CDK4/6 Inhibitors in Patients Aged 80 and Older with HR+/HER2− Metastatic Breast Cancer: A Real-World Multicenter Study

: Background: Older adults aged ≥80 with HR+/HER2- metastatic breast cancer (MBC) are often underrepresented in clinical trials, leaving clinicians with limited data to guide treatment decisions. Given the increasing prevalence of this age group, real-world evidence is crucial to inform therapeutic strategies. Methods: We performed a multicenter retrospective study across seven Italian oncology units, focusing on patients aged 80 or above who received CDK4/6 inhibitors in combination with endocrine therapy between January 2020 and May 2024. Baseline characteristics, treatment details, and adverse events were recorded. Primary endpoints were progression-free survival (PFS) and overall survival (OS); toxicity was assessed using CTCAE v5.0. Follow-up was estimated using the reverse Kaplan-Meier method. Results: Eighty patients were included, with a median age of 83. Over half had visceral metastases, and 41.0% were frail (G8 ≤ 14). Approximately 44.0% of patients started treatment at a reduced dose. The median PFS was 13 months (95.0% CI 9.3-18.0), and the median OS was 15 months (95.0% CI 11.8-18.2). Hematologic toxicity was frequent, with neutropenia occurring in 58.8% (25.0% grade ≥ 3) and anemia in 12.5% (2.5% grade ≥ 3). Asthenia was reported in 16.2% (5.0% grade ≥ 3). Diarrhea occurred in 3.8% overall, mainly in patients treated with abemaciclib (42.9% any grade; 14.3% grade ≥ 3). ALT/AST elevations were observed in 8.8% (1.2% grade ≥ 3), and QTc prolongation in 2.5% (1.2% grade ≥ 3, all with ribociclib). Grade ≥ 3 events were uncommon and generally manageable. Conclusions: CDK4/6 inhibitor-based therapy is feasible in patients aged ≥80 years, with outcomes and tolerability comparable to those observed in younger elderly. Our results highlight the importance of individualized treatment strategies in this oldest-old population.