OS Therapies Announces Statistically Significant Positive Final 2-Year Overall Survival Data from Phase 2b Trial of OST-HER2 in the Prevention or Delay of Recurrent, Fully-Resected, Pulmonary Metastatic Osteosarcoma

Rhea-AI Summary

OS Therapies (NYSE American: OSTX) reported statistically significant final 2-year overall survival (OS) data from its Phase 2b OST-HER2 trial in prevention/delay of recurrent, fully-resected pulmonary metastatic osteosarcoma. 75% (27/36) of treated patients achieved 2-year OS versus 40% historical control (p < 0.0001); 5 patients were lost to follow-up. 100% of patients with 12-month event-free survival reached 2-year OS. Biomarker correlation data are expected in November 2025. Regulatory filings targeted: MHRA MAA Dec 2025, FDA BLA Jan 2026, EMA MAA Q1 2026. Investor call set for Oct 13, 2025.

Positive

• 2-year overall survival 75% versus 40% historical (p<0.0001)
• 27 evaluable patients achieved 2-year OS out of 36
• 100% of 12-month EFS patients reached 2-year OS
• Planned MHRA MAA filing in December 2025
• Planned FDA BLA filing under Accelerated Approval in January 2026

Negative

• Non-randomized study using historical control
• Small evaluable cohort of 36 patients
• Five patients were lost to follow-up
• Biomarker correlation data pending November 2025

Insights

Clinical Development & Regulatory Strategist

Phase 2b shows statistically significant 2-year overall survival benefit; regulatory filings planned within months.

OS Therapies reports that OST-HER2 produced 2-year overall survival of 75% (27/36 evaluable) versus 40% in a historical control (p < 0.0001), and that all patients reaching 12‑month Event Free Survival subsequently reached 2-year overall survival. The dataset is non-randomized and includes five patients lost to follow-up; safety is described as "safe and well-tolerated."

The company states it held productive regulatory discussions with MHRA, FDA, and EMA representatives who indicated overall survival, supported by biomarker data, may support conditional approval. The company plans biomarker correlation results in November 2025 and intends to file an MAA to MHRA in December 2025, a BLA to FDA in January 2026, and an MAA to EMA in Q1 2026. These concrete milestones create near-term, monitorable regulatory steps.

Key risks and dependencies include the non-randomized nature of the trial and reliance on historical controls, the pending biomarker correlation data expected in November 2025, and formal feedback or decisions from regulators after the planned filings in December 2025–Q1 2026. Watch the November biomarker readout, the regulatory meeting outcomes in December, and the formal acceptances or review designations after each filing within the next 3–6 months.

• 75% of OST-HER2 treated patients achieved 2-year overall survival compared with 40% in the historical control group (p < 0.0001)
• 100% of patients who achieved 12 month event free survival achieved 2 year overall survival

New York, New York--(Newsfile Corp. - October 10, 2025) - OS Therapies Inc. (NYSE American: OSTX) ("OS Therapies" or "the Company"), a clinical-stage cancer immunotherapy and antibody drug conjugate biotechnology company, today announced statistically significant positive final 2-year overall survival data from the Company's Phase 2b trial of off-the-shelf immunotherapy candidate OST-HER2 in the prevention or delay of recurrent, fully-resected, pulmonary metastatic osteosarcoma (the "Fully Resected Metastatic Osteosarcoma Trial"). 75% (27 out of 36 evaluable patients; 5 patients were lost to follow-up) of OST-HER2 treated patients achieved 2-year overall survival as measured from most recent pulmonary resection, compared with 40% of historical control patients¹ (p < 0.0001) Subgroup analyses showed that 100% of patients who achieved 12-month Event Free Survival (EFS) achieved 2-year overall survival, whereas 59% of patients who did not achieve EFS achieved 2-year overall survival.

"The overall survival data in this non-randomized OST-HER2 study is encouraging as the results show it was safe and well-tolerated," said Dr. Peter Anderson, pediatric oncologist at Cleveland Clinic Children's. "New treatments are needed for treating metastatic osteosarcoma, so I am hopeful this could become an option in the near future." Dr. Anderson is a scientific advisor for OS Therapies.

"The continued outperformance in overall survival of OST-HER2 when compared with historical control is exactly what we were hoping for when we started the Company in 2018," said Paul Romness, MPH, Chairman & CEO of OS Therapies. "We have had highly productive regulatory meetings with the United Kingdom's (UK) Medicines and Healthcare products Regulatory Agency (MHRA), the United States (US) Food & Drug Administration (FDA) and the European Medicines Agency's (EMA) Dutch rapporteur. Representatives of all three regulatory agencies, in various ways, indicated that overall survival may be an appropriate clinical endpoint to support a conditional marketing authorization, especially when supported with biomarker data. The rationale for overall survival combined with biomarkers is largely a result of the compelling data recently published in canine osteosarcoma by researchers at the University of Pennsylvania."

Dr. Robert Petit, Chief Medical & Scientific Officer for OS Therapies: "We are now prospectively analyzing patient samples from the clinical trial to confirm that clinical outcomes are indeed correlated with immune system biomarker activation. We expect that data to be available to us in November 2025, in time for our meetings we expect to hold with each of these regulatory agencies in December 2025. We remain on track to file a conditional Marketing Authorization Application (MAA) to MHRA in December 2025, a Biologics Licensing Application (BLA) under the Accelerated Approval Program to FDA in January 2026, and a MAA to EMA in the first quarter of 2026."

The Company will host an investor conference call on Monday, October 13, 2025 at 8:30am ET to discuss the data collected to date from the ongoing Fully Resected Metastatic Osteosarcoma Trial and the regulatory path to conditional marketing authorization(s) for OST-HER2 in the United Kingdom, the United States and the European Union. Dial-in details for the conference call / webcast will be available on the News/Investors page of the Company's website by 6am ET on the morning of the conference call.

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. 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 Rare Pediatric Disease Designation (RPDD) from the U.S. Food & Drug Administration and Fast-Track and Orphan Drug designations from the U.S. FDA and 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. The Company anticipates submitting a Biologics Licensing Application (BLA) to the U.S. FDA for OST-HER2 in osteosarcoma in 2025 and, if approved, would become eligible to receive a Priority Review Voucher that it could then sell. 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.

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.

Forward-Looking Statements
Statements in this press release about future expectations, plans and prospects, as well as any other statements regarding matters that are not historical facts, may constitute forward-looking statements within the meaning of the federal securities laws. These forward-looking statements and terms such as "anticipate," "expect," "intend," "may," "will," "should" or other comparable terms involve risks and uncertainties because they relate to events and depend on circumstances that will occur in the future. Those statements include statements regarding the intent, belief or current expectations of OS Therapies and members of its management, as well as the assumptions on which such statements are based. OS Therapies cautions readers that forward-looking statements are based on management's expectations and assumptions as of the date of this press release and are subject to certain risks and uncertainties that could cause actual results to differ materially, including, but not limited to the approval of OST-HER2 by the U.S. FDA and other risks and uncertainties described in "Risk Factors" in the Company's most recent Annual Report on Form 10-K, most recent Quarterly Report on Form 10-Q and other subsequent documents the Company files with the Securities and Exchange Commission. Any forward-looking statements contained in this press release speak only as of the date hereof, and, except as required by the federal securities laws, OS Therapies specifically disclaims any obligation to update any forward-looking statement, whether as a result of new information, future events or otherwise.

OS Therapies Contact Information:

Investor Relations
Harrison Seidner, PhD
WaterSeid Partners
OSTX@waterseid.com

Public Relations
Jessica Starman, MBA
Elev8 New Media
media@ostherapies.com

https://x.com/OSTherapies
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https://www.linkedin.com/company/os-therapies/

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¹ A.H. Aljubran, A. Griffin, M. Pintilie, M. Blackstein; Osteosarcoma in adolescents and adults: survival analysis with and without lung metastases; Annals of Oncology; Volume 20, Issue 6; 2009;1136-41

To view the source version of this press release, please visit https://www.newsfilecorp.com/release/269942

FAQ

What were OSTX OST-HER2 Phase 2b 2-year overall survival results on October 10, 2025?

OST-HER2 showed 75% 2-year overall survival (27/36 evaluable) versus 40% historical control with p < 0.0001.

How many patients were evaluable in the OSTX OST-HER2 Phase 2b trial?

There were 36 evaluable patients in the reported analysis; 5 patients were lost to follow-up.

When will OSTX release biomarker data that may support OST-HER2 regulatory filings?

The company expects biomarker correlation data to be available in November 2025.

What regulatory filings and timelines did OSTX announce for OST-HER2?

Targets are a MHRA MAA in Dec 2025, a FDA BLA under Accelerated Approval in Jan 2026, and an EMA MAA in Q1 2026.

Will OSTX host an investor call to discuss the OST-HER2 data?

Yes. An investor conference call and webcast is scheduled for Oct 13, 2025 at 8:30am ET.

How did 12-month event-free survival correlate with 2-year overall survival in the OSTX trial?

All patients who achieved 12-month EFS also achieved 2-year overall survival in the reported cohort.