OS Therapies Expects Phase 3 Study For The OST-HER2 Metastatic Osteosarcoma Program To Begin Near End Of Q3 2026

OS Therapies, Inc. (NYSE: OSTX ) ("OS Therapies" or "the Company"), the world leader in gene-edited, Listeria-based cancer immunotherapies, today provides a third quarter 2026 corporate outlook.

The outlook focuses on upcoming clinical and regulatory milestones for the Company's lead program OST-HER2 in the prevention or delay of recurrence in fully resected, pulmonary metastatic osteosarcoma (the "Metastatic Osteosarcoma Program"), advancements with the Company's animal health-focused subsidiary OS Animal Health, Inc. (OSAH) and the Company's financial position as key regulatory and corporate milestones come into focus.

OST-HER2 Metastatic Osteosarcoma Program OS Therapies recently announced full alignment with the European Medicines Agency (EMA), the U.K.'s Medicines and Healthcare products Regulatory Agency (MHRA) and Australia's Therapeutic Goods Administration (TGA) with respect to the necessary requirements to be granted Conditional Marketing Authorisation Applications (CMAAs).

The key outstanding milestones are 3-year overall survival data, which EMA, MHRA and TGA have aligned around to support early market access, and the Pediatric Investigation Plans (PIP) for each jurisdiction.

Prior to being granted CMAAs in Europe, the U.K. or Australia, the Company must have commenced a confirmatory Phase 3 clinical study.

This Phase 3 study is expected to initiate towards the end of the third quarter of 2026, initially in Australia only and after early market access is decided upon then in the U.K., Europe and the United States.

MHRA and TGA have both agreed to allow the Company to initiate the Phase 3 study with the same OST-HER2 drug product batch that was used in the Phase 2b trial and is currently being administered to Expanded Access patients in the United States.

Full alignment has been reached with EMA, MHRA and TGA in the following areas: Non-Clinical Chemistry, Manufacturing and Controls (CMC) Clinical Safety Confirmatory Phase 3 protocol design Historical Control Comparator Initiation of the confirmatory Phase 3 protocol with existing Phase 2 drug product OS Therapies was granted Advanced Therapy Medicinal Products (ATMP) designation in Europe and the U.K., which permitted access to additional resources from local regulators to support CMAA submissions.

The Company is making significant progress towards alignment on the PIP – including a first round of positive feedback from the EMA PDCO - and has received a submission window of September 30, 2026 from EMA for the CMAA.

With MHRA and TGA, the Company is in ongoing dialogue to identify the earliest possible date to complete CMAA submissions and is making significant progress towards PIPs. "Given that 100% of patients who achieved 1-year event free survival (EFS) also achieved overall survival at the 2-year and 2.5-year timepoints, and the knowledge that 100% of patients who did not achieve 1-year EFS while achieving 2-year overall survival also achieved 2.5-year overall survival, we believe the sustained clinical benefit data package emerging for OST-HER2 will be compelling for regulators in comparison with the bleak 3-year overall survival outlook metastatic osteosarcoma patients have," said Dr.

Craig Eagle, Chief Medical Advisor and member of the OS Therapies Board of Directors. "While the 1-year EFS data favored OST-HER2-treated patients in a statistically significant manner when compared with historical control data, the biomarker data was highly correlated with 2.5 year overall survival." As a result of the initiation of the CMAA process with EMA, coordination has begun between EMA and the U.S.

Food & Drug Administration (FDA) with respect to the OST-HER2 regulatory dossier.

OS Therapies completed the submission to FDA of the non-clinical and clinical Biologics License Application (BLA) modules in January 2026, and has since completed a Type C Phase 3 protocol design meeting where alignment was achieved on the design of the Phase 3 protocol, as well as alignment on outstanding non-clinical, CMC and clinical safety matters.

The Company recently released 2.5-year overall survival clinical efficacy data and is awaiting the date for the requested Type B Statistical Methods meeting being scheduled to align on the methods to evaluate clinical efficacy for the 2.5-year timepoint, the use of historical control and/or OST-400 as appropriate comparator(s) to interpret clinical efficacy, Rolling Review and Regenerative Medicine Advanced Therapy (RMAT) designation.

OS Therapies expects this meeting to set the stage for 3-year overall survival data to be used to support the complete BLA submission.

Following that Type B Statistical Methods meeting, the Company expects to hold its previously announced invited Type B Pre-BLA meeting in early Fall of 2026, following the completion of the remaining aspects of the BLA submission, to review the entire BLA dossier in an effort to be granted early market access under Accelerated Approval in the U.S..

FDA accepted the Company's letter of intent for the Commissioner's National Priority Review Voucher (CNPV) program in January 2026.

OSAH Clinical, Regulatory and Corporate Progress The Company recently announced the peer-reviewed publication of new data showing OST-HER2 combined with radiation significantly improved survival in frontline canine osteosarcoma when compared against radiation alone.

This adds to previously published data showing that OST-HER2 was effective in treating fully resected metastatic osteosarcoma, as well as unresectable metastatic osteosarcoma when compared with the current standards of care.

Canine osteosarcoma data is what originally catalyzed the OST-HER2 human clinical development program.

Following CMC alignment with FDA, EMA, MHRA and TGA, the Company is now in a strong position to review with the U.S.

Department of Agriculture (USDA) its regulatory strategy to bring OST-HER2 back into the market to treat canine osteosarcoma.

The Company expects to hold a meeting with USD