07:28 AM EDT, 08/27/2025 (MT Newswires) -- Merck ( MRK ) said Wednesday its phase 3 trial for investigational patritumab deruxtecan targeting metastatic breast cancer has dosed its first patient.

The trial evaluates the efficacy and safety of investigational patritumab deruxtecan versus investigator's choice of treatment in patients with unresectable locally advanced or metastatic hormone receptor positive, HER2 negative breast cancer with disease progression, following endocrine and CDK4/6 inhibitor therapy in either the adjuvant or first-line metastatic settings, Merck ( MRK ) said.

The trial will enroll about 1,000 patients across Asia, Europe, North America, and South America, the company said.

Patritumab deruxtecan is being jointly developed and commercialized by Daiichi Sankyo and Merck ( MRK ), except in Japan where Daiichi Sankyo will maintain exclusive rights. Daiichi Sankyo will be solely responsible for manufacturing and supply, Merck ( MRK ) said.