Real-world treatment patterns and adherence to adjuvant abemaciclib among patients with early hormone receptor–positive (HR+) breast cancer (BC).

530 Background: Adjuvant abemaciclib (2 years) is approved in combination with endocrine therapy (ET) for node-positive high-risk early HR+ BC. Although it improves invasive disease-free and overall survival, its long duration and toxicities increase treatment complexity. As adjuvant regimens become more intensive, long-term adherence is critical for real-world effectiveness. However, data outside clinical trials are limited. We examined treatment patterns and factors associated with adherence to abemaciclib in a younger population of patients with early BC. Methods: We conducted a retrospective cohort study using the Merative MarketScan Commercial Claims and Encounters database. Patients aged 18-64 years with early BC who received abemaciclib between 2017 and 2024 were identified. Index date was the date of the first abemaciclib claim. We included patients who underwent breast surgery within one year before the index date and had continuous enrollment for 6 months before and one year after index date. We excluded patients with metastatic disease prior to the index date. Medication adherence was assessed using the proportion of days covered (PDC), with adherence defined as PDC ≥80%. Multivariable logistic regression was used to examine factors associated with abemaciclib adherence at one and two years. Results: Among 640 patients (median age 51 years) treated with adjuvant abemaciclib (median follow up 1.5 years), 55% were adherent to therapy at one year. Adherence declined from 100% in month 1, to 66%, 57%, 55%, and 50% in months 2, 6, 12, and 24 (p<0.001). Overall, 84.5% of patients initiated abemaciclib at 150 mg, and 47.2% had no dose adjustments during treatment. On multivariable analysis of one-year adherence, patients prescribed lower abemaciclib doses (50 or 100mg) were less likely to be adherent compared to those prescribed 150 mg (aOR=0.52; 95%CI 0.31–0.90). Adherence to ET was also associated with higher abemaciclib adherence (aOR=3.63; 95%CI 2.55–5.18). Polypharmacy was associated with reduced abemaciclib adherence among patients taking two (aOR=0.56; 95%CI 0.37–0.85) or ≥3 drug classes (aOR=0.51; 95%CI 0.34–0.78) compared with 0–1 drug class. Patients with high-deductible health plans (HDHP) had higher odds of adherence compared to those with PPO insurance (aOR=2.10; 95%CI 1.23–3.59). At two years (n=213), younger age and adherence to ET were significantly associated with greater abemaciclib adherence. Conclusions: Abemaciclib adherence was low, 55% in year 1, and 50% in year 2. Although lower abemaciclib doses and polypharmacy were associated with lower odds of adherence, ET adherence was associated with higher odds. Adherence in HDHP patients may reflect demographics (White, young, higher income). These findings highlight the need for close patient monitoring and strategies to support long-term adherence to adjuvant treatment.