IMPORTANCE: Treatment options for patients with disease progression after 
treatment with trastuzumab, pertuzumab, and ado-trastuzumab emtansine (T-DM1) 
are limited. Tucatinib is an oral, potent, human epidermal growth factor 
receptor 2 (HER2)-specific tyrosine kinase inhibitor (TKI) being developed as a 
novel treatment for ERBB2/HER2-positive breast cancer.
OBJECTIVE: To determine the maximum tolerated dosage of tucatinib in combination 
with T-DM1 in the treatment of patients with ERBB2/HER2-positive metastatic 
breast cancer with and without brain metastases.
DESIGN, SETTING, AND PARTICIPANTS: In this phase 1b open-label, multicenter, 
clinical trial, 57 participants enrolled between January 22, 2014, and June 22, 
2015, were 18 years of age or older with ERBB2/HER2-positive metastatic breast 
cancer previously treated with trastuzumab and a taxane. Data were analyzed 
between January and March 2018.
INTERVENTIONS: Tucatinib 300 mg or 350 mg administered orally twice per day for 
21 days and T-DM1 3.6 mg/kg administered intravenously once every 21 days.
MAIN OUTCOMES AND MEASURES: Safety assessments, pharmacokinetics, and response 
were assessed using RECIST 1.1 every 2 cycles for 6 cycles, followed by every 3 
cycles.
RESULTS: Fifty-seven T-DM1-naive patients (median [IQR] 51 [44.0-63.0] years of 
age) who had undergone a median of 2 earlier HER2 therapies (range, 1-3) were 
treated. The tucatinib maximum tolerated dosage was determined to be 300 mg 
administered twice per day with dose-limiting toxic reactions seen at 350 mg 
twice per day. Pharmacokinetic analysis showed that there was no drug-drug 
interaction with T-DM1. Adverse events seen among the 50 patients treated at the 
maximum tolerated dosage regardless of causality included nausea (36 patients; 
72%), diarrhea (30 patients; 60%), fatigue (28 patients; 56%), epistaxis (22 
patients; 44%), headache (22 patients; 44%), vomiting (21 patients; 42%), 
constipation (21 patients; 42%), and decreased appetite (20 patients; 40%); the 
majority of adverse events were grade 1 or 2. Tucatinib-related toxic reactions 
that were grade 3 and above included thrombocytopenia (7 patients; 14%) and 
hepatic transaminitis (6 patients; 12%).
CONCLUSIONS AND RELEVANCE: In this study, tucatinib in combination with T-DM1 
appeared to have acceptable toxicity and to show preliminary antitumor activity 
among heavily pretreated patients with ERBB2/HER2-positive metastatic breast 
cancer with and without brain metastases.
TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT01983501.