Real-world clinical outcomes and toxicity in metastatic breast cancer patients treated with palbociclib and endocrine therapy.

PURPOSE: Real-world data are critical to demonstrate the reproducibility of evidence and external generalizability of randomized clinical trials. Palbociclib is an oral small-molecule inhibitor of cyclin-dependent kinases 4 and 6 that has been shown to improve progression-free survival (PFS) when combined with letrozole or fulvestrant in phase 3 clinical trials. We evaluated real-world outcomes in metastatic breast cancer patients who received palbociclib in combination with endocrine therapy in routine clinical practice.
METHODS: Records of patients with advanced hormone receptor (HR)-positive breast cancer treated with palbociclib at the Cleveland Clinic health system from February, 2015 to December, 2017 were retrospectively reviewed. The primary end point was PFS.
RESULTS: In this cohort, 411 women were included. The median age and follow-up times were 53.5 years and 10.2 months, respectively. The median PFS for palbociclib plus letrozole was 15.1 months for patients treated in first line, 10.5 months in second line, and 4.2 months in third line and beyond. For patients who received fulvestrant plus palbociclib, the median PFS in first, second, and third line and beyond were 11.6, 12.3, and 6.4 months, respectively. The most common adverse events were hematologic, with grade 3-4 neutropenia occurring in 58% of patients. Thirty-one (8%) patients permanently discontinued palbociclib due to adverse events.
CONCLUSIONS: Among patients with HR-positive advanced breast cancer, the estimated PFS in patients treated with fulvestrant and palbociclib was comparable to a previously reported phase 3 trial. However, the median PFS with letrozole and palbociclib was shorter than previously reported data from phase 2 and 3 trials. Palbociclib toxicity was very manageable, with a low drug discontinuation rate.