Abstract
Anti-HER2 agents, such as trastuzumab, lapatinib, trastuzumab emtansine (T-DM1), and pertuzumab, are standard agents in the treatment of breast cancer overexpressing the human epidermal growth factor receptor 2 (HER2). Trastuzumab is the first approved HER2-targeted agent. Subsequent developments include agents with different mechanisms. In this paper, we review the results of clinical trials of T-DM1, a new anti-HER2 agent, with a focus on Japanese patients with breast cancer. On the basis of results from a Phase I study (JO22591), the maximum tolerated dose was determined to be 3.6 mg/kg every 3 weeks for both Japanese and western patients. In a Phase II study (JO22997), the overall response rate was 38.4% (90% confidence interval 28.8-48.6). T-DM1 was well tolerated in Japanese patients; however, the incidence of grade 3 or 4 thrombocytopenia was higher than that observed in earlier western studies, but was not associated with clinically important symptoms. Pharmacokinetic parameters for T-DM1 and its metabolites were consistent with those reported previously from a Phase I or II study in non-Japanese patients, and the data obtained showed no suggestion of ethnic differences. Several Phase III studies of T-DM1 are ongoing throughout the world, including in Japanese patients with breast cancer.