OS Therapies Files New Patent Application Covering Biomarkers of the Immune Response to Listeria Monocytogenes

"The biomarker data from patients who responded to OST-HER2 treatment shows tremendous innate and adaptive immune responses that result in a significantly improved ability to fight cancer," said Paul Romness, MPH, Chair & CEO of OS Therapies. "While not all metastatic osteosarcoma is HER2 positive, a recently published study reported that over 80% of the patients were found to have HER2 positive metastatic lesions^[1]. In our Phase 2b trial, 100% of the patients that did achieve 1-year event free survival (EFS) exhibited the distinct immune signature covered in our patent application and went of to achieve 2-year overall survival. We believe this meets the pharmacodynamic/response biomarker criteria outlined in the U.S. Food & Drug Administration's (FDA)'s Biomarkers, EndpointS, and other Tools (BEST)^[2] glossary as a surrogate clinical endpoint to support a Biologics License Application under the Accelerated Approval Program^[3]. This pharmacodynamic/response biomarker also provides us with a strong basis to think about additional clinical applications of OST-HER2 beyond osteosarcoma, such as in HER2 positive breast cancer and bladder cancer.

Mr. Romness continued, "Notwithstanding the potential for future indication expansion, we are now laser focused on our packed Q2-2026 regulatory meeting schedule with FDA, EMA, MHRA and TGA in preparation for gaining accelerated market access for OST-HER2 in the fully-resected pulmonary metastatic setting later this year in the U.S., Europe and the U.K. with the potential to expand beyond these jurisdictions in 2027."

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 ATMP from the EMA. Under the RPDD program, if the Company receives a Biologics License Application (BLA) in the United States, it will become eligible to receive a Priority Review Voucher (PRV) that it intends to sell. The most recent publicly disclosed PRV transaction occurred in February 2026 at a reported value of $205 million. The Company is seeking to obtain a BLA under the Accelerated Approval Program for OST-HER2 in osteosarcoma in the second half of 2026.

About OS Therapies

OS Therapies is a clinical stage oncology company focused on the identification, development, and commercialization of treatments for Osteosarcoma (OS) and other solid tumors. The Company is the world leader in listeria-based cancer immunotherapies. OST-HER2, the Company's lead asset, is an immunotherapy leveraging the immune-stimulatory effects of Listeria bacteria to initiate a strong immune response targeting the HER2 protein. OST-HER2 has received Orphan Drug Designation (ODD), Fast Track Designation (FTD) and Rare Pediatric Disease Designation (RPDD) from the U.S. Food & Drug Administration and has received ODD, FTD and ATMP from the European Medicines Agency. The Company reported positive data in its Phase 2b clinical trial of OST-HER2 in recurrent, fully resected, lung metastatic osteosarcoma, demonstrating statistically significant benefit in the 12-month event free survival (EFS) primary endpoint of the study and the overall survival (OS) secondary endpoint. The Company anticipates receiving a Biologics License Application (BLA) from the U.S. FDA for OST-HER2 in osteosarcoma in 2026 and, if approved, would become eligible to receive a Priority Review Voucher that it could then sell. The Company also anticipates receiving Conditional Marketing Authorisations from the U.K.'s Medicines and Healthcare products Regulatory Agency and the EMA for OST-HER2 in 2026. OST-HER2 has completed a Phase 1 clinical study primarily in breast cancer patients, in addition to showing preclinical efficacy data in various models of breast cancer. OST-HER2 has been conditionally approved by the U.S. Department of Agriculture for the treatment of canines with osteosarcoma. The Company also anticipates reading out data from a Phase 1b study of OST-504 in castration resistant prostate cancer in the first half of 2026.

In addition, OS Therapies is 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 silicone Si-Linker and Conditionally Active Payload (CAP) technology, enabling the delivery of multiple payloads per linker. For more information, please visit www.ostherapies.com.

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