Tolerability of palbociclib in younger and older patients with advanced breast cancer.

Introduction: Palbociclib is a small-molecule cyclin-dependent kinase 4/6 inhibitor used to treat hormone receptor-positive, human epidermal growth factor receptor-2 negative advanced breast cancer. Patient-specific factors impacting dose reductions or discontinuations are unknown.

Methods: The primary objective was to evaluate the association of age (<60 vs. ≥60 years) with palbociclib dose reductions or discontinuations secondary to neutropenia. This single-center, retrospective chart review included hormone receptor-positive, human epidermal growth factor receptor-2 negative advanced breast cancer patients ≥18 years treated with palbociclib between April 2015 and May 2020. Patients <60 years at the time of palbociclib initiation were in the younger group and patients ≥60 years were in the older group.

Results: Among the 107 patients included, younger patients were less likely than older patients to have a palbociclib starting dose <125 mg (0% vs. 11.9%,  = 0.02). Differences in palbociclib dose reductions or treatment discontinuations secondary to neutropenia were not detected (35.4% vs. 42.4%,  = 0.55). Neither the total number of palbociclib dose reductions (none: 54.2% vs. 49.1%, one: 33.3% vs. 42.4%, two: 12.5% vs. 8.5%,  = 0.61), nor the final dose of palbociclib (125 mg: 54.2% vs. 40.7%, 100 mg: 29.2% vs. 27.1%, 75 mg: 16.7% vs. 32.2%,  = 0.17) differed between younger and older patients.

Conclusions: Age (<60 vs. ≥60 years) was not associated with the rate of palbociclib dose reductions or discontinuations secondary to neutropenia. Older (≥60 years) patients were more likely to start palbociclib at lower doses which may impact neutropenia and non-neutropenic intolerance.