OST-HER2 (AOST-2121)

June 8, 2026: OS Therapies, Inc. (NYSE: OSTX) said it aligned with the FDA and U.K. MHRA on the design of a planned confirmatory Phase 3 trial of OST-HER2 to prevent or delay recurrence in fully resected pulmonary metastatic osteosarcoma. The company said the Phase 3 must be initiated before any U.S. Accelerated Approval BLA grant and before Conditional Marketing Authorisation Applications in the U.K., Europe and Australia.

The company expects the Phase 3 to commence late in Q3 2026 in Australia after submitting under the TGA Clinical Trial Notification scheme, with the intent to be eligible for regulatory decisions in Q4 (year-end) and patient access in 2027. TGA and MHRA agreed OST-HER2 Phase 3 can be initiated using remaining Phase 2b drug product. FDA, MHRA, EMA and TGA aligned on the CMC plan and potency assays; regulators also accepted a patent-pending pharmacodynamic biomarker signature as a surrogate clinical efficacy endpoint for Phase 3.

OS Therapies said the EMA has begun rolling review, and it expects U.S. decisions on rolling review and RMAT designation after an upcoming Type B pre-BLA meeting. The company has begun filing a U.S. BLA in 2026; it noted OST-HER2 previously showed positive Phase 2b results on the 12-month EFS primary endpoint and OS secondary endpoint.