Tolerance to CD4/6 inhibitor therapy as first line therapy among ER+ patients with metastatic breast cancer.

e13020 Background: CDK4/6 inhibitors are crucial in managing hormone receptor-positive, HER2-negative metastatic breast cancer (MBC), significantly improving progression-free survival (PFS). However, toxicities may necessitate dose reductions or treatment changes. This study examines the influence of demographic factors, including age, race, and area deprivation index (ADI), as well as clinical characteristics such as white blood cell count (WBC), on dose reductions and treatment changes in women receiving first-line CDK4/6 therapy for MBC. Methods: This retrospective study included patients treated with CDK4/6 inhibitors as first-line therapy for MBC at UPMC Hillman Cancer Center between July 1, 2015, and January 11, 2025. Demographics, including age (dichotomized at 59 years), race (White/Black), and ADI (scores 0–100, lower = more deprivation), were analyzed for differences in medication type, dose reductions, WBC at therapy start, transaminase elevation, and therapy duration. Outcomes included time to progression, survival, and death. Descriptive and comparative statistics were applied. Results: Among 158 patients, 81% were White, 7.4% Black, and 1% Other. The mean ADI was 60.01 (SD 24.8), and the mean age at diagnosis was 58.6 years (range 27–83). Black patients were nearly 10 years younger than White patients at diagnosis (49.8 vs. 59.6 years). First-line therapies included palbociclib (54.9%), ribociclib (10.9%), and abemaciclib (9.7%). White patients were more likely to receive palbociclib (76.9% vs. 11% for Black patients), while Black patients were more often prescribed abemaciclib (55.5% vs. 11.5%). Dose reductions occurred in 34.9%, with neutropenia, fatigue, and rash as common reasons, but no significant differences were observed by race, age, or ADI. Mean WBC (7.0; SD 3.3) at therapy start and transaminase elevation (8.6%) were similar across groups. The mean therapy duration was 902 days (SD 801). White patients had a longer duration compared to Black patients (943 vs. 470 days, p=0.06). Of the cohort, 16% (n=28) were deceased, with no differences by demographics. Therapy duration was significantly associated with survival (p<0.01). Race was associated with survival (p=0.05), and age at diagnosis was linked to race (p=0.01). Conclusions: Racial disparities in treatment patterns and outcomes, including shorter therapy durations for Black patients, underscore the need for personalized strategies and equitable care in metastatic breast cancer.