A single institution experience with palbociclib toxicity requiring dose modifications.

Purpose: Since the widespread implementation of adding palbociclib to endocrine therapy in clinical practice, myelosuppression is becoming increasingly recognized as a toxicity that may lead to dose modification. We aimed to characterize toxicities observed with palbociclib resulting in dose modifications and prescriber preferences in modifying palbociclib dosage in response to treatment-related toxicities outside the context of a clinical trial.

Methods: We conducted a single institution, retrospective study of treatment-related adverse events (AEs) resulting in modifications in dose and schedule and the methods by which dose modifications occurred in patients with advanced hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancer receiving palbociclib and endocrine therapy.

Results: From 2/2015 to 10/2016, 100 patients were identified for inclusion in this study. Treatment with palbociclib and endocrine therapy resulted in dose modifications in 38.0% of patients due to AEs with 18.4% requiring subsequent dose changes. Most palbociclib dose modifications occurred during the first 2 cycles. Grade 3-4 neutropenia accounted for 54.8% events of palbociclib dose modification. Most providers (65.8%) dose reduced palbociclib from 125 mg to 100 mg as their preferred method of dose modification, while others dose reduced from 125 mg to 75 mg (10.5%) and altered the schedule to 125 mg every other day (7.9%). A comparable rate of palbociclib dose modifications and subsequent dose changes were identified in an age ≥ 65 subgroup. In this group, dose adjustments were most commonly from grade 3-4 neutropenia, occurred mainly during cycle 1, and were most frequently addressed by dose reduction from 125 to 100 mg.

Conclusions: Neutropenia remains the predominant cause for palbociclib dose modification and most modifications occur within the first two cycles. Older age (≥ 65) does not affect palbociclib tolerance. Our findings provide context outside of a clinical trial that inform ongoing studies evaluating the safety and feasibility of palbociclib-based therapies.