• FDA confirmed that data from single-arm studies in rare diseases, such as in ultra-rare deadly pediatric cancer osteosarcoma, could support a Biologics Licensing Application (BLA) under Accelerated Approval Program pathway, as stated in FDA guidance documents
• FDA proposed confirmatory study design include additional osteosarcoma disease settings such as prevention of recurrence following primary tumor resection, where a randomized controlled study may be feasible and that the confirmatory study would have to be underway prior to granting a BLA
• FDA indicated that the use of canine data to support confirmation that pre-specified immune biomarker activation is correlated with treatment-emergent human clinical benefit will depend on biomarkers chosen for evaluation & clinical validation thereof
• FDA requested follow-up meeting once immune activation biomarker analysis is available to review data within context of pending Company-planned BLA filing
• Key Opinion Leader Dr. Peter Anderson, who participated in the meeting following FDA's request that a key opinion leader be present to help educate the agency on the standard of care in osteosarcoma, noted that immunotherapies such as OST-HER2 are the most promising treatment category being investigated
• Company anticipates immune activation biomarker analysis data will be released during week of the JP Morgan Healthcare Conference in January 2026
• Company reiterates end of January 2026 timeline for planned BLA submission, with FDA-proposed meetings to review biomarker data & additional clinical data, as well as final confirmatory study design, expected to occur after BLA filing
• FDA feedback received remains in alignment with the Company's international regulatory strategy - including the United Kingdom and European Union

New York, New York--(Newsfile Corp. - December 15, 2025) - OS Therapies Inc. (NYSE American: OSTX) ("OS Therapies" or "the Company"), the world leader in listeria-based cancer immunotherapies, today announced that it held a successful Type C meeting with the United States Food & Drug Administration (FDA) regarding the Phase 2b human clinical trial of OST-HER2 in the prevention or delay of recurrent, fully-resected, pulmonary metastatic osteosarcoma (the 'Metastatic Osteosarcoma Program'). The Company made significant progress in understanding the parameters under which the agency would consider clinical efficacy data supported by treatment-emergent immune biomarker activation could be supportive of a BLA under the Accelerated Approval Program and reiterates its intent to file BLA documents with FDA by the end January 2026.

OS Therapies' lead product candidate, OST-HER2, is an innovative immunotherapy using a HER2 bioengineered form of the bacteria Listeria monocytogenes (Lm) to trigger a strong immune response against cancer cells, whether primary tumor, metastatic disease or micro-metastases expressing HER2. OST-HER2 has been shown to activate CD8 T-Cells, CD4 T-Cells, NK cells and dendritic cells, against cancer. Peer-reviewed literature investigating predictors of improved clinical outcomes in osteosarcoma patients suggest that increased T-cell counts and NK cells are independently associated with improved clinical outcomes.¹

Key highlights from the Type C FDA meeting include:

1. FDA confirmed that data from a single arm study in ultra-rare pediatric cancer osteosarcoma could support a BLA under Accelerated Approval Program pathway, as stated in FDA guidance documents
2. FDA suggested a confirmatory study design to include additional osteosarcoma disease settings such as prevention of recurrence following primary tumor resection, where a randomized controlled study may be feasible and that the confirmatory study would have to be underway prior to granting a BLA
3. FDA indicated that the use of canine data to support confirmation that pre-specified immune biomarker activation is correlated with treatment-emergent human clinical benefit will depend on biomarkers chosen for evaluation & clinical validation thereof
4. FDA requested follow-up meeting once the forthcoming pre-specified immune activation biomarker analysis is available to review data within context of pending Company-planned BLA filing

"There has been no improvement in the standard of care in osteosarcoma in over 40 years," said Dr. Robert Petit, Chief Medical & Scientific Officer of OS Therapies. "Canine models of osteosarcoma are the most predictive models of cancer in the field of oncology due to near-identical clinical presentation, metastatic occurrence and genetic expression & disease mutation drivers. Leveraging clinical and immune data against canine osteosarcoma lung metastases to predict a treatment effect in humans is one of the most fruitful clinical research opportunities we have available. It is important that we leverage the field Comparative Oncology to help accelerate new therapies to osteosarcoma patients who desperately need new treatment options to improve survival outcomes."

The Company expects to complete the proposed pre-specified immune activation biomarker analysis in the forthcoming weeks, with data expected to be made public during the week of the JP Morgan Healthcare Conference in January 2026. The Company reiterates that it intends to submit BLA for Accelerated Approval by the end of January 2026. The Company intends to accept FDA's request for two (2) additional meetings to review 1) the forthcoming pre-specified biomarker data analysis and 2) a further updated proposed confirmatory study design in separate meetings (a Type D meeting for pre-specified biomarker analysis follow-up and a pre-IND meeting for further updated confirmatory clinical trial design follow-up) following the submission of the BLA. The Company expects that following the submission of the BLA and the two invited meetings that the Company will request a pre-BLA meeting prior to FDA making a final decision of the Company's Accelerated Approval request. The Company has already begun preparations to to initiate a confirmatory clinical study immediately prior to receiving Accelerated Approval prior to the end of the third quarter of 2026 for the Metastatic Osteosarcoma Program.

"We were very pleased that we made meaningful progress with the FDA on the potential of using immune activation biomarker data as a surrogate endpoint of clinical efficacy and on the proposed design of a confirmatory study design required to be initiated prior to received Accelerated Approval," said Paul Romness, Chairman & CEO of OS Therapies. "Dr. Anderson's meaningful contribution helped shape the conversation about the significant unmet medical need and inform the agency about the critical value of Comparative Oncology in potentially helping speed therapies to market in osteosarcoma. We expect not only to have biomarker data from our human clinical study to share with FDA, but also to have additional biomarker data from our canine program in frontline osteosarcoma which is the disease setting that FDA suggested may be suitable to conduct a confirmatory randomized study as part of an agreement with the agency regarding Accelerated Approval."

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 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 2026 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.

¹ https://link.springer.com/article/10.1186/s12957-025-03784-4

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