Purpose The management of endocrine therapy resistance is one of the most challenging facets of advanced breast cancer treatment. Palbociclib is an inhibitor of cyclin-dependent kinases 4 and 6 approved for the treatment of hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced or metastatic breast cancer in combination with fulvestrant in postmenopausal women with disease progression following endocrine therapy. However, treatment responsiveness of tumors to palbociclib after multiple lines of endocrine therapy is not clearly established. The purpose of this study was to determine the efficacy of palbociclib and letrozole in patients pretreated with one or more lines of endocrine therapy. Methods This was a single-center, retrospective cohort study of all postmenopausal hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer patients who received palbociclib and letrozole as a second-line endocrine therapy or beyond (and no prior cyclin-dependent kinases 4 and 6 inhibitor therapy) between February 1, 2015, and July 31, 2016. The primary objective was to evaluate time to treatment failure of palbociclib in combination with letrozole as a second-line of therapy or beyond. Results Fifty-three patients meeting eligibility criteria were included in the analysis. For the primary outcome, the median time to treatment failure of palbociclib and letrozole was 6.3 months (95% CI 3.1–7.4 months). Progression-free survival of palbociclib and letrozole therapy was 6.4 months (95% CI 4.9–8.3 months). Conclusions Palbociclib and letrozole therapy is a viable, effective treatment option for metastatic breast cancer patients who were not exposed to cyclin-dependent kinases 4 and 6 inhibitors as a first-line endocrine therapy. The benefits of palbociclib and letrozole therapy were seen without excessive toxicity, and although neutropenia was common, it may be managed with dose reduction.
Development of CDK4/6 Inhibitors: A Five Years Update