A Study of Tucatinib vs. Placebo in Combination With Capecitabine & Trastuzumab in Patients With Advanced HER2+ Breast Cancer

This is a randomized, international, multi-center study in patients with progressive unresectable locally advanced or metastatic HER2+ breast cancer who have had prior treatment with trastuzumab, pertuzumab and T-DM1. There are two phases to this trial: the Double-blind Phase and the Unblinded Phase. In the Double-blind phase, participants were randomized in a 2:1 ratio to receive tucatinib or placebo in combination with capecitabine and trastuzumab. In the Unblinded Phase, patients on placebo may be offered tucatinib. Stratification factors include presence or history of treated or untreated brain metastases or brain lesions of equivocal significance (yes/no), Eastern Cooperative Oncology Group (ECOG) Performance Status (0 vs. 1), and region of world (US vs. Canada vs. Rest of World). Safety assessments will be performed at a minimum of once every three weeks throughout study treatment and 30 days after the last dose of study drugs. Laboratory assessments will be performed locally at sites. Left ventricular ejection fraction will be assessed by MUGA or ECHO at screening and once every 12 weeks thereafter. For the blinded phase, contrast brain MRI was performed at baseline. Efficacy assessments (CT of chest, abdomen and pelvis at a minimum) utilized RECIST 1.1 and included patients with evaluable tumors defined as measurable target lesions and non-measurable non-target lesions. RECIST assessment was performed at baseline, every 6 weeks for the first 24 weeks, and then every 9 weeks thereafter. Repeat MRI of the brain was required on this same schedule only in those patients with brain metastases identified at baseline. All treatment decisions were made based upon investigator assessment. All patients underwent a repeat MRI of the brain within 30 days of the end of treatment unless previously performed at time of disease progression. For the unblinded phase, RECIST assessments will be performed per standard clinical practice as determined by investigator with a maximum interval of 12 weeks.