Background: Appropriate adverse event (AE) management and maintenance of therapeutic intensity are necessary to achieve therapeutic benefits of CDK4/6 inhibitors (palbociclib and abemaciclib) in hormone receptor-positive, HER2-negative metastatic/recurrent breast cancer. Objective: This study was aimed at clarifying the effect of AEs associated with palbociclib and abemaciclib on treatment. Methods: A total of 62 and 49 patients were prescribed palbociclib and abemaciclib, respectively, at our hospital from January 1, 2018 to June 30, 2023. The rate and reasons for treatment discontinuation, interruption of administration, and changes in dose and dosing schedule, treatment duration, and relative dose intensity (RDI) were compared between the groups of patients prescribed the 2 treatments. Results: Treatment discontinuation due to AEs occurred more frequently with abemaciclib (12 patients) because of interstitial lung disease and hepatic and renal events than with palbociclib (5 patients; P = .008). Administration was interrupted in 57 (91.9%) and 35 (71.4%) patients treated with palbociclib and abemaciclib, respectively (P = .004). Dose reduction occurred in 37 (67.3%) and 19 (47.5%) patients treated with palbociclib and abemaciclib, respectively (P = .053). The median [range] treatment duration was 301 [21-1643] days for palbociclib and 238 [70-1526] days for abemaciclib (log-rank test, P = .581). The median RDI was 59.7% and 59.6% for palbociclib and abemaciclib, respectively (P = .539). Although the AEs of palbociclib and abemaciclib affected the treatment considerably, the treatment duration and RDI were similar. Conclusion: CDK4/6 inhibitors should be selected based on the tolerability and manageability of each AE.
Keywords: CDK4/6 inhibitor; HER2-negative; hormone receptor-positive; interruption of administration; metastatic/recurrent breast cancer.