Bosutinib in combination with the aromatase inhibitor letrozole: A phase II trial in postmenopausalwomen evaluating first-line endocrine therapy in locally advanced or metastatic hormone receptor-positive/HER2-negative breast cancer

Background. Endocrine therapy resistance in hormone receptorpositive (HR+) breast cancer (BC) may involve crosstalk between HRs and growth factor signaling pathways. We evaluated bosutinib, a dual Src/Abl tyrosine kinase inhibitor that has previously demonstrated some antitumor activity in BC, plus letrozole as first-line endocrine therapy in locally advanced or metastatic HR+/HER2- BC. Methods. Sixteen postmenopausal women were enrolled in a phase II study evaluating the safety/efficacy of bosutinib plus letrozole. In the single-arm safety/dose-confirming lead-in (part 1), patients received oral bosutinib at 400 mg/day plus letrozole at 2.5mg/day; adverse events (AEs)anddose-limiting toxicities (DLTs) were monitored, and initial efficacy was assessed. A randomized efficacy/safety phase (part 2) was planned to evaluate the combination versus letrozole monotherapy. Results. Fifteen of 16 subjects experienced treatment-related AEs, most commonly diarrhea (69%). Treatment-related hepatotoxicity AEs (primarily alanine aminotransferase [ALT] or aspartate aminotransferase [AST] elevations) occurred in 6 of 16 patients (38%). Four of 15 evaluable patients (27%) experienced a DLT (grade 3/4 ALT/AST elevations, n = 2; grade 3 rash, n = 1; grade 3 diarrhea or vomiting, n = 1), including 1 Hy's law hepatotoxicity case. All DLTs resolved following treatment discontinuation. One patient achieved confirmed partial response; one had stable disease for >24 weeks. Study termination occurred before part 2. Conclusion. The unfavorable risk-benefit ratio did not warrant further investigation of bosutinib plus letrozole.