OST-HER2 (AOST-2121)

September 21, 2026: OS Therapies (NYSE: OSTX) said the U.K. MHRA will lead early market access regulatory coordination for OST-HER2 under Project Orbis for prevention or delay of recurrence in fully resected, pulmonary metastatic osteosarcoma. Following a recent MHRA Scientific Advice Meeting, the company said it has alignment to start a confirmatory Phase 3 trial in the U.K. using requalified Phase 2b drug product, with initiation planned for mid-Q4 2026, and expects to submit a U.K. Conditional Marketing Authorisation Application (CMAA) in the coming weeks/Q4 2026.

The company also said FDA aligned on the amended adaptive Phase 3 design at a Type C Statistical Methods Meeting, supporting a non-Orbis Accelerated Approval pathway. OS Therapies said FDA/EMA would allow Phase 3 expansion in their jurisdictions once Phase 3-ready/commercial-ready product is available, expected in 2027.

For data support, MHRA granted access to the U.K. Clinical Practice Research Datalink/national patient registry to strengthen comparator data in the OST-400 natural history database while Phase 3 is ongoing. The company said ongoing BLA filing completion is pending in Q4 2026 and it awaits FDA decisions on RMAT, Rolling Review, and CNPV. It reiterated prior positive Phase 2b results, stating OST-HER2 showed clinically significant benefit on the 12-month EFS primary endpoint and OS secondary endpoint.