Predictive Factors of Dose Reduction Among Patients Treated With Palbociclib for Advanced or Metastatic Breast Cancer in a Real-World Setting

Presenting Authors: Sandra Savignac, PharmD, MSc, CIUSSS de l’Est-de-l’Île-de-Montréal, Quebec, Canada; Marianne Boyer, BPharm, MSc, BCOP, Centre hospitalier de l’Université de Montréal, Quebec, Canada

Co-Authors: Christine Messier, BPharm, MSc, BCOP, and Marie-Lawrence Monfette, PharmD, MSc, BCOP, Centre hospitalier de l’Université de Montréal, Quebec, Canada; Nathalie Letarte, PharmD, MSc, BCOP, Faculté de pharmacie, Université de Montréal, Centre hospitalier de l’Université de Montréal, Quebec, Canada

BACKGROUND: The Canadian palbociclib monograph recommends a starting dose of 125 mg/day and subsequent dose reductions during treatment if adverse events occur. However, in practice, clinicians sometimes choose to initiate palbociclib at a lower dose to prevent the occurrence of adverse events in patients who are considered more likely to have them.

OBJECTIVES: This study aimed to identify baseline patient characteristics that may predict the need for palbociclib dose reduction during treatment. The secondary objectives were to describe treatment patterns and clinical outcomes, including progression-free survival (PFS), in patients treated with palbociclib at the Centre hospitalier de l’Université de Montréal (CHUM). The study also aimed to identify the frequency and type of interventions carried out by oncology pharmacists for these patients.

METHODS: This single-center, retrospective medical chart review study included adults diagnosed with hormone receptor–positive, human epidermal growth factor 2–negative, advanced or metastatic breast cancer treated with palbociclib in combination with letrozole or fulvestrant between April 2019 and June 2022 at CHUM. Logistic regression was performed to identify the factors predicting dose reduction during treatment. The Kaplan-Meier method was used to estimate PFS. Log-rank tests were performed for comparison.

RESULTS: A total of 149 patients were included: 59.7% were treated with palbociclib plus letrozole, and 40.3% were treated with palbociclib plus fulvestrant. Most patients (65.1%) received palbociclib as first-line therapy. The median age was 66 years, and 1.3% of patients were men. Palbociclib was initiated at 125 mg/day and 75 mg/day or 100 mg/day in 90.6% and 9.4% of patients, respectively. Palbociclib dose reductions occurred in 37.6% of all patients, mostly as a result of neutropenia. Only 2 factors were predictive of dose reduction: age ≥70 years (odds ratio [OR], 2.1; 95% confidence interval [CI], 1.063-4.148; P=.033) and absolute neutrophil count (ANC) <3.5 × 10⁹/L before initiating treatment (OR, 3.120; 95% CI, 1.516-6.422; P=.002). Exploratory analyses showed that the median PFS was 17.3 months; patients with dose reduction had a better median PFS (20.4 vs 14.1 months; P=.112). Pharmacists carried out on average 14.4 interventions per patient, mainly to answer questions from oncology nurses (17.5%).

CONCLUSION: These real-world data suggest that patients aged ≥70 years and those whose ANC before initiating treatment is <3.5 × 10⁹/L may benefit from an up-front dose reduction because they are at higher risk for a dose reduction of palbociclib during treatment.

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