Subsequent Events	6 Months Ended
Jun. 30, 2026
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events Innovent License Agreement On July 8, 2026, the Company entered into the Innovent Agreement, pursuant to which Innovent granted the Company an exclusive (even as to Innovent and its affiliates, subject to Innovent’s right to directly or indirectly conduct certain research, non-clinical development and manufacturing activities), sublicensable right and license under certain patent rights and know-how to research, develop, manufacture, and commercialize Innovent’s proprietary monoclonal antibody targeting CD40L, which is referred to as SP001, and certain backup monoclonal antibodies and derivative monoclonal antibodies, in each case targeting CD40L and products containing the Licensed Compounds worldwide, excluding the Innovent Territory. Innovent also granted the Company a non-exclusive, sublicensable license under the Licensed Intellectual Property to research, non-clinically develop and manufacture the Licensed Compounds and Licensed Products in the Innovent Territory in connection with the Company’s exploitation of the Licensed Compounds and Licensed Products worldwide, excluding the Innovent Territory (the territory excluding the Innovent Territory, the “Licensed Territory”). Under the Innovent Agreement, the Company granted Innovent an exclusive (even as to the Company and its affiliates, subject to the Company’s right to directly or indirectly conduct certain research, non-clinical development and manufacturing activities), sublicensable right and license under certain patent rights and know-how (the “Company Intellectual Property”) to develop, manufacture, and commercialize the Licensed Compounds and Licensed Products in the Innovent Territory. The Company also granted Innovent a non-exclusive, sublicensable license under the Company Intellectual Property to research, non-clinically develop and manufacture the Licensed Compounds and Licensed Products in the Innovent Territory. Each party has agreed that, for five years from the date of the Innovent Agreement, it will not, directly or indirectly, alone or with or for any other person or entity, clinically develop or commercialize any monoclonal antibody, bispecific antibody, or multispecific antibody targeting CD40L in the Licensed Territory. If, within three years from the date of the Innovent Agreement, Innovent or any of its affiliates initiates development of a bispecific or multispecific antibody targeting CD40L, in specified circumstances the Company has an option to include that antibody as a Licensed Compound under the Innovent Agreement subject to certain financial terms. If the Company declines to exercise that option, then Innovent is free to clinically develop and commercialize that antibody in the Licensed Territory. The Company has agreed to use commercially reasonable efforts to develop, obtain regulatory approval for, and commercialize a Licensed Product in at least one indication in the United States and any one of France, Germany, Italy, Spain, the United Kingdom or Japan. The Company has agreed to file an IND application for a Licensed Product in the United States within 12 months from the date of the Innovent Agreement, subject to extensions for specified justifiable delays. Under the terms of the Innovent Agreement, the Company is obligated to pay to Innovent a $35.0 million upfront payment, as well as aggregate milestone payments of up to approximately $1.05 billion upon the achievement of certain development, regulatory and commercial milestones. The Company is also obligated to pay Innovent tiered royalties ranging from a high single-digit percentage to a mid-teen-digit percentage on annual net sales of all Licensed Products. The Company is obligated to pay royalties on a Licensed Product-by-Licensed Product and country-by-country basis until the latest of: (i) the expiration of the last valid claim of the licensed patents covering the composition of matter of the Licensed Compound contained in such Licensed Product in such country; (ii) the expiration of the last applicable regulatory exclusivity right with respect to such Licensed Product in such country; and (iii) 11 years following the first commercial sale of such Licensed Product in such country (each, a “Royalty Term”). The royalty rate is subject to specified reductions on a Licensed Product-by-Licensed Product and country-by-country basis under specified circumstances. The Innovent Agreement contemplates that the Company will enter into ancillary arrangements with Innovent, including a clinical supply agreement and a pharmacovigilance agreement. The Company has also granted Innovent a right of first negotiation to be a secondary manufacturer of the Licensed Compounds and Licensed Products for commercialization in the Licensed Territory. Unless earlier terminated, the Innovent Agreement will expire on the expiration of the last to expire Royalty Term. Unless the Innovent Agreement is earlier terminated, on expiration of each applicable Royalty Term, the Company will have a fully paid-up, irrevocable and perpetual license under the Licensed Intellectual Property to develop, manufacture and commercialize each applicable Licensed Product in the applicable country, which license will become non-exclusive after a specified point in time. Either party may terminate the Innovent Agreement for the other party’s uncured material breach, following a customary notice and cure period, or insolvency. Innovent may terminate the Innovent Agreement if the Company challenges the scope, validity, or enforceability of the licensed patents. The Company may terminate the Innovent Agreement for any reason upon 90 days’ written notice to Innovent prior to the first commercial sale of a Licensed Product, or upon 150 days’ written notice to Innovent after the first commercial sale of a Licensed Product. Additionally, if the Company has the right to terminate the Innovent Agreement for certain uncured material breaches by Innovent, the Company may, in lieu of termination, elect to keep the Innovent Agreement in force and reduce the Company’s subsequent payment obligations. Upon a termination of the Innovent Agreement prior to expiration of an applicable Royalty Term, the Company’s licenses from Innovent will terminate and Innovent will have the right to obtain a license under the Company Intellectual Property, with a right to sublicense, with respect to specified terminated products, which license will be either non-exclusive and fully paid-up or exclusive and subject to milestone and royalty obligations, at Innovent’s election. Royalty Financing Transaction On July 8, 2026 (the “Closing Date”), the Company, Spero Holdings SPV, LLC, a Delaware limited liability company and wholly owned subsidiary of the Company (“Holdings”), and Spero SPV, LLC, a Delaware limited liability company and wholly owned subsidiary of Holdings (“Issuer”), entered into the following agreements with affiliates of HCRx: (i) a non-recourse Note Purchase and Guaranty Agreement (the “NPA”), by and among Holdings, Issuer, the purchasers party thereto (the “NPA Purchasers”) and HCR Spero SPV, LLC, as the purchaser representative for such NPA Purchasers (the “NPA Purchaser Representative”), (ii) a Royalty and Milestone Payment Interest Purchase and Sale Agreement (the “RPA”), by and among Issuer, the purchasers party thereto (the “RPA Purchasers”), and HCR Spero SPV, LLC, as the purchaser representative for such RPA Purchasers (the “RPA Purchaser Representative”) and (iii) a Limited Recourse Guaranty (the “Limited Guaranty”), by the Company in favor of the NPA Purchaser Representative and RPA Purchaser Representative. Under the terms of the NPA, Issuer sold to the NPA Purchasers, senior secured notes in an aggregate principal amount of $105,000,000 (the “Notes”), which Notes (i) were issued net of an original issue discount in the amount of $3,150,000, which, after giving effect to the RPA Purchase Price referenced below, reduces the aggregate $105,000,000 in gross proceeds of the NPA and RPA by 1.5%, (ii) accrue interest at 10% per annum, which interest, to the extent not paid in cash from available GSK Proceeds (as defined below) in accordance with the terms of the NPA, is capitalized and added to the principal amount of the Notes on a quarterly basis and (iii) mature nine years after the Closing Date. The Notes are secured by substantially all of Holdings’ and Issuer’s assets (collectively, the “Collateral”), which includes the equity interest of Issuer held by Holdings and rights to a portion of the future GSK Proceeds arising under the GSK Agreement, but excludes intellectual property as to which the grant of security interests is restricted. The Notes and other obligations under the NPA are generally payable solely from the GSK Proceeds, unless voluntarily prepaid at the option of Issuer prior to maturity with a premium or subject to certain mandatory prepayment triggers and foreclosure rights. Under the terms of the RPA, Issuer sold to the RPA Purchasers 65% of the GSK Proceeds arising after the payment in full of the Notes (the “Purchased Proceeds”) in exchange for a cash payment of $1,575,000 (the “RPA Purchase Price”). Issuer’s obligations under the RPA are secured by a customary back-up lien in the Purchased Proceeds and proceeds thereof. The Company retains an interest in 35% of the GSK Proceeds arising after payment in full of the Notes. In connection with the NPA and RPA, the Company will provide certain servicing, management and administrative functions on behalf of Holdings and Issuer. Each of the NPA, RPA and Limited Guaranty contain certain customary terms and conditions, including representations and warranties, indemnities, affirmative and negative covenants, and events of default. Upon the occurrence and during the continuance of an event of default under the NPA, the NPA Purchaser Representative may declare the Notes and other obligations under the NPA to be immediately due and payable, and if such Notes and other obligations are not repaid, foreclose on the Collateral and use the proceeds thereof to repay the Notes, and, in limited circumstances, seek payment from the Company pursuant to the Limited Guaranty. The obligations of the Company in respect of the Limited Guaranty in circumstances where the Company is liable are, except in very limited circumstances, subject to a cap and limited to contract breaches, defaults and other liabilities customary for similarly structured royalty financing transactions.

Subsequent Events

6 Months Ended

Jun. 30, 2026

12.

Subsequent Events

Innovent License Agreement

On July 8, 2026, the Company entered into the Innovent Agreement, pursuant to which Innovent granted the Company an exclusive (even as to Innovent and its affiliates, subject to Innovent’s right to directly or indirectly conduct certain research, non-clinical development and manufacturing activities), sublicensable right and license under certain patent rights and know-how to research, develop, manufacture, and commercialize Innovent’s proprietary monoclonal antibody targeting CD40L, which is referred to as SP001, and certain backup monoclonal antibodies and derivative monoclonal antibodies, in each case targeting CD40L and products containing the Licensed Compounds worldwide, excluding the Innovent Territory. Innovent also granted the Company a non-exclusive, sublicensable license under the Licensed Intellectual Property to research, non-clinically develop and manufacture the Licensed Compounds and Licensed Products in the Innovent Territory in connection with the Company’s exploitation of the Licensed Compounds and Licensed Products worldwide, excluding the Innovent Territory (the territory excluding the Innovent Territory, the “Licensed Territory”).

Under the Innovent Agreement, the Company granted Innovent an exclusive (even as to the Company and its affiliates, subject to the Company’s right to directly or indirectly conduct certain research, non-clinical development and manufacturing activities), sublicensable right and license under certain patent rights and know-how (the “Company Intellectual Property”) to develop, manufacture, and commercialize the Licensed Compounds and Licensed Products in the Innovent Territory. The Company also granted Innovent a non-exclusive, sublicensable license under the Company Intellectual Property to research, non-clinically develop and manufacture the Licensed Compounds and Licensed Products in the Innovent Territory.

Each party has agreed that, for five years from the date of the Innovent Agreement, it will not, directly or indirectly, alone or with or for any other person or entity, clinically develop or commercialize any monoclonal antibody, bispecific antibody, or multispecific antibody targeting CD40L in the Licensed Territory. If, within three years from the date of the Innovent Agreement, Innovent or any of its affiliates initiates development of a bispecific or multispecific antibody targeting CD40L, in specified circumstances the Company has an option to include that antibody as a Licensed Compound under the Innovent Agreement subject to certain financial terms. If the Company declines to exercise that option, then Innovent is free to clinically develop and commercialize that antibody in the Licensed Territory.

The Company has agreed to use commercially reasonable efforts to develop, obtain regulatory approval for, and commercialize a Licensed Product in at least one indication in the United States and any one of France, Germany, Italy, Spain, the United Kingdom or Japan. The Company has agreed to file an IND application for a Licensed Product in the United States within 12 months from the date of the Innovent Agreement, subject to extensions for specified justifiable delays.

Under the terms of the Innovent Agreement, the Company is obligated to pay to Innovent a $35.0 million upfront payment, as well as aggregate milestone payments of up to approximately $1.05 billion upon the achievement of certain development, regulatory and commercial milestones.

The Company is also obligated to pay Innovent tiered royalties ranging from a high single-digit percentage to a mid-teen-digit percentage on annual net sales of all Licensed Products. The Company is obligated to pay royalties on a Licensed Product-by-Licensed Product and country-by-country basis until the latest of: (i) the expiration of the last valid claim of the licensed patents covering the composition of matter of the Licensed Compound contained in such Licensed Product in such country; (ii) the expiration of the last applicable regulatory exclusivity right with respect to such Licensed Product in such country; and (iii) 11 years following the first commercial sale of such Licensed Product in such country (each, a “Royalty Term”). The royalty rate is subject to specified reductions on a Licensed Product-by-Licensed Product and country-by-country basis under specified circumstances.

The Innovent Agreement contemplates that the Company will enter into ancillary arrangements with Innovent, including a clinical supply agreement and a pharmacovigilance agreement. The Company has also granted Innovent a right of first negotiation to be a secondary manufacturer of the Licensed Compounds and Licensed Products for commercialization in the Licensed Territory.

Unless earlier terminated, the Innovent Agreement will expire on the expiration of the last to expire Royalty Term. Unless the Innovent Agreement is earlier terminated, on expiration of each applicable Royalty Term, the Company will have a fully paid-up, irrevocable and perpetual license under the Licensed Intellectual Property to develop, manufacture and commercialize each applicable Licensed Product in the applicable country, which license will become non-exclusive after a specified point in time. Either party may terminate the Innovent Agreement for the other party’s uncured material breach, following a customary notice and cure period, or insolvency. Innovent may terminate the Innovent Agreement if the Company challenges the scope, validity, or enforceability of the licensed patents. The Company may terminate the Innovent Agreement for any reason upon 90 days’ written notice to Innovent prior to the first commercial sale of a Licensed Product, or upon 150 days’ written notice to Innovent after the first commercial sale of a Licensed Product. Additionally, if the Company has the right to terminate the Innovent Agreement for certain uncured material breaches by Innovent, the Company may, in lieu of termination, elect to keep the Innovent Agreement in force and reduce the Company’s subsequent payment obligations. Upon a termination of the Innovent Agreement prior to expiration of an applicable Royalty Term, the Company’s licenses from Innovent will terminate and Innovent will have the right to obtain a license under the Company Intellectual Property, with a right to sublicense, with respect to specified terminated products, which license will be either non-exclusive and fully paid-up or exclusive and subject to milestone and royalty obligations, at Innovent’s election.

Royalty Financing Transaction

On July 8, 2026 (the “Closing Date”), the Company, Spero Holdings SPV, LLC, a Delaware limited liability company and wholly owned subsidiary of the Company (“Holdings”), and Spero SPV, LLC, a Delaware limited liability company and wholly owned subsidiary of Holdings (“Issuer”), entered into the following agreements with affiliates of HCRx: (i) a non-recourse Note Purchase and Guaranty Agreement (the “NPA”), by and among Holdings, Issuer, the purchasers party thereto (the “NPA Purchasers”) and HCR Spero SPV, LLC, as the purchaser representative for such NPA Purchasers (the “NPA Purchaser Representative”), (ii) a Royalty and Milestone Payment Interest Purchase and Sale Agreement (the “RPA”), by and among Issuer, the purchasers party thereto (the “RPA Purchasers”), and HCR Spero SPV, LLC, as the purchaser representative for such RPA Purchasers (the “RPA Purchaser Representative”) and (iii) a Limited Recourse Guaranty (the “Limited Guaranty”), by the Company in favor of the NPA Purchaser Representative and RPA Purchaser Representative.

Under the terms of the NPA, Issuer sold to the NPA Purchasers, senior secured notes in an aggregate principal amount of $105,000,000 (the “Notes”), which Notes (i) were issued net of an original issue discount in the amount of $3,150,000, which, after giving effect to the RPA Purchase Price referenced below, reduces the aggregate $105,000,000 in gross proceeds of the NPA and RPA by 1.5%, (ii) accrue interest at 10% per annum, which interest, to the extent not paid in cash from available GSK Proceeds (as defined below) in accordance with the terms of the NPA, is capitalized and added to the principal amount of the Notes on a quarterly basis and (iii) mature nine years after the Closing Date. The Notes are secured by substantially all of Holdings’ and Issuer’s assets (collectively, the “Collateral”), which includes the equity interest of Issuer held by Holdings and rights to a portion of the future GSK Proceeds arising under the GSK Agreement, but excludes intellectual property as to which the grant of security interests is restricted. The Notes and other obligations under the NPA are generally payable solely from the GSK Proceeds, unless voluntarily prepaid at the option of Issuer prior to maturity with a premium or subject to certain mandatory prepayment triggers and foreclosure rights.

Under the terms of the RPA, Issuer sold to the RPA Purchasers 65% of the GSK Proceeds arising after the payment in full of the Notes (the “Purchased Proceeds”) in exchange for a cash payment of $1,575,000 (the “RPA Purchase Price”). Issuer’s obligations under the RPA are secured by a customary back-up lien in the Purchased Proceeds and proceeds thereof. The Company retains an interest in 35% of the GSK Proceeds arising after payment in full of the Notes.

In connection with the NPA and RPA, the Company will provide certain servicing, management and administrative functions on behalf of Holdings and Issuer.

Each of the NPA, RPA and Limited Guaranty contain certain customary terms and conditions, including representations and warranties, indemnities, affirmative and negative covenants, and events of default. Upon the occurrence and during the continuance of an event of default under the NPA, the NPA Purchaser Representative may declare the Notes and other obligations under the NPA to be immediately due and payable, and if such Notes and other obligations are not repaid, foreclose on the Collateral and use the proceeds thereof to repay the Notes, and, in limited circumstances, seek payment from the Company pursuant to the Limited Guaranty. The obligations of the Company in respect of the Limited Guaranty in circumstances where the Company is liable are, except in very limited circumstances, subject to a cap and limited to contract breaches, defaults and other liabilities customary for similarly structured royalty financing transactions.