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OS Therapies is moving ahead with plans for an accelerated approval pathway after its off-the-shelf candidate for people who have had surgery to treat osteosarcoma succeeded in a single-arm mid-stage study. The New York-based biotech’s HER2-targeted immunotherapy, dubbed OST-HER2, achieved statistical significance in the Phase 2b primary endpoint of event-free survival (EFS) at 12 months, according to a Wednesday release. The drugmaker’s shares $OSTX were up by around 48% at the start of trading on Wednesday. Around one-third of 39 evaluable subjects aged 12 to 39 with recently resected osteosarcoma with metastases that had recurred in the lungs were free of disease recurrence following treatment with OST-HER2. This compares well with the 20% EFS seen with historical control, according to the drugmaker. In the trial’s secondary survival endpoint, interim results showed 91% of patients given OST-HER2 were still alive at one year compared with 80% for historical control, good for a p-value of 0.07. The data come after OS Therapies announced in October that the final patient in the trial had completed their last visit. A month later, the company secured $6 million in a private placement. The biotech is now planning to engage with the FDA on a potential accelerated approval path for the treatment candidate, CEO Paul Romness said in the release. Romness has previously held roles at Johnson & Johnson, Amgen and Boehringer Ingelheim. There are currently no FDA-approved treatments to prevent recurrent, fully resected, lung metastatic osteosarcoma. The disease often starts in the bones of the legs or arms and affects just 1,000 people in the US each year, but is the most common malignant bone tumor in children and young adults. Around half of all patients are diagnosed with lung metastasis at some point after surgery and chemotherapy, according to OS Therapies. Other drugmakers developing treatments for osteosarcoma include Umoja Biopharma, which is advancing an in vivo CAR-T therapy called UB-TT170 for the disease. Denmark-based ADC biotech Adcendo is homing in on targeting uPARAP , a collagen receptor that’s overexpressed in cancers, including osteosarcoma. Elsewhere, OS Therapies’ OST-HER2 is also being developed for breast cancer and other solid tumors, although a Phase 2a trial in those indications is currently on hold as the company waits for an osteosarcoma approval. The asset is designed to stimulate the body’s immune system to fight cancer better by training HER2-specific T cells to detect and destroy cancer cells. Editor’s note: This article was updated to add the company’s stock movement.