Spero Therapeutics (SPRO) 10-Q 5 Aug 21 2021 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2021	Jul. 29, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
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		32,146,847
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	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 71,562	$ 85,209
Marketable securities	27,656	41,697
Other receivables	1,851	5,330
Tax incentive receivable, current	346	846
Prepaid expenses and other current assets	4,901	6,063
Total current assets	106,316	139,145
Property and equipment, net	1,362	1,669
Tax incentive receivable	0	311
Operating lease right of use assets	6,705	7,114
Other assets	5,211	5,212
Total assets	119,594	153,451
Current liabilities:
Accounts payable	1,632	1,155
Accrued expenses and other current liabilities	9,007	12,241
Operating lease liabilities	1,008	947
Deferred Revenue, Current	1,179	0
Total current liabilities	12,826	14,343
Non-current operating lease liabilities	6,430	6,891
Deferred revenue, non-current	9,928	0
Other long-term liabilities	158	177
Total liabilities	29,342	21,411
Commitments and contingencies (Note 8)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized, 3,218,152 shares issued and outstanding as of June 30, 2021 and 3,218,287 shares issued and outstanding as of December 31, 2020	3	3
Common stock, $0.001 par value; 60,000,000 shares authorized as of June 30, 2021 and December 31, 2020; 29,699,147 shares issued and outstanding as of June 30, 2021 and 29,260,247 shares issued and outstanding as of December 31, 2020	30	29
Additional paid-in capital	405,923	409,722
Accumulated deficit	(315,702)	(277,707)
Accumulated other comprehensive gain (loss)	(2)	(7)
Total stockholders' equity	90,252	132,040
Total liabilities and stockholders' equity	$ 119,594	$ 153,451

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (Unaudited) - $ / shares	Jun. 30, 2021	Dec. 31, 2020
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	3,218,152
Preferred stock, shares/units outstanding		3,218,287
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	60,000,000	60,000,000
Common stock, shares/units issued	29,699,147	29,260,247
Common stock, shares/units outstanding	29,699,147	29,260,247

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues:
Grant revenue	$ 3,042	$ 1,676	$ 10,342	$ 3,208
Collaboration revenue	$ 2,106	$ 51	$ 2,106	$ 220
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Total revenues	$ 5,148	$ 1,727	$ 12,448	$ 3,428
Operating expenses:
Research and development	14,461	15,656	32,865	36,092
General and administrative	9,229	4,547	17,528	8,633
Total operating expenses	23,690	20,203	50,393	44,725
Loss from operations	(18,542)	(18,476)	(37,945)	(41,297)
Other income (expense):
Interest income	82	86	180	323
Other income (expense), net	(112)	889	(230)	215
Total other income (expense), net	(30)	975	(50)	538
Net loss	$ (18,572)	$ (17,501)	$ (37,995)	$ (40,759)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.63)	$ (0.85)	$ (1.29)	$ (2.06)
Weighted average common shares outstanding, basic and diluted:	29,675,399	20,633,402	29,545,496	20,095,415
Comprehensive loss:
Net loss	$ (18,572)	$ (17,501)	$ (37,995)	$ (40,759)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	1	34	5	(1)
Net unrealized gains (losses) on securities	1	34	5	(1)
Total comprehensive loss	$ (18,571)	$ (17,467)	$ (37,990)	$ (40,760)

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Cash flows from operating activities:
Net loss	$ (18,572)	$ (17,501)	$ (37,995)	$ (40,759)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization			307	379
Non-cash lease cost			402	224
Share-based compensation	2,163	1,214	4,172	2,324
Unrealized foreign currency transaction (gain) loss			10	(251)
Accretion of discount on marketable securities			124	(59)
Changes in operating assets and liabilities:
Other receivables			3,479	3,727
Prepaid expenses and other current assets			1,258	(4,334)
Tax incentive receivables			800	(252)
Other assets			(8)	(1,761)
Accounts payable			478	(2,567)
Accrued expenses and other current liabilities			(3,314)	(4,857)
Deferred revenue			(1,356)	0
Other long-term liabilities			(19)	(53)
Operating lease liability			(400)	14
Net cash used in operating activities			(32,046)	(48,225)
Cash flows from investing activities:
Purchases of marketable securities			(12,578)	(3,995)
Proceeds from maturities of marketable securities			26,500	50,350
Purchases of property and equipment			0	(94)
Net cash provided by (used in) investing activities			13,922	46,261
Cash flows from financing activities:
Proceeds From Issuance Of Common Stock			4,253	7,271
Proceeds from equity offering			4,253	7,271
Payment of offering and financing costs			(146)	(502)
Proceeds from stock option exercises			370	808
Net cash provided by financing activities			4,477	37,576
Net increase (decrease) in cash and cash equivalents			(13,647)	35,612
Cash, cash equivalents and restricted cash at beginning of period			85,209	29,730	$ 29,730
Cash, cash equivalents and restricted cash at end of period	$ 71,562	$ 65,342	71,562	65,342	85,209
Supplemental disclosure of non-cash activities:
Offering and financing costs in accounts payable and accruals			80		$ 0
Rights Offering [Member]
Cash flows from financing activities:
Proceeds From Issuance Of Common Stock			0	9,416
Proceeds from equity offering			0	9,416
Series C Preferred Stock [Member]
Cash flows from financing activities:
Payment of offering and financing costs			(130)	(41)
Series C Preferred Stock [Member] | Rights Offering [Member]
Cash flows from financing activities:
Proceeds from issuance of Series C Preferred Stock related to Rights Offering			$ 0	$ 20,583

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Series C Preferred Stock [Member]	Series A, B, C and D Convertible Preferred Stock [Member]	Series A, B and C Convertible Preferred Stock [Member]	Series A, B and C Convertible Preferred Stock [Member]Series C Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Series C Preferred Stock [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2019	$ 74,574					$ 19	$ 273,966		$ (199,427)	$ 16
Balances, Shares at Dec. 31, 2019				2,720		19,190,695
Issuance of common stock upon the exercise of stock options	808						808
Issuance of common stock upon the exercise of stock options, shares						131,933
Issuance of common stock, net of financing/offering costs and net of issuance costs	16,226	$ 20,542				$ 2	16,224	$ 20,542
Issuance of common stock and preferred stock, net of financing/offering costs, shares					2,287	1,691,467
Accretion of Series C preferred stock					$ 549			(549)
Beneficial conversion feature of Series C preferred stock					$ (549)			$ 549
Share-based compensation expense	2,324						2,324
Unrealized gain (loss) on available-for-sale securities	(1)									(1)
Net loss	(40,759)								(40,759)
Balances at Jun. 30, 2020	73,714					$ 21	313,864		(240,186)	15
Balances, Shares at Jun. 30, 2020				5,007		21,014,095
Balances at Mar. 31, 2020	82,427					$ 20	305,111		(222,685)	(19)
Balances, Shares at Mar. 31, 2020				5,007		20,328,340
Issuance of common stock upon the exercise of stock options	269						269
Issuance of common stock upon the exercise of stock options, shares						40,537
Issuance of common stock, net of financing/offering costs and net of issuance costs	7,271					$ 1	7,270
Issuance of common stock and preferred stock, net of financing/offering costs, shares						645,218
Share-based compensation expense	1,214						1,214
Unrealized gain (loss) on available-for-sale securities	34									34
Net loss	(17,501)								(17,501)
Balances at Jun. 30, 2020	73,714					$ 21	313,864		(240,186)	15
Balances, Shares at Jun. 30, 2020				5,007		21,014,095
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	$ 370						370
Issuance of common stock upon the exercise of stock options, shares	46,715					46,715
Issuance of common stock, net of financing/offering costs and net of issuance costs	$ 4,123					$ 1	4,122
Issuance of common stock and preferred stock, net of financing/offering costs, shares						257,185
Conversion of convertible preferred stock to common stock, shares			(135)			135,000
Premium for commitment to sell common stock							12,463
Share-based compensation expense	4,172						4,172
Unrealized gain (loss) on available-for-sale securities	5									5
Net loss	(37,995)								(37,995)
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
Balances at Mar. 31, 2021	118,846		$ 3			$ 30	415,946		(297,130)	(3)
Balances, Shares at Mar. 31, 2021			3,218,152			29,663,946
Issuance of common stock upon the exercise of stock options	277						277
Issuance of common stock upon the exercise of stock options, shares						35,201
Premium for commitment to sell common stock	12,463						12,463
Share-based compensation expense	2,163						2,163
Unrealized gain (loss) on available-for-sale securities	1									1
Net loss	(18,572)								(18,572)
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

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	Mar. 05, 2020	Jun. 30, 2021	Jun. 30, 2020
Payment of financing/offering costs		$ 146	$ 502
Series C Preferred Stock [Member]
Payment of financing/offering costs	$ 100	$ 130	41
Common Stock [Member]
Payment of financing/offering costs			$ 462

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	6 Months Ended
	Jun. 30, 2021
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 treatments in high unmet need areas involving multi-drug resistant, or MDR, bacterial infections and rare diseases. The Company’s most advanced product candidate, tebipenem pivoxil hydrobromide or tebipenem HBr, is designed to be the first oral carbapenem-class antibiotic for use in adults to treat MDR Gram-negative infections. Treatment with effective orally administrable antibiotics may prevent hospitalizations for serious infections and enable earlier, more convenient and cost-effective treatment of patients after hospitalization. The Company is also developing SPR720, a novel oral antibiotic designed for the treatment of a rare, orphan disease caused by pulmonary non-tuberculous mycobacterial infections, or NTM disease. In addition, the Company has a Potentiator technology, that includes an IV-administered product candidate, SPR206, being developed to treat MDR Gram-negative infections in the hospital. 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 a concluded collaboration agreement, funding from government contracts, licensing agreements and through the sale of the Company’s common and preferred stock. The Company has incurred recurring losses since inception, including net losses of $ 18.6 million and $ 17.5 million for the three months ended June 30, 2021 and 2020, respectively, and $ 38.0 million and $ 40.8 million for the six months ended June 30, 2021 and 2020, respectively. In addition, as of June 30, 2021, the Company had an accumulated deficit of $ 315.7 million. The Company expects to continue to generate operating losses for the foreseeable future. In accordance with Accounting Standards Update (“ASU”) 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that these consolidated financial statements are issued. Based on the Company’s current operating plan and existing cash, cash equivalents and marketable securities, the Company has determined that there is substantial doubt regarding its ability to continue as a going concern. 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, 2020 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying unaudited condensed consolidated financial statements as of June 30, 2021, and for the three and six months ended June 30, 2021, 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, 2020, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, 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 June 30, 2021, and results of operations for the three and six months ended June 30, 2021, and cash flows for the six months ended June 30, 2021 and 2020 have been made. The results of operations for the three and six months ended June 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021 .

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2021
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 MDR bacterial infections. 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 June 30, 2021 , and December 31, 2020, 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 Effective January 1, 2019 , the Company adopted ASC Topic 842, Leases (“ASC 842”), using the modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the previous guidance in ASC 840, Leases (“ASC 840”). 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 June 30, 2021, the Company had no short-term leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew. The Company monitors its plans to renew its material leases on a quarterly basis. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate (“IBR”), which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, and in a similar economic environment. Since the Company does not have any debt and has not been rated by any major credit rating agency, the Company’s IBR was estimated by developing a synthetic credit rating for the Company. The Company has elected to account for lease and non-lease components together as a single lease component. Other Assets Other assets consist of long-term prepayments and deposits. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:  Level 1—Quoted prices in active markets for identical assets or liabilities.  Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.  Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. Revenue Recognition – Collaboration Revenue Effective June 30, 2021, the Company entered into a licensing agreement that is 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 arrangement in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangement. 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. In June 2019, the Company entered into a collaboration agreement with the Bill and Melinda Gates Medical Research Institute (the “Gates MRI”) and concluded that the agreement is within the scope of the accounting guidance for collaboration arrangements (see Note 10). Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company recognizes the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred. 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 (see Note 11). 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. 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 measures all share-based awards 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 six months ended June 30, 2021 and 2020, 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. and excludes net income (loss) attributable to non-controlling interests. 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 six months ended June 30, 2021, or to our net deferred tax assets as of June 30, 2021 . Recently Issued and Adopted Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No 2016-13, Financial Instruments – Credit Losses (Topic 326). The Accounting Standards Codification 326, Financial Instruments- Credit Losses (“ASC 326”) requires a financial asset measured at amortized cost basis to be presented at the net amount expected to be collected. Under ASU 2016-13, the Company is required to use a current expected credit loss (“CECL”) model that immediately recognizes an estimate of credit losses that are expected to occur over the life of the financial instruments that are in the scope of the update, including trade receivables. The updated guidance also amends the previous other-than-temporary impairment model for available-for-sale debt securities by requiring the recognition of impairments related to credit losses through an allowance account and limits the amount of credit loss to the difference between a security’s amortized cost basis and its fair value. In addition, the length of time a security has been in an unrealized loss position no longer impacts the determination of whether a credit loss exists. The Company adopted the guidance on January 1, 2020 with no impact. For available-for-sale securities, the updated guidance was applied prospectively. In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (a consensus of the FASB Emerging Issues Task Force) (“ASU 2018-15”). The amendments in ASU 2018-15 align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The Company adopted this standard as of January 1, 2020 , on a prospective basis. The adoption did no t have a material impact on the Company’s consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. This standard modifies certain disclosure requirements on fair value measurements. This standard became effective for the Company on January 1, 2020 , and did no t have a material impact on its disclosures. For the new disclosures regarding our Level 3 instruments, please read Note 3, Fair Value Measurements, to these condensed consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 . This standard makes targeted improvements for collaborative arrangements as follows:  Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers, when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements;  Adds unit-of-account guidance to ASC 808, Collaborative Arrangements, to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and  Precludes a company from presenting transactions with collaborative arrangement participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. This standard became effective for the Company on January 1, 2020 , and did no t have a material impact on its condensed consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes , amended guidance on the accounting and reporting of income taxes. The guidance is intended to simplify the accounting for income taxes by removing exceptions related to certain intraperiod tax allocations and deferred tax liabilities; clarifying guidance primarily related to evaluating the step-up tax basis for goodwill in a business combination; and reflecting enacted changes in tax laws or rates in the annual effective tax rate. The amended guidance is effective for interim and annual periods in 2021. Early adoption is permitted. The application of the amendments in the new guidance are to be applied on a retrospective basis, on a modified retrospective basis through a cumulative-effect adjustment to retained earnings or prospectively, depending on the amendment. This standard became effective for the Company on January 1, 2021 , and did no t have a material impact on its condensed consolidated financial statements and related disclosures.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	6 Months Ended
	Jun. 30, 2021
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 June 30, 2021 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 70,896 $ — $ 70,896 Total cash equivalents — 70,896 — 70,896 Marketable securities: Corporate bonds — 12,663 — 12,663 Commercial paper — 14,993 — 14,993 Total marketable securities — 27,656 — 27,656 Total cash equivalents and marketable securities $ — $ 98,552 $ — $ 98,552 Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 73,488 $ — $ 73,488 Commercial paper — 5,998 — 5,998 Corporate bonds — 3,006 — 3,006 Total cash equivalents — 82,492 — 82,492 Marketable securities: Corporate bonds — 13,221 — 13,221 Commercial paper — 28,476 — 28,476 Total marketable securities — 41,697 — 41,697 Total cash equivalents and marketable securities $ — $ 124,189 $ — $ 124,189 Excluded from the tables above is cash of $ 0.7 million and $ 2.7 million as of June 30, 2021, and December 31, 2020, respectively. During the six months ended June 30, 2021 , there were no transfers between Level 1, Level 2 and Level 3 categories. Marketable Securities The Company’s marketable securities are classified as Level 2 assets under the fair value hierarchy as these assets were primarily determined from independent pricing sources, which generally derive security prices from recently reported trades for identical or similar securities. The Company evaluated debt securities with unrealized losses for any expected credit losses and determined unrealized losses on these securities were related to non-credit factors. Additionally, the Company currently does no t intend to and is not required to sell these investments prior to an anticipated recovery in value. The following table summarizes the gross unrealized gains and losses of the Company’s marketable securities as of June 30, 2021, and December 31, 2020 (in thousands): June 30, 2021 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Assets: Corporate bonds $ 12,665 $ — $ ( 2 ) $ 12,663 Commercial paper 14,993 — — 14,993 $ 27,658 $ — $ ( 2 ) $ 27,656 December 31, 2020 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Assets: Corporate bonds $ 13,227 $ — $ ( 6 ) $ 13,221 Commercial paper 28,476 — — 28,476 $ 41,703 $ — $ ( 6 ) $ 41,697 As of June 30, 2021, and December 31, 2020, all of the Company’s marketable securities had remaining contractual maturity dates of one year or less from the respective consolidated balance sheet date.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	6 Months Ended
	Jun. 30, 2021
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 June 30, 2021 and December 31, 2020 (in thousands): June 30, 2021 December 31, 2020 Accrued external research and development expenses $ 4,752 7,035 Accrued payroll and related expenses 2,495 3,918 Accrued professional fees 1,560 1,066 Accrued other 200 222 Total Accrued expenses and other current liabilities $ 9,007 $ 12,241

Equity Transactions

Equity Transactions	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Equity Transactions	5. Equity Transactions Underwritten Public Offering On September 15, 2020, the Company completed an underwritten public offering of an aggregate of 4,785,000 shares of its common stock, and an aggregate of 3,215,000 shares of newly designated Series D Convertible Preferred Stock (“Series D Preferred Stock”). The price to the public in the offering was $ 10.00 per share with respect to the common stock and the Series D Preferred Stock. In addition, under the terms of the Underwriting Agreement, the Company granted the underwriters an option, exercisable for 30 days, to purchase up to 1,200,000 additional shares of common stock. The shares of Series D Preferred Stock are convertible on a one-to-one basis into shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series D Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99 % of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series D Preferred Stock will receive a payment equal to $ 0.001 per share of Series D Preferred Stock before any proceeds are distributed to the holders of common stock and equal to any distributions to the holders of Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock. Shares of Series D Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series D Preferred Stock will be required to amend the terms of the Series D Preferred Stock. As such, the Company has classified the Series D Preferred Stock within permanent equity in its consolidated balance sheet. The offering closed on September 15, 2020 with an aggregate public offering price of $ 80.0 million. Aggregate net proceeds from the offering were $ 74.7 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. Additionally, pursuant to the Underwriting Agreement, on October 1, 2020, the Company issued and sold 1,200,000 shares of common stock at the price of $ 10.00 per share pursuant to the underwriters’ exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of approximately $ 11.2 million after deducting underwriting discounts and commissions. Rights Offering On February 11, 2020, the Company announced a rights offering pursuant to which it distributed to holders of its common stock and Series A Convertible Preferred Stock (“Series A Preferred Stock”) and Series B Convertible Preferred Stock (“Series B Preferred Stock”), at no charge, non-transferable subscription rights to purchase shares of Spero common stock and Series C Convertible Preferred Stock (“Series C Preferred Stock”), with an aggregate offering value of $ 30.0 million. For each share of common stock (including shares of common stock issuable upon conversion of the Company’s outstanding shares of Series A Preferred Stock and Series B Preferred Stock) owned by holders of record as of 5:00 p.m., New York time, on February 10, 2020, the holders of such shares received 0.152 rights to purchase shares of Spero common stock (subject to the aggregate offering threshold and certain ownership limitations). Each whole right allowed holders to subscribe for one share of common stock at the subscription price equal to $ 9.00 per whole share (or an equivalent number of shares of Series C Preferred Stock). The total number of subscription rights issued to each stockholder was rounded down to the nearest whole number. The Rights Offering was fully backstopped by certain affiliates of BVF Partners L.P. (“BVF”), which agreed to purchase, at a minimum, their respective as-converted pro rata share of the offered shares under the Rights Offering, plus an additional amount of Common Stock or Series C Preferred Stock that are not subscribed by other purchasers in the Rights Offering, for a total of up to $ 30.0 million. At the closing of the rights offering on March 5, 2020, a total of 1,046,249 shares of the Company’s common stock and 2,287 shares of Series C Preferred Stock were issued for aggregate gross proceeds of $ 30.0 million. The aggregate issue costs related to the offering were $ 0.5 million. $ 20.6 million of the aggregate gross proceeds relates to the issuance of Series C and the associated issuance costs are $ 0.1 million. Each share of Series C Preferred Stock is convertible into 1,000 shares of Spero common stock at the election of the holder, subject to beneficial ownership conversion limits applicable to the Series C Preferred Stock. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series C Preferred Stock will receive a payment equal to $ 0.001 per share of Series C Preferred Stock before any proceeds are distributed to the holders of Common Stock and equal to any distributions to the holders of the Series A Preferred Stock and Series B Preferred Stock. The Series C Preferred Stock have no voting rights, except as required by law. The Series C Preferred Stock does not have any mandatory redemption rights or other redemption rights that would be outside of the Company’s control. As such, the Company has classified the Series C Preferred Stock within permanent equity in its consolidated balance sheet. Upon issuance, each share of Series C Preferred Stock included an embedded beneficial conversion feature. The beneficial conversion feature arose because the market price of the Company’s common stock on the date of issuance of the Series C Preferred Stock was $ 9.22 per share as compared to an effective conversion price of the Series C Preferred Stock of $ 8.98 per share. As a result, the Company recorded the intrinsic value of the beneficial conversion feature of $ 0.5 million as a discount on the Series C Preferred Stock at issuance. Because the Series C Preferred Stock is immediately convertible upon issuance and does not include mandatory redemption provisions, the discount on the Series C Preferred Stock was immediately accreted.

Common Stock

Common Stock	6 Months Ended
	Jun. 30, 2021
Equity [Abstract]
Common Stock	6. Common Stock On December 3, 2018, the Company filed a universal shelf registration statement on Form S-3 (Registration No. 333-228661) with the SEC, which was declared effective on December 11, 2018, and pursuant to which the Company registered for sale up to $ 200.0 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 $ 50.0 million of its common stock available for issuance pursuant to an “at-the-market” offering program sales agreement that it entered into with Cantor Fitzgerald & Co. (“Cantor”). Under the sales agreement, Cantor was permitted to 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 prospectus underlying the “at-the-market” offering program was terminated on September 9, 2020 in connection with the Company’s underwritten public offering that was completed in September 2020. At such time, the Company had raised approximately $ 15.4 million in sales of its common stock under the “at-the-market” offering program, prior to deducting sales commissions, and had remaining available capacity of approximately $ 34.6 million. On November 13, 2020, the Company reinstated the “at-the-market” offering program with a capacity of up to $ 34.0 million by filing an updated prospectus. On March 11, 2021, the Company entered into a new sales agreement with 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. Under the new sales agreement, Cantor may sell shares of the Company’s common stock by any method permitted by law deemed to be an “at the market” offering as defined in Rule 415 of the Securities Act, subject to the terms of the new 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. 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. 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. In addition, under the terms of the Pfizer Purchase Agreement, the shares are subject to a lock-up restriction, such that Pfizer will not, subject to certain limited exceptions, without the prior approval of the Company, sell or otherwise dispose of the shares until one year after the date of the closing of the sale of the shares under the Pfizer Purchase Agreement. 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 10 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. During the six months ended June 30, 2021 the Company sold 257,185 shares of its common stock under its “at-the-market” offering sales agreement at an average price of approximately $ 17.05 per share for aggregate gross proceeds of approximately $ 4.4 million prior to deducting sales commissions.

Share-Based Compensation

Share-Based Compensation	6 Months Ended
	Jun. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-Based Compensation	7. Share-Based Compensation The Company maintains two equity compensation plans: the 2017 Stock Incentive Plan (the “2017 Plan”) and the 2019 Inducement Equity Incentive Plan (the “2019 Inducement Plan”). In June 2020, the board of directors approved an increase of 700,000 shares of common stock for issuance under the 2019 Inducement Plan. Performance-based awards During 2019, the Company granted 100,000 options and 50,000 restricted stock units (“RSUs”) containing the same performance-based vesting criteria. These options and RSUs (the “Performance Awards”) are subject to performance-based vesting eligibility and a subsequent partial time-based vesting schedule. Specifically, the Performance Awards are eligible for vesting based on the achievement of performance criteria, each representing a 25 % vesting opportunity if achieved within a specified time during the performance period (the “Performance Period”), and relating to (i) the release of tebipenem HBr top-line data; (ii) FDA acceptance of a tebipenem HBr New Drug Application; (iii) non-dilutive financing; and (iv) equity financing. Following the Performance Period, Performance Awards determined to be eligible for vesting as a result of achievement of the performance criteria will vest as follows: (a) 50 % of the eligible award will vest immediately, and (b) the remaining eligible award will vest (i) in the case of options, in equal monthly installments ending two years after the Performance Period expiration, and (ii) in the case of RSUs, on such two year anniversary. In January 2021, the Company cancelled the performance-based awards due to the non-achievement of the performance-based vesting criteria, and the awards were added back to the shares of common stock available for issuance under the 2017 Plan. None of the outstanding options had vested and no compensation expense associated with performance-based awards was recognized as of June 30, 2021. The following table summarizes the activity of options and RSUs under the 2017 Plan containing performance-based vesting criteria during the six months ended June 30, 2021: Number of Number of Outstanding as of December 31, 2020 63,107 30,561 Granted - - Exercised - - Forfeited or cancelled ( 63,107 ) ( 30,561 ) Outstanding as of June 30, 2021 - - 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 $ 14.05 and $ 6.70 per option for those options granted during the six months ended June 30, 2021 and 2020 respectively. The following table summarizes stock option activity under all equity plans (excluding RSUs) during the six months ended June 30, 2021: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2020 3,682,233 $ 9.10 7.84 $ 37,881 Granted 1,169,615 18.28 — — Exercised ( 46,715 ) 8.33 — — Forfeited or cancelled ( 225,425 ) 13.83 — — Outstanding as of June 30, 2021 4,579,708 $ 11.22 7.78 $ 17,689 Outstanding as of June 30, 2021 - vested and 4,579,708 $ 11.22 7.78 $ 17,689 Exercisable at June 30, 2021 2,260,040 $ 8.19 6.67 $ 13,066 As of June 30, 2021, a total of 5,549,873 shares have been authorized and reserved for issuance under all equity plans and 463,627 shares were available for future issuance under such plans. 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 June 30, Six Months Ended June 30, 2021 2020 2021 2020 Research and development expenses $ 895 $ 559 $ 1,735 $ 1,055 General and administrative expenses 1,268 655 2,437 1,269 Total $ 2,163 $ 1,214 $ 4,172 $ 2,324

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. 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 10). 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 June 30, 2021 or December 31, 2020. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses, as incurred, the costs related to any such legal proceedings.

Government Contracts

Government Contracts	6 Months Ended
	Jun. 30, 2021
Contractors [Abstract]
Government Contracts	9. Government Contracts BARDA In July 2018, the Company was awarded a contract from Biomedical Advanced Research and Development Authority (“BARDA”) of up to $ 44.2 million to develop tebipenem HBr for the treatment of complicated urinary tract infections (“cUTI”) caused by antibiotic resistant Gram-negative bacteria and for assessment against biodefense pathogens. The award committed initial funding of $ 15.7 million over a three-year base period from July 1, 2018 to June 30, 2021 for cUTI development activities. In May 2019, the contract was modified to include additional funding of approximately $ 2.5 million for the development of tebipenem HBr, increasing the amount of 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 $ 34.1 million and extended the period of performance through November 1, 2021. The balance of the award is subject to BARDA exercising a second option, which would entail funding of $ 12.7 million and is exercisable by BARDA subject to, among other things, satisfactory progress and results from the biodefense studies described below. As part of an inter-agency collaboration between BARDA and the Defense Threat Reduction Agency (“DTRA”), a series of studies to assess the efficacy of tebipenem HBr in the treatment of infections caused by biodefense threats such as anthrax, plague and melioidosis will be conducted under the direction of Spero. The FDA requires data from a human pneumonic disease as supportive evidence of human efficacy when developing an antibiotic to treat a pulmonary biothreat infection under 21 CFR 314.600, “The Animal Rule,” the scope of which the BARDA award includes the assessment of tebipenem HBr levels in the lung of healthy volunteers as well as a proof of concept clinical trial in pneumonia patients. 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 $ 56.8 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 $ 1.9 million and $ 1.6 million of revenue under the BARDA award during the three months ended June 30, 2021 and 2020, respectively, and recognized $ 8.2 million and $ 2.8 million of revenue under the BARDA award during the six months ended June 30, 2021 and 2020. U.S. Department of Defense On July 1, 2019, the Company received a $ 5.9 million award from the DoD Congressionally Directed Medical Research Programs (“CDMRP”) Joint Warfighter Medical Research Program. The funding 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 non-human primate toxicology study, and microbiological surveillance studies that would be required for a potential New Drug Application, or NDA, submission with the U.S. Food and Drug Administration for SPR206. The Company recognized $ 1.1 million and $ 1.8 million in revenue under this agreement during the three and six months ended June 30, 2021, respectively, and recognized immaterial revenue under this agreement during the three and six months ended June 30, 2020. NIAID In May 2021, the Company was awarded a five-year contract from the U.S. National Institute of Allergy and Infectious Diseases, or 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 June 30, 2021, funding for the base period totaling $ 2.1 million has been committed. The Company recognized immaterial revenue under this agreement during both the three and six months ended June 30, 2021. In February 2017, the Company was awarded a grant from NIAID, under its Small Business Innovation Research program, over a two-year period from March 1, 2017 to February 28, 2019, to conduct additional preclinical studies of SPR720, the Company’s novel oral bacterial gyrase inhibitor, for the treatment of non-tuberculous mycobacterial infections. The award was structured as a 12-month $ 0.6 million base period and a $ 0.4 million option period. Through December 31, 2017, only the base period funds were committed. In February 2018, NIAID exercised the $ 0.4 million 12-month option period. In January 2019, the period of performance for this award was extended through February 28, 2020 and during the year ended December 31, 2020, this award was closed out. The Company did no t recognize revenue under this agreement during the both the three months ended June 30, 2021 and 2020 , respectively. The Company did no t recognize revenue under this agreement during the six months ended June 30, 2021 and recognized approximately $ 0.1 million of revenue under this agreement during the six months ended June 30, 2020. 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 June 30, 2021, funding for the base period and the first two option periods totaling $ 5.9 million have been committed. In March 2021, a contract modification was executed, and the period of performance for this award was extended until June 2021 . The Company recognized less than $ 0.1 million of revenue under this agreement during both the three months ended June 30, 2021 and 2020, respectively, and recognized $ 0.4 million and $ 0.3 million during the six months ended June 30, 2021 and 2020 , respectively.

License, Collaboration and Serv

License, Collaboration and Service Agreements	6 Months Ended
	Jun. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License, Collaboration and Service Agreements	10. 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 milestone payments of up to $ 5.8 million upon the achievement of specified clinical and regulatory milestones and a payment of £ 5.0 million ( $ 6.9 million and $ 6.8 million as of June 30, 2021 and December 31, 2020, respectively) 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 six months ended June 30, 2021 and 2020, 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 six months ended June 30, 2021 , the Company did no t record any research and development expense related to the achievement of regulatory milestones for SPR720. 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 certain know-how and a license to research, develop, manufacture and sell products for a proprietary compound in the licensed territory. In exchange for the know-how and 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. As part of the agreement, the Company is obligated to make future milestone payments of up to $ 2.0 million as of June 30, 2021 upon the achievement of specified clinical and 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 $ 7.5 million. 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 condensed consolidated balance sheet as of June 30, 2021. 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 factors, 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 and the 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 Potentiator, 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 be making more significant investments in the development of SPR206 beyond what was contemplated at the time of the Original Everest License Agreement. The Original Everest License Agreement provided that the Company could receive up to $ 59.5 million upon achievement of certain milestones. The Amended Everest License Agreement provides that the Company may receive up to $38.0 million upon achievement of certain milestones, of which $2.8 million has been received to date. The Company may receive milestones of up to $ 1.5 million if the Company chooses to complete a future clinical study, of which the Company received approximately $ 0.8 million upon the initiation of the Bronchoalveolar Lavage (BAL) clinical trial of SPR206 in June 2021. 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. Gates MRI Collaboration Agreement In June 2019, the Company entered into a collaboration with Gates MRI to develop SPR720 for the treatment of lung infections caused by Mycobacterium tuberculosis. In furtherance of the Gates MRI’s charitable purposes, the Company also granted to Gates MRI a no-cost, exclusive license to develop, manufacture and commercialize SPR720 for the treatment of tuberculosis (“TB”) in low- and middle- income countries. The Gates MRI is responsible for formulating and funding its own research plan for the development of SPR720 for TB. As such, Gates MRI will conduct and fund preclinical and clinical studies for the development of SPR720 against TB. In addition, Gates MRI and the Company will jointly design and manage certain collaborative research activities, which the Company will perform and which will be funded by the Gates MRI. Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company will recognize the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred. The Company recorded $ 0.6 million and $ 1.1 million during the three and six months ended June 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 is classified as a prepaid asset on the Company’s balance sheet and is being amortized over a service period of approximately 34 months . The Company has paid Savior an additional $ 5.1 million for facility build out costs, which is classified as a long-term asset on the Company’s balance sheet as of June 30, 2021. 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 will be 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, as presented below. 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. The transaction price to be allocated to the identified performance obligations was determined to be the $ 12.5 million premium on the Company's commitment to sell common stock to Pfizer under the Pfizer Purchase Agreement at a price per share in excess of fair value. The allocation was performed based on the relative standalone selling prices of the performance obligations. The following table shows the performance obligations and the transaction price allocated to those obligations (in millions) : Transaction Price Performance Obligations Allocated Recognition Method License and know-how transfer (1) $ 1.4 Fully satisfied; recognized upon delivery of the license Research and development services related to upcoming milestones (2) 11.1 Recognized over time as services are delivered $ 12.5 1. The standalone selling price for the license and know-how was determined by the income approach utilizing a discounted cash flow. The key assumptions in our estimate of the standalone selling price for the license and know-how include the probability of technological and regulatory success, our estimate of future product revenues, and the discount rate, among others. 2. The standalone selling price for the research and development services was estimated based on our estimate of costs to be incurred to fulfill our obligations associated with the performance of the research and development services, plus a reasonable margin. 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, we determined they were fully constrained as of June 30, 2021 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. As 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, the Company recognized $ 1.4 million of revenue from the contract during the three and six months ended June 30, 2021. The remaining transaction price balance of $ 11.1 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 June 30, 2021, the research and development services related to the second performance obligation were expected to be recognized over a period of approximately five years. As of June 30, 2021, the Company recorded $ 11.1 million in deferred revenue, which was not included in the condensed consolidated statement of cash flows and $ 1.4 million in revenue associated with the Pfizer Purchase Agreement, which was treated as a non-cash operating activity.

Australia Research and Developm

Australia Research and Development Tax Incentive	6 Months Ended
	Jun. 30, 2021
Research And Development [Abstract]
Australia Research and Development Tax Incentive	11. Australia Research and Development Tax Incentive The Australian government has established a research and development tax incentive to encourage industry investment in research and development, which is available to companies incorporated under Australian law that have core research and development activities. In September 2016, the Company established Spero Potentiator Australia Pty Limited to carry out certain research and development activities. As this subsidiary meets the eligibility requirements of the Australian tax law, it is eligible to receive a 43.5 % tax incentive for qualified research and development activities. The Company did no t record a reduction during both the three and six months ended June 30, 2021 , and recorded less than $ 0.1 million and $ 0.3 million during the three and six months ended June 30, 2020, respectively, as a reduction to research and development expenses in the consolidated statements of operations and comprehensive loss associated with this tax incentive, representing 43.5 % of the Company’s qualified research and development spending in Australia. The tax incentive refund is denominated in Australian dollars and, therefore, the associated tax incentive receivable is re-measured to U.S. dollars as of each reporting date. The Company’s tax incentive receivables from the Australian government totaled $ 0.3 million and $ 1.2 million as of June 30, 2021, and December 31, 2020 , respectively.

Net Loss per Share

Net Loss per Share	6 Months Ended
	Jun. 30, 2021
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 June 30, Six Months Ended June 30, 2021 2020 2021 2020 Numerator: Net loss $ ( 18,572 ) $ ( 17,501 ) $ ( 37,995 ) $ ( 40,759 ) Deemed dividend — — — ( 549 ) Net loss attributable to common stockholders $ ( 18,572 ) $ ( 17,501 ) $ ( 37,995 ) $ ( 41,308 ) Denominator: Weighted average common shares outstanding, basic and diluted 29,675,399 20,633,402 29,545,496 20,095,415 Net loss per share, basic and diluted $ ( 0.63 ) $ ( 0.85 ) $ ( 1.29 ) $ ( 2.06 ) The net loss applicable to common stockholders for the six months ended June 30, 2020 did not equal net loss due to the accretion of the beneficial conversion feature of Series C Preferred Stock in the amount of $ 0.5 million. The beneficial conversion feature was initially recorded as a discount on the Series C Preferred Stock with a corresponding amount recorded to Additional Paid-in Capital. The discount on the Series C Preferred Stock was then immediately written off as a deemed dividend as the Series C Preferred Stock does not have a stated redemption date and is immediately convertible at the option of the holder. 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 June 30, Six Months Ended June 30, 2021 2020 2021 2020 Options to purchase common stock 4,579,708 3,671,200 4,579,708 3,671,200 Unvested restricted stock units — 30,561 — 30,561 Series A convertible preferred stock (as converted to common shares) — 1,720,000 — 1,720,000 Series B convertible preferred stock (as converted to common shares) 938,000 1,000,000 938,000 1,000,000 Series C convertible preferred stock (as converted to common shares) 2,214,000 2,287,000 2,214,000 2,287,000 Series D convertible preferred stock (as converted to common shares) 3,215,000 — 3,215,000 — 10,946,708 8,708,761 10,946,708 8,708,761

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2021
Subsequent Events [Abstract]
Subsequent Event	13. Subsequent Events The Company has evaluated, for potential recognition and disclosure, events that occurred prior to the date in which the condensed consolidated financial statements were available to be issued. Other than as disclosed below, there were no material subsequent events. On July 1, 2021 the Company sold 2,362,348 shares of its common stock to Pfizer as at price of $ 16.93 for an aggregate purchase price of $ 40.0 million prior to deducting offering expenses under the Pfizer Purchase Agreement, which was effective as of June 30, 2021. Subsequent to June 30, 2021, the Company sold 70,000 shares of its common stock under the at-the-market offering program sales agreement with Cantor at an average price of approximately $ 16.27 per share for aggregate gross proceeds of approximately $ 1.1 million prior to deducting sales commissions and offering expenses.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2021
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 MDR bacterial infections. 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 June 30, 2021 , and December 31, 2020, 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 Effective January 1, 2019 , the Company adopted ASC Topic 842, Leases (“ASC 842”), using the modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the previous guidance in ASC 840, Leases (“ASC 840”). 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 June 30, 2021, the Company had no short-term leases with terms of one year or less. Options to renew a lease are not included in the Company’s initial lease term assessment unless there is reasonable certainty that the Company will renew. The Company monitors its plans to renew its material leases on a quarterly basis. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate (“IBR”), which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, and in a similar economic environment. Since the Company does not have any debt and has not been rated by any major credit rating agency, the Company’s IBR was estimated by developing a synthetic credit rating for the Company. The Company has elected to account for lease and non-lease components together as a single lease component.
Other Assets	Other Assets Other assets consist of long-term prepayments and deposits.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:  Level 1—Quoted prices in active markets for identical assets or liabilities.  Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.  Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities.
Revenue Recognition - Collaboration Revenue	Revenue Recognition – Collaboration Revenue Effective June 30, 2021, the Company entered into a licensing agreement that is 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 arrangement in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangement. 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. In June 2019, the Company entered into a collaboration agreement with the Bill and Melinda Gates Medical Research Institute (the “Gates MRI”) and concluded that the agreement is within the scope of the accounting guidance for collaboration arrangements (see Note 10). Due to the cost-funded nature of the payments and the Company’s assessment that it does not have a vendor/customer relationship with the Gates MRI, the Company recognizes the funding received under the agreement as a reduction to the research and development expenses incurred, as the related expenses are incurred.
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 (see Note 11). 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.
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 measures all share-based awards 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 six months ended June 30, 2021 and 2020, 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. and excludes net income (loss) attributable to non-controlling interests. 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 six months ended June 30, 2021, or to our net deferred tax assets as of June 30, 2021 .
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) No 2016-13, Financial Instruments – Credit Losses (Topic 326). The Accounting Standards Codification 326, Financial Instruments- Credit Losses (“ASC 326”) requires a financial asset measured at amortized cost basis to be presented at the net amount expected to be collected. Under ASU 2016-13, the Company is required to use a current expected credit loss (“CECL”) model that immediately recognizes an estimate of credit losses that are expected to occur over the life of the financial instruments that are in the scope of the update, including trade receivables. The updated guidance also amends the previous other-than-temporary impairment model for available-for-sale debt securities by requiring the recognition of impairments related to credit losses through an allowance account and limits the amount of credit loss to the difference between a security’s amortized cost basis and its fair value. In addition, the length of time a security has been in an unrealized loss position no longer impacts the determination of whether a credit loss exists. The Company adopted the guidance on January 1, 2020 with no impact. For available-for-sale securities, the updated guidance was applied prospectively. In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (a consensus of the FASB Emerging Issues Task Force) (“ASU 2018-15”). The amendments in ASU 2018-15 align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The Company adopted this standard as of January 1, 2020 , on a prospective basis. The adoption did no t have a material impact on the Company’s consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. This standard modifies certain disclosure requirements on fair value measurements. This standard became effective for the Company on January 1, 2020 , and did no t have a material impact on its disclosures. For the new disclosures regarding our Level 3 instruments, please read Note 3, Fair Value Measurements, to these condensed consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 . This standard makes targeted improvements for collaborative arrangements as follows:  Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers, when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements;  Adds unit-of-account guidance to ASC 808, Collaborative Arrangements, to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and  Precludes a company from presenting transactions with collaborative arrangement participants that are not directly related to sales to third parties with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. This standard became effective for the Company on January 1, 2020 , and did no t have a material impact on its condensed consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes , amended guidance on the accounting and reporting of income taxes. The guidance is intended to simplify the accounting for income taxes by removing exceptions related to certain intraperiod tax allocations and deferred tax liabilities; clarifying guidance primarily related to evaluating the step-up tax basis for goodwill in a business combination; and reflecting enacted changes in tax laws or rates in the annual effective tax rate. The amended guidance is effective for interim and annual periods in 2021. Early adoption is permitted. The application of the amendments in the new guidance are to be applied on a retrospective basis, on a modified retrospective basis through a cumulative-effect adjustment to retained earnings or prospectively, depending on the amendment. This standard became effective for the Company on January 1, 2021 , and did no t have a material impact on its condensed consolidated financial statements and related disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2021
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)	6 Months Ended
	Jun. 30, 2021
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 June 30, 2021 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 70,896 $ — $ 70,896 Total cash equivalents — 70,896 — 70,896 Marketable securities: Corporate bonds — 12,663 — 12,663 Commercial paper — 14,993 — 14,993 Total marketable securities — 27,656 — 27,656 Total cash equivalents and marketable securities $ — $ 98,552 $ — $ 98,552 Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ — $ 73,488 $ — $ 73,488 Commercial paper — 5,998 — 5,998 Corporate bonds — 3,006 — 3,006 Total cash equivalents — 82,492 — 82,492 Marketable securities: Corporate bonds — 13,221 — 13,221 Commercial paper — 28,476 — 28,476 Total marketable securities — 41,697 — 41,697 Total cash equivalents and marketable securities $ — $ 124,189 $ — $ 124,189
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 June 30, 2021, and December 31, 2020 (in thousands): June 30, 2021 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Assets: Corporate bonds $ 12,665 $ — $ ( 2 ) $ 12,663 Commercial paper 14,993 — — 14,993 $ 27,658 $ — $ ( 2 ) $ 27,656 December 31, 2020 Amortized Cost Gross Unrealized Gross Unrealized Fair Value Assets: Corporate bonds $ 13,227 $ — $ ( 6 ) $ 13,221 Commercial paper 28,476 — — 28,476 $ 41,703 $ — $ ( 6 ) $ 41,697

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	6 Months Ended
	Jun. 30, 2021
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 June 30, 2021 and December 31, 2020 (in thousands): June 30, 2021 December 31, 2020 Accrued external research and development expenses $ 4,752 7,035 Accrued payroll and related expenses 2,495 3,918 Accrued professional fees 1,560 1,066 Accrued other 200 222 Total Accrued expenses and other current liabilities $ 9,007 $ 12,241

Share-Based Compensation (Table

Share-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2021
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 six months ended June 30, 2021: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2020 3,682,233 $ 9.10 7.84 $ 37,881 Granted 1,169,615 18.28 — — Exercised ( 46,715 ) 8.33 — — Forfeited or cancelled ( 225,425 ) 13.83 — — Outstanding as of June 30, 2021 4,579,708 $ 11.22 7.78 $ 17,689 Outstanding as of June 30, 2021 - vested and 4,579,708 $ 11.22 7.78 $ 17,689 Exercisable at June 30, 2021 2,260,040 $ 8.19 6.67 $ 13,066
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 June 30, Six Months Ended June 30, 2021 2020 2021 2020 Research and development expenses $ 895 $ 559 $ 1,735 $ 1,055 General and administrative expenses 1,268 655 2,437 1,269 Total $ 2,163 $ 1,214 $ 4,172 $ 2,324
Performance-based Vesting Criteria [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Activity of Options and RSUs of Performance-Based Options	The following table summarizes the activity of options and RSUs under the 2017 Plan containing performance-based vesting criteria during the six months ended June 30, 2021: Number of Number of Outstanding as of December 31, 2020 63,107 30,561 Granted - - Exercised - - Forfeited or cancelled ( 63,107 ) ( 30,561 ) Outstanding as of June 30, 2021 - -

License, Collaboration and Se_2

License, Collaboration and Service Agreements (Tables)	6 Months Ended
	Jun. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Performance Obligation Along with Standalone Selling Price and Transaction Price Allocated	: Transaction Price Performance Obligations Allocated Recognition Method License and know-how transfer (1) $ 1.4 Fully satisfied; recognized upon delivery of the license Research and development services related to upcoming milestones (2) 11.1 Recognized over time as services are delivered $ 12.5 1. The standalone selling price for the license and know-how was determined by the income approach utilizing a discounted cash flow. The key assumptions in our estimate of the standalone selling price for the license and know-how include the probability of technological and regulatory success, our estimate of future product revenues, and the discount rate, among others. 2. The standalone selling price for the research and development services was estimated based on our estimate of costs to be incurred to fulfill our obligations associated with the performance of the research and development services, plus a reasonable margin.

Net Loss per Share (Tables)

Net Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2021
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 June 30, Six Months Ended June 30, 2021 2020 2021 2020 Numerator: Net loss $ ( 18,572 ) $ ( 17,501 ) $ ( 37,995 ) $ ( 40,759 ) Deemed dividend — — — ( 549 ) Net loss attributable to common stockholders $ ( 18,572 ) $ ( 17,501 ) $ ( 37,995 ) $ ( 41,308 ) Denominator: Weighted average common shares outstanding, basic and diluted 29,675,399 20,633,402 29,545,496 20,095,415 Net loss per share, basic and diluted $ ( 0.63 ) $ ( 0.85 ) $ ( 1.29 ) $ ( 2.06 )
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 June 30, Six Months Ended June 30, 2021 2020 2021 2020 Options to purchase common stock 4,579,708 3,671,200 4,579,708 3,671,200 Unvested restricted stock units — 30,561 — 30,561 Series A convertible preferred stock (as converted to common shares) — 1,720,000 — 1,720,000 Series B convertible preferred stock (as converted to common shares) 938,000 1,000,000 938,000 1,000,000 Series C convertible preferred stock (as converted to common shares) 2,214,000 2,287,000 2,214,000 2,287,000 Series D convertible preferred stock (as converted to common shares) 3,215,000 — 3,215,000 — 10,946,708 8,708,761 10,946,708 8,708,761

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Nature Of Business And Basis Of Presentation [Abstract]
Net loss	$ 18,600	$ 17,500	$ 38,000	$ 40,800
Accumulated deficit	$ 315,702		$ 315,702		$ 277,707

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Summary Of Significant Accounting Policies [Line Items]
Off-balance sheet risk description	As of June 30, 2021, and December 31, 2020, 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	80.00%
Percentage of adjusted taxable income plus business interest income	50.00%
Percentage of limitation on adjusted taxable income plus business interest income under 2017 tax act	30.00%
CARES act Percentage of corporate charitable deduction limitation on taxable income	25.00%
CARES act eligible cost-recovery period	15 years
CARES act percentage of bonus depreciation	100.00%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
CARES act net operating loss carryback period	5 years
ASU No. 2016-12 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change In Accounting Principle Accounting Standards Update Adopted	true
Change In Accounting Principle Accounting Standards Update Adoption Date	Jan. 1, 2019
Change In Accounting Principle Accounting Standards Update Immaterial Effect	true
ASU No. 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change In Accounting Principle Accounting Standards Update Adopted	true
Change In Accounting Principle Accounting Standards Update Adoption Date	Jan. 1, 2020
Change In Accounting Principle Accounting Standards Update Immaterial Effect	true
ASU No. 2018-15 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change In Accounting Principle Accounting Standards Update Adopted	true
Change In Accounting Principle Accounting Standards Update Adoption Date	Jan. 1, 2020
Change In Accounting Principle Accounting Standards Update Immaterial Effect	true
Change in accounting principle, accounting standards update, transition option elected [Extensible List]	us-gaap:AccountingStandardsUpdate201815ProspectiveMember
ASU No. 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change In Accounting Principle Accounting Standards Update Adopted	true
Change In Accounting Principle Accounting Standards Update Adoption Date	Jan. 1, 2020
Change In Accounting Principle Accounting Standards Update Immaterial Effect	true
ASU No. 2018-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change In Accounting Principle Accounting Standards Update Adopted	true
Change In Accounting Principle Accounting Standards Update Adoption Date	Jan. 1, 2020
Change In Accounting Principle Accounting Standards Update Immaterial Effect	true
ASU No. 2019-12 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change In Accounting Principle Accounting Standards Update Adopted	true
Change In Accounting Principle Accounting Standards Update Adoption Date	Jan. 1, 2021
Change In Accounting Principle Accounting Standards Update Immaterial Effect	true

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Detail)	6 Months Ended
	Jun. 30, 2021
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	Jun. 30, 2021	Dec. 31, 2020
Cash equivalents:
Total cash equivalents	$ 700	$ 2,700
Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Total cash equivalents	70,896	82,492
Marketable securities:
Total marketable securities	27,656	41,697
Total cash equivalents and marketable securities	98,552	124,189
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents	0	0
Marketable securities:
Total marketable securities	0	0
Total cash equivalents and marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents	70,896	82,492
Marketable securities:
Total marketable securities	27,656	41,697
Total cash equivalents and marketable securities	98,552	124,189
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents	0	0
Marketable securities:
Total marketable securities	0	0
Total cash equivalents and marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Total cash equivalents	70,896	73,488
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	70,896	73,488
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]
Cash equivalents:
Total cash equivalents		5,998
Marketable securities:
Total marketable securities	12,663	28,476
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents		0
Marketable securities:
Total marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents		5,998
Marketable securities:
Total marketable securities	12,663	28,476
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents		0
Marketable securities:
Total marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Cash equivalents:
Total cash equivalents		3,006
Marketable securities:
Total marketable securities	14,993	13,221
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 1 [Member]
Cash equivalents:
Total cash equivalents		0
Marketable securities:
Total marketable securities	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 2 [Member]
Cash equivalents:
Total cash equivalents		3,006
Marketable securities:
Total marketable securities	14,993	13,221
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 3 [Member]
Cash equivalents:
Total cash equivalents		0
Marketable securities:
Total marketable securities	$ 0	$ 0

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Total cash	$ 700,000	$ 2,700,000
Fair value assets level 1 to level 2 transfers	0
Fair value assets level 2 to level 1 transfers	0
Fair value liabilities level 1 to level 2 transfers	0
Fair value liabilities level 2 to level 1 transfers	0
Transfer of financial asset into level 3 of fair value	0
Transfer of financial liabilities into level 3 of fair value	$ 0

Fair Value Measurements and M_5

Fair Value Measurements and Marketable Securities - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 27,658	$ 41,703
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(6)
Fair Value	27,656	41,697
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,665	13,227
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(6)
Fair Value	12,663	13,221
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	14,993	28,476
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 14,993	$ 28,476

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued external research and development expenses	$ 4,752	$ 7,035
Accrued payroll and related expenses	2,495	3,918
Accrued professional fees	1,560	1,066
Accrued other	200	222
Total Accrued expenses and other current liabilities	$ 9,007	$ 12,241

Equity Transactions - Underwrit

Equity Transactions - Underwritten Public Offering - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Oct. 01, 2020	Sep. 15, 2020	Mar. 05, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Class Of Stock [Line Items]
Proceeds from equity offering						$ 4,253	$ 7,271
Series D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Preferred stock, conversion basis						The shares of Series D Preferred Stock are convertible on a one-to-one basis into shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series D Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99% of the total number of shares of common stock then issued and outstanding, subject to certain exceptions.
Maximum percentage of common stock holding upon conversion of preferred stock		9.99%
Preferred stock, liquidation per share		$ 0.001
Voting rights				no
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares			1,046,249		645,218	257,185	1,691,467
Proceeds from equity offering	$ 11,200
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Underwritten offering, exercisable term		30 days
Gross proceeds from issuance of common and preferred stock		$ 80,000
Net proceeds from issuance of common and preferred stock		$ 74,700
Underwritten Public Offering [Member] | Series D Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares		3,215,000
Share price		$ 10
Underwritten Public Offering [Member] | Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	1,200,000	4,785,000
Share price	$ 10	$ 10
Underwritten Public Offering [Member] | Maximum [Member]
Class Of Stock [Line Items]
Additional number of common stock issuable		1,200,000

Equity Transactions - Rights Of

Equity Transactions - Rights Offering - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 05, 2020	Feb. 11, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Feb. 28, 2021	Dec. 31, 2020
Class Of Stock [Line Items]
Common stock, par value				$ 0.001			$ 0.001
Aggregate issue costs				$ 146	$ 502
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares	1,046,249		645,218	257,185	1,691,467
Aggregate issue costs					$ 462
Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of Common Stock, conversion of Preferred Stock to Common Stock	$ 20,600	$ 30,000
Right to purchase common stock shares		0.152
Common stock, par value		$ 9
Issuance of stock, shares	2,287
Aggregate issue costs	$ 100			$ 130	$ 41
Number of shares issuable upon conversion				1,000		73,000
Preferred stock, liquidation per share				$ 0.001
Voting rights				no
Share price	$ 9.22
Intrinsic value of beneficial conversion feature				$ 500
Common Stock and Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of Common Stock, conversion of Preferred Stock to Common Stock	$ 30,000
Amount of agreed to purchase offered shares and not subscribed shares value under rights offering				$ 30,000
Aggregate issue costs	$ 500
Series A Convertible Preferred Stock [Member]
Class Of Stock [Line Items]
Share price	$ 8.98

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($)	Oct. 01, 2020	Sep. 09, 2020	Mar. 05, 2020	Feb. 28, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 11, 2021	Nov. 13, 2020	Dec. 11, 2018
Class Of Stock [Line Items]
Issuance of stock, value					$ 7,271,000	$ 4,123,000	$ 16,226,000
Proceeds from sale of common stock to related party						$ 4,253,000	7,271,000
Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares			2,287
Issuance of stock, value							$ 20,542,000
Share price			$ 9.22
Preferred stock, conversion basis				73
Number of shares issuable upon conversion				73,000		1,000
Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares			1,046,249		645,218	257,185	1,691,467
Issuance of stock, value					$ 1,000	$ 1,000	$ 2,000
Proceeds from sale of common stock to related party	$ 11,200,000
Pfizer Inc [Member]
Class Of Stock [Line Items]
Issuance of stock, shares				2,362,348
Issuance of stock, value				$ 27,500,000
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						$ 40,000,000
At-The-Market Offering Program [Member]
Class Of Stock [Line Items]
Proceeds from sale of common stock to related party						$ 4,400,000
At-The-Market Offering Program [Member] | Common Stock [Member]
Class Of Stock [Line Items]
Issuance of stock, shares						257,185
Share price						$ 17.05
At-The-Market Offering Program [Member] | Sales Agreement [Member] | Cantor Fitzgerald & Co [Member]
Class Of Stock [Line Items]
Common stock value remaining available for issuance		$ 34,600,000
Issuance of stock, shares						70,000
Share price						$ 16.27
Proceeds from sale of common stock to related party		$ 15,400,000				$ 1,100,000
At-The-Market Offering Program [Member] | Sales Agreement [Member] | Cantor Fitzgerald & Co [Member] | Maximum [Member]
Class Of Stock [Line Items]
Authorized offering value								$ 300,000,000		$ 200,000,000
Common stock available for issuance								$ 75,000,000	$ 34,000,000	$ 50,000,000
Series B Preferred Stock [Member]
Class Of Stock [Line Items]
Preferred stock, conversion basis				62
Number of shares issuable upon conversion				62,000

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2021USD ($)Plan$ / sharesshares	Jun. 30, 2020$ / shares	Dec. 31, 2019shares	Dec. 31, 2020shares
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	29,699,147			29,260,247
Share based compensation options granted	1,169,615
Number of options vested	0
Compensation expense recognized | $	$ 0
Weighted average grant-date fair value of awards granted | $ / shares	$ 14.05	$ 6.70
Number of shares authorized for issuance	5,549,873
Common stock reserved for issuance	463,627
Performance-based Vesting Criteria [Member] | Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation options granted			100,000
Options vesting period			2 years
Performance-based Vesting Criteria [Member] | Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted shares in addition to stock option			50,000
Options vesting period			2 years
Performance-based Vesting Criteria 1 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive units vesting			25.00%
Performance-based Vesting Criteria 2 [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive units vesting			50.00%
2019 Inducement Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in shares of common stock for issuance	700,000

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Activity of Options and RSUs of Performance-Based Options (Details)	6 Months Ended
	Jun. 30, 2021shares
Number of Shares
Outstanding as of December 31, 2020	3,682,233
Share based compensation options granted	1,169,615
Issuance of common stock upon the exercise of stock options, shares	46,715
Outstanding as of June 30, 2021	4,579,708
2017 Plan [Member] | Number Of Performance Based Option Shares [Member]
Number of Shares
Outstanding as of December 31, 2020	63,107
Share based compensation options granted	0
Issuance of common stock upon the exercise of stock options, shares	0
Forfeited or cancelled	(63,107)
Outstanding as of June 30, 2021	0
2017 Plan [Member] | Number Of Performance Based RSUs Shares [Member]
Number of Shares
Outstanding as of December 31, 2020	30,561
Share based compensation options granted	0
Issuance of common stock upon the exercise of stock options, shares	0
Forfeited or cancelled	(30,561)
Outstanding as of June 30, 2021	0

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
Number of Shares
Outstanding as of December 31, 2020 | shares	3,682,233
Granted | shares	1,169,615
Exercised | shares	(46,715)
Forfeited or cancelled | shares	(225,425)
Outstanding as of June 30, 2021 | shares	4,579,708	3,682,233
Outstanding as of June 30, 2021 - vested and expected to vest | shares	4,579,708
Exercisable at June 30, 2021 | shares	2,260,040
Weighted Average Exercise Price
Outstanding as of December 31, 2020 | $ / shares	$ 9.10
Granted | $ / shares	18.28
Exercised | $ / shares	8.33
Forfeited or cancelled | $ / shares	13.83
Outstanding as of June 30, 2021 | $ / shares	11.22	$ 9.10
Outstanding as of June 30, 2021 - vested and expected to vest | $ / shares	11.22
Exercisable at June 30, 2021 | $ / shares	$ 8.19
Weighted Average Contractual Term
Outstanding	7 years 9 months 10 days	7 years 10 months 2 days
Outstanding as of June 30, 2021 - vested and expected to vest	7 years 9 months 10 days
Exercisable at June 30, 2021	6 years 8 months 1 day
Aggregate Intrinsic Value
Outstanding as of December 31, 2020 | $	$ 17,689	$ 37,881
Outstanding as of June 30, 2021 | $	17,689	$ 37,881
Outstanding as of June 30, 2021 - vested and expected to vest | $	17,689
Exercisable at June 30, 2021 | $	$ 13,066

Share-Based Compensation - Su_3

Share-Based Compensation - Summary of Share-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	$ 2,163	$ 1,214	$ 4,172	$ 2,324
Research and Development Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	895	559	1,735	1,055
General and Administrative Expenses [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation	$ 1,268	$ 655	$ 2,437	$ 1,269

Government Contracts - Addition

Government Contracts - Additional Information (Detail)

Jul. 01, 2019USD ($)

May 31, 2021USD ($)Option

Jul. 31, 2018USD ($)

Feb. 28, 2018USD ($)

Feb. 28, 2017USD ($)

Jun. 30, 2016USD ($)Option

Jun. 30, 2021USD ($)

Jun. 30, 2020USD ($)

Jun. 30, 2021USD ($)Option

Jun. 30, 2020USD ($)

Dec. 31, 2020

Feb. 28, 2019

Jan. 31, 2020USD ($)

May 31, 2019USD ($)

Government Contracts [Line Items]

Contract term

2 years

Grant revenue

$ 5,148,000

$ 1,727,000

$ 12,448,000

$ 3,428,000

BARDA and DTRA [Member]

Government Contracts [Line Items]

Maximum potential funding from government contract

56,800,000

56,800,000

Potential grant amount exercised

12,700,000

12,700,000

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

Government Contracts [Line Items]

Grant revenue

1,100

1,800,000

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

Government Contracts [Line Items]

Award received for clinical development

$ 5,900,000

Non-dilutive funding period

4 years

BARDA [Member]

Government Contracts [Line Items]

Potential contract amount awarded

$ 44,200,000

BARDA [Member] | Base Period Contracts [Member]

Government Contracts [Line Items]

Potential amount initial funding awarded

$ 15,700,000

$ 18,200,000

Potential amount additional funding awarded

2,500,000

Overall potential award increased

$ 46,800,000

Contract term

3 years

BARDA [Member] | Grant [Member]

Government Contracts [Line Items]

Grant revenue

1,900

1,600,000

8,200,000

2,800,000

BARDA [Member] | First Option [Member]

Government Contracts [Line Items]

Potential amount additional funding awarded

$ 15,900,000

Potential amount increase in committed funding

34,100,000

BARDA [Member] | Second Option [member]

Government Contracts [Line Items]

Potential grant amount awarded

$ 12,700,000

DTRA [Member] | Maximum [Member]

Government Contracts [Line Items]

Potential amount additional funding awarded

10,000,000

10,000,000

Niaid [Member]

Government Contracts [Line Items]

Contract term

5 years

12 months

Maximum potential funding from government contract

$ 23,400,000

Contact extension term

Feb. 28, 2020

Number of option period for funding from government contract | Option

5

Committed amount from government contract

$ 2,100,000

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

Government Contracts [Line Items]

Maximum potential funding from government contract

$ 6,500,000

Number of option period for funding from government contract | Option

3

Contact extension term

2021-06

Niaid [Member] | Base Period Contracts [Member]

Government Contracts [Line Items]

Potential grant amount awarded

$ 600,000

Niaid [Member] | Grant [Member]

Government Contracts [Line Items]

Grant revenue

0

0

$ 0

Niaid [Member] | Grant [Member] | Maximum [Member]

Government Contracts [Line Items]

Grant revenue

100,000

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

Government Contracts [Line Items]

Grant revenue

$ 100,000

$ 100,000

$ 400,000

$ 300,000

Niaid [Member] | Option Period Contracts [Member]

Government Contracts [Line Items]

Contract term

12 months

Potential grant amount awarded

$ 400,000

Potential grant amount exercised

$ 400,000

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

$ 5,900,000

License, Collaboration and Se_3

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

Jan. 15, 2021

Jan. 04, 2019USD ($)

Jun. 30, 2021USD ($)$ / shares

Feb. 28, 2021USD ($)shares

Nov. 30, 2018USD ($)

Jun. 30, 2017USD ($)

Jun. 30, 2016USD ($)

Jun. 30, 2016GBP (£)

May 31, 2016USD ($)

Jun. 30, 2021USD ($)$ / shares

Jun. 30, 2020USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Jun. 30, 2021USD ($)$ / sharesshares

Jun. 30, 2020USD ($)

Dec. 31, 2020USD ($)$ / shares

Oct. 31, 2017USD ($)

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Issuance of stock, value

$ 7,271,000

$ 4,123,000

$ 16,226,000

Common stock, par value | $ / shares

$ 0.001

$ 0.001

$ 0.001

$ 0.001

Proceeds from sale of common stock to related party

$ 4,253,000

7,271,000

Revenue recognized

$ 2,106,000

51,000

2,106,000

220,000

Deferred revenue

$ 9,928,000

9,928,000

9,928,000

$ 0

Premium associated freestanding equity

12,500,000

Pfizer Inc [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Issuance of stock, shares | shares

2,362,348

Issuance of stock, value

$ 27,500,000

Remaining transaction price allocated to the performance obligations

12,500,000

12,500,000

12,500,000

Cantab Related Agreements [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payment upon achievement of specified commercial milestone

£ 5

6,900,000

$ 6,800,000

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

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

Potential milestone payments upon achievement of specified condition

$ 1,000,000

License agreement fixed assets related payments

$ 1,600,000

Meiji License Agreement [Member] | Maximum [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payment upon achievement of specified clinical and regulatory milestones

$ 2,000,000

Sublicense fee payable to counter party

$ 7,500,000

Everest License Agreement [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Period of option granted on an agreement

12 months

Upfront payment received

$ 3,000,000

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

$ 2,000,000

Receivable amount upon achievement of certain milestone

59,500,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

800,000

2,000,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 1,000,000

Everest License Agreement [Member] | Maximum [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Future milestone payments

$ 1,500,000

Agreement termination period upon written notice

180 days

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

600,000

$ 1,100,000

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

$ 5,100,000

Pfizer License and Share Purchase Agreements [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Issuance of stock, shares | shares

2,362,348

Issuance of stock, value

$ 27,500,000

Remaining transaction price allocated to the performance obligations

11,100,000

11,100,000

11,100,000

Revenue recognized

1,400,000

1,400,000

Deferred revenue

$ 11,100,000

$ 11,100,000

11,100,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 0

Issuance of stock, shares | shares

2,362,348

Share price | $ / shares

$ 16.93

$ 16.93

$ 16.93

Proceeds from sale of common stock to related party

$ 40,000,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Receivable amount upon achievement of certain milestone

$ 80,000,000

License, Collaboration and Se_4

License, Collaboration and Service Agreements - Schedule of Performance Obligation Along with Standalone Selling Price and Transaction Price Allocated (Details)	6 Months Ended
	Jun. 30, 2021USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Performance Obligation Allocated Transaction Price	$ 12,500
License and Know-How Transfer [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Performance Obligation Allocated Transaction Price	1,400
Research And Development Services Related To Upcoming Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Performance Obligation Allocated Transaction Price	$ 11,100

Australia Research and Develo_2

Australia Research and Development Tax Incentive - Additional Information (Detail) - Spero Potentiator Australia Pty Limited [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Australia [Member]
Schedule Of Income Tax [Line Items]
Tax incentive receivable	$ 300		$ 300		$ 1,200
Research and Development Expenses [Member]
Schedule Of Income Tax [Line Items]
Percentage of refundable tax incentive			43.50%
Reduction to research and development expenses	$ 0		$ 0	$ 300
Research and Development Expenses [Member] | Maximum [Member]
Schedule Of Income Tax [Line Items]
Reduction to research and development expenses		$ 100

Net Loss per Share - Summary of

Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net loss	$ (18,572)	$ (17,501)	$ (37,995)	$ (40,759)
Deemed dividend	0	0	0	(549)
Net loss attributable to common stockholders	$ (18,572)	$ (17,501)	$ (37,995)	$ (41,308)
Denominator:
Weighted average common shares outstanding, basic and diluted	29,675,399	20,633,402	29,545,496	20,095,415
Net loss per share, basic and diluted	$ (0.63)	$ (0.85)	$ (1.29)	$ (2.06)

Net Loss per Share - Additional

Net Loss per Share - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2020USD ($)
Series C Preferred Stock [Member]
Earnings Per Share [Line Items]
Accretion of beneficial conversion feature	$ 0.5

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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	10,946,708	8,708,761	10,946,708	8,708,761
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	4,579,708	3,671,200	4,579,708	3,671,200
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	0	30,561	0	30,561
Series A Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	1,720,000	0	1,720,000
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	938,000	1,000,000	938,000	1,000,000
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	2,214,000	2,287,000	2,214,000	2,287,000
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	3,215,000	0	3,215,000	0

Subsequent Events (Additional I

Subsequent Events (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 01, 2021	Sep. 09, 2020	Feb. 28, 2021	Jun. 30, 2021	Jun. 30, 2020
Subsequent Event [Line Items]
Proceeds from sale of common stock to related party				$ 4,253	$ 7,271
Pfizer Inc [Member]
Subsequent Event [Line Items]
Stock Issued During Period Shares New Issues			2,362,348
Pfizer Purchase Agreement [Member] | Pfizer Inc [Member]
Subsequent Event [Line Items]
Stock Issued During Period Shares New Issues				2,362,348
Shares Issued Price Per Share				$ 16.93
Proceeds from sale of common stock to related party				$ 40,000
Pfizer Purchase Agreement [Member] | Pfizer Inc [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Stock Issued During Period Shares New Issues	2,362,348
Shares Issued Price Per Share	$ 16.93
Proceeds from sale of common stock to related party	$ 40,000
At The Market Offering Program [Member]
Subsequent Event [Line Items]
Proceeds from sale of common stock to related party				$ 4,400
At The Market Offering Program [Member] | Sales Agreement [Member] | Cantor Fitzgerald Co [Member]
Subsequent Event [Line Items]
Stock Issued During Period Shares New Issues				70,000
Shares Issued Price Per Share				$ 16.27
Proceeds from sale of common stock to related party		$ 15,400		$ 1,100