Phase I Study to Assess the Combination of Afatinib with Trastuzumab in Patients with Advanced or Metastatic HER2-Positive Breast Cancer

Purpose: The HER2 mAb, trastuzumab, is a standard therapy for patients with HER2-positive breast cancer before acquired resistance. Afatinib, an irreversible, oral, small-molecule ErbB family blocker, shows clinical activity in trastuzumab-refractory HER2-positive breast cancer. Experimental Design: This phase I study used a 3+3 dose escalation to determine the MTD of oral once-daily afatinib in combination with the recommended dose of intravenous trastuzumab (4 mg/kg week 1; 2 mg/kg/wk thereafter). Adult women with confirmed advanced/metastatic HER2-positive breast cancer were eligible. Results: Of 18 patients treated, 16 received daily afatinib 20 mg and two 30 mg. Overall, 4 of 13 and 2 of 2 patients receiving afatinib 20 mg and 30 mg, respectively, experienced dose-limiting toxicity (DLT; all CTCAE grade 3 diarrhea). Most frequent treatment- related adverse events were diarrhea (94%), rash (56%), and fatigue (56%). Overall, pharmacokinetic profiles of afatinib and trastuzumab in combination were consistent with the known characteristics of each alone. Overall, objective response and disease control rates were 11% and 39%, respectively, with median progression-free survival 111.0 days (95% confidence interval, 56.0-274.0). Conclusions: The MTD of afatinib was 20 mg daily combined with the recommended weekly dose of trastuzumab, with 1 of 6 patients showing DLTs in the dose escalation. However, additional DLTs occurred in the dose-expansion phase meaning that this MTD cannot be recommended for phase II development without strict diarrhea management. There was no evidence suggesting relevant pharmacokinetic drug-drug interactions. Signs of clinical activity were seen in trastuzumab-resistant HER2-positive breast cancer, suggesting further investigation with optimal diarrhea management is warranted. (C) 2014 AACR.