Palbociclib in endocrine-resistant metastatic breast cancer: real-world outcomes.

Background: Palbociclib is a cornerstone treatment for patients with hormone receptor-positive and human epidermal growth factor receptor 2-negative, metastatic breast cancer.

Research design and methods: This retrospective cohort study included 140 patients who were resistant to endocrine therapy and received palbociclib in combination with endocrine therapy. It is aimed to evaluate the efficacy and tolerability of palbociclib and to identify factors influencing progression-free survival and overall survival.

Results: A total of 140 were enrolled in the study; 57 and 83 patients were primary and secondary resistance to endocrine therapy, respectively. In the whole cohort, regarding progression-free survival; low Ki-67 and incidence of moderate-to-severe (grade 3/4) neutropenia were associated with longer median progression-free survival with p-values of 0.004 (3.7 vs. 8.4 months) and 0.026 (4.7 vs. 6.5 months), respectively. Regarding overall survival, duration on palbociclib (≥12 months) had longer overall survival compared with patients who received palbociclib <12 months (p = 0.0012). The overall survival was longer in the secondary-resistant group than in the primary-resistant group (log-rank test p = 0.0003).

Conclusion: In a metastatic setting, low Ki-67 and moderate-to-severe neutropenia are associated with better survival outcomes in endocrine resistant patients treated with palbociclib (Clinical trial registration: www.clinicaltrials.gov identifier is NCT06338644).