Theseus Pharmaceuticals (THRX) 10-Q 15 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 01, 2021
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	001-40869
Entity Registrant Name	THESEUS PHARMACEUTICALS, INC.
Entity Tax Identification Number	83-0712806
Entity Address State Or Province	MA
Entity Address, Address Line One	245 Main Street
Entity Address, City or Town	Cambridge
Entity Incorporation, State or Country Code	DE
Entity Address, Postal Zip Code	02142
City Area Code	857
Local Phone Number	400-9491
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	THRX
Security Exchange Name	NASDAQ
Entity Current Reporting Status	No
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		38,702,650
Entity Central Index Key	0001745020
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 90,404	$ 8,457
Prepaid expenses and other current assets	1,082	113
Total current assets	91,486	8,570
Property and equipment, net		1
Other assets	6,133	43
Total assets	97,619	8,614
Current liabilities:
Accounts payable	2,392	1,132
Accrued expenses and other current liabilities	3,288	463
Total current liabilities	5,680	1,595
Restricted stock liability, net of current portion	903
Total liabilities	6,583	1,595
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	141,181	41,289
Stockholders' deficit:
Additional paid-in capital	2,777
Accumulated deficit	(52,922)	(34,270)
Total stockholders' deficit	(50,145)	(34,270)
Total liabilities, redeemable convertible preferred stock and stockholders' deficit	97,619	8,614
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	41,289	$ 41,289
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable convertible preferred stock:
Redeemable convertible preferred stock	$ 99,892

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Redeemable convertible preferred stock, shares authorized	33,701,081	22,136,987
Redeemable convertible preferred stock, shares issued	25,475,905	16,734,179
Redeemable convertible preferred stock, shares outstanding	25,475,905	16,734,179
Redeemable convertible preferred stock, preference in liquidation	$ 122,237	$ 22,137
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	41,083,993	29,054,797
Common stock, shares issued	2,054,555	1,708,625
Common stock, shares outstanding	2,054,555	1,708,625
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	22,136,987	22,136,987
Redeemable convertible preferred stock, shares issued	16,734,179	16,734,179
Redeemable convertible preferred stock, shares outstanding	16,734,179	16,734,179
Redeemable convertible preferred stock, preference in liquidation	$ 22,137	$ 22,137
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	11,564,094	11,564,094
Redeemable convertible preferred stock, shares issued	8,741,726	0
Redeemable convertible preferred stock, shares outstanding	8,741,726	0
Redeemable convertible preferred stock, preference in liquidation	$ 100,100

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating expenses:
Research and development	$ 4,996	$ 1,275	$ 13,306	$ 3,470
General and administrative	2,378	154	5,371	478
Total operating expenses	7,374	1,429	18,677	3,948
Loss from operations	(7,374)	(1,429)	(18,677)	(3,948)
Other income (expense), net:
Change in fair value of preferred stock tranche rights				(3,969)
Change in fair value of anti-dilution rights				(1,190)
Other income (expense), net	2		25
Total other income (expense), net	2		25	(5,159)
Net loss	(7,372)	(1,429)	(18,652)	(9,107)
Comprehensive loss	(7,372)	(1,429)	(18,652)	(9,107)
Net loss attributable to common stockholders-basic (Note 13)	(7,372)	(1,429)	(18,652)	(9,107)
Net loss attributable to common stockholders- diluted (Note 13)	$ (7,372)	$ (1,429)	$ (18,652)	$ (9,107)
Net loss per share attributable to common stockholders-basic and diluted	$ (5.16)	$ (2.12)	$ (18.69)	$ (18.11)
Weighted-average common stock outstanding-basic and diluted	1,428,054	672,911	997,949	502,804

Condensed Statements of Redeema

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK	SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED DEFICIT	Total
Beginning balance at Dec. 31, 2019	$ 6,857
Beginning balance, shares at Dec. 31, 2019	6,668,818
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs	$ 4,855
Issuance of redeemable convertible preferred stock, net of issuance costs, shares	2,640,603
Ending balance at Mar. 31, 2020	$ 11,712
Ending balance, shares at Mar. 31, 2020	9,309,421
Beginning balance at Dec. 31, 2019				$ 813	$ (10,220)	$ (9,407)
Beginning balance, shares at Dec. 31, 2019			288,568
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock, shares			134,156
Stock-based compensation				31		31
Net loss					(2,265)	(2,265)
Ending balance at Mar. 31, 2020				844	(12,485)	(11,641)
Ending balance, shares at Mar. 31, 2020			422,724
Beginning balance at Dec. 31, 2019	$ 6,857
Beginning balance, shares at Dec. 31, 2019	6,668,818
Ending balance at Sep. 30, 2020	$ 23,377
Ending balance, shares at Sep. 30, 2020	13,710,427
Beginning balance at Dec. 31, 2019				813	(10,220)	(9,407)
Beginning balance, shares at Dec. 31, 2019			288,568
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss						(9,107)
Ending balance at Sep. 30, 2020				1,081	(19,327)	(18,246)
Ending balance, shares at Sep. 30, 2020			724,060
Beginning balance at Mar. 31, 2020	$ 11,712
Beginning balance, shares at Mar. 31, 2020	9,309,421
Ending balance at Jun. 30, 2020	$ 11,712
Ending balance, shares at Jun. 30, 2020	9,309,421
Beginning balance at Mar. 31, 2020				844	(12,485)	(11,641)
Beginning balance, shares at Mar. 31, 2020			422,724
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock, shares			223,758
Stock-based compensation				203		203
Net loss					(5,413)	(5,413)
Ending balance at Jun. 30, 2020				1,047	(17,898)	(16,851)
Ending balance, shares at Jun. 30, 2020			646,482
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs	$ 11,665
Issuance of redeemable convertible preferred stock, net of issuance costs, shares	4,401,006
Ending balance at Sep. 30, 2020	$ 23,377
Ending balance, shares at Sep. 30, 2020	13,710,427
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock, shares			77,578
Stock-based compensation				34		34
Net loss					(1,429)	(1,429)
Ending balance at Sep. 30, 2020				1,081	(19,327)	(18,246)
Ending balance, shares at Sep. 30, 2020			724,060
Beginning balance at Dec. 31, 2020	$ 41,289					$ 41,289
Beginning balance, shares at Dec. 31, 2020	16,734,179	0				16,734,179
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs		$ 99,892
Issuance of redeemable convertible preferred stock, net of issuance costs, shares		8,741,726
Ending balance at Mar. 31, 2021	$ 41,289	$ 99,892
Ending balance, shares at Mar. 31, 2021	16,734,179	8,741,726
Beginning balance at Dec. 31, 2020					(34,270)	$ (34,270)
Beginning balance, shares at Dec. 31, 2020			882,789
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock, shares			85,430
Stock-based compensation				44		44
Net loss					(4,473)	(4,473)
Ending balance at Mar. 31, 2021				44	(38,743)	(38,699)
Ending balance, shares at Mar. 31, 2021			968,219
Beginning balance at Dec. 31, 2020	$ 41,289					$ 41,289
Beginning balance, shares at Dec. 31, 2020	16,734,179	0				16,734,179
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs, shares		8,741,726
Ending balance at Sep. 30, 2021	$ 41,289	$ 99,892				$ 141,181
Ending balance, shares at Sep. 30, 2021	16,734,179	8,741,726				25,475,905
Beginning balance at Dec. 31, 2020					(34,270)	$ (34,270)
Beginning balance, shares at Dec. 31, 2020			882,789
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss						(18,652)
Ending balance at Sep. 30, 2021				2,777	(52,922)	(50,145)
Ending balance, shares at Sep. 30, 2021			1,139,079
Beginning balance at Mar. 31, 2021	$ 41,289	$ 99,892
Beginning balance, shares at Mar. 31, 2021	16,734,179	8,741,726
Ending balance at Jun. 30, 2021	$ 41,289	$ 99,892
Ending balance, shares at Jun. 30, 2021	16,734,179	8,741,726
Beginning balance at Mar. 31, 2021				44	(38,743)	(38,699)
Beginning balance, shares at Mar. 31, 2021			968,219
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock, shares			85,430
Stock-based compensation				1,440		1,440
Net loss					(6,807)	(6,807)
Ending balance at Jun. 30, 2021				1,484	(45,550)	(44,066)
Ending balance, shares at Jun. 30, 2021			1,053,649
Ending balance at Sep. 30, 2021	$ 41,289	$ 99,892				$ 141,181
Ending balance, shares at Sep. 30, 2021	16,734,179	8,741,726				25,475,905
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock, shares			85,430
Stock-based compensation				1,293		$ 1,293
Net loss					(7,372)	(7,372)
Ending balance at Sep. 30, 2021				$ 2,777	$ (52,922)	$ (50,145)
Ending balance, shares at Sep. 30, 2021			1,139,079

Condensed Statements of Redee_2

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
Issuance costs		$ 3	$ 3
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK
Issuance costs	$ 208

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (18,652)	$ (9,107)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	1	2
Stock-based compensation expense	2,777	268
Change in fair value of preferred stock tranche rights		3,969
Change in fair value of anti-dilution rights		1,190
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(969)	7
Accounts payable	837
Accrued expenses and other current liabilities	1,367	78
Other assets	(2,640)	(21)
Net cash used in operating activities	(17,279)	(3,614)
Cash flows from financing activities:
Payment of initial public offering costs	(1,683)
Proceeds from early exercise of options	1,395
Net cash provided by financing activities	99,604	7,994
Net increase in cash and cash equivalents	82,325	4,380
Cash and cash equivalents at beginning of year	8,457	1,317
Cash and cash equivalents at end of period	90,782	5,697
Supplemental disclosure of cash flows:
Deferred financing costs in accounts payable	423
Deferred financing costs in accrued expenses	965
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
Cash flows from financing activities:
Proceeds from issuance redeemable convertible preferred stock, net of issuance costs		$ 7,994
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK
Cash flows from financing activities:
Proceeds from issuance redeemable convertible preferred stock, net of issuance costs	$ 99,892

Condensed Statements of Cash _2

Condensed Statements of Cash Flows (Parenthetical) $ in Thousands	Sep. 30, 2021USD ($)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 90,404
Restricted cash (included in other assets)	378
Total cash, cash equivalents, and restricted cash	$ 90,782

Nature of the Business

Nature of the Business	9 Months Ended
	Sep. 30, 2021
Nature of the Business
Nature of the Business	1. Nature of the Business Theseus Pharmaceuticals, Inc. (“Theseus” or the “Company”) is a biopharmaceutical company focused on improving the lives of cancer patients through the discovery, development and commercialization of transformative targeted therapies. The Company was incorporated in December 2017 under the laws of the State of Delaware, and its principal offices are in Cambridge, Massachusetts. On September 27, 2021, the Company effected a one-for-1.32286 reverse stock split of shares of the Company’s common stock and convertible preferred stock. All of the share and per share amounts included in the accompanying condensed financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Shares of common stock underlying outstanding stock options were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with the terms of the appropriate securities agreements. Stockholders entitled to fractional shares as a result of the reverse stock split received a cash payment in lieu of receiving fractional shares. On October 6, 2021, the Company’s registration statements on Form S-1 relating to its initial public offering of its common stock were declared effective by the Securities and Exchange Commission (“SEC”). In the initial public offering (“IPO”), which closed on October 12, 2021, the Company issued and sold 10,000,200 shares of common stock at a public offering price of $16.00 per share. The aggregate gross proceeds, before deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company, were approximately $160.0 million. Upon the closing of the IPO, all of the Company’s outstanding shares of redeemable convertible preferred stock automatically converted into 25,475,905 shares of common stock. On October 25, 2021, the underwriters partially exercised their over-allotment option to purchase an additional 1,171,990 shares of common stock for $16.00 per share. The exercise of the over-allotment option closed on October 27, 2021, and the aggregate gross proceeds, before deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company, were approximately $18.8 million. The remainder of the underwriters’ over-allotment option expired unexercised. In connection with the closing of the IPO, the Company amended and restated its certificate of incorporation to among other things: (a) authorize 500,000,000 shares of voting common stock; (b) eliminate all references to the previously existing series of redeemable convertible preferred stock; (c) authorize 50,000,000 shares of preferred stock that may be issued from time to time by the Company’s board of directors (the “Board”) in one or more series. Basis of Presentation The Company’s condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the ASC and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The accompanying condensed financial statements and footnotes to the financial statements have been prepared on the same basis as the most recently audited annual financial statements and, in the opinion of management, reflect all normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of September 30, 2021 and the results of its operations and its cash flows for the nine-month periods ended September 30, 2021 and 2020. The results for the three- and nine-month periods ended September 30, 2021 are not necessarily indicative of results to be expected for the year ending December 31, 2021, any other interim periods, or any future year or period. These condensed financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2020 included in the Company’s final prospectus for its IPO dated October 6, 2021, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Securities Act”), with the SEC on October 7, 2021. Liquidity The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. The Company’s development programs 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 and infrastructure and extensive compliance-reporting capabilities. Because of the numerous risks and uncertainties associated with product development, the Company is unable to predict the timing or amount of increased expenses or when or if the Company will be able to achieve or maintain profitability. Even if the Company is able to generate revenue from product sales, the Company may not become profitable. If the Company fails to become profitable or is unable to sustain profitability on a continuing basis, then the Company may be unable to continue its operations at planned levels and be forced to reduce or terminate its operations. The Company expects to incur substantial operating losses and negative cash flows from operations for the foreseeable future. In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the accompanying condensed financial statements are described in the Company’s audited financial statements for the years ended December 31, 2020 and 2019, included in the Company’s final prospectus for its IPO dated October 6, 2021, filed pursuant to Rule 424(b)(4) under the Securities Act with the SEC on October 7, 2021. There have been no material changes in the Company’s significant accounting policies during the nine months ended September 30, 2021, except as noted below. Unaudited Interim Financial Information The accompanying condensed balance sheet as of September 30, 2021, and the condensed statements of operations and comprehensive loss, redeemable convertible preferred stock and stockholders’ deficit for the three- and nine-month periods ended September 30, 2021 and 2020 and statements of cash flows for the nine months ended September 30, 2021 and 2020 are unaudited. The condensed interim financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of September 30, 2021 and the results of its operations for the three- and nine-month periods ended September 30, 2021 and 2020 and its cash flows for the nine months ended September 30, 2021 and 2020. The financial data and other information disclosed in these notes related to the three- and nine-month periods ended September 30, 2021 and 2020 are also unaudited. The results for the three- and nine-month periods ended September 30, 2021 are not necessarily indicative of results to be expected for the full year or for any other subsequent interim period. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Estimates and judgments are based on historical information and other market-specific or various relevant assumptions, including in certain circumstances, future projections, that management believes to be reasonable under the circumstances. Actual results could differ materially from estimates. Significant estimates and assumptions are used for, but not limited to, the accruals for research and development expenses, the determination of fair value of equity instruments, preferred stock tranche rights and the anti-dilution right, and for periods prior to the completion of the IPO, stock-based compensation expense. Cash and Cash Equivalents Cash includes cash in readily available checking accounts. Cash is carried at cost, which approximates its fair value. The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents are carried at fair market value. Restricted Cash Restricted cash consists of a restricted cash deposit of $0.4 million which serves as collateral for a letter of credit issued to the landlord of the Company’s leased facility for a security deposit upon entering into the lease in September 2021. The Company classified this amount as restricted cash in the accompanying condensed balance sheet within other assets as of September 30, 2021. As of December 31, 2020, the Company held no restricted cash. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. For the three and nine months ended September 31, 2021 and 2020, there were no differences between net loss and comprehensive loss. Deferred Issuance Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred issuance costs until such financings are consummated. After consummation of such an equity financing, these costs are recorded as a reduction of the proceeds generated as a result of the offering. Should the planned equity financing be abandoned, the deferred issuance costs, currently recorded within other assets, will be expensed immediately as a charge to operating expenses in the statements of operations and comprehensive loss. The Company recorded deferred issuance costs of $3.1 million within other assets as of September 30, 2021, and recorded no deferred issuance costs as of December 31, 2020. Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial statements and disclosures. ​ In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) .

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2021
Fair Value of Financial Assets and Liabilities
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities Assets and liabilities measured at fair value on a recurring basis are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, ​ ​ ​ DESCRIPTION ​ 2021 ​ LEVEL 1 ​ LEVEL 2 ​ LEVEL 3 Assets ​ ​ ​ ​ ​ Cash equivalents ​ $ 89,526 ​ $ 89,526 ​ $ — ​ $ — Total financial assets ​ $ 89,526 ​ $ 89,526 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ During the year ended December 31, 2020, there were no assets or liabilities measured at fair value. During the nine months ended September 30, 2021 and the year ended December 31, 2020, there were no transfers between fair value levels. The fair values of the Company’s cash equivalents, consisting of its money market funds, are based on quoted market prices in active markets without any valuation adjustment. The carrying values of other current assets, accounts payable, and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities. Tranche Rights The Company determined that its obligations to issue, and the Company’s investors’ obligation to purchase, additional shares of Series A convertible preferred stock (“Series A”) at a fixed price (i.e. the issuance price) pursuant to the Tranche Rights (see Note 8) in three subsequent tranches following the initial closing of the Series A financing represented freestanding financial instruments. The Company issued 2,267,813 and 3,779,688 shares in January 2020 and July 2020, respectively, pursuant to these rights, at a purchase price of $1.32 per share. The final tranche was cancelled upon termination of the Series A preferred stock purchase agreement (“Series A Agreement”) in December 2020. The Tranche Rights were classified as a liability on the Company’s balance sheets and initially recorded at fair value. The Tranche Rights were subsequently revalued until the tranches were settled, with changes in fair value for each reporting period recognized in other income (expense), net in the statement of operations and comprehensive loss. Upon the purchase of the Tranche Right shares, the fair value of the related Tranche Right was recognized as Series A redeemable convertible preferred stock. The obligation was fully satisfied in December 2020 when the Series A Agreement was terminated, with the remaining value recognized in the statement of operations and comprehensive loss. As such, the Company does not have a Tranche Right liability recorded as of December 31, 2020 or September 30, 2021. The fair value of Tranche Rights was based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. A change in the assumptions related to the valuation of the Tranche Rights could have a significant impact on the value of the obligation. Each Tranche Right was valued as a forward contract. The values were determined using a probability-weighted present value calculation. In determining the fair values of the Tranche Rights, estimates and assumptions impacting fair value included the estimated future values of the Company’s Series A, discount rates, estimated time to tranche closing, and probability of each tranche closing. The Company determined the per share future value of the Series A shares by back-solving to the initial proceeds of the Series A financing. The Company remeasured each Tranche Right at each reporting period and prior to settlement. Anti-dilution Right In accordance with a license agreement between the Company and ARIAD Pharmaceuticals, Inc. (“ARIAD”), the Company was obligated to issue to ARIAD additional Series A shares for no consideration upon the issuance of certain Milestone Shares (see Note 8) to certain investors (“Anti-dilution Right”). The Company determined that the Anti-dilution Right was a freestanding financial instrument. The freestanding financial instrument was classified as an asset or liability on the Company’s balance sheets and initially recorded at fair value. The fair value of the Anti-dilution Right was based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. A change in the assumptions related to the valuation of the Anti-dilution Rights could have a significant impact on the value of the obligation. The Anti-dilution Right was valued as a forward contract. The value was determined using a probability-weighted present value calculation. In determining the fair values of the obligation, estimates and assumptions impacting fair value included the estimated future values of the Company’s Series A shares, discount rates, estimated time to share issuance and probability of each share issuance. The Company determined the per share future value of the Series A shares by back-solving to the initial proceeds of the Series A financing. The Anti-dilution Right was subsequently revalued until anti-dilution shares were issued or the Anti-dilution Right was terminated, with changes in fair value for each reporting period recognized in other income (expense), net in the statements of operations and comprehensive loss. Upon issuance of the Anti-dilution shares, the fair value of the Anti-dilution Right was recognized as Series A redeemable convertible preferred stock. In accordance with the Anti-dilution Right, the Company issued ARIAD 372,791 and 621,318 additional Series A shares in January 2020 and July 2020, respectively, upon the issuance of Milestone Shares to its investors. The obligation was fully satisfied in December 2020 in conjunction with the termination of the Series A Agreement; as such, the Company does not have an Anti-dilution Right liability as of December 31, 2020 or September 30, 2021. ARIAD did not receive additional Series A shares upon the termination of the Series A Agreement. The Company remeasured the Tranche Rights and the Anti-dilution Right at each reporting period and prior to settlement. The following reflects the ranges of significant quantitative inputs used in the valuation of the Tranche Rights and the Anti-dilution Right during 2020: ​ ​ ​ ​ ​ ​ ​ ​ FOR THE YEAR ENDED ​ ​ DECEMBER 31, 2020 Stand-alone Series A Preferred Stock price (spot price) ​ $1.84 - $5.93 Risk-free rate ​ 1.6% Discount factor ​ 0.9923 - 1.0 Time to milestone event (years) ​ 0.0 years - 0.5 years Probability of tranche closing ​ 0% - 100% ​ ​ ​ ​ The following table provides a rollforward of the aggregate fair value of the Company’s Tranche Rights and Anti-dilution Right (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ PREFERRED STOCK ANTI-DILUTION ​ ​ TRANCHE RIGHTS ​ RIGHT Balance as of December 31, 2019 ​ $ 2,220 ​ $ 1,142 Change in fair value ​ 3,969 ​ 1,190 Fair value recognized as Series A upon settlement of right ​ (6,189) ​ (2,332) Balance as of December 31, 2020 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2021
Accrued Expenses
Accrued Expenses	4. Accrued Expenses Accrued expenses consisted of the following as of September 30, 2021 and as of December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, DECEMBER 31, ​ ​ 2021 ​ 2020 Accrued research and development ​ $ 664 ​ $ 310 Accrued legal ​ 112 ​ 143 Accrued compensation and benefits ​ 738 ​ 10 Accrued other ​ 1,280 ​ — Restricted stock liability, current ​ 494 ​ — Total accrued expenses and other current liabilities ​ $ 3,288 ​ $ 463 ​ ​ ​ ​ ​ ​ ​ ​

Property and Equipment, net

Property and Equipment, net	9 Months Ended
	Sep. 30, 2021
Property and Equipment, net
Property and Equipment, net	5. Property and Equipment, net Property and equipment, net consisted of the following as of September 30, 2021 and as of December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, DECEMBER 31, ​ ​ 2021 ​ 2020 Computer equipment ​ $ — ​ $ 6 Less: accumulated depreciation ​ — ​ (5) Property and equipment, net ​ $ — ​ $ 1 ​ ​ ​ ​ ​ ​ ​ ​ Depreciation expense for the nine months ended September 30, 2021 and 2020 was approximately $1,000 and $2,000, respectively. There were no impairments recorded to date.

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2021
License Agreements
License Agreements	6. License Agreements Agreement Description In June 2018, the Company entered into a license agreement with ARIAD, for an exclusive, transferable (subject to certain restrictions), sublicensable (subject to certain conditions), worldwide license, under certain of ARIAD’s patent rights, know-how and compounds and a certain ARIAD chemical library, to develop, use, manufacture, market and commercialize certain compounds, and products that contain such compounds, that are therapeutically useful for the treatment of diseases and disorders in humans, including with respect to c- KIT, a type of receptor tyrosine kinase and tumor marker (also known as CD117 and stem cell factor receptor). Pursuant to the license agreement, in exchange for an exclusive license, a non-exclusive license, and certain laboratory equipment, the Company issued to ARIAD 621,318 shares of Series A and a non-exclusive license to certain intellectual property. In addition, the Company provided to ARIAD the Anti-dilution Right described in Note 3. The non-exclusive licenses were granted to the parties in order to administratively facilitate the flow of data between the parties without infringement on existing patents; all of the economic value of the agreement is concentrated in the exclusive license. The laboratory equipment the Company acquired was not material. The Company accounted for the agreement as an asset acquisition. The transaction price was measured as the fair value of the 621,318 shares of Series A issued to ARIAD and the fair value of the Anti-Dilution Right (as described in Note 3), or approximately $1.4 million. The Company determined that the licensed intellectual property represented in-process research and development assets with no alternative future use. As a result, the cost to license the intellectual property of $1.4 million was recognized as research and development expense on the Company’s statements of operations in the year-ended December 31, 2018. The Company is required to pay ARIAD tiered royalty payments that are low- to mid-single digits of the Company’s future net sales and those of its sublicensees of each product comprising a licensed ARIAD compound in each country. The Company is also responsible for costs relating to the prosecution and maintenance of the licensed patents. The agreement contains anti-stacking and generic competition provisions on the royalties whereby the Company may deduct a percentage of the amounts due for royalties from its payments if the Company enters into a third-party license agreement, and may reduce the rates in the event a generic product is being marketed and sold by a third party and the average net sales as measured over a specified period of time are at least a certain percentage lower than the average net sales during the a specified period of time immediately prior to the launch of the generic product. The term of the agreement commenced in June 2018 and unless earlier terminated as provided in the agreement for breach of terms by either party or for convenience by the Company with advanced written notice, shall continue in full force and effect, on a country-by-country and product-by-product basis until the date on which the royalty term in such country with respect to such product expires. The royalty term is the period from the first commercial sale of such product in such country until the later of (a) the expiry of all patents that cover the product in such country or (b) ten years after the first commercial sale. The license agreement terminates, on a product-by-product and country-by-country basis, on expiration of the royalty term for such product for the applicable country. Thereafter, the licenses from ARIAD to the Company with respect to such product for such country will convert to a fully paid, royalty-free, irrevocable and perpetual license.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies.
Commitments and Contingencies	7. Commitments and Contingencies Legal Proceedings The Company may from time to time be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the nine months ended September 30, 2021 or the year ended December 31, 2020, and no material legal proceedings are currently pending or, to the best of the Company’s knowledge, threatened. ​ Leases On September 16, 2021, the Company entered into an agreement to lease 7,351 square feet of office space in Cambridge, Massachusetts. The lease term is seven years , with a one-time option right to extend the term five additional years, and is expected to commence in the first quarter of 2022. The annual base rent under the lease is approximately $0.8 million for the first lease year and is subject to annual increases of 3% thereafter. The Company provided a security deposit in the form of a letter of credit in the amount of approximately $0.4 million upon signing, pursuant to the terms of the lease. ​ The future minimum payments required under the lease as of September 30, 2021 are as follows (in thousands): ​ ​ ​ ​ ​ Year Ending September 30, ​ ​ 2021 (3 months) ​ $ — 2022 ​ ​ 757 2023 ​ ​ 780 2024 ​ ​ 803 2025 ​ ​ 827 Thereafter ​ ​ 2,635 Total minimum lease payments ​ $ 5,802 ​

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2021
Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock	8. Redeemable Convertible Preferred Stock In June 2018, the Company entered into the Series A Agreement, under which it agreed to issue up to 13,203,025 shares of Series A in an initial closing and three subsequent milestone-based tranches, inclusive of the Anti-dilution Right. Under the Series A Agreement, the Company initially issued 3,779,688 shares to certain investors at a price of $1.32 per share for proceeds of $4.7 million, net of issuance costs of $0.3 million. The Series A Agreement provided for three additional tranche closings based on the achievement of three defined milestones (the “Tranche Rights”), pursuant to which certain investors were required to purchase, and the Company to sell, up to 7,559,376 additional shares of Series A at a price of $1.32 per share upon the achievement or waiver of the defined milestone. All Series A shares issued as a result of the achievement or waiver of a milestone are referred to as “Milestone Shares”. The Company concluded that the obligation and right to make future issuances of Series A shares under the Tranche Rights met the definition of a freestanding financial instrument, as the rights were legally detachable from the Series A (see Note 3). In January 2020 and July 2020, the Company sold 2,267,813 and 3,779,688 Milestone Shares, respectively, to certain investors at a price of $1.32 per share for proceeds of $8.0 million as a result of the achievement of two milestones. The requirement to purchase the remaining Milestone Shares was cancelled in December 2020 upon the determination by the Company that the program would be cancelled. In conjunction with the Series A shares issued to certain investors, the Company issued shares to ARIAD pursuant to the ARIAD Agreement (Note 6). The Company issued 621,318 shares to ARIAD as part of the initial Series A closing in June 2018 in exchange for access to ARIAD’s intellectual property. Upon the issuance of Milestone Shares to certain investors in January 2020 and July 2020, the Company issued 372,791 and 621,318 Series A shares, respectively, to ARIAD as part of ARIAD’s Anti-dilution Right (see Note 3). The Company concluded that the Anti-dilution Right that represented the obligation to make future issuances of Series A to ARIAD met the definition of a freestanding financial instrument, as the rights were separately exercisable and legally detachable from the underlying license rights. In August 2019, the Series A Agreement was amended to authorize the sale of 2,267,812 additional Series A shares to a certain majority stockholder of the Company in the form of an operating cash closing. These shares were sold for $1.32 per share, for gross proceeds of $3.0 million and recognized at their fair value of $2.6 million. In December 2020, the Series A Agreement was further amended to authorize the sale of 3,023,751 additional shares of Series A to the same investor in the form of a second operating cash closing. These shares were sold for a purchase price of $1.32 per share, for gross proceeds of $4.0 million and recognized at their fair value of $17.9 million. Immediately after the second operating cash closing in December 2020, the Series A Agreement was terminated; as such, as of December 31, 2020, no additional Series A shares were authorized to be issued. No terms of the existing Series A shares were amended in conjunction with these transactions, and the operating cash closes were intended to provide for a certain investor to be able to purchase additional preferred stock shares outside of those issued in conjunction with milestone achievement and to provide the Company with cash to cover operating expenses. In January 2021, the Company entered into the Series B Preferred Stock Purchase Agreement (“Series B Agreement”) whereby the Company agreed to issue 8,741,726 shares of Series B redeemable convertible preferred stock (“Series B”) at $11.45 per share to designated investors. The certificate of incorporation was amended and restated in connection with the execution of the Series B Agreement and authorized the new Series B shares. During the nine months ended September 30, 2021, the Company issued 8,741,726 shares of Series B for gross proceeds of $100.1 million. As of September 30, 2021 and December 31, 2020, the redeemable convertible preferred stock (“preferred stock”) consisted of the following (in thousands, except share amounts): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, 2021 ​ ​ PREFERRED ​ PREFERRED STOCK ​ ​ ​ ​ ​ COMMON STOCK ​ ​ STOCK ​ ISSUED AND ​ CARRYING ​ LIQUIDATION ​ ISSUABLE UPON ​ AUTHORIZED OUTSTANDING VALUE VALUE CONVERSION Series A 22,136,987 16,734,179 ​ $ 41,289 ​ $ 22,137 16,734,179 Series B 11,564,094 8,741,726 ​ 99,892 ​ 100,100 8,741,726 Total 33,701,081 25,475,905 ​ $ 141,181 ​ $ 122,237 25,475,905 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ DECEMBER 31, 2020 ​ ​ PREFERRED ​ PREFERRED STOCK ​ ​ ​ ​ ​ COMMON STOCK ​ ​ STOCK ​ ISSUED AND ​ CARRYING ​ LIQUIDATION ​ ISSUABLE UPON ​ AUTHORIZED OUTSTANDING VALUE VALUE CONVERSION Series A 22,136,987 16,734,179 ​ $ 41,289 ​ $ 22,137 16,734,179 Series B — — ​ — ​ — — Total 22,136,987 16,734,179 ​ $ 41,289 ​ $ 22,137 16,734,179 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The holders of Series A and Series B shares have the following rights and preferences: Voting Rights The holder of each share of preferred stock shall have the right to one vote for each share of common stock into which such preferred stock could then be converted, and with respect to such vote, such holder shall have full voting rights and powers equal to the voting rights and powers of the holders of common stock, and shall be entitled, notwithstanding any provision hereof, to notice of any stockholders’ meeting in accordance with the bylaws of this corporation. Dividends The holders of shares of preferred stock are entitled to receive dividends, out of any assets legally available therefor, prior and in preference to any declaration or payment of any dividend on the common stock by the Company at a rate of 8.0% of the original issue price per annum, payable when, as and if declared by the Company’s board of directors (the “Board”). Such dividends shall not be cumulative. After payment of such dividends, any additional dividends or distributions shall be distributed among all holders of common stock and preferred stock in proportion to the number of shares of common stock that would be held by each such holder if all shares of preferred stock were converted to common stock at the then effective conversion rate. Liquidation Rights In the event of any liquidation, dissolution or winding up of the Company, whether voluntary or involuntary, asset transfer, merger or acquisition (a “Liquidation Event”), the holders of preferred stock shall be entitled to receive out of the proceeds or assets of this corporation available for distribution to its stockholders, prior and in preference to any distribution of the proceeds of such liquidation event to the holders of common stock by reason of their ownership thereof, an amount per share equal to the sum of the applicable original issue price. Conversion Each share of preferred stock shall automatically be converted into shares of common stock at the conversion rate at the time in effect for such series of preferred stock immediately upon the earlier of: (i) the closing of the Company’s sale of its common stock in a firm commitment underwritten public offering pursuant to an effective registration statement on Form S-1 under the Securities Act of 1933, as amended, that results in at least $30 million of gross proceeds and in connection with such offering the common stock is listed for trading on the Nasdaq Stock Market’s National Market, the New York Stock Exchange or another exchange or marketplace approved by the Board; or, (ii) the date, or the occurrence of an event, specified by vote or written consent or agreement of the holders of a majority of the then outstanding shares of preferred stock. The conversion price of the preferred stock will be subject to a broad-based weighted average anti-dilution adjustment in the event that the Company issues additional equity securities (other than the issuance of shares reserved under any employee incentive plan and certain other customary exceptions) at a purchase price less than the applicable conversion price. Redemption The preferred stock is not redeemable at the option of the holder thereof except for in the event of a Liquidation Event if the corporation does not effect a dissolution under the general corporation law within 90 days after such Liquidation Event.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2021
Common Stock.
Common Stock	9. Common Stock The Company was authorized to issue 41,083,993 and 29,054,797 and shares of $0.0001 par value common stock as of September 30, 2021 and as of December 31, 2020, respectively. The voting, dividend and liquidation rights of the holders of the Company’s common stock are subject to and qualified by the rights, powers and preference of the holders of the preferred stock set forth above. Voting Rights Each share of common stock entitles the holder to one vote, together with the holders of preferred stock, on all matters submitted to the stockholders for a vote. Dividends Holders of common stock shall be entitled to receive dividends if and when they are declared by the Board and after all holders of preferred stock have been paid according to their rights described in Note 8. As of September 30, 2021, no cash dividends have been declared or paid. Liquidation Rights After payment to the holders of preferred stock of their liquidation preferences, the remaining assets of the Company are distributed to the holders of common stock. Restricted Stock The Company issued restricted common stock to its founders in May 2018, which vest monthly over five years through 2023. At issuance, these shares also contained certain performance-based vesting criteria which were associated with the milestone events for the Series A shares, two of which were achieved in 2020 (see Note 8). In December 2020 in conjunction with the Series A termination, the final performance-based vesting criteria was waived, leaving only service-based vesting criteria remaining as of September 30, 2021 and as of December 31, 2020. As of the nine months ended September 30, 2021 and as of the year ended December 31, 2020, the Company has reserved the following shares of common stock for potential conversion of outstanding preferred stock, the vesting of restricted stock and the exercise of stock options: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, DECEMBER 31, ​ ​ 2021 ​ 2020 Preferred Stock 25,475,905 16,734,179 Unvested restricted stock 915,476 825,836 Options to purchase common stock 4,699,003 1,852,141 Total 31,090,384 19,412,156 ​ ​ ​ ​ ​ ​

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Stock-Based Compensation
Stock-Based Compensation	10. Stock-Based Compensation 2018 Stock Incentive Plan The Company adopted the 2018 Stock Incentive Plan (the “Plan”) on June 13, 2018 for the issuance of stock options and other stock-based awards. As of September 30, 2021 and December 31, 2020, the number of shares reserved for issuance under the Plan as approved by the Board was 6,026,908 and 2,474,920 shares, respectively. There were 981,975 and 622,779 shares available for future grant under the Plan as of September 30, 2021 and December 31, 2020, respectively. The Plan is administered by the Board. The exercise prices, vesting and other restrictions are determined at the discretion of the Board, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the common stock on the date of grant. Stock options awarded under the Plan expire ten years after the grant date unless the Board sets a shorter term. Vesting periods for awards under the plans are determined at the discretion of the Board. Stock options granted to employees and nonemployees typically vest over four years. Shares of restricted stock awards granted to employees, officers, members of the Board, advisors, and consultants of the Company typically vest over five years. Certain executives who are option holders are able to early exercise stock option awards, even prior to full vesting conditions being met. If and when this occurs, the stock option becomes outstanding restricted stock, and remains restricted until the remaining vesting terms are met. The Company can repurchase these early unvested exercised options. During the nine months ended September 30, 2021, 345,930 options to purchase common stock were exercised early and they continue to vest as restricted stock awards over the requisite service period. The Company has recognized the proceeds received of $1.4 million as a liability as of September 30, 2021. 2021 Equity Incentive Plan In September 2021, the Company’s board of directors adopted, and its stockholders approved, the Theseus Pharmaceuticals, Inc. 2021 Equity Incentive Plan (the “2021 Plan”), which became effective on October 6, 2021. The 2021 Plan provides for the grant of stock options, restricted shares, restricted stock units, and other types of equity awards. The aggregate number of shares of common stock reserved for issuance under the 2021 Plan is equal to the sum of (a) 4,395,080 shares of common stock, plus (b) up to 4,699,003 shares of common stock subject to awards granted under the Plan that are outstanding on October 6, 2021 that are subsequently forfeited, expire or lapse unexercised or unsettled and shares of common stock issued pursuant to awards granted under the Plan that are outstanding on October 6, 2021 and that are subsequently forfeited to or reacquired by the Company, and (c) the additional annual increase in shares of common stock reserved for issuance under the 2021 Plan. On the first day of each fiscal year of the Company during the term of the 2021 Plan, commencing on January 1, 2023 and concluding on (and including) January 1, 2031, the aggregate number of shares of common stock reserved for issuance under the 2021 Plan shall automatically increase by a number equal to the lesser of (a) five percent (5%) of the total number of shares of common stock actually issued and outstanding on the last day of the preceding fiscal year or (b) a number of shares of common stock determined by the Company’s board of directors. The Company shall reserve and keep available such number of shares of common stock as will be sufficient to satisfy the requirements of the 2021 Plan. Employee Stock Purchase Plan ​ In September 2021, the Company’s board of directors adopted, and its stockholders approved, the Theseus Pharmaceuticals, Inc. 2021 Employee Stock Purchase Plan (the “ESPP”), which became effective on October 6, 2021. The ESPP is administered by the Company’s board of directors or by one or more committees to which the Company’s board of directors delegates such administration. The number of shares of common stock reserved for issuance under the ESPP is 400,000, plus the additional annual increase in shares of common stock reserved for issuance under the ESPP. On the first day of each fiscal year of the Company during the term of the ESPP, commencing on January 1, 2023 and concluding on January 1, 2041, the aggregate number of shares of common stock reserved for issuance under the ESPP shall automatically increase by a number equal to the lesser of (i) one percent (1%) of the total number of shares of common stock actually issued and outstanding on the last day of the preceding fiscal year, and (ii) a number of shares of common stock determined by the Company’s board of directors. Shares of common stock issued pursuant to the ESPP may be authorized but unissued shares or treasury shares. ​ Stock Option Valuation The assumptions that the Company used in Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ FOR THE NINE MONTHS ENDED FOR THE YEAR ENDED ​ ​ SEPTEMBER 30, 2021 ​ DECEMBER 31, 2020 Risk-free interest rate 1.03% ​ 0.51% Expected term (in years) 5.2 - 6.1 ​ 5.97 - 6.00 Expected volatility 76.14% - 83.56% ​ 98.17% - 98.21% Expected dividend yield 0.00% ​ 0.00% ​ ​ ​ ​ ​ ​ A summary of option activity under the Plan during the nine months ended September 30, 2021 is as follows (in thousands except share, per share data and contractual terms): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ WEIGHTED-AVERAGE WEIGHTED-AVERAGE ​ ​ ​ ​ ​ PER-SHARE ​ REMAINING ​ AGGREGATE ​ ​ SHARES ​ EXERCISE PRICE ​ CONTRACTUAL TERM ​ INTRINSIC VALUE Outstanding as of December 31, 2020 ​ 1,852,141 ​ $ 0.32 9.98 ​ $ 6,566 Granted ​ 3,195,931 ​ 5.10 ​ Exercised ​ (345,930) ​ 4.03 ​ 569 Cancelled ​ (3,139) ​ ​ 4.03 ​ ​ ​ ​ ​ Outstanding as of September 30, 2021 ​ 4,699,003 ​ $ 3.29 9.48 ​ 43,079 Options vested and exercisable as of September 30, 2021 ​ 421,850 ​ $ 1.17 9.21 ​ $ 4,762 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ There were no options vested as of December 31, 2020. 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 for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The weighted-average per share grant date fair value of options granted during the nine months ended September 30, 2021 and 2020 was $4.67 and $0, respectively. Stock-based compensation expense for options granted of $2.5 million was recorded for the nine months ended September 30, 2021. As of September 30, 2021, there was $18.9 million of unrecognized stock-based compensation expense related to unvested stock options. The unrecognized stock-based compensation expense is estimated to be recognized over a period of 3.4 years as of September 30, 2021. The total fair value of options vested during the nine months ended September 30, 2021 and 2020 was $1.5 million and $0, respectively. Shares of Restricted Common Stock The Company has granted shares of restricted common stock with the number of shares that vest based on the lesser of the time-based vesting over five years or the achievement of certain milestones. A summary of restricted stock activity under the Plan during the nine months ended September 30, 2021 and the year ended December 31, 2020 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ WEIGHTED-AVERAGE ​ ​ ​ ​ PER-SHARE GRANT-DATE ​ ​ SHARES ​ FAIR VALUE Unvested shares at December 31, 2020 825,836 ​ $ 0.83 Vested (256,290) ​ 0.83 Unvested shares issued as a result of early option exercise 345,930 ​ 2.14 Unvested shares at September 30, 2021 915,476 ​ $ 2.66 ​ ​ ​ ​ ​ ​ ​ In 2019, the restricted common stock awards were modified to add a new performance condition for the operating cash closing of the Series A financing, which was ultimately achieved in 2019. In 2020, the restricted common stock awards were modified to waive the final requirement to achieve a certain milestone. As a result, as of September 30, 2021, all of the unvested restricted common stock awards are only subject to time-based vesting through the end of the requisite service period. As of September 30, 2021 and December 31, 2020, respectively, there was $0.1 million and $0.2 million of unrecognized stock-based compensation expense related to unvested restricted stock. The unrecognized stock-based compensation expense is estimated to be recognized over a period of 1.7 years as of September 30, 2021. The total fair value of restricted stock vested during the nine months ended September 30, 2021 and the year ended December 31, 2020 was $2.6 million and $1.9 million, respectively. The Company recorded stock-based compensation expense for restricted stock of $0.3 million during both the nine months ended September 30, 2021 and 2020. Stock-based Compensation Expense Total stock-based compensation expense recorded as research and development and general and administrative expenses, respectively, for employees, directors and non-employees during the three and nine months ended September 30, 2021 and 2020 was as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ THREE MONTHS ENDED SEPTEMBER 30, ​ NINE MONTHS ENDED SEPTEMBER 30, ​ 2021 2020 2021 2020 Research and development ​ $ 585 ​ $ 26 ​ $ 1,479 ​ $ 207 General and administrative ​ ​ 708 ​ ​ 8 ​ ​ 1,298 ​ ​ 61 ​ ​ $ 1,293 ​ $ 34 ​ $ 2,777 ​ $ 268 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Taxes
Income Taxes	11. Income Taxes Income taxes for the nine months ended September 30, 2021 and 2020 have been calculated based on an estimated annual effective tax rate and certain discrete items. For the nine months ended September 30, 2021 and 2020, no income tax was recorded, as the Company is forecasting an ordinary pre-tax loss for the year ending December 31, 2021 and maintains a full valuation allowance against its net deferred tax assets. The Company has never been examined by the Internal Revenue Service or any other jurisdiction for any tax years and, as such, all years within the applicable statutes of limitations are potentially subject to audit. ​

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions
Related Party Transactions	12. Related Party Transactions ​ Iain Dukes is a founding member of the Company and has served as a member of the Board since June 2018. From June 2018 until April 2021, Dr. Dukes served as the Company's Chief Executive Officer under a consulting agreement, and from April 2021 until September 2021, Dr. Dukes served as the Executive Chairman. On September 15, 2021, Dr. Dukes transitioned into his role as Chairman of the Board. During the nine months ended September 30, 2021, Dr. Dukes earned compensation of $0.2 million, of which $30,000 was payable as of period end. As of September 30, 2021, Dr. Dukes owned 388,324 shares of the Company's common stock, and held options to purchase an additional 441,959 shares of common stock.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2021
Net Loss Per Share
Net Loss Per Share	13. Net Loss Per Share Basic and diluted loss per share is computed by dividing net loss attributable to common stockholders by the weighted-average common shares outstanding (in thousands, except share and per share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ THREE MONTHS ENDED SEPTEMBER 30, ​ NINE MONTHS ENDED SEPTEMBER 30, ​ 2021 2020 2021 2020 Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss ​ $ (7,372) ​ $ (1,429) ​ $ (18,652) ​ $ (9,107) Net loss attributable to common stockholders - basic and diluted ​ $ (7,372) ​ $ (1,429) ​ $ (18,652) ​ $ (9,107) Denominator: ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common stock outstanding - basic and diluted ​ 1,428,054 ​ 672,911 ​ 997,949 ​ 502,804 Net loss per share attributable to common stockholders - basic and diluted ​ $ (5.16) ​ $ (2.12) ​ $ (18.69) ​ $ (18.11) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The Company’s potentially dilutive securities, which include preferred stock, unvested restricted stock and stock options, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted-average number of shares of common stock outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following from the computation of diluted net loss per share attributable to common stockholders for the nine months ended September 30, 2021 and 2020 because including them would have had an anti-dilutive effect: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ NINE MONTHS ENDED SEPTEMBER 30, ​ ​ 2021 2020 Preferred Stock ​ 25,475,905 ​ 13,710,427 ​ Unvested restricted stock 915,476 984,567 Options to purchase common stock ​ 4,699,003 ​ — ​ ​ 31,090,384 14,694,994 ​ ​ ​ ​ ​ ​ ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Summary of Significant Accounting Policies
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying condensed balance sheet as of September 30, 2021, and the condensed statements of operations and comprehensive loss, redeemable convertible preferred stock and stockholders’ deficit for the three- and nine-month periods ended September 30, 2021 and 2020 and statements of cash flows for the nine months ended September 30, 2021 and 2020 are unaudited. The condensed interim financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of September 30, 2021 and the results of its operations for the three- and nine-month periods ended September 30, 2021 and 2020 and its cash flows for the nine months ended September 30, 2021 and 2020. The financial data and other information disclosed in these notes related to the three- and nine-month periods ended September 30, 2021 and 2020 are also unaudited. The results for the three- and nine-month periods ended September 30, 2021 are not necessarily indicative of results to be expected for the full year or for any other subsequent interim period.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Estimates and judgments are based on historical information and other market-specific or various relevant assumptions, including in certain circumstances, future projections, that management believes to be reasonable under the circumstances. Actual results could differ materially from estimates. Significant estimates and assumptions are used for, but not limited to, the accruals for research and development expenses, the determination of fair value of equity instruments, preferred stock tranche rights and the anti-dilution right, and for periods prior to the completion of the IPO, stock-based compensation expense.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash includes cash in readily available checking accounts. Cash is carried at cost, which approximates its fair value. The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents are carried at fair market value.
Restricted Cash	Restricted Cash Restricted cash consists of a restricted cash deposit of $0.4 million which serves as collateral for a letter of credit issued to the landlord of the Company’s leased facility for a security deposit upon entering into the lease in September 2021. The Company classified this amount as restricted cash in the accompanying condensed balance sheet within other assets as of September 30, 2021. As of December 31, 2020, the Company held no restricted cash.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. For the three and nine months ended September 31, 2021 and 2020, there were no differences between net loss and comprehensive loss.
Deferred Issuance Costs	Deferred Issuance Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred issuance costs until such financings are consummated. After consummation of such an equity financing, these costs are recorded as a reduction of the proceeds generated as a result of the offering. Should the planned equity financing be abandoned, the deferred issuance costs, currently recorded within other assets, will be expensed immediately as a charge to operating expenses in the statements of operations and comprehensive loss. The Company recorded deferred issuance costs of $3.1 million within other assets as of September 30, 2021, and recorded no deferred issuance costs as of December 31, 2020.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial statements and disclosures. ​ In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) .

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value of Financial Assets and Liabilities
Schedule of assets and liabilities measured at fair value on a recurring basis	Assets and liabilities measured at fair value on a recurring basis are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, ​ ​ ​ DESCRIPTION ​ 2021 ​ LEVEL 1 ​ LEVEL 2 ​ LEVEL 3 Assets ​ ​ ​ ​ ​ Cash equivalents ​ $ 89,526 ​ $ 89,526 ​ $ — ​ $ — Total financial assets ​ $ 89,526 ​ $ 89,526 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of significant quantitative inputs used in the valuation of the Tranche Rights and the Anti-dilution Right	​ ​ ​ ​ ​ ​ ​ ​ FOR THE YEAR ENDED ​ ​ DECEMBER 31, 2020 Stand-alone Series A Preferred Stock price (spot price) ​ $1.84 - $5.93 Risk-free rate ​ 1.6% Discount factor ​ 0.9923 - 1.0 Time to milestone event (years) ​ 0.0 years - 0.5 years Probability of tranche closing ​ 0% - 100% ​ ​ ​
Schedule of of the aggregate fair value of the company's tranche rights and anti-dilution right	The following table provides a rollforward of the aggregate fair value of the Company’s Tranche Rights and Anti-dilution Right (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ PREFERRED STOCK ANTI-DILUTION ​ ​ TRANCHE RIGHTS ​ RIGHT Balance as of December 31, 2019 ​ $ 2,220 ​ $ 1,142 Change in fair value ​ 3,969 ​ 1,190 Fair value recognized as Series A upon settlement of right ​ (6,189) ​ (2,332) Balance as of December 31, 2020 ​ $ — ​ $ — ​ ​ ​ ​ ​ ​ ​

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2021
Accrued Expenses
Schedule of Accrued Expenses	Accrued expenses consisted of the following as of September 30, 2021 and as of December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, DECEMBER 31, ​ ​ 2021 ​ 2020 Accrued research and development ​ $ 664 ​ $ 310 Accrued legal ​ 112 ​ 143 Accrued compensation and benefits ​ 738 ​ 10 Accrued other ​ 1,280 ​ — Restricted stock liability, current ​ 494 ​ — Total accrued expenses and other current liabilities ​ $ 3,288 ​ $ 463 ​ ​ ​ ​ ​ ​ ​

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	9 Months Ended
	Sep. 30, 2021
Property and Equipment, net
Schedule of Property and Equipment, net	Property and equipment, net consisted of the following as of September 30, 2021 and as of December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, DECEMBER 31, ​ ​ 2021 ​ 2020 Computer equipment ​ $ — ​ $ 6 Less: accumulated depreciation ​ — ​ (5) Property and equipment, net ​ $ — ​ $ 1 ​ ​ ​ ​ ​ ​ ​

Commitments and Contingencies -

Commitments and Contingencies - Future minimum payments (Tables)	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies.
Schedule of future minimum payments required under the lease	​ ​ ​ ​ ​ Year Ending September 30, ​ ​ 2021 (3 months) ​ $ — 2022 ​ ​ 757 2023 ​ ​ 780 2024 ​ ​ 803 2025 ​ ​ 827 Thereafter ​ ​ 2,635 Total minimum lease payments ​ $ 5,802

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2021
Redeemable Convertible Preferred Stock
Schedule of redeemable convertible preferred stock	As of September 30, 2021 and December 31, 2020, the redeemable convertible preferred stock (“preferred stock”) consisted of the following (in thousands, except share amounts): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, 2021 ​ ​ PREFERRED ​ PREFERRED STOCK ​ ​ ​ ​ ​ COMMON STOCK ​ ​ STOCK ​ ISSUED AND ​ CARRYING ​ LIQUIDATION ​ ISSUABLE UPON ​ AUTHORIZED OUTSTANDING VALUE VALUE CONVERSION Series A 22,136,987 16,734,179 ​ $ 41,289 ​ $ 22,137 16,734,179 Series B 11,564,094 8,741,726 ​ 99,892 ​ 100,100 8,741,726 Total 33,701,081 25,475,905 ​ $ 141,181 ​ $ 122,237 25,475,905 ​

Common Stock (Tables)

Common Stock (Tables)	9 Months Ended
	Sep. 30, 2021
Common Stock.
Schedule of common reserved shares of common stock for issuance	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ SEPTEMBER 30, DECEMBER 31, ​ ​ 2021 ​ 2020 Preferred Stock 25,475,905 16,734,179 Unvested restricted stock 915,476 825,836 Options to purchase common stock 4,699,003 1,852,141 Total 31,090,384 19,412,156 ​ ​ ​ ​ ​

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Stock-Based Compensation
Schedule of stock option valuation assumptions	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ FOR THE NINE MONTHS ENDED FOR THE YEAR ENDED ​ ​ SEPTEMBER 30, 2021 ​ DECEMBER 31, 2020 Risk-free interest rate 1.03% ​ 0.51% Expected term (in years) 5.2 - 6.1 ​ 5.97 - 6.00 Expected volatility 76.14% - 83.56% ​ 98.17% - 98.21% Expected dividend yield 0.00% ​ 0.00% ​ ​ ​ ​ ​
Schedule of summary of option activity	A summary of option activity under the Plan during the nine months ended September 30, 2021 is as follows (in thousands except share, per share data and contractual terms): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ WEIGHTED-AVERAGE WEIGHTED-AVERAGE ​ ​ ​ ​ ​ PER-SHARE ​ REMAINING ​ AGGREGATE ​ ​ SHARES ​ EXERCISE PRICE ​ CONTRACTUAL TERM ​ INTRINSIC VALUE Outstanding as of December 31, 2020 ​ 1,852,141 ​ $ 0.32 9.98 ​ $ 6,566 Granted ​ 3,195,931 ​ 5.10 ​ Exercised ​ (345,930) ​ 4.03 ​ 569 Cancelled ​ (3,139) ​ ​ 4.03 ​ ​ ​ ​ ​ Outstanding as of September 30, 2021 ​ 4,699,003 ​ $ 3.29 9.48 ​ 43,079 Options vested and exercisable as of September 30, 2021 ​ 421,850 ​ $ 1.17 9.21 ​ $ 4,762 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of summary of restricted stock activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ WEIGHTED-AVERAGE ​ ​ ​ ​ PER-SHARE GRANT-DATE ​ ​ SHARES ​ FAIR VALUE Unvested shares at December 31, 2020 825,836 ​ $ 0.83 Vested (256,290) ​ 0.83 Unvested shares issued as a result of early option exercise 345,930 ​ 2.14 Unvested shares at September 30, 2021 915,476 ​ $ 2.66 ​ ​ ​ ​ ​ ​
Schedule of stock-based compensation expense	Total stock-based compensation expense recorded as research and development and general and administrative expenses, respectively, for employees, directors and non-employees during the three and nine months ended September 30, 2021 and 2020 was as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ THREE MONTHS ENDED SEPTEMBER 30, ​ NINE MONTHS ENDED SEPTEMBER 30, ​ 2021 2020 2021 2020 Research and development ​ $ 585 ​ $ 26 ​ $ 1,479 ​ $ 207 General and administrative ​ ​ 708 ​ ​ 8 ​ ​ 1,298 ​ ​ 61 ​ ​ $ 1,293 ​ $ 34 ​ $ 2,777 ​ $ 268 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Net Loss Per Share
Schedule of basic and diluted loss per share	Basic and diluted loss per share is computed by dividing net loss attributable to common stockholders by the weighted-average common shares outstanding (in thousands, except share and per share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ THREE MONTHS ENDED SEPTEMBER 30, ​ NINE MONTHS ENDED SEPTEMBER 30, ​ 2021 2020 2021 2020 Numerator: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss ​ $ (7,372) ​ $ (1,429) ​ $ (18,652) ​ $ (9,107) Net loss attributable to common stockholders - basic and diluted ​ $ (7,372) ​ $ (1,429) ​ $ (18,652) ​ $ (9,107) Denominator: ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common stock outstanding - basic and diluted ​ 1,428,054 ​ 672,911 ​ 997,949 ​ 502,804 Net loss per share attributable to common stockholders - basic and diluted ​ $ (5.16) ​ $ (2.12) ​ $ (18.69) ​ $ (18.11) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of antidilutive securities excluded from computation of diluted net loss per share	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ NINE MONTHS ENDED SEPTEMBER 30, ​ ​ 2021 2020 Preferred Stock ​ 25,475,905 ​ 13,710,427 ​ Unvested restricted stock 915,476 984,567 Options to purchase common stock ​ 4,699,003 ​ — ​ ​ 31,090,384 14,694,994 ​ ​ ​ ​ ​ ​

Nature of the Business (Details

Nature of the Business (Details) $ / shares in Units, $ in Thousands	Oct. 25, 2021USD ($)$ / sharesshares	Oct. 06, 2021USD ($)$ / sharesshares	Sep. 27, 2021	Sep. 30, 2021USD ($)$ / sharesshares	Jun. 30, 2021USD ($)	Mar. 31, 2021USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Sep. 30, 2021USD ($)$ / sharesshares	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)$ / sharesshares
Reverse split			1.32286
Common stock, par value | $ / shares				$ 0.0001						$ 0.0001		$ 0.0001
Common stock, shares authorized				41,083,993						41,083,993		29,054,797
Redeemable convertible preferred stock, shares authorized				33,701,081						33,701,081		22,136,987
Accumulated deficit | $				$ (52,922)						$ (52,922)		$ (34,270)
Net loss | $				(7,372)	$ (6,807)	$ (4,473)	$ (1,429)	$ (5,413)	$ (2,265)	(18,652)	$ (9,107)
Net cash used in operations | $										(17,279)	(3,614)
Cash and cash equivalents | $				$ 90,404			$ 5,697			$ 90,404	$ 5,697	$ 8,457
IPO
Shares issued		10,000,200
Share price per share | $ / shares		$ 16
Aggregate gross proceeds | $		$ 160,000
Shares converted to common stock		25,475,905
Over-Allotment Option
Number of shares	1,171,990
Aggregate gross proceeds | $	$ 18,800
Common stock, par value | $ / shares	$ 16
Common stock, shares authorized		500,000,000
Redeemable convertible preferred stock, shares authorized		50,000,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Restricted cash deposit	$ 400,000	$ 0
Other assets
Deferred issuance costs	$ 3,100,000	$ 0

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Recurring basis (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020	Jul. 31, 2020
Assets
Total financial assets		$ 0
Liabilities measured at fair value		0
Transfer of assets from level 1 to level 2	$ 0	0
Transfer of assets from level 2 to level 1			$ 0
Transfer of liabilities from level 1 to level 2	0	0
Transfer of liabilities from level 2 to level 1	0	$ 0
Recurring
Assets
Cash equivalents	89,526,000
Total financial assets	89,526,000
Recurring | Level 1
Assets
Cash equivalents	89,526,000
Total financial assets	$ 89,526,000

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Narrative (Details) - SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK	1 Months Ended				3 Months Ended
	Jul. 31, 2020$ / sharesshares	Jun. 30, 2020shares	Jan. 31, 2020$ / sharesshares	Jun. 30, 2018tranche$ / sharesshares	Sep. 30, 2020shares	Mar. 31, 2020shares
Fair Value of Financial Assets and Liabilities
Number of subsequent milestone-based tranches | tranche				3
Number of shares issued					4,401,006	2,640,603
Certain investors
Fair Value of Financial Assets and Liabilities
Number of shares issued	3,779,688	3,779,688	2,267,813	3,779,688
Share price per share | $ / shares	$ 1.32		$ 1.32	$ 1.32
ARIAD Pharmaceuticals, Inc
Fair Value of Financial Assets and Liabilities
Number of shares issued	621,318		372,791	621,318

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Significant quantitative inputs used in the valuation (Details)	Dec. 31, 2020Y$ / shares
Stand-alone Series A Preferred Stock price (spot price) | Minimum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input | $ / shares	1.84
Stand-alone Series A Preferred Stock price (spot price) | Maximum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input | $ / shares	5.93
Risk-free rate
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input	0.016
Discount factor | Minimum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input	0.9923
Discount factor | Maximum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input	1
Time to milestone event (years) | Minimum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input | Y	0
Time to milestone event (years) | Maximum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input | Y	0.5
Probability of tranche closing | Minimum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input	0
Probability of tranche closing | Maximum
Fair Value of Financial Assets and Liabilities
Tranche rights and the anti-dilution right measurement input	1

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Aggregate fair value of the company's tranche rights and anti-dilution right (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Preferred stock tranche rights
Roll-forward of the aggregate fair value of the company
Balance as of December 31, 2019	$ 2,220
Change in fair value	3,969
Fair value recognized as Series A upon settlement of right	(6,189)
Anti-dilution right
Roll-forward of the aggregate fair value of the company
Balance as of December 31, 2019	1,142
Change in fair value	1,190
Fair value recognized as Series A upon settlement of right	$ (2,332)

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accrued Expenses
Accrued research and development	$ 664	$ 310
Accrued legal	112	143
Accrued compensation and benefits	738	10
Accrued other	1,280
Restricted stock liability, current	494
Total accrued expenses and other current liabilities	$ 3,288	$ 463

Property and Equipment, net (De

Property and Equipment, net (Details) $ in Thousands	Dec. 31, 2020USD ($)
Property and Equipment, net
Less: accumulated depreciation	$ (5)
Property and equipment, net	1
Computer equipment
Property and Equipment, net
Computer equipment	$ 6

Property and Equipment, net - A

Property and Equipment, net - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Property and Equipment, net
Depreciation expense	$ 1,000	$ 2,000
Impairments recorded to date	$ 0

License Agreements (Details)

License Agreements (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2018
License Agreements [Line Items]
Cost of license included in research and development expense	$ 4,996	$ 1,275		$ 13,306	$ 3,470
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
License Agreements [Line Items]
Number of shares issued		4,401,006	2,640,603
License agreement with ARIAD
License Agreements [Line Items]
Transaction price				$ 1,400
Cost of license included in research and development expense						$ 1,400
Minimum threshold period of royalty term				10 years
License agreement with ARIAD | SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
License Agreements [Line Items]
Number of shares issued				621,318

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Sep. 16, 2021USD ($)ft²
Commitments and Contingencies.
Office space leased | ft²	7,351
Lease term	7 years
Option to extend	true
Renewal term	5 years
Lease expense for the first year	$ 0.8
Lease subject to annual increase (as percentage)	3.00%
Security deposit in the form of line of credit	$ 0.4

Commitments and Contingencies_2

Commitments and Contingencies - Future minimum payments (Details) $ in Thousands	Sep. 30, 2021USD ($)
Commitments and Contingencies.
2022	$ 757
2023	780
2024	803
2025	827
Thereafter	2,635
Total minimum lease payments	$ 5,802

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended							3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 31, 2021$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares	Jul. 31, 2020USD ($)Milestone$ / sharesshares	Jun. 30, 2020shares	Jan. 31, 2020USD ($)Milestone$ / sharesshares	Aug. 31, 2019USD ($)$ / sharesshares	Jun. 30, 2018USD ($)Milestonetranche$ / sharesshares	Mar. 31, 2021shares	Sep. 30, 2020shares	Mar. 31, 2020shares	Sep. 30, 2021USD ($)shares	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)$ / sharesshares
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable Convertible Preferred Stock
Shares authorized for issuance | shares							13,203,025
Number of subsequent milestone-based tranches | tranche							3
Number of shares issued | shares									4,401,006	2,640,603
Proceeds from issuance | $												$ 7,994
Issuance costs | $											$ 3
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK | Certain investors
Redeemable Convertible Preferred Stock
Number of shares issued | shares			3,779,688	3,779,688	2,267,813		3,779,688
Share price per share | $ / shares			$ 1.32		$ 1.32		$ 1.32
Proceeds from issuance | $			$ 8,000		$ 8,000		$ 4,700
Issuance costs | $							$ 300
Number of additional tranches closings based on achievement | tranche							3
Number of defined milestone | Milestone							3
Number of additional shares issued | shares							7,559,376
Number of milestones achieved | Milestone			2		2
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK | Majority shareholders
Redeemable Convertible Preferred Stock
Shares authorized for issuance | shares						2,267,812							0
Share price per share | $ / shares		$ 1.32				$ 1.32							$ 1.32
Gross proceeds | $		$ 4,000				$ 3,000
Number of additional shares issued | shares		3,023,751											3,023,751
Fair value | $		$ 17,900				$ 2,600							$ 17,900
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK | ARIAD Pharmaceuticals, Inc
Redeemable Convertible Preferred Stock
Number of shares issued | shares			621,318		372,791		621,318
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable Convertible Preferred Stock
Number of shares issued | shares								8,741,726			8,741,726
Proceeds from issuance | $											$ 99,892
Gross proceeds | $											100,100
Issuance costs | $											$ 208
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK | Designated investors
Redeemable Convertible Preferred Stock
Number of shares issued | shares	8,741,726
Share price per share | $ / shares	$ 11.45

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Classifications (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Redeemable Convertible Preferred Stock
PREFERRED STOCK AUTHORIZED	33,701,081			22,136,987
PREFERRED STOCK ISSUED	25,475,905			16,734,179
PREFERRED STOCK OUTSTANDING	25,475,905			16,734,179
CARRYING VALUE	$ 141,181			$ 41,289
LIQUIDATION VALUE	$ 122,237			$ 22,137
COMMON STOCK ISSUABLE UPON CONVERSION	25,475,905			16,734,179
SERIES A REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable Convertible Preferred Stock
PREFERRED STOCK AUTHORIZED	22,136,987			22,136,987
PREFERRED STOCK ISSUED	16,734,179			16,734,179
PREFERRED STOCK OUTSTANDING	16,734,179	16,734,179	16,734,179	16,734,179	13,710,427	9,309,421	9,309,421	6,668,818
CARRYING VALUE	$ 41,289	$ 41,289	$ 41,289	$ 41,289	$ 23,377	$ 11,712	$ 11,712	$ 6,857
LIQUIDATION VALUE	$ 22,137			$ 22,137
COMMON STOCK ISSUABLE UPON CONVERSION	16,734,179			16,734,179
SERIES B REDEEMABLE CONVERTIBLE PREFERRED STOCK
Redeemable Convertible Preferred Stock
PREFERRED STOCK AUTHORIZED	11,564,094			11,564,094
PREFERRED STOCK ISSUED	8,741,726			0
PREFERRED STOCK OUTSTANDING	8,741,726	8,741,726	8,741,726	0
CARRYING VALUE	$ 99,892	$ 99,892	$ 99,892
LIQUIDATION VALUE	$ 100,100
COMMON STOCK ISSUABLE UPON CONVERSION	8,741,726

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock - Rights and preferences (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)Vote
Redeemable Convertible Preferred Stock
Voting rights per share | Vote	1
Dividends rate	8.00%
Minimum gross proceeds | $	$ 30

Common Stock (Details)

Common Stock (Details)	9 Months Ended
	Sep. 30, 2021USD ($)Vote$ / sharesshares	Dec. 31, 2020Milestone$ / sharesshares
Common stock, authorized | shares	41,083,993	29,054,797
Common stock, par value | $ / shares	$ 0.0001	$ 0.0001
Number of votes per common stock | Vote	1
Common stock, cash dividend | $	$ 0
Restricted Common Stock
Vesting period	5 years
Number of milestone events achieved | Milestone		2

Common Stock - Reserve for futu

Common Stock - Reserve for future issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020
Common Stock.
Preferred Stock	25,475,905	16,734,179
Unvested restricted stock	915,476	825,836
Options to purchase common stock	4,699,003	1,852,141
Total	31,090,384	19,412,156

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - USD ($) $ in Thousands	Oct. 06, 2021	Sep. 30, 2021	Dec. 31, 2020
Stock-Based Compensation
Shares reserved under the Plan		31,090,384	19,412,156
Proceeds from options exercised		$ 1,395
Options to purchase common stock
Stock-Based Compensation
Options to purchase common stock, exercised		345,930
Options granted		3,195,931
Stock-based compensation expense		$ 2,500
Restricted Common Stock
Stock-Based Compensation
Vesting period		5 years
2018 Stock Incentive Plan
Stock-Based Compensation
Shares reserved under the Plan		6,026,908	2,474,920
Shares available for future grant under the Plan		981,975	622,779
Threshold percentage of exercise price		100.00%
Expiration period		10 years
Options to purchase common stock, exercised		345,930
Proceeds from options exercised		$ 1,400
2018 Stock Incentive Plan | Options to purchase common stock
Stock-Based Compensation
Vesting period		4 years
2018 Stock Incentive Plan | Restricted Common Stock
Stock-Based Compensation
Vesting period		5 years
2021 Plan | Subsequent event
Stock-Based Compensation
Common stock reserved for issuance	4,395,080
Common stock granted	4,699,003
Percentage increase of common stock reserved for issuance	5.00%
2021 ESPP | Subsequent event
Stock-Based Compensation
Common stock reserved for issuance	400,000
Percentage increase of common stock reserved for issuance	1.00%

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Valuation (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Stock-Based Compensation
Risk-free interest rate	1.03%	0.51%
Expected volatility, Minimum	76.14%	98.17%
Expected volatility, Maximum	83.56%	98.21%
Expected dividend yield	0.00%	0.00%
Minimum
Stock-Based Compensation
Expected term (in years)	5 years 2 months 12 days	5 years 11 months 19 days
Maximum
Stock-Based Compensation
Expected term (in years)	6 years 1 month 6 days	6 years

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of option activity (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Shares
Outstanding as of beginning of period (in shares)	1,852,141
Outstanding as of end of period (in shares)	4,699,003		1,852,141
Options to purchase common stock
Shares
Outstanding as of beginning of period (in shares)	1,852,141
Granted (in shares)	3,195,931
Exercised (in shares)	(345,930)
Cancelled (in shares)	(3,139)
Outstanding as of end of period (in shares)	4,699,003		1,852,141
Options vested and exercisable (in shares)	421,850
Weighted-average per-share exercise price
Outstanding as of beginning of period (in per share)	$ 0.32
Granted (in per share)	5.10
Exercised (in per share)	4.03
Cancelled (in per share)	4.03
Outstanding as of end of period (in per share)	3.29		$ 0.32
Options vested and exercisable (in per share)	$ 1.17
Additional Disclosures
Weighted-average remaining contractual term	9 years 5 months 23 days		9 years 11 months 23 days
Weighted-average remaining contractual term, vested and exercisable	9 years 2 months 15 days
Aggregate intrinsic value	$ 43,079,000		$ 6,566,000
Aggregate intrinsic value, Exercised	569,000
Aggregate intrinsic value, vested and exercisable	$ 4,762,000
Options vested			0
Weighted-average per share grant date fair value of options granted	$ 4.67	$ 0
Unrecognized stock-based compensation expense related to unvested stock options	$ 18,900,000
Stock-based compensation expense	$ 2,500,000
Recognition period	3 years 4 months 24 days
Total fair value of options vested	$ 1,500,000	$ 0

Stock-Based Compensation - Shar

Stock-Based Compensation - Shares of Restricted Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Shares
Unvested shares, Beginning balance (in shares)				825,836
Unvested shares, Ending balance (in shares)	915,476	825,836		915,476		825,836
Weighted-average per-share grant-date fair value
Stock-based compensation expense	$ 1,293		$ 34	$ 2,777	$ 268
Restricted Common Stock
Stock-Based Compensation
Vesting period				5 years
Shares
Unvested shares, Beginning balance (in shares)				825,836
Vested (in shares)				(256,290)
Unvested shares issued as a result of early option exercise (in shares)				345,930
Unvested shares, Ending balance (in shares)	915,476	825,836		915,476		825,836
Weighted-average per-share grant-date fair value
Unvested shares, Beginning balance (in per share)				$ 0.83
Vested (in per share)				0.83
Unvested shares issued as a result of early option exercise (in per share)				2.14
Unvested shares, Ending balance (in per share)	$ 2.66	$ 0.83		$ 2.66		$ 0.83
Unrecognized stock-based compensation expense related to unvested restricted stock	$ 100	$ 200		$ 100		$ 200
Recognition period				1 year 8 months 12 days
Total fair value of restricted stock vested				$ 2,600		$ 1,900
Stock-based compensation expense		$ 300		$ 300

Stock-Based Compensation - Expe

Stock-Based Compensation - Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Stock-Based Compensation
Stock-based compensation expense	$ 1,293	$ 34	$ 2,777	$ 268
Research and development
Stock-Based Compensation
Stock-based compensation expense	585	26	1,479	207
General and administrative
Stock-Based Compensation
Stock-based compensation expense	$ 708	$ 8	$ 1,298	$ 61

Income Taxes (Details)

Income Taxes (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Income Taxes
Income tax	$ 0	$ 0

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Related Party Transactions
Common stock, shares issued	2,054,555	1,708,625
Chief Executive Officer
Related Party Transactions
Amount of earned compensation	$ 200,000
Amount of earned compensation payable	$ 30,000
Common stock, shares issued	388,324
Options to purchase additional shares of common stock (in shares)	441,959

Net Loss Per Share - Basic and

Net Loss Per Share - Basic and diluted loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net loss	$ (7,372)	$ (6,807)	$ (4,473)	$ (1,429)	$ (5,413)	$ (2,265)	$ (18,652)	$ (9,107)
Net loss attributable to common stockholders - basic	(7,372)			(1,429)			(18,652)	(9,107)
Net loss attributable to common stockholders - diluted	$ (7,372)			$ (1,429)			$ (18,652)	$ (9,107)
Denominator:
Weighted-average common stock outstanding-basic	1,428,054			672,911			997,949	502,804
Net loss per share attributable to common stockholders-basic	$ (5.16)			$ (2.12)			$ (18.69)	$ (18.11)

Net Loss Per Share - Antidiluti

Net Loss Per Share - Antidilutive securities (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded since there inclusion would be anti-dilutive	31,090,384	14,694,994
Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded since there inclusion would be anti-dilutive	25,475,905	13,710,427
Unvested restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded since there inclusion would be anti-dilutive	915,476	984,567
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded since there inclusion would be anti-dilutive	4,699,003