Beyond the 5-year mark: adherence to and continuation of extended adjuvant endocrine therapy in non-metastatic breast cancer patients.

Purpose: Extended adjuvant endocrine therapy (AET) beyond 5 years (up to 10 years) has been shown to be beneficial for some women with non-metastatic, hormone-receptor positive breast cancer. Much of our current understanding of adherence to and continuation with AET is derived from examining the 5 years after AET initiation and is limited beyond the first 5 years. To address this limitation, we conducted a retrospective cohort study examining AET adherence and time of continuation beyond 5 years among a large, real-world population in Northern California.

Methods: We evaluated adherence to and continuation of extended AET among a cohort of 13,675 women diagnosed with Stage I-III, estrogen receptor positive (ER+) breast cancer between 2008 and 2017 treated at Kaiser Permanente Northern California. Adherence (medication possession ratio > 80%) and discontinuation (last pill possession date before a 180-day gap) of AET were examined.

Results: Annual AET adherence declined gradually each year from 79% in year 1-60% in year 5 and dropped dramatically to 23% in year 6. Following year 6, annual adherence again declined gradually and was 10% in year 10. Similarly, there was a striking decline in AET continuation between year 5 (52%) and year 6 (20%). Only 4.5% continued to the end of year 10.

Conclusions: We observed a dramatic drop in both adherence to and continuation with AET following year 5 into year 6. This potentially represents an inflection point for intervention to improve adherence to and continuation with extended AET among this patient population.