Phase 1b clinical trial of ado-trastuzumab emtansine and ribociclib for HER2-positive metastatic breast cancer

Abstract

Patients with HER2+ metastatic breast cancer are often treated with a multitude of therapies in the metastatic setting, and additional strategies to prolong responses to anti-HER2 therapies are needed. Preclinical evidence suggests synergy between cyclin-dependent kinase 4 and 6 (CDK 4/6) inhibitors and anti-HER2 therapies. We conducted a phase 1b study of ribociclib and ado-trastuzumab emtansine (T-DM1) in patients with advanced/metastatic HER2-positive breast cancer previously treated with trastuzumab and a taxane in any setting, with four or fewer prior lines of therapy in the metastatic setting. A standard 3\u2009+\u20093 dose-escalation design was used to evaluate various doses of ribociclib in combination with T-DM1, starting at 300\u2009mg. The primary objective was to determine the maximum tolerated dose and/or recommended phase 2 dose (RP2D) of ribociclib in combination with T-DM1. A total of 12 patients were enrolled. During dose-escalation, patients received doses of ribociclib of 300\u2009mg ( n \u2009=\u20093), 400\u2009mg ( n \u2009=\u20093), 500\u2009mg ( n \u2009=\u20093), and 600\u2009mg ( n \u2009=\u20093). No dose-limiting toxicities were observed. The majority of toxicities were Grade 1 and 2, and the most common Grade 3 toxicities were neutropenia (33%), leukopenia (33%), and anemia (25%). After a median follow-up of 12.4 months, the median PFS was 10.4 months (95% confidence interval, 2.7–19.3). Based on the pharmacokinetic analysis, adverse events, and dose reductions, 400\u2009mg was determined to be the RP2D for ribociclib given on days 8–21 of a 21-day cycle with T-DM1.