ARS Pharmaceuticals (SPRY) 10-Q 13 May 21 2021 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2021	May 10, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Entity Registrant Name	Silverback Therapeutics, Inc.
Entity Central Index Key	0001671858
Document Period End Date	Mar. 31, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Title of 12(b) Security	Common Stock
Trading Symbol	SBTX
Security Exchange Name	NASDAQ
Entity File Number	001-39756
Entity Tax Identification Number	81-1489190
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	500 Fairview Ave N
Entity Address, Address Line Two	Suite 600
Entity Address, City or Town	Seattle
Entity Address, State or Province	WA
Entity Address, Postal Zip Code	98109
City Area Code	206
Local Phone Number	456-2900
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		34,905,511

Condensed Balance Sheet

Condensed Balance Sheet - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 374,205	$ 386,569
Prepaid expenses and other current assets	4,141	4,087
Total current assets	378,346	390,656
Property and equipment, net	1,603	1,618
Restricted cash	350	350
Right-of-use asset	1,900	2,180
Total assets	382,199	394,804
Current liabilities:
Accounts payable	4,374	2,583
Accrued expenses	5,799	5,278
Term loan payable, net	495	844
Current portion of lease liability	927	896
Total current liabilities	11,595	9,601
Lease liability, net of current portion	2,055	2,326
Total liabilities	13,650	11,927
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred Stock, $0.0001 par value per share; 10,000,000 shares authorized at March 31, 2021 and December 31, 2020; no shares issued and outstanding at March 31, 2021 and December 31, 2020
Common stock, $0.0001 par value per share; 200,000,000 shares authorized at March 31, 2021 and December 31, 2020, 34,903,497 and 34,801,537 shares issued, and 34,827,204 and 34,701,274 shares outstanding at March 31, 2021 and December 31, 2020, respectively	3	3
Additional paid-in capital	484,147	479,608
Accumulated deficit	(115,601)	(96,734)
Total stockholders' equity	368,549	382,877
Total liabilities, and stockholders' equity	$ 382,199	$ 394,804

Condensed Balance Sheet (Parent

Condensed Balance Sheet (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock , shares authorized	10,000,000	10,000,000
Preferred stock, shares, issued	0	0
Preferred stock, shares, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares, issued	34,903,497	34,801,537
Common stock, shares, outstanding	34,827,204	34,701,274

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating expenses:
Research and development	$ 12,239	$ 4,414
General and administrative	6,646	828
Total operating expenses	18,885	5,242
Loss from operations	(18,885)	(5,242)
Interest income (expense), net	18	(37)
Net loss and comprehensive loss	$ (18,867)	$ (5,279)
Net loss per share applicable to common stockholders, basic and diluted	$ (0.54)	$ (7.89)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	34,773,950	669,033

Condensed Statements of Redeema

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Redeemable Convertible Preferred Stock [Member]
Temporary equity, Beginning balance at Dec. 31, 2019					$ 53,174
Temporary equity, Beginning balance, shares at Dec. 31, 2019					15,714,283
Beginning balance at Dec. 31, 2019	$ (58,777)		$ 5,010	$ (63,787)
Beginning balance, shares at Dec. 31, 2019		664,431
Issuance of Series B redeemable convertible preferred stock for cash, net					$ 21,458
Issuance of Series B redeemable convertible preferred stock for cash, net, Shares					10,027,666
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes					$ 10,095
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes, Shares					4,673,388
Exercise of common stock options and vesting of early exercised common stock options	6		6
Exercise of common stock options and vesting of early exercised common stock options, Shares		5,350
Stock-based compensation	47		47
Net loss and comprehensive loss	(5,279)			(5,279)
Temporary Equity, Ending balance at Mar. 31, 2020					$ 84,727
Temporary Equity, Ending balance, shares at Mar. 31, 2020					30,415,337
Ending balance at Mar. 31, 2020	(64,003)		5,063	(69,066)
Ending balance, shares at Mar. 31, 2020		669,781
Beginning balance at Dec. 31, 2020	$ 382,877	$ 3	479,608	(96,734)
Beginning balance, shares at Dec. 31, 2020	34,701,274	34,701,274
Exercise of common stock options and vesting of early exercised common stock options	$ 254		254
Exercise of common stock options and vesting of early exercised common stock options, Shares	101,960	125,930
Stock-based compensation	$ 4,285		4,285
Net loss and comprehensive loss	(18,867)			(18,867)
Ending balance at Mar. 31, 2021	$ 368,549	$ 3	$ 484,147	$ (115,601)
Ending balance, shares at Mar. 31, 2021	34,827,204	34,827,204

Condensed Statements of Redee_2

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Series B Redeemable Convertible Preferred Stock [Member]
Stock issuance costs	$ 76

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net loss	$ (18,867)	$ (5,279)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	186	139
Amortization of debt issuance costs	1	20
Stock-based compensation expense	4,285	47
Non-cash lease expense	280	262
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(54)	92
Accounts payable and accrued expenses	2,208	(2,697)
Lease liability	(240)	(212)
Net cash used in operating activities	(12,201)	(7,628)
Cash flows from investing activities:
Purchase of property and equipment	(35)	(33)
Net cash used in investing activities	(35)	(33)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		21,659
Principal payments on term loan payable	(350)	(350)
Proceeds from exercise of common stock options	222	6
Net cash (used in) provided by financing activities	(128)	21,315
Change in cash, cash equivalents, and restricted cash	(12,364)	13,654
Cash, cash equivalents, and restricted cash at beginning of period	386,919	10,526
Cash, cash equivalents, and restricted cash at end of period	374,555	24,180
Supplemental disclosure of cash flow information:
Purchases of property and equipment included in accounts payable and accrued expenses	136
Change in early exercise liability included in accounts payable and accrued expenses	$ 32
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes		10,095
Unpaid issuance costs and amounts payable to investors for redeemable converteible preferred stock included in accounts payable and accrued liabilities		$ 201

Nature of Business

Nature of Business	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Nature of Business	1. Nature of Business Silverback Therapeutics, Inc. (“Silverback” or “the Company”) is a clinical-stage biopharmaceutical company focused on leveraging its proprietary ImmunoTAC technology platform to develop systemically delivered and tissue targeted therapeutics for the treatment of cancer, chronic viral infections, and other serious diseases. The Company’s platform enables us to strategically pair proprietary linker-payloads that modulate key disease-modifying pathways with monoclonal antibodies directed at specific disease sites. The Company was formed in Seattle, Washington and incorporated in the state of Delaware on January 4, 2016. Initial Public Offering and Related Transaction On December 3, 2020, the Company’s registration statement on Form S-1 No. 333-250009) In connection with the IPO, all 77,074,884 shares of redeemable convertible preferred stock outstanding at the time of the IPO converted into 20,758,098 shares of the Company’s common stock. Risks and Uncertainties The Company is subject to a number of inherent risks which include, but are not limited to, the need to obtain adequate additional funding, possible failure of clinical trials or other events demonstrating a lack of clinical safety or efficacy of its product candidates, dependence on key personnel, reliance on third-party service providers for manufacturing drug product and conducting clinical trials, the ability to successfully secure its proprietary technology, and risks related to the regulatory approval and commercialization of a product candidate. Additionally, the development and commercialization of new drug products is highly competitive. Products or technologies developed by competitors may diminish or render obsolete the Company’s existing products under development. Liquidity and Capital Resources The accompanying financial statements have been prepared assuming that the Company will continue as a going concern. The Company has incurred net operating losses since its inception and had an accumulated deficit of $115.6 million as of March 31, 2021. The Company had cash and cash equivalents of $374.2 million as of March 31, 2021 and has not generated positive cash flows from operations. To date, the Company has funded its operations primarily through the issuance of redeemable convertible preferred stock, convertible notes, and the sale of common stock in connection with the IPO. The Company’s currently available cash and cash equivalents as of March 31, 2021 are sufficient to meet its anticipated cash requirements for the 12 months following the date the financial statements are issued. Management considers that there are no conditions or events, in the aggregate, that raise substantial doubt about the entity’s ability to continue as a going concern for a period of at least 12 months from the date the financial statements are issued. Management expects operating losses to continue for the foreseeable future. There can be no assurance that the Company will ever earn revenues or achieve profitability, or if achieved, that they will be sustained on a continuing basis. In addition, the manufacturing, clinical and preclinical development activities as well as the commercialization of the Company’s products, if approved, will require significant additional financing. The Company may be unable to secure such financing when needed, or if available, such financings may be under terms that are unfavorable to the Company or the current stockholders. If the Company is unable to raise additional funds when needed, it may be required to delay, reduce the scope of, or eliminate development programs, which may adversely affect its business and operations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“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 Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). Unaudited Interim Condensed Financial Statements The accompanying condensed balance sheet as of March 31, 2021, and condensed statements of operations and comprehensive loss, condensed statements of cash flows, and condensed statements of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three months ended March 31, 2021 and 2020, are unaudited. The balance sheet as of December 31, 2020 was derived from the audited financial statements as of and for the year ended December 31, 2020. The unaudited interim condensed financial statements have been prepared on a basis consistent with the audited annual financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, consisting solely of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2021, and the condensed results of its operations and its cash flows for the three months ended March 31, 2021 and 2020. The financial data and other information disclosed in these notes related to the three months ended March 31, 2021 and 2020 are also unaudited. The condensed results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the full year ending December 31, 2021 or any other period. Use of Estimates The preparation of the Company’s financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to accruals for research and development expenses, valuation of equity awards, and valuation allowances for deferred tax assets. These estimates and assumptions are based on current facts, historical experience and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results may differ materially and adversely from these estimates. The full extent to which the coron a concerning COVID-19 and treat COVID-19, as the COVID-19 pandemic Fair Value of Financial Instruments Cash and cash equivalents, and restricted cash are carried at fair value. Financial instruments, including accounts payable and accrued expenses are carried at cost, which approximates fair value given their short-term nature. Term loan payable is carried at cost, which approximates fair value as its effective interest rate approximates current market rates. Cash and Cash Equivalents Cash equivalents are comprised of short-term, highly-liquid investments with maturities of 90 days or less at the date of purchase. At March 31, 2021 and December 31, 2021, the Company’s cash equivalents consisted of money market funds. Restricted Cash Restricted cash consists of a deposit securing a collateral letter of credit issued in connection with the Company’s facility operating lease. The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the condensed balance sheets that sum to the amounts shown in the condensed statements of cash flows (in thousands): March 31, 1 December 31, Cash and cash equivalents $ 374,205 $ 386,569 Restricted cash 350 350 Total cash and cash equivalents and restricted cash $ 374,555 $ 386,919 Concentrations of Credit Risk The Company is subject to credit risk from holding its cash and cash equivalents at one commercial bank. The Company limits its exposure to credit losses by investing in money market funds through a U.S. bank with high credit ratings. Cash may consist of deposits held with banks that may at times exceed federally insured limits, however, exposure to credit risk in the event of default by the financial institution is limited to the extent of amounts recorded on the balance sheets. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. Leases Leases consist of the Company’s operating lease. In accordance with ASC 842, Leases, the Company determines if an arrangement is a lease at inception and evaluates each lease agreement to determine whether the lease is an operating or finance lease. For leases where the Company is the lessee, right-of-use non-lease Research and Development Expenses All research and development costs are expensed in the period incurred. Research and development expenses consist primarily of direct and indirect costs incurred in connection with the development of the Company’s ImmunoTAC technology platform, discovery efforts, and preclinical study and clinical trial activities related to the Company’s program pipeline, including the Company’s lead product candidate, SBT6050, and other pipeline programs, including SBT6290 and SBT8230. Direct costs include expenses incurred under agreements with contract research organizations (“CROs”) and other vendors that conduct the Company’s preclinical and clinical activities, expenses associated with manufacturing the Company’s product candidates including under agreements with contract development and manufacturing organizations (“CDMOs”) and other vendors, and consulting fees. Indirect costs include personnel-related expenses, consisting of employee salaries, bonuses, benefits, and stock-based compensation expense and recruiting costs for personnel engaged in research and development activities, facility and equipment related expenses, consisting of indirect and allocated expenses for rent, depreciation, and equipment maintenance, and other unallocated research and development expenses incurred in connection with the Company’s research and development programs, including laboratory materials and supplies and license fees. Research and development expenses are charged to operating expenses as incurred when these expenditures relate to the Company’s research and development efforts and have no alternative future uses. The Company is obligated to make upfront payments upon execution of certain research and development agreements. Advance payments, including nonrefundable amounts, for goods or services that will be used or rendered for future research and development activities are capitalized until such goods are delivered or the related services are performed, or such time when the Company does not expect the goods to be delivered or services to be performed. The Company estimates the period over which such services will be performed and the level of effort to be expended in each period. If actual timing of performance or the level of effort varies from the estimate, the Company will adjust the amounts recorded accordingly. Since inception, the Company has not experienced any material differences between accrued or prepaid costs and actual costs. Stock-Based Compensation The Comprehensive Loss Comprehensive loss is defined as a change in equity during a period from transactions and other events and circumstances from non-owner sources. The Company’s comprehensive loss was equal to net loss for the three months ended March 31, 2021 and 2020. Net Loss Per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive given the net loss of the Company. For purposes of this calculation, redeemable convertible preferred stock, stock options, employee stock purchase rights, and unvested common stock subject to repurchase are considered to be common stock equivalents but are not included in the calculations of diluted net loss per share for the periods presented as their effect would be antidilutive. Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with certain new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it is (1) no longer an emerging growth company or (2) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recently Issued Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company follows authoritative accounting guidance, which among other things, defines fair value, establishes a consistent framework for measuring fair value, and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as the exchange price that would be received to sell an asset or paid to transfer a liability (at exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The three levels of inputs that may be used to measure fair value include: Level 1: Quoted prices in active markets for identical assets or liabilities. The Company’s Level 1 assets consist of money market funds. Level 2: Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities in active markets or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Unobservable inputs that are supported by little or no market activity. The following table identifies the Company’s assets and liabilities that were measured at fair value on a recurring basis (in thousands): March 31, 2021 Level 1 Level 2 Level 3 Assets: Money market funds $ 374,205 $ — $ — December 31, 2020 Level 1 Level 2 Level 3 Assets: Money market funds $ 386,369 $ — $ — There were no transfers between the Level 1 and Level 2 categories or into or out of the Level 3 category during the periods presented.

Accrued Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2021
Payables and Accruals [Abstract]
Accrued Expenses	4. Accrued Expenses Accrued expenses consisted of the following (in thousands): March 31, December 31, 2021 2020 Research and development expenses $ 3,610 $ 2,063 Employee compensation and benefits 1,666 2,634 Professional services and other 523 581 Total accrued expenses $ 5,799 $ 5,278

Leases

Leases	3 Months Ended
	Mar. 31, 2021
Leases [Abstract]
Leases	5. Leases The Company leases an office and laboratory space in Seattle, Washington. The components of lease expense and related cash flows were as follows (in thousands): Three Months Ended March 31, 2021 2020 Lease expense Operating lease expense $ 347 $ 347 Variable lease expense 106 86 Total lease expense $ 453 $ 433 Operating cash outflows from operating leases $ 421 $ 397 The remaining term on the Company’s lease was 1.7 years as of March 31, 2021. To compute the present value of the lease liability, the Company used a discount rate of 8.5%. Future minimum commitments due under the operating lease agreement as of March 31, 2021 are as follows (in thousands): Years Ending December 31, Amount 2021 (remaining 9 months) $ 824 2022 2,454 Thereafter — Total undiscounted lease payments 3,278 Present value adjustment (297 ) Total present value of lease payments $ 2,981

Term Loan Payable

Term Loan Payable	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Term Loan Payable	6. Term Loan Payable In November 2016, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”) and borrowed $3.5 million as a term loan. The outstanding principal amount of the term loan accrues interest at an annual rate of 1.75% per annum. At closing, the Company incurred de minimis debt issuance costs and owed a final payment fee of $0.3 million, both of which are amortized to interest expense over the remaining term of the debt under the effective interest method. The effective interest rate of the Company’s term loan is 5.14%. Interest expense under the term loan totaled less than $0.1 million for each of the three months ended March 31, 2021 and 2020. The term loan is collateralized by the Company’s tangible and intangible assets, excluding intellectual property. The proceeds are to be used as working capital and to fund general business requirements. The agreement includes customary nonfinancial covenants and events of default that include, among other things, non-payment, inaccuracy The term loan’s original maturity date was November 1, 2020. However, in April 2020, the Company amended the loan and security agreement to defer principal payments for six months and extend the maturity date to May 1, 2021. There were no costs or additional warrant issuances in connection with this amendment. The Company accounted for the amendment as a debt modification and is amortizing the remaining debt discount over the remaining term. On May 1, 2021, the Company made its final scheduled payment to SVB under the loan and security agreement including the final payment fee.

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	3 Months Ended
	Mar. 31, 2021
Stockholders' Equity Note [Abstract]
Stockholders' Equity (Deficit)	7. Stockholders’ Equity (Deficit) Redeemable Convertible Preferred Stock Prior to its conversion to common stock in December 2020, the Company’s redeemable convertible preferred stock was classified as mezzanine equity on the Company’s balance sheets as the shares are contingently redeemable upon a deemed liquidation such as a change in control and in that event, there is no guarantee that all stockholders would be entitled to receive the same form of consideration. No accretion to redemption value was recorded during the three months ended March 31, 2020 as a deemed liquidation event was not considered probable. In March 2020, the Company issued 14,701,054 shares of its Series B redeemable convertible preferred stock, including 4,673,388 shares issued upon conversion of then outstanding convertible notes and accrued interest, and 10,027,666 shares issued for cash at a purchase price of $2.16 per share, resulting in gross proceeds of $21.5 million. The Series B purchase agreement provided that the Company would issue, and the Series B holders would purchase, an additional 21,732,862 shares of the Company’s Series B redeemable convertible preferred stock across two tranches for aggregate proceeds of $46.9 million in the event that certain agreed upon milestones were achieved or the preferred majority approved their closing. As of March 31, 2020, these future Series B tranches did not meet the definition of freestanding instruments or the definition of derivatives, therefore, they were not accounted for separately or bifurcated. Common Stock The Company has reserved shares of common stock for the following potential future issuances: March 31, December 31, 2021 2020 Shares underlying outstanding equity awards 6,308,582 6,316,569 Shares available for future equity award grants 3,152,910 1,506,806 Shares underlying early exercised equity awards 76,293 100,263 Shares underlying ESPP withholdings 19,126 4,393 Total 9,556,911 7,928,031

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Stock-based compensation expense recognized for all equity awards has been reported in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended 2021 2020 Research and development expense $ 1,925 $ 31 General and administrative expense 2,360 16 Total stock-based compensation expense $ 4,285 $ 47 Stock Option Awards As of March 31, 2021, the Company’s equity incentive plans authorized a total of Stock Options Outstanding Shares Subject to Options Outstanding Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (Years) Aggregate Intrinsic Value Balance at December 31, 2020 6,316,569 $ 10.93 9.5 $ 223,647 Granted 93,973 $ 48.03 Exercised (101,960 ) $ 2.18 $ 4,227 Cance l — $ — Balance at March 31, 2021 6,308,582 $ 11.63 9.3 $ 202,325 Vested at March 31, 2021 708,833 $ 6.97 8.7 $ 25,956 The total fair value of shares vested during the three months ended March 31, 2021 and 2020 was $3.6 million and less than $0.1 million, respectively. The aggregate intrinsic value in the table above is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the Company’s common stock for all options that were in-the-money March 31, 2021 was $33.32 The grant date fair value of stock options was estimated using a Black-Scholes option pricing model with the following weighted-average assumptions: Three Months March 31, 2021 Expected term (in years) 6.0 Expected volatility 83% Risk-free interest rate 0.72% Expected dividend yield — The fair value of stock options was determined using the Black-Scholes option-pricing model and the assumptions below. Each of these inputs is subjective and generally requires significant judgement. Fair Value of Common Stock. Expected Term. mid-point Expected Volatility. Risk-free Interest Rate. Expected Dividend Yield.

Licensing Agreement

Licensing Agreement	3 Months Ended
	Mar. 31, 2021
Licensing Agreement [Abstract]
Licensing Agreement	9. Licensing Agreement Cell Line License Agreement with WuXi Biologics (Hong Kong) Limited In October 2019, the Company entered into a cell line license agreement with WuXi Biologics (Hong Kong) Limited (“WuXi Bio”). Under the license agreement, WuXi Bio granted the Company a non-exclusive, know-how In the event the Company manufactures its commercial supplies of a product produced by the Licensed Cell Line using a manufacturer other than WuXi Bio or its affiliates, the Company will become obligated to pay WuXi Bio aggregate milestone payments, upon achievement of certain sales milestones, of up to $10.8 million. The Company has the right to terminate the license by giving at least six months prior written notice to WuXi Bio and paying all amounts due to them through the termination date. In the event the Company fails to pay all amounts due to WuXi Bio under the license agreement, and fails to pay the amounts within 30 days after receiving written notice of such failure, WuXi Bio may terminate the license with 45 days written notice to the Company. In the event either party commits a material breach under the license and fails to cure the breach within 30 days after receiving written notice from the other party of such breach, either party may terminate the license immediately upon written notice to the other party.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Legal Proceedings From time to time, the Company may have certain contingent liabilities that arise in the ordinary course of its business activities. The Company accrues a liability for such matters when it is probable that future expenditures will be made and can be reasonably estimated. Significant judgment is required to determine both probability and the estimated amount. Indemnifications In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless, and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance coverage that reduces its exposure and enables the Company to recover a portion of any future amounts paid. The Company believes the estimated fair value of these indemnification agreements in excess of applicable insurance coverage is minimal. COVID-19 The global COVID-19 COVID-19 COVID-19 non-lab COVID-19 COVID-19

Employee Benefit Plans

Employee Benefit Plans	3 Months Ended
	Mar. 31, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Employee Benefit Plans	1 1 The Company maintains a retirement plan, which is qualified under section 401(k) of the Internal Revenue Code of 1986, as amended, for the Company’s U.S. employees. The plan allows eligible employees to defer, at the employee’s discretion, pretax compensation up to the IRS annual limits. The Company does not match contributions made by employees.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	12. Net Loss Per Share Attributable to Common Stockholders The following outstanding shares of potentially dilutive securities were excluded from the computation of the diluted net loss per share attributable to common stockholders for the periods presented because their effect would have been anti-dilutive: Three Months Ended March 31, 2021 2020 Redeemable convertible preferred stock — 8,191,580 Common stock options 6,308,582 550,176 Unvested common stock 76,293 5,095 Common stock warrants — 9,154 ESPP withholdings 19,126 — Total potentially dilutive shares 6,404,001 8,756,005

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Polices)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“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 Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). Unaudited Interim Condensed Financial Statements The accompanying condensed balance sheet as of March 31, 2021, and condensed statements of operations and comprehensive loss, condensed statements of cash flows, and condensed statements of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three months ended March 31, 2021 and 2020, are unaudited. The balance sheet as of December 31, 2020 was derived from the audited financial statements as of and for the year ended December 31, 2020. The unaudited interim condensed financial statements have been prepared on a basis consistent with the audited annual financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, consisting solely of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of March 31, 2021, and the condensed results of its operations and its cash flows for the three months ended March 31, 2021 and 2020. The financial data and other information disclosed in these notes related to the three months ended March 31, 2021 and 2020 are also unaudited. The condensed results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the full year ending December 31, 2021 or any other period.
Use of Estimates	Use of Estimates The preparation of the Company’s financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to accruals for research and development expenses, valuation of equity awards, and valuation allowances for deferred tax assets. These estimates and assumptions are based on current facts, historical experience and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results may differ materially and adversely from these estimates. The full extent to which the coron a concerning COVID-19 and treat COVID-19, as the COVID-19 pandemic
Fair Value of Financial Instruments	Fair Value of Financial Instruments Cash and cash equivalents, and restricted cash are carried at fair value. Financial instruments, including accounts payable and accrued expenses are carried at cost, which approximates fair value given their short-term nature. Term loan payable is carried at cost, which approximates fair value as its effective interest rate approximates current market rates.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash equivalents are comprised of short-term, highly-liquid investments with maturities of 90 days or less at the date of purchase. At March 31, 2021 and December 31, 2021, the Company’s cash equivalents consisted of money market funds.
Restricted Cash	Restricted Cash Restricted cash consists of a deposit securing a collateral letter of credit issued in connection with the Company’s facility operating lease. The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the condensed balance sheets that sum to the amounts shown in the condensed statements of cash flows (in thousands): March 31, 1 December 31, Cash and cash equivalents $ 374,205 $ 386,569 Restricted cash 350 350 Total cash and cash equivalents and restricted cash $ 374,555 $ 386,919
Concentrations of Credit Risk	Concentrations of Credit Risk The Company is subject to credit risk from holding its cash and cash equivalents at one commercial bank. The Company limits its exposure to credit losses by investing in money market funds through a U.S. bank with high credit ratings. Cash may consist of deposits held with banks that may at times exceed federally insured limits, however, exposure to credit risk in the event of default by the financial institution is limited to the extent of amounts recorded on the balance sheets. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.
Leases	Leases Leases consist of the Company’s operating lease. In accordance with ASC 842, Leases, the Company determines if an arrangement is a lease at inception and evaluates each lease agreement to determine whether the lease is an operating or finance lease. For leases where the Company is the lessee, right-of-use non-lease
Research and Development Expenses	Research and Development Expenses All research and development costs are expensed in the period incurred. Research and development expenses consist primarily of direct and indirect costs incurred in connection with the development of the Company’s ImmunoTAC technology platform, discovery efforts, and preclinical study and clinical trial activities related to the Company’s program pipeline, including the Company’s lead product candidate, SBT6050, and other pipeline programs, including SBT6290 and SBT8230. Direct costs include expenses incurred under agreements with contract research organizations (“CROs”) and other vendors that conduct the Company’s preclinical and clinical activities, expenses associated with manufacturing the Company’s product candidates including under agreements with contract development and manufacturing organizations (“CDMOs”) and other vendors, and consulting fees. Indirect costs include personnel-related expenses, consisting of employee salaries, bonuses, benefits, and stock-based compensation expense and recruiting costs for personnel engaged in research and development activities, facility and equipment related expenses, consisting of indirect and allocated expenses for rent, depreciation, and equipment maintenance, and other unallocated research and development expenses incurred in connection with the Company’s research and development programs, including laboratory materials and supplies and license fees. Research and development expenses are charged to operating expenses as incurred when these expenditures relate to the Company’s research and development efforts and have no alternative future uses. The Company is obligated to make upfront payments upon execution of certain research and development agreements. Advance payments, including nonrefundable amounts, for goods or services that will be used or rendered for future research and development activities are capitalized until such goods are delivered or the related services are performed, or such time when the Company does not expect the goods to be delivered or services to be performed. The Company estimates the period over which such services will be performed and the level of effort to be expended in each period. If actual timing of performance or the level of effort varies from the estimate, the Company will adjust the amounts recorded accordingly. Since inception, the Company has not experienced any material differences between accrued or prepaid costs and actual costs.
Stock-Based Compensation	Stock-Based Compensation The
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as a change in equity during a period from transactions and other events and circumstances from non-owner sources. The Company’s comprehensive loss was equal to net loss for the three months ended March 31, 2021 and 2020.
Net Loss Per Share Attributable to Common Stockholders	Net Loss Per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive given the net loss of the Company. For purposes of this calculation, redeemable convertible preferred stock, stock options, employee stock purchase rights, and unvested common stock subject to repurchase are considered to be common stock equivalents but are not included in the calculations of diluted net loss per share for the periods presented as their effect would be antidilutive.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with certain new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it is (1) no longer an emerging growth company or (2) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates.
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Issued Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Assets and Liabilities Measured at Fair Value On Recurring Basis	The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the condensed balance sheets that sum to the amounts shown in the condensed statements of cash flows (in thousands): March 31, 1 December 31, Cash and cash equivalents $ 374,205 $ 386,569 Restricted cash 350 350 Total cash and cash equivalents and restricted cash $ 374,555 $ 386,919

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value On Recurring Basis	The following table identifies the Company’s assets and liabilities that were measured at fair value on a recurring basis (in thousands): March 31, 2021 Level 1 Level 2 Level 3 Assets: Money market funds $ 374,205 $ — $ — December 31, 2020 Level 1 Level 2 Level 3 Assets: Money market funds $ 386,369 $ — $ —

Accrued Expenses (Tables)

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2021
Payables and Accruals [Abstract]
Summary of Accrued Expenses	Accrued expenses consisted of the following (in thousands): March 31, December 31, 2021 2020 Research and development expenses $ 3,610 $ 2,063 Employee compensation and benefits 1,666 2,634 Professional services and other 523 581 Total accrued expenses $ 5,799 $ 5,278

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2021
Leases [Abstract]
Summary of Operating Lease Cost	The components of lease expense and related cash flows were as follows (in thousands): Three Months Ended March 31, 2021 2020 Lease expense Operating lease expense $ 347 $ 347 Variable lease expense 106 86 Total lease expense $ 453 $ 433 Operating cash outflows from operating leases $ 421 $ 397
Summary of Future Minimum Commitments Due Under the Operating Lease Agreement	Future minimum commitments due under the operating lease agreement as of March 31, 2021 are as follows (in thousands): Years Ending December 31, Amount 2021 (remaining 9 months) $ 824 2022 2,454 Thereafter — Total undiscounted lease payments 3,278 Present value adjustment (297 ) Total present value of lease payments $ 2,981

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Tables)	3 Months Ended
	Mar. 31, 2021
Stockholders' Equity Note [Abstract]
Summary of Common Shares Reserved for Future Issuance	The Company has reserved shares of common stock for the following potential future issuances: March 31, December 31, 2021 2020 Shares underlying outstanding equity awards 6,308,582 6,316,569 Shares available for future equity award grants 3,152,910 1,506,806 Shares underlying early exercised equity awards 76,293 100,263 Shares underlying ESPP withholdings 19,126 4,393 Total 9,556,911 7,928,031

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Summary of Share-based Compensation Expense	Stock-based compensation expense recognized for all equity awards has been reported in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended 2021 2020 Research and development expense $ 1,925 $ 31 General and administrative expense 2,360 16 Total stock-based compensation expense $ 4,285 $ 47
Summary of Activity Related to Stock Option Grants	A summary of the Company’s stock option activity for the three months ended March 31, 2021 is as follows (in thousands, except share and per share data and years): Stock Options Outstanding Shares Subject to Options Outstanding Weighted- Average Exercise Price Weighted- Average Remaining Contractual Life (Years) Aggregate Intrinsic Value Balance at December 31, 2020 6,316,569 $ 10.93 9.5 $ 223,647 Granted 93,973 $ 48.03 Exercised (101,960 ) $ 2.18 $ 4,227 Cance l — $ — Balance at March 31, 2021 6,308,582 $ 11.63 9.3 $ 202,325 Vested at March 31, 2021 708,833 $ 6.97 8.7 $ 25,956
Summary of Fair Value of Each Option Grant Estimated Throughout Year Using Black-Scholes Option-pricing Model	The grant date fair value of stock options was estimated using a Black-Scholes option pricing model with the following weighted-average assumptions: Three Months March 31, 2021 Expected term (in years) 6.0 Expected volatility 83% Risk-free interest rate 0.72% Expected dividend yield —

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Summary of Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	The following outstanding shares of potentially dilutive securities were excluded from the computation of the diluted net loss per share attributable to common stockholders for the periods presented because their effect would have been anti-dilutive: Three Months Ended March 31, 2021 2020 Redeemable convertible preferred stock — 8,191,580 Common stock options 6,308,582 550,176 Unvested common stock 76,293 5,095 Common stock warrants — 9,154 ESPP withholdings 19,126 — Total potentially dilutive shares 6,404,001 8,756,005

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Dec. 03, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accumulated deficit		$ (115,601)	$ (96,734)
Cash and cash equivalents		$ 374,205	$ 386,569
Common stock, shares, issued		34,903,497	34,801,537
Liquidity and Capital Resources [Member]
Cash and cash equivalents		$ 374,200
Redeemable Convertible Preferred Stock [Member]
Redeemable convertible preferred stock outstanding				30,415,337	15,714,283
IPO [Member]
Offering price per share	$ 21
Net proceeds after deducting underwriting discounts and commissions and offering expenses	$ 255,300
Common stock, shares, issued	13,225,000
IPO [Member] | Common Stock [Member]
Conversion of preferred stock, common shares issued	20,758,098
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Redeemable convertible preferred stock outstanding	77,074,884

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Summary of reconciliation of cash and cash equivalents and restricted cash reported within the condensed balance sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Cash and cash equivalents	$ 374,205	$ 386,569
Restricted cash	350	350
Total cash and cash equivalents and restricted cash	374,555	386,919	$ 24,180	$ 10,526
Restricted Cash [Member]
Cash and cash equivalents	374,205	386,569
Restricted cash	350	350
Total cash and cash equivalents and restricted cash	$ 374,555	$ 386,919

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Recurring [Member] - Money Market Funds [Member] - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 374,205	$ 386,369
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - Fair Value, Recurring [Member] $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value,assets transfers from Level 1 to Level 2	$ 0
Fair value,assets transfers from Level 2 to Level 1	0
Fair value, liabilities transfers from Level 1 to Level 2	0
Fair value, liabilities transfers from Level 2 to Level 1	0
Fair Value, assets transfers into (out of) Level 3	$ 0

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Research and development expenses	$ 3,610	$ 2,063
Employee compensation and benefits	1,666	2,634
Professional services and other	523	581
Total accrued expenses	$ 5,799	$ 5,278

Leases - Summary of Operating

Leases - Summary of Operating Lease Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Lease, Cost [Abstract]
Operating lease expense	$ 347	$ 347
Variable lease expense	106	86
Total lease expense	453	433
Operating cash outflows from operating leases	$ 421	$ 397

Leases - Summary of Future Mini

Leases - Summary of Future Minimum Commitments Due Under Operating Lease Agreement (Detail) $ in Thousands	Mar. 31, 2021USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021 (remaining 9 months)	$ 824
2022	2,454
Total undiscounted lease payments	3,278
Present value adjustment	(297)
Total present value of lease payments	$ 2,981

Leases - Additional Information

Leases - Additional Information (Detail)	Mar. 31, 2021
Leases [Abstract]
Operating Lease Weighted Average Remaining Lease Term	1 year 8 months 12 days
Operating Lease Weighted Average Discount Rate	8.50%

Term Loan Payable - Additional

Term Loan Payable - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Apr. 30, 2020	Nov. 30, 2016	Mar. 31, 2021	Mar. 31, 2020
Debt Instrument [Line Items]
Debt issuance costs			$ 1	$ 20
Term Loan [Member] | Silicon Valley Bank [Member] | Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Term loan		$ 3,500
Annual Interest rate		1.75%
Debt issuance costs		$ 300
Effective interest rate			5.14%
Interest Expense			$ 100	$ 100
Maturity date		Nov. 1, 2020
Term Loan [Member] | Silicon Valley Bank [Member] | Amended Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Maturity date	May 1, 2021

Stockholders' Equity (Deficit_2

Stockholders' Equity (Deficit) - Summary of Common Shares Reserved for Future Issuance  (Detail) - shares	Mar. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Total	9,556,911	7,928,031
Shares Underlying Outstanding Equity Awards [Member]
Class of Stock [Line Items]
Total	6,308,582	6,316,569
Shares Available For Future Equity Award Grants [Member]
Class of Stock [Line Items]
Total	3,152,910	1,506,806
Shares Underlying Early Exercised Equity Awards [Member]
Class of Stock [Line Items]
Total	76,293	100,263
Shares Underlying ESPP Withholdings [Member]
Class of Stock [Line Items]
Total	19,126	4,393

Stockholders' Equity (Deficit_3

Stockholders' Equity (Deficit) - Additional Information (Detail) - Redeemable Convertible Preferred Stock [Member] $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)$ / sharesshares
Class of Stock [Line Items]
Accretion to Redemption value | $	$ 0
Issuance of Series B redeemable convertible preferred stock	10,027,666
Redeemable Stock Excersible On Agreed Upon Milestones [Member]
Class of Stock [Line Items]
Issuance of Series B redeemable convertible preferred stock	21,732,862
Proceeds from Issuance of Convertible Preferred Stock | $	$ 46,900
Series B [Member]
Class of Stock [Line Items]
Issuance of Series B redeemable convertible preferred stock	14,701,054
Issuance of Series B redeemable convertible preferred stock for cash	4,673,388
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes	10,027,666
Preferred stock, Par or Stated Value Per Share | $ / shares	$ 2.16

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Share-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 4,285	$ 47
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	1,925	31
General and administrative expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 2,360	$ 16

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Activity Related to Stock Option Grants (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Subject to Options Outstanding, Beginning balance	6,316,569
Shares Subject to Options Outstanding, Granted	93,973	0
Shares Subject to Options Outstanding, Exercised	(101,960)
Shares Subject to Options Outstanding, Ending balance	6,308,582		6,316,569
Shares Subject to Options Outstanding Vested, Ending balance	708,833
Weighted- Average Exercise Price, Beginning balance	$ 10.93
Weighted- Average Exercise Price, Granted	48.03
Weighted- Average Exercise Price, Exercised	2.18
Weighted- Average Exercise Price, Ending balance	11.63		$ 10.93
Weighted- Average Exercise Price Vested, Ending balance	$ 6.97
Weighted- Average Remaining Contractual Life (Years)	9 years 3 months 18 days		9 years 6 months
Weighted- Average Remaining Contractual Life (Years), Vested	8 years 8 months 12 days
Aggregate Intrinsic Value, Beginning balance	$ 223,647
Aggregate Intrinsic Value, Exercised	4,227
Aggregate Intrinsic Value, Ending balance	202,325		$ 223,647
Aggregate Intrinsic Value, Vested Ending balance	$ 25,956

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Fair Value of Each Option Grant Estimated Throughout Year Using Black-Scholes Option-pricing Model (Detail)	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Expected term (in years)	6 years
Expected volatility	83.00%
Risk-free interest rate	0.72%
Expected dividend yield	0.00%

Stock Based Compensation - Addi

Stock Based Compensation - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount	$ 59,600,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	3 years 1 month 28 days
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	9,564,798
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	6,308,582		6,316,569
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	6,404,001	8,756,005
Fair Value of Shares Vested	$ 3,600,000	$ 100,000
Share Based Compensation Number of Options Granted	93,973	0
Share Based Compensation Expected Dividend Yeild		$ 0
Shares Available for Future Equity Award Grants	9,556,911		7,928,031
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 33.32
Stock Option Awards [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Shares Available for Future Equity Award Grants	3,152,910
Unvested Common Stock [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	76,293	5,095

Licensing Agreement - Additiona

Licensing Agreement - Additional Information (Details) - WuXi Bio [Member] - USD ($)	Oct. 31, 2019	Mar. 31, 2021
Aggregate milestone payments upon achievement of certain sales milestones		$ 10,800,000
Agreement payment and license Term desciption		In the event the Company fails to pay all amounts due to WuXi Bio under the license agreement, and fails to pay the amounts within 30 days after receiving written notice of such failure, WuXi Bio may terminate the license with 45 days written notice to the Company.
Research and Development Expense [Member]
License Fees paid	$ 150,000

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Summary of Antidilutive Securities Excluded from Computation of Earnings Per Share Amount (Detail) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	6,404,001	8,756,005
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares		8,191,580
Common stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	6,308,582	550,176
Unvested Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	76,293	5,095
Common stock warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares		9,154
ESPP withholdings [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	19,126