PROJECT SUMMARY-ABSTRACT Poor medication adherence is a leading cause of preventable morbidity and mortality, and patients of low socioeconomic status (SES) are up to 65% more likely to be non-adherent. Breast cancer patients living in high SES areas have 84% five-year survival compared with 68% for low SES, and there are large SES-based differences in breast cancer treatment, particularly use and adherence to hormonal therapy (HT), that likely contribute to these survival differences. In this proposal, we seek to investigate mechanisms for the differences in HT adherence, focusing on potentially remediable health delivery issues. Specifically, we aim to 1) Examine the extent of SES disparities in adherence to oral hormonal breast cancer therapies, and their association with mortality disparities among an all-age, national cohort of women with incident breast cancer, 2) Decompose potential mechanisms (ability to pay, medication complexity, and pharmacy accessibility) of adherence disparities and 3) Simulate the effect of alternative care strategies and policies on HT adherence, breast cancer mortality and SES disparities therein. These aims will be carried out by analyzing a contemporary cohort of women with incident breast cancer identified from the Optum U.S. dataset, which contains sociodemographic, economic (including net worth), medical, and pharmaceutical information for over 35 million patients covered by private and public insurance, which we will link to pharmacy distance measures. We will begin by examining the relationship between measures of SES, adherence and mortality, and then seek to apportion the SES disparities in adherence according to the contribution of each potential factor (or set of factors), using the Oaxaca-Blinder regression-based decomposition approach. Finally, having quantified the net contribution of each factor on SES-specific adherence and differentials therein, we will use parameter estimates generated in Aim 1 to simulate the anticipated effect of alternative policies or interventions (changing subsidy thresholds, improving refill synchronization) in reducing disparities in breast cancer treatment and outcomes. Our goal is to provide policymakers, plan administrators, researchers and advocacy societies with an improved understanding of the health delivery factors that might explain SES disparities in breast cancer outcomes, with an eye to developing effective strategies to eliminate them. By focusing on the experience of a large and diverse cohort of breast cancer patients of all ages, this study will address an important and timely issue, and directly inform efforts to determine how best to allocate resources in cancer as well as other chronic conditions in order to obtain the greatest value and equity.