OS Animal Health Announces Peer-Reviewed Publication of Clinical Trial Data for OST-HER2 Plus Palliative Radiation as Frontline Treatment in Canine Osteosarcoma

• 20% of OST-HER2 treated dogs survived 2-year vs. 1% in control group (p = 0.00995)

• Over 15,000 dogs per year diagnosed with osteosarcoma

• Company expects to submit U.S. Department of Agriculture meeting request to gain alignment on regulatory path towards conditional approval in the third quarter of 2026

• OS Therapies reiterates plan to distribute OS Animal Health shares to shareholders

• OS Animal Health plans crowdfunding to expand shareholder base to meet U.S. national exchange initial listing standards, with indications of interest now being received

Rockville, Maryland--(Newsfile Corp. - June 26, 2026) - OS Animal Health, Inc. ("OS Animal Health" or "the Company"), the wholly-owned animal health subsidiary of OS Therapies, Inc. (NYSE:OSTX) ("OS Therapies"), the world leader in gene-edited, Listeria-based cancer immunotherapies, today announced the peer-reviewed publication of data from the Phase 2 clinical trial of OST-HER2, together with palliative radiation therapy ("RT"), aimed at preventing amputation in frontline canine osteosarcoma. The median overall survival time in the OST-HER2 +RT treated dogs was 159 days vs. 124 days in the RT-alone control group (p = 0.0237). Survival at 2 years significantly favors OST-HER2+RT treated dogs when compared with RT-alone 0.00995. The Company intends to meet with the U.S. Department of Agriculture, the regulatory agency that governs the licensure of biologic drugs for companion animals, in the third quarter to align on a regulatory path to market for OST-HER2 in canines now that there is alignment with global regulators on manufacturing requirements for OST-HER2 in humans. A link to the publication can be found at: https://www.cell.com/molecular-therapy-family/oncology/fulltext/S2950-3299(26)00119-0.

Survival for OST-HER2+radiation vs. radiation-alone in frontline canine osteosarcoma

OS Therapies reiterates its plan to distribute OS Animal Health shares to OS Therapies shareholders following OS Animal Health's previously proposed 'go-public' transaction for which a Form S-1 offering was filed confidentially in the first quarter of 2026. The Company has been advised by potential underwriters to expand the Company's shareholder base while still private in preparation for the transaction. OS Animal Health plans to initiate a crowdfunding in the third quarter of 2026 to expand its shareholder base in order to meet the initial listing standards for a U.S. national stock exchange. Indications of interest from potential investors in the proposed crowdfunding are being received at: https://www.mrcrowd.com/ttw/osah

"The data from this pilot study is very exciting and represents the first evidence that it may be possible for some dogs to avoid amputation and survive with adequate limb function and good quality of life beyond what is expected with palliative radiation alone," said Dr. Nicola J. Mason, the Paul A. James and Charles A. Gilmore Endowed Chair Professor and Professor of Medicine and Pathobiology at the University of Pennsylvania School of Veterinary Medicine and lead author of the study. "Although many dogs do extremely well physically with amputation, there is often a reluctance to amputate especially in large breeds given their size and the almost inevitable development of metastases and associated poor prognosis. While small, this study shows that a subset of immunotherapy treated dogs can control their primary disease, and experience delay in development of metastases without amputation and opens the door to a potential change in the standard of care (SOC) in the treatment of osteosarcoma. We believe that recent data showing improved antigen presentation and anti-cancer cellular immune activity in treated dogs and humans, combined with clinical data showing increased survival in various disease settings, support the concept that OST-HER2 may represent a breakthrough treatment for all stages of osteosarcoma. We intend to continue to study OST-HER2 alone, as well as in combination with potential complementary therapies, to find the best solution for canine patients dealing with this deadly disease in the hope that future findings may be translated into further human clinical development."

Over 15,000 dogs per year are diagnosed with osteosarcoma in the United States. Despite the introduction of amputation, chemotherapy and radiation therapy into the treatment paradigm, prognosis remains extremely poor. Comparative Oncology, the field in which naturally-occurring cancers in animals are studied to identify whether progress could be translated to human cancers, has identified canine osteosarcoma as the most promising area where translation of advancements in dogs may be possible to humans due to high genetic homology of the cancers, shared environment, congruent immune system responses and near-identical clinical disease-course.

"The field of canine oncology treatment is gaining significant momentum as the large animal health companies begin to launch molecularly-targeted therapies for dogs," said Edward Robb, DVT, President of OS Animal Health. "OST-HER2 is the anchor product candidate from the listeria platform that can potentially begin to make a near-term commercial impact in osteosarcoma. However, the opportunity for OS Animal Health is much broader than that. With the advent of molecularly targeted therapies gaining traction in animal health, the idea that OST-HER2 could become the standard for all HER2 positive canine cancers is something we will begin to look at closely. If that translates into clinical success, we could then advance listeria product candidates from that platform that target other cancer antigens. We believe this platform is ideally suited as the 'go-to' molecularly targeted immunotherapy for veterinarians."

About OS Animal Health

OS Animal Health (OSAH) is a wholly owned subsidiary of OS Therapies, Inc. (NYSE:OSTX) dedicated to improving outcomes for pets with cancer. The Company is focused on developing and commercializing OST-HER2, an off-the-shelf immunotherapy candidate for canine osteosarcoma that has previously received conditional approval from the USDA for the treatment of this aggressive bone cancer in dogs. Operating with a separate, animal-health-focused strategy and capital structure, OS Animal Health is working to re-establish USDA conditional approval and bring innovative immunotherapies to the veterinary oncology market, with the goal of extending and improving the lives of dogs living with osteosarcoma, and animals living with other HER2 positive cancers.

About Our Intended Offering

OS Animal Health is "testing the waters" to gauge investor interest in an offering under Regulation Crowdfunding. No money or other consideration is being solicited - if sent, it will not be accepted. No offer to buy securities will be accepted, and no part of the purchase price will be received until a Form C offering statement is filed with the SEC and through an approved intermediary platform. Any indication of interest involves no obligation or commitment of any kind.

About OS Therapies

OS Therapies is a clinical stage oncology company focused on the identification, development, and commercialization of treatments for Osteosarcoma (OS) and other solid tumors. The Company is the world leader in gene-edited, Listeria-based cancer immunotherapies. OST-HER2, the Company's lead asset, is an immunotherapy leveraging the immune-stimulatory effects of Listeria bacteria to initiate a strong immune response targeting the HER2 protein. OST-HER2 is designed to target two mutated extracellular epitopes and one mutated intracellular epitope of the HER2 oncogene, requiring only one of these three epitopes to be present in a tumor (or micro-metastasis) to trigger the desired immune response. OST-HER2 has received Orphan Drug Designation (ODD), Fast Track Designation (FTD) and Rare Pediatric Disease Designation (RPDD) from the U.S. Food & Drug Administration and has received ODD, FTD and ATMP from the European Medicines Agency.

The Company reported positive data in its Phase 2b clinical trial of OST-HER2 in recurrent, fully resected, lung metastatic osteosarcoma, demonstrating clinically significant benefit in the 12-month event free survival (EFS) primary endpoint of the study and the overall survival (OS) secondary endpoint. The Company is seeking a Biologics License Application (BLA) from the U.S. FDA for OST-HER2 in osteosarcoma in 2026 and, if approved, would become eligible to receive a Priority Review Voucher that it could then sell. The Company also anticipates receiving Conditional Marketing Authorisation Applications from the U.K.'s Medicines and Healthcare products Regulatory Agency and the EMA for OST-HER2 in 2026. OST-HER2 has completed a Phase 1 clinical study primarily in breast cancer patients, in addition to showing preclinical efficacy data in various models of breast cancer. OST-HER2 has been conditionally approved by the U.S. Department of Agriculture for the treatment of canines with osteosarcoma.

In addition, OS Therapies is advancing its next-generation Antibody Drug Conjugate (ADC) and Drug Conjugates (DC), known as tunable ADC (tADC), which features tunable, tailored antibody-linker-payload candidates. This platform leverages the Company's proprietary silicone Si-Linker and Conditionally Active Payload (CAP) technology, enabling the delivery of multiple payloads per linker. For more information, please visit www.ostherapies.com.

Forward-Looking Statements

Statements in this press release about future expectations, plans and prospects, as well as any other statements regarding matters that are not historical facts, may constitute forward-looking statements within the meaning of the federal securities laws. These forward-looking statements and terms such as "anticipate," "expect," "intend," "may," "will," "should" or other comparable terms involve risks and uncertainties because they relate to events and depend on circumstances that will occur in the future. Those statements include statements regarding the intent, belief or current expectations of OS Therapies and members of its management, as well as the assumptions on which such statements are based. OS Therapies cautions readers that forward-looking statements are based on management's expectations and assumptions as of the date of this press release and are subject to certain risks and uncertainties that could cause actual results to differ materially, including, but not limited to the potential approval of OST-HER2 by the U.S. FDA, licensure by the U.S. Department of Agriculture and other risks and uncertainties described in "Risk Factors" in the Company's most recent Annual Report on Form 10-K and other subsequent documents the Company files with the Securities and Exchange Commission. The proposal for a future distribution of OS Animal Health shares and national market listing are preliminary and not assured. Any forward-looking statements contained in this press release speak only as of the date hereof, and, except as required by the federal securities laws, OS Therapies specifically disclaims any obligation to update any forward-looking statement, whether as a result of new information, future events or otherwise.

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