Adherence to adjuvant endocrine therapy: is it a factor for ethnic differences in breast cancer outcomes in New Zealand?

Abstract

Purpose: Despite the benefits of adjuvant endocrine therapy for hormone receptor positive breast cancer, many women are non-adherent or discontinue endocrine treatment early. We studied differences in adherence to adjuvant endocrine therapy by ethnicity in a cohort of New Zealand women with breast cancer and its impact on breast cancer outcomes.

Methods: We analysed data on women (n = 1149) with newly diagnosed hormone receptor positive, non-metastatic, invasive breast cancer who were treated with adjuvant endocrine therapy in the Waikato during 2005-2011. Linked data from the Waikato Breast Cancer Registry and National Pharmaceutical Database were examined to identify differences by ethnicity in adherence to adjuvant endocrine therapy and the effect of sub-optimal adherence on cancer recurrence and mortality.

Results: Overall, a high level of adherence of ≥80% was observed among 70.4% of women, which declined from 76.8% to 59.3% from the first to fifth year of treatment. Māori women were significantly more likely to be sub-optimally adherent (<80%) compared with European women (crude rate 37% vs. 28%, p = 0.005, adjusted OR = 1.51, 95% CI 1.04-2.17). Sub-optimal adherence was associated with a significantly higher risk of breast cancer mortality (HR = 1.77, 95% CI 1.05-2.99) and recurrence (HR = 2.14, 95% CI 1.46-3.14).

Conclusions: Sub-optimal adherence to adjuvant endocrine therapy was a likely contributor for breast cancer mortality inequity between Māori and European women, and highlights the need for future research to identify effective ways to increase adherence in Māori women.

Keywords: Breast cancer; Endocrine therapy; Ethnicity; Inequity; Medication adherence.