Real‐world study of palbociclib combined with endocrine therapy for patients with metastatic breast cancer: A comparison of subsequent treatment patterns and HER2 expression analysis

Abstract Background Cyclin‐dependent kinase 4/6 inhibitors combined with endocrine therapy (ET) comprise the standard treatment for patients with hormone receptor‐positive and human epidermal growth factor 2 (HER2)‐negative metastatic breast cancer. The optimal systematic treatment after progression on palbociclib and the role of HER2 expression among these patients remain unclear. Methods The authors retrospectively identified 361 patients who received palbociclib combined with ET. Progression‐free survival (PFS) and overall survival (OS) were analyzed based on subsequent treatments and HER2 status (PFS sub and OS sub , respectively). PFS1 and OS1 were calculated from palbociclib administration to disease progression/death and death from any cause, respectively. PFS sub and OS sub were calculated from subsequent treatment initiation. Results The median PFS1 and OS1 were 10.2 and 39.9 months, respectively. The median PFS sub and OS sub of 111 patients (54.7%) who received chemotherapy were 4.9 months and 20.0 months, respectively, whereas those of 89 patients (43.8%) who received endocrine backbone therapy were 5.9 months and 29.3 months, respectively. Among them, 31 patients (15.3%) who received abemaciclib combined with new ET showed better PFS sub and OS sub (12.2 months and not reached, respectively). The median PFS1 was significantly shorter in the HER2‐low subgroup than in the HER2‐zero subgroup among patients who received second‐line or later palbociclib (6.1 vs. 7.8 months; p = .040) but did not differ among patients who received first‐line palbociclib. Conclusions Various regimens after palbociclib use were received. An improvement was noted in PFS among patients who received endocrine backbone therapy relative to chemotherapy, which may have been secondary to the receipt of chemotherapy by patients with more aggressive disease. HER2 status was not related to the effect of first‐line palbociclib, but it may play a role in later lines.