Vet Oncology: OS AH Unveils Phase 2 Success for Gene-Edited Listeria Immunotherapy in Dogs

Pet Animals Oncology segment is witnessing increased traction with a number of start ups and human pharma spin offs taking a stab at this therapeutic segment, on account of huge un-met opportunities.

In a milestone development for comparative oncology, OS Animal Health—the veterinary-focused subsidiary of OS Therapies (NYSE American: OSTX)—has announced the peer-reviewed publication of positive data from its Phase 2 clinical trial of OST-HER2.

Published in a leading veterinary oncology journal, the data validates a highly anticipated, gene-edited, Listeria-based cancer immunotherapy specifically evaluated as a frontline treatment for canine osteosarcoma.

The study investigated the therapeutic efficacy of OST-HER2 administered in tandem with palliative radiation therapy (RT) with an ambitious clinical goal: preventing mandatory limb amputation while simultaneously delaying lethal pulmonary metastases in newly diagnosed canine patients.

• 0% of OST-HER2 treated dogs survived 2-year vs. 1% in control group (p = 0.00995)

• Over 15,000 dogs per year diagnosed with osteosarcoma

• Company expects to submit U.S. Department of Agriculture meeting request to gain alignment on regulatory path towards conditional approval in the third quarter of 2026

• OS Therapies reiterates plan to distribute OS Animal Health shares to shareholders

• OS Animal Health plans crowdfunding to expand shareholder base to meet U.S. national exchange initial listing standards, with indications of interest now being received

Efficacy: Expanding Survival Timelines

Canine osteosarcoma is an aggressive bone cancer affecting more than 15,000 dogs annually in the United States alone. Under current standards of care involving palliative radiation alone, the prognosis is universally poor, with a historical baseline trajectory toward nearly 100% mortality within a year due to rapid metastatic spread to the lungs.

The published Phase 2 data demonstrates a statistically significant deviation from those historical control baselines: Survival Benchmarks: OST-HER2 + Radiation vs. Radiation-Alone

• Median Overall Survival (OS): Dogs treated with the combined OST-HER2 and radiation regimen achieved a median overall survival time of 159 days, compared to just 124 days in the radiation-alone control group ($p = 0.0237$).

• 500-Day Survival Vector: Longer-term tracking showed that 27% of the immunotherapy cohort survived to day 500, compared to a meager 2% in the control group ($p = 0.0108$).

• Two-Year Survival Curve: At the 2-year milestone, 20% of the OST-HER2 treated dogs remained alive with sustained limb function, while the control group dropped to just 1% ($p = 0.00995$).

  1. Antigen-Presenting Infiltration: Upon intravenous injection, the modified Listeria vector is rapidly engulfed by the immune system’s professional Antigen-Presenting Cells (APCs), such as macrophages and dendritic cells.

  2. Triple-Epitope Activation: Once inside the APC, the vector expresses a specialized chimeric fusion protein designed to target three distinct regions of the HER2 oncogene—two mutated extracellular epitopes and one intracellular epitope. This design requires the target tumor or microscopic lung metastasis to display only one of the three signatures to trigger an attack.

  3. Sustained T-Cell Proliferation: The activated APCs trigger a massive cellular immune response, causing HER2-specific cytotoxic T-cells to multiply. These T-cells travel through the blood to infiltrate bone tumors and eliminate microscopic cancer colonies before they establish macro-metastatic lung nodules.

Breaking Tumor Shielding: Concurrently, the Listeria-driven activation suppresses local Regulatory T-cells (Tregs) and Myeloid-Derived Suppressor Cells (MDSCs), stripping away the protective microenvironment that typically isolates solid tumors from immune clearance.

Comparative Oncology and the Regulatory Roadmap

Beyond its direct veterinary value, the success of the canine trial serves as a vital proof-of-concept for human applications. Because the genetic and cellular architecture of osteosarcoma in canines is highly comparable to human pediatric osteosarcoma, scientists are utilizing these veterinary data sets to isolate predictive blood biomarkers within the interferon-gamma pathway to accelerate parallel human trials.

With peer-reviewed clinical validation established, OS Animal Health is adjusting its regulatory timeline. The company plans to submit a formal meeting request to the U.S. Department of Agriculture (USDA) Center for Veterinary Biologics in the third quarter of 2026.

The upcoming regulatory session will focus on aligning manufacturing standards and clinical frameworks to secure a definitive path toward USDA Conditional Approval, aiming to bring this targeted molecular immunotherapy to the commercial veterinary oncology marketplace.