Tamura K, et al. - With all patients who had HER2-positive advanced-stage breast cancer with prior treatment with trastuzumab emtansine who received trastuzumab deruxtecan at the recommended doses for expansion, researchers reported the safety and initial activity results of their open-label, dose-escalation and dose-expansion phase 1 trial with. At least one dose of trastuzumab deruxtecan at the recommended doses for expansion was received by 115 of 118 patients with HER2-positive breast cancer. Outcomes suggest a manageable safety profile with trastuzumab deruxtecan, and it showed preliminary activity in patients with HER2-positive breast cancer pretreated with trastuzumab emtansine. Results justify further development in phase 2 and 3 clinical trials for HER2-positive breast cancer. Frequent grade 3 or worse treatment-emergent adverse events included anemia and decreased neutrophil, white blood cell, and platelet counts.