Palbociclib added to ongoing endocrine therapy for hormone receptor-positive HER2-negative metastatic breast cancer: A case report series

Palbociclib is a potent cyclin-dependent kinase (CDK)4/6 inhibitor that disrupts cell cycle progression and has been recently approved in combination with an aromatase inhibitor or fulvestrant as first- and second-line treatment in hormone receptor (HR)+, human epidermal growth factor receptor (HER)2- metastatic breast cancer. There is evidence that palbociclib may reverse endocrine therapy resistance and that it may also be added to ongoing endocrine therapy beyond progression to obtain clinical benefit. The aim of the present study was to explore this possibility in 5 patients who received palbociclib + fulvestrant following disease progression while under treatment with fulvestrant alone. The median progression-free survival was not reached during a median follow-up of 25 months, and the most frequent best response was stable disease. Three patients remained under treatment on the last re-evaluation. All patients had highly endocrine-sensitive disease and had previously received fulvestrant for ≥12 months. The hypothesis that a selected subpopulation of patients with HR+/HER2- metastatic breast cancer may benefit from the addition of palbociclib to ongoing endocrine therapy beyond disease progression merits further investigation.