Social Determinants of Health (SDoH) and Other Predictors in Treatment Initiation with CDK4/6 Inhibitors in Medicare Patients with HR+/HER2– Metastatic Breast Cancer (MBC)

OBJECTIVES: In HR+/HER2– MBC, the cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) have replaced endocrine therapy alone as the new standard of care; however, several barriers to treatment initiation still exist. In this study, we used the Andersen’s Behavioral Model (ABM) of health services use to assess SDoH and other factors associated with the initiation of CDK4/6i therapy in the United States Medicare population.

METHODS: Using a retrospective cohort design, women aged ≥65 years with HR+/HER2– MBC (diagnosed: 2015-2017) were selected from SEER-Medicare database. Eligible patients had continuous enrollment in Medicare Parts A/B/D during follow-up. Outcome was defined as time from MBC diagnosis to CDK4/6i therapy initiation and assessed descriptively using Kaplan-Meier analysis. Predictors were classified into predisposing, need, and enabling factors (including area-level SDoH measures) based on the ABM, and their effect on the outcome was assessed using the Fine and Gray hazard modeling technique.

RESULTS: Of 752 eligible women, 352 (46.8%) initiated CDK4/6i therapy after MBC diagnosis. The median time to CDK4/6i therapy initiation was 27.9 months (95% confidence interval [CI]=20.0-34.6). In the adjusted analysis, predictors significantly associated with CDK4/6i therapy initiation included: median household income (HHI), high vs. low (hazard ratio [HR]=1.70; 95%CI=1.03-2.81); percentage population with only Medicare coverage, high vs. low (HR=1.54; 95%CI=1.04-2.27); age group, ≥80yrs vs. 65-69yrs (HR=0.53; 95%CI=0.38-0.73); bone metastases, yes vs. no (HR=1.34; 95%CI=1.03-1.75); and diagnosis year, 2017 vs. 2015 (HR=1.67; 95%CI=1.27-2.2).

CONCLUSIONS: In this real-world study, older Medicare patients with HR+/HER2– MBC residing in areas with high median HHI and high percentage of population with Medicare coverage had higher rate of initiating CDK4/6i therapy, suggesting inequitable access to these novel, effective treatments and a need for policy intervention. Patients diagnosed more recently and those with bone metastases had higher CDK4/6i therapy initiation rate; however, those in the older age group had lower rate, possibly relating to frailty concerns.