(RW) post-progression outcomes after first-line (1L) treatment with ribociclib + an aromatase inhibitor (AI) vs AI alone in US patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2–) metastatic breast cancer (MBC).

e13044 Background: Ribociclib remains the only cyclin-dependent kinase 4/6 inhibitor to consistently demonstrate overall survival (OS) benefits in the MBC setting, with statistically significant and clinically meaningful OS results reported in all three MONALEESA studies in HR+/HER2− MBC. However, because of the short follow-up period observed among RW patients treated with ribociclib, comprehensive evaluation of OS remains limited. Hence, RW progression-free survival 2 (rwPFS2) and chemotherapy-free survival (rwCFS) can serve as reliable post-progression endpoints. Methods: This retrospective study utilized deidentified electronic health record–derived data from the Flatiron Health Research Database: 5,649 US female patients (including 3,466 patients ≥65 years of age) with HR+/HER2− MBC who initiated 1L ribociclib in combination with an AI or AI monotherapy between January 1, 2020 and January 31, 2025 with follow-up through July 31, 2025. Analyses of rwPFS2 (time from start of 1L to first disease progression during 2L, or death during 1L or 2L, whichever occurred first) and rwCFS were performed using unadjusted and adjusted time-to-event methods, including the Kaplan-Meier method and Cox proportional hazards regression. To account for differences in baseline characteristics, inverse probability of treatment weighting with stabilized weights was used (sIPTW). Key patient demographics and clinical characteristics were used for adjustment; details to be provided in the full presentation. Results: After sIPTW, median rwPFS2 was extended by 14.1 months with ribociclib plus AI compared with AI monotherapy (hazard ratio [HR], 0.58 [0.52–0.65]) in the overall cohort. Among patients aged ≥65 years, the extension was 16.7 months (HR, 0.58 [0.49–0.68]). Moreover, median rwCFS improved by 12.5 months with ribociclib plus AI vs AI alone in the overall cohort (HR, 0.64 [0.57–0.72]) and by 13.3 months in those aged ≥65 years (HR, 0.60 [0.51–0.71]). Conclusions: These RW findings augment clinical trial results, reinforcing that ribociclib provides consistent and substantial post-progression survival benefits in patients with HR+/HER2− MBC, including patients aged ≥ 65 years.