LAUSR

Background: In treating human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer, the efficacy of capecitabine combined with HER2-directed agents such as trastuzumab and lapatinib is supported by some evidence. The combination of T-DM1 and S-1, another oral 5-FU, may be a safe alternative treatment for metastatic breast cancer. Objectives: The optimal dose of S-1 was evaluated in combination with T-DM1 for patients with HER2-positive advanced or metastatic breast cancer. The safety and clinical response of this combination treatment were also assessed. Methods: This 3 + 3 dose-escalation study of S-1 given for the first 2 of 3 weeks, in combination with T-DM1 (3.6 mg/kg given every 3 weeks) to patients with trastuzumab-pretreated HER2-positive advanced or metastatic breast cancer was designed to evaluate the dose-limiting toxicity (DLT) occurrence in the first cycle. We also evaluated the safety and clinical activity of this combination treatment in multiple cycles. Two different dose levels of S-1 (65 and 80 mg/m2/day) were planned, although the capecitabine arm was abandoned because of slow recruitment. Results: Twelve out of the 13 patients enrolled were evaluable for DLT. One DLT (grade ≥3 non-hematological adverse events) occurred at dose level 0, leading to the expansion of this cohort to 6 patients, with an additional DLT (≥7 days discontinuation of medication), while no DLT occurred at dose level 1. As a result, the maximum tolerable dose of S-1 was determined to be 80 mg/m2/day for 14 days with T-DM1 3.6 mg/kg, repeated every 3 weeks. Two patients had grade 3 thrombocytopenia at dose level 0, and 1 patient at dose level 1. Conclusions: S-1 can be safely combined with the clinically relevant dose of T-DM1 in patients with HER2-positive advanced or metastatic breast cancer. Further evaluation with a larger sample size is required for efficacy assessment.