Factors Associated with Adherence to Adjuvant Endocrine Therapy Among Privately Insured and Newly Diagnosed Breast Cancer Patients: A Quantile Regression Analysis.

BACKGROUND: Adherence to adjuvant endocrine therapy (AET) for estrogen receptor-positive breast cancer remains suboptimal, which suggests that women are not getting the full benefit of the treatment to reduce breast cancer recurrence and mortality. The majority of studies on adherence to AET focus on identifying factors among those women at the highest levels of adherence and provide little insight on factors that influence medication use across the distribution of adherence.

OBJECTIVE: To understand how factors influence adherence among women across low and high levels of adherence.

METHODS: A retrospective evaluation was conducted using the Truven Health MarketScan Commercial Claims and Encounters Database from 2007-2011. Privately insured women aged 18-64 years who were recently diagnosed and treated for breast cancer and who initiated AET within 12 months of primary treatment were assessed. Adherence was measured as the proportion of days covered (PDC) over a 12-month period. Simultaneous multivariable quantile regression was used to assess the association between treatment and demographic factors, use of mail order pharmacies, medication switching, and out-of-pocket costs and adherence. The effect of each variable was examined at the 40th, 60th, 80th, and 95th quantiles.

RESULTS: Among the 6,863 women in the cohort, mail order pharmacies had the greatest influence on adherence at the 40th quantile, associated with a 29.6% (95% CI = 22.2-37.0) higher PDC compared with retail pharmacies. Out-of-pocket cost for a 30-day supply of AET greater than $20 was associated with an 8.6% (95% CI = 2.8-14.4) lower PDC versus $0-$9.99. The main factors that influenced adherence at the 95th quantile were mail order pharmacies, associated with a 4.4% higher PDC (95% CI = 3.8-5.0) versus retail pharmacies, and switching AET medication 2 or more times, associated with a 5.6% lower PDC versus not switching (95% CI = 2.3-9.0).

CONCLUSIONS: Factors associated with adherence differed across quantiles. Addressing the use of mail order pharmacies and out-of-pocket costs for AET may have the greatest influence on improving adherence among those women with low adherence.

DISCLOSURES: This research was supported by a Ruth L. Kirschstein National Research Service Award for Individual Predoctoral Fellowship grant from the National Cancer Institute (grant number F31 CA174338), which was awarded to Farias. Additionally, Farias was funded by a Postdoctoral Fellowship at the University of Texas School of Public Health Cancer Education and Career Development Program through the National Cancer Institute (NIH Grant R25 CA57712). The other authors declare no conflicts of interest.

DISCLAIMER: The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health. Farias was primarily responsible for the study concept and design, along with Hansen and Zeliadt and with assistance from the other authors. Farias, Hansen, and Zeliadt took the lead in data interpretation, assisted by the other authors. The manuscript was written by Farias, along with Thompson and assisted by the other authors, and was revised by Ornelas, Li, and Farias, with assistance from the other authors.