Phase 2 Study of Standard Chemotherapy With Trastuzumab, Plus or Minus Pertuzumab, for Pre-treated Metastatic Breast Cancer

The current preferred first-line therapy for patients with HER2-overexpressing metastatic breast cancer (MBC) is a taxane plus trastuzumab plus pertuzumab, based on results from the CLEOPATRA trial. For patients with disease that has progressed on trastuzumab and a taxane, ado-trastuzumab emtansine (T-DM1) was recently approved based on results from the EMILIA trial showing superiority in this setting compared with capecitabine plus lapatinib. However, the standard for first-line therapy may change again in the near future, when results become available from the MARIANNE trial, which is evaluating T-DM1 alone or in combination with pertuzumab as upfront therapy. Two important questions that may be raised by the findings of this study are whether pertuzumab is effective as second- or later-line therapy following single-agent T-DM1, and whether pertuzumab administered beyond progression on prior pertuzumab therapy is of clinical benefit as trastuzumab has been proven to be. The study will seek to determine the efficacy of chemotherapy (physician's choice of vinorelbine, taxane \[paclitaxel, docetaxel or nab paclitaxel\] or capecitabine) plus trastuzumab vs chemotherapy (physician's choice) plus trastuzumab plus pertuzumab in women with HER2-overexpressing MBC that has been previously treated with T-DM1 in the metastatic setting. We hypothesize that the addition of pertuzumab to trastuzumab plus chemotherapy will improve median progression-free survival (PFS), compared to trastuzumab plus chemotherapy alone, as second- or later-line therapy in patients who have received prior T-DM1. Patients will be stratified according to whether they have received prior pertuzumab versus not. We will also explore whether continuing treatment with pertuzumab in patients who have been previously treated with pertuzumab improves PFS.