949	$ 25,563
Marketable securities	173,365	210,658
Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	213,314	236,221
Recurring | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	30,013	25,563
Recurring | U.S. Treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	9,936
Marketable securities	20,249	14,215
Recurring | Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	107,861	113,447
Recurring | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	13,431	17,889
Recurring | Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	31,824	65,107
Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	30,013	25,563
Recurring | Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	30,013	25,563
Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	183,301	210,658
Recurring | Level 2 | U.S. Treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	9,936
Marketable securities	20,249	14,215
Recurring | Level 2 | Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	107,861	113,447
Recurring | Level 2 | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	13,431	17,889
Recurring | Level 2 | Government Agency Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 31,824	$ 65,107

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($)
Fair Value Disclosures [Abstract]
Fair value transfers between levels	$ 0

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	$ 39,949	$ 25,563
Cash equivalents, Gross unrealized Gains	0	0
Cash equivalents, Gross unrealized Losses	0	0
Cash equivalents, Fair Value	39,949	25,563
Marketable securities, Amortized Cost	173,799	210,794
Marketable securities, Gross Unrealized Gains	8	131
Marketable securities, Gross Unrealized Losses	(442)	(267)
Marketable securities, Fair Value	173,365	210,658
Cash Equivalents And Marketable Securities Amortized Cost Basis	213,748	236,357
Total cash equivalents and marketable securities, Gross Unrealized Gains	8	131
Cash Equivalents And Marketable Securities Accumulated Gross Unrealized Loss Before Tax	(442)	(267)
Cash Equivalents And Marketable Securities Fair Value Disclosure	213,314	236,221
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	13,589	17,891
Marketable securities, Gross Unrealized Gains	0	8
Marketable securities, Gross Unrealized Losses	(158)	(10)
Marketable securities, Fair Value	13,431	17,889
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	108,066	113,492
Marketable securities, Gross Unrealized Gains	7	90
Marketable securities, Gross Unrealized Losses	(212)	(135)
Marketable securities, Fair Value	107,861	113,447
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	30,013	25,563
Cash equivalents, Gross unrealized Gains	0	0
Cash equivalents, Gross unrealized Losses	0	0
Cash equivalents, Fair Value	30,013	25,563
Government Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, Amortized Cost	31,878	65,182
Marketable securities, Gross Unrealized Gains	0	23
Marketable securities, Gross Unrealized Losses	(54)	(98)
Marketable securities, Fair Value	31,824	65,107
U.S. Treasury securities
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	9,936
Cash equivalents, Gross unrealized Gains	0
Cash equivalents, Gross unrealized Losses	0
Cash equivalents, Fair Value	9,936
Marketable securities, Amortized Cost	20,266	14,229
Marketable securities, Gross Unrealized Gains	1	10
Marketable securities, Gross Unrealized Losses	(18)	(24)
Marketable securities, Fair Value	$ 20,249	$ 14,215

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($) Securities
Investments, Debt and Equity Securities [Abstract]
Sale of marketable securities	$ 0
Number of debt securities, unrealized loss position, less than 12 months | Securities	4,600
Aggregate fair value of securities, unrealized loss position for less than 12 months	$ 125,700
Aggregate fair value of securities, unrealized loss position for greater than 12 months	31,300
Aggregate fair value of securities, remaining contractual maturity of greater than one year	$ 31,400

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Property Plant And Equipment [Line Items]
Total property and equipment	$ 17,939	$ 17,976
Less: accumulated depreciation	(13,174)	(12,760)
Property and equipment, net	4,765	5,216
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	9,645	9,682
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	600	600
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	393	393
Software
Property Plant And Equipment [Line Items]
Total property and equipment	199	199
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 7,102	$ 7,102

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 479	$ 566

Additional Balance Sheet Deta_3

Additional Balance Sheet Detail - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Balance Sheet Related Disclosures [Abstract]
Prepaid expenses	$ 3,524	$ 3,658
Prepaid sign-on bonuses subject to vesting provisions	30	71
Interest income receivable	1,084	1,712
Total prepaid expenses and other current assets	$ 4,638	$ 5,441

Additional Balance Sheet Deta_4

Additional Balance Sheet Detail - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Balance Sheet Related Disclosures [Abstract]
External research and development	$ 2,325	$ 3,164
Payroll and benefits	1,615	4,712
Professional services	480	454
Other	385	396
Total accrued expenses and other current liabilities	$ 4,805	$ 8,726

Preferred Stock - Additional In

Preferred Stock - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Temporary Equity [Line Items]
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Series B Preferred Stock
Temporary Equity [Line Items]
Temporary equity, dividends declared	$ 0

Common Stock - Additional Infor

Common Stock - Additional Information (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($) Vote $ / shares shares	Dec. 31, 2023 $ / shares shares
Equity [Abstract]
Common stock, shares authorized | shares	200,000,000	200,000,000
Common stock, par value | $ / shares	$ 0.001	$ 0.001
Common stock voting rights	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders
Number of common stock voting rights | Vote	1
Dividends declared or paid | $	$ 0

Common Stock - Schedule of Pote

Common Stock - Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock (Details) - shares	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Class Of Stock [Line Items]
Shares reserved for future issuance	17,591,700	15,388,773
Outstanding Stock Options
Class Of Stock [Line Items]
Shares reserved for future issuance	12,289,352	9,972,217
Restricted stock Units
Class Of Stock [Line Items]
Shares reserved for vesting of restricted stock units	128,709	75,017
2019 Stock Incentive Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	3,008,629	3,157,537
2019 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	1,774,696	1,346,125
2022 Stock Inducement Plan
Class Of Stock [Line Items]
Shares reserved for future issuance	390,314	837,877

Stock-based Compensation Expe_3

Stock-based Compensation Expense - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions					3 Months Ended
	Jan. 01, 2024	May 18, 2023	Mar. 08, 2023	Jul. 02, 2019	Mar. 31, 2024	Dec. 31, 2023	Feb. 28, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance					17,591,700	15,388,773
Unrecognized stock-based compensation expense					$ 42.8
Unrecognized stock-based compensation expense, weighted average period expect to recognized					2 years 11 months 26 days
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance				252,142	1,774,696
Increase in number of shares reserved for issuance (shares)	428,571			428,571
Increase in number of shares reserved for issuance, percent of common stock outstanding				1%
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance					12,289,352	9,972,217
2016 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance					0
2019 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance				2,017,142	3,008,629
Increase in number of shares reserved for issuance (shares)	2,000,000			2,000,000
Increase in number of shares reserved for issuance, percent of common stock outstanding				4%
Weighted average grant date fair value					$ 6.23
Total intrinsic value of stock options exercised					$ 0.8
2019 Stock Incentive Plan | Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period					4 years
Contractual term					10 years
2022 Inducement Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for future issuance					390,314		1,750,000
Increase in number of shares reserved for issuance (shares)		1,400,000	2,000,000

Stock-based Compensation Expe_4

Stock-based Compensation Expense - Summary of Stock Option Activity (Details) - 2019 Plan and 2016 Plan - Employee Stock Option - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Outstanding, Beginning balance	9,972,217
Number of Shares, Granted	2,629,058
Number of Shares, Exercised	(214,094)
Number of Shares, Cancelled	(97,829)
Number of Shares, Outstanding, Ending balance	12,289,352	9,972,217
Number of Shares, Exercisable	3,365,888
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 8.44
Weighted Average Exercise Price, Granted	7.59
Weighted Average Exercise Price, Exercised	7.71
Weighted Average Exercise Price, Cancelled	9.22
Weighted Average Exercise Price, Outstanding, Ending balance	8.26	$ 8.44
Weighted Average Exercise Price, Exercisable	$ 12.71
Weighted Average Remaining Contractual Term (in years), Outstanding	8 years 7 months 20 days	8 years 5 months 26 days
Weighted Average Remaining Contractual Term (in years), Exercisable	7 years 2 months 8 days
Aggregate Intrinsic Value, Outstanding	$ 35,107,034	$ 15,114,074
Aggregate Intrinsic Value, Exercisable	$ 2,509,520

Stock-based Compensation Expe_5

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
	Mar. 31, 2024	Mar. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	4.10%	3.80%
Expected dividend yield	0%	0%
Expected term (years)	6 years	6 years
Expected stock price volatility	103.10%	94.30%

Stock-based Compensation Expe_6

Stock-based Compensation Expense - Summary of Restricted Stock Activity outside of 2019 and 2016 Plan (Details) - Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan - Restricted stock Units	3 Months Ended
	Mar. 31, 2024 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance | shares	75,017
Number of Shares, Granted | shares	70,445
Number of Shares, Vested | shares	(11,550)
Number of Shares, Cancelled | shares	(5,203)
Number of Shares, Unvested, Ending balance | shares	128,709
Weighted Average Grant Date Fair Value, Unvested, Beginning balance | $ / shares	$ 10.77
Weighted Average Grant Date Fair Value, Granted | $ / shares	6.95
Weighted Average Grant Date Fair Value, Vested | $ / shares	12.53
Weighted Average Grant Date Fair Value, Cancelled | $ / shares	9.38
Weighted Average Grant Date Fair Value, Unvested, Ending balance | $ / shares	$ 8.58

Stock-based Compensation Expe_7

Stock-based Compensation Expense - Summary of Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,916	$ 4,253
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2,587	3,408
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,329	$ 845

Collaboration and License Agr_2

Collaboration and License Agreements - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2021	Dec. 31, 2020	Jul. 31, 2020
Acceleron Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues		$ 0.3
Deferred Revenue	$ 0		$ 0
MyoKardia Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	10
Deferred Revenue			0.2
Reimbursement Payment Received	7.7		7.2
Unbilled accounts receivable related to reimbursable research and development cost			0.5
Upfront payment							$ 10
Payment for prepaid research funding	$ 2.5		$ 2.5				2.5
Prepaid research funding payment received						$ 2.5
MyoKardia Collaboration and License Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment for certain identified targets							298.5
Milestone payment for certain other identified targets							$ 150
MyoKardia Collaboration and License Agreement | Clinical and Regulatory Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone				$ 2.5	$ 2.5

Right of Reference and License

Right of Reference and License Agreement - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) shares
GSK Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones	$ 5
Specified clinical and regulatory milestones phase two clinical trial	2.5
GSK Agreement | Maximum
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones	37.5
Specified sales milestone	60
CAMP4 Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones	35
CAMP4 Agreement | Maximum
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Specified clinical and regulatory milestones	$ 35
Series B Convertible Preferred Stock | GSK Agreement
Research And Development Assets Acquired Other Than Through Business Combination [Line Items]
Shares issued for asset acquisition | shares	12,500,000

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2024 USD ($)
26 Landsdowne Street | Accounting Standards Update 2016-02 [Member]
Related Party Transaction [Line Items]
2024	$ 1,939	[1]
2025	2,649
2026	2,729
2027	2,811
Thereafter	1,426
Total minimum lease payments	11,554
Less: imputed interest	(1,417)
future minimum lease payments due	10,137
125 Sidney Street
Related Party Transaction [Line Items]
Total minimum lease payments	$ 0
[1] Amounts are for the nine months ending December 31, 2024.

Leases - Schedule of Minimum Re

Leases - Schedule of Minimum Rental Commitments Under Non-cancelable Leases (Details) $ in Thousands	Mar. 31, 2024 USD ($)
26 Landsdowne Street | Accounting Standards Update 2016-02 [Member]
Related Party Transaction [Line Items]
Total minimum lease payments	$ 11,554
Less: imputed interest	1,417
Operating Lease, Liability, Total	10,137
125 Sidney Street
Related Party Transaction [Line Items]
Total minimum lease payments	$ 0

Leases (Additional Information)

Leases (Additional Information) (Details) $ in Thousands		1 Months Ended	3 Months Ended
	Nov. 30, 2017 USD ($) SquareFeet	Nov. 30, 2021 USD ($) SquareFeet	Mar. 31, 2024 USD ($)
Related Party Transaction [Line Items]
Operating lease expense			$ 500
Variable lease expense			200
26 Landsdowne Street
Related Party Transaction [Line Items]
Office and laboratory space | SquareFeet	28,731
Extended lease term	5 years
Lease term	10 years
Total lease commitment	$ 25,100
Leasehold improvements	5,000
Security deposit and letter of credit amount	$ 1,100
125 Sidney Street
Related Party Transaction [Line Items]
Office and laboratory space | SquareFeet		12,196
Total lease commitment		$ 1,700
Operating lease expense			200
Variable lease expense			0
Lease payments due			$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Indemnification Agreement
Commitments And Contingencies [Line Items]
Accrued liabilities for indemnification agreements	$ 0	$ 0
Legal Proceedings
Commitments And Contingencies [Line Items]
Expenses incurred for legal proceedings	$ 0

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Retirement Benefits [Abstract]
Defined contribution plan name	401(k)
Contributions to the 401(k) Plan	$ 0.2	$ 0.2

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
	Mar. 31, 2024	Mar. 31, 2023
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	12,418,061	6,981,222
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	12,289,352	6,876,679
Unvested restricted stock units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation of diluted net loss per share	128,709	104,543

Subsequent Events (Additional I

Subsequent Events (Additional Information) (Details) - Subsequent Event [Member] - Sanofi [Member] $ in Millions	May 11, 2024 USD ($)
Subsequent Event [Line Items]
Development and regulatory milestone payments	$ 975
Upfront Payment	$ 80