OSE Immunotherapeutics to Implement a Flexible Bridge Equity Financing

• First step of a global financing strategy, as the Company is actively engaging with institutional investors in the United States and in Europe
• Gross proceeds could amount to €19.3 million based on current share price¹ over a 24-month period
• Cash runway to be extended to end of December 2026
• Subsequent filing of the 2025 Universal Registration Document now scheduled on June 3, 2026, to include this financing

NANTES, France, May 28, 2026 – 9:00pm CET - OSE Immunotherapeutics SA (ISIN: FR0012127173; Mnemo: OSE) (the "Company" or “OSE”), today announces the signature of a term sheet to implement a flexible bridge equity financing with IRIS Capital Investment (“IRIS”), known as SmartATM®. Under this financing, which is expected to be signed in the coming days, IRIS would commit to subscribe up to 4,000,000 new ordinary shares of the Company (ie. approximately 17.0% of OSE’s current share capital), under certain conditions, at a 5% discount over a period of up to 24 months.

“Since the change in governance last October, we have designed a new 3-year strategic plan, more pragmatic, more realistic and in line with shareholders’ expectations. We have selected chronic pouchitis as a new rare indication for lusvertikimab, while we develop a new subcutaneous formulation to move forward with our clinical development in ulcerative colitis. In parallel, we are moving very nicely with the recruitment of Tedopi’s Phase 3 trial Artemia in non-small cell lung cancer”, commented Marc Le Bozec, Chief Executive Officer of OSE Immunotherapeutics. “We have been actively engaging with institutional investors for the past few months, with the ambition to finance this 3-year strategic plan with the support of long-term Tier-1 specialized investors. This bridge financing is the first step towards this ambition.”

Terms and Conditions of the Bridge Equity Financing with IRIS (the “Financing”)

The Financing would be structured via the issuance of warrants giving the right to subscribe new ordinary shares of the Company (the “Warrants”). The Board of Directors of OSE Immunotherapeutics, using the delegation granted to it under the 21st resolution of the shareholders' Meeting held on September 30, 2025, would proceed with the issuance, without preferential subscription rights for shareholders, of 4,000,000 Warrants to IRIS.

Each Warrant would allow IRIS to subscribe one new ordinary share of the Company, at its own initiative and subject to the conditions set out in the appendix to this press release, at an exercise price equal to 95% of the lowest daily Volume-Weighted Average Price (VWAP) observed in the 3 trading days immediately preceding the exercise date.

1 Daily Volume-Weighted Average Price on May 27, 2026, ie. €4.8297. This amount is not guaranteed, as it depends, among others, on the share price.

The main terms and conditions of the Warrants are described in the appendix to this press release.

An upfront payment of €2 million would be made by IRIS upon the signing of the Financing.

Based on current share price, and assuming that the Company elects to use all of the Financing, the gross proceeds would amount to approximately €19.3 million (including the upfront payment), although this amount is not guaranteed.

The Company would retain total control over the use of the Financing and can at any time, after the €2 million upfront payment has been fully set off against the exercise price of the Warrants (or otherwise reimbursed to IRIS), limit the number of Warrants to be exercised upon a certain period of time, modify the exercise floor price, pause and/or terminate it, at no cost, to adapt to market conditions and cash requirements.

In addition, customary events of default would give IRIS an option to terminate the Financing. Although, there would be no financial covenant.

The issuance of the Warrants would be subject neither to a prospectus requiring an approval from the French financial market authority (Autorité des Marchés Financiers, or the “AMF”), nor to the publication of an information document to be filed with the AMF.

The Company would regularly publish on its website the number of new shares issued pursuant to the exercise of the Warrants.

Cash Runway and Financing Strategy

To date, the Company does not have sufficient net working capital to meet its obligations over the next 12 months.

The Company’s cash and cash equivalents totaled €17.0 million as of March 31, 2026. After the completion of the Financing and assuming related gross proceeds of €19.3 million over a 24-month period, and based on its current plans, the Company estimates that its cash and cash equivalents will be sufficient to fund its operations until the end of December 2026. This cash runway does not include any potential future milestone payments from existing partnerships.

The Financing is the first step of a global financing strategy aiming at securing OSE’s 3-year Strategic Plan and the Company is actively engaging with institutional investors in the United States and in Europe.

To further extend its runway beyond 2026, the Company continues to evaluate several complementary options, including a potential new strategic partnership involving one of its proprietary assets, equity offering to institutional investors, restructuring of its existing debt, and potential milestone payments from current partnerships.

Although the Company is confident in its ability to meet its short-term financing objectives, there is no guarantee that it will be able to obtain the necessary financing to meet its needs or to obtain funds at attractive terms and conditions to finance all of its activities on a 12-month horizon.

The Company has deferred the publication of its audited consolidated financial statements and its Universal Registration Document for the fiscal year 2025 to June 3, 2026, after market close and signing of the Financing. Certification of the 2025 consolidated financial statements by the Company's auditors would take place before the filing of the Universal Registration Document with the AMF on June 3, 2026. Statutory auditors are expected to include a going concern qualification in their certification report.

ABOUT OSE IMMUNOTHERAPEUTICS

OSE Immunotherapeutics is a biotech company dedicated to developing first-in-class assets in immuno-oncology (IO) and immuno￾inflammation (I&I) that address the unmet patient needs of today and tomorrow. We partner with leading academic institutions and biopharmaceutical companies in our efforts to develop and bring to the market transformative medicines for people with serious diseases. OSE Immunotherapeutics is based between Nantes and Paris and is listed on Euronext. Additional information about OSE Immunotherapeutics assets is available on the Company’s website: www.ose-immuno.com. Follow us on LinkedIn.

Contacts

OSE Immunotherapeutics: investors@ose-immuno.com

FP2COM (Media Relations): Florence Portejoie: fportejoie@fp2com.fr I +33 6 07 768 283

Disclaimer

The Company’s independent auditors have not yet audited nor have they expressed any opinion or any other form of assurance on these unaudited consolidated financial results, in particular OSE Immunotherapeutics has not yet obtained assurance from its auditors that the financial statements will be certified without qualification. The audit procedures by the statutory auditors of the Company are in progress.

Forward-looking statements

This press release contains express or implied information and statements that might be deemed forward-looking information and statements in respect of OSE Immunotherapeutics. They do not constitute historical facts. These information and statements include financial projections that are based upon certain assumptions and assessments made by OSE Immunotherapeutics’ management considering its experience and its perception of historical trends, current economic and industry conditions, expected future developments and other factors they believe to be appropriate.

These forward-looking statements include statements typically using conditional and containing verbs such as “expect”, “anticipate”, “believe”, “target”, “plan”, or “estimate”, their declensions and conjugations and words of similar import. Although the OSE Immunotherapeutics management believes that the forward-looking statements and information are reasonable, the OSE Immunotherapeutics’ shareholders and other investors are cautioned that the completion of such expectations is by nature subject to various risks, known or not, and uncertainties which are difficult to predict and generally beyond the control of OSE Immunotherapeutics. These risks could cause actual results and developments to differ materially from those expressed in or implied or projected by the forward￾looking statements. These risks include those discussed or identified in the public filings made by OSE Immunotherapeutics with the AMF. Such forward-looking statements are not guarantees of future performance. This press release includes only summary information and should be read with the OSE Immunotherapeutics Universal Registration Document filed with the AMF on April 30, 2025, including the annual financial report for the fiscal year 2024, available on the OSE Immunotherapeutics’ website. Other than as required by applicable law, OSE Immunotherapeutics issues this press release at the date hereof and does not undertake any obligation to update or revise the forward-looking information or statements.

Furthermore, the AMF recommends companies that implement equity lines or other staggered capital increases to, in particular, include the following standard warning:

The Company is implementing an equity financing line in the form of warrants to subscribe new ordinary shares with IRIS. After receiving the shares issued upon exercise of the Warrants, IRIS does not intend to remain a shareholder of the Company. The shares resulting from the exercise of the Warrants will generally be sold on the market at short notice, which can exert significant downward pressure on the share price. Shareholders may suffer a loss of their invested capital due to a significant decrease in the value of the Company's shares, as well as significant dilution due to the large number of shares issued to IRIS. Investors are advised to exercise great caution before deciding to invest in the Company’s securities. The Company reminds the investors that the Financing would not be the first equity line implemented by the Company. The Investors are also invited to review the risk factors relating to this transaction, as set out in this press release.

Key terms and conditions of the Financing

Main characteristics of the Warrants

General principles of the financing line

Notes

1. Daily Volume-Weighted Average Price on May 27, 2026, ie. €4.8297. This amount is not guaranteed, as it depends, among others, on the share price.