Spero Therapeutics (SPRO) 10-Q 15 May 24 2024 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2024	May 06, 2024
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Title of 12(b) Security	Common Stock, $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		53,986,639
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 Quarterly Report	true
Document Transition Report	false

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 82,271	$ 76,333
Collaboration receivable, current - related party	49,198	49,152
Other receivables	5,017	1,545
Prepaid expenses and other current assets	3,668	4,178
Total current assets	140,154	131,208
Property and equipment, net	2	2
Operating lease right of use assets	3,902	4,155
Collaboration receivable, non-current - related party	23,211	46,590
Other assets	434	435
Total assets	167,703	182,390
Current liabilities:
Accounts payable	3,066	1,378
Accrued expenses and other current liabilities	5,380	6,557
Operating lease liabilities	1,725	1,718
Income taxes payable	387	387
Deferred revenue, current	2,868	2,132
Deferred revenue, current - related party	28,395	24,981
Total current liabilities	41,821	37,153
Non-current operating lease liabilities	3,519	3,825
Deferred revenue, non-current	9,948	10,825
Deferred revenue, non-current - related party	16,128	23,606
Other long-term liabilities	38	87
Total liabilities	71,454	75,496
Commitments and contingencies (Note 7)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized, no shares issued and outstanding as of March 31, 2024 and December 31, 2023	0	0
Common stock, $0.001 par value; 120,000,000 shares authorized as of March 31, 2024 and December 31, 2023; 53,869,139 shares issued and outstanding as of March 31, 2024 and 52,999,680 shares issued and outstanding as of December 31, 2023	54	53
Additional paid-in capital	499,936	497,913
Accumulated deficit	(403,741)	(391,072)
Total stockholders' equity	96,249	106,894
Total liabilities and stockholders' equity	$ 167,703	$ 182,390

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (Unaudited) - $ / shares	Mar. 31, 2024	Dec. 31, 2023
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	0
Preferred stock, shares/units outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares/units issued	53,869,139	52,999,680
Common stock, shares/units outstanding	53,869,139	52,999,680

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenues:
Grant revenue	$ 5,063	$ 1,329
Collaboration revenue - related party	4,064	517
Collaboration revenue	$ 140	$ 223
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember
Total revenues	$ 9,267	$ 2,069
Operating expenses:
Research and development	17,332	8,979
General and administrative	5,917	7,317
Total operating expenses	23,249	16,296
Loss from operations	(13,982)	(14,227)
Other income (expense):
Interest income	1,327	963
Other income (expense), net	(14)	(2)
Total other income (expense), net	1,313	961
Net loss and comprehensive loss	$ (12,669)	$ (13,266)
Net loss per share attributable to common stockholders, basic	$ (0.24)	$ (0.25)
Net loss per share attributable to common stockholders, diluted	$ (0.24)	$ (0.25)
Weighted average common shares outstanding, basic	53,524,037	52,527,018
Weighted average common shares outstanding, diluted	53,524,037	52,527,018

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Cash flows from operating activities:
Net loss	$ (12,669)	$ (13,266)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	0	113
Non-cash lease cost	254	236
Share-based compensation	2,023	2,170
Changes in operating assets and liabilities:
Collaboration receivable, current and non-current - related party	23,333	0
Other receivables	(3,472)	(781)
Prepaid expenses and other current assets	510	(164)
Other assets	1	0
Accounts payable	1,688	(34)
Accrued expenses and other current liabilities	(1,177)	(111)
Deferred revenue, current and non-current	(141)	(224)
Deferred revenue - related party, current and non-current	(4,064)	(516)
Other long-term liabilities	(49)	(11)
Operating lease liability	(299)	(265)
Net cash provided by (used in) operating activities	5,938	(12,853)
Cash flows from investing activities:
Net decrease in cash and cash equivalents:	5,938	(12,853)
Cash, cash equivalents and restricted cash at beginning of period	76,333	109,107
Cash, cash equivalents and restricted cash at end of period	$ 82,271	$ 96,254

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2022	$ 75,934	$ 52	$ 489,760	$ (413,878)
Balances, Shares at Dec. 31, 2022		52,456,195
Issuance of common stock upon the vesting of restricted stock units	1	$ 1
Issuance of common stock upon the vesting of restricted stock units, shares		115,618
Share-based compensation expense	2,170		2,170
Net loss	(13,266)			(13,266)
Balances at Mar. 31, 2023	64,839	$ 53	491,930	(427,144)
Balances, Shares at Mar. 31, 2023		52,571,813
Balances at Dec. 31, 2023	$ 106,894	$ 53	497,913	(391,072)
Balances, Shares at Dec. 31, 2023		52,999,680
Issuance of common stock upon the exercise of stock options, shares	0
Issuance of common stock upon the vesting of restricted stock units	$ 1	$ 1
Issuance of common stock upon the vesting of restricted stock units, shares		869,459
Share-based compensation expense	2,023		2,023
Net loss	(12,669)			(12,669)
Balances at Mar. 31, 2024	$ 96,249	$ 54	$ 499,936	$ (403,741)
Balances, Shares at Mar. 31, 2024		53,869,139

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pay vs Performance Disclosure
Net Income (Loss)	$ (12.7)	$ (13.3)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Mar. 31, 2024
Trading Arrangements, by Individual
Name	Satyavrat Shukla
Title	President and Chief Executive Officer
Rule 10b5-1 Arrangement Adopted	true
Termination Date	February 15, 2024

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2024
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 and developing novel treatments for rare diseases and diseases caused by multi-drug resistant (“MDR”) bacterial infections with high unmet need. The Company's wholly-owned 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 partnered programs consist of SPR206 and tebipenem HBr. SPR206 is an IV-administered antibiotic 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 complicated urinary tract infections (“cUTIs”) including pyelonephritis, caused by certain microorganisms, in adult patients who have limited oral treatment options. 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 its collaboration and licensing agreements, funding from government contracts and through the sale of the Company’s common and preferred stock. The Company has incurred recurring losses, including net losses of $ 12.7 million and $ 13.3 million for the three months ended March 31, 2024 and 2023, respectively. In addition, as of March 31, 2024, the Company had an accumulated deficit of $ 403.7 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 quarterly 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 quarterly consolidated financial statements. 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 may seek additional funding through public or private financings, debt financing, collaboration agreements, government grants or other avenues. 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. Interim Financial Information The consolidated balance sheet at December 31, 2023 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying unaudited condensed consolidated financial statements as of March 31, 2024, and for the three months ended March 31, 2024, have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2023, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, on file with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of March 31, 2024, and results of operations for the three months ended March 31, 2024 and 2023, and cash flows for the three months ended March 31, 2024 and 2023 have been made. The results of operations for the three months ended March 31, 2024 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2024 .

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies 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 and developing 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 March 31, 2024 and December 31, 2023, the Company had no off-balance sheet risks, including but not limited, to 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. 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 Revenu e 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. When an arrangement contains payment terms that are extended beyond one year, a significant financing component may exist. The Company assessed its revenue-generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component exists in certain arrangements. The significant financing component is calculated as the difference between the stated value and present value of the milestones payable and is recognized as interest income over the extended payment period. Judgment is used in determining: (1) whether the financing component in a license agreement is significant and, if so, (2) the discount rate used in calculating the significant financing component. 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 judgment in the determination of the significant assumptions relating to forecasted future revenues, development timelines, 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. 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. External Research and Development 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. The Company recognizes external research and development costs based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its service providers. This process involves reviewing open contracts and purchase orders, communicating with applicable personnel to identify services that have been performed on the Company’s behalf, and estimating the level of service performed and the associated cost incurred for the service when the Company has not yet been invoiced or otherwise notified of actual costs. 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 . 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, Compensation—Stock Compensation, 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. 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 Income (Loss) Comprehensive income (loss) includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the three months ended March 31, 2024 and 2023, there were no components of other comprehensive income (loss), and therefore comprehensive loss is the same as net loss. Net Income (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. Recently Issued and Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“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. On March 6, 2024, the SEC approved a rule that will require registrants to provide certain climate-related information in their registration statements and annual reports. The rule requires information about a registrant's climate-related risks that are reasonably likely to have a material impact on its business, results of operations, or financial condition. The required information about climate-related risks also includes disclosure of a registrant's greenhouse gas emissions. In addition, the rules will require registrants to present certain climate-related financial metrics in their audited financial statements. The Company does not expect this rule to have a material impact on its consolidated financial statements, but it will require increased disclosures within the notes to the consolidated financial statements and related disclosures. In December 2023, the FASB issued ASU 2023-09, Improvements to Income Tax Disclosures. This ASU is related to the disclosure of rate reconciliation and income taxes paid. This guidance becomes effective for annual periods beginning after December 15, 2024 with early adoption permitted and should be applied on a prospective basis. The Company does not expect this standard to have a material impact on its consolidated financial statements, but it will require increased disclosures within the notes to the consolidated financial statements. In November 2023, the FASB issued ASU 2023-07, Improvements to Reportable Segment Disclosures. This ASU is related to reportable segment disclosure requirements, primarily through enhanced disclosures about significant segment expenses. This guidance becomes effective for annual periods beginning after December 15, 2023 and interim periods beginning after December 15, 2024, with early adoption permitted and should be applied on a retrospective basis. The Company does not expect this standard to have a material impact on its consolidated financial statements, but it will require increased disclosures within the notes to the consolidated financial statements.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 81,596 $ — $ 81,596 Total cash equivalents — 81,596 — 81,596 Fair Value Measurements at December 31, 2023 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 75,628 $ — $ 75,628 Total cash equivalents — 75,628 — 75,628 Excluded from the tables above is cash of $ 0.7 million and $ 0.7 million as of March 31, 2024 and December 31, 2023, respectively. During the three months ended March 31, 2024 , there were no transfers between Level 1, Level 2 and Level 3 categories.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended
	Mar. 31, 2024
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	4. Accrued Expenses and Other Current Liabilities The following table presents the Company’s accrued expenses and other current liabilities as of March 31, 2024 and December 31, 2023 (in thousands): March 31, 2024 December 31, 2023 Accrued payroll and related expenses $ 1,302 $ 3,339 Accrued external research and development expenses 2,995 2,274 Accrued professional fees 875 708 Accrued other 208 236 Total Accrued expenses and other current liabilities $ 5,380 $ 6,557

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Common Stock	5. Common Stock “At-the-Market” Offering On March 11, 2021, the Company entered into a Controlled Equity Offering 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), which became effective on March 29, 2021 (the “2021 Form S-3”) , 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 under the Sales Agreement. The 2021 Form S-3 expired on March 29, 2024. The Company filed a new universal shelf registration statement on Form S-3 with the SEC on March 15, 2024, which became effective on March 22, 2024, 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 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 of 1933, as amended (the “Securities Act”), subject to the terms of the Sales Agreement. During the three months ended March 31, 2024 and 2023, the Company did not sell any shares of its common stock under its Sales Agreement.

Share-Based Compensation

Share-Based Compensation	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Share-Based Compensation	6. Share-Based Compensation The Company maintains two equity compensation plans, the 2017 Stock Incentive Plan (as amended, the “2017 Plan”) and the 2019 Inducement Equity Incentive Plan (as amended, the “2019 Inducement Plan”), which provide for the grant of stock-based awards to its directors, officers and employees. The equity plans provide for the grant of non-qualified and incentive stock options, as well as restricted stock units (“RSUs”), restricted stock and other stock-based awards. As of March 31, 2024, a total of 15,345,127 shares of common stock have been authorized and reserved for issuance under all of the Company's equity plans and 2,749,693 shares of common stock were available for future issuance under such plans. Stock Options The weighted-average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $ 1.12 and $ 1.29 per option for those options granted during the three months ended March 31, 2024 and 2023, respectively. The following table summarizes stock option activity under all equity plans (excluding RSUs) during the three months ended March 31, 2024: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2023 2,865,594 $ 10.89 5.76 $ 1 Granted 35,604 1.52 Exercised — — Forfeited or cancelled ( 20,135 ) 13.66 Outstanding as of March 31, 2024 2,881,063 $ 10.75 5.51 $ 8 Outstanding as of March 31, 2024 - vested and 2,881,063 $ 10.75 5.51 $ 8 Exercisable at March 31, 2024 2,488,009 $ 10.37 5.22 $ 2 As of March 31, 2024 , there was approximately $ 3.6 million of total unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted-average period of approximately 1.37 years. Restricted Stock Units The Company granted 3,123,496 RSUs to employees during the three months ended March 31, 2024. The following table summarizes RSU activity under all equity plans during the three months ended March 31, 2024: Number of Weighted Average Grant Date Fair Value Outstanding as of December 31, 2023 5,368,807 $ 2.45 Granted 3,123,496 1.57 Vested and released ( 869,459 ) 2.73 Forfeited or cancelled ( 149,600 ) 1.93 Outstanding as of March 31, 2024 7,473,244 $ 2.06 As of March 31, 2024 , there was approximately $ 13.7 million of total unrecognized compensation expense related to RSUs, which is expected to be recognized over a weighted-average period of approximately 3.22 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 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 Inducement Plan, as applicable. Performance-Based Awards 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 were awarded based on certain performance criteria relating to pipeline execution, business development, and financial stewardship. As these performance criteria were deemed to be achieved by May 31, 2023, 70,001 of the Inducement PSUs vested in September 2023 and the remaining 69,999 of the Inducement PSUs will vest in September 2024 upon fulfilment of the service condition. The Company recognized less than $ 0.1 million of expense associated with the Inducement PSUs during the three months ended March 31, 2024. The following table summarizes Inducement PSU activity under all equity plans during the three months ended March 31, 2024: Number of Inducement Weighted Average Grant Date Fair Value Outstanding as of December 31, 2023 69,999 $ 1.08 Granted — — Vested and released — — Forfeited or cancelled — — Outstanding as of March 31, 2024 69,999 $ 1.08 Share-Based Compensation Expense The Company recorded share-based compensation expense related to incentive stock options, nonqualified stock options, stock grants, and stock-based awards in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended March 31, 2024 2023 Research and development expenses $ 690 $ 798 General and administrative expenses 1,333 1,372 Total $ 2,023 $ 2,170

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies As a public biotechnology company, the Company operates in a regulated environment, and from time to time, is party to various legal proceedings and receives regulatory inquiries arising in the ordinary course of business. The costs and outcome of litigation, regulatory, investigatory or other proceedings cannot be predicted with certainty, and some lawsuits, claims, actions or proceedings may be disposed of unfavorably to the Company and could have a material adverse effect on the Company’s results of operations or financial condition. In addition, intellectual property disputes often have a risk of injunctive relief which, if imposed against the Company, could materially and adversely affect its financial condition or results of operations. If a matter is both probable to result in a material liability and the amount of loss can be reasonably estimated, the Company accrues the estimated loss. Disclosure is provided when a loss is considered probable, but the loss is not reasonably estimable and when a material loss is reasonably possible but not probable. If such a loss is not probable or cannot be reasonably estimated, a liability is not recorded. 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 9). Operating Leases The Company has entered into an operating lease agreement with respect to its corporate headquarters located at 675 Massachusetts Avenue, Cambridge, Massachusetts. 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 and its executive officers 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 executive 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 March 31, 2024 or December 31, 2023. Legal Proceedings Two putative class action lawsuits were filed against the Company and certain of its current and former executive 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 (“Consolidated Putative Class Action”). The parties filed an Amended Complaint on December 5, 2022, purported to be brought on behalf of stockholders who purchased our common stock from September 8, 2020 through May 2, 2022. The Amended Complaint generally alleges that the Company and certain of its current and former officers violated Sections 10(b) and/or 20(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and Rule 10b-5 promulgated thereunder by making allegedly false and/or misleading statements concerning the New Drug Application (“NDA”) for tebipenem HBr in an effort to lead investors to believe that the drug would receive approval from the FDA. Plaintiffs seek unspecified damages, interest, attorneys’ fees, and other costs. The Company filed a fully-briefed Motion to Dismiss on June 21, 2023. The Court has not yet ruled on the Motion. A stockholder derivative action was filed against the Company, as nominal defendant, and certain of the Company's current and former officers in the United States District Court for the District of Delaware, captioned Marti v. Mahadevia, et al., Case. No. 1:23-cv-01133-RGA (the “Derivative Complaint”), on October 11, 2023. The plaintiffs both purport to be current stockholders, and the allegations are primarily the same as those made in the Consolidated Putative Class Action. The Derivative Complaint was transferred to the Eastern District of New York on November 13, 2023. A second stockholder derivative action was filed against the Company, as nominal defendant, and certain of its current and former officers in the Supreme Court of the State of New York, Kings County, captioned Heil v. Mahadevia, et al., Case. No. 505153/2024 (the “Second Derivative Complaint”), on February 21, 2024. The Second Derivative Complaint makes primarily the same allegations as the First Derivative Complaint, and the Consolidated Putative Class Action. The plaintiffs in both derivative suits have agreed to a stay pending decision on the class action, subject to court approval. The Company denies any allegations of wrongdoing and intends to vigorously defend against these lawsuits. 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 these actions. 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	3 Months Ended
	Mar. 31, 2024
Contractors [Abstract]
Government Contracts	8. 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 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 and extended the period of performance through November 1, 2021. BARDA subsequently committed additional funding of $ 0.6 million, increasing the total committed funding for the base period and first contract option to $ 34.7 million and extended the period of performance for this first contract option through December 31, 2025. On January 19, 2022, the Company announced that BARDA exercised a third option under the contract. The new option increased the total amount of committed funding by $ 12.9 million to approximately $ 47.6 million, increasing the total potential contract value to $ 60.3 million. On September 30, 2022, remaining funding from the $ 12.9 million option was reallocated to a new option to support upcoming clinical trials. 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 of March 31, 2024, 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. As part of an inter-agency collaboration between BARDA and the Defense Threat Reduction Agency (“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 the Company. 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 may provide up to $ 69.7 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 the Company’s achievement of specified milestones. The Company recognized $ 4.9 million and $ 0.5 million of revenue under this agreement during the three months ended March 31, 2024 and 2023, respectively. 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 $ 23.5 million over a base period and six option periods. As of March 31, 2024 , funding for the base period totaling $ 2.2 million, funding for Option 1 of $ 4.0 million and funding for Option 3 of $ 0.8 million has been committed. The Company recognized $ 0.2 million and $ 0.9 million under this agreement during the three months ended March 31, 2024 and 2023, respectively.

License, Collaboration and Serv

License, Collaboration and Service Agreements	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License, Collaboration and Service Agreements	9. 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 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, the Company 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 three months ended March 31, 2024 and 2023, 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 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 the Company to add any such countries to the GSK Territory. Under the terms of the GSK License Agreement, in November 2022, the Company received an upfront payment of $ 66.0 million for GSK to secure rights to the medicine. In July 2023, the Company received written agreement from the FDA, under a special protocol assessment (“SPA”), on the design and size of PIVOT-PO, a pivotal Phase 3 clinical trial of tebipenem HBr in patients with cUTI, including acute pyelonephritis. Under the terms of the GSK License Agreement, the Company received a $ 30.0 million development milestone payment during the third quarter of 2023. In December 2023, the Company commenced enrollment in PIVOT-PO with its first patient, first visit. Under the terms of the GSK License Agreement, the Company was entitled to receive a $ 95.0 million development milestone payable in four equal semiannual installments. In February 2024, the Company received the first installment payment of $ 23.8 million for such development milestone. Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) GSK's submission of a new drug application with the FDA for tebipenem HBr $ 25.0 Total potential 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 In July 2023, the Company entered into Amendment 1 to the GSK License Agreement, which updated the technology transfer terms of the GSK License Agreement. In December 2023, the Company entered into Amendment 2 to the GSK License Agreement, which added a country to the locations for PIVOT-PO. Under the terms of Amendment 2, the Company may receive up to an additional $ 4.3 million in tranched milestones based on activities in such country. In March 2024, the Company entered into Amendment 3 to the GSK License Agreement, which assigns its rights to Product Trademarks (as defined in Amendment 3 to the GSK License Agreement) to GSK. 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) ten years following the date of the first commercial sale of such licensed product in such country (the “GSK Royalty Term”). During the GSK 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 the Company 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. At contract inception, the total transaction price was $ 64.7 million, which included the initial payment of $ 66.0 million in the fourth quarter of 2022 and the discount of $ 1.3 million related to the stock purchase agreement (“GSK SPA”) with Glaxo Group Limited, an affiliate of GSK. 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 remaining $ 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 Company recognized 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 September 21, 2022 (the “GSK Effective Date”) and GSK could begin to use and benefit from the license at the GSK Effective Date. The $ 30.0 million milestone payment received by the Company under the GSK License Agreement, was accounted for as variable consideration under ASC 606 and was added to the transaction price in the third quarter of 2023. Of this $ 30.0 million milestone, $ 21.2 million was recognized upon achievement of the milestone and the remaining $ 8.8 million was allocated to the research and development services performance obligation and will be recognized over time as the services are delivered, on a cumulative catch-up basis. The Company is entitled to receive the $ 95.0 million milestone payment in four equal semiannual installments under the GSK License Agreement. This milestone was accounted for as variable consideration under ASC 606 and was added to the transaction price in the fourth quarter of 2023. The Company determined that a significant financing component of $ 2.5 million exists related to extended payments terms granted to GSK. The Company presents effects of the financing component separately from collaboration revenue - related party as a component of interest income in its consolidated statement of operations. Of the $ 95.0 million milestone, $ 64.7 million was recognized upon achievement of the milestone in the fourth quarter of 2023, and the remaining amount after the $ 2.5 million significant financing component was allocated to the research and development services performance obligation and will be recognized over time as the services are delivered, on a cumulative catch-up basis. The milestone installment payments are classified as collaboration receivable - related party on the Company's consolidated balance sheet as of March 31, 2024. The Company received the first milestone installment payment of $ 23.8 million in the first quarter of 2024. The potential future 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 March 31, 2024 and not included in the transaction price. The Company can also earn sales-based royalties. Pursuant to Amendment 2 to the GSK License Agreement, the Company allocated $ 3.2 million of the total potential additional milestones to the research and development services obligation, as those development milestones were considered probable of achievement. These potential milestones were accounted for as variable consideration under ASC 606 and were added to the transaction price in the fourth quarter of 2023 and will be recognized over time as services are delivered. In total and inclusive of the above, the Company recognized $ 4.1 million and $ 0.5 million during the three months ended March 31, 2024 and 2023, respectively, related to the performance obligations, which was recorded as collaboration revenue - related party on its consolidated statement of operations. The remaining transaction price balance of approximately $ 44.5 million from the GSK License Agreement allocated to the research and development services performance obligation has been recorded as deferred revenue - related party in the condensed consolidated balance sheets. As of March 31, 2024, 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 sheets as of March 31, 2024 . In October 2021, the Company paid a $ 1.0 million milestone payment to Meiji upon submission of an NDA to the FDA for tebipenem HBr. The Company was obligated 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 during the year ended December 31, 2022, and the Company paid the remaining $ 0.9 million in the fourth quarter of 2023. The Company recorded these amounts as research and development expenses in the Company's consolidated statement of operations. The Company is obligated to make future milestone payments of up to $ 1.0 million as of March 31, 2024 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. 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. SPR206 Agreements Cantab License Agreement In June 2016, the Company entered into a stock purchase agreement (the “Cantab Agreement”) with Pro Bono Bio PLC, a corporation organized under the laws of England, and its affiliates, including PBB Distributions Limited (“PBB”), Cantab Anti-Infectives Ltd. and New Pharma License Holdings Limited. Under the Cantab Agreement, 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 ( $ 6.3 million as of March 31, 2024) 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 three months ended March 31, 2024 and 2023, the Company did no t record any research and development expense related to the achievement of regulatory milestones for SPR206, as no milestones were met or are probable of being met as of the balance sheet date. The Cantab Agreement continues 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 Medicines II Limited (“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 “Everest Licensed Products”), in Greater China (which includes Mainland China, Hong Kong and Macau), South Korea and certain Southeast Asian countries (the “Everest Territory”). The Company retained development, manufacturing and commercialization rights with respect to SPR206 and Everest Licensed Products in the rest of the world and also retained the right to develop or manufacture SPR206 and Everest Licensed Products in the Everest 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 Everest 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. In addition, under the Amended Everest License Agreement, the Company assigned patents in the Everest 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 Everest Licensed Products in the Everest Territory following regulatory approval of SPR206. Everest has the right to sublicense to affiliates and third parties in the Everest Territory. Everest is responsible for all costs related to developing, obtaining regulatory approval of and commercializing SPR206 and Everest Licensed Products in the Everest Territory, and is obligated to use commercially reasonable efforts to develop, manufacture and commercialize Everest 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 Everest Licensed Products in the Everest 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 Everest Licensed Product-by-Everest-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 Everest 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 Everest Licensed Product. As of March 31, 2024 , remaining future milestone payments of $ 34.0 million are fully constrained, and will be recognized when and if achievement of those milestones becomes probable. The Company did no t recognize revenue under this agreement during both the three months ended March 31, 2024 and 2023. Pfizer License and Share Purchase Agreements On June 30, 2021, the Company and Pfizer Inc. (“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 “Pfizer 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. 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 Pfizer 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 Pfizer Licensed Products in the Pfizer Territory. Pfizer is responsible for commercializing SPR206 and the Pfizer 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 March 31, 2024 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 $ 0.1 million and $ 0.2 million of revenue from the contract during the three months ended March 31, 2024 and 2023, respectively. The r emaining transaction price balance of approximately $ 12.8 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 sheets. As of March 31, 2024 , 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	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss per Share	10. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders of the Company was calculated as follows (in thousands, except share and per share amounts): Three Months Ended March 31, 2024 2023 Numerator: Net loss attributable to common stockholders $ ( 12,669 ) $ ( 13,266 ) Denominator: Weighted average common shares outstanding, basic and diluted 53,524,037 52,527,018 Net loss per share, basic and diluted $ ( 0.24 ) $ ( 0.25 ) 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 the Company is the same. The Company excluded the following potential shares of common stock, 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: Three Months Ended March 31, 2024 2023 Options to purchase common stock 2,881,063 3,998,991 Unvested RSUs and PSUs 7,543,243 4,374,103 10,424,306 8,373,094

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
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 and developing 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 March 31, 2024 and December 31, 2023, the Company had no off-balance sheet risks, including but not limited, to 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.
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 Revenu e 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. When an arrangement contains payment terms that are extended beyond one year, a significant financing component may exist. The Company assessed its revenue-generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component exists in certain arrangements. The significant financing component is calculated as the difference between the stated value and present value of the milestones payable and is recognized as interest income over the extended payment period. Judgment is used in determining: (1) whether the financing component in a license agreement is significant and, if so, (2) the discount rate used in calculating the significant financing component. 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 judgment in the determination of the significant assumptions relating to forecasted future revenues, development timelines, 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.
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.
External Research and Development Costs and Accruals	External Research and Development 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. The Company recognizes external research and development costs based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its service providers. This process involves reviewing open contracts and purchase orders, communicating with applicable personnel to identify services that have been performed on the Company’s behalf, and estimating the level of service performed and the associated cost incurred for the service when the Company has not yet been invoiced or otherwise notified of actual costs. 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 .
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, Compensation—Stock Compensation, 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. 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 Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the three months ended March 31, 2024 and 2023, there were no components of other comprehensive income (loss), and therefore comprehensive loss is the same as net loss.
Net Income (Loss) per Share	Net Income (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.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“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. On March 6, 2024, the SEC approved a rule that will require registrants to provide certain climate-related information in their registration statements and annual reports. The rule requires information about a registrant's climate-related risks that are reasonably likely to have a material impact on its business, results of operations, or financial condition. The required information about climate-related risks also includes disclosure of a registrant's greenhouse gas emissions. In addition, the rules will require registrants to present certain climate-related financial metrics in their audited financial statements. The Company does not expect this rule to have a material impact on its consolidated financial statements, but it will require increased disclosures within the notes to the consolidated financial statements and related disclosures. In December 2023, the FASB issued ASU 2023-09, Improvements to Income Tax Disclosures. This ASU is related to the disclosure of rate reconciliation and income taxes paid. This guidance becomes effective for annual periods beginning after December 15, 2024 with early adoption permitted and should be applied on a prospective basis. The Company does not expect this standard to have a material impact on its consolidated financial statements, but it will require increased disclosures within the notes to the consolidated financial statements. In November 2023, the FASB issued ASU 2023-07, Improvements to Reportable Segment Disclosures. This ASU is related to reportable segment disclosure requirements, primarily through enhanced disclosures about significant segment expenses. This guidance becomes effective for annual periods beginning after December 15, 2023 and interim periods beginning after December 15, 2024, with early adoption permitted and should be applied on a retrospective basis. The Company does not expect this standard to have a material impact on its consolidated financial statements, but it will require increased disclosures within the notes to the consolidated financial statements.

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 81,596 $ — $ 81,596 Total cash equivalents — 81,596 — 81,596 Fair Value Measurements at December 31, 2023 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 75,628 $ — $ 75,628 Total cash equivalents — 75,628 — 75,628

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2024
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	The following table presents the Company’s accrued expenses and other current liabilities as of March 31, 2024 and December 31, 2023 (in thousands): March 31, 2024 December 31, 2023 Accrued payroll and related expenses $ 1,302 $ 3,339 Accrued external research and development expenses 2,995 2,274 Accrued professional fees 875 708 Accrued other 208 236 Total Accrued expenses and other current liabilities $ 5,380 $ 6,557

Share-Based Compensation (Table

Share-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2024
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Stock Option Activity	The following table summarizes stock option activity under all equity plans (excluding RSUs) during the three months ended March 31, 2024: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2023 2,865,594 $ 10.89 5.76 $ 1 Granted 35,604 1.52 Exercised — — Forfeited or cancelled ( 20,135 ) 13.66 Outstanding as of March 31, 2024 2,881,063 $ 10.75 5.51 $ 8 Outstanding as of March 31, 2024 - vested and 2,881,063 $ 10.75 5.51 $ 8 Exercisable at March 31, 2024 2,488,009 $ 10.37 5.22 $ 2
Summary of Share-Based Compensation Expense	The Company recorded share-based compensation expense related to incentive stock options, nonqualified stock options, stock grants, and stock-based awards in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended March 31, 2024 2023 Research and development expenses $ 690 $ 798 General and administrative expenses 1,333 1,372 Total $ 2,023 $ 2,170
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Activity of Options and RSUs of Performance-Based Options	The following table summarizes RSU activity under all equity plans during the three months ended March 31, 2024: Number of Weighted Average Grant Date Fair Value Outstanding as of December 31, 2023 5,368,807 $ 2.45 Granted 3,123,496 1.57 Vested and released ( 869,459 ) 2.73 Forfeited or cancelled ( 149,600 ) 1.93 Outstanding as of March 31, 2024 7,473,244 $ 2.06
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Activity of Options and RSUs of Performance-Based Options	The following table summarizes Inducement PSU activity under all equity plans during the three months ended March 31, 2024: Number of Inducement Weighted Average Grant Date Fair Value Outstanding as of December 31, 2023 69,999 $ 1.08 Granted — — Vested and released — — Forfeited or cancelled — — Outstanding as of March 31, 2024 69,999 $ 1.08

License, Collaboration and Se_2

License, Collaboration and Service Agreements (Tables)	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of remaining potential milestone payments	Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) GSK's submission of a new drug application with the FDA for tebipenem HBr $ 25.0 Total potential 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)	3 Months Ended
	Mar. 31, 2024
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 the Company was calculated as follows (in thousands, except share and per share amounts): Three Months Ended March 31, 2024 2023 Numerator: Net loss attributable to common stockholders $ ( 12,669 ) $ ( 13,266 ) Denominator: Weighted average common shares outstanding, basic and diluted 53,524,037 52,527,018 Net loss per share, basic and diluted $ ( 0.24 ) $ ( 0.25 )
Summary of Shares Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential shares of common stock, 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: Three Months Ended March 31, 2024 2023 Options to purchase common stock 2,881,063 3,998,991 Unvested RSUs and PSUs 7,543,243 4,374,103 10,424,306 8,373,094

Subsequent Events (Tables)

Subsequent Events (Tables)	3 Months Ended
	Mar. 31, 2024
Subsequent Events [Abstract]
Schedule of remaining potential milestone payments	Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) GSK's submission of a new drug application with the FDA for tebipenem HBr $ 25.0 Total potential 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

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Nature Of Business And Basis Of Presentation [Abstract]
Net loss	$ 12,700	$ 13,300
Accumulated deficit	$ 403,741		$ 391,072

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Off-balance sheet risk description	As of March 31, 2024 and December 31, 2023, the Company had no off-balance sheet risks, including but not limited, to foreign exchange contracts, option contracts, or other hedging arrangements.
Off-balance sheet risk	$ 0	$ 0
Percentage of tax benefit recognized	50%

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	Mar. 31, 2024	Dec. 31, 2023
Cash equivalents:
Total cash equivalents	$ 700	$ 700
Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Total cash equivalents	81,596	75,628
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents	81,596	75,628
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Total cash equivalents	81,596	75,628
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	81,596	75,628
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	$ 0	$ 0

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Total cash	$ 0.7	$ 0.7
Fair Value, Liabilities of Level 2 to Level 1 Transfers Amount	$ 0

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued payroll and related expenses	$ 1,302	$ 3,339
Accrued external research and development expenses	2,995	2,274
Accrued professional fees	875	708
Accrued other	208	236
Total Accrued expenses and other current liabilities	$ 5,380	$ 6,557

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Mar. 15, 2024	Dec. 31, 2023	Mar. 11, 2021
Class Of Stock [Line Items]
Common Stock, Value, Issued, Total	$ 54		$ 53
At-The-Market Offering Program [Member] | Maximum [Member]
Class Of Stock [Line Items]
Authorized offering value		$ 300,000
Common stock available for issuance		$ 75,000
At-The-Market Offering Program [Member] | Cantor Fitzgerald & Co [Member] | Maximum [Member]
Class Of Stock [Line Items]
Authorized offering value				$ 300,000
Common stock available for issuance				$ 75,000

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended			3 Months Ended
	Sep. 30, 2024 shares	Sep. 30, 2023 shares	Sep. 30, 2022 shares	Mar. 31, 2024 USD ($) Plan Installments $ / shares shares	Mar. 31, 2023 $ / shares	Dec. 31, 2023 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in shares of common stock for issuance				53,869,139		52,999,680
Share based compensation options granted				35,604
Common stock, par value | $ / shares				$ 0.001		$ 0.001
2017 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of equity compensation plans | Plan				2
Number of shares authorized for issuance				15,345,127
Common stock reserved for issuance				2,749,693
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense | $				$ 3.6
Options vesting period				1 year 4 months 13 days
Weighted average grant-date fair value of awards granted | $ / shares				$ 1.12	$ 1.29
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation options granted				3,123,496
Unrecognized compensation expense | $				$ 13.7
Options vesting period				3 years 2 months 19 days
Vested and released				869,459
Weighted average grant-date fair value of awards granted | $ / shares				$ 1.57
Right to receive, number of share				1
Equal monthly installments | Installments				4
Performance-Based Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted shares in addition to stock option			140,000
Vested and released		70,001
Performance-Based Awards [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense recognized | $				$ 0.1
Performance-Based Awards [Member] | Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vested and released	69,999

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Number of Shares
Outstanding as of December 31, 2023	2,865,594
Granted	35,604
Exercised	0
Forfeited or cancelled	(20,135)
Outstanding as of March 31,2024	2,881,063	2,865,594
Outstanding as of March 31, 2024 - vested and expected to vest	2,881,063
Exercisable at March 31, 2024	2,488,009
Weighted Average Exercise Price
Outstanding as of December 31, 2023	$ 10.89
Granted	1.52
Exercised	0
Forfeited or cancelled	13.66
Outstanding as of March 31, 2024	10.75	$ 10.89
Outstanding as of March 31, 2024 - vested and expected to vest	10.75
Exercisable at March 31, 2024	$ 10.37
Weighted Average Contractual Term
Outstanding as of March 31, 2024	5 years 6 months 3 days	5 years 9 months 3 days
Outstanding as of March 31, 2024 - vested and expected to vest	5 years 6 months 3 days
Exercisable at March 31, 2024	5 years 2 months 19 days
Outstanding as of March 31,2024	$ 8	$ 1
Outstanding as of March 31, 2024 - vested and expected to vest	8
Exercisable at March 31, 2024	$ 2

Share-Based Compensation - Rest

Share-Based Compensation - Restricted Stock Units (Details)	3 Months Ended
	Mar. 31, 2024 $ / shares shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Outstanding as of December 31, 2023	2,865,594
Granted	35,604
Outstanding as of March 31,2024	2,881,063
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Outstanding as of December 31, 2023	5,368,807
Granted	3,123,496
Vested and released	(869,459)
Forfeited or cancelled	(149,600)
Outstanding as of March 31,2024	7,473,244
Weighted Average Grant Date Fair Value of Outstanding as of December 31, 2023 | $ / shares	$ 2.45
Weighted average grant-date fair value of awards granted | $ / shares	1.57
Weighted average grant-date fair value of awards vested and released | $ / shares	2.73
Weighted average grant-date fair value of awards forfeited or cancelled | $ / shares	1.93
Weighted Average Grant Date Fair Value of Outstanding as of March 31, 2024 | $ / shares	$ 2.06

Share-Based Compensation - Perf

Share-Based Compensation - Performance-Based awards (Details)	3 Months Ended
	Mar. 31, 2024 $ / shares shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Outstanding as of December 31, 2023	2,865,594
Granted	35,604
Outstanding as of March 31,2024	2,881,063
Performance Shares [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Outstanding as of December 31, 2023	69,999
Granted	0
Vested and released	0
Forfeited or cancelled	0
Outstanding as of March 31,2024	69,999
Weighted Average Grant Date Fair Value of Outstanding as of December 31, 2023 | $ / shares	$ 1.08
Weighted average grant-date fair value of awards granted | $ / shares	0
Weighted average grant-date fair value of awards vested and released | $ / shares	0
Weighted average grant-date fair value of awards forfeited or cancelled | $ / shares	0
Weighted Average Grant Date Fair Value of Outstanding as of March 31, 2024 | $ / shares	$ 1.08

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	$ 2,023	$ 2,170
Research and development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	690	798
General and Administrative Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	$ 1,333	$ 1,372

Government Contracts - Addition

Government Contracts - Additional Information (Detail) $ in Thousands	1 Months Ended		3 Months Ended
	May 31, 2021 USD ($) Option	Jul. 31, 2018 USD ($)	Mar. 31, 2024 USD ($) Option	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Jan. 19, 2022 USD ($)	Jan. 31, 2020 USD ($)	May 31, 2019 USD ($)
Government Contracts [Line Items]
Grant revenue			$ 9,267	$ 2,069
BARDA and DTRA [Member]
Government Contracts [Line Items]
Maximum potential funding from government contract			69,700
Potential grant amount exercised			12,700
BARDA [Member]
Government Contracts [Line Items]
Potential contract amount awarded		$ 44,200				$ 60,300
Potential amount additional funding awarded						12,900
Overall potential award increased						$ 47,600
Award received for clinical development					$ 12,900
BARDA [Member] | Base Period Contracts [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded		$ 15,700
Potential amount additional funding awarded								$ 2,500
Overall potential award increased								46,800
Contract term		3 years
BARDA [Member] | Base Period Contracts [Member] | Maximum [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded								18,200
BARDA [Member] | Base Period Contracts [Member] | Minimum [Member]
Government Contracts [Line Items]
Potential amount initial funding awarded								$ 15,700
BARDA [Member] | Grant [Member]
Government Contracts [Line Items]
Grant revenue			4,900	500
BARDA [Member] | First Option [Member]
Government Contracts [Line Items]
Potential amount additional funding awarded							$ 15,900
Potential Amount Subsequently Funding Awarded							600
Potential amount increase in committed funding							$ 34,700
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
Niaid [Member] | SPR206 [Member]
Government Contracts [Line Items]
Contract term	5 years
Potential amount increase in committed funding			2,200
Grant revenue			200	$ 900
Award received for clinical development	$ 23,500
Number of option period for funding from government contract | Option	6
Niaid [Member] | First Option [Member] | SPR206 [Member]
Government Contracts [Line Items]
Potential amount increase in committed funding			$ 4,000
Number of option period for funding from government contract | Option			1
Niaid [Member] | Third Option [Member] | SPR206 [Member]
Government Contracts [Line Items]
Potential amount increase in committed funding			$ 800
Number of option period for funding from government contract | Option			3

License, Collaboration and Se_3

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

1 Months Ended

3 Months Ended

12 Months Ended

Jan. 15, 2021 USD ($)

Jun. 30, 2017 USD ($)

Jun. 30, 2016 USD ($)

Jun. 30, 2016 GBP (£)

May 31, 2016 USD ($)

Mar. 31, 2024 USD ($) $ / shares shares

Mar. 31, 2023 USD ($)

Sep. 30, 2022 USD ($)

Dec. 31, 2020 USD ($)

Mar. 31, 2019 USD ($)

Dec. 31, 2018 USD ($)

Dec. 31, 2023 USD ($) $ / shares

Dec. 31, 2022 USD ($)

Sep. 30, 2023 USD ($)

Jul. 31, 2023 USD ($)

Nov. 07, 2022 USD ($)

Oct. 31, 2017 USD ($)

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Common stock, par value | $ / shares

$ 0.001

$ 0.001

Revenue recognized

$ 140,000

$ 223,000

Milestone payment

225,000,000

Total Milestone

$ 95,000,000

Deferred revenue, current

2,868,000

2,132,000

GSK SPA [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Remaining transaction price allocated to the performance obligations

64,700,000

Milestone payment

21,200,000

Total Milestone

30,000,000

Initial Payments to Perform the Obligations

$ 66,000,000

Discount on Obligations

1,300,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

19,000,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Remaining transaction price allocated to the performance obligations

64,700,000

Performance Obligation Allocated Transaction Price

45,700,000

Vertex License Agreement [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

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

$ 0

0

Contract termination period

10 years

Nonrefundable upfront payments

$ 500,000

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

$ 80,200,000

Contract termination period if no material development or commercialization occurs

1 year

GSK License Agreement [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

$ 4,100,000

500,000

Deferred revenue, current

44,500,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Total potential additional milestones

3,200,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payment

23,800,000

$ 4,300,000

$ 66,000,000

Reduction In Subsequent Payments by GSK to the Company

50%

GSK License Agreement [Member] | GSK SPA [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payment

23,800,000

64,700,000

$ 30,000,000

Total Received Milestone

95,000,000

$ 30,000,000

Significant financing component

2,500,000

Total Milestone

95,000,000

Research and development services performance obligation

8,800,000

2,500,000

Meiji License Agreement [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Nonrefundable upfront payments

$ 600,000

Sublicense fee payable to counter party

7,500,000

Potential milestone payments upon achievement of specified condition

$ 1,000,000

License agreement fixed assets related payments

$ 1,600,000

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

1,000,000

Future milestone payments

1,000,000

Sublicence Fee Paid to Counter Party

$ 6,600,000

Remaining Sublicence Fee Paid to Counter Party

$ 900,000

Cantab Related Agreements [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payment upon achievement of specified commercial milestone

£ 5

6,300,000

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

$ 0

0

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

$ 5,800,000

Everest License Agreement [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Period of option granted on an agreement

12 months

Upfront payment received

$ 3,000,000

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

$ 2,000,000

Revenue recognized

$ 0

0

Milestone payment

34,000,000

Everest License Agreement [Member] | SPR 206 [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

2,000,000

Milestones payment received

$ 2,000,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 1,000,000

Everest License Agreement [Member] | Maximum [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Agreement termination period upon written notice

180 days

Everest License Agreement [Member] | Maximum [Member] | SPR 206 [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Receivable amount upon achievement of certain milestone

$ 38,000,000

$ 59,500,000

Everest License Agreement [Member] | Minimum [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Agreement termination period upon written notice

90 days

Pfizer License and Share Purchase Agreements [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Receivable amount upon achievement of certain milestone

$ 900,000

Milestones payment received

5,000,000

Issuance of stock, shares | shares

2,362,348

Issuance of stock, value

$ 27,500,000

Remaining transaction price allocated to the performance obligations

12,500,000

Revenue recognized

100,000

$ 200,000

Premium associated freestanding equity

12,500,000

Performance Obligation Allocated Transaction Price

5,000,000

Deferred revenue, current

12,800,000

Pfizer License and Share Purchase Agreements [Member] | SPR 206 [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Receivable amount upon achievement of certain milestone

$ 80,000,000

Issuance of stock, shares | shares

2,362,348

Share price | $ / shares

$ 16.93

Proceeds from sale of common stock to related party

$ 40,000,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

11,100,000

$ 4,100,000

Pfizer License and Share Purchase Agreements [Member] | License and Know-How Transfer [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Performance Obligation Allocated Transaction Price

$ 1,400,000

License, Collaboration and Se_4

License, Collaboration and Service Agreements - Schedule of remaining potential milestone payments (Details) $ in Millions	Mar. 31, 2024 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 225
Sales milestone events	1,000
NDA Submission [Member] | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	25
Potential Commercial Milestone Payments | Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 150

Restructuring - Additional Info

Restructuring - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Restructuring Cost and Reserve [Line Items]
Common stock to be issued	$ 54	$ 53
Performance-Based Awards [Member] | Minimum [Member]
Restructuring Cost and Reserve [Line Items]
Compensation expense recognized	$ 100

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	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Net loss attributable to common stockholders	$ (12,669)	$ (13,266)
Denominator:
Weighted average common shares outstanding, basic	53,524,037	52,527,018
Weighted average common shares outstanding, diluted	53,524,037	52,527,018
Net loss per share attributable to common stockholders, basic	$ (0.24)	$ (0.25)
Net loss per share attributable to common stockholders, diluted	$ (0.24)	$ (0.25)

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	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	10,424,306	8,373,094
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	2,881,063	3,998,991
Unvested RSUs and PSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	7,543,243	4,374,103

Subsequent Events (Additional I

Subsequent Events (Additional Information) (Details) - shares	Mar. 31, 2024	Dec. 31, 2023
Subsequent Event [Line Items]
Common stock authorized for issuance	120,000,000	120,000,000