Rationale for the Initiation, Outcomes, and Characteristics of Chemotherapy Following CDK4/6 Inhibitors in Breast Cancer: A Real-World Cohort Study

Simple Summary This study investigated one of the treatment options for advanced breast cancer patients after they completed standard therapy that combines hormone therapy and specific targeted agents called cyclin-dependent kinase 4/6 inhibitors. We wanted to understand why in real-world patients start chemotherapy after finishing standard initial treatment, how they respond to it, and what factors might affect their outcomes. We found that chemotherapy was often used when patients faced dissemination to internal organs with the risk of further progression, but it provided limited benefits. We suggest that modern drugs recommended by guidelines before chemotherapy should be better reimbursed in Poland. This research could help doctors make better treatment decisions and improve future clinical trials.Abstract The standard therapy for hormone-receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer includes the use of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) with endocrine therapy. The optimal post-CDK4/6i treatment sequence is unclear. This cohort study evaluated the initiation, characteristics, and outcomes of chemotherapy following CDK4/6i-based treatment. Among the 227 patients who began CDK4/6i therapy, 114 completed it. Seventy-nine female patients received further treatment, including 55 receiving chemotherapy. The average age was 60.1 years. Post-CDK4/6i chemotherapy was typically (69.1%) first-line due to an impending visceral crisis. The median progression-free survival (mPFS) was 3.0 months (range 0.5-18.9), and the median overall survival (mOS) was 8.3 months (0.5-26.1). The median OS from the end of CDK4/6i treatment was 12.4 months (1.5-26.8). In univariate analysis, neither mPFS nor mOS was associated with age, tumor grade, receptor status, Ki67 status, time from diagnosis to CDK4/6i cessation, therapy line, or CDK4/6i type. Dose reduction occurred in 12 patients (21.8%), and chemotherapy was ceased due to adverse events in 8 patients (14.6%). Chemotherapy showed limited benefit regardless of the regimen. The role of chemotherapy may evolve with broader CDK4/6i use in adjuvant treatment.