OS Therapies Secures Alignment for Phase 3 Trial of OST-HER2

OS Therapies, Inc. (NYSE American: OSTX), the world leader in gene-edited, Listeria-based cancer immunotherapies, has made significant progress in its regulatory efforts for the treatment of metastatic osteosarcoma. The company has secured alignment with the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) on the design of a pending Phase 3 study for its lead asset, OST-HER2, which is aimed at preventing or delaying the recurrence of fully resected, pulmonary metastatic osteosarcoma.

According to the company, the recent Type C Statistical Methods Meeting with the FDA resulted in full alignment, paving the way for the initiation of a confirmatory Phase 3 trial in the United Kingdom (U.K.) with the existing Phase 2 drug product. This alignment is expected to trigger the submission of a Conditional Marketing Authorisation Application (CMAA) in the coming weeks, with the Phase 3 trial set to initiate in mid-Q4/'26 following the requalification of the material used in the Phase 2b study.

The U.K.'s Medicines and Healthcare products Regulatory Agency (MHRA) has taken the lead in the international regulatory review process for OST-HER2 under Project Orbis, an initiative started in May 2019 to provide a framework for concurrent submission and review of oncology products among international partners. Since 2021, 29 MHRA-led oncology products have been approved by the FDA via Project Orbis, underscoring the U.K.'s role in facilitating early market access.

MHRA has granted OS Therapies access to the U.K. national patient registry, the UK Clinical Practice Research Datalink (CPRD), to strengthen comparator data during the Phase 3 trial. This access is expected to further improve the interpretability of data during the early market access phase. The company continues to engage with the U.S. FDA on the non-Orbis Accelerated Approval route, with the FDA aligning on the proposed adaptive design of the pending Phase 3 study.

In addition to the regulatory progress, OS Therapies has completed the resubmission of a Regenerative Medicine Advanced Therapy (RMAT) request and received a Commissioner's National Priority Review Voucher (CNPV) letter of intent from the FDA. The company is also seeking a Conditional Marketing Authorization Application from MHRA in the U.K. under Project Orbis for OST-HER2 in metastatic osteosarcoma in the fourth quarter of 2026, and plans to obtain a Biologics License Application (BLA) under the Accelerated Approval Program in the U.S. immediately thereafter.

The company reported positive data in its Phase 2b clinical trial of OST-HER2, demonstrating clinically significant benefit in the 12-month event-free survival (EFS) primary endpoint and the overall survival (OS) secondary endpoint. The trial results have been pivotal in advancing the company's efforts to secure regulatory approval for OST-HER2.

OS Therapies is also 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 silicon Si-Linker and Conditionally Active Payload (CAP) technology, enabling the delivery of multiple payloads per linker.

The company's lead asset, OST-HER2, has received Orphan Drug Designation (ODD), Fast Track Designation (FTD), and Rare Pediatric Disease Designation (RPDD) from the FDA. It has also received ODD, FTD, and Advanced Therapy Medicinal Products (ATMP) designations from the EMA and MHRA. The company anticipates receiving Conditional Marketing Authorisation Applications from the U.K. and the EMA for OST-HER2 in 2026.

In other developments, OS Therapies has completed dosing in a Phase 1 study of OST-504 for castration-resistant prostate cancer. The company has also reported that a recent PRV sale occurred in August 2026 for $220 million. However, there can be no assurance that the company would realize a comparable value, if any, in connection with any future PRV sale.

Overall, OS Therapies is making significant strides in its regulatory and clinical development efforts, with a focus on advancing its lead asset, OST-HER2, for the treatment of metastatic osteosarcoma.