OS Therapies Completes Final Treatment in OST-HER2 Osteosarcoma Phase 2b Trial

OS Therapies Incorporated (NYSE American: OSTX), a clinical-stage biopharmaceutical company specializing in ADC and Immunotherapy research, has announced that the last patient enrolled in the AOST-2121 clinical trial (NCT04974008) for OST-HER2 in recurrent, resected Osteosarcoma (OS) has completed their final treatment. The final radiological imaging evaluation for this patient is anticipated by early in the fourth quarter of 2024 as part of the 12-month Event-Free Survival (EFS) primary endpoint analysis. The company plans to close all clinical trial sites and lock the database in preparation for data analysis and a topline data readout expected in the fourth quarter of 2024.

OST-HER2 is a biologic therapeutic candidate that uses a Listeria monocytogenes (Lm) vector-based immunotherapeutic vaccine to prevent metastasis, delay recurrence, and increase overall survival in osteosarcoma patients. The AOST-2121 clinical trial aims to prove the efficacy of OST-HER2 in patients who have already experienced recurrent disease and are at high risk of recurrence. Participants receive 16 doses of OST-HER2 once every three weeks, followed by a four-week post-treatment monitoring period, totaling 52 weeks. Radiographic evaluations are conducted throughout the treatment period.

The proposed action mechanism of OST-HER2 involves stimulating innate and adaptive immune responses via the Lm vector, generating T-cells that target and destroy micro-metastases potentially leading to recurrent osteosarcoma. These T-cells specifically target HER2 expressed by tumor cells, killing them and releasing additional tumor targets for the immune system. Currently, no approved adjuvant treatments exist for recurrent osteosarcoma in the United States.

The AOST-2121 study has successfully enrolled 41 patients receiving OST-HER2 at 21 clinical trial sites across the U.S. The primary study endpoints include a 12-month Event-Free Survival (EFS) and a 36-month Overall Survival (OS), with interim OS evaluations at 12 and 24 months. Topline EFS data, interim 2-year OS data, and additional secondary analyses are anticipated to be reported in the fourth quarter of 2024. Over the past four decades, no novel therapeutic interventions approved by the FDA have significantly improved clinical outcomes for osteosarcoma patients.

The inclusion of data from the final patient, along with that from Patient #40, will contribute to interim results set to be announced in conjunction with ASCO 2024. This is alongside the Phase I clinical data previously reported for breast cancer, which the company plans to target following osteosarcoma. OS Therapies extends gratitude to the patients, families, clinicians, researchers, assistants, and the entire osteosarcoma community for their support of this significant trial.

OS Therapies is focused on developing and commercializing treatments for osteosarcoma and other solid tumors. The company's lead asset, OST-HER2, leverages the immune-stimulatory effects of Listeria bacteria to elicit a robust immune response against the HER2 protein. The company has completed enrollment for a 41-patient Phase 2b clinical trial of OST-HER2 in resected, recurrent osteosarcoma, with results expected in the fourth quarter of 2024. OST-HER2 has also completed a Phase 1 clinical study primarily involving breast cancer patients and has shown strong preclinical efficacy in various breast cancer models.

In addition to OST-HER2, OS Therapies is advancing its next-generation Antibody Drug Conjugate (ADC) platform, known as tunable ADC (tADC). This platform features tailored antibody-linker-payload candidates utilizing the company's proprietary silicone linker technology, enabling the delivery of multiple payloads per linker.

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