Real-world treatment outcomes in patients with HR+ HER2- advanced breast cancer treated with CDK4/6 inhibitors and endocrine therapy.

1067 Background: The addition of cyclin-dependent kinases 4 and 6 inhibitors (CDK4/6i) to endocrine therapy (ET) improves progression-free (PFS) and overall survival (OS) in hormone receptor-positive, HER2-negative advanced breast cancer (HR+/HER2- ABC) in 1 st and 2 nd line setting. The international guidelines advise using 1 st line CDK4/6i; however, its superiority over 2 nd line use is debatable. Methods: We retrospectively analyzed clinical data of HR+/HER2- ABC pts administered palbociclib (PAL), ribociclib (RIB) or abemaciclib (ABM) added to either 1 st or 2 nd line ET. Results: A total of 1511 pts (PAL=507; RIB=724; ABM=280) from 11 cancer Polish centers treated between Sep 2017 and Dec 2023. Median follow-up was 29.6 (95%CI 28.2 to 30.9) months (m). Mean age was 62.6±11.9, 58.9±12.2 and 61.4±10.8 years in PAL, RIB and ABM, respectively. 815 patients (53.9%) had visceral and 1109 (73.4%) bone metastases. 1254 pts (83%) received 1 st line CDK4/6i, in 841 (67%) combined with aromatase inhibitor (AI) and in 413 (33%) with fulvestrant (FUL). Second-line CDK4/6i was used in 257 pts (17%); 59 (23%) and 198 (67%) with AI and FUL, respectively. There was no difference in PFS, PFS2, and OS between three CDK4/6i in combination with AIs in 1 st and 2 nd line setting. Median PFS (95%CI) for 1 st line AI with PAL, RIB, ABM were 29.0 (21.6-37.0), 32.0 (24.5-36.9), not reached (N/R) (21.9-N/R) m; PFS2: 37.0 (28.8-N/R), 33.0 (29.5-40.2), N/R (28.9-N/R); OS: 48.9 (42.9-N/R), 48.6 (41.9-N/R), N/R (36.7-N/R). Median PFS (95%CI) for 2 nd line AI with PAL, RIB, ABM were 11.9 (9.49-N/R), 10.8 (8.27-N/R), 11.0 (6.43-N/R); PFS2: 22.7 (16.8-N/R), 32.3 (24.9-N/R), 31.0 (13.2-N/R); OS: 27.4 (24.4-N/R), 36.7 (30.1-N/R), 45.5 (29.0-N/R) (no statistical significance). In contrast, RIB+FUL was superior to PAL+FUL and ABM+FUL in both lines (Table). The safety profile was concordant with the reported data for ET + CDK4/6i. Conclusions: Identifying predictive factors of response is essential for the rational use of CDK4/6i and ET in HR+/HER2- ABC. Table: see text