Spero Therapeutics (SPRO) 10-Q 14 Nov 22 2022 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2022	Nov. 10, 2022
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q3
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	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		51,776,053
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	Sep. 30, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 50,446	$ 112,584
Marketable securities	0	33,818
Other receivables	914	2,280
Tax incentive receivable, current	39	361
Prepaid expenses and other current assets	1,488	8,829
Total current assets	52,887	157,872
Property and equipment, net	494	1,026
Operating lease right of use assets	5,356	6,530
Other assets	5,739	5,644
Total assets	64,476	171,072
Current liabilities:
Accounts payable	2,582	1,101
Accrued expenses and other current liabilities	8,093	14,350
Operating lease liabilities	1,683	1,362
Deferred Revenue, Current	3,726	1,857
Derivative liability, current	141	0
Total current liabilities	16,225	18,670
Liability related to the sale of future royalties, non-current	0	48,414
Non-current operating lease liabilities	5,218	5,973
Deferred revenue, non-current	10,442	8,786
Derivative Liability, Noncurrent	0	802
Other long-term liabilities	107	138
Total liabilities	31,992	82,783
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 September 30, 2022 and 3,218,152 shares issued and outstanding as of December 31, 2021	0	3
Common stock, $0.001 par value; 120,000,000 shares authorized as of September 30, 2022 and December 31, 2021; 42,856,623 shares issued and outstanding as of September 30, 2022 and 32,393,738 shares issued and outstanding as of December 31, 2021	43	32
Additional paid-in capital	473,090	455,719
Accumulated deficit	(440,649)	(367,463)
Accumulated other comprehensive gain (loss)	0	(2)
Total stockholders' equity	32,484	88,289
Total liabilities and stockholders' equity	$ 64,476	$ 171,072

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (Unaudited) - $ / shares	Sep. 30, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares/units issued	0	3,218,152
Preferred stock, shares/units outstanding	0	3,218,152
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares/units issued	42,856,623	32,393,738
Common stock, shares/units outstanding	42,856,623	32,393,738

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Revenues:
Grant revenue	$ 924	$ 2,356	$ 3,843	$ 12,698
Collaboration revenue	$ 1,082	$ 708	$ 2,225	$ 2,814
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Total revenues	$ 2,006	$ 3,064	$ 6,068	$ 15,512
Operating expenses:
Research and development	7,360	14,436	32,504	47,301
General and administrative	6,632	11,152	29,988	28,680
Restructuring	(152)	0	11,697	0
Total operating expenses	13,840	25,588	74,189	75,981
Loss from operations	(11,834)	(22,524)	(68,121)	(60,469)
Other income (expense):
Interest income	208	90	391	270
Other income (expense), net	(23)	(87)	(46)	(317)
Interest expense related to the sale of future royalties	0	0	(2,605)	0
Loss on extinguishment of liability related to the sale of future royalties	0	0	(3,581)	0
Change in fair value of derivative liability	(26)	0	776	0
Total other income (expense), net	159	3	(5,065)	(47)
Net loss	$ (11,675)	$ (22,521)	$ (73,186)	$ (60,516)
Net loss per share, basic and diluted	$ (0.33)	$ (0.70)	$ (2.16)	$ (1.99)
Net loss per share, basic and diluted	$ (0.33)	$ (0.70)	$ (2.16)	$ (1.99)
Weighted average common shares outstanding, basic and diluted	35,882,076	32,132,500	33,834,198	30,417,305
Weighted average common shares outstanding, basic and diluted:	35,882,076	32,132,500	33,834,198	30,417,305
Comprehensive loss:
Net loss	$ (11,675)	$ (22,521)	$ (73,186)	$ (60,516)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	0	0	2	5
Net unrealized gains (losses) on securities	0	0	2	5
Total comprehensive loss	$ (11,675)	$ 22,521	$ (73,184)	$ (60,511)

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Cash flows from operating activities:
Net loss	$ (11,675)	$ (22,521)	$ (73,186)	$ (60,516)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization			740	455
Non-cash lease cost			446	611
Impairment of assets			635	0
Share-based compensation	2,106	2,330	6,626	6,502
Unrealized foreign currency transaction (gain) loss			8	17
Accretion of discount on marketable securities			13	197
Change in fair value of derivative liability			(776)	0	$ (200)
Non-cash interest expense associated with the sale of future royalties			2,605	0
Loss on extinguishment of liability related to the sale of future royalties	0	0	3,581	0
Changes in operating assets and liabilities:
Other receivables			1,366	3,088
Prepaid expenses and other current assets			7,133	2,758
Tax incentive receivables			312	797
Other assets			(2)	7
Accounts payable			1,483	2,007
Accrued expenses and other current liabilities			(6,257)	(3,185)
Deferred revenue, current and non-current			3,525	10,898
Other long-term liabilities			(31)	(29)
Operating lease liability			(434)	(605)
Net cash used in operating activities			(52,213)	(36,998)
Cash flows from investing activities:
Purchases of marketable securities			(26,970)	(13,601)
Proceeds from maturities of marketable securities			60,777	44,548
Net cash provided by (used in) investing activities			33,807	30,947
Cash flows from financing activities:
Repayment of liability related to the sale of future royalties			(54,485)	0
Proceeds from the issuance of common stock, net of issuance costs			10,334	5,357
Payment of offering and financing costs			0	(411)
Proceeds from stock option exercises			419	1,218
Net cash provided by financing activities			(43,732)	33,701
Net increase (decrease) in cash and cash equivalents			(62,138)	27,650
Cash, cash equivalents and restricted cash at beginning of period			112,584	85,209	85,209
Cash, cash equivalents and restricted cash at end of period	$ 50,446	$ 112,859	50,446	112,859	$ 112,584
Supplemental disclosure of non-cash activities:
Offering and financing costs in accounts payable and accruals			0	2,300
Pfizer Purchase Agreement [Member]
Cash flows from financing activities:
Proceeds from the issuance of common stock, net of issuance costs			$ 0	$ 27,537

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Pfizer Purchase Agreement [Member]	Series A, B, C and D Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Restricted Stock Units (RSUs) [Member]	Common Stock [Member] Pfizer Purchase Agreement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Pfizer Purchase Agreement [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2020	$ 132,040		$ 3	$ 29			$ 409,722		$ (277,707)	$ (7)
Balances, Shares at Dec. 31, 2020			3,218,287	29,260,247
Issuance of common stock upon the exercise of stock options	1,218						1,218
Issuance of common stock upon the exercise of stock options, shares				135,571
Issuance of common stock, net of issuance costs	5,357	$ 27,289		$ 1		$ 2	5,356	$ 27,287
Issuance of common stock, net of issuance costs, shares				327,185		2,362,348
Conversion of convertible preferred stock to common stock, shares			(135)	135,000
Share-based compensation expense	6,502						6,502
Unrealized gain (loss) on available-for-sale securities	5									5
Net loss	(60,516)								(60,516)
Balances at Sep. 30, 2021	111,895		$ 3	$ 32			450,085		(338,223)	(2)
Balances, Shares at Sep. 30, 2021			3,218,152	32,220,351
Balances at Jun. 30, 2021	90,252		$ 3	$ 30			405,923		(315,702)	(2)
Balances, Shares at Jun. 30, 2021			3,218,152	29,699,147
Issuance of common stock upon the exercise of stock options	826						826
Issuance of common stock upon the exercise of stock options, shares				88,856
Issuance of common stock, net of issuance costs	1,105	$ 39,903				$ 2	1,105	$ 39,901
Issuance of common stock, net of issuance costs, shares				70,000		2,362,348
Share-based compensation expense	2,330						2,330
Unrealized gain (loss) on available-for-sale securities	0
Net loss	(22,521)								(22,521)
Balances at Sep. 30, 2021	111,895		$ 3	$ 32			450,085		(338,223)	(2)
Balances, Shares at Sep. 30, 2021			3,218,152	32,220,351
Balances at Dec. 31, 2021	88,289		$ 3	$ 32			455,719		(367,463)	(2)
Balances, Shares at Dec. 31, 2021			3,218,152	32,393,738
Issuance of common stock upon the exercise of stock options	$ 419						419
Issuance of common stock upon the exercise of stock options, shares	56,120			56,120
Issuance of common stock upon the vesting of RSUs, shares					75,287
Issuance of common stock, net of issuance costs	$ 10,331			$ 5			10,326
Issuance of common stock, net of issuance costs, shares				3,964,478
Conversion of convertible preferred stock to common stock	3		$ (3)	$ 6
Conversion of convertible preferred stock to common stock, shares			(3,218,152)	6,367,000
Share-based compensation expense	6,626						6,626
Unrealized gain (loss) on available-for-sale securities	2									$ 2
Net loss	(73,186)								(73,186)
Balances at Sep. 30, 2022	32,484			$ 43			473,090		(440,649)
Balances, Shares at Sep. 30, 2022				42,856,623
Balances at Jun. 30, 2022	35,923		$ 2	$ 35			464,860		(428,974)
Balances, Shares at Jun. 30, 2022			2,275,214	35,067,477
Issuance of common stock upon the vesting of RSUs, shares					75,287
Issuance of common stock, net of issuance costs	6,128			$ 4			6,124
Issuance of common stock, net of issuance costs, shares				3,226,859
Conversion of convertible preferred stock to common stock	2		$ (2)	$ 4
Conversion of convertible preferred stock to common stock, shares			(2,275,214)	4,487,000
Share-based compensation expense	2,106						2,106
Unrealized gain (loss) on available-for-sale securities	0
Net loss	(11,675)								(11,675)
Balances at Sep. 30, 2022	$ 32,484			$ 43			$ 473,090		$ (440,649)
Balances, Shares at Sep. 30, 2022				42,856,623

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)
Payment of financing/offering costs		$ 411
Common Stock [Member] | Pfizer Purchase Agreement [Member]
Payment of financing/offering costs	$ 100	200
Premium on issued of common stock	$ 12,500	$ 12,500

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2022
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Spero Therapeutics, Inc., together with its consolidated subsidiaries (the “Company” or “Spero”), is a multi-asset, clinical-stage biopharmaceutical company focused on identifying, developing, and commercializing novel treatments for bacterial infections, including multi-drug resistant (“MDR”) bacterial infections, and rare diseases. The Company's lead product candidate, SPR720, is an oral antimicrobial agent in development for the treatment of nontuberculous mycobacterial (“NTM”) pulmonary disease, a rare orphan disease. The Company's partnership directed programs consist of SPR206 and tebipenem HBr, which through the Company's business development efforts, have partnership relationships supporting their ongoing development. SPR206 is an IV-administered agent being developed as an innovative option to treat MDR Gram-negative bacterial infections in the hospital setting. Tebipenem HBr is designed to be the first broad-spectrum oral carbapenem-class antibiotic for use to treat certain bacterial infections that cause complicated urinary tract infections (“cUTIs”) including pyelonephritis, caused by certain microorganisms, in adult patients who have limited oral treatment options. The Company 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 pandemic caused by COVID-19 has resulted, and is likely to continue to result, in significant national and global economic disruption and may adversely affect our business. The Company has experienced impacts to its clinical and development timelines due to the worldwide spread of COVID-19 and its variants. However, to date, the Company has not experienced material impacts to liquidity, nor has it incurred impairment of any assets as a result of COVID-19 or its variants. The Company continues to monitor this situation and the possible effects on its business, results of operations and financial condition, including manufacturing, clinical trials, research and development costs and employee-related amounts. 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 agreements, funding from government contracts, licensing agreements and through the sale of the Company’s common and preferred stock. The Company has incurred recurring losses since inception, including net losses of $ 11.7 million and $ 22.5 million for the three months ended September 30, 2022 and 2021, respectively, and $ 73.2 million and $ 60.5 million for the nine months ended September 30, 2022 and 2021, respectively. In addition, as of September 30, 2022, the Company had an accumulated deficit of $ 440.6 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, cash equivalents and marketable securities and the initial upfront payments of $ 66.0 million the Company is entitled to receive under the GSK License Agreement and $ 9.0 million the Company received under the GSK SPA (as defined below) 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. Beyond that point, the Company will require additional funding to fund the development of its product candidates through regulatory approval and commercialization, and to support its continued operations. The Company will seek additional funding through public or private financings, debt financing, collaboration agreements, government grants or other venues. The COVID-19 pandemic has resulted in ongoing volatility in financial markets. If our access to capital is restricted or associated borrowing costs increase as a result of developments in financial markets, including relating to the COVID-19 pandemic or its variants, our operations and financial condition could be adversely impacted. 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. The accompanying consolidated financial statements do not include any adjustments that might result from the outcome of this uncertainty. Accordingly, the consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. Interim Financial Information The consolidated balance sheet at December 31, 2021 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying unaudited condensed consolidated financial statements as of September 30, 2022, and for the three and nine months ended September 30, 2022, 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, 2021, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2021, 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 September 30, 2022, and results of operations for the three and nine months ended September 30, 2022, and cash flows for the nine months ended September 30, 2022 and 2021 have been made. The results of operations for the three and nine months ended September 30, 2022 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2022 .

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2022
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual for clinical trial costs and other research and development expenses and the valuation of share-based awards. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including expenses, manufacturing, clinical trials, research and development costs and employee-related amounts, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain it or treat COVID-19, as well as the economic impact on local, regional, national and international customers and markets. The Company has contemplated the impact of COVID-19 within its financial statements and is not aware of any specific event or circumstance that would require the Company to update estimates, judgments or revise the carrying value of any assets or liabilities. There may be changes to those estimates in future periods. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on identifying, developing and commercializing novel treatments for bacterial infections, including MDR bacterial infections, and rare diseases. All of the Company’s tangible assets are held in the United States. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains most of its cash and cash equivalents at one accredited financial institution. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. As of September 30, 2022 , and December 31, 2021, the Company had no off-balance sheet risk such as foreign exchange contracts, option contracts, or other hedging arrangements. Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents include cash held in banks and money market instruments. Marketable Securities Marketable securities consist of investments in corporate obligations with original maturities greater than 90 days. The Company considers its portfolio of investments to be available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Investments with maturities beyond one year are generally classified as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value are included as a component of other income (expense), net based on the specific identification method. Any credit impairments are recorded through an allowance account. Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense are recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Computer software and equipment 3 years Office furniture and equipment 7 years Manufacturing equipment 5 years Leasehold improvements Shorter of life of lease or 5 years Costs for capital assets not yet placed into service are capitalized as construction in progress and are depreciated in accordance with the above guidelines once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. The Company periodically evaluates whether events and circumstances have occurred that may warrant revision of the estimated useful life of property and equipment. Leases At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. As of September 30, 2022, the Company had no short-term leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew. The Company monitors its plans to renew its material leases on a quarterly basis. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate (“IBR”), which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, and in a similar economic environment. Since the Company does not have any debt and has not been rated by any major credit rating agency, the Company’s IBR was estimated by developing a synthetic credit rating for the Company. The Company has elected to account for lease and non-lease components together as a single lease component. Other Assets Other assets consist of long-term prepayments and deposits. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities . Revenue Recognition – Collaboration 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. Any future out-licensing agreements entered into by the Company and additional third parties shall also be evaluated under Topic 606. Terms of these arrangements include various payment types, typically including one or more of the following: upfront license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and/or royalties on net sales of licensed products. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations under the agreement; (iii) determine the transaction price, including constraint on variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) determine how the revenue will be recognized for each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. The SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangements. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method. In determining the accounting treatment for these arrangements, the Company develops assumptions to determine the stand-alone selling price for each performance obligation in the contract. These assumptions may include forecasted revenues, development timelines, discount rates and probabilities of technical and regulatory success . Government Tax Incentives For available government tax incentives that the Company may earn without regard to the existence of taxable income and that require the Company to forego tax deductions or the use of future tax credits and net operating loss carryforwards, the Company classifies the funding recognized as a reduction of the related qualifying research and development expenses incurred. Since the fourth quarter of 2016, the Company’s operating subsidiary in Australia has met the eligibility requirements to receive a tax incentive for qualifying research and development activities. The Company recognizes these incentives as a reduction of research and development expenses in the consolidated statements of operations and comprehensive loss in the same period that the related qualifying expenses are incurred. Reductions of research and development expense recognized upon incurring qualifying expenses in advance of receipt of tax incentive payments are recorded in the consolidated balance sheet as tax incentive receivables . Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including personnel salaries, share-based compensation and benefits, allocated facilities costs, depreciation, manufacturing expenses, costs related to the Company’s government contract and grant arrangements, and external costs of outside vendors engaged to conduct preclinical development activities, clinical trials as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Clinical Trial and other Research Contract Costs and Accruals The Company has entered into various research and development contracts with clinical research organizations and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. There may be instances in which payments made to these vendors exceed the level of service provided and will result in a prepayment of the expense. The Company records accruals for estimated ongoing research and clinical trial costs based on the services received and efforts expended pursuant to multiple contracts with these vendors. When evaluating the adequacy of the accrued liabilities, the Company analyzes the progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs . Restructuring The Company has made estimates and judgments regarding the amount and timing of its restructuring expense and liability, including current and future period termination benefits and other exit costs to be incurred when related actions take place. Restructuring charges are reflected in the Company's consolidated statements of income. Actual results may differ from these estimates. 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 Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. For the three and nine months ended September 30, 2022 and 2021, these changes related to unrealized gains and losses on the Company’s available-for-sale marketable securities. Net Loss per Share The Company follows the two-class method when computing net income (loss) per share, as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Net income (loss) per share attributable to common stockholders is calculated based on net income (loss) attributable to Spero Therapeutics, Inc. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents. Income Taxes In response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act (CARES Act) was signed into law in March 2020. The CARES Act lifts certain deduction limitations originally imposed by the Tax Cuts and Jobs Act of 2017 (“2017 Tax Act”). Corporate taxpayers may carryback net operating losses (“NOLs”) originating during 2018 through 2020 for up to five years , which was not previously allowed under the 2017 Tax Act. The CARES Act also eliminates the 80 % of taxable income limitations by allowing corporate entities to fully utilize NOL carryforwards to offset taxable income in 2018, 2019 or 2020. Taxpayers may generally deduct interest up to the sum of 50 % of adjusted taxable income plus business interest income ( 30 % limit under the 2017 Tax Act) for tax years beginning January 1, 2019 and 2020. The CARES Act allows taxpayers with alternative minimum tax credits to claim a refund in 2020 for the entire amount of the credits instead of recovering the credits through refunds over a period of years, as originally enacted by the 2017 Tax Act. In addition, the CARES Act raises the corporate charitable deduction limit to 25 % of taxable income and makes qualified improvement property generally eligible for 15-year cost-recovery and 100 % bonus depreciation. The enactment of the CARES Act did not result in any material adjustments to our income tax provision for the three and nine months ended September 30, 2022, or to our net deferred tax assets as of September 30, 2022 . Liability Related to the Sale of Future Royalties Prior to repayment, as discussed further in Note 10, the Company treated the liability related to the sale of future royalties as a debt instrument, amortized under the effective interest rate method over the estimated life of the revenue streams. The Company recognized interest expense thereon using the effective rate, which was based on its current estimates of future revenues over the life of the arrangement. The Company periodically assessed its expected revenues using internal projections, imputed interest on the carrying value of the deferred royalty obligation and recorded interest expense using the imputed effective interest rate. To the extent its estimates of future revenues were greater or less than previous estimates or the estimated timing of such payments was materially different than previous estimates, the Company accounted for any such changes by adjusting the effective interest rate on a prospective basis, with a corresponding impact to the reclassification of the deferred royalty obligation. The assumptions used in determining the expected repayment term of the deferred royalty obligation and amortization period of the issuance costs required that the Company made estimates that could impact the short-term and long-term classification of such costs, as well as the period over which such costs were amortized. Derivative Liability In connection with certain transactions, the Company has identified certain embedded derivatives, which are recorded as liabilities on the Company’s consolidated balance sheet and are remeasured to fair value at each reporting date until the derivative is settled. Changes in the fair value of the derivative liabilities are recognized as other income (expense) in the consolidated statement of operations and comprehensive loss. Recently Issued and Adopted Accounting Pronouncements Other accounting standards that have been issued by the Financial Accounting Standards Board or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on our financial statements upon adoption.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	9 Months Ended
	Sep. 30, 2022
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair Value Measurements and Marketable Securities The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis (in thousands): Fair Value Measurements at September 30, 2022 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 49,569 $ — $ 49,569 Total cash equivalents — 49,569 — 49,569 Total cash equivalents and marketable securities — 49,569 — 49,569 Liabilities: Derivative liability $ — $ — $ 141 $ 141 $ — $ — $ 141 $ 141 Fair Value Measurements at December 31, 2021 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 109,316 $ — $ 109,316 Corporate bonds — 2,701 — 2,701 Total cash equivalents — 112,017 — 112,017 Marketable securities: Corporate bonds — 11,479 — 11,479 Commercial paper — 22,339 — 22,339 Total marketable securities — 33,818 — 33,818 Total cash equivalents and marketable securities $ — $ 145,835 $ — $ 145,835 Liabilities: Derivative liability $ — $ — $ 802 $ 802 $ — $ — $ 802 $ 802 Excluded from the tables above is cash of $ 0.9 million and $ 0.6 million as of September 30, 2022, and December 31, 2021, respectively. During the nine months ended September 30, 2022 , there were no transfers between Level 1, Level 2 and Level 3 categories. Marketable Securities The Company’s marketable securities are classified as Level 2 assets under the fair value hierarchy as these assets were primarily determined from independent pricing sources, which generally derive security prices from recently reported trades for identical or similar securities. The Company evaluated debt securities with unrealized losses for any expected credit losses and determined unrealized losses on these securities were related to non-credit factors. Additionally, the Company currently does no t intend to and is not required to sell these investments prior to an anticipated recovery in value. The Company did not have any assets classified as marketable securities as of September 30, 2022 . The following table summarizes the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2021 (in thousands): December 31, 2021 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Assets: Corporate bonds $ 11,481 $ — $ ( 2 ) $ 11,479 Commercial paper 22,339 — — 22,339 $ 33,820 $ — $ ( 2 ) $ 33,818 As of December 31, 2021, all of the Company’s marketable securities had remaining contractual maturity dates of one year or less from the respective consolidated balance sheet date. Embedded Derivative Liability related to change of control In connection with the termination of the Revenue Interest Agreement (as defined below) (see Note 10), the Company classified $ 0.1 million as a derivative liability on its condensed consolidated balance sheet because of an embedded feature that represents a conditional obligation to pay HCR (as defined below) an additional cash amount upon a change of control. The Company will remeasure the derivative liability to fair value at each reporting date until it expires on December 31, 2022, and recognize changes in the fair value of the derivative liability as a component of other income (expense) in the condensed consolidated statement of operations and comprehensive loss. The Company valued the change of control provision under the Revenue Interest Termination Agreement, dated June 7, 2022, by and between the Company and HCR, using a series of Black-Scholes-Merton option pricing models. The assumptions used in the valuation model include (1) the Company's estimates of the probability of a change of control event occurring prior to or as of December 31, 2022, (2) the Company's closing stock price as of June 7, 2022, (3) the Company's fully-diluted number of shares outstanding as of June 7, 2022, (4) volatility, (5) risk-free rate, and (6) the Company's credit-risk-adjusted discount rate. The fair value of the derivative liability both upon issuance in June 2022 and as of September 30, 2022 is $ 0.1 million and is classified as Level 3 liability under the fair value hierarchy. Liability related to the sale of future royalties As of December 31, 2021, in connection with the liability related to the sale of future royalties, the Company classified $ 1.0 million at inception of its Revenue Interest Financing Agreement as a derivative liability on its consolidated balance sheet because there were embedded instruments that represented a conditional obligation to pay HCR the final payment, which is 250 % of the Investment Amount, upon an event of default or change of control. The fair value of the derivative liability upon issuance in October 2021 was $ 1.0 million, and was classified as Level 3 liability under the fair value hierarchy. As of December 31, 2021 the fair value of the derivative liability decreased by $ 0.2 million to $ 0.8 million, primarily due to the passage of time and changes in the market volatility and underlying credit risk inputs. The fair value for the liability related to the sale of future royalties at the time of the initial transaction was based on the Company's current estimates of future royalties expected to be paid to HCR over the remaining patent life of the product, which were considered Level 3 inputs (see Note 10).

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2022
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 September 30, 2022 and December 31, 2021 (in thousands): September 30, 2022 December 31, 2021 Accrued external research and development expenses $ 1,219 $ 6,315 Accrued restructuring expenses 967 — Accrued payroll and related expenses 4,860 5,884 Accrued professional fees 836 909 Accrued other 211 1,242 Total Accrued expenses and other current liabilities $ 8,093 $ 14,350

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2022
Equity [Abstract]
Common Stock	. Common Stock “At-the-Market” Offering On March 11, 2021, the Company entered into a new sales agreement with Cantor Fitzgerald & Co. (“Cantor”) and filed a new universal shelf registration statement on Form S-3 (Registration No. 333-254170), and pursuant to which the Company registered for sale up to $ 300.0 million of any combination of its common stock, preferred stock, debt securities, warrants, rights and/or units from time to time and at prices and on terms that the Company may determine, including up to $ 75.0 million of its common stock available for issuance pursuant to the new “at-the-market” offering program sales agreement that it entered into with Cantor (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. The Company’s universal shelf registration statement on Form S-3 (Registration No. 333-254170) became effective on March 29, 2021 and its prior sales agreement with Cantor terminated automatically at such time. During the nine months ended September 30, 2022, the Company sold 3,964,478 shares of its common stock under its Sales Agreement at an average price of approximately $ 2.69 per share for aggregate gross proceeds of approximately $ 10.7 million prior to deducting sales commissions. Convertible Preferred Stock In February 2021, a holder of the Company’s Series B Preferred Stock elected to convert 62 shares of Series B Preferred Stock into 62,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series B Preferred Stock. In addition, a holder of the Company’s Series C Preferred Stock elected to convert 73 shares of Series C Preferred Stock into 73,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series C Preferred Stock. In June 2022, a holder of the Company’s Series B Preferred Stock elected to convert 938 shares of Series B Preferred Stock into 938,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series B Preferred Stock. In addition, a holder of the Company’s Series D Preferred Stock elected to convert 942,000 shares of Series D Preferred Stock into 942,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series D Preferred Stock. In September 2022, a holder of the Company’s Series C Preferred Stock elected to convert the remaining 2,214 shares of Series C Preferred Stock into 2,214,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series C Preferred Stock. In addition, a holder of the Company’s Series D Preferred Stock elected to convert the remaining 2,273,000 shares of Series D Preferred Stock into 2,273,000 shares of the Company’s common stock, pursuant to such holder’s rights under the certificate of designation for such Series D Preferred Stock. As of September 30, 2022, all of the Company's shares of preferred stock have been converted to common stock. Pfizer License and Share Purchase Agreements On June 30, 2021, the Company agreed to sell 2,362,348 shares of common stock to Pfizer Inc. (“Pfizer”) pursuant to a Share Purchase Agreement (the “Pfizer Purchase Agreement”), at a price of $ 16.93 per share, which represented a premium over the most recent closing price on June 30, 2021, for an aggregate purchase price of $ 40.0 million. No shareholder approval was required for the sale of the shares. Pfizer is an accredited investor as defined in the Securities Act, and the shares were sold pursuant to exemptions from registration under Regulation D of the Securities Act. The Company has not filed a registration statement with the SEC covering the resale of the shares and such securities may not be offered or sold in the United States absent registration or an exemption from registration under the Securities Act and any applicable state securities laws. The fair market value of 2,362,348 shares of the Company's common stock issued to Pfizer under the Pfizer Purchase Agreement was $ 27.5 million. The common stock issued under the Pfizer Purchase Agreement were valued using an option pricing valuation model as the shares are subject to certain holding period restrictions. The Company accounted for the associated premium of $ 12.5 million as a freestanding equity-linked instrument under ASC 815. The premium was allocated as consideration for the Company's license agreement with Pfizer (the “Pfizer License Agreement”) and evaluated under ASC 606. The premium was determined not to be constrained and was included in the calculation of the total transaction price related to the Pfizer License Agreement as of June 30, 2021. Refer to Note 9 for further discussion. The closing of the sale of the shares pursuant to the Pfizer Purchase Agreement occurred on July 1, 2021. Upon closing, the Company recorded the fair market value of the shares issued in stockholders’ equity in its condensed consolidated balance sheet. Charter Amendment On August 17, 2021, the Company filed a Certificate of Amendment to its Amended and Restated Certificate of Incorporation with the Secretary of State of the State of Delaware to increase the number of shares of the Company’s common stock authorized for issuance from 60,000,000 shares to 120,000,000 shares (the “Charter Amendment”). The Charter Amendment was approved by the Company’s stockholders at the Annual Meeting held on August 17, 2021.

Share-Based Compensation

Share-Based Compensation	9 Months Ended
	Sep. 30, 2022
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 (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. In September 2022, the Company's shareholders approved an amendment to the 2017 Plan to increase the number of shares of the Company’s common stock authorized for issuance under the 2017 Plan by 2,000,000 shares. In August 2021, the Company's shareholders approved amendments to the 2017 Plan. The amendments provide for the following: (i) increases the number of shares of the Company’s common stock authorized for issuance under the 2017 Plan by 3,170,254 shares, (ii) removes the “evergreen” provision historically included in the 2017 Plan, and (iii) makes certain other amendments. 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 $ 7.94 and $ 13.57 per option for those options granted during the nine months ended September 30, 2022 and 2021 respectively. The following table summarizes stock option activity under all equity plans (excluding RSUs) during the nine months ended September 30, 2022: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2021 4,890,716 $ 11.75 7.62 $ 24,263 Granted 956,604 11.12 Exercised ( 56,120 ) 7.48 Forfeited or cancelled ( 1,775,539 ) 13.23 Outstanding as of September 30, 2022 4,015,661 $ 11.00 5.48 $ 2 Outstanding as of September 30, 2022 - vested and 4,015,661 $ 11.00 5.48 $ 2 Exercisable at September 30, 2022 2,606,480 $ 9.65 4.73 $ — As of September 30, 2022, a total of 10,720,127 shares have been authorized and reserved for issuance under all equity plans and 4,806,802 shares were available for future issuance under such plans. Restricted Stock Units The Company granted 1,115,357 RSUs to employees during the nine months ended September 30, 2022. T he following table summarizes RSU activity under all equity plans during the nine months ended September 30, 2022: Number of Weighted Average Grant Date Fair Value Outstanding as of December 31, 2021 513,690 17.08 Granted 1,115,357 7.01 Vested and released ( 75,287 ) 17.07 Forfeited or cancelled ( 408,000 ) 14.16 Outstanding as of September 30, 2022 1,145,760 8.32 As of September 30, 2022 , there was approximately $ 8.6 million of total unrecognized compensation expense related to RSUs, which is expected to be recognized over a weighted-average period of approximately 2.92 years. The fair value of the RSUs is determined on the date of grant based on the market price of the Company’s common stock on that date. Each RSU represents the right to receive one share of the Company’s common stock, $ 0.001 par value per share, upon vesting. 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. Performance-Based Awards In July 2022, as part of the retention awards referenced in Footnote 11, the Company granted a number of shares of common stock containing performance-based vesting criteria ("performance-based awards") to be issued on May 31, 2023, with a value of $ 1.7 million based on the common stock price at such time, subject to the discretion of the Company's Board or Human Capital Management Committee to pay in cash or a combination of cash and stock. Performance-based awards will be recorded as a liability on the Company's condensed consolidated balance sheet and accrued over time as achievement of performance metrics become probable. The performance-based awards are eligible for vesting based on the achievement of certain performance criteria by May 31, 2023 relating to pipeline execution, business development, and financial stewardship. Performance-based awards for which the performance criteria have not been achieved as specified by May 31, 2023 will lapse and be forfeited. The performance-based awards will be subject to acceleration of vesting in the event of termination of employment without cause by the Company or by the executive for good reason (each as defined in the executive’s employment agreement). In September 2022, the Company approved an award of 140,000 performance-based stock units as part of an executive inducement grant ("Inducement PSUs"). The Inducement PSUs are eligible for vesting based on the same performance criteria, above. To the extent these performance criteria are met by May 31, 2023, 50 % of the Inducement PSUs will vest on September 12, 2023 and 50 % will vest on September 12, 2024. The Company recognized $ 0.5 million of compensation expense associated with performance-based awards and an insignificant amount of expense associated with the Inducement PSUs during the nine months ended September 30, 2022, as the performance conditions were considered probable of achievement. 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 September 30, Nine Months Ended September 30, 2022 2021 2022 2021 Research and development expenses $ 730 $ 1,059 $ 2,595 $ 2,794 General and administrative expenses 1,376 1,271 4,031 3,708 Total $ 2,106 $ 2,330 $ 6,626 $ 6,502

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2022
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies License Agreements The Company has entered into license agreements with various parties under which it is obligated to make contingent and non-contingent payments (see Note 9). Operating Leases The Company has entered into an operating lease agreement with U.S. REIF Central Plaza Massachusetts, LLC with respect to its corporate headquarters located at 675 Massachusetts Avenue, Cambridge, Massachusetts. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements that will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of September 30, 2022 or December 31, 2021. Legal Proceedings Two putative class action lawsuits were filed against the Company and certain of its officers in the United States District Court for the Eastern District of New York, one captioned Richard S. Germond v. Spero Therapeutics, Inc., Ankit Mahadevia, and Satyavrat Shukla, Case No. 1:22-cv-03125, filed on May 26, 2022, and the other captioned Kashif Memon v. Spero Therapeutics, Inc., Ankit Mahadevia, and Satyavrat Shukla Case No. 1:22-cv-04154, filed on July 15, 2022. The parties moved to consolidate the two complaints on July 22, 2022, which were ordered consolidated on August 5, 2022. Both Mr. Memon and stockholder Nabil Saad filed motions for appointment of lead plaintiff/lead counsel on July 25, 2022. Mr. Saad withdrew his opposition on August 15, 2022, and Mr. Memon was appointed lead plaintiff on September 19, 2022. The complaint purports to be brought on behalf of stockholders who purchased the Company’s common stock from May 6, 2021 through May 2, 2022 (Mr. Memon’s complaint alleged a six-month longer class period than the complaint originally filed by Mr. Germond). The complaint generally alleges that the Company and certain of its officers violated Sections 10(b) and/or 20(a) of the Securities Exchange Act of 1934 and Rule 10b-5 promulgated thereunder by making allegedly false and/or misleading statements concerning the New Drug Application for tebipenem HBr in an effort to lead investors to believe that the drug would receive approval from the FDA. The complaint seeks unspecified damages, interest, attorneys’ fees, and other costs. Mr. Memon must file an amended complaint on or before December 5, 2022. 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	9 Months Ended
	Sep. 30, 2022
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, increasing the total committed funding to $ 34.0 million and extended the period of performance through November 1, 2021. In October 2022, BARDA extended the period of performance for the first contract option through December 15, 2023. As of December 31, 2021, the balance of the award was subject to BARDA exercising a second option which would entail funding of $ 12.7 million and is exercisable by BARDA subject to, among other things, satisfactory progress and results from the biodefense studies described below. On January 19, 2022, the Company announced that BARDA exercised a new option under the contract. The new option increases the total amount of committed funding by $ 12.9 million to approximately $ 46.9 million, increasing the total potential contract value to $ 59.7 million. The additional $ 12.9 million option is expected to provide support for a clinical trial and related activities for orally administered tebipenem HBr's use in treating pediatric patients with cUTIs, including acute pyelonephritis. On September 30, 2022, remaining funding from the option originally supporting a clinical trial in pediatric patients was reprogrammed to support clinical trials for patients with cUTIs, including acute pyelonephritis. The period of performance for this new option extends through December 31, 2025 and does not change the total amount of committed funding or potential contract value. As part of an inter-agency collaboration between BARDA and the 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 Spero. DTRA provides up to $ 10.0 million, in addition to the total potential award from BARDA, to cover the cost of the nonclinical biodefense aspects of the collaborative program for tebipenem HBr. Together, BARDA and DTRA will provide up to $ 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 Spero’s achievement of specified milestones. The Company recognized $ 0.5 million and $ 0.7 million of revenue under the BARDA award during the three months ended September 30, 2022 and 2021, respectively, and recognized $ 1.9 million and $ 8.9 million of revenue under the BARDA award during the nine months ended September 30, 2022 and 2021, respectively. U.S. Department of Defense On July 1, 2019, the Company received a $ 5.9 million award from the U.S. Department of Defense (“DoD”) Congressionally Directed Medical Research Programs (“CDMRP”) Joint Warfighter Medical Research Program. The funding will support the further clinical development of SPR206. The award commits non-dilutive funding of $ 5.9 million over a four-year period to cover the costs of select Phase 1 pharmacology studies, a 28-day GLP NHP toxicology study, and microbiological surveillance studies that would be required for a potential NDA submission with the FDA for SPR206. The Company recognized less than $ 0.1 million and $ 1.5 million in revenue under this agreement during the three months ended September 30, 2022 and 2021, respectively, and recognized $ 1.0 million and $ 3.3 million under this agreement during the nine months ended September 30, 2022 and 2021, respectively. All activities under this award were completed and the award was closed on August 14, 2022. NIAID In May 2021, the Company was awarded a five-year contract from the U.S. National Institute of Allergy and Infectious Diseases (“NIAID”) under the Agency’s Omnibus Broad Agency Announcement No. HHS-NIH-NIAID-BAA2020-1 award mechanism to support further development of SPR206. Funding will be used to offset certain expenses related to manufacturing, clinical, non-clinical and regulatory activities. The Company can receive up to $ 23.4 million over a base period and five option periods. As of September 30, 2022 , funding for the base period totaling $ 2.1 million has been committed. The Company recognized $ 0.3 million and $ 1.0 million under this agreement during the three and nine months ended September 30, 2022, respectively, and recognized $ 0.1 million under this agreement during both the three and nine months ended September 30, 2021. In June 2016, the Company entered into agreements with Pro Bono Bio PLC (“PBB”), a corporation organized under the laws of England, and certain of its affiliates, including PBB Distributions Limited and Cantab Anti-Infectives Limited (“CAI”), in order to acquire certain intellectual property and government funding arrangements relating to SPR206. Under these agreements, CAI agreed to submit a request to NIAID to novate the then CAI-held NIAID contract to Spero, which was finalized in December 2017. The NIAID contract provides for development funding of up to $ 6.5 million over a base period and three option periods. As of December 31, 2021, funding for the base period and the first two option periods totaling $ 5.9 million had been committed. In March 2021, a contract modification was executed and the performance period for this award was extended until June 15, 2021. The Company did no t recognize revenue under this agreement during the three and nine months ended September 30, 2022 . The Company did no t recognize revenue under this agreement during the three months ended September 30, 2021 and recognized $ 0.4 million during the nine months ended September 30, 2021 .

License, Collaboration and Serv

License, Collaboration and Service Agreements	9 Months Ended
	Sep. 30, 2022
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. Cantab License Agreements Under the Cantab Agreements, the Company is obligated to make future milestone payments of up to $ 5.8 million upon the achievement of specified clinical and regulatory milestones and a payment of £ 5.0 million ( $ 5.5 million as of September 30, 2022) upon the achievement of a specified commercial milestone. In addition, the Company agreed to pay to PBB royalties, on a product-by-product and country-by-country basis, of a low single-digit percentage based on net sales of products licensed under the agreement. During both the three and nine months ended September 30, 2022 and 2021, the Company did no t record any research and development expense related to the achievement of regulatory milestones for SPR206. The Cantab Agreements continue indefinitely, with royalty payment obligations thereunder continuing on a product-by-product and country-by-country basis until the later of ten years after the first commercial sale of such product in such country or the expiration in such country of the last to expire valid claim of any of the applicable patents. Vertex License Agreement In May 2016, the Company entered into an agreement with Vertex Pharmaceuticals Incorporated (“Vertex”) whereby Vertex granted the Company certain know-how and a sublicense to research, develop, manufacture and sell products for a proprietary compound, as well as a transfer of materials. In exchange for the know-how, sublicense and materials, Spero paid Vertex an upfront, one-time, nonrefundable, non-creditable fee of $ 0.5 million, which was recognized as research and development expense. As part of the agreement, the Company is obligated to make future milestone payments of up to $ 80.2 million upon the achievement of specified clinical, regulatory and commercial milestones and to pay Vertex tiered royalties, on a product-by-product and country-by-country basis, of a mid-single-digit to low double-digit percentage based on net sales of products licensed under the agreement. During the three and nine months ended September 30, 2022 and 2021, the Company did no t record any research and development expense related to the achievement of regulatory milestones for SPR720 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 . Meiji License Agreement In June 2017, the Company entered into agreements with Meiji Seika Pharma Co. Ltd. (“Meiji”), a Japanese corporation, whereby Meiji granted to the Company a license under certain patents, know-how and regulatory documentation to research, develop, manufacture and sell products containing a proprietary compound in the licensed territory. In exchange for the license, the Company paid Meiji an upfront, one-time, nonrefundable, non-creditable fee of $ 0.6 million, which was recognized as research and development expense. In October 2017, the Company paid a $ 1.0 million milestone payment to Meiji upon the enrollment of the first patient in the Company’s Phase 1 clinical trial of tebipenem HBr. The payment was recorded as research and development expense in the statement of operations and comprehensive loss for the year ended December 31, 2017. The Company paid Meiji approximately $ 1.6 million during the fourth quarter of 2018 related to fixed assets which will be used in manufacturing related activities at Meiji. This equipment has been capitalized as property and equipment in the consolidated balance sheet as of September 30, 2022 . 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. As part of the agreement, the Company is obligated to make future milestone payments of up to $ 1.0 million as of September 30, 2022 upon the achievement of specified regulatory milestones, to pay royalties, on a product-by-product and country-by-country basis, of a low single-digit percentage based on net sales of products licensed under the agreement and to pay Meiji a low double-digit percentage of any sublicense fees received by the Company up to a maximum amount of $ 7.5 million. 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. 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 “Licensed Products”), in Greater China (which includes Mainland China, Hong Kong and Macau), South Korea and certain Southeast Asian countries (the “Territory”). The Company retained development, manufacturing and commercialization rights with respect to SPR206 and Licensed Products in the rest of the world and also retained the right to develop or manufacture SPR206 and Licensed Products in the Territory for use outside the Territory. In addition to the license grant with respect to SPR206, the Company, through its wholly owned subsidiary, Spero Potentiator, Inc., a Delaware corporation, granted Everest a 12 -month exclusive option to negotiate with it for an exclusive license to develop, manufacture and commercialize SPR741 in the Territory. Under the terms of the Original Everest License Agreement, the Company received an upfront payment of $ 3.0 million that was recognized in the first quarter of 2019, comprised of a $ 2.0 million payment to license SPR206 and $ 1.0 million for the exclusive option to negotiate a license to develop SPR741. The Company also received a milestone payment of $ 2.0 million in the fourth quarter of 2020 upon completion and delivery of the results of a clinical study. On January 15, 2021, the Company entered into an amended and restated license agreement (“the Amended Everest License Agreement”) with Everest and Spero Potentiator, Inc., which amended and restated in its entirety the Original Everest License Agreement. The Amended Everest License Agreement modifies the dates and values of certain milestone events related to development and commercialization of SPR206. Everest will now be making more significant investments in the development of SPR206 beyond what was contemplated at the time of the Original Everest License Agreement. The Original Everest License Agreement provided that the Company could receive up to $ 59.5 million upon achievement of certain milestones. The Amended Everest License Agreement provides that the Company may receive up to $ 38.0 million upon achievement of certain milestones, of which $ 1.3 million has been received to date. The Company received milestones of $ 1.5 million related to a clinical study for SPR206, of which the Company received approximately $ 0.8 million upon the initiation of the Bronchoalveolar Lavage (“BAL”) clinical trial of SPR206 in June 2021 and received the remaining $ 0.8 million upon the delivery of the clinical study report in the second quarter of 2022. In addition, under the Amended Everest License Agreement, the Company assigned patents in the Territory to Everest, rather than licensing such patents to Everest, and the option related to SPR741 and the related provisions have been removed. Under the terms of the Amended Everest License Agreement, the Company is also entitled to receive high single-digit to low double-digit royalties on net sales, if any, of Licensed Products in the Territory following regulatory approval of SPR206. Everest has the right to sublicense to affiliates and third parties in the Territory. Everest is responsible for all costs related to developing, obtaining regulatory approval of and commercializing SPR206 and Licensed Products in the Territory, and is obligated to use commercially reasonable efforts to develop, manufacture and commercialize Licensed Products, including to achieve certain specified diligence milestones within agreed-upon periods. A joint development committee will be established between the Company and Everest to coordinate and review the development, manufacturing and commercialization plans with respect to Licensed Products in the Territory. Unless earlier terminated due to certain material breaches of the contract, or otherwise, the Amended Everest License Agreement will expire on a jurisdiction-by-jurisdiction and Licensed Product-by-Licensed Product basis upon the latest to occur of expiration of the last valid claim under a licensed patent in such jurisdiction, the expiration of regulatory exclusivity in such jurisdiction or ten years after the first commercial sale of such Licensed Product in such jurisdiction. The Amended Everest License Agreement may be terminated in its entirety by Everest upon 90 or 180 days’ prior written notice, depending on the stage of development of the initial Licensed Product. The Company did no t recognize revenue during the three months ended September 30, 2022 and recognized $ 0.8 million during the nine months ended September 30, 2022 related to this agreement. The Company recognized $ 0.5 million and $ 1.3 million during the three and nine months ended September 30, 2021, respectively, related to this agreement. Gates MRI Collaboration Agreement In June 2019, the Company entered into a collaboration with the Bill and Melinda Gates Research Institute (the “Gates MRI”) to develop SPR720 for the treatment of lung infections caused by Mycobacterium tuberculosis. In furtherance of the Gates MRI’s charitable purposes, the Company also granted to Gates MRI a no-cost, exclusive license to develop, manufacture and commercialize SPR720 for the treatment of tuberculosis (“TB”) in low- and middle- income countries. The Gates MRI is responsible for formulating and funding its own research plan for the development of SPR720 for TB. As such, Gates MRI will conduct and fund preclinical and clinical studies for the development of SPR720 against TB. In addition, Gates MRI and the Company will jointly design and manage certain collaborative research activities, which the Company will perform and which will be funded by the Gates MRI. Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company will recognize the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred. The Company did no t record a reduction to research and development expense as no activities were funded by Gates MRI during the three and nine months ended September 30, 2022 . The Company recorded $ 0.3 million and $ 1.4 million during the three and nine months ended September 30, 2021, respectively, as a reduction to research and development expense related to activities funded by Gates MRI. Savior Service Agreement In November 2018, the Company entered into a service agreement with Savior Lifetec Corporation (“Savior”) to perform technology transfer, process development, analytical method development and testing and formulation development for tebipenem HBr. Per the terms of the agreement, the Company paid Savior a non-refundable supervision fee of approximately $ 2.0 million to manage the buildout of a commercial manufacturing facility. The supervision fee was classified as a prepaid asset on the Company’s balance sheet and was fully amortized over a service period of approximately 34 months as of December 31, 2021. The Company has paid Savior an additional $ 5.3 million for facility build out costs, which is classified as a long-term asset on the Company’s balance sheet as of September 30, 2022. Pfizer License and Share Purchase Agreements On June 30, 2021, the Company and Pfizer entered into the Pfizer License Agreement and the Pfizer Purchase Agreement. Under the terms of the Pfizer License Agreement, the Company granted Pfizer an exclusive royalty-bearing license to develop, manufacture and commercialize SPR206 or products that contain SPR206 (the “Licensed Products”) globally with some territorial exceptions (the “Pfizer Territory”). The Pfizer Territory excludes the United States and the Asian markets previously licensed to Everest, those being the People’s Republic of China, including Hainan Island, the Hong Kong Special Administrative Region of the People’s Republic of China, and the Macau Special Administrative Region of the People’s Republic of China, Taiwan, the Republic of Korea (South Korea), the Republic of Singapore, Malaysian Federation, Kingdom of Thailand, the Republic of Indonesia, Socialist Republic of Vietnam and the Republic of the Philippines). Under the terms of the Pfizer Purchase Agreement, Pfizer purchased 2,362,348 shares of the Company’s common stock at a price of $ 16.93 per share for a total investment of $ 40.0 million. Under the terms of the Pfizer License Agreement, the Company received no other upfront payments but is eligible to receive up to $ 80.0 million in development and sales milestones, and may also receive high single-digit to low double-digit royalties on net sales of SPR206 in the Pfizer Territory. Achievement of these payments cannot be guaranteed. The Company and Pfizer agree that upon Pfizer’s request, the parties will negotiate in good faith regarding procuring a clinical or commercial supply of the compound. The fair market value of 2,362,348 shares of the Company's common stock issued to Pfizer under the Pfizer Purchase Agreement was determined to be $ 27.5 million. The common stock issued under the Pfizer Purchase Agreement were valued using an option pricing valuation model as the shares are subject to certain holding period restrictions. The Company accounted for the associated premium of $ 12.5 million as a freestanding equity-linked instrument under ASC 815. The premium was allocated as consideration for the Pfizer License Agreement and evaluated under ASC 606. The premium was determined not to be constrained and was included in the calculation of the total transaction price related to the Pfizer License Agreement as of June 30, 2021. The Company is responsible for all costs related to developing and obtaining regulatory approval of SPR206 and Licensed Products in the Pfizer Territory, with a focus on the European market, and is obligated to use commercially reasonable efforts, including to achieve certain specified diligence milestones within agreed-upon periods. A joint development committee was established between the Company and Pfizer to coordinate and review the development, manufacturing and commercialization plans with respect to Licensed Products in the Pfizer Territory. Pfizer is responsible for commercializing SPR206 and the Licensed Products in the Pfizer Territory. Unless earlier terminated due to certain material breaches of the contract or by Pfizer’s convenience, or otherwise, the Pfizer License Agreement will expire on a jurisdiction-by-jurisdiction and licensed product-by-licensed product basis after ten years from the effective date. The Pfizer License Agreement will automatically renew for an additional ten-year term unless terminated. Accounting Analysis and Revenue Recognition The Company determined that Pfizer is a customer and that the Pfizer License Agreement is within the scope of ASC 606 as licensing intellectual property and performing ongoing research and development services are ordinary activities that are ongoing and central to the Company’s operations. Accordingly, in determining the appropriate amount of revenue to be recognized, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including the constraint on variable consideration; (iv) allocated the transaction price to the identified performance obligations in proportion to their SSP; and (v) recognized revenue when each performance obligation was deemed to be satisfied. Based on that evaluation, the Company identified two performance obligations, license and know-how transfer and research and development services related to upcoming milestones. The Company determined that the supply agreement is a customer option and not a material right, as the pricing to Pfizer is not at a significant discount. Furthermore, Pfizer has the right to use third parties to manufacture the compound, or to manufacture the compound itself. At contract inception, $ 1.4 million of the then transaction price of $ 12.5 million was allocated to the license and know-how transfer performance obligations, which was fully satisfied and recognized as revenue upon delivery of the license. The additional $ 11.1 million was allocated to the research and development services obligation and is being recognized over time as services are delivered. In the third quarter of 2022, upon the completion of a milestone related to regulatory engagement for SPR206, Pfizer communicated its approval that the milestone was achieved, and the Company received $ 5.0 million under the Pfizer License Agreement, which the Company accounted for as variable consideration under ASC 606 and was added to the transaction price in the third quarter of 2022. Of this $ 5.0 million milestone, $ 0.9 million was recognized during the third quarter of 2022 and the remaining $ 4.1 million was allocated to the research and development services performance obligation and will be recognized over time as the services are delivered. The potential license maintenance fees and development milestone payments from the Pfizer License Agreement will be accounted for as variable consideration under ASC 606. Given the uncertain nature of these payments, the Company determined they were fully constrained as of September 30, 2022 and not included in the transaction price. The Company can also earn sales-based royalties. The Company recognizes revenue for the license performance obligation at a point in time, that is upon transfer of the license to Pfizer. Control of the license was transferred on the Effective Date and Pfizer could begin to use and benefit from the license at the Effective Date. In total, and inclusive of the above, the Company recognized $ 1.1 million and $ 1.5 million of revenue from the contract during the three and nine months ended September 30, 2022 , respectively, and recognized $ 0.2 million and $ 1.6 million of revenue from the contract during the three and nine months ended September 30, 2021, respectively. The r emaining transaction price balance and subsequent milestone achievement of approximately $ 14.2 million from the Pfizer Purchase Agreement allocated to the research and development services performance obligation has been recorded as deferred revenue in the condensed consolidated balance sheet. As of September 30, 2022 , the research and development services related to the second performance obligation were expected to be recognized as costs are incurred over the project development timeframe.

Liability Related to the Sale o

Liability Related to the Sale of Future Royalties	9 Months Ended
	Sep. 30, 2022
Revenue Recognition and Deferred Revenue [Abstract]
Liability Related to the Sale of Future Royalties	10. Liability Related to the Sale of Future Royalties On September 29, 2021, the Company entered into a Revenue Interest Agreement with certain entities managed by HealthCare Royalty Management, LLC (“HCR”), pursuant to which the Company sold to HCR the right to receive certain royalty payments from the Company for a purchase price of up to $ 125.0 million. The Company evaluated the terms of the Revenue Interest Agreement and concluded that the features of the investment amount were similar to those of a debt instrument. The Company received gross proceeds of $ 50.0 million from HCR at an initial funding on October 19, 2021 (the “Initial Investment Amount”). As such, the Company accounted for this transaction as long-term debt as of December 31, 2021. The Company was entitled to receive an additional $ 50.0 million upon FDA approval of tebipenem HBr on or before December 31, 2022 (the “Second Investment Amount”), and an additional $ 25.0 million subject to the mutual agreement of the Company and HCR and if the Company meets certain minimum tebipenem HBr product sales thresholds in the United States within 12 months from commercial launch (the “Third Investment Amount,” and together with the Initial Investment Amount and the Second Investment Amount, collectively, the “Investment Amount”). On June 7, 2022, the Company entered into a Revenue Interest Termination Agreement (the “Termination Agreement”) with HCR and HCR Collateral Management, LLC, as collateral agent for HCR under the Revenue Interest Agreement, pursuant to which the parties mutually terminated the Revenue Interest Agreement and Security Agreement between the parties (together, the HCR Agreements”), and certain other related ancillary agreements, arrangements or understandings under or contemplated by the HCR Agreements. The Company was released from all of its obligations and the rights to any future revenues and collateral reverted back to the Company in return for a cash payment of $ 54.5 million and a potential additional cash amount contingent upon the occurrence of a change of control event. If a change of control event occurs on or prior to December 31, 2022, then the Company will be obligated to pay to HCR an additional amount within 15 days following the consummation of such change of control transaction, calculated based on the fully-diluted equity value of the Company. If the change of control value is less than $ 100 million, the change of control payment would be $ 4 million, if $ 100 million to $ 200 million, the change of control payment would be $ 4 million plus 6 % of the change of control value in excess of $ 100 million and if above $ 200 million, the change of control payment would be $ 10 million plus 3 % of the change of control value in excess of $ 200 million. The change of control payment is not cumulative, such that only the highest applicable change of control value shall apply in calculating the change of control payment. The rights to this contingent payment expire on December 31, 2022. The contingent payment meets the accounting definition of a derivative under ASC 815 and therefore a liability of $ 0.1 million is recorded on the Company’s condensed consolidated balance sheet as of September 30, 2022 (see Note 3). The Company recognized a loss on the extinguishment of $ 3.6 million, as reported on the associated condensed consolidated statement of operations and comprehensive loss. The following table presents the changes in the liability related to the sale of future royalties under the Revenue Interest Agreement with HCR as of September 30, 2022 (in thousands): Liability related to sale of future royalties, as of December 31, 2021 $ 48,414 Repayment of liability ( 54,485 ) Loss on extinguishment of liability 3,581 Establish change of control derivative liability ( 115 ) Interest expense recognized 2,605 Liability related to sale of future royalties, as of September 30, 2022 $ —

Restructuring

Restructuring	9 Months Ended
	Sep. 30, 2022
Restructuring and Related Activities [Abstract]
Restructuring	11. Restructuring On May 3, 2022, the Company implemented a strategic restructuring initiative and corresponding reduction in workforce. The restructuring initiative and corresponding reduction in workforce was designed to reduce costs and reallocate resources towards the Company’s clinical development programs for SPR720 and SPR206, while maintaining key personnel needed to help preserve the value of the Company’s tebipenem HBr program. The restructuring reduced the Company’s workforce from 146 full-time employees as of December 31, 2021 to 41 full-time employees following the restructuring. During the nine months ended September 30, 2022, the Company recognized a restructuring charge of approximately $ 11.7 million, the majority of which was incurred in the second quarter of 2022. During the three months ended September 30, 2022, the Company recognized a restructuring charge of $( 0.2 ) million related to restructuring estimate adjustments. Restructuring charges included approximately $ 8.6 million of employee related termination costs, lease impairment expense of $ 0.6 million and $ 2.5 million of other discontinuation costs such as contract termination fees. The following tables summarize the restructuring related charges by line item within the Company’s consolidated statements of operations where they were recorded during the three and nine months ended September 30, 2022 (in thousands): Three Months Ended September 30, 2022 Research and development General and administrative Total Severance and other employee costs $ ( 53 ) $ ( 52 ) $ ( 105 ) Lease impairment - - - Other - ( 47 ) ( 47 ) Total restructuring charges $ ( 53 ) $ ( 99 ) $ ( 152 ) Nine Months Ended September 30, 2022 Research and development General and administrative Total Severance and other employee costs $ 3,872 $ 4,680 $ 8,552 Lease impairment 372 263 635 Other 116 2,394 2,510 Total restructuring charges $ 4,360 $ 7,337 $ 11,697 The restructuring charge was included in accrued expenses and other current liabilities in the Company’s condensed consolidated balance sheet. Activity for the quarter is summarized as follows (amounts in thousands): As of September 30, 2022 Balance as of December 31, 2021 $ — Charge to expense 11,697 Payments made ( 7,250 ) Write-offs and impairments ( 3,480 ) Balance as of September 30, 2022 $ 967 As of September 30, 2022, the Company had $ 1.0 million remaining in accrued expenses related to restructuring costs on its condensed consolidated balance sheet, which will be paid by the end of the first quarter of 2023. Retention Awards In June 2022, upon recommendation of the Company's Human Capital Management Committee (the “HCM Committee”), the Board of Directors approved retention awards for employees of the Company. Subject to remaining actively employed with the Company through May 31, 2023, the aggregate retention awards include (i) a cash bonus of $ 1.1 million payable on November 30, 2022 and $ 0.3 million to be paid in cash or as a fully vested RSU grant of the same value and (ii) a cash bonus of $ 3.4 million payable on May 31, 2023 and $ 0.9 million to be paid in cash or as a fully vested RSU grant of the same value. These amounts are accrued as services are performed through May 31, 2023. Executive Retention Awards On July 1, 2022, upon recommendation of the Company's HCM Committee, the Board of Directors approved a cash and RSU retention award to certain members of the Company's executive leadership team consisting of the following: • Subject to the certain members of the Company's executive leadership team remaining actively employed with the Company through May 31, 2023, they shall receive an aggregate of: (i) a cash bonus equal to $ 0.9 million payable on November 30, 2022 and (ii) if certain performance criteria are achieved, a number of shares of common stock to be issued to them on May 31, 2023 having a value of $ 1.7 million based on the common stock price at such time, subject to the discretion of the Board or the HCM Committee to pay in cash or a combination of cash and stock. The RSUs are eligible for vesting based on the achievement of certain performance criteria by May 31, 2023 relating to pipeline execution, business development, and financial stewardship. RSUs for which the performance criteria have not been achieved as specified by May 31, 2023 will lapse and be forfeited. The RSUs will be subject to acceleration of vesting in the event of termination of employment without cause by the Company or by the executive for good reason (each as defined in the executive’s employment agreement). These awards will be accrued as services are incurred through May 31, 2023. Awards with performance criteria will be accrued as performance metrics are met. During the nine months ended September 30, 2022, the Company recognized $ 0.5 million of compensation expense associated with these awards (see Note 6) .

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2022
Earnings Per Share [Abstract]
Net Loss per Share	12. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2022 2021 2022 2021 Numerator: Net loss $ ( 11,675 ) $ ( 22,521 ) $ ( 73,186 ) $ ( 60,516 ) Net loss attributable to common stockholders $ ( 11,675 ) $ ( 22,521 ) $ ( 73,186 ) $ ( 60,516 ) Denominator: Weighted average common shares outstanding, basic and diluted 35,882,076 32,132,500 33,834,198 30,417,305 Net loss per share, basic and diluted $ ( 0.33 ) $ ( 0.70 ) $ ( 2.16 ) $ ( 1.99 ) The Company excluded potentially dilutive securities from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended September 30, Nine Months Ended September 30, 2022 2021 2022 2021 Options to purchase common stock 4,015,661 4,747,203 4,015,661 4,747,203 Unvested restricted stock units 1,145,760 445,040 1,145,760 445,040 Series B convertible preferred stock (as converted to common shares) — 938,000 — 938,000 Series C convertible preferred stock (as converted to common shares) — 2,214,000 — 2,214,000 Series D convertible preferred stock (as converted to common shares) — 3,215,000 — 3,215,000 5,161,421 11,559,243 5,161,421 11,559,243

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2022
Subsequent Events [Abstract]
Subsequent Event	13. Subsequent Events Tebipenem HBr License Agreement and Share Purchase Agreement with GSK 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 and Meiji’s regulatory documents to develop, manufacture and commercialize tebipenem pivoxil and tebipenem HBr and products that contain tebipenem pivoxil and tebipenem HBr (the “GSK Licensed Products”) in all territories, except certain Asian countries previously licensed to Meiji (Japan, Bangladesh, Brunei, Cambodia, China, Indonesia, Laos, Malaysia, Myanmar, Philippines, Singapore, South Korea, Taiwan, Thailand and Vietnam(the “Meiji Territory”)) (the “GSK Territory”). If the Company's license with Meiji is terminated, or if Meiji forfeits or loses its rights to develop, manufacture and commercialize tebipenem HBr and products that contain tebipenem HBr in any countries in the Meiji Territory, then GSK will have an exclusive first right to negotiate with Spero to add any such countries to the GSK Territory. Under the terms of the GSK License Agreement, the Company is entitled to receive an upfront payment of $ 66.0 million for GSK to secure rights to the medicine. Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) Delivery of Phase 3 program $ 150.0 Total commercial milestone payments based on first sale (US/EU) $ 150.0 Total potential sales milestone payments $ 225.0 Royalties Low-single digit to low-double digit (if sales exceed $ 1.0 billion) tiered royalties on net product sales. Royalties are subject to reduction in the event of third-party licenses, entry of a generic product or expiration of patent and regulatory exclusivity prior to the tenth (10th) 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 additional further development, including Phase III regulatory filing and commercialization activities for tebipenem HBr in the balance of the GSK Territory outside of the United States. 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 will be 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 (10) years following the date of the first commercial sale of such licensed product in such country (the “Royalty Term”). During the Royalty Term, the Company has agreed not to develop, manufacture or commercialize any oral carbapenem for any indication or any oral antibiotic for cUTI; this restriction does not apply to any third party which acquires control of Spero after the date of the GSK License Agreement if certain conditions are met. The Company has the right to terminate the GSK License Agreement upon a material breach by, or bankruptcy of, GSK. GSK has the right to terminate the GSK License Agreement at any time upon a specified number of days’ notice or upon a material breach by, or bankruptcy of, the Company. In addition, in the event that GSK has the right to terminate the GSK License Agreement due to a breach by the Company, GSK may elect not to terminate the GSK License Agreement and in lieu thereof may assume the responsibility and expense of development of tebipenem HBr in the United States, in which event GSK’s obligation to make further development payments to the Company would cease, and/or to reduce all subsequent commercial and sales milestone payments and royalty payments otherwise due by GSK to the Company under the GSK License Agreement by fifty percent ( 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. Share Purchase Agreement Concurrently with the execution of the GSK License Agreement, on September 21, 2022 (the “GSK Effective Date”), the Company entered into a stock purchase agreement (the “GSK SPA”) with Glaxo Group Limited (“GGL”), an affiliate of GSK, which closed on November 7, 2022 (the “GSK Closing Date”), and pursuant to which GGL purchased on the GSK Closing Date 7,450,000 shares (the “GSK Shares”) of the Company’s common stock at a purchase price of approximately $ 1.20805 per share, for an aggregate purchase price of $ 9.0 million. The GSK SPA contains certain standstill, lock-up and registration rights provisions. The Shares were issued and sold without registration under the Securities Act in reliance on the exemptions provided by Section 4(a)(2) of the Securities Act and/or Regulation D promulgated thereunder and in reliance on similar exemptions under applicable state laws. "At-the-Market" Offering Subsequent to September 30, 2022 and through and including November 10, 2022, the Company sold an additional 1,567,505 shares of its common stock under the "at-the-market" offering program Sales Agreement with Cantor at an average price of approximately $ 2.05 per share for aggregate gross proceeds of approximately $ 3.2 million prior to deducting sales commissions.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2022
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual for clinical trial costs and other research and development expenses and the valuation of share-based awards. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including expenses, manufacturing, clinical trials, research and development costs and employee-related amounts, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain it or treat COVID-19, as well as the economic impact on local, regional, national and international customers and markets. The Company has contemplated the impact of COVID-19 within its financial statements and is not aware of any specific event or circumstance that would require the Company to update estimates, judgments or revise the carrying value of any assets or liabilities. There may be changes to those estimates in future periods. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on identifying, developing and commercializing novel treatments for bacterial infections, including MDR bacterial infections, and rare diseases. All of the Company’s tangible assets are held in the United States.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains most of its cash and cash equivalents at one accredited financial institution. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. As of September 30, 2022 , and December 31, 2021, the Company had no off-balance sheet risk such as foreign exchange contracts, option contracts, or other hedging arrangements.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents include cash held in banks and money market instruments.
Marketable Securities	Marketable Securities Marketable securities consist of investments in corporate obligations with original maturities greater than 90 days. The Company considers its portfolio of investments to be available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Investments with maturities beyond one year are generally classified as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value are included as a component of other income (expense), net based on the specific identification method. Any credit impairments are recorded through an allowance account.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense are recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Computer software and equipment 3 years Office furniture and equipment 7 years Manufacturing equipment 5 years Leasehold improvements Shorter of life of lease or 5 years Costs for capital assets not yet placed into service are capitalized as construction in progress and are depreciated in accordance with the above guidelines once placed into service. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. The Company periodically evaluates whether events and circumstances have occurred that may warrant revision of the estimated useful life of property and equipment.
Leases	Leases At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and short-term and long-term lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. As of September 30, 2022, the Company had no short-term leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew. The Company monitors its plans to renew its material leases on a quarterly basis. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate (“IBR”), which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, and in a similar economic environment. Since the Company does not have any debt and has not been rated by any major credit rating agency, the Company’s IBR was estimated by developing a synthetic credit rating for the Company. The Company has elected to account for lease and non-lease components together as a single lease component.
Other Assets	Other Assets Other assets consist of long-term prepayments and deposits.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities .
Revenue Recognition - Collaboration Revenue	Revenue Recognition – Collaboration 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. Any future out-licensing agreements entered into by the Company and additional third parties shall also be evaluated under Topic 606. Terms of these arrangements include various payment types, typically including one or more of the following: upfront license fees; development, regulatory and commercial milestone payments; payments for manufacturing supply services; and/or royalties on net sales of licensed products. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations under the agreement; (iii) determine the transaction price, including constraint on variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) determine how the revenue will be recognized for each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. The SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangements. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method. In determining the accounting treatment for these arrangements, the Company develops assumptions to determine the stand-alone selling price for each performance obligation in the contract. These assumptions may include forecasted revenues, development timelines, discount rates and probabilities of technical and regulatory success .
Government Tax Incentives	Government Tax Incentives For available government tax incentives that the Company may earn without regard to the existence of taxable income and that require the Company to forego tax deductions or the use of future tax credits and net operating loss carryforwards, the Company classifies the funding recognized as a reduction of the related qualifying research and development expenses incurred. Since the fourth quarter of 2016, the Company’s operating subsidiary in Australia has met the eligibility requirements to receive a tax incentive for qualifying research and development activities. The Company recognizes these incentives as a reduction of research and development expenses in the consolidated statements of operations and comprehensive loss in the same period that the related qualifying expenses are incurred. Reductions of research and development expense recognized upon incurring qualifying expenses in advance of receipt of tax incentive payments are recorded in the consolidated balance sheet as tax incentive receivables .
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including personnel salaries, share-based compensation and benefits, allocated facilities costs, depreciation, manufacturing expenses, costs related to the Company’s government contract and grant arrangements, and external costs of outside vendors engaged to conduct preclinical development activities, clinical trials as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Clinical Trial and other Research Contract Costs and Accruals	Clinical Trial and other Research Contract Costs and Accruals The Company has entered into various research and development contracts with clinical research organizations and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. There may be instances in which payments made to these vendors exceed the level of service provided and will result in a prepayment of the expense. The Company records accruals for estimated ongoing research and clinical trial costs based on the services received and efforts expended pursuant to multiple contracts with these vendors. When evaluating the adequacy of the accrued liabilities, the Company analyzes the progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs .
Restructuring	Restructuring The Company has made estimates and judgments regarding the amount and timing of its restructuring expense and liability, including current and future period termination benefits and other exit costs to be incurred when related actions take place. Restructuring charges are reflected in the Company's consolidated statements of income. Actual results may differ from these estimates.
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 Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. For the three and nine months ended September 30, 2022 and 2021, these changes related to unrealized gains and losses on the Company’s available-for-sale marketable securities.
Net Loss per Share	Net Loss per Share The Company follows the two-class method when computing net income (loss) per share, as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. Net income (loss) per share attributable to common stockholders is calculated based on net income (loss) attributable to Spero Therapeutics, Inc. Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting net income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents.
Income Taxes	Income Taxes In response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act (CARES Act) was signed into law in March 2020. The CARES Act lifts certain deduction limitations originally imposed by the Tax Cuts and Jobs Act of 2017 (“2017 Tax Act”). Corporate taxpayers may carryback net operating losses (“NOLs”) originating during 2018 through 2020 for up to five years , which was not previously allowed under the 2017 Tax Act. The CARES Act also eliminates the 80 % of taxable income limitations by allowing corporate entities to fully utilize NOL carryforwards to offset taxable income in 2018, 2019 or 2020. Taxpayers may generally deduct interest up to the sum of 50 % of adjusted taxable income plus business interest income ( 30 % limit under the 2017 Tax Act) for tax years beginning January 1, 2019 and 2020. The CARES Act allows taxpayers with alternative minimum tax credits to claim a refund in 2020 for the entire amount of the credits instead of recovering the credits through refunds over a period of years, as originally enacted by the 2017 Tax Act. In addition, the CARES Act raises the corporate charitable deduction limit to 25 % of taxable income and makes qualified improvement property generally eligible for 15-year cost-recovery and 100 % bonus depreciation. The enactment of the CARES Act did not result in any material adjustments to our income tax provision for the three and nine months ended September 30, 2022, or to our net deferred tax assets as of September 30, 2022 .
Liability Related to the Sale of Future Royalties	Liability Related to the Sale of Future Royalties Prior to repayment, as discussed further in Note 10, the Company treated the liability related to the sale of future royalties as a debt instrument, amortized under the effective interest rate method over the estimated life of the revenue streams. The Company recognized interest expense thereon using the effective rate, which was based on its current estimates of future revenues over the life of the arrangement. The Company periodically assessed its expected revenues using internal projections, imputed interest on the carrying value of the deferred royalty obligation and recorded interest expense using the imputed effective interest rate. To the extent its estimates of future revenues were greater or less than previous estimates or the estimated timing of such payments was materially different than previous estimates, the Company accounted for any such changes by adjusting the effective interest rate on a prospective basis, with a corresponding impact to the reclassification of the deferred royalty obligation. The assumptions used in determining the expected repayment term of the deferred royalty obligation and amortization period of the issuance costs required that the Company made estimates that could impact the short-term and long-term classification of such costs, as well as the period over which such costs were amortized.
Derivative Liability	Derivative Liability In connection with certain transactions, the Company has identified certain embedded derivatives, which are recorded as liabilities on the Company’s consolidated balance sheet and are remeasured to fair value at each reporting date until the derivative is settled. Changes in the fair value of the derivative liabilities are recognized as other income (expense) in the consolidated statement of operations and comprehensive loss.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements Other accounting standards that have been issued by the Financial Accounting Standards Board or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on our financial statements upon adoption.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2022
Accounting Policies [Abstract]
Estimated Useful Lives of Assets	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense are recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Laboratory equipment 5 years Computer software and equipment 3 years Office furniture and equipment 7 years Manufacturing equipment 5 years Leasehold improvements Shorter of life of lease or 5 years

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2022
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis (in thousands): Fair Value Measurements at September 30, 2022 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 49,569 $ — $ 49,569 Total cash equivalents — 49,569 — 49,569 Total cash equivalents and marketable securities — 49,569 — 49,569 Liabilities: Derivative liability $ — $ — $ 141 $ 141 $ — $ — $ 141 $ 141 Fair Value Measurements at December 31, 2021 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 109,316 $ — $ 109,316 Corporate bonds — 2,701 — 2,701 Total cash equivalents — 112,017 — 112,017 Marketable securities: Corporate bonds — 11,479 — 11,479 Commercial paper — 22,339 — 22,339 Total marketable securities — 33,818 — 33,818 Total cash equivalents and marketable securities $ — $ 145,835 $ — $ 145,835 Liabilities: Derivative liability $ — $ — $ 802 $ 802 $ — $ — $ 802 $ 802
Summary of Gross Unrealized Gains and Losses of Marketable Securities	The following table summarizes the gross unrealized gains and losses of the Company’s marketable securities as of December 31, 2021 (in thousands): December 31, 2021 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Assets: Corporate bonds $ 11,481 $ — $ ( 2 ) $ 11,479 Commercial paper 22,339 — — 22,339 $ 33,820 $ — $ ( 2 ) $ 33,818

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2022
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 September 30, 2022 and December 31, 2021 (in thousands): September 30, 2022 December 31, 2021 Accrued external research and development expenses $ 1,219 $ 6,315 Accrued restructuring expenses 967 — Accrued payroll and related expenses 4,860 5,884 Accrued professional fees 836 909 Accrued other 211 1,242 Total Accrued expenses and other current liabilities $ 8,093 $ 14,350

Share-Based Compensation (Table

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2022
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 nine months ended September 30, 2022: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2021 4,890,716 $ 11.75 7.62 $ 24,263 Granted 956,604 11.12 Exercised ( 56,120 ) 7.48 Forfeited or cancelled ( 1,775,539 ) 13.23 Outstanding as of September 30, 2022 4,015,661 $ 11.00 5.48 $ 2 Outstanding as of September 30, 2022 - vested and 4,015,661 $ 11.00 5.48 $ 2 Exercisable at September 30, 2022 2,606,480 $ 9.65 4.73 $ —
Summary of Activity of Options and RSUs of Performance-Based Options	he following table summarizes RSU activity under all equity plans during the nine months ended September 30, 2022: Number of Weighted Average Grant Date Fair Value Outstanding as of December 31, 2021 513,690 17.08 Granted 1,115,357 7.01 Vested and released ( 75,287 ) 17.07 Forfeited or cancelled ( 408,000 ) 14.16 Outstanding as of September 30, 2022 1,145,760 8.32
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 September 30, Nine Months Ended September 30, 2022 2021 2022 2021 Research and development expenses $ 730 $ 1,059 $ 2,595 $ 2,794 General and administrative expenses 1,376 1,271 4,031 3,708 Total $ 2,106 $ 2,330 $ 6,626 $ 6,502

Liability Related to the Sale_2

Liability Related to the Sale of Future Royalties (Tables)	9 Months Ended
	Sep. 30, 2022
Revenue Recognition and Deferred Revenue [Abstract]
Schedule of Changes in the Liability Related to the Sale of Future Royalties	The following table presents the changes in the liability related to the sale of future royalties under the Revenue Interest Agreement with HCR as of September 30, 2022 (in thousands): Liability related to sale of future royalties, as of December 31, 2021 $ 48,414 Repayment of liability ( 54,485 ) Loss on extinguishment of liability 3,581 Establish change of control derivative liability ( 115 ) Interest expense recognized 2,605 Liability related to sale of future royalties, as of September 30, 2022 $ —

Restructuring (Tables)

Restructuring (Tables)	9 Months Ended
	Sep. 30, 2022
Restructuring and Related Activities [Abstract]
Schedule Restructuring charges	The following tables summarize the restructuring related charges by line item within the Company’s consolidated statements of operations where they were recorded during the three and nine months ended September 30, 2022 (in thousands): Three Months Ended September 30, 2022 Research and development General and administrative Total Severance and other employee costs $ ( 53 ) $ ( 52 ) $ ( 105 ) Lease impairment - - - Other - ( 47 ) ( 47 ) Total restructuring charges $ ( 53 ) $ ( 99 ) $ ( 152 ) Nine Months Ended September 30, 2022 Research and development General and administrative Total Severance and other employee costs $ 3,872 $ 4,680 $ 8,552 Lease impairment 372 263 635 Other 116 2,394 2,510 Total restructuring charges $ 4,360 $ 7,337 $ 11,697 The restructuring charge was included in accrued expenses and other current liabilities in the Company’s condensed consolidated balance sheet. Activity for the quarter is summarized as follows (amounts in thousands): As of September 30, 2022 Balance as of December 31, 2021 $ — Charge to expense 11,697 Payments made ( 7,250 ) Write-offs and impairments ( 3,480 ) Balance as of September 30, 2022 $ 967

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2022
Earnings Per Share [Abstract]
Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2022 2021 2022 2021 Numerator: Net loss $ ( 11,675 ) $ ( 22,521 ) $ ( 73,186 ) $ ( 60,516 ) Net loss attributable to common stockholders $ ( 11,675 ) $ ( 22,521 ) $ ( 73,186 ) $ ( 60,516 ) Denominator: Weighted average common shares outstanding, basic and diluted 35,882,076 32,132,500 33,834,198 30,417,305 Net loss per share, basic and diluted $ ( 0.33 ) $ ( 0.70 ) $ ( 2.16 ) $ ( 1.99 )
Summary of Shares Excluded From the Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended September 30, Nine Months Ended September 30, 2022 2021 2022 2021 Options to purchase common stock 4,015,661 4,747,203 4,015,661 4,747,203 Unvested restricted stock units 1,145,760 445,040 1,145,760 445,040 Series B convertible preferred stock (as converted to common shares) — 938,000 — 938,000 Series C convertible preferred stock (as converted to common shares) — 2,214,000 — 2,214,000 Series D convertible preferred stock (as converted to common shares) — 3,215,000 — 3,215,000 5,161,421 11,559,243 5,161,421 11,559,243

Subsequent Events (Tables)

Subsequent Events (Tables)	9 Months Ended
	Sep. 30, 2022
Subsequent Events [Abstract]
Schedule of remaining potential milestone payments	Under the terms of the GSK License Agreement, the Company is entitled to receive an upfront payment of $ 66.0 million for GSK to secure rights to the medicine. Remaining potential payments are milestone and royalty based, and are as follows (in millions): Event Milestone payments (up to) Delivery of Phase 3 program $ 150.0 Total commercial milestone payments based on first sale (US/EU) $ 150.0 Total potential sales milestone payments $ 225.0 Royalties Low-single digit to low-double digit (if sales exceed $ 1.0 billion) tiered royalties on net product sales.

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Nov. 07, 2022	Dec. 31, 2021
Subsequent Event [Line Items]
Net loss	$ 11,700	$ 22,500	$ 73,200	$ 60,500
Accumulated deficit	440,649		440,649			$ 367,463
Common stock to be issued	$ 43		$ 43			$ 32
Subsequent Events
Subsequent Event [Line Items]
Milestone payment					$ 225,000
GSK License Agreement [Member] | GSK [Member] | Subsequent Events
Subsequent Event [Line Items]
Milestone payment					66,000
GGL [Member] | Subsequent Events
Subsequent Event [Line Items]
Common stock to be issued					$ 9,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Dec. 31, 2021
Summary Of Significant Accounting Policies [Line Items]
Off-balance sheet risk description	As of September 30, 2022, and December 31, 2021, the Company had no off-balance sheet risk such as foreign exchange contracts, option contracts, or other hedging arrangements.
Off-balance sheet risk	$ 0	$ 0
CARES act eliminated percentage on limitation of taxable income	80%
Percentage of adjusted taxable income plus business interest income	50%
Percentage of limitation on adjusted taxable income plus business interest income under 2017 tax act	30%
CARES act Percentage of corporate charitable deduction limitation on taxable income	25%
CARES act eligible cost-recovery period	15 years
CARES act percentage of bonus depreciation	100%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
CARES act net operating loss carryback period	5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Detail)	9 Months Ended
	Sep. 30, 2022
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	5 years
Computer Software and Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	3 years
Office Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	7 years
Manufacturing Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Estimated useful life	Shorter of life of lease or 5 years

Fair Value Measurements and M_3

Fair Value Measurements and Marketable Securities - Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Cash equivalents:
Total cash equivalents	$ 900	$ 600
Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Total cash equivalents	49,569	112,017
Marketable securities:
Total marketable securities		33,818
Total cash equivalents and marketable securities	49,569	145,835
Derivative liability	141	802
Derivative liabilities	141	802
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents	0	0
Marketable securities:
Total marketable securities		0
Total cash equivalents and marketable securities	0	0
Derivative liability	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents	49,569	112,017
Marketable securities:
Total marketable securities		33,818
Total cash equivalents and marketable securities	49,569	145,835
Derivative liability	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	0	0
Marketable securities:
Total marketable securities		0
Total cash equivalents and marketable securities	0	0
Derivative liability	141	802
Derivative liabilities	141	802
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Total cash equivalents	49,569	109,316
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents	0	0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents	49,569	109,316
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	$ 0	0
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Total marketable securities		22,339
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 1 [Member]
Marketable securities:
Total marketable securities		0
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 2 [Member]
Marketable securities:
Total marketable securities		22,339
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 3 [Member]
Marketable securities:
Total marketable securities		0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Cash equivalents:
Total cash equivalents		2,701
Marketable securities:
Total marketable securities		11,479
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents		0
Marketable securities:
Total marketable securities		0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents		2,701
Marketable securities:
Total marketable securities		11,479
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents		0
Marketable securities:
Total marketable securities		$ 0

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended		12 Months Ended
	Oct. 31, 2021	Sep. 30, 2022	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Jun. 07, 2022
Fair Value Disclosures [Abstract]
Total cash		$ 900,000	$ 900,000		$ 600,000
Fair Value, Liabilities of Level 2 to Level 1 Transfers Amount			0
Transfer of financial asset into level 3 of fair value			0
Transfer of financial liabilities into level 3 of fair value		0
Commitments Amount					$ 1,000,000
Percent of aggregate payment received					250%
Fair value of the derivative liability	$ 1,000,000		100,000
Change in fair value of derivative liability			776,000	$ 0	$ 200,000
Derivative Liability		$ 100,000	$ 100,000		$ 800,000	$ 54,500,000

Fair Value Measurements and M_5

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

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 1,219	$ 6,315
Accrued restructuring expenses	967	0
Accrued payroll and related expenses	4,860	5,884
Accrued professional fees	836	909
Accrued other	211	1,242
Total Accrued expenses and other current liabilities	$ 8,093	$ 14,350

Equity Transactions - Underwrit

Equity Transactions - Underwritten Public Offering - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Class Of Stock [Line Items]
Proceeds from equity offering			$ 10,334	$ 5,357
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	3,226,859	70,000	3,964,478	327,185

Equity Transactions - Rights Of

Equity Transactions - Rights Offering - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Feb. 28, 2021
Class Of Stock [Line Items]
Issuance of Common Stock, conversion of Preferred Stock to Common Stock	$ 2		$ 3
Common stock, par value	$ 0.001		$ 0.001		$ 0.001
Aggregate issue costs			$ 0	$ 411
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of Common Stock, conversion of Preferred Stock to Common Stock	$ 4		$ 6
Issuance of stock, shares	3,226,859	70,000	3,964,478	327,185
Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Number of shares issuable upon conversion						73,000

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Feb. 28, 2021	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Aug. 17, 2021	Mar. 11, 2021
Class Of Stock [Line Items]
Common stock, shares authorized				120,000,000		120,000,000		120,000,000
Issuance of stock, value				$ 6,128	$ 1,105	$ 10,331	$ 5,357
Proceeds from sale of common stock to related party						$ 10,334	$ 5,357
Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Preferred stock, conversion basis			73
Number of shares issuable upon conversion			73,000
Series B Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	938		62
Number of shares issuable upon conversion	938,000		62,000
Series C Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares						2,214
Number of shares issuable upon conversion				2,214,000		2,214,000
Series D Preferred Stock [Member]
Class Of Stock [Line Items]
Preferred stock, conversion basis	942,000					2,273,000
Number of shares issuable upon conversion	942,000			2,273,000		2,273,000
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares				3,226,859	70,000	3,964,478	327,185
Issuance of stock, value				$ 4		$ 5	$ 1
Maximum [Member]
Class Of Stock [Line Items]
Common stock, shares authorized									120,000,000
Minimum [Member]
Class Of Stock [Line Items]
Common stock, shares authorized									60,000,000
Pfizer Inc Member
Class Of Stock [Line Items]
Issuance of stock, shares			2,362,348
Issuance of stock, value			$ 27,500
Pfizer Purchase Agreement [Member]
Class Of Stock [Line Items]
Issuance of stock, value			$ 12,500
Pfizer Purchase Agreement [Member] | Pfizer Inc Member
Class Of Stock [Line Items]
Issuance of stock, shares		2,362,348
Share price		$ 16.93
Proceeds from sale of common stock to related party		$ 40,000
At-The-Market Offering Program [Member]
Class Of Stock [Line Items]
Proceeds from sale of common stock to related party						$ 10,700
At-The-Market Offering Program [Member] | Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares						3,964,478
Share price				$ 2.69		$ 2.69
At-The-Market Offering Program [Member] | Sales Agreement [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		9 Months Ended
	May 31, 2023	Jul. 01, 2022 USD ($)	Sep. 30, 2022 $ / shares shares	Aug. 31, 2021 shares	Sep. 30, 2022 USD ($) Plan Installments $ / shares shares	Sep. 30, 2021 $ / shares	Dec. 31, 2021 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of equity compensation plans | Plan					2
Increase in shares of common stock for issuance			42,856,623		42,856,623		32,393,738
Share based compensation options granted					956,604
Weighted average grant-date fair value of awards granted | $ / shares					$ 7.94	$ 13.57
Number of shares authorized for issuance			10,720,127		10,720,127
Common stock reserved for issuance			4,806,802		4,806,802
Common stock, par value | $ / shares			$ 0.001		$ 0.001		$ 0.001
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation options granted					1,115,357
Unrecognized compensation expense | $					$ 8.6
Options vesting period					2 years 11 months 1 day
Number of options vested					75,287
Weighted average grant-date fair value of awards granted | $ / shares					$ 7.01
Right to receive, number of share					1
Common stock, par value | $ / shares			$ 0.001		$ 0.001
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
Compensation expense recognized | $					$ 0.5
Number of common stock award shares granted | $		$ 1.7
Performance-based Vesting Criteria 1 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive units vesting	50%
Performance-based Vesting Criteria 2 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive units vesting	50%
2017 Stock Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in shares of common stock authorized for issuance			2,000,000	3,170,254

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2022	Dec. 31, 2021
Number of Shares
Outstanding as of December 31, 2021	4,890,716
Granted	956,604
Exercised	(56,120)
Forfeited or cancelled	(1,775,539)
Outstanding as of September 30, 2022	4,015,661	4,890,716
Outstanding as of September 30, 2022 - vested and expected to vest	4,015,661
Exercisable at September 30, 2022	2,606,480
Weighted Average Exercise Price
Outstanding as of December 31, 2021	$ 11.75
Granted	11.12
Exercised	7.48
Forfeited or cancelled	13.23
Outstanding as of September 30, 2022	11	$ 11.75
Outstanding as of September 30, 2022 - vested and expected to vest	11
Exercisable at September 30, 2022	$ 9.65
Weighted Average Contractual Term
Outstanding	5 years 5 months 23 days	7 years 7 months 13 days
Outstanding as of September 30, 2022 - vested and expected to vest	5 years 5 months 23 days
Exercisable at September 30, 2022	4 years 8 months 23 days
Outstanding as of December 31, 2021	$ 2	$ 24,263
Outstanding as of September 30, 2022	2	$ 24,263
Outstanding as of September 30, 2022 - vested and expected to vest	2
Exercisable at September 30, 2022	$ 0

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Restricted Stock Units Activity Under All Plans (Details) - $ / shares	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Outstanding as of December 31, 2021	4,890,716
Granted	956,604
Outstanding as of September 30, 2022	4,015,661
Weighted average grant-date fair value of awards granted	$ 7.94	$ 13.57
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Outstanding as of December 31, 2021	513,690
Granted	1,115,357
Vested and released	(75,287)
Forfeited or cancelled	(408,000)
Outstanding as of September 30, 2022	1,145,760
Weighted Average Grant Date Fair Value of Outstanding as of December 31, 2021	$ 17.08
Weighted average grant-date fair value of awards granted	7.01
Weighted average grant-date fair value of awards vested and released	17.07
Weighted average grant-date fair value of awards forfeited or cancelled	14.16
Weighted Average Grant Date Fair Value of Outstanding as of September 30, 2022	$ 8.32

Share-Based Compensation - Su_3

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	$ 2,106	$ 2,330	$ 6,626	$ 6,502
Research and Development Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	730	1,059	2,595	2,794
General and Administrative Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	$ 1,376	$ 1,271	$ 4,031	$ 3,708

Government Contracts - Addition

Government Contracts - Additional Information (Detail) $ in Thousands

1 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

Jul. 01, 2019 USD ($)

May 31, 2021 USD ($) Option

Jul. 31, 2018 USD ($)

Jun. 30, 2016 USD ($) Option

Sep. 30, 2022 USD ($)

Sep. 30, 2021 USD ($)

Sep. 30, 2022 USD ($)

Sep. 30, 2021 USD ($)

Dec. 31, 2021 USD ($) Option

Jan. 19, 2022 USD ($)

Jan. 31, 2020 USD ($)

May 31, 2019 USD ($)

Government Contracts [Line Items]

Grant revenue

$ 2,006

$ 3,064

$ 6,068

$ 15,512

BARDA and DTRA [Member]

Government Contracts [Line Items]

Maximum potential funding from government contract

69,700

69,700

Potential grant amount exercised

12,700

12,700

Grant [Member] | DoD, CDMRP and JWMRP [Member] | Maximum [Member]

Government Contracts [Line Items]

Grant revenue

100

1,000

Grant [Member] | DoD, CDMRP and JWMRP [Member] | Minimum [Member]

Government Contracts [Line Items]

Grant revenue

1,500

3,300

SPR206 [Member] | DoD, CDMRP and JWMRP [Member]

Government Contracts [Line Items]

Award received for clinical development

$ 5,900

Non-dilutive funding period

4 years

BARDA [Member]

Government Contracts [Line Items]

Potential contract amount awarded

$ 44,200

$ 59,700

Potential amount additional funding awarded

12,900

Overall potential award increased

$ 46,900

BARDA [Member] | Base Period Contracts [Member]

Government Contracts [Line Items]

Potential amount initial funding awarded

$ 15,700

$ 18,200

Potential amount additional funding awarded

2,500

Overall potential award increased

$ 46,800

Contract term

3 years

BARDA [Member] | Grant [Member]

Government Contracts [Line Items]

Grant revenue

500

700

1,900

8,900

BARDA [Member] | First Option [Member]

Government Contracts [Line Items]

Potential amount additional funding awarded

$ 15,900

Potential amount increase in committed funding

34,000

BARDA [Member] | Second Option [member]

Government Contracts [Line Items]

Potential grant amount awarded

$ 12,700

DTRA [Member] | Maximum [Member]

Government Contracts [Line Items]

Potential amount additional funding awarded

10,000

10,000

Niaid [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]

Government Contracts [Line Items]

Maximum potential funding from government contract

$ 6,500

Number of option period for funding from government contract | Option

3

Niaid [Member] | Grant [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]

Government Contracts [Line Items]

Grant revenue

0

0

0

400

Niaid [Member] | SPR206 [Member]

Government Contracts [Line Items]

Contract term

5 years

Grant revenue

$ 300

$ 100

1,000

$ 100

Award received for clinical development

$ 23,400

Number of option period for funding from government contract | Option

5

Committed amount from government contract

$ 2,100

Niaid [Member] | First Option [Member] | Spero Cantab [Member] | Cantab Anti Infectives Ltd. [Member] | Cantab Agreements [Member]

Government Contracts [Line Items]

Number of option period for funding from government contract | Option

2

Committed amount from government contract

$ 5,900

License, Collaboration and Se_2

License, Collaboration and Service Agreements - Additional Information (Detail) $ / shares in Units, £ in Millions		1 Months Ended						3 Months Ended					9 Months Ended
	Jan. 15, 2021 USD ($)	Jun. 30, 2021 USD ($)	Nov. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 GBP (£)	May 31, 2016 USD ($)	Sep. 30, 2022 USD ($) $ / shares	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) $ / shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of stock, value								$ 6,128,000	$ 1,105,000				$ 10,331,000	$ 5,357,000
Common stock, par value | $ / shares								$ 0.001					$ 0.001		$ 0.001
Proceeds from sale of common stock to related party													$ 10,334,000	5,357,000
Remaining transaction price allocated to the performance obligations								$ 12,500,000					12,500,000
Revenue recognized								1,082,000	708,000				2,225,000	2,814,000
Deferred revenue								10,442,000					10,442,000		$ 8,786,000
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
License and Know-How Transfer [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Performance Obligation Allocated Transaction Price													1,400,000
Cantab Related Agreements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified commercial milestone						£ 5							5,500,000
License agreement research and development expense related to achievement of regulatory milestones								0	0				0	0
Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones					$ 5,800,000
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				$ 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
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					$ 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
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
Potential milestone payments upon completion and delivery of results of a clinical study										$ 2,000,000
Revenue recognized								0	500,000				$ 800,000	1,300,000
Everest License Agreement [Member] | SPR 206 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 800,000									2,000,000
Potential milestone payments upon completion and delivery of results of a clinical study	$ 1,500,000	$ 800,000
Milestones payment received	$ 1,300,000
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					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
Gates MRI [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expense								0	300,000				0	1,400,000
Savior Service Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Supervision fee amortization service period			34 months
Savior Service Agreement [Member] | Prepaid Asset [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable supervision fee related to commercial manufacturing facility			$ 2,000,000
Savior Service Agreement [Member] | Long-term Asset [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Service agreement additional payment related to facility build out costs													5,300,000
Pfizer License and Share Purchase Agreements [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivable amount upon achievement of certain milestone								900,000					$ 900,000
Milestones payment received								5,000,000
Issuance of stock, shares | shares													2,362,348
Issuance of stock, value													$ 27,500,000
Revenue recognized								1,100,000	$ 200,000				1,500,000	$ 1,600,000
Deferred revenue								14,200,000					14,200,000
Premium associated freestanding equity													12,500,000
Performance Obligation Allocated Transaction Price								$ 5,000,000
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					$ 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								$ 4,100,000

License, Collaboration and Se_3

License, Collaboration and Service Agreements - Schedule of Performance Obligation Along with Standalone Selling Price and Transaction Price Allocated (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2022
Pfizer License and Share Purchase Agreements [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Performance Obligation Allocated Transaction Price	$ 5
License and Know-How Transfer [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Performance Obligation Allocated Transaction Price		$ 1.4
Research And Development Services Related To Upcoming Milestones [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Performance Obligation Allocated Transaction Price		$ 11.1
Research And Development Services Related To Upcoming Milestones [Member] | Pfizer License and Share Purchase Agreements [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Performance Obligation Allocated Transaction Price	$ 4.1

Liability Related to the Sale_3

Liability Related to the Sale of Future Royalties (Additional Information) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Dec. 31, 2022	Jun. 07, 2022	Dec. 31, 2021	Oct. 19, 2021
Deferred Revenue Arrangement [Line Items]
Liability Related to the Sale of Future Royalties	$ 100		$ 54,500	$ 800
Liability for contingent payment	100
HCR [Member]
Deferred Revenue Arrangement [Line Items]
Purchase price	125,000
Initial investment amount					$ 50,000
Loss on the extinguishment	$ 3,581
Related Party Transaction, Rate	6%
Related Party Transaction, Expenses	$ 4,000
Forecast [Member] | HCR [Member]
Deferred Revenue Arrangement [Line Items]
Additional investment amount		$ 50,000
Alternative Investment		$ 25,000
Maximum [Member] | HCR [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Costs	200,000
Minimum [Member] | HCR [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Costs	100,000
Statement of Operations and Comprehensive Loss [Member]
Deferred Revenue Arrangement [Line Items]
Loss on the extinguishment	3,600
Sales Agreement [Member] | Maximum [Member]
Deferred Revenue Arrangement [Line Items]
Change control value in excess amount	200,000
Revenue Interest Financing Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Transaction, Expenses	10,000
Revenue Interest Financing Agreement [Member] | HCR [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Transaction, Expenses	4,000
Revenue Interest Financing Agreement [Member] | Maximum [Member]
Deferred Revenue Arrangement [Line Items]
Change control value in excess amount	$ 100,000
Revenue Interest Financing Agreement [Member] | Minimum [Member] | HCR [Member]
Deferred Revenue Arrangement [Line Items]
Related Party Transaction, Rate	3%
Related Party Costs	$ 100,000

Liability Related to the Sale_4

Liability Related to the Sale of Future Royalties - Schedule of Changes in the Liability Related to the Sale of Future Royalties (Details) - HCR [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2022 USD ($)
Deferred Revenue Arrangement [Line Items]
Liability related to sale of future royalties, beginning balance	$ 48,414
Repayment of liability	(54,485)
Loss on the extinguishment	3,581
Establish change of control derivative liability	(115)
Interest expense recognized	2,605
Liability related to sale of future royalties, ending balance	$ 0

Restructuring - Additional Info

Restructuring - Additional Information (Details) $ in Thousands			3 Months Ended		9 Months Ended
	May 31, 2023 USD ($)	Nov. 30, 2022 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) Employee
Restructuring Cost and Reserve [Line Items]
Restructuring Charges			$ (152)	$ 0	$ 11,697	$ 0
Employee related termination costs					8,600
Lease impairment expense					600
Contract termination fees			2,500		2,500
Outstanding Accrued Expense			1,000		1,000
Common stock to be issued			$ 43		43		$ 32
Performance-Based Awards [Member]
Restructuring Cost and Reserve [Line Items]
Compensation expense recognized					$ 500
Human Capital Management Committee [Member] | Subsequent Event [Member]
Restructuring Cost and Reserve [Line Items]
Aggregate Retention Awards Cash Bonus	$ 3,400	$ 1,100
Cash	900	300
Tebipenem HBr Program [Member]
Restructuring Cost and Reserve [Line Items]
Number Of Employees Before Restructuring Initiative | Employee							146
Number Of Employees After Restructuring Initiative | Employee							41
Executive Retention Awards Member | Subsequent Event [Member]
Restructuring Cost and Reserve [Line Items]
Cash Bonus		$ 900
Common stock to be issued	$ 1,700

Restructuring - Restructuring c

Restructuring - Restructuring charges (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	$ (105)		$ 8,552
Lease impairment	0		635
Other	(47)		2,510
Total restructuring charges	(152)	$ 0	11,697	$ 0
Research and Development Expense [Member]
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	(53)		3,872
Lease impairment	0		372
Other	0		116
Total restructuring charges	(53)		4,360
General and Administrative Expense [Member]
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	(52)		4,680
Lease impairment	0		263
Other	(47)		2,394
Total restructuring charges	$ (99)		$ 7,337

Restructuring -Schedule of Rest

Restructuring -Schedule of Restructuring charges included in Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Restructuring and Related Activities [Abstract]
Balance at beginning of period			$ 0
Charge to expense	$ (152)	$ 0	11,697	$ 0
Payments made			(7,250)
Write-offs and impairments			(3,480)
Balance at end of period	$ 967		$ 967

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		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Net loss	$ (11,675)	$ (22,521)	$ (73,186)	$ (60,516)
Net loss attributable to common stockholders	$ (11,675)	$ (22,521)	$ (73,186)	$ (60,516)
Denominator:
Weighted average common shares outstanding, basic and diluted	35,882,076	32,132,500	33,834,198	30,417,305
Weighted average common shares outstanding, basic and diluted:	35,882,076	32,132,500	33,834,198	30,417,305
Net loss per share, basic and diluted	$ (0.33)	$ (0.70)	$ (2.16)	$ (1.99)
Net loss per share, basic and diluted	$ (0.33)	$ (0.70)	$ (2.16)	$ (1.99)

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		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	5,161,421	11,559,243	5,161,421	11,559,243
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	4,015,661	4,747,203	4,015,661	4,747,203
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1,145,760	445,040	1,145,760	445,040
Series B Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	938,000	0	938,000
Series C Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	2,214,000	0	2,214,000
Series D Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	3,215,000	0	3,215,000

Subsequent Events (Additional I

Subsequent Events (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 10, 2022	Nov. 07, 2022	Sep. 30, 2022	Dec. 31, 2021
Subsequent Event [Line Items]
Common stock to be issued			$ 43	$ 32
Common Stock, Par or Stated Value Per Share			$ 0.001	$ 0.001
Subsequent Event [Member]
Subsequent Event [Line Items]
Milestone payment		$ 225,000
GGL [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Common stock to be issued		$ 9,000
Stock Issued During Period, Shares, New Issues		7,450,000
Shares Issued, Price Per Share		$ 1.20805
GSK License Agreement [Member] | GSK [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Milestone payment		$ 66,000
Reduction In Subsequent Payments by GSK to the Company		50%
At The Market Offering Program [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Stock Issued During Period, Shares, New Issues	1,567,505
Shares Issued, Price Per Share	$ 2.05
Proceeds From Issuance Follow On Public Offering Gross	$ 3,200

Subsequent Events - Schedule of

Subsequent Events - Schedule of remaining potential milestone payments (Details) - Subsequent Events $ in Millions	Nov. 07, 2022 USD ($)
Subsequent Event [Line Items]
Milestone payment	$ 225
Sales milestone events	1,000
Delivery of phase III programme | Maximum [Member]
Subsequent Event [Line Items]
Milestone payment	150
Commercial Milestone Payments | Maximum [Member]
Subsequent Event [Line Items]
Milestone payment	$ 150