Clinical Experience with Abemaciclib in Patients Previously Treated with Another CDK 4/6 Inhibitor in a Tertiary Hospital: A Case Series Study

SIMPLE SUMMARY: This research aimed to study the effectiveness and safety of a drug called abemaciclib in patients with metastatic breast cancer (MBC) who had previously received similar treatments. The medical records of 11 patients who were given abemaciclib after their disease worsened despite prior therapies were analyzed. The results demonstrated that abemaciclib, alone or in combination with tamoxifen, showed positive effects against MBC. On average, patients experienced six months without disease progression, and some even had significant improvements, such as one patient achieving complete resolution of liver metastases. The most observed side effects, including diarrhea and fatigue, were generally mild. These findings might add further evidence that abemaciclib could be a safe and effective treatment option for MBC patients who have not responded to previous therapies. This research provides valuable insights for making informed decisions regarding future treatment approaches in the medical community. ABSTRACT: The three approved cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, including abemaciclib, have shown differences in their preclinical, pharmacological, and clinical data. Abemaciclib stands out for its broader target range and more rapid and intense activity. It has demonstrated efficacy as a monotherapy or in combination with tamoxifen in endocrine-refractory metastatic breast cancer (MBC) patients with prior chemotherapy. However, the clinical data on abemaciclib after exposure to previous CDK4/6 inhibitors are limited. In this single-center retrospective case series, we identified all patients who received abemaciclib until February 2022 after experiencing documented progression on palbociclib or ribociclib. The safety profile and clinical outcomes of abemaciclib treatment in this specific patient cohort were evaluated. Eleven patients were included in this retrospective case series, nine receiving abemaciclib with tamoxifen. Eight patients had visceral involvement, and the median age was 69 (ranging from 42 to 84). The median time from the end of prior CDK4/6 inhibitor treatment to abemaciclib initiation was 17.5 months (ranging from 3 to 41 months). Patients had undergone a median of three prior therapies (ranging from 1 to 7), including chemotherapy in 54.5% of cases. The median follow-up time was six months (ranging from 1 to 22 months). The median progression-free survival (PFS) was 8 months (95% CI 3.9–12). Five patients continued abemaciclib treatment, and one patient with liver metastases achieved a complete hepatic response. The most common adverse events were diarrhea (72.7%, no grade ≥ 3) and asthenia (27.3%, no grade ≥ 3). Our preliminary findings suggest that abemaciclib could be an effective and safe treatment option for MBC patients who have previously received palbociclib or ribociclib.