A Study of Tucatinib vs. Placebo in Combination With Ado-trastuzumab Emtansine (T-DM1) for Patients With Advanced or Metastatic HER2+ Breast Cancer

This study is designed to evaluate the efficacy and safety of tucatinib in combination with T-DM1 in participants with unresectable locally-advanced or metastatic HER2+ breast cancer who have had prior treatment with a taxane and trastuzumab in any setting. Prior pertuzumab treatment is permitted, but not required. Participants will be randomized in a 1:1 manner to receive 21-day cycles of either tucatinib or placebo in combination with T-DM1. While on study treatment, participants will be assessed for progression every 6 weeks for the first 24 weeks, and every 9 weeks thereafter, irrespective of dose holds or interruptions. Study treatment will continue until unacceptable toxicity, disease progression, withdrawal of consent, or study closure. After completion of study treatment and after occurrence of disease progression, participants in both arms of the study will continue to be followed for survival until study closure or withdrawal of consent.