Theseus Pharma stops enrollment in Phase 1/2 study and THE-630 development

Theseus Pharmaceuticals is discontinuing enrollment in the ongoing phase 1/2 study and terminating development of THE-630 in patients with gastrointestinal stromal tumors, or GIST. With 2 out of 6 patients experiencing a hand-foot skin reaction – HFSR – dose-limiting toxicity, the 27 mg dose exceeds the maximum tolerated dose – MTD -. As a result, the company has made the decision to terminate the development of THE-630 in GIST. Patients currently enrolled in the trial will continue to receive THE-630 until a treatment discontinuation criterion is met. As of July 10, , 32 patients have been treated with THE-630 across 7 dose levels. Six patients developed grade 1 to 3 HFSR. No significant skin toxicity was observed in preclinical toxicology studies. The company is analyzing trial data to inform the feasibility of developing low dose THE-630 for KIT-associated mast cell-driven inflammatory indications. Theseus plans to nominate a development candidate from this series in the first half of 2024. A development candidate nomination from the BCR-ABL program is also expected in H1 2024. The company has $234M in cash, cash equivalents, and marketable securities as of June 30, 2023, with its runway extended into 2026.