OS Therapies Gains Full MHRA and EMA Alignment on Conditional Marketing Applications for OST-HER2 in Osteosarcoma
核心洞察
OS Therapies achieved full alignment with the UK MHRA and EU EMA on its conditional marketing authorisation applications for OST-HER2 in resected pulmonary metastatic osteosarcoma (搜索).
Regulators accepted historical-control overall survival data from peer-reviewed literature as appropriate for the orphan indication, supporting the pending submissions.
The company expects to complete the UK CMAA submission within weeks and to begin the required confirmatory Phase 3 trial in the fourth quarter of 2026.
OS Therapies, Inc. (NYSE American: OSTX) announced it has achieved full alignment with the U.K. Medicines and Healthcare products Regulatory Agency (搜索) (MHRA) on its pending Conditional Marketing Authorisation Application (CMAA) for OST-HER2 in the orphan indication of prevention or delay of recurrence in fully resected, pulmonary metastatic osteosarcoma (搜索). The agreement covers both the use of historical control data and the design of the confirmatory Phase 3 trial required before a CMAA can be granted.
According to the company, the MHRA agreed that comparable historical control data — derived from a systematic evaluation of all suitable available peer-reviewed literature by independent biostatistics advisors and presented at its most recent Scientific Advice Meeting (SAM) — was appropriate for the application. The MHRA's supportive advice aligns with the agency's recently published Rare Disease Regulatory Framework, as well as advice from the European Medicines Agency (搜索) (EMA) pertaining to a forthcoming CMAA via the pan-European Centralised Procedure. The EMA also recently informed the company that a previously requested Scientific Advice Working Party (SAWP) meeting was no longer required prior to CMAA submission.
Confirmatory Phase 3 Design and Submission Timeline
Full alignment was also reached on the proposed design of the confirmatory Phase 3 trial, including the proportion of patients to be dosed with remaining Phase 2 drug product versus forthcoming Phase 3 material. The Phase 3 trial must have commenced prior to the granting of a CMAA. OS Therapies expects to complete the CMAA submission in the coming weeks and intends to commence the confirmatory Phase 3 as soon as possible in the fourth quarter of 2026, with site outreach already underway.
"With this full alignment now in hand, and with it the recognition from international regulators that the standard of care in Osteosarcoma (搜索) has not meaningfully changed in the last forty years, we are bringing this to the U.S. Food & Drug Administration (FDA)," said Paul Romness, MPH, Chairman & CEO of OS Therapies. He described the alignment with MHRA and EMA as "a major achievement" that paves the way for the upcoming CMAA submissions and the initiation of the confirmatory Phase 3 trial.
U.S. Regulatory Pathway and Designations
In the United States, the FDA is considering an ongoing Biologics License Application (BLA) submission that began in January 2026, with potential early market access under the Accelerated Approval Program. The FDA participates in the Orbis Project, which coordinates approvals between international regulators including the U.K. OS Therapies intends to request Rolling Review of the BLA following its upcoming mid-September 2026 FDA Type C Statistical Methods Meeting.
OST-HER2 has received Orphan Drug Designation (ODD), Fast Track Designation (FTD) and Rare Pediatric Disease Designation (RPDD) from the FDA, and ODD, FTD and Advanced Therapy Medicinal Products (ATMP — the RMAT equivalent in the EU and UK) status from the EMA and MHRA. Under the RPDD program, if the company is granted a BLA in the United States, it would become eligible to receive a Priority Review Voucher (PRV) that it intends to sell. The most recent PRV sale occurred in August 2026 for $220 million, though the company cautioned there can be no assurance it would realize comparable value in any future sale. OS Therapies has completed resubmission of a Regenerative Medicine Advanced Therapy (RMAT) request, and its Commissioner's National Priority Review Voucher (CNPV) letter of intent has been accepted by the FDA.
The company is seeking a BLA under the Accelerated Approval Program in the U.S. and CMAA in Europe, the U.K. and Australia for OST-HER2 in metastatic osteosarcoma (搜索) in the fourth quarter of 2026.
Mechanism and Phase 2b Data
OST-HER2 is an immunotherapy that leverages the immune-stimulatory effects of Listeria bacteria to initiate a strong immune response targeting the HER2 (搜索) protein. It 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.
The company reported positive data from its Phase 2b clinical trial of OST-HER2 in the prevention or delay of recurrence in fully resected, pulmonary metastatic osteosarcoma (搜索), demonstrating clinically significant benefit in the 12-month event-free survival (EFS) primary endpoint and the overall survival (OS) secondary endpoint. OST-HER2 has also completed a Phase 1 clinical study primarily in breast cancer patients and has shown preclinical efficacy data in various models of breast cancer. It was previously conditionally approved by the U.S. Department of Agriculture for the treatment of canines with osteosarcoma.
Broader Pipeline
Beyond OST-HER2, OS Therapies has completed dosing in a Phase 1 study of OST-504 (搜索) for castration-resistant prostate cancer (搜索). The company is also advancing a next-generation antibody-drug conjugate (ADC) and drug conjugate platform known as tunable ADC (tADC), featuring tunable, tailored antibody-linker-payload candidates. The platform leverages the company's proprietary silicon Si-Linker and Conditionally Active Payload (CAP) technology, enabling the delivery of multiple payloads per linker.