Adherence to endocrine therapy including ovarian suppression: A large observational cohort study of US women with early breast cancer.

<bold>Background: </bold>Recent clinical trials support adding ovarian suppression (OS) to oral endocrine therapy (ET) for premenopausal women with early breast cancer. The adoption of OS among real-world populations and the impact of OS on ET adherence have not been evaluated.<bold>Methods: </bold>This study examined a retrospective, observational cohort of women under the age of 50 years with incident early breast cancer from 2001 to 2016. The IBM MarketScan Commercial insurance claims database was used to identify new users of ET with or without OS and to track discontinuation of or adherence to ET. In all, 21,948 women filled at least 1 prescription for ET within 12 months of their diagnosis after a washout period of 12 months with no prior claims. Patients who received an aromatase inhibitor without a synchronous OS drug were excluded.<bold>Results: </bold>Use of OS increased over time and reached 11.3% in 2016. In an unadjusted analysis, 40.2% of ET+OS users discontinued ET early, whereas 48.8% of tamoxifen-alone users did. In adjusted analyses, ET+OS users had a similar likelihood of discontinuing ET in comparison with tamoxifen-alone users (hazard ratio, 0.92; 95% confidence interval, 0.83-1.03). Approximately 30% of women had low adherence over the first year of use. The likelihood of high adherence was similar, regardless of OS exposure.<bold>Conclusions: </bold>The use of OS among young, commercially insured patients with breast cancer increased over time in agreement with recent clinical trial results but remained relatively low. Nonadherence to ET was common, but the use of OS was not associated with lower adherence to ET in this observational, nonrandomized cohort. These findings may reassure oncologists that use of OS does not endanger ET adherence, although prospective studies are needed for confirmation. [ABSTRACT FROM AUTHOR]