Spero Therapeutics (SPRO) 10-K 30 Mar 23 2022 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Mar. 01, 2023
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Title of 12(b) Security	Common Stock, $0.001 par value per share
Trading Symbol	SPRO
Security Exchange Name	NASDAQ
Entity Registrant Name	SPERO THERAPEUTICS, INC.
Entity Central Index Key	0001701108
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		52,571,813
Entity Public Float	$ 22
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity File Number	001-38266
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-4590683
Entity Address, Address Line One	675 Massachusetts Avenue,
Entity Address, Address Line Two	14th Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	857
Local Phone Number	242-1600
Document Annual Report	true
Document Transition Report	false
ICFR Auditor Attestation Flag	false
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE None.
Auditor Name	PricewaterhouseCoopers LLP
Auditor Location	Boston, Massachusetts
Auditor Firm ID	238

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 109,107	$ 112,584
Marketable securities	0	33,818
Other receivables	1,084	2,641
Prepaid expenses and other current assets	3,379	8,829
Total current assets	113,570	157,872
Property and equipment, net	368	1,026
Operating lease right-of-use assets	5,124	6,530
Other assets	5,740	5,644
Total assets	124,802	171,072
Current liabilities:
Accounts payable	617	1,101
Accrued expenses and other current liabilities	8,973	14,350
Operating lease liabilities	1,690	1,362
Deferred Revenue, Current	10,369	1,857
Total current liabilities	21,649	18,670
Liability related to the sale of future royalties	0	48,414
Non-current operating lease liabilities	4,957	5,973
Deferred revenue, non-current	22,166	8,786
Derivative liability	0	802
Other long-term liabilities	96	138
Total liabilities	48,868	82,783
Commitments and contingencies (Note 12)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized, no shares issued and outstanding as of December 31, 2022 and 3,218,152 shares issued and outstanding as of December 31, 2021	0	3
Common stock, $0.001 par value; 120,000,000 shares authorized as of December 31, 2022 and December 31, 2021; 52,456,195 shares issued and outstanding as of December 31, 2022 and 32,393,738 shares issued and outstanding as of December 31, 2021	52	32
Additional paid-in capital	489,760	455,719
Accumulated deficit	(413,878)	(367,463)
Accumulated other comprehensive gain (loss)	0	(2)
Total stockholders' equity	75,934	88,289
Total liabilities and stockholders' equity	$ 124,802	$ 171,072

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares/units issued	0	3,218,152
Preferred stock, shares/units outstanding	0	3,218,152
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares/units issued	52,456,195	32,393,738
Common stock, shares/units outstanding	52,456,195	32,393,738

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Revenues:
Grant revenue	$ 4,930	$ 15,186
Collaboration revenue	$ 48,579	$ 3,070
Type of Revenue [Extensible List]	License [Member]	License [Member]
Total revenues	$ 53,509	$ 18,256
Operating expenses:
Research and development	47,593	64,526
General and administrative	36,483	41,701
Restructuring Charges	11,630	0
Total operating expenses	95,706	106,227
Loss from operations	(42,197)	(87,971)
Other income (expense):
Interest income	1,106	346
Other income (expense), net	(55)	(395)
Interest expense related to the sale of future royalties	(2,605)	(1,940)
Loss on extinguishment of liability related to the sale of future royalties	(3,581)	0
Change in fair value of derivative liability	917	204
Total other income (expense), net	(4,218)	(1,785)
Net loss	$ (46,415)	$ (89,756)
Net loss per share attributable to common stockholders, basic	$ (1.23)	$ (2.91)
Net loss per share attributable to common stockholders, diluted	$ (1.23)	$ (2.91)
Weighted average common shares outstanding, basic	37,585,075	30,895,756
Weighted Average Number of Shares Outstanding, Diluted	37,585,075	30,895,756
Comprehensive loss:
Net loss	$ (46,415)	$ (89,756)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	2	5
Unrealized gain (loss) on available-for-sale securities	2	5
Total comprehensive loss	$ (46,413)	$ (89,751)

CONSOLIDATED STATEMENTS OF CONV

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Pfizer Purchase Agreement [Member]	GSK [Member]	Series A, B, C and D Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Pfizer Purchase Agreement [Member]	Common Stock [Member] GSK [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Pfizer Purchase Agreement [Member]	Additional Paid-in Capital [Member] GSK [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2020	$ 132,040			$ 3	$ 29			$ 409,722			$ (277,707)	$ (7)
Balance, Shares at Dec. 31, 2020				3,218,287	29,260,247
Issuance of common stock upon the exercise of stock options	1,450							1,450
Issuance of common stock upon the exercise of stock options, shares					160,674
Issuance of common stock and preferred stock net of offering costs and issuance costs	7,831	$ 27,289			$ 1	$ 2		7,830	$ 27,287
Issuance of common stock and preferred stock net of offering costs, shares					475,469	2,362,348
Conversion of convertible preferred stock to common stock, shares				(135)	135,000
Share-based compensation expense	9,430							9,430
Unrealized gain (loss) on available-for-sale securities	5											5
Net loss	(89,756)										(89,756)
Balances at Dec. 31, 2021	88,289			$ 3	$ 32			455,719			(367,463)	(2)
Balance, shares at Dec. 31, 2021				3,218,152	32,393,738
Issuance of common stock upon the exercise of stock options	$ 420							420
Issuance of common stock upon the exercise of stock options, shares	56,120				56,120
Issuance of common stock and preferred stock net of offering costs and issuance costs	$ 14,234		$ 10,278		$ 7		$ 7	14,227		$ 10,271
Issuance of common stock and preferred stock net of offering costs, shares					5,963,294		7,450,000
Conversion of convertible preferred stock to common stock	3			$ (3)	$ 6
Conversion of convertible preferred stock to common stock, shares				(3,218,152)	6,367,000
Share-based compensation expense	9,123							9,123
Unrealized gain (loss) on available-for-sale securities	2											$ 2
IssuanceOfCommonStockUponTheVestingOfRsu	0
Issuance of common stock upon the vesting of RSUs					226,043
Net loss	(46,415)										(46,415)
Balances at Dec. 31, 2022	$ 75,934				$ 52			$ 489,760			$ (413,878)
Balance, shares at Dec. 31, 2022					52,456,195

CONSOLIDATED STATEMENTS OF CO_2

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Payment of offering costs	$ 0	$ 248
GSK [Member]
common stock issued discount value	$ 1,300
Common Stock [Member] | Pfizer Purchase Agreement [Member]
Payments of financing costs		200
Premium on issued of common stock		$ 12,500

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net loss	$ (46,415)	$ (89,756)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	869	646
Non-cash lease cost	678	471
Impairment of assets	635	0
Share-based compensation	9,123	9,430
Unrealized foreign currency transaction loss	10	13
Accretion of premium (discount) on marketable securities	13	251
Change in fair value of derivative liabilities	(917)	(204)
Non-cash interest expense associated with the sale of future royalties	2,605	1,940
Loss on extinguishment of liability related to the sale of future royalties	3,581	0
Changes in operating assets and liabilities:
Other receivables	1,196	3,050
Prepaid expenses and other current assets	5,242	(2,808)
Tax incentive receivables	351	782
Other assets	(3)	(319)
Accounts payable	(484)	(53)
Accrued expenses and other current liabilities	(5,377)	2,109
Deferred revenue, current and non-current	21,892	10,643
Other long-term liabilities	(42)	(39)
Operating lease liabilities	(688)	(503)
Net cash used in operating activities	(7,731)	(64,347)
Cash flows from investing activities:
Purchases of marketable securities	(26,970)	(43,915)
Proceeds from maturities of marketable securities	60,777	51,548
other	0	39
Net cash provided by investing activities	33,807	7,672
Cash flows from financing activities:
Repayment of liability related to the sale of future royalties	(54,485)	0
Proceeds from sale of future royalties, net	0	49,750
Transaction costs from sale of future royalties	0	(2,210)
Royalty payments	0	(60)
Payment of offering costs	0	(248)
Proceeds from stock option exercises	420	1,450
Net cash provided by financing activities	(29,553)	84,050
Net increase (decrease) in cash and cash equivalents	(3,477)	27,375
Cash and cash equivalents at beginning of period	112,584	85,209
Cash and cash equivalents at end of period	109,107	112,584
Proceeds from the issuance of common stock, net of commissions
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of issuance costs	14,234	7,831
Proceeds from the issuance of common stock related to the GSK Share Purchase Agreement
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of issuance costs	10,278	0
Pfizer Purchase Agreement [Member]
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of issuance costs	$ 0	$ 27,537

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2022
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Spero Therapeutics, Inc., together with its consolidated subsidiaries (the “Company” or “Spero”), is a multi-asset, clinical-stage biopharmaceutical company focused on identifying, developing, and commercializing novel treatments for bacterial infections, including multi-drug resistant (“MDR”) bacterial infections, and rare diseases. The Company's lead product candidate, SPR720, is an oral antimicrobial agent in development for the treatment of nontuberculous mycobacterial (“NTM”) pulmonary disease, a rare orphan disease. The Company's partnership-directed programs consist of SPR206 and tebipenem HBr. SPR206 is an IV-administered agent being developed as an innovative option to treat MDR Gram-negative bacterial infections in the hospital setting. Tebipenem HBr is designed to be the first broad-spectrum oral carbapenem-class antibiotic for use to treat certain bacterial infections that cause complicated urinary tract infections (“cUTIs”) including pyelonephritis, caused by certain microorganisms, in adult patients who have limited oral treatment options. The Company was formed as Spero Therapeutics, LLC in December 2013 under the laws of the State of Delaware . On June 30, 2017 , through a series of transactions, Spero Therapeutics, LLC merged with and into Spero Therapeutics, Inc. (formerly known as Spero OpCo, Inc.), a Delaware corporation. The Company is subject to risks and uncertainties common to 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, risks of failure or unsatisfactory results of nonclinical studies and clinical trials, the need to obtain marketing approval for its product candidates, the need to successfully commercialize and gain market acceptance of its product candidates and the ability to secure additional capital to fund operations. The Company’s product candidates will require additional 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. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying consolidated financial statements of the Company have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its consolidated subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. Since inception, the Company has funded its operations with proceeds from sales of preferred units (including bridge units, which converted into preferred units), payments received in connection with a concluded collaboration agreement, funding from government contracts, licensing agreements and through the sale of the Company’s common and preferred stock. The Company has incurred recurring losses since inception, including net losses of $ 46.4 million and $ 89.8 million for the years ended December 31, 2022 and 2021, respectively. In addition, as of December 31, 2022, the Company had an accumulated deficit of $ 413.9 million. The Company expects to continue to generate operating losses for the foreseeable future. In accordance with Accounting Standards Update (“ASU”) 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. As of the issuance date of these annual consolidated financial statements, the Company expects its current operating plan, existing cash and cash equivalents will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of these annual consolidated financial statements. Beyond that point, the Company will require additional funding to fund the development of its product candidates through regulatory approval and commercialization, and to support its continued operations. The Company will seek additional funding through public or private financings, debt financing, collaboration agreements, government grants or other venues. There is no assurance that the Company will be successful in obtaining sufficient funding on acceptable terms, if at all, and it could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could materially adversely affect its business prospects or its ability to continue operations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2022
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with 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. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual for clinical trial costs and other research and development expenses and the valuation of share-based awards. There may be changes to those estimates in future periods. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on identifying, developing and commercializing novel treatments for bacterial infections, including MDR bacterial infections, and rare diseases. All of the Company’s tangible assets are held in the United States. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains most of its cash and cash equivalents at one accredited financial institution. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. As of December 31, 2022, and 2021, the Company had no off-balance sheet risk such as foreign exchange contracts, option contracts, or other hedging arrangements. Cash Equivalents 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 and cash equivalents include cash held in banks and money market instruments. Marketable Securities Marketable securities consist of investments in corporate obligations with original maturities greater than 90 days. The Company considers its portfolio of investments to be available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Investments with maturities beyond one year are generally classified as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value are included as a component of other income (expense), net based on the specific identification method. Any credit impairments are recorded through an allowance account. Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense are recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Computer software and equipment 3 years Office furniture and equipment 7 years Manufacturing equipment 5 years Leasehold improvements Shorter of life of lease or 5 years Costs for capital assets not yet placed into service are capitalized as construction in progress and are depreciated in accordance with the above guidelines once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. The Company periodically evaluates whether events and circumstances have occurred that may warrant revision of the estimated useful life of property and equipment. Leases At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. As of December 31, 2022 and 2021, the Company had no short-term leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew. The Company monitors its plans to renew its material leases on a quarterly basis. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate (“IBR”), which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, and in a similar economic environment. Since the Company does not have any debt and has not been rated by any major credit rating agency, the Company’s IBR was estimated by developing a synthetic credit rating for the Company. The Company has elected to account for lease and non-lease components together as a single lease component. Other Assets Other assets consist of long-term prepayments and deposits. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an 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. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Revenue Recognition - Collaboration Revenue The Company has entered into licensing agreements that are evaluated under Accounting Standards Codification, Topic 606 (“Topic 606”), Revenue from Contracts with Customers, through which the Company licenses certain of its product candidates’ rights to a third party. Terms of these arrangements include various payment types, typically including one or more of the following: upfront license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and/or royalties on net sales of licensed products. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations under the agreement; (iii) determine the transaction price, including constraint on variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) determine how the revenue will be recognized for each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. The SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangements. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method. In determining the accounting treatment for these arrangements, the Company develops assumptions to determine the stand-alone selling price for each performance obligation in the contract. The Company develops the estimated standalone selling price for the license using a discounted cash flow model. To develop this model, the Company applies significant judgement in the determination of the significant assumptions relating to forecasted future revenues, development time lines, the discount rate, and probabilities of technical and regulatory success. The Company develops the estimated standalone selling price for the research and development services using a discounted cash flow model. The assumptions to develop the estimated standalone selling price for the related research and development services include estimates of costs to be incurred to fulfill its obligations associated with the performance of the research and development services, plus a reasonable margin. Government Tax Incentives For available government tax incentives that the Company may earn without regard to the existence of taxable income and that require the Company to forego tax deductions or the use of future tax credits and net operating loss carryforwards, the Company classifies the funding recognized as a reduction of the related qualifying research and development expenses incurred. Since the fourth quarter of 2016, the Company’s operating subsidiary in Australia has met the eligibility requirements to receive a tax incentive for qualifying research and development activities. The Company recognizes these incentives as a reduction of research and development expenses in the consolidated statements of operations and comprehensive loss in the same period that the related qualifying expenses are incurred. Reductions of research and development expense recognized upon incurring qualifying expenses in advance of receipt of tax incentive payments are recorded in the consolidated balance sheet as tax incentive receivables. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including personnel salaries, share-based compensation and benefits, allocated facilities costs, depreciation, manufacturing expenses, costs related to the Company’s government contract and grant arrangements, and external costs of outside vendors engaged to conduct preclinical development activities, clinical trials as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. In June 2019, the Company entered into a collaboration agreement with the Bill and Melinda Gates Medical Research Institute (the “Gates MRI”) and concluded that the agreement is within the scope of the accounting guidance for collaboration arrangements (see Note 14). Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company recognizes the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred. Clinical Trial and other Research Contract Costs and Accruals The Company has entered into various research and development contracts with clinical research organizations and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. There may be instances in which payments made to these vendors exceed the level of service provided and will result in a prepayment of the expense. The Company records accruals for estimated ongoing research and clinical trial costs based on the services received and efforts expended pursuant to multiple contracts with these vendors. When evaluating the adequacy of the accrued liabilities, the Company analyzes the progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Restructuring The Company made estimates and judgments regarding the amount and timing of its restructuring expense and liability, including one-time termination benefits and other exit costs accounted for upon the announcement of the restructuring in May 2022. Restructuring charges are reflected in the Company's consolidated statements of income. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Share-Based Compensation The Company issues stock-based awards to employees and directors in the form of stock options and restricted stock units. The Company measures and recognizes compensation expense for its stock-based awards granted to its employees and directors based on the estimated grant date fair value in accordance with ASC 718 and determines the fair value of restricted stock units based on the fair value of its common stock. The Company measures all share-based options granted to employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. Compensation expense of those awards is recognized over the requisite service period, which is generally the vesting period of the respective award. The Company records the expense for awards with service-based conditions using the straight-line method over the requisite service period, net of any actual forfeitures. The Company has also granted certain awards subject to performance-based vesting eligibility and a subsequent partial time-based vesting schedule. Performance-based awards will be recorded as an accrued liability on the Company's condensed consolidated balance sheet and accrued over time as achievement of performance metrics become probable. The Company classifies share-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. For the years ended December 31, 2022 and 2021, these changes related to unrealized gains and losses on the Company’s available-for-sale marketable securities. There were no reclassifications out of comprehensive loss for the years ended December 31, 2022 and 2021. Net Loss per Share The Company follows the two-class method when computing net income (loss) per share, as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Net income (loss) per share attributable to common stockholders is calculated based on net income (loss) attributable to Spero Therapeutics, Inc. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50 % likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. On December 22, 2017, the Tax Cuts and Jobs Act ("TCJA") was signed into law. Under the TCJA provisions, effective with tax years beginning on or after January 1, 2022, taxpayers can no longer immediately expense qualified research and development expenditures, including all direct, indirect, overhead and software development costs. Taxpayers are now required to capitalize and amortize these costs over five years for research conducted within the United States or 15 years for research conducted abroad. Liability Related to the Sale of Future Royalties Prior to repayment, as discussed further in Note 9, the Company treated the liability related to the sale of future royalties as a debt instrument, amortized under the effective interest rate method over the estimated life of the revenue streams. The Company recognized interest expense thereon using the effective rate, which was based on its estimates of future revenues over the life of the arrangement. The Company periodically assessed its expected revenues using internal projections, imputed interest on the carrying value of the deferred royalty obligation and recorded interest expense using the imputed effective interest rate. To the extent its estimates of future revenues were greater or less than previous estimates or the estimated timing of such payments was materially different than previous estimates, the Company accounted for any such changes by adjusting the effective interest rate on a prospective basis, with a corresponding impact to the reclassification of the deferred royalty obligation. The assumptions used in determining the expected repayment term of the deferred royalty obligation and amortization period of the issuance costs required that the Company made estimates that could impact the short-term and long-term classification of such costs, as well as the period over which such costs were amortized. Derivative Liability In connection with certain transactions, the Company has identified certain embedded derivatives, which are recorded as liabilities on the Company’s consolidated balance sheet and are remeasured to fair value at each reporting date until the derivative is settled. Changes in the fair value of the derivative liabilities are recognized as other income (expense) in the consolidated statement of operations and comprehensive loss. Recently Issued and Adopted 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 noted, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its condensed consolidated financial statements and disclosures. In June 2022, the FASB issued ASU 2022-03, Fair Value Measurement of Equity Securities Subject to Contractual Sale Restrictions . The ASU clarifies that a contractual restriction on the sale of an equity security is not considered part of the unit of account of the equity security and, therefore, is not considered in measuring fair value. The ASU also clarifies that an entity cannot, as a separate unit of account, recognize and measure a contractual sale restriction and requires specific disclosures for equity securities subject to contractual sale restrictions. These changes will become effective for the Company after December 15, 2023. The Company early adopted this standard during the fourth quarter of 2022 and it did not have a material impact on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes , amended guidance on the accounting and reporting of income taxes. The guidance is intended to simplify the accounting for income taxes by removing exceptions related to certain intraperiod tax allocations and deferred tax liabilities; clarifying guidance primarily related to evaluating the step-up tax basis for goodwill in a business combination; and reflecting enacted changes in tax laws or rates in the annual effective tax rate. The amended guidance is effective for interim and annual periods in 2021. Early adoption is permitted. The application of the amendments in the new guidance are to be applied on a retrospective basis, on a m

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	12 Months Ended
	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair Value Measurements and Marketable Securities The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis (in thousands): Fair Value Measurements at December 31, 2022 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 108,227 $ — $ 108,227 Total cash equivalents — 108,227 — 108,227 Fair Value Measurements at December 31, 2021 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 109,316 $ — $ 109,316 Corporate bonds $ — 2,701 — 2,701 Total cash equivalents — 112,017 — 112,017 Marketable securities: Corporate bonds — 11,479 — 11,479 Commercial paper — 22,339 — 22,339 Total marketable securities — 33,818 — 33,818 Total cash equivalents and marketable securities $ — $ 145,835 $ — $ 145,835 Liabilities: Derivative liability $ — $ — $ 802 $ 802 $ — $ — $ 802 $ 802 Excluded from the tables above is cash of $ 0.9 million and $ 0.6 million as of December 31, 2022 and 2021, respectively. During the years ended December 31, 2022 and 2021, there were no transfers between Level 1, Level 2 and Level 3 categories. Marketable Securities The Company’s marketable securities are classified as Level 2 assets under the fair value hierarchy as these assets were primarily determined from independent pricing sources, which generally derive security prices from recently reported trades for identical or similar securities. The Company evaluated debt securities with unrealized losses for any expected credit losses and determined unrealized losses on these securities were related to non-credit factors. Additionally, the Company currently does no t intend to and is not required to sell these investments prior to an anticipated recovery in value. The Company did not have any assets classified as marketable securities as of December 31, 2022 . The following table summarizes the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2021 (in thousands): December 31, 2021 Amortized Gross Gross Fair Value Assets: Corporate bonds 11,481 — ( 2 ) 11,479 Commercial paper 22,339 — — 22,339 $ 33,820 $ — $ ( 2 ) $ 33,818 As of December 31, 2021, all of the Company’s marketable securities had remaining contractual maturity dates of one year or less from the respective consolidated balance sheet date. Embedded Derivative Liability related to change of control During the year ended December 31, 2022, in connection with the termination of the Revenue Interest Agreement (as defined below) (see Note 9), the Company classified $ 0.1 million as a derivative liability on its condensed consolidated balance sheet because of an embedded feature that represented a conditional obligation to pay HCR (as defined below) an additional cash amount upon a change of control. The Company remeasured the derivative liability to fair value at each reporting date until it expired on December 31, 2022, and recognized changes in the fair value of the derivative liability as a component of other income (expense) in the condensed consolidated statement of operations and comprehensive loss. The Company valued the change of control provision under the Revenue Interest Termination Agreement, dated June 7, 2022, by and between the Company and HCR, using a series of Black-Scholes-Merton option pricing models. The assumptions used in the valuation model include (1) the Company's estimates of the probability of a change of control event occurring prior to or as of December 31, 2022, (2) the Company's common stock closing stock price as of June 7, 2022, (3) the Company's fully-diluted number of shares of common stock outstanding as of June 7, 2022, (4) volatility, (5) risk-free rate, and (6) the Company's credit-risk-adjusted discount rate. The fair value of the derivative liability upon issuance in June 2022 was $ 0.1 million, and was classified as Level 3 liability under the fair value hierarchy. As of December 31, 2022 the fair value of the derivative liability expired, and as such, was reduced by $ 0.1 million to zero . Liability related to the sale of future royalties During the year ended December 31, 2021, in connection with the liability related to the sale of future royalties, the Company classified a derivative liability on its consolidated balance sheet at inception of its Revenue Interest Financing Agreement because there were embedded instruments that represented a conditional obligation to pay HCR the final payment. The fair value of the derivative liability upon issuance in October 2021 was $ 1.0 million, and was classified as Level 3 liability under the fair value hierarchy. As of December 31, 2021 the fair value of the derivative liability decreased by $ 0.2 million to $ 0.8 million, primarily due to the passage of time and changes in the market volatility and underlying credit risk inputs. The Revenue Interest Agreement was terminated on June 7, 2022, and as such, the fair value of this derivative liability was reduced to zero. The fair value for the liability related to the sale of future royalties at the time of the initial transaction was based on the Company's current estimates of future royalties expected to be paid to HCR over the remaining patent life of the product, which were considered Level 3 inputs (see Note 9).

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2022 2021 Leasehold improvements $ 1,636 $ 1,636 Manufacturing equipment 1,338 1,338 Computer software and equipment 437 507 Office furniture and equipment 209 425 3,620 3,906 Less: Accumulated depreciation and amortization ( 3,252 ) ( 2,880 ) $ 368 $ 1,026 Property and equipment decreases during the year ended December 31, 2022 were primarily related to impairments due to our strategic restructuring announced in May 2022. Property and equipment increases during the year ended December 31, 2021 primarily related to office furniture and equipment and construction-in-progress related to the expansion of Company’s leased office space (see Note 5). Depreciation and amortization expense related to property and equipment was $ 0.7 million and $ 0.6 million for the years ended December 31, 2022 and 2021, respectively.

Leases

Leases	12 Months Ended
	Dec. 31, 2022
Leases [Abstract]
Leases	5. Leases Operating Leases The Company has entered into, and subsequently amended, an operating lease agreement with U.S. REIF Central Plaza Massachusetts, LLC with respect to its corporate headquarters located at 675 Massachusetts Avenue, Cambridge, Massachusetts where the Company leases approximately 23,400 square feet of office space. The Company's lease extends through July 2027 . For the years ended December 31, 2022 and 2021, the components of operating lease expense were as follows (in thousands): Operating lease expense Statement of Operations Location December 31, 2022 December 31, 2021 Fixed operating lease expense Research and development expense $ 824 $ 869 General and administrative expense 772 681 Variable operating lease expense Research and development expense 56 76 General and administrative expense 59 143 Total operating lease expense $ 1,711 $ 1,769 Supplemental cash flow information related to the Company’s operating leases for the years ended December 31, 2022 and 2021, was as follows (in thousands): December 31, 2022 December 31, 2021 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 1,514 $ 1,525 The following table presents the lease balances within the consolidated balance sheet, weighted average remaining lease term, and the weighted average discount rates related to the Company’s operating and finance leases as of December 31, 2022 and 2021 (in thousands, except for the weighted average remaining lease term and the weighted average discount rate): Lease Assets and Liabilities Classification December 31, 2022 December 31, 2021 Assets Operating Operating lease right-of-use assets $ 5,124 $ 6,530 Financing Property and equipment, net 201 468 Total leased assets $ 5,325 $ 6,998 Liabilities Current Operating Operating lease liabilities $ 1,690 $ 1,362 Non-Current Operating Non-current operating lease liabilities 4,957 5,973 Total lease liabilities $ 6,647 $ 7,335 Weighted average remaining lease term (in years) 4.6 5.6 Weighted average discount rate 9.8 % 9.8 % The following table presents the maturity of the Company’s operating lease liabilities as of December 31, 2022 (in thousands): Years Ending December 31, 2023 1,690 2024 1,718 2025 1,746 2026 1,956 2027 1,156 Total future minimum lease payments 8,266 Less imputed interest ( 1,619 ) Total operating lease liabilities $ 6,647

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2022 December 31, 2021 Accrued external research and development expenses $ 1,823 $ 6,315 Accrued payroll and related expenses 5,797 5,884 Accrued professional fees 696 909 Accrued restructuring expenses 448 - Accrued other 209 1,242 Total Accrued expenses and other current liabilities $ 8,973 $ 14,350

Equity

Equity	12 Months Ended
	Dec. 31, 2022
Equity [Abstract]
Equity	. Equity Convertible Preferred Shares Series A Convertible Preferred Shares The Company has designated 2,220 of the 10,000,000 authorized shares of preferred stock as Series A Preferred Stock. Each share of Series A Preferred Stock is convertible into 1,000 shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series A Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99 % of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series A Preferred Stock will receive a payment equal to $ 0.001 per share of Series A Preferred Stock before any proceeds are distributed to the holders of common stock. Shares of Series A Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series A Preferred Stock will be required to amend the terms of the Series A Preferred Stock. As such, the Company classified the Series A Preferred Stock within permanent equity in its consolidated balance sheet. Series B Convertible Preferred Shares The Company has designated 1,000 of the 10,000,000 authorized shares of preferred stock as Series B Preferred Stock. Each share of Series B Preferred Stock is convertible into 1,000 shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series B Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99 % of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series B Preferred Stock will receive a payment equal to $ 0.001 per share of Series B Preferred Stock before any proceeds are distributed to the holders of common stock and equal to any distributions to the holders of Series A Preferred Stock. Shares of Series B Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series B Preferred Stock will be required to amend the terms of the Series B Preferred Stock. As such, the Company has classified the Series B Preferred Stock within permanent equity in its consolidated balance sheet. Series C Convertible Preferred Shares The Company has designated 3,333 of the 10,000,000 authorized shares of preferred stock as Series C Preferred Stock. Each share of Series C Preferred Stock is convertible into 1,000 shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series C Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99 % of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series C Preferred Stock will receive a payment equal to $ 0.001 per share of Series C Preferred Stock before any proceeds are distributed to the holders of common stock and equal to any distributions to the holders of Series A Preferred Stock and Series B Preferred Stock. Shares of Series C Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series C Preferred Stock will be required to amend the terms of the Series C Preferred Stock. As such, the Company has classified the Series C Preferred Stock within permanent equity in its consolidated balance sheet. Series D Convertible Preferred Shares The Company has designated 3,215,000 of the 10,000,000 authorized shares of preferred stock as Series D Preferred Stock. The shares of Series D Preferred Stock are convertible on a one-to-one basis into shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series D Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99 % of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series D Preferred Stock will receive a payment equal to $ 0.001 per share of Series D Preferred Stock before any proceeds are distributed to the holders of common stock and equal to any distributions to the holders of Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock. Shares of Series D Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series D Preferred Stock will be required to amend the terms of the Series D Preferred Stock. As such, the Company has classified the Series D Preferred Stock within permanent equity in its consolidated balance sheet. Conversions In February 2021, a holder of the Company’s Series B Preferred Stock elected to convert 62 shares of Series B Preferred Stock into 62,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series B Preferred Stock. In addition, a holder of the Company’s Series C Preferred Stock elected to convert 73 shares of Series C Preferred Stock into 73,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series C Preferred Stock. In June 2022, a holder of the Company’s Series B Preferred Stock elected to convert 938 shares of Series B Preferred Stock into 938,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series B Preferred Stock. In addition, a holder of the Company’s Series D Preferred Stock elected to convert 942,000 shares of Series D Preferred Stock into 942,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series D Preferred Stock. In September 2022, a holder of the Company’s Series C Preferred Stock elected to convert the remaining 2,214 shares of Series C Preferred Stock into 2,214,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series C Preferred Stock. In addition, a holder of the Company’s Series D Preferred Stock elected to convert the remaining 2,273,000 shares of Series D Preferred Stock into 2,273,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series D Preferred Stock. As of December 31, 2022, all of the Company's shares of preferred stock have been converted to common stock. Common Stock “At-the-Market” Offering On March 11, 2021, the Company entered into a sales agreement (the “Sales Agreement”) with Cantor Fitzgerald & Co. (“Cantor”) and filed a universal shelf registration statement on Form S-3 (Registration No. 333-254170), and pursuant to which the Company registered for sale up to $ 300.0 million of any combination of its common stock, preferred stock, debt securities, warrants, rights and/or units from time to time and at prices and on terms that the Company may determine, including up to $ 75.0 million of its common stock available for issuance pursuant to the “at-the-market” offering program Sales Agreement. Under the Sales Agreement, Cantor may sell shares of the Company’s common stock by any method permitted by law deemed to be an “at the market” offering as defined in Rule 415 of the Securities Act, subject to the terms of the Sales Agreement. The Company’s universal shelf registration statement on Form S-3 (Registration No. 333-254170) became effective on March 29, 2021. During year ended December 31, 2022 the Company sold 5,963,294 shares of its common stock under the Sales Agreement at an average price of approximately $ 2.46 per share for aggregate gross proceeds of approximately $ 14.7 million prior to deducting sales commissions. Subsequent to December 31, 2022, the Company did not sell any shares of its common stock under the Sales Agreement. GSK License and Share Purchase Agreements Concurrently with the execution of the GSK License Agreement, on September 21, 2022 (the “GSK Effective Date”), the Company entered into a stock purchase agreement (the “GSK SPA”) with Glaxo Group Limited (“GGL”), an affiliate of GSK, which closed on November 7, 2022 (the “GSK Closing Date”), and pursuant to which GGL purchased on the GSK Closing Date 7,450,000 shares (the “GSK Shares”) of the Company’s common stock at a purchase price of approximately $ 1.20805 per share, which represented a discount on the closing price on November 4, 2022, for an aggregate purchase price of $ 9.0 million. The GSK SPA contains certain standstill, lock-up and registration rights provisions. Upon closing, the Company recorded the fair market value of the shares issued in stockholders’ equity in its condensed consolidated balance sheet. The fair market value of 7,450,000 shares of the Company's common stock issued under the GSK SPA was $ 10.3 million. The common stock issued under the GSK SPA were valued using an option pricing valuation model as the shares are subject to certain holding period restrictions. The Company accounted for the associated discount of $ 1.3 million as a freestanding equity-linked instrument under ASC 815. The discount was allocated as consideration for the GSK License Agreement and evaluated under ASC 606. The discount was determined not to be constrained and was included in the calculation of the total transaction price related to the GSK License Agreement as of the effective date of the transaction. Refer to Note 14 for further discussion. The GSK Shares were issued and sold without registration under the Securities Act in reliance on the exemptions provided by Section 4(a)(2) of the Securities Act and/or Regulation D promulgated thereunder and in reliance on similar exemptions under applicable state laws. Refer to Note 14 for further discussion. Pfizer License and Share Purchase Agreements On June 30, 2021, the Company agreed to sell 2,362,348 shares of common stock to Pfizer Inc. (“Pfizer”) pursuant to a Share Purchase Agreement (the “Pfizer Purchase Agreement”), at a price of $ 16.93 per share, which represented a premium over the most recent closing price on June 30, 2021, for an aggregate purchase price of $ 40.0 million. No shareholder approval was required for the sale of the shares. Pfizer is an accredited investor as defined in the Securities Act, and the shares were sold pursuant to exemptions from registration under Regulation D of the Securities Act. The Company has not filed a registration statement with the SEC covering the resale of the shares and such securities may not be offered or sold in the United States absent registration or an exemption from registration under the Securities Act and any applicable state securities laws. The fair market value of 2,362,348 shares of the Company's common stock issued to Pfizer under the Pfizer Purchase Agreement was $ 27.5 million. The common stock issued under the Pfizer Purchase Agreement were valued using an option pricing valuation model as the shares are subject to certain holding period restrictions. The Company accounted for the associated premium of $ 12.5 million as a freestanding equity-linked instrument under ASC 815. The premium was allocated as consideration for the Company's license agreement with Pfizer (the “Pfizer License Agreement”) and evaluated under ASC 606. The premium was determined not to be constrained and was included in the calculation of the total transaction price related to the Pfizer License Agreement as of June 30, 2021. Refer to Note 14 for further discussion. The closing of the sale of the shares pursuant to the Pfizer Purchase Agreement occurred on July 1, 2021. Upon closing, the Company recorded the fair market value of the shares issued in stockholders’ equity in its condensed consolidated balance sheet. Charter Amendment On August 17, 2021, the Company filed a Certificate of Amendment to its Amended and Restated Certificate of Incorporation with the Secretary of State of the State of Delaware to increase the number of shares of the Company’s common stock authorized for issuance from 60,000,000 shares to 120,000,000 shares (the “Charter Amendment”). The Charter Amendment was approved by the Company’s stockholders at the annual meeting of stockholders held on August 17, 2021.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]
Share-Based Compensation	8. Share-Based Compensation 2017 Stock Incentive Plan On June 28, 2017, the Company’s shareholders approved the 2017 Stock Incentive Plan (the “2017 Plan”). The 2017 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock grants and stock-based awards. The 2017 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100 % of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years . The number of shares initially reserved for issuance under the 2017 Plan was 1,785,416 shares of common stock. The shares of common stock underlying any awards that are forfeited, cancelled, repurchased or are otherwise terminated by the Company under the 2017 Plan will be added back to the shares of common stock available for issuance under the 2017 Plan. On October 18, 2017, the Company’s shareholders approved an amendment to the 2017 Plan, which became effective upon the completion of the Company’s initial public offering, to increase the total number of shares reserved for issuance under the 2017 Plan from 1,785,416 to 2,696,401 . Additionally, the number of shares of common stock that may be issued under the 2017 Plan would be automatically increased on each January 1, beginning with the fiscal year ending December 31, 2019 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the lowest of (i) 607,324 shares of common stock, (ii) 4 % of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors or compensation committee. On August 17, 2021, the Company's shareholders approved amendments to the 2017 Plan. The amendments provided for the following: (i) increased the number of shares of the Company’s common stock authorized for issuance under the 2017 Plan by 3,170,254 shares, (ii) removed the “evergreen” provision historically included in the 2017 Plan, and (iii) made certain other amendments. In September 2022, the Company's shareholders approved an amendment to the 2017 Plan to increase the number of shares of the Company’s common stock authorized for issuance under the 2017 Plan by 2,000,000 shares. As of December 31, 2022 , there were 4,382,875 shares remaining available to be issued under the 2017 Plan, as amended. 2019 Equity Incentive Plan On March 11, 2019, the Company adopted the 2019 Inducement Equity Incentive Plan (the “2019 Inducement Plan”) to reserve 331,500 shares of its common stock to be used exclusively for grants of awards to individuals that were not previously employees or directors of the Company as a material inducement to such individuals’ entry into employment with Spero within the meaning of Rule 5635(c)(4) of the Nasdaq Listing Rules. The terms and conditions of the 2019 Inducement Plan are substantially similar to those of the 2017 Plan. In June 2020, the board of directors approved an amendment to the 2019 Inducement Plan to increase the number of shares of common stock authorized for issuance under the 2019 Inducement Plan by 700,000 shares. In December 2022, the board of directors approved an amendment to the 2019 Inducement Plan to increase the number of shares of common stock authorized for issuance under the 2019 Inducement Plan by 875,000 shares. As of December 31, 2022 , there were 1,030,096 shares remaining available to be issued under the 2019 Inducement Plan, as amended. The following table summarizes stock option activity for all of our plans during 2022: 2017 Plan 2019 Inducement Plan Total Number of Stock Options Outstanding as of December 31, 2021 3,893,605 997,111 4,890,716 Granted 956,604 — 956,604 Exercised ( 56,120 ) — ( 56,120 ) Forfeited or cancelled ( 1,161,257 ) ( 650,844 ) ( 1,812,101 ) Outstanding as of December 31, 2022 3,632,832 346,267 3,979,099 As of December 31, 2022 , a total of 11,595,127 shares have been authorized and reserved for issuance under all equity plans and 5,412,971 shares were available for future issuance under such plans. Stock Option Valuation The fair value of stock options is estimated using the Black-Scholes option-pricing model. The Company does not have sufficient company-specific historical and implied volatility information and it therefore estimates its expected share volatility based on the historical volatility of a set of publicly traded peer companies. The Company expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The Company has estimated the expected term of the Company’s stock option awards utilizing the “simplified” method for awards that qualify as “plain-vanilla.” The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The assumptions that the Company used in the Black-Scholes option-pricing model to determine the fair value of stock option awards granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2022 2021 Risk-free interest rate 1.7 % 0.8 % Expected term (in years) 6.2 6.2 Expected volatility 82.7 % 92.6 % Expected dividend yield 0.0 % 0.0 % The following table summarizes details regarding stock options granted under our equity incentive plans for the year ended December 31, 2022: Number of Shares Weighted Average Exercise Price Weighted Average Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2021 4,890,716 $ 11.75 7.62 $ 24,263 Granted 956,604 11.12 Exercised ( 56,120 ) 7.48 Forfeited or cancelled ( 1,812,101 ) 13.21 Outstanding as of December 31, 2022 3,979,099 $ 10.99 5.27 $ 1 Outstanding as of December 31, 2022 - vested and expected to vest 3,979,099 $ 10.99 5.27 $ 1 Exercisable at December 31, 2022 2,688,861 $ 9.76 4.58 $ — The weighted average grant-date fair value of stock options granted during the year ended December 31, 2022 was $ 7.94 per share. The weighted average grant-date fair value of awards granted during the year ended December 31, 2021 was $ 13.37 per share. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2022 and 2021 was approximately $ 0.1 million and $ 1.2 million, respectively. The Company satisfies stock option exercises with newly issued shares of its common stock. As of December 31, 2022 , total unrecognized compensation cost related to unvested stock option grants was approximately $ 11.4 million. This amount is expected to be recognized over a weighted average period of approximately 2.42 years. Restricted Stock Units The Company granted 1,420,750 restricted stock units (“RSUs”) to employees during the year ended December 31, 2022. The following table summarizes RSU activity under all equity plans (excluding performance-based RSUs) during the year months ended December 31, 2022: Number of Weighted Average Grant Date Fair Value Outstanding as of December 31, 2021 513,690 $ 17.08 Granted 1,420,750 5.88 Vested and released ( 226,043 ) 7.00 Forfeited or cancelled ( 408,000 ) 14.16 Outstanding as of December 31, 2022 1,300,397 $ 7.51 As of December 31, 2022, there was approximately $ 8.1 million of total unrecognized compensation expense related to RSUs, which is expected to be recognized over a weighted-average period of approximately 2.83 years. The fair value of the RSUs is determined on the date of grant based on the market price of the Company’s common stock on that date. Each RSU represents the right to receive one share of the Company’s common stock, $ 0.001 par value per share, upon vesting. Other than RSUs granted as retention awards, the RSUs vest in four equal annual installments, subject to the individual’s continued service to the Company through the applicable vesting date, and are subject to the terms and conditions of the Company’s form of RSU agreement under the 2017 Plan and 2019 Plan. Performance-Based awards In July 2022, as part of the retention awards referenced in Footnote 10, the Company issued awards containing performance-based vesting criteria ("performance-based awards") to be issued on May 31, 2023, with a value of $ 1.7 million based on the common stock price at such time, subject to the discretion of the Company's Board or Compensation Committee to pay in cash or a combination of cash and stock. Performance-based awards will be recorded as an accrued liability on the Company's condensed consolidated balance sheet and accrued over time as achievement of performance metrics become probable. The performance-based awards are eligible for vesting based on the achievement of certain performance criteria by May 31, 2023 relating to pipeline execution, business development, and financial stewardship. Performance-based awards for which the performance criteria have not been achieved as specified by May 31, 2023 will lapse and be forfeited. The performance-based awards will be subject to acceleration of vesting in the event of termination of employment without cause by the Company or by the executive for good reason (each as defined in the executive’s employment agreement). In September 2022, the Company approved an award of 140,000 performance-based stock units as part of an executive inducement grant ("Inducement PSUs"). The Inducement PSUs are eligible for vesting based on the same performance criteria, above. To the extent these performance criteria are met by May 31, 2023, 50 % of the Inducement PSUs will vest on September 12, 2023 and 50 % will vest on September 12, 2024. The Company recognized $ 1.0 million of compensation expense associated with performance-based awards and an insignificant amount of expense associated with the Inducement PSUs during the year ended December 31, 2022, as the performance conditions were considered probable of achievement. Share-Based Compensation Expense The Company recorded share-based compensation expense, for both RSUs and stock options in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2022 2021 Research and development expenses $ 3,678 $ 4,163 General and administrative expenses 5,445 5,267 Total $ 9,123 $ 9,430

Liability Related to the Sale o

Liability Related to the Sale of Future Royalties	12 Months Ended
	Dec. 31, 2022
Revenue Recognition and Deferred Revenue [Abstract]
Liability Related to the Sale of Future Royalties	9. Liability Related to the Sale of Future Royalties On September 29, 2021, the Company entered into a Revenue Interest Financing Agreement (“Revenue Interest Agreement”) with certain entities managed by HealthCare Royalty Management, LLC (“HCR”), pursuant to which the Company sold to HCR the right to receive certain royalty payments from the Company for a purchase price of up to $ 125.0 million. The Company evaluated the terms of the Revenue Interest Agreement and concluded that the features of the investment amount were similar to those of a debt instrument. The Company received gross proceeds of $ 50.0 million from HCR at an initial funding on October 19, 2021. As such, the Company accounted for this transaction as long-term debt as of December 31, 2021. The Company was entitled to receive an additional $ 50.0 million upon FDA approval of tebipenem HBr on or before December 31, 2022, and an additional $ 25.0 million subject to the mutual agreement of the Company and HCR and if the Company met certain minimum tebipenem HBr product sales thresholds in the United States within 12 months from commercial launch. On June 7, 2022, the Company entered into a Revenue Interest Termination Agreement (the “Termination Agreement”) with HCR and HCR Collateral Management, LLC, as collateral agent for HCR under the Revenue Interest Agreement, pursuant to which the parties mutually agreed to terminate the Revenue Interest Agreement between the parties, and certain other related ancillary agreements, arrangements or understandings under or contemplated by the Revenue Interest Agreement and Security Agreement between the Company and HCR. The Company was released from all of its obligations and the rights to any future revenues and collateral reverted back to the Company in return for a cash payment of $ 54.5 million and a potential additional cash amount contingent upon the occurrence of a change of control event. If a change of control event had occurred on or prior to December 31, 2022, the Company would have been obligated to pay to HCR an additional amount within 15 days following the consummation of such change of control transaction, calculated based on the fully-diluted equity value of the Company. If the change of control value was less than $ 100 million, the change of control payment would be $ 4 million, if $ 100 million to $ 200 million, the change of control payment would be $ 4 million plus 6 % of the change of control value in excess of $ 100 million and if above $ 200 million, the change of control payment would be $ 10 million plus 3 % of the change of control value in excess of $ 200 million. The change of control payment is not cumulative, such that only the highest applicable change of control value would apply in calculating the change of control payment. The rights to this contingent payment expired on December 31, 2022. The Company recognized a loss on the extinguishment of $ 3.6 million, as reported on the associated condensed consolidated statement of operations and comprehensive loss. The following table presents the changes in the liability related to the sale of future royalties under the Revenue Interest Agreement with HCR as of December 31, 2022 (in thousands): December 31, 2022 Liability related to sale of future royalties, as of December 31, 2021 $ 48,414 Repayment of liability ( 54,485 ) Loss on extinguishment of liability 3,581 Establish change of control derivative liability ( 115 ) Interest expense recognized 2,605 Liability related to sale of future royalties, as of December 31, 2022 $ —

Restructuring

Restructuring	12 Months Ended
	Dec. 31, 2022
Restructuring and Related Activities [Abstract]
Restructuring	10. Restructuring On May 3, 2022, the Company implemented a strategic restructuring initiative and corresponding reduction in workforce. The restructuring initiative and corresponding reduction in workforce was designed to reduce costs and reallocate resources towards the Company’s clinical development programs for SPR720 and SPR206, while maintaining key personnel needed to help preserve the value of the Company’s tebipenem HBr program. The restructuring reduced the Company’s workforce from 146 full-time employees as of December 31, 2021 to 41 full-time employees as of the end of the second quarter of 2022 following the restructuring. During the year ended December 31, 2022, the Company recognized a restructuring charge of $ 11.6 million, the majority of which was incurred in the second quarter of 2022. Restructuring charges included approximately $ 8.6 million of employee related termination costs and $ 3.0 million of other discontinuation costs such as contract termination fees and lease impairment expense. The following tables summarize the restructuring related charges by line item within the Company’s consolidated statements of operations where they were recorded during the year ended December 31, 2022: Year Ended December 31, 2022 Research and development General and administrative Total Severance and other employee costs $ 3,872 $ 4,680 $ 8,552 Other 488 2,590 3,078 Total restructuring charges $ 4,360 $ 7,270 $ 11,630 The restructuring charge was included in accrued expenses and other current liabilities in the Company’s condensed consolidated balance sheet. Activity for the quarter is summarized as follows (amounts in thousands): As of December 31, 2022 Balance as of December 31, 2021 $ — Charge to expense 11,630 Payments made ( 7,702 ) Write-offs and impairments ( 3,480 ) Balance as of December 31, 2022 $ 448 As of December 31, 2022, the Company had $ 0.4 million remaining in accrued expenses related to restructuring costs on its condensed consolidated balance sheet, of which the majority will be paid by the end of the first quarter of 2023. Retention Awards In June 2022, upon recommendation of the Company's Compensation Committee, the Board of Directors approved retention awards for employees of the Company. Subject to remaining actively employed with the Company through May 31, 2023, the aggregate retention awards include (i) a cash bonus of $ 1.1 million, which was paid on November 30, 2022 and $ 0.2 million as fully vested RSU grants of the same value issued on November 30, 2022 and (ii) a cash bonus of $ 3.2 million payable on May 31, 2023 and $ 0.7 million to be paid in cash or as a fully vested RSU grant of the same value. These amounts are accrued as services are performed through May 31, 2023. On July 1, 2022, upon recommendation of the Company's Compensation Committee, the Board of Directors approved a cash and RSU retention award to certain members of the Company's executive leadership team consisting of the following: • Subject to the certain members of the Company's executive leadership team remaining actively employed with the Company through May 31, 2023, they shall receive an aggregate of: (i) a cash bonus equal to $ 0.9 million, which was paid on November 30, 2022 and (ii) if certain performance criteria are achieved, a number of shares of common stock to be issued to them on May 31, 2023 having a value of $ 1.7 million based on the common stock price at such time, subject to the discretion of the Board or the Compensation Committee to pay in cash or a combination of cash and stock. The RSUs are eligible for vesting based on the achievement of certain performance criteria by May 31, 2023 relating to pipeline execution, business development, and financial stewardship. RSUs for which the performance criteria have not been achieved as specified by May 31, 2023 will lapse and be forfeited. The RSUs will be subject to acceleration of vesting in the event of termination of employment without cause by the Company or by the executive for good reason (each as defined in the executive’s employment agreement). These awards will be accrued as services are incurred through May 31, 2023. Awards with performance criteria will be accrued as performance metrics are met. During the year ended December 31, 2022, the Company recognized $ 1.0 million of compensation expense associated with these awards (see Note 8) .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes During the years ended December 31, 2022 and 2021, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period due to its uncertainty of realizing a benefit from those items. The domestic and foreign components of loss before income taxes were as follows (in thousands): Year Ended December 31, 2022 2021 Domestic $ ( 46,249 ) $ ( 89,565 ) Foreign ( 166 ) ( 191 ) Loss before income taxes $ ( 46,415 ) $ ( 89,756 ) A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2022 2021 Federal statutory income tax rate ( 21.0 ) ( 21.0 ) Federal and state research and development tax credit ( 4.3 ) ( 3.1 ) State taxes, net of federal benefit ( 2.3 ) ( 7.0 ) Nondeductible items 1.0 0.6 Increase in deferred tax asset valuation allowance 26.6 30.5 Effective income tax rate — — Net deferred tax assets as of December 31, 2022 and 2021 consisted of the following (in thousands): December 31, 2022 2021 Net operating loss carryforwards $ 78,619 $ 83,849 Research and development tax credit carryforwards 14,294 12,375 Capitalized research and development expenses, net 11,613 — Other 10,264 6,220 Total deferred tax assets 114,790 102,444 Valuation allowance ( 114,790 ) ( 102,444 ) Net deferred tax assets $ — $ — As of December 31, 2022, the Company had U.S. federal and state net operating loss carryforwards of $ 291.9 million and $ 282.9 million, respectively, which may be available to offset future income tax liabilities. The federal NOLs of $ 73.0 million will expire at various dates from 2033 to 2037 and approximately $ 218.9 million can be carried forward indefinitely. The state NOLs begin to expire in 2033 and will expire at various dates through 2039 . In addition, as of December 31, 2022 , the Company had foreign net operating loss carryforwards of $ 4.6 million, which may be available to offset future income tax liabilities and do not expire. As of December 31, 2022 , the Company also had federal and state research and development tax credit carryforwards of $ 11.5 million and $ 2.8 million, respectively, which may be available to offset future income tax liabilities and begin to expire in 2033 and 2028 , respectively. Utilization of the U.S. net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50 % over a three-year period. If the Company experiences a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. The Company completed an analysis under Section 382 through December 31, 2022 and concluded it had experienced ownership changes in the past. However, the ownership changes did not result in a limitation that at this time would materially reduce the total amount of net operating loss carryforwards and credits that can be utilized. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2022 and 2021. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2022 and 2021 related primarily to the increase in net operating loss carryforwards and research and development tax credit carryforwards, and were as follows (in thousands): December 31, 2022 2021 Valuation allowance as of beginning of year $ ( 102,444 ) $ ( 75,039 ) Increases recorded to income tax provision ( 12,346 ) ( 27,405 ) Valuation allowance as of end of year $ ( 114,790 ) $ ( 102,444 ) The Company has no t recorded any amounts for unrecognized tax benefits as of December 31, 2022 or 2021. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision. As of December 31, 2022 or 2021, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts had been recognized in the Company’s statement of operations and comprehensive loss. The Company has not, as yet, conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. The Company had filed separate U.S. income tax returns return for each of its subsidiaries prior to its reorganization in 2015. The Company now files U.S. income tax returns as a U.S. consolidated group. In Massachusetts, the Company files income tax returns as a combined group except for its Massachusetts Securities Corporation subsidiary, which is a separate income tax filing. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities remains open for all years since 2019. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state authorities to the extent utilized in a future period. No federal or state tax audits are currently in process. In response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act (“CARES Act”), was signed into law in the United States in March 2020. The CARES Act adjusted a number of provisions of the tax code, including the calculation and eligibility of certain deductions and the treatment of net operating losses and tax credits. The enactment of the CARES Act did not result in any material adjustments to the Company’s income tax provision for the year ended December 31, 2022, or to the Company’s net deferred tax assets as of December 31, 2022. On December 22, 2017, the Tax Cuts and Jobs Act ("TCJA") was signed into law. Under the TCJA provisions, effective with tax years beginning on or after January 1, 2022, taxpayers can no longer immediately expense qualified research and development expenditures, including all direct, indirect, overhead and software development costs. Taxpayers are now required to capitalize and amortize these costs over five years for research conducted within the United States or 15 years for research conducted abroad. As a result, the Company capitalized $ 47.6 million of research and development expenses for the year ended December 31, 2022.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies License Agreements The Company has entered into license agreements with various parties under which it is obligated to make contingent and non-contingent payments (see Note 14). Operating Leases The Company has entered into an operating lease agreement with U.S. REIF Central Plaza Massachusetts, LLC with respect to its corporate headquarters located at 675 Massachusetts Avenue, Cambridge, Massachusetts (see Note 5). Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements that will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2022 or 2021. Legal Proceedings Two putative class action lawsuits were filed against the Company and certain of its officers in the United States District Court for the Eastern District of New York, one captioned Richard S. Germond v. Spero Therapeutics, Inc., Ankit Mahadevia, and Satyavrat Shukla, Case No. 1:22-cv-03125, filed on May 26, 2022, and the other captioned Kashif Memon v. Spero Therapeutics, Inc., Ankit Mahadevia, and Satyavrat Shukla Case No. 1:22-cv-04154, filed on July 15, 2022. The parties moved to consolidate the two complaints on July 22, 2022, which were ordered consolidated on August 5, 2022. Both Mr. Memon and stockholder Nabil Saad filed motions for appointment of lead plaintiff/lead counsel on July 25, 2022. Mr. Saad withdrew his opposition on August 15, 2022, and Mr. Memon was appointed lead plaintiff on September 19, 2022. The complaint purports to be brought on behalf of stockholders who purchased the Company’s common stock from May 6, 2021 through May 2, 2022 (Mr. Memon’s complaint alleged a six-month longer class period than the complaint originally filed by Mr. Germond). The complaint generally alleges that the Company and certain of its officers violated Sections 10(b) and/or 20(a) of the Securities Exchange Act of 1934 and Rule 10b-5 promulgated thereunder by making allegedly false and/or misleading statements concerning the New Drug Application for tebipenem HBr in an effort to lead investors to believe that the drug would receive approval from the FDA. The complaint seeks unspecified damages, interest, attorneys’ fees, and other costs. Mr. Memon filed an amended complaint on December 5, 2022. The amended complaint generally alleges the same arguments provided in the original complaint. The Company notified the Court of its intention to file a motion to dismiss the amended complaint in its entirety. The Court held a conference on February 22, 2023 and set a date of March 31, 2023 for the Company to file a motion to dismiss. The Company denies any allegations of wrongdoing and intends to vigorously defend against this lawsuit. However, there is no assurance that the Company will be successful in its defense or that insurance will be available or adequate to fund any settlement or judgment or the litigation costs of this action. Moreover, the Company is unable to predict the outcomes or reasonably estimate a range of possible loss at this time. Additional lawsuits against the Company and certain of its officers or directors may be filed in the future. If additional similar complaints are filed, absent new or different allegations that are material, the Company will not necessarily announce such additional filings.

Government Contracts

Government Contracts	12 Months Ended
	Dec. 31, 2022
Contractors [Abstract]
Government Contracts	13. Government Contracts BARDA In July 2018, the Company was awarded a contract from Biomedical Advanced Research and Development Authority (“BARDA”) of up to $ 44.2 million to develop tebipenem HBr for the treatment of complicated urinary tract infections (“cUTI”) caused by antibiotic resistant Gram-negative bacteria and for assessment against biodefense pathogens. The award committed initial funding of $ 15.7 million over a three-year base period from July 1, 2018 to June 30, 2021 for cUTI development activities. In May 2019, the contract was modified to include additional funding of approximately $ 2.5 million for the development of tebipenem HBr, increasing the amount of the initial committed funding from $15.7 million to approximately $ 18.2 million and increasing the overall potential award to $ 46.8 million. In January 2020, BARDA exercised its first contract option for additional committed funding of $ 15.9 million, increasing the total committed funding to $ 34.0 million and extended the period of performance through November 1, 2021. In October 2022, BARDA extended the period of performance for the first contract option through December 15, 2023. As of December 31, 2022, the balance of the award was subject to BARDA exercising a second option which would entail funding of $ 12.7 million and is exercisable by BARDA subject to, among other things, satisfactory progress and results from the biodefense studies described below. On January 19, 2022, the Company announced that BARDA exercised a new option under the contract. The new option increases the total amount of committed funding by $ 12.9 million to approximately $ 46.9 million, increasing the total potential contract value to $ 59.7 million. In September 2022, remaining funding from the $ 12.9 million option was reprogrammed to support clinical trials for patients with cUTIs, including acute pyelonephritis. The period of performance for this new option extends through December 31, 2025 and does not change the total amount of committed funding or potential contract value. As part of an inter-agency collaboration between BARDA and the DTRA, a series of studies to assess the efficacy of tebipenem HBr in the treatment of infections caused by biodefense threats such as anthrax, plague and melioidosis will be conducted under the direction of Spero. DTRA provides up to $ 10.0 million, in addition to the total potential award from BARDA, to cover the cost of the nonclinical biodefense aspects of the collaborative program for tebipenem HBr. Together, BARDA and DTRA will provide up to $6 9.7 million in total funding for the clinical development and biodefense assessment of tebipenem HBr, of which $ 12.7 million is subject to the exercise of options by BARDA and Spero’s achievement of specified milestones. During the years ended December 31, 2022 and 2021, the Company recognized $ 2.2 million and $ 9.9 million of revenue under this agreement, respectively. U.S. Department of Defense On July 1, 2019, the Company received a $ 5.9 million award from the DoD Congressionally Directed Medical Research Programs (“CDMRP”) Joint Warfighter Medical Research Program. The funding supported the further clinical development of SPR206. The award committed non-dilutive funding of $ 5.9 million over a four-year period to cover the costs of select Phase 1 pharmacology studies, a 28-day GLP NHP toxicology study, and microbiological surveillance studies that would be required for a potential NDA submission with the FDA for SPR206. During the years ended December 31, 2022 and 2021, the Company recognized $ 1.0 million and $ 4.5 million in revenue under this agreement, respectively. As of December 31, 2022, all activities under this award were completed and this award was closed out. NIAID In May 2021, the Company was awarded a five-year contract from the U.S. National Institute of Allergy and Infectious Diseases (“NIAID”) under the Agency’s Omnibus Broad Agency Announcement No. HHS-NIH-NIAID-BAA2020-1 award mechanism to support further development of SPR206. Funding will be used to offset certain expenses related to manufacturing, clinical, non-clinical and regulatory activities. The Company can receive up to $ 23.4 million over a base period and five option periods. As of December 31, 2022, funding for the base period totaling $ 6.2 million has been committed. In December 2022, the contract was modified to include additional funding of approximately $ 0.1 million increasing the amount of base period committed funding from $ 2.1 million to $ 2.2 million. This increased the total potential contract value to $ 23.5 million. In October 2022, NIAID exercised its first option under the contract, committing $ 4.0 million for SPR206 through April 2025. The Company recognized $ 1.7 million and $ 0.4 million under this agreement during the years ended December 31, 2022 and 2021, respectively. In June 2016, the Company entered into agreements with Pro Bono Bio PLC (“PBB”), a corporation organized under the laws of England, and certain of its affiliates, including PBB Distributions Limited and Cantab Anti-Infectives Limited (“CAI”), in order to acquire certain intellectual property and government funding arrangements relating to SPR206. Under these agreements, CAI agreed to submit a request to NIAID to novate the then CAI-held NIAID contract to Spero, which was finalized in December 2017. The NIAID contract provided development funding of up to $ 6.5 million over a base period and three option periods. In March 2021, a contract modification was executed and the performance period for this award was extended through June 15, 2021. As of December 31, 2021, funding for the base period and the first two option periods totaling $ 5.9 million had been committed. The Company did not recognize revenue under this agreement during the year ended December 31, 2022 and recognized $ 0.4 million in revenue under this agreement, during the year ended December 31, 2021.

License, Collaboration and Serv

License, Collaboration and Service Agreements	12 Months Ended
	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License, Collaboration and Service Agreements	14. License, Collaboration and Service Agreements The Company has certain obligations under license agreements with third parties that include annual maintenance fees and payments that are contingent upon achieving various development, regulatory and commercial milestones. Pursuant to these license agreements, the Company is required to make milestone payments if certain development, regulatory and commercial milestones are achieved, and may have certain additional research funding obligations. Also, pursuant to the terms of each of these license agreements, when and if commercial sales of a product commence, the Company will pay royalties to its licensors on net sales of the respective products. SPR720 Agreements Gates MRI In June 2019, the Company entered into a collaboration with Gates MRI to develop SPR720 for the treatment of lung infections caused by Mycobacterium tuberculosis. In furtherance of the Gates MRI’s charitable purposes, the Company also granted to Gates MRI a no-cost, exclusive license to develop, manufacture and commercialize SPR720 for the treatment of tuberculosis (“TB”) in low- and middle- income countries. The Gates MRI is responsible for formulating and funding its own research plan for the development of SPR720 for TB. As such, Gates MRI will conduct and fund preclinical and clinical studies for the development of SPR720 against TB. In addition, Gates MRI and the Company will jointly design and manage certain collaborative research activities, which the Company will perform and which will be funded by the Gates MRI. Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company will recognize the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred. The Company did not record a reduction to research and development expense as no activities were funded by Gates MRI during the year ended December 31, 2022. The Company did not record a reduction to research and development expense related to activities funded by Gates MRI during the year ended December 31, 2022 and recorded a $ 1.5 million reduction to research and development expense related to activities funded by Gates MRI during the year ended December 31, 2021. Vertex License Agreement In May 2016, the Company entered into an agreement with Vertex Pharmaceuticals Incorporated (“Vertex”) whereby Vertex granted the Company certain know-how and a sublicense to research, develop, manufacture and sell products for a proprietary compound, as well as a transfer of materials. In exchange for the know-how, sublicense and materials, Spero paid Vertex an upfront, one-time, nonrefundable, non-creditable fee of $ 0.5 million, which was recognized as research and development expense. As part of the agreement, the Company is obligated to make future milestone payments of up to $ 80.2 million upon the achievement of specified clinical, regulatory and commercial milestones and to pay Vertex tiered royalties, on a product-by-product and country-by-country basis, of a mid-single-digit to low double-digit percentage based on net sales of products licensed under the agreement. During the years ended December 31, 2022 and 2021, the Company did no t record any research and development expense under this agreement and the next milestone under this agreement is not accrued because it is not yet probable. The agreement continues in effect until the expiration of all payment obligations thereunder, with royalty payment obligations continuing on a product-by-product and country-by-country basis until the later of ten years after the first commercial sale of such product in such country or the date of expiration in such country of the last to expire applicable patent. Further, Vertex has the right to terminate the agreement if provided with notification from the Company of intent to cease all development or if no material development or commercialization efforts occur for one year . Tebipenem HBr Agreements GSK License and Share Purchase Agreement On November 7, 2022, the Company closed the transactions contemplated by the GSK License Agreement, which was entered into on September 21, 2022. Pursuant to the terms of the GSK License Agreement, the Company granted GSK an exclusive royalty-bearing license, with the right to grant sublicenses, under the Company’s intellectual property and regulatory documents and a sublicense under certain intellectual property of Meiji Seika Pharma Co. Ltd. (“Meiji”) and Meiji’s regulatory documents to develop, manufacture and commercialize tebipenem pivoxil and tebipenem HBr and products that contain tebipenem pivoxil and tebipenem HBr (the “GSK Licensed Products”) in all territories, except certain Asian countries previously licensed to Meiji (Japan, Bangladesh, Brunei, Cambodia, China, Indonesia, Laos, Malaysia, Myanmar, Philippines, Singapore, South Korea, Taiwan, Thailand and Vietnam (the “Meiji Territory”)) (the “GSK Territory”). If the Company's license with Meiji is terminated, or if Meiji forfeits or loses its rights to develop, manufacture and commercialize tebipenem HBr and products that contain tebipenem HBr in any countries in the Meiji Territory, then GSK will have an exclusive first right to negotiate with Spero to add any such countries to the GSK Territory. Under the terms of the GSK License Agreement, the Company received an upfront payment of $ 66.0 million for GSK to secure rights to the medicine. Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) From FDA acceptance of clinical protocol, commencement of Phase 3 study through NDA submission $ 150.0 (in tranched milestones) Total commercial milestone payments based on first sale (US/EU) $ 150.0 Total potential sales milestone payments $ 225.0 Royalties Low-single digit to low-double digit (if sales exceed $ 1.0 billion) tiered royalties on net product sales Royalties are subject to reduction in the event of third-party licenses, entry of a generic product or expiration of patent and regulatory exclusivity prior to the tenth anniversary of the first commercial sale of a GSK Licensed Product in a particular country. The Company will be responsible for the execution and costs of the follow-up Phase 3 clinical trial of tebipenem HBr. GSK will be responsible for the execution and costs of any additional further development, including additional Phase 3 regulatory filing and commercialization activities for tebipenem HBr in the GSK Territory. The Company will also be responsible for providing and paying for the clinical supply of tebipenem HBr while GSK will be responsible for the costs of the commercial supply of tebipenem HBr. A joint development committee has been established between GSK and the Company to coordinate and review development activities for tebipenem HBr in the United States. Unless earlier terminated due to certain material breaches of the GSK License Agreement or by GSK for convenience, or otherwise, the GSK License Agreement will expire on a jurisdiction-by-jurisdiction and GSK Licensed Product-by-GSK Licensed Product basis on the latest to occur of (i) loss of patent exclusivity, (ii) loss of regulatory exclusivity or (iii) the Royalty Term. During the Royalty Term, the Company has agreed not to develop, manufacture or commercialize any oral carbapenem for any indication or any oral antibiotic for cUTI; this restriction does not apply to any third party which acquires control of Spero after the date of the GSK License Agreement if certain conditions are met. The Company has the right to terminate the GSK License Agreement upon a material breach by, or bankruptcy of, GSK. GSK has the right to terminate the GSK License Agreement at any time upon a specified number of days’ notice or upon a material breach by, or bankruptcy of, the Company. In addition, in the event that GSK has the right to terminate the GSK License Agreement due to a breach by the Company, GSK may elect not to terminate the GSK License Agreement and in lieu thereof may assume the responsibility and expense of development of tebipenem HBr in the United States, in which event GSK’s obligation to make further development payments to the Company would cease, and/or to reduce all subsequent commercial and sales milestone payments and royalty payments otherwise due by GSK to the Company under the GSK License Agreement by 50 %. The GSK License Agreement contains representations and warranties, other covenants, indemnification provisions and other terms and conditions customary for transactions of the type contemplated by the GSK License Agreement. In support of certain of its rights to indemnification, GSK also has certain rights to suspend payments otherwise owed to the Company, as well as the right to offset payments otherwise owed to the Company against certain indemnifiable claims. Accounting Analysis and Revenue Recognition The Company determined that GSK is a customer and that the GSK License Agreement is within the scope of ASC 606 as licensing intellectual property and performing ongoing research and development services are ordinary activities that are ongoing and central to the Company’s operations. Accordingly, in determining the appropriate amount of revenue to be recognized, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including the constraint on variable consideration; (iv) allocated the transaction price to the identified performance obligations in proportion to their SSP; and (v) recognized revenue when each performance obligation was deemed to be satisfied. Based on that evaluation, the Company identified two performance obligations, related to the license and to research and development services. The Company developed the estimated SSP for the license using a discounted cash flow model. In developing this estimate, the Company applied significant judgment in the determination of the significant assumptions relating to forecasted future cash flows, the discount rate, and the probability of success. The SSP for the research and development services was estimated based on the Company's estimate of costs to be incurred to fulfill its obligations associated with the performance of the research and development services, plus a reasonable margin. The total transaction price is $ 64.7 million, which includes the initial payment of $ 66.0 million and the discount of $ 1.3 million related to the GSK SPA (see Note 7). At contract inception, $ 45.7 million of the initial $ 64.7 million was allocated to the license transfer performance obligation, which was fully satisfied and recognized as revenue upon delivery of the license. The additional $ 19.0 million was allocated to the research and development services obligation and is being recognized over time as services are delivered, estimated to be over a three year period. The potential development milestone payments from the GSK License Agreement will be accounted for as variable consideration under ASC 606. Given the uncertain nature of these payments, the Company determined they were fully constrained as of December 31, 2022 and not included in the transaction price. The Company can also earn sales-based royalties. The Company recognizes revenue for the license performance obligation at a point in time, that is upon transfer of the license to GSK. Control of the license was transferred on the GSK Effective Date and GSK could begin to use and benefit from the license at the GSK Effective Date. In total, the Company recognized $ 46.1 million during the year ended December 31, 2022 related to the GSK License Agreement, which included $ 45.7 million recognized upfront upon the license and know-how transfer and $ 0.4 million related to the research and development service obligation. The remaining transaction price balance of approximately $ 18.6 million from the GSK License Agreement allocated to the research and development services performance obligation has been recorded as deferred revenue in the condensed consolidated balance sheet. As of December 31, 2022, the research and development services related to the second performance obligation were expected to be recognized as costs are incurred over the project development timeframe. Meiji License Agreement In June 2017, the Company entered into agreements with Meiji, whereby Meiji granted to the Company a license under certain patents, know-how and regulatory documentation to research, develop, manufacture and sell products containing a proprietary compound in the licensed territory. In exchange for the license, the Company paid Meiji an upfront, one-time, nonrefundable, non-creditable fee of $ 0.6 million, which was recognized as research and development expense. In October 2017, the Company paid a $ 1.0 million milestone payment to Meiji upon the enrollment of the first patient in the Company’s Phase 1 clinical trial of tebipenem HBr. The payment was recorded as research and development expense in the statement of operations and comprehensive loss for the year ended December 31, 2017. The Company paid Meiji approximately $ 1.6 million during the fourth quarter of 2018 related to fixed assets which will be used in manufacturing related activities at Meiji. This equipment has been capitalized as property and equipment in the consolidated balance sheet as of December 31, 2022. In October 2021 , the Company paid a $ 1.0 million milestone payment to Meiji upon submission of a NDA to the FDA for tebipenem HBr. As part of the agreement, the Company is obligated to make future milestone payments of up to $ 1.0 million as of December 31, 2022 upon the achievement of specified regulatory milestones, to pay royalties, on a product-by-product and country-by-country basis, of a low single-digit percentage based on net sales of products licensed under the agreement and to pay Meiji a low double-digit percentage of any sublicense fees received by the Company up to a maximum amount of $ 7.5 million, of which the Company paid $ 6.6 million as of December 31, 2022 and was recorded as research and development expense in the Company's consolidated statement of operations. The agreement continues in effect until the expiration of all payment obligations thereunder (including royalty payments and licensee revenue) on a product-by-product and country-by-country basis, unless earlier terminated by the parties. Pursuant to the terms of the agreement, in addition to each party’s right to terminate the agreement upon the other party’s material breach (if not cured within a specified period after receipt of notice) or insolvency, the Company also has unilateral termination rights (i) in the event that the Company abandons the development and commercialization of tebipenem HBr for efficacy, safety, legal or business reasons, and (ii) under certain circumstances arising out of the head license with a global pharmaceutical company. Savior Service Agreement In November 2018, the Company entered into a service agreement with Savior Lifetec Corporation (“Savior”) to perform technology transfer, process development, analytical method development and testing and formulation development for tebipenem HBr. Per the terms of the agreement, the Company paid Savior a non-refundable supervision fee of approximately $ 2.0 million to manage the buildout of a commercial manufacturing facility. The supervision fee was classified as a prepaid asset on the Company’s balance sheet and was fully amortized as of December 31, 2021. The Company has paid Savior an additional $ 5.3 million for facility build out costs, which is classified as a long-term asset on the Company’s balance sheet as of December 31, 2022. SPR206 Agreements Cantab License Agreements Under the Cantab Agreements, the Company is obligated to make future milestone payments of up to $ 5.8 million upon the achievement of specified clinical and regulatory milestones and a payment of £ 5.0 million ( $ 6.0 million as of December 31, 2022) upon the achievement of a specified commercial milestone. In addition, the Company agreed to pay to PBB royalties, on a product-by-product and country-by-country basis, of a low single-digit percentage based on net sales of products licensed under the agreement. During both the years ended December 31, 2022 and 2021, the Company did no t record any research and development expense related to the achievement of regulatory milestones for SPR206. The Cantab Agreements continue indefinitely, with royalty payment obligations thereunder continuing on a product-by-product and country-by-country basis until the later of ten years after the first commercial sale of such product in such country or the expiration in such country of the last to expire valid claim of any of the applicable patents. Everest Medicines License Agreement On January 4, 2019, the Company, through its wholly owned subsidiary New Pharma License Holdings Limited (“NPLH”), entered into a license agreement (the “Original Everest License Agreement”), with Everest. Under the terms of the Original Everest License Agreement, the Company granted Everest an exclusive license to develop, manufacture and commercialize SPR206 or products that contain SPR206 (the “Licensed Products”), in Greater China (which includes Mainland China, Hong Kong and Macau), South Korea and certain Southeast Asian countries (the “Territory”). The Company retained development, manufacturing and commercialization rights with respect to SPR206 and Licensed Products in the rest of the world and also retained the right to develop or manufacture SPR206 and Licensed Products in the Territory for use outside the Territory. In addition to the license grant with respect to SPR206, the Company, through its wholly owned subsidiary, Spero Potentiator, Inc., a Delaware corporation, granted Everest a 12 -month exclusive option to negotiate with it for an exclusive license to develop, manufacture and commercialize SPR741 in the Territory. Under the terms of the Original Everest License Agreement, the Company received an upfront payment of $ 3.0 million that was recognized in the first quarter of 2019, comprised of a $ 2.0 million payment to license SPR206 and $ 1.0 million for the exclusive option to negotiate a license to develop SPR741. The Company also received a milestone payment of $ 2.0 million in the fourth quarter of 2020 upon completion and delivery of the results of a clinical study. On January 15, 2021, the Company entered into an amended and restated license agreement (“the Amended Everest License Agreement”) with Everest and Spero Potentiator, Inc., which amended and restated in its entirety the Original Everest License Agreement. The Amended Everest License Agreement modifies the dates and values of certain milestone events related to development and commercialization of SPR206. Everest will now be making more significant investments in the development of SPR206 beyond what was contemplated at the time of the Original Everest License Agreement. The Original Everest License Agreement provided that the Company could receive up to $ 59.5 million upon achievement of certain milestones. The Amended Everest License Agreement provides that the Company may receive up to $ 38.0 million upon achievement of certain milestones, of which $ 2.0 million has been received to date. The Company received milestones of $ 1.5 million related to a clinical study for SPR206, of which the Company received approximately $ 0.8 million upon the initiation of the BAL clinical trial of SPR206 in June 2021 and received the remaining $ 0.7 million upon the delivery of the clinical study report in the second quarter of 2022. In addition, under the Amended Everest License Agreement, the Company assigned patents in the Territory to Everest, rather than licensing such patents to Everest, and the option related to SPR741 and the related provisions have been removed. Under the terms of the Amended Everest License Agreement, the Company is also entitled to receive high single-digit to low double-digit royalties on net sales, if any, of Licensed Products in the Territory following regulatory approval of SPR206. Everest has the right to sublicense to affiliates and third parties in the Territory. Everest is responsible for all costs related to developing, obtaining regulatory approval of and commercializing SPR206 and Licensed Products in the Territory, and is obligated to use commercially reasonable efforts to develop, manufacture and commercialize Licensed Products, including to achieve certain specified diligence milestones within agreed-upon periods. A joint development committee has been established between the Company and Everest to coordinate and review the development, manufacturing and commercialization plans with respect to Licensed Products in the Territory. Unless earlier terminated due to certain material breaches of the contract, or otherwise, the Amended Everest License Agreement will expire on a jurisdiction-by-jurisdiction and Licensed Product-by-Licensed Product basis upon the latest to occur of expiration of the last valid claim under a licensed patent in such jurisdiction, the expiration of regulatory exclusivity in such jurisdiction or ten years after the first commercial sale of such Licensed Product in such jurisdiction. The Amended Everest License Agreement may be terminated in its entirety by Everest upon 90 or 180 days ’ prior written notice, depending on the stage of development of the initial Licensed Product. To date, all performance obligations under the contract were fully satisfied. As of December 31, 2022 remaining future milestone payments of $ 34.0 are fully constrained, and will be recognized when and if achievement of those milestones becomes probable. During the year ended December 31, 2022 , the Company recognized approximately $ 0.7 million of revenue under this agreement related to certain milestone achievements. During the year ended December 31, 2021, the Company recognized $ 1.3 million of revenue related to this agreement. Pfizer License and Share Purchase Agreements On June 30, 2021, the Company and Pfizer entered into the Pfizer License Agreement and the Pfizer Purchase Agreement. Under the terms of the Pfizer License Agreement, the Company granted Pfizer an exclusive royalty-bearing license to develop, manufacture and commercialize SPR206 or products that contain SPR206 (the “Licensed Products”) globally with some territorial exceptions (the “Pfizer Territory”). The Pfizer Territory excludes the United States and the Asian markets previously licensed to Everest, those being the People’s Republic of China, including Hainan Island, the Hong Kong Special Administrative Region of the People’s Republic of China, and the Macau Special Administrative Region of the People’s Republic of China, Taiwan, the Republic of Korea (South Korea), the Republic of Singapore, Malaysian Federation, Kingdom of Thailand, the Republic of Indonesia, Socialist Republic of Vietnam and the Republic of the Philippines). Under the terms of the Pfizer Purchase Agreement, Pfizer purchased 2,362,348 shares of the Company’s common stock at a price of $ 16.93 per share for a total investment of $ 40.0 million. Under the terms of the Pfizer License Agreement, the Company received no other upfront payments but is eligible to receive up to $ 80.0 million in development and sales milestones, and may also receive high single-digit to low double-digit royalties on net sales of SPR206 in the Pfizer Territory. Achievement of these payments cannot be guaranteed. The Company and Pfizer agree that upon Pfizer’s request, the parties will negotiate in good faith regarding procuring a clinical or commercial supply of the compound. The fair market value of 2,362,348 shares of the Company's common stock issued to Pfizer under the Pfizer Purchase Agreement was determined to be $ 27.5 million. The common stock issued under the Pfizer Purchase Agreement were valued using an option pricing valuation model as the shares are subject to certain holding period restrictions. The Company accounted for the associated premium of $ 12.5 million as a freestanding equity-linked instrument under ASC 815. The premium was allocated as consideration for the Pfizer License Agreement and evaluated under ASC 606. The premium was determined not to be constrained and was included in the calculation of the total transaction price related to the Pfizer License Agreement as of June 30, 2021. The Company is responsible for all costs related to developing and obtaining regulatory approval of SPR206 and Licensed Products in the Pfizer Territory, with a focus on the European market, and is obligated to use commercially reasonable efforts, including to achieve certain specified diligence milestones within agreed-upon periods. A joint development committee was established between the Company and Pfizer to coordinate and review the development, manufacturing and commercialization plans with respect to Licensed Products in the Pfizer Territory. Pfizer is responsible for commercializing SPR206 and the Licensed Products in the Pfizer Territory. Unless earlier terminated due to certain material breaches of the contract or by Pfizer’s convenience, or otherwise, the Pfizer License Agreement will expire on a jurisdiction-by-jurisdiction and licensed product-by-licensed product basis after ten years from the effective date. The Pfizer License Agreement will automatically renew for an additional ten-year term unless terminated. Accounting Analysis and Revenue Recognition The Company determined that Pfizer is a customer and that the Pfizer License Agreement is within the scope of ASC 606 as licensing intellectual property and performing ongoing research and development services are ordinary activities that are ongoing and central to the Company’s operations. Accordingly, in determining the appropriate amount of revenue to be recognized, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including the constraint on variable consideration; (iv) allocated the transaction price to the identified performance obligations in proportion to their SSP; and (v) recognized revenue when each performance obligation was deemed to be satisfied. Based on that evaluation, the Company identified two performance obligations, license and know-how transfer and research and development services related to upcoming milestones. The Company determined that the supply agreement is a customer option and not a material right, as the pricing to Pfizer is not at a significant discount. Furthermore, Pfizer has the right to use third parties to manufacture the compound, or to manufacture the compound itself. At contract inception, $ 1.4 million of the then transaction price of $ 12.5 million was allocated to the license and know-how transfer performance obligations, which was fully satisfied and recognized as revenue upon delivery of the license. The additional $ 11.1 million was allocated to the research and development services obligation and is being recognized over time as services are delivered. In the third quarter of 2022, upon the completion of a milestone related to regulatory engagement for SPR206, Pfizer communicated its approval that the milestone was achieved, and the Company received $ 5.0 million under the Pfizer License Agreement, which the Company accounted for as variable consideration under ASC 606 and was added to the transaction price in the third quarter of 2022. Of this $ 5.0 million milestone, $ 0.9 million was recognized during the third quarter of 2022 and the remaining $ 4.1 million was allocated to the research and development services performance obligation and will be recognized over time as the services are delivered. The potential license maintenance fees and development milestone payments from the Pfizer License Agreement will be accounted for as variable consideration under ASC 606. Given the uncertain nature of these payments, the Company determined they were fully constrained as of December 31, 2022 and not included in the transaction price. The Company can also earn sales-based royalties. The Company recognizes revenue for the license performance obligation at a point in time, that is upon transfer of the license to Pfizer. Control of the license was transferred on the Effective Date and Pfizer could begin to use and benefit from the license at the Effective Date. In total, and inclusive of the above, the Company recognized $ 1.8 million during both the twelve months ended December 31, 2022 and 2021, respectively. The remaining transaction price balance of approximately $ 13.9 million from the Pfizer Purchase Agreement allocated to the research and development services performance obligation has been recorded as deferred revenue in the condensed consolidated balance sheet. As of December 31, 2022, the research and development services related to the second performance obligation were expected to be recognized as costs are incurred over the project development timeframe.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2022
Earnings Per Share [Abstract]
Net Loss per Share	15. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts): Year Ended December 31, 2022 2021 Numerator: Net loss $ ( 46,415 ) $ ( 89,756 ) Net loss attributable to common stockholders $ ( 46,415 ) $ ( 89,756 ) Denominator: Weighted average common shares outstanding, basic and diluted 37,585,075 30,895,756 Net loss per share, basic and diluted $ ( 1.23 ) $ ( 2.91 ) The Company excluded potentially dilutive securities 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 common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2022 2021 Options to purchase common stock 3,979,099 4,890,716 Unvested restricted stock units 1,300,397 513,690 Series A convertible preferred stock (as converted to common shares) — — Series B convertible preferred stock (as converted to common shares) — 938,000 Series C convertible preferred stock (as converted to common shares) — 2,214,000 Series D convertible preferred stock (as converted to common shares) — 3,215,000 Total 5,279,496 11,771,406

Retirement Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2022
Retirement Benefits [Abstract]
Retirement Plan	16. Retirement Plan The Company has a defined-contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pre-tax basis. As currently established, the Company is not required to make any contributions to the 401(k) Plan. The Company made matching contributions to the 401(k) Plan of $ 0.4 million and $ 0.5 million during the years ended December 31, 2022 and 2021, respectively.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2022
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events The Company has evaluated, for potential recognition and disclosure, events that occurred prior to the date at which the consolidated financial statements were available to be issued. There were no material subsequent events. Subsequent to December 31, 2022, the Company did no t sell any shares of its common stock under the Sales Agreement.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2022
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with 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. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual for clinical trial costs and other research and development expenses and the valuation of share-based awards. There may be changes to those estimates in future periods. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on identifying, developing and commercializing novel treatments for bacterial infections, including MDR bacterial infections, and rare diseases. All of the Company’s tangible assets are held in the United States.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains most of its cash and cash equivalents at one accredited financial institution. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. As of December 31, 2022, and 2021, the Company had no off-balance sheet risk such as foreign exchange contracts, option contracts, or other hedging arrangements.
Cash Equivalents	Cash Equivalents 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 and cash equivalents include cash held in banks and money market instruments.
Marketable Securities	Marketable Securities Marketable securities consist of investments in corporate obligations with original maturities greater than 90 days. The Company considers its portfolio of investments to be available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Investments with maturities beyond one year are generally classified as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value are included as a component of other income (expense), net based on the specific identification method. Any credit impairments are recorded through an allowance account.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense are recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Computer software and equipment 3 years Office furniture and equipment 7 years Manufacturing equipment 5 years Leasehold improvements Shorter of life of lease or 5 years Costs for capital assets not yet placed into service are capitalized as construction in progress and are depreciated in accordance with the above guidelines once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. The Company periodically evaluates whether events and circumstances have occurred that may warrant revision of the estimated useful life of property and equipment.
Leases	Leases At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. As of December 31, 2022 and 2021, the Company had no short-term leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew. The Company monitors its plans to renew its material leases on a quarterly basis. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate (“IBR”), which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, and in a similar economic environment. Since the Company does not have any debt and has not been rated by any major credit rating agency, the Company’s IBR was estimated by developing a synthetic credit rating for the Company. The Company has elected to account for lease and non-lease components together as a single lease component.
Other Assets	Other Assets Other assets consist of long-term prepayments and deposits.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an 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. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.
Revenue Recognition - Collaboration Revenue	Revenue Recognition - Collaboration Revenue The Company has entered into licensing agreements that are evaluated under Accounting Standards Codification, Topic 606 (“Topic 606”), Revenue from Contracts with Customers, through which the Company licenses certain of its product candidates’ rights to a third party. Terms of these arrangements include various payment types, typically including one or more of the following: upfront license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and/or royalties on net sales of licensed products. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations under the agreement; (iii) determine the transaction price, including constraint on variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) determine how the revenue will be recognized for each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. The SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangements. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method. In determining the accounting treatment for these arrangements, the Company develops assumptions to determine the stand-alone selling price for each performance obligation in the contract. The Company develops the estimated standalone selling price for the license using a discounted cash flow model. To develop this model, the Company applies significant judgement in the determination of the significant assumptions relating to forecasted future revenues, development time lines, the discount rate, and probabilities of technical and regulatory success. The Company develops the estimated standalone selling price for the research and development services using a discounted cash flow model. The assumptions to develop the estimated standalone selling price for the related research and development services include estimates of costs to be incurred to fulfill its obligations associated with the performance of the research and development services, plus a reasonable margin.
Government Tax Incentives	Government Tax Incentives For available government tax incentives that the Company may earn without regard to the existence of taxable income and that require the Company to forego tax deductions or the use of future tax credits and net operating loss carryforwards, the Company classifies the funding recognized as a reduction of the related qualifying research and development expenses incurred. Since the fourth quarter of 2016, the Company’s operating subsidiary in Australia has met the eligibility requirements to receive a tax incentive for qualifying research and development activities. The Company recognizes these incentives as a reduction of research and development expenses in the consolidated statements of operations and comprehensive loss in the same period that the related qualifying expenses are incurred. Reductions of research and development expense recognized upon incurring qualifying expenses in advance of receipt of tax incentive payments are recorded in the consolidated balance sheet as tax incentive receivables.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including personnel salaries, share-based compensation and benefits, allocated facilities costs, depreciation, manufacturing expenses, costs related to the Company’s government contract and grant arrangements, and external costs of outside vendors engaged to conduct preclinical development activities, clinical trials as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. In June 2019, the Company entered into a collaboration agreement with the Bill and Melinda Gates Medical Research Institute (the “Gates MRI”) and concluded that the agreement is within the scope of the accounting guidance for collaboration arrangements (see Note 14). Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company recognizes the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred.
Clinical Trial and other Research Contract Costs and Accruals	Clinical Trial and other Research Contract Costs and Accruals The Company has entered into various research and development contracts with clinical research organizations and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. There may be instances in which payments made to these vendors exceed the level of service provided and will result in a prepayment of the expense. The Company records accruals for estimated ongoing research and clinical trial costs based on the services received and efforts expended pursuant to multiple contracts with these vendors. When evaluating the adequacy of the accrued liabilities, the Company analyzes the progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Restructuring	Restructuring The Company made estimates and judgments regarding the amount and timing of its restructuring expense and liability, including one-time termination benefits and other exit costs accounted for upon the announcement of the restructuring in May 2022. Restructuring charges are reflected in the Company's consolidated statements of income.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Share-Based Compensation	Share-Based Compensation The Company issues stock-based awards to employees and directors in the form of stock options and restricted stock units. The Company measures and recognizes compensation expense for its stock-based awards granted to its employees and directors based on the estimated grant date fair value in accordance with ASC 718 and determines the fair value of restricted stock units based on the fair value of its common stock. The Company measures all share-based options granted to employees and directors based on the fair value on the date of grant using the Black-Scholes option-pricing model. Compensation expense of those awards is recognized over the requisite service period, which is generally the vesting period of the respective award. The Company records the expense for awards with service-based conditions using the straight-line method over the requisite service period, net of any actual forfeitures. The Company has also granted certain awards subject to performance-based vesting eligibility and a subsequent partial time-based vesting schedule. Performance-based awards will be recorded as an accrued liability on the Company's condensed consolidated balance sheet and accrued over time as achievement of performance metrics become probable. The Company classifies share-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. For the years ended December 31, 2022 and 2021, these changes related to unrealized gains and losses on the Company’s available-for-sale marketable securities. There were no reclassifications out of comprehensive loss for the years ended December 31, 2022 and 2021.
Net Loss per Share	Net Loss per Share The Company follows the two-class method when computing net income (loss) per share, as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Net income (loss) per share attributable to common stockholders is calculated based on net income (loss) attributable to Spero Therapeutics, Inc. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50 % likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. On December 22, 2017, the Tax Cuts and Jobs Act ("TCJA") was signed into law. Under the TCJA provisions, effective with tax years beginning on or after January 1, 2022, taxpayers can no longer immediately expense qualified research and development expenditures, including all direct, indirect, overhead and software development costs. Taxpayers are now required to capitalize and amortize these costs over five years for research conducted within the United States or 15 years for research conducted abroad.
Liability Related To The Sale of Future Royalties	Liability Related to the Sale of Future Royalties Prior to repayment, as discussed further in Note 9, the Company treated the liability related to the sale of future royalties as a debt instrument, amortized under the effective interest rate method over the estimated life of the revenue streams. The Company recognized interest expense thereon using the effective rate, which was based on its estimates of future revenues over the life of the arrangement. The Company periodically assessed its expected revenues using internal projections, imputed interest on the carrying value of the deferred royalty obligation and recorded interest expense using the imputed effective interest rate. To the extent its estimates of future revenues were greater or less than previous estimates or the estimated timing of such payments was materially different than previous estimates, the Company accounted for any such changes by adjusting the effective interest rate on a prospective basis, with a corresponding impact to the reclassification of the deferred royalty obligation. The assumptions used in determining the expected repayment term of the deferred royalty obligation and amortization period of the issuance costs required that the Company made estimates that could impact the short-term and long-term classification of such costs, as well as the period over which such costs were amortized.
Derivative Liability	Derivative Liability In connection with certain transactions, the Company has identified certain embedded derivatives, which are recorded as liabilities on the Company’s consolidated balance sheet and are remeasured to fair value at each reporting date until the derivative is settled. Changes in the fair value of the derivative liabilities are recognized as other income (expense) in the consolidated statement of operations and comprehensive loss.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted 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 noted, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its condensed consolidated financial statements and disclosures. In June 2022, the FASB issued ASU 2022-03, Fair Value Measurement of Equity Securities Subject to Contractual Sale Restrictions . The ASU clarifies that a contractual restriction on the sale of an equity security is not considered part of the unit of account of the equity security and, therefore, is not considered in measuring fair value. The ASU also clarifies that an entity cannot, as a separate unit of account, recognize and measure a contractual sale restriction and requires specific disclosures for equity securities subject to contractual sale restrictions. These changes will become effective for the Company after December 15, 2023. The Company early adopted this standard during the fourth quarter of 2022 and it did not have a material impact on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes , amended guidance on the accounting and reporting of income taxes. The guidance is intended to simplify the accounting for income taxes by removing exceptions related to certain intraperiod tax allocations and deferred tax liabilities; clarifying guidance primarily related to evaluating the step-up tax basis for goodwill in a business combination; and reflecting enacted changes in tax laws or rates in the annual effective tax rate. The amended guidance is effective for interim and annual periods in 2021. Early adoption is permitted. The application of the amendments in the new guidance are to be applied on a retrospective basis, on a modified retrospective basis through a cumulative-effect adjustment to retained earnings or prospectively, depending on the amendment. This standard became effective for the Company on January 1, 2021, and did not have a material impact on its consolidated financial statements and related disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2022
Accounting Policies [Abstract]
Estimated Useful Lives of Assets	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense are recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Computer software and equipment 3 years Office furniture and equipment 7 years Manufacturing equipment 5 years Leasehold improvements Shorter of life of lease or 5 years

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis (in thousands): Fair Value Measurements at December 31, 2022 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 108,227 $ — $ 108,227 Total cash equivalents — 108,227 — 108,227 Fair Value Measurements at December 31, 2021 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 109,316 $ — $ 109,316 Corporate bonds $ — 2,701 — 2,701 Total cash equivalents — 112,017 — 112,017 Marketable securities: Corporate bonds — 11,479 — 11,479 Commercial paper — 22,339 — 22,339 Total marketable securities — 33,818 — 33,818 Total cash equivalents and marketable securities $ — $ 145,835 $ — $ 145,835 Liabilities: Derivative liability $ — $ — $ 802 $ 802 $ — $ — $ 802 $ 802
Summary of Gross Unrealized Gains and Losses of Marketable Securities	The following table summarizes the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2021 (in thousands): December 31, 2021 Amortized Gross Gross Fair Value Assets: Corporate bonds 11,481 — ( 2 ) 11,479 Commercial paper 22,339 — — 22,339 $ 33,820 $ — $ ( 2 ) $ 33,818

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Components of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2022 2021 Leasehold improvements $ 1,636 $ 1,636 Manufacturing equipment 1,338 1,338 Computer software and equipment 437 507 Office furniture and equipment 209 425 3,620 3,906 Less: Accumulated depreciation and amortization ( 3,252 ) ( 2,880 ) $ 368 $ 1,026

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2022
Leases [Abstract]
Summary of Components of Operating Lease Expense	For the years ended December 31, 2022 and 2021, the components of operating lease expense were as follows (in thousands): Operating lease expense Statement of Operations Location December 31, 2022 December 31, 2021 Fixed operating lease expense Research and development expense $ 824 $ 869 General and administrative expense 772 681 Variable operating lease expense Research and development expense 56 76 General and administrative expense 59 143 Total operating lease expense $ 1,711 $ 1,769
Summary of Supplemental Cash Flow Information Related to Operating Leases	Supplemental cash flow information related to the Company’s operating leases for the years ended December 31, 2022 and 2021, was as follows (in thousands): December 31, 2022 December 31, 2021 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 1,514 $ 1,525
Summary of Lease Balances Related to Operating and Finance Leases Except Weighted Average Remaining Lease Term And Discount Rate	The following table presents the lease balances within the consolidated balance sheet, weighted average remaining lease term, and the weighted average discount rates related to the Company’s operating and finance leases as of December 31, 2022 and 2021 (in thousands, except for the weighted average remaining lease term and the weighted average discount rate): Lease Assets and Liabilities Classification December 31, 2022 December 31, 2021 Assets Operating Operating lease right-of-use assets $ 5,124 $ 6,530 Financing Property and equipment, net 201 468 Total leased assets $ 5,325 $ 6,998 Liabilities Current Operating Operating lease liabilities $ 1,690 $ 1,362 Non-Current Operating Non-current operating lease liabilities 4,957 5,973 Total lease liabilities $ 6,647 $ 7,335 Weighted average remaining lease term (in years) 4.6 5.6 Weighted average discount rate 9.8 % 9.8 %
Summary of Maturity of Operating Lease Liabilities	The following table presents the maturity of the Company’s operating lease liabilities as of December 31, 2022 (in thousands): Years Ending December 31, 2023 1,690 2024 1,718 2025 1,746 2026 1,956 2027 1,156 Total future minimum lease payments 8,266 Less imputed interest ( 1,619 ) Total operating lease liabilities $ 6,647

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2022
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2022 December 31, 2021 Accrued external research and development expenses $ 1,823 $ 6,315 Accrued payroll and related expenses 5,797 5,884 Accrued professional fees 696 909 Accrued restructuring expenses 448 - Accrued other 209 1,242 Total Accrued expenses and other current liabilities $ 8,973 $ 14,350

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock Option Activity	The following table summarizes stock option activity for all of our plans during 2022: 2017 Plan 2019 Inducement Plan Total Number of Stock Options Outstanding as of December 31, 2021 3,893,605 997,111 4,890,716 Granted 956,604 — 956,604 Exercised ( 56,120 ) — ( 56,120 ) Forfeited or cancelled ( 1,161,257 ) ( 650,844 ) ( 1,812,101 ) Outstanding as of December 31, 2022 3,632,832 346,267 3,979,099 The following table summarizes details regarding stock options granted under our equity incentive plans for the year ended December 31, 2022: Number of Shares Weighted Average Exercise Price Weighted Average Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2021 4,890,716 $ 11.75 7.62 $ 24,263 Granted 956,604 11.12 Exercised ( 56,120 ) 7.48 Forfeited or cancelled ( 1,812,101 ) 13.21 Outstanding as of December 31, 2022 3,979,099 $ 10.99 5.27 $ 1 Outstanding as of December 31, 2022 - vested and expected to vest 3,979,099 $ 10.99 5.27 $ 1 Exercisable at December 31, 2022 2,688,861 $ 9.76 4.58 $ —
Summary of Activity of Options and RSUs of Performance-Based Options	The following table summarizes RSU activity under all equity plans (excluding performance-based RSUs) during the year months ended December 31, 2022: Number of Weighted Average Grant Date Fair Value Outstanding as of December 31, 2021 513,690 $ 17.08 Granted 1,420,750 5.88 Vested and released ( 226,043 ) 7.00 Forfeited or cancelled ( 408,000 ) 14.16 Outstanding as of December 31, 2022 1,300,397 $ 7.51
Summary of Assumptions Used to Determine Fair Value of Stock Option Awards Granted to Employees and Directors	The assumptions that the Company used in the Black-Scholes option-pricing model to determine the fair value of stock option awards granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2022 2021 Risk-free interest rate 1.7 % 0.8 % Expected term (in years) 6.2 6.2 Expected volatility 82.7 % 92.6 % Expected dividend yield 0.0 % 0.0 %
Summary of Share-Based Compensation Expense	The Company recorded share-based compensation expense, for both RSUs and stock options in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2022 2021 Research and development expenses $ 3,678 $ 4,163 General and administrative expenses 5,445 5,267 Total $ 9,123 $ 9,430

Liability Related to the Sale_2

Liability Related to the Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2022
Revenue Recognition and Deferred Revenue [Abstract]
Schedule of Changes in the Liability Related to the Sale of Future Royalties	The following table presents the changes in the liability related to the sale of future royalties under the Revenue Interest Agreement with HCR as of December 31, 2022 (in thousands): December 31, 2022 Liability related to sale of future royalties, as of December 31, 2021 $ 48,414 Repayment of liability ( 54,485 ) Loss on extinguishment of liability 3,581 Establish change of control derivative liability ( 115 ) Interest expense recognized 2,605 Liability related to sale of future royalties, as of December 31, 2022 $ —

Restructuring (Tables)

Restructuring (Tables)	12 Months Ended
	Dec. 31, 2022
Restructuring and Related Activities [Abstract]
Schedule Restructuring charges	The following tables summarize the restructuring related charges by line item within the Company’s consolidated statements of operations where they were recorded during the year ended December 31, 2022: Year Ended December 31, 2022 Research and development General and administrative Total Severance and other employee costs $ 3,872 $ 4,680 $ 8,552 Other 488 2,590 3,078 Total restructuring charges $ 4,360 $ 7,270 $ 11,630 The restructuring charge was included in accrued expenses and other current liabilities in the Company’s condensed consolidated balance sheet. Activity for the quarter is summarized as follows (amounts in thousands): As of December 31, 2022 Balance as of December 31, 2021 $ — Charge to expense 11,630 Payments made ( 7,702 ) Write-offs and impairments ( 3,480 ) Balance as of December 31, 2022 $ 448

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Schedule of Domestic and Foreign Components of Loss before Income Taxes	The domestic and foreign components of loss before income taxes were as follows (in thousands): Year Ended December 31, 2022 2021 Domestic $ ( 46,249 ) $ ( 89,565 ) Foreign ( 166 ) ( 191 ) Loss before income taxes $ ( 46,415 ) $ ( 89,756 )
Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2022 2021 Federal statutory income tax rate ( 21.0 ) ( 21.0 ) Federal and state research and development tax credit ( 4.3 ) ( 3.1 ) State taxes, net of federal benefit ( 2.3 ) ( 7.0 ) Nondeductible items 1.0 0.6 Increase in deferred tax asset valuation allowance 26.6 30.5 Effective income tax rate — —
Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2022 and 2021 consisted of the following (in thousands): December 31, 2022 2021 Net operating loss carryforwards $ 78,619 $ 83,849 Research and development tax credit carryforwards 14,294 12,375 Capitalized research and development expenses, net 11,613 — Other 10,264 6,220 Total deferred tax assets 114,790 102,444 Valuation allowance ( 114,790 ) ( 102,444 ) Net deferred tax assets $ — $ —
Summary of Changes in Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2022 and 2021 related primarily to the increase in net operating loss carryforwards and research and development tax credit carryforwards, and were as follows (in thousands): December 31, 2022 2021 Valuation allowance as of beginning of year $ ( 102,444 ) $ ( 75,039 ) Increases recorded to income tax provision ( 12,346 ) ( 27,405 ) Valuation allowance as of end of year $ ( 114,790 ) $ ( 102,444 )

License, Collaboration and Se_2

License, Collaboration and Service Agreements (Tables)	12 Months Ended
	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of remaining potential milestone payments	Under the terms of the GSK License Agreement, the Company received an upfront payment of $ 66.0 million for GSK to secure rights to the medicine. Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) From FDA acceptance of clinical protocol, commencement of Phase 3 study through NDA submission $ 150.0 (in tranched milestones) Total commercial milestone payments based on first sale (US/EU) $ 150.0 Total potential sales milestone payments $ 225.0 Royalties Low-single digit to low-double digit (if sales exceed $ 1.0 billion) tiered royalties on net product sales

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2022
Earnings Per Share [Abstract]
Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts): Year Ended December 31, 2022 2021 Numerator: Net loss $ ( 46,415 ) $ ( 89,756 ) Net loss attributable to common stockholders $ ( 46,415 ) $ ( 89,756 ) Denominator: Weighted average common shares outstanding, basic and diluted 37,585,075 30,895,756 Net loss per share, basic and diluted $ ( 1.23 ) $ ( 2.91 )
Summary of Shares Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2022 2021 Options to purchase common stock 3,979,099 4,890,716 Unvested restricted stock units 1,300,397 513,690 Series A convertible preferred stock (as converted to common shares) — — Series B convertible preferred stock (as converted to common shares) — 938,000 Series C convertible preferred stock (as converted to common shares) — 2,214,000 Series D convertible preferred stock (as converted to common shares) — 3,215,000 Total 5,279,496 11,771,406

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Nov. 07, 2022	Sep. 21, 2022	Mar. 11, 2021
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporation month and year	2013-12
State of incorporation	DE
Net loss	$ (46,415)	$ (89,756)
Accumulated deficit	413,878	367,463
Milestone payment	225,000
Common stock to be issued	$ 52	$ 32
Spero Therapeutics LLC [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Merger date	Jun. 30, 2017
GSK License Agreement [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock to be issued				$ 9,000
GSK License Agreement [Member] | Glaxo Smith Kline [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Milestone payment			$ 66,000
At-The-Market Offering Program [Member] | Cantor Fitzgerald & Co [Member] | Maximum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Authorized offering value					$ 300,000
Common stock available for issuance					$ 75,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Summary Of Significant Accounting Policies [Line Items]
Off-balance sheet risk description	As of December 31, 2022, and 2021, the Company had no off-balance sheet risk such as foreign exchange contracts, option contracts, or other hedging arrangements.
Off-balance sheet risk	$ 0	$ 0
Percentage of tax benefit recognized	50%
Reclassification out of Accumulated Other Comprehensive Income [Member]
Summary Of Significant Accounting Policies [Line Items]
Debt securities, available-for-sale, unrealized gains (losses)	$ 0	$ 0
ASU No. 2016-12 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
ASU No. 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
ASU No. 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
ASU No. 2018-15 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
ASU No. 2018-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2022
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	5 years
Computer Software and Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	3 years
Office Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	7 years
Manufacturing Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	Shorter of life of lease or 5 years

Fair Value Measurements and M_3

Fair Value Measurements and Marketable Securities - Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Jun. 07, 2022	Dec. 31, 2021
Cash equivalents:
Total cash equivalents	$ 900		$ 600
Derivative liabilities:
Derivative liability	0	$ 54,500	802
Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Total cash equivalents	108,227		112,017
Marketable securities:
Total marketable securities			33,818
Total cash equivalents and marketable securities			145,835
Derivative liabilities:
Derivative liability			802
Derivative liabilities			802
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents	0		0
Marketable securities:
Total marketable securities			0
Total cash equivalents and marketable securities			0
Derivative liabilities:
Derivative liability			0
Derivative liabilities			0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents	108,227		112,017
Marketable securities:
Total marketable securities			33,818
Total cash equivalents and marketable securities			145,835
Derivative liabilities:
Derivative liability			0
Derivative liabilities			0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	0		0
Marketable securities:
Total marketable securities			0
Total cash equivalents and marketable securities			0
Derivative liabilities:
Derivative liability			802
Derivative liabilities			802
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Total cash equivalents	108,227		109,316
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents	0		0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents	108,227		109,316
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	$ 0		0
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Total marketable securities			22,339
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents			0
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 2 [Member]
Marketable securities:
Total marketable securities			22,339
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents			0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Cash equivalents:
Total cash equivalents			2,701
Marketable securities:
Total marketable securities			11,479
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents			0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents			2,701
Marketable securities:
Total marketable securities			11,479
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents			$ 0

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Jun. 30, 2022	Oct. 31, 2021	Dec. 31, 2022	Dec. 31, 2021	Jun. 07, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash			$ 900	$ 600
Fair Value, Liabilities of Level 2 to Level 1 Transfers Amount			0	0
Transfer of financial asset into level 3 of fair value			0	0
Transfer of financial liabilities into level 3 of fair value			0
Fair value of the derivative liability	$ 100	$ 1,000
Change in fair value of derivative liability			917	204
Derivative liability			$ 0	$ 802	$ 54,500
Anti-dilution rights outstanding			5,279,496	11,771,406
HCR [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability			$ 100
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value of derivative liability			100	$ 800
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value of derivative liability			$ 0	$ 200

Fair Value Measurements and M_5

Fair Value Measurements and Marketable Securities - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) $ in Thousands	Dec. 31, 2021 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 33,820
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Value	33,818
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,481
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Value	11,479
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	22,339
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	$ 22,339

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,620	$ 3,906
Less: Accumulated depreciation and amortization	(3,252)	(2,880)
Property and equipment, net	368	1,026
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,636	1,636
Manufacturing Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,338	1,338
Computer Software and Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	437	507
Office Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 209	$ 425

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 869	$ 646
Loss related to write off	635	0
Manufacturing Equipment [Member]
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 700	$ 600

Leases - Additional Information

Leases - Additional Information (Detail) - Office Space [Member] - ft²			1 Months Ended	12 Months Ended
	May 04, 2020	Dec. 16, 2019	Aug. 31, 2015	Dec. 31, 2022
Lessee Lease Description [Line Items]
Operating lease, existence of option to extend			true
Lease Agreements [Member] | Cambridge, Massachusetts [Member] | U.S. REIF Central Plaza Massachusetts, LLC [Member]
Lessee Lease Description [Line Items]
Operating lease expiration date				2027-07
Operating lease, existence of option to extend	true	true
Area of office space				23,400

Leases - Summary of Components

Leases - Summary of Components of Operating Lease Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Lessee Lease Description [Line Items]
Total operating lease expense	$ 1,711	$ 1,769
Research and Development Expense [Member]
Lessee Lease Description [Line Items]
Fixed operating lease expense	824	869
Variable operating lease expense	56	76
General and Administrative Expense [Member]
Lessee Lease Description [Line Items]
Fixed operating lease expense	772	681
Variable operating lease expense	$ 59	$ 143

Leases - Summary of Supplementa

Leases - Summary of Supplemental Cash Flow Information Related to Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 1,514	$ 1,525

Leases - Summary of Lease Balan

Leases - Summary of Lease Balances Related to Operating and Finance Leases Except Weighted Average Remaining Lease Term And Discount Rate (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Assets
Operating lease right-of-use assets	$ 5,124	$ 6,530
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Property, Plant and Equipment, Net	Property, Plant and Equipment, Net
Financing	$ 201	$ 468
Total leased assets	5,325	6,998
Liabilities
Operating lease liabilities	1,690	1,362
Non-current operating lease liabilities	4,957	5,973
Total lease liabilities	$ 6,647	$ 7,335
Weighted average remaining lease term (in years)	4 years 7 months 6 days	5 years 7 months 6 days
Weighted average discount rate	9.80%	9.80%

Leases - Summary of Maturity of

Leases - Summary of Maturity of Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
2023	$ 1,690
2024	1,718
2025	1,746
2026	1,956
2027	1,156
Total future minimum lease payments	8,266
Less imputed interest	(1,619)
Total operating lease liabilities	$ 6,647	$ 7,335

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 1,823	$ 6,315
Accrued payroll and related expenses	5,797	5,884
Accrued professional fees	696	909
Accrued restructuring expenses	448	0
Accrued other	209	1,242
Accrued expenses and other current liabilities	$ 8,973	$ 14,350

Equity Transactions - Underwrit

Equity Transactions - Underwritten Public Offering - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Series D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	3,215,000
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	5,963,294	475,469

Equity Transactions - Rights Of

Equity Transactions - Rights Offering - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Class Of Stock [Line Items]
Issuance of Common Stock, conversion of Preferred Stock to Common Stock	$ 3
Common stock, par value	$ 0.001	$ 0.001
Aggregate issuance costs	$ 0	$ 248
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of Common Stock, conversion of Preferred Stock to Common Stock	$ 6
Issuance of stock, shares	5,963,294	475,469
Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	2,220

Equity - Additional Information

Equity - Additional Information (Detail) - USD ($)		1 Months Ended		9 Months Ended	12 Months Ended
	Sep. 21, 2022	Jun. 30, 2022	Feb. 28, 2021	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021	Aug. 17, 2021	Jun. 30, 2021	Mar. 11, 2021
Class Of Stock [Line Items]
Preferred stock, shares authorized					10,000,000	10,000,000
Issuance of common stock and preferred stock net of offering costs and issuance costs					$ 14,234,000	$ 7,831,000
Common Stock, Shares Authorized					120,000,000	120,000,000
Common Stock, Value, Issued					$ 52,000	$ 32,000
Maximum [Member]
Class Of Stock [Line Items]
Common Stock, Shares Authorized							120,000,000
Minimum [Member]
Class Of Stock [Line Items]
Common Stock, Shares Authorized							60,000,000
Pfizer Inc [Member]
Class Of Stock [Line Items]
Issuance of stock, shares			2,362,348
Issuance of common stock and preferred stock net of offering costs and issuance costs			$ 27,500,000
GSK SPA [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	7,450,000
Issuance of common stock and preferred stock net of offering costs and issuance costs	$ 10,300,000
GSK License Agreement [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	7,450,000
Share price	$ 1.20805
Issuance of common stock and preferred stock net of offering costs and issuance costs	$ 1,300,000
Common Stock, Value, Issued	$ 9,000,000
Pfizer Purchase Agreement [Member]
Class Of Stock [Line Items]
Issuance of common stock and preferred stock net of offering costs and issuance costs			$ 12,500
Pfizer Purchase Agreement [Member] | Pfizer Inc [Member]
Class Of Stock [Line Items]
Issuance of stock, shares		2,362,348
Share price								$ 16.93
Proceeds from sale of common stock to related party		$ 40,000,000
At-The-Market Offering Program [Member]
Class Of Stock [Line Items]
Issuance of stock, shares					5,963,294
Share price					$ 2.46
Gross proceeds from issuance of common and preferred stock					$ 14,700,000
At-The-Market Offering Program [Member] | Cantor Fitzgerald & Co [Member] | Maximum [Member]
Class Of Stock [Line Items]
Authorized offering value									$ 300,000,000
Common stock available for issuance									$ 75,000,000
Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares					2,220
Preferred stock, shares authorized					10,000,000
Number of shares issuable upon conversion					1,000
Maximum percentage of common stock holding upon conversion of preferred stock					9.99%
Preferred stock, liquidation per share					$ 0.001
Voting rights					no
Series B Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares		938	62		1,000
Preferred stock, shares authorized					10,000,000
Number of shares issuable upon conversion		938,000	62,000		1,000
Maximum percentage of common stock holding upon conversion of preferred stock					9.99%
Preferred stock, liquidation per share					$ 0.001
Voting rights					no
Series C Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares		2,214			3,333
Preferred stock, shares authorized					10,000,000
Number of shares issuable upon conversion			73,000	2,214,000	1,000
Maximum percentage of common stock holding upon conversion of preferred stock					9.99%
Preferred stock, liquidation per share					$ 0.001
Voting rights					no
Preferred stock, conversion basis			73
Series D Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares					3,215,000
Preferred stock, shares authorized					10,000,000
Number of shares issuable upon conversion		942,000		2,273,000
Maximum percentage of common stock holding upon conversion of preferred stock					9.99%
Preferred stock, liquidation per share					$ 0.001
Voting rights					no
Preferred stock, conversion basis		942,000		2,273,000	The shares of Series D Preferred Stock are convertible on a one-to-one basis into shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series D Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99% of the total number of shares of common stock then issued and outstanding, subject to certain exceptions.
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares					5,963,294	475,469
Issuance of common stock and preferred stock net of offering costs and issuance costs					$ 7,000	$ 1,000

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Millions

1 Months Ended

9 Months Ended

12 Months Ended

May 31, 2023

Jul. 01, 2022 USD ($)

Aug. 17, 2021 shares

Jun. 28, 2017 shares

Dec. 31, 2022 USD ($) $ / shares shares

Jun. 30, 2020 shares

Sep. 30, 2022 shares

Dec. 31, 2022 USD ($) Installments $ / shares shares

Dec. 31, 2021 USD ($) $ / shares shares

Dec. 31, 2019 shares

Mar. 11, 2019 shares

Oct. 18, 2017 shares

Oct. 17, 2017 shares

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of shares authorized for issuance

11,595,127

11,595,127

Common stock reserved for issuance

5,412,971

5,412,971

Share based compensation options granted

956,604

Common stock, shares/units issued

52,456,195

52,456,195

32,393,738

Common Stock, Par or Stated Value Per Share | $ / shares

$ 0.001

$ 0.001

$ 0.001

Compensation expense recognized | $

$ 1

Aggregate intrinsic value of stock options exercised | $

$ 0.1

$ 1.2

Stock option exercised

56,120

Stock Option [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Weighted average grant-date fair value of awards granted | $ / shares

$ 7.94

$ 13.37

Total unrecognized compensation cost related to unvested share-based awards | $

$ 11.4

$ 11.4

Weighted average period

2 years 5 months 1 day

Restricted Stock Units (RSUs) [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Options vesting period

2 years 9 months 29 days

Unrecognized compensation expense | $

$ 8.1

Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Equal Monthly Installments | Installments

4

Share-based Compensation Arrangement by Share-based Payment Award, Right to Receive Share, Number

1

Share based compensation options granted

1,420,750

Common Stock, Par or Stated Value Per Share | $ / shares

$ 0.001

$ 0.001

Vested and released

226,043

Weighted average grant-date fair value of awards granted | $ / shares

$ 5.88

Performance-Based Awards [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of common stock award shares granted | $

$ 1.7

Granted shares in addition to stock option

140,000

Compensation expense recognized | $

$ 1

Performance-based Vesting Criteria 1 [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Percentage of incentive units vesting

50%

Performance-based Vesting Criteria 2 [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Percentage of incentive units vesting

50%

2017 Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of shares authorized for issuance

1,785,416

2,696,401

1,785,416

Number of shares to be incremented

607,324

Percentage of outstanding shares of common stock to be incremented

4%

Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized

2,000,000

Share based compensation options granted

956,604

Common stock, shares/units issued

4,382,875

4,382,875

Stock option exercised

56,120

2017 Plan [Member] | Maximum [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Exercise price per share of stock options as a percentage of fair market value of share of common stock

100%

2017 Plan [Member] | Minimum [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Options vesting period

10 years

2019 Inducement Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized

875,000

700,000

Common stock reserved for issuance

331,500

Common stock, shares/units issued

1,030,096

1,030,096

Two Thousands And Seventeen Stock Incentive Plan [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized

3,170,254

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Stock Option Activity (Detail)	12 Months Ended
	Dec. 31, 2022 shares
Number of Shares
Outstanding as of December 31, 2021	4,890,716
Granted	956,604
Exercised	(56,120)
Forfeited or cancelled	(1,812,101)
Outstanding as of December 31, 2022	3,979,099
2017 Plan [Member]
Number of Shares
Outstanding as of December 31, 2021	3,893,605
Granted	956,604
Exercised	(56,120)
Forfeited or cancelled	(1,161,257)
Outstanding as of December 31, 2022	3,632,832
2019 Inducement Plan [Member]
Number of Shares
Outstanding as of December 31, 2021	997,111
Forfeited or cancelled	(650,844)
Outstanding as of December 31, 2022	346,267

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Activity of Options and RSUs of Performance-Based Options (Details)	12 Months Ended
	Dec. 31, 2022 shares
Number of Shares
Outstanding as of December 31, 2021	4,890,716
Granted	956,604
Exercised	(56,120)
Outstanding as of December 31, 2022	3,979,099
2017 Plan [Member]
Number of Shares
Outstanding as of December 31, 2021	3,893,605
Granted	956,604
Exercised	(56,120)
Outstanding as of December 31, 2022	3,632,832

Share-Based Compensation - Su_3

Share-Based Compensation - Summary of Assumptions Used to Determine Fair Value of Stock Option Awards Granted to Employees and Directors (Detail)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Share-Based Payment Arrangement [Abstract]
Risk-free interest rate	1.70%	0.80%
Expected term (in years)	6 years 2 months 12 days	6 years 2 months 12 days
Expected volatility	82.70%	92.60%
Expected dividend yield	0%	0%

Share-Based Compensation - Su_4

Share-Based Compensation - Summary of Stock Option Granted Under Equity Incentive Plans (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Number of Shares
Outstanding as of December 31, 2021	4,890,716
Granted	956,604
Exercised	(56,120)
Forfeited or cancelled	(1,812,101)
Outstanding as of December 31, 2022	3,979,099	4,890,716
Outstanding as of December 31, 2022 - vested and expected to vest	3,979,099
Exercisable at December 31, 2022	2,688,861
Weighted Average Exercise Price
Outstanding as of December 31, 2021	$ 11.75
Granted	11.12
Exercised	7.48
Forfeited or cancelled	13.21
Outstanding as of December 31, 2022	10.99	$ 11.75
Outstanding as of December 31, 2022 - vested and expected to vest	10.99
Exercisable at December 31, 2022	$ 9.76
Weighted Average Contractual Term
Outstanding as of December 31, 2021	5 years 3 months 7 days	7 years 7 months 13 days
Outstanding as of December 31, 2022 - vested and expected to vest	5 years 3 months 7 days
Exercisable at December 31, 2022	4 years 6 months 29 days
Outstanding as of December 31, 2021	$ 1	$ 24,263
Outstanding as of December 31, 2022 - vested and expected to vest	1
Exercisable at December 31, 2022	$ 0

Share-Based Compensation - Su_5

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 9,123	$ 9,430
Research and Development Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	3,678	4,163
General and Administrative Expense [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 5,445	$ 5,267

Share-Based Compensation - Rest

Share-Based Compensation - Restricted Stock Units (Details)	12 Months Ended
	Dec. 31, 2022 $ / shares shares
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Outstanding as of December 31, 2021	4,890,716
Granted	956,604
Outstanding as of December 31, 2022	3,979,099
Restricted Stock Units (RSUs) [Member]
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Outstanding as of December 31, 2021	513,690
Granted	1,420,750
Vested and released	(226,043)
Forfeited or cancelled	(408,000)
Outstanding as of December 31, 2022	1,300,397
Weighted Average Grant Date Fair Value of Outstanding as of December 31, 2021 | $ / shares	$ 17.08
Weighted average grant-date fair value of awards granted | $ / shares	5.88
Weighted average grant-date fair value of awards vested and released | $ / shares	7
Weighted average grant-date fair value of awards forfeited or cancelled | $ / shares	14.16
Weighted Average Grant Date Fair Value of Outstanding as of December 31, 2022 | $ / shares	$ 7.51

Liability Related to the Sale_3

Liability Related to the Sale of Future Royalties- Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Jun. 07, 2022	Oct. 19, 2021
Deferred Revenue Arrangement [Line Items]
Derivative liability	$ 0	$ 802	$ 54,500
Related Party Transaction, Expenses from Transactions with Related Party	10,000
Loss on extinguishment of liability related to the sale of future royalties	3,581	$ 0
Revenue Interest Financing Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Change of Control Value in Excess Amount	100,000
Maximum [Member] | Sales Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Change of Control Value in Excess Amount	200,000
HCR [Member]
Deferred Revenue Arrangement [Line Items]
Purchase price	125,000
Initial investment amount				$ 50,000
Additional investment amount	50,000
Alternative investment	25,000
Derivative liability	54,485
Related Party Transaction, Expenses from Transactions with Related Party	$ 4,000
Related Party Transaction, Rate	6%
HCR [Member] | Maximum [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Costs	$ 200,000
HCR [Member] | Minimum [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Costs	$ 100,000
Related Party Transaction, Rate	3%

Liability Related to the Sale_4

Liability Related to the Sale of Future Royalties - Schedule of Changes in the Liability Related to the Sale of Future Royalties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Jun. 07, 2022	Dec. 31, 2021
Deferred Revenue Arrangement [Line Items]
Less initial derivative liability (recorded as discount)	$ 0	$ (54,500)	$ (802)
HealthCare Royalty Management, LLC [Member]
Deferred Revenue Arrangement [Line Items]
Liability related to sale of future royalties, beginning balance	48,414
Less initial derivative liability (recorded as discount)	(54,485)
Less payments made	3,581
Establish change of control derivative liability	(115)
Plus Interest expense accrued/ recognized	2,605
Liability related to sale of future royalties, ending balance	$ 0

Non-Controlling Interests - Spe

Non-Controlling Interests - Spero Gyrase - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Noncontrolling Interest [Line Items]
Common stock issued, aggregate fair value	$ 14,234	$ 7,831
Research and development expense	$ 47,593	$ 64,526

Restructuring (Additional Infor

Restructuring (Additional Information) (Details) $ in Thousands			12 Months Ended
	May 31, 2023 USD ($)	Nov. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($) Employee
Restructuring Cost and Reserve [Line Items]
Restructuring Charges			$ 11,630	$ 0
Employee related termination costs			8,600
Contract termination fees			3,000
Outstanding Accrued Expense			400
Common stock to be issued			52	$ 32
Compensation expense recognized			1,000
Human Capital Management Committee [Member]
Restructuring Cost and Reserve [Line Items]
Aggregate Retention Awards Cash Bonus		$ 1,100
Cash		200
Human Capital Management Committee [Member] | Subsequent Event [Member]
Restructuring Cost and Reserve [Line Items]
Aggregate Retention Awards Cash Bonus	$ 3,200
Cash	700
Tebipenem HBr Program [Member]
Restructuring Cost and Reserve [Line Items]
Number Of Employees Before Restructuring Initiative | Employee				146
Number Of Employees After Restructuring Initiative | Employee				41
Executive Retention Awards Member
Restructuring Cost and Reserve [Line Items]
Cash Bonus		$ 900
Executive Retention Awards Member | Subsequent Event [Member]
Restructuring Cost and Reserve [Line Items]
Common stock to be issued	$ 1,700
Performance-Based Awards [Member]
Restructuring Cost and Reserve [Line Items]
Compensation expense recognized			$ 1,000

Restructuring - Restructuring c

Restructuring - Restructuring charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	$ 8,552
Other	3,078
Restructuring	11,630	$ 0
Research and Development Expense [Member]
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	3,872
Other	488
Restructuring	4,360
General and Administrative Expense [Member]
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	4,680
Other	2,590
Restructuring	$ 7,270

Restructuring - Schedule of Res

Restructuring - Schedule of Restructuring charges included in Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Restructuring and Related Activities [Abstract]
Restructuring Reserve, Beginning Balance	$ 0
Charge to expense	11,630	$ 0
Payments made	7,702
Write-offs and impairments	3,480
Restructuring Reserve, Ending Balance	$ 448	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Income Taxes [Line Items]
Income tax benefits for the net operating losses	$ 0	$ 0
Federal net operating loss carryforwards	291,900
State net operating loss carryforwards	282,900
Federal net operating loss carryforwards, subject to expiration	73,000
Federal net operating loss carryforwards, not subject to expiration	218,900
Foreign net operating loss carryforwards	4,600
FederalTax credit carryforward	11,500
StateTax credit carryforward	$ (2,800)
Maximum increase in percentage ownership of stockholders or public groups	50%
Ownership change, increase in ownership percentage, term	3 years
Unrecognized tax benefits	$ 0	0
Accrued interest or penalties related to uncertain tax positions	$ 0	$ 0
Federal corporate income tax rate	21%	21%
Capitalized research and development expenses	$ 11,613	$ 0
Software Development Costs [Member]
Income Taxes [Line Items]
Capitalized research and development expenses	$ 47,600
Domestic Country [Member]
Income Taxes [Line Items]
Tax credit carryforward expiration period	2033
Domestic Country [Member] | Earliest Tax Year [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration year	2033
Domestic Country [Member] | Latest Tax Year [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration year	2037
State And Local Jurisdiction [Member]
Income Taxes [Line Items]
Tax credit carryforward expiration period	2028
State And Local Jurisdiction [Member] | Earliest Tax Year [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration year	2033
State And Local Jurisdiction [Member] | Latest Tax Year [Member]
Income Taxes [Line Items]
Net operating loss carryforwards expiration year	2039

Income Taxes - Schedule of Dome

Income Taxes - Schedule of Domestic and Foreign Components of Loss before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Domestic	$ (46,249)	$ (89,565)
Foreign	(166)	(191)
Loss before income taxes	$ (46,415)	$ (89,756)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(21.00%)	(21.00%)
Federal and state research and development tax credit	(4.30%)	(3.10%)
State taxes, net of federal benefit	(2.30%)	(7.00%)
Nondeductible items	1%	0.60%
Increase in deferred tax asset valuation allowance	26.60%	30.50%
Effective income tax rate	0%	0%

Income Taxes - Net Deferred Tax

Income Taxes - Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 78,619	$ 83,849
Research and development tax credit carryforwards	14,294	12,375
Capitalized research and development expenses	11,613	0
Other	10,264	6,220
Total deferred tax assets	114,790	102,444
Valuation allowance	(114,790)	(102,444)	$ (75,039)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ (102,444)	$ (75,039)
Increases recorded to income tax provision	(12,346)	(27,405)
Valuation allowance as of end of year	$ (114,790)	$ (102,444)

Government Contracts - Addition

Government Contracts - Additional Information (Detail) $ in Thousands		1 Months Ended			12 Months Ended
	Jul. 01, 2019 USD ($)	May 31, 2021 USD ($) Option	Jul. 31, 2018 USD ($)	Jun. 30, 2016 USD ($) Option	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($) Option	Sep. 30, 2022 USD ($)	Jan. 19, 2022 USD ($)	Jan. 31, 2020 USD ($)	May 31, 2019 USD ($)
Government Contracts [Line Items]
Grant revenue					$ 53,509	$ 18,256
BARDA and DTRA [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract					9,700
BARDA and Speros Achievement of Specified Milestones [Member]
Government Contracts [Line Items]
Potential grant amount exercised					12,700
Grant [Member] | Minimum [Member] | DoD, CDMRP and JWMRP [Member]
Government Contracts [Line Items]
Grant revenue						4,500
SPR206 [Member] | DoD, CDMRP and JWMRP [Member]
Government Contracts [Line Items]
Award received for clinical development	$ 5,900
Non-dilutive funding period	4 years
BARDA [Member]
Government Contracts [Line Items]
Potential contract amount awarded			$ 44,200					$ 59,700
Potential amount additional funding awarded								12,900
Overall potential award increased								$ 46,900
Award received for clinical development							$ 12,900
BARDA [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded			$ 15,700							$ 18,200
Potential amount additional funding awarded										2,500
Overall potential award increased										$ 46,800
Contract term			3 years
BARDA [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue					2,200	9,900
BARDA [Member] | First Option [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded									$ 15,900
Potential amount increase in committed funding									34,000
BARDA [Member] | Second Option [member]
Government Contracts [Line Items]
Potential grant amount awarded									$ 12,700
DTRA [Member] | Maximum [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded					10,000
U.S. Department of Defense [Member] | Grant [Member] | Minimum [Member]
Government Contracts [Line Items]
Grant revenue					1,000
Niaid [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded					100
Niaid [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract				$ 6,500
Number of option period for funding from government contract | Option				3
Niaid [Member] | Grant [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Grant revenue						400
Niaid [Member] | SPR206 [Member]
Government Contracts [Line Items]
Contract term		5 years
Potential amount increase in committed funding					6,200
Maximum potential funding from government contract					23,500
Grant revenue					1,700	$ 400
Award received for clinical development		$ 23,400
Number of option period for funding from government contract | Option		5
Niaid [Member] | SPR206 [Member] | Maximum [Member]
Government Contracts [Line Items]
Potential amount increase in committed funding					2,200
Niaid [Member] | SPR206 [Member] | Minimum [Member]
Government Contracts [Line Items]
Potential amount increase in committed funding					2,100
Niaid [Member] | First Option [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]
Government Contracts [Line Items]
Number of option period for funding from government contract | Option						2
Committed amount from government contract						$ 5,900
Niaid [Member] | First Option [Member] | SPR206 [Member]
Government Contracts [Line Items]
Committed amount from government contract					$ 4,000

License, Collaboration and Se_3

License, Collaboration and Service Agreements - Additional Information (Detail) $ / shares in Units, $ in Thousands, £ in Millions

1 Months Ended

3 Months Ended

12 Months Ended

Sep. 21, 2022 USD ($) $ / shares shares

Jan. 15, 2021 USD ($)

Jun. 30, 2021 USD ($)

Feb. 28, 2021 USD ($) shares

Nov. 30, 2018 USD ($)

Jun. 30, 2017 USD ($)

Jun. 30, 2016 USD ($)

Jun. 30, 2016 GBP (£)

May 31, 2016 USD ($)

Sep. 30, 2022 USD ($)

Dec. 31, 2020 USD ($)

Mar. 31, 2019 USD ($)

Dec. 31, 2018 USD ($)

Dec. 31, 2022 USD ($) $ / shares shares

Dec. 31, 2021 USD ($)

Nov. 07, 2022 USD ($)

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred Revenue, Current

$ 10,369

$ 1,857

Revenue recognized

48,579

3,070

Common stock issued, aggregate fair value

14,234

7,831

Remaining Transaction Price Allocated To Performance Obligations

12,500

Milestone payment

225,000

Pfizer Inc [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Issuance of common stock and preferred stock net of offering costs, shares | shares

2,362,348

Common stock issued, aggregate fair value

$ 27,500

GSK SPA [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Transaction price allocated to the performance obligations

64,700

Initial Payments Amount to Perform the Obligations

66,000

Discount on Performance of Obligations

1,300

Issuance of common stock and preferred stock net of offering costs, shares | shares

7,450,000

Common stock issued, aggregate fair value

$ 10,300

License and Know-How Transfer [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

1,400

License and Know-How Transfer [Member] | GSK SPA [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Transaction price allocated to the performance obligations

64,700

Performance Obligation Allocated Transaction Price

45,700

Research And Development Services Related To Upcoming Milestones [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

11,100

Research And Development Services Related To Upcoming Milestones [Member] | GSK SPA [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

19,000

Cantab Related Agreements [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Potential milestone payment upon achievement of specified commercial milestone

£ 5

6,000

License agreement research and development expense related to achievement of regulatory milestones

$ 0

Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones

$ 5,800

Vertex License Agreement [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract termination period

10 years

Nonrefundable upfront payments

$ 500

Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones

$ 80,200

License agreement payment of research and development expense related to achievement of regulatory milestones

$ 0

0

Contract termination period if no material development or commercialization occurs

1 year

Meiji License Agreement [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Nonrefundable upfront payments

$ 600

Sublicense fee payable to counter party

$ 7,500

Sublicence Fee Paid

6,600

Potential milestone payments upon achievement of specified condition

$ 1,000

License agreement fixed assets related payments

$ 1,600

Potential milestone payments upon completion and delivery of results of a clinical study

1,000

Future milestone payments

$ 1,000

Everest License Agreement [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Period of option granted on an agreement

12 months

Upfront payment received

$ 3,000

Potential milestone payments upon completion and delivery of results of a clinical study

$ 2,000

Revenue recognized

$ 700

1,300

Milestone payment

34,000

Everest License Agreement [Member] | SPR206 [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 800

2,000

Potential milestone payments upon completion and delivery of results of a clinical study

$ 1,500

$ 700

milestones payment received

$ 2,000

Everest License Agreement [Member] | SPR 741 [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 1,000

Everest License Agreement [Member] | Maximum [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Agreement termination period upon written notice

180 days

Everest License Agreement [Member] | Maximum [Member] | SPR206 [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Receivable amount upon achievement of certain milestone

$ 38,000

59,500

Everest License Agreement [Member] | Minimum [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Agreement termination period upon written notice

90 days

Pfizer License and Share Purchase Agreements [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Receivable amount upon achievement of certain milestone

$ 900

milestones payment received

5,000

Deferred Revenue, Current

13,900

Revenue recognized

$ 1,800

Issuance of common stock and preferred stock net of offering costs, shares | shares

2,362,348

Common stock issued, aggregate fair value

$ 27,500

Associated premium as a freestanding equity-linked instrument under ASC 815

12,500

Performance Obligation Allocated Transaction Price

5,000

Pfizer License and Share Purchase Agreements [Member] | SPR206 [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 0

Issuance of common stock and preferred stock net of offering costs, shares | shares

2,362,348

Share price | $ / shares

$ 16.93

Proceeds from sale of common stock to related party

$ 40,000

Receivable amount upon achievement of certain milestone

80,000

Pfizer License and Share Purchase Agreements [Member] | Research And Development Services Related To Upcoming Milestones [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

$ 4,100

Gates MRI [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Reduction to research and development expense

$ 1,500

Savior Service Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Non-refundable supervision fee related to commercial manufacturing facility

$ 2,000

Savior Service Agreement [Member] | Long-term Asset [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Service agreement additional payment related to facility build out costs

5,300

GSK License Agreement [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred Revenue, Current

18,600

Revenue recognized

46,100

Issuance of common stock and preferred stock net of offering costs, shares | shares

7,450,000

Share price | $ / shares

$ 1.20805

Common stock issued, aggregate fair value

$ 1,300

GSK License Agreement [Member] | Glaxo Smith Kline [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payment

$ 66,000

Change in subsequent payments

50%

GSK License Agreement [Member] | License and Know-How Transfer [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue recognized

45,700

GSK License Agreement [Member] | Research And Development Services Related To Upcoming Milestones [Member]

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

$ 400

License, Collaboration and Se_4

License, Collaboration and Service Agreements - Schedule of remaining potential milestone payments (Details) $ in Millions	Dec. 31, 2022 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 225
Sales milestone events	1,000
Commercial milestone payments | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	150
NDA Submission [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 150

Net Loss per Share - Summary of

Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Numerator:
Net loss	$ (46,415)	$ (89,756)
Net loss attributable to common stockholders	$ 46,415	$ (89,756)
Denominator:
Weighted average common shares outstanding, basic	37,585,075	30,895,756
Weighted average common shares outstanding, diluted	37,585,075	30,895,756
Net loss per share attributable to common stockholders, basic	$ (1.23)	$ (2.91)
Net loss per share,diluted	$ (1.23)	$ (2.91)

Net Loss per Share - Summary _2

Net Loss per Share - Summary of Shares Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	5,279,496	11,771,406
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	3,979,099	4,890,716
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,300,397	513,690
Series A Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	0
Series B Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	938,000
Series C Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	2,214,000
Series D Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	3,215,000

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
Matching contributions to 401(k) plan	$ 0.4	$ 0.5

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Jan. 19, 2022	Mar. 11, 2021	Jul. 31, 2018
BARDA [Member]
Subsequent Event [Line Items]
Potential amount additional funding awarded		$ 12.9
Overall potential award increased		46.9
Potential contract amount awarded		$ 59.7		$ 44.2
At-The-Market Offering Program [Member]
Subsequent Event [Line Items]
Issuance of stock, shares	5,963,294
At-The-Market Offering Program [Member] | Cantor Fitzgerald & Co [Member] | Maximum [Member]
Subsequent Event [Line Items]
Common stock available for issuance			$ 75
Sales Agreement [Member]
Subsequent Event [Line Items]
Issuance of stock, shares	0