Racial disparities in overall survival after the introduction of cyclin-dependent kinase 4/6 inhibitors for patients with hormone receptor-positive, HER2 negative metastatic breast cancer

Abstract Purpose: CDK4/6i combined with endocrine therapy have improved HR+/HER2- metastatic breast cancer (MBC) outcomes. However, it is still unclear whether the response to CDK4/6i is similar for all races. Therefore, we aimed to assess overall survival (OS) trends stratified by race in patients with HR+/HER2- MBC after the approval of CDK4/6i, as part of the standard of care, in 2015. Methods: We performed a population-based study using the SEER database. Patients with HR+/HER2- MBC were divided into two time-based cohorts: 1) 2011-2013, corresponding to the pre-CDK4/6i era, and 2) 2015-2017 to the post-CDK4/6i era. We used propensity score matching and identified 2,684 patients in each cohort that matched in several characteristics. Kaplan-Meier curves and Cox proportional hazard models were used to estimate 2-year OS. Additionally, we compared survival between groups with a stratified log-rank test. Lastly, we compared OS and breast cancer death risk between cohorts (stratified by race) using Fine and Gray model. Results: The 2-year OS rate was 65% for the post-CDK4/6i group and 62% for the pre-CDK4/6i cohort (stratified log-rank p=0.025). The 2-year OS for NHW patients improved in the post-CDK4/6i era compared to the pre-CDK4/6i era (67% vs. 63%, p=0.033). However, OS did not improve for NHB (54% vs. 54%, p=0.876) or Hispanic (67% vs 65%, p=0.617) groups. Conclusions: Our study confirms that outcomes for HR+/HER2- MBC have improved after CDK4/6i were introduced in 2015. However, this effect is primarily driven by the improved OS in NHW patients, without significant improvement in OS in NHB or Hispanics.