CytomX Therapeutics Inc. (NASDAQ: CTMX) reported positive Phase 1 expansion data for its varsetatug masetecan treatment in late-line metastatic colorectal cancer. The data comes from the ongoing CTMX-2051-101 Phase 1 study with a January 16, 2026 data cutoff.

The study showed confirmed response rates of 32% at the 10 mg/kg dose and 20% at the 8.6 mg/kg dose administered every three weeks. Estimated progression-free survival was 7.1 months at 10 mg/kg and 6.8 months at 8.6 mg/kg.

A total of 93 patients with late-line metastatic colorectal cancer were enrolled in the study, with 56 patients evaluable for efficacy at expansion doses. Patients had previously received a median of 3 prior lines of therapy in the metastatic setting, and 96% had previously been treated with irinotecan.

The most common treatment-related adverse event was diarrhea, which the company described as generally manageable and reversible. In dose optimization cohorts using an updated prophylactic regimen, Grade 3 diarrhea occurred in 10% of 20 patients receiving 8.6 mg/kg and 10 mg/kg doses.

CytomX plans to engage with the FDA mid-year to discuss potential registrational trial design for late-line colorectal cancer. The company has initiated a Phase 1 study evaluating the treatment in combination with bevacizumab and plans to start a Phase 1b/2 chemotherapy combination study by the end of 2026.

Varsetatug masetecan is a masked, conditionally activated antibody-drug conjugate targeting epithelial cell adhesion molecule (EpCAM). The treatment was discovered in collaboration with ImmunoGen, now part of AbbVie.