Mersana Therapeutics (MRSN) said Friday initial clinical data from the phase 1 study of its lead candidate, emiltatug ledadotin, targeting B7-H4, showed clinical activity across multiple tumor types and was well tolerated among patients.

The company said the dose escalation portion of the trial enrolled 130 patients with advanced/metastatic cancers, including types of breast cancer, ovarian cancer, and endometrial cancer. Many patients were heavily pretreated, with a median of 4.5 prior therapy lines.

The drugmaker said that the confirmed objective response rate at intermediate doses was 23% across all B7-H4 high tumors and 23% in B7-H4 high TNBC patients.

At higher doses, the objective response rate was 22%, with 78% of patients showing 30% or more tumor reduction in target lesions, it added.

Mersana said it will continue to explore higher doses and implement proteinuria mitigation strategies.

Shares of the company rose 20% in premarket activity.