T-DM1 and Tucatinib Compared With T-DM1 Alone in Preventing Relapses in People With High Risk HER2-positive breast cancer

Purpose: Is the combination of T-DM1 and a newer drug tucatinib, better than usual treatment with T-DM1 alone at preventing your cancer from returning? Treatment with T-DM1 and placebo or T-DM1 and tucatinib, for up to 14 cycles, unless the breast cancer returns or the side effects become too severe. If the breast cancer returns while on the study, the treatment will be changed. After completion of T-DM1 and/or placebo, or T-DM1 and tucatinib, the study doctor will continue to follow the patients condition with clinic visits every 6 months for 10 years and watch for side effects and for signs of breast cancer returning.