Palbociclib in Combination with Endocrine Therapy in Patients with Metastatic Breast Cancer in a Real-World Population: Impact of Dose-Intensity, Dose Reductions and Cycle Delays on Efficacy

<b>Purpose</b>: With the addition of palbociclib to endocrine therapy, many hormone receptor-positive (HR+) metastatic breast cancer (mBC) patients experience toxicities that can lead to dose reductions and cycle delays. We examined the actual doses of palbociclib received by patients and their treatment responses. These dose adjustments, made at the physician's discretion, are not always consistent with pharmaceutical company recommendations. The aim of this study was to assess the influence of dose adjustments on dose intensity and treatment response in our patients. <b>Methods</b>: Records of patients with HR+ mBC treated with palbociclib between December 2016 and January 2019 at the Sainte-Catherine Institute were retrospectively reviewed. Dose intensity was defined as the total dose of palbociclib received by each patient during the first six months of treatment. Anticipated dose reductions and extended cycle delays were recorded. Treatment response at six months and survival were assessed using statistical analyses. <b>Results</b>: A total of 131 women were included; the median age was 67 years. Forty-six patients (35%) experienced an anticipated dose reduction or an extended cycle delay during the first six months of treatment. Logistic regression analysis showed that factors correlated with six-month treatment response included anticipated dose reduction or extended cycle delay (OR = 14.6, 95% CI 3.74-97.4, <i>p</i> < 0.001), cycle delay > 4 weeks (OR = 5.94, 95% CI 1.58-21, <i>p</i> = 0.01), initial dosage < 125 mg (OR = 4.09, 95% CI 1.13-13.7, <i>p</i> = 0.034), and six-month dose intensity < 14,250 mg (OR = 26.0, 95% CI 4.91-481, <i>p</i> < 0.001). <b>Conclusions</b>: In this real-world assessment of clinical outcomes in French patients with HR+ mBC treated with palbociclib, a palbociclib dose intensity lower than recommended-particularly due to cycle delays longer than four weeks-was associated with an increased risk of six-month disease progression.