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

523 Background: Extending adjuvant endocrine therapy (AET) beyond five years has been shown to be beneficial for women with non-metastatic, hormone-responsive breast cancer. While many studies have examined adherence to (taking medication according to prescribed regimen) and continuation of (taking medication for prescribed duration) AET, most have focused on the first five years of use and data beyond that period are lacking. To address this knowledge gap, we conducted a large retrospective cohort study to determine adherence to and continuation of AET beyond the first 5 years among a cohort of non-metastatic breast cancer patients treated in a community setting. Methods: We estimated adherence to and continuation of extended AET (tamoxifen or aromatase inhibitor) among 13,675 women at Kaiser Permanente Northern California (KPNC), an integrated healthcare system caring for approximately 4.5 million members. The cohort consisted of women diagnosed with Stage I-III, estrogen receptor positive (ER+) breast cancer and were treated at KPNC from 2008 to 2017. Adherence was defined as medication possession ratio >80% for each 12-month period during follow-up. Continuation was defined as time to last pill possession date before a 180-day gap of AET and was based on the Kaplan-Meier estimator. Both adherence and continuation were examined for the 10-year period after initiation. Results: Of the 13,675 eligible women who initiated AET, 81% were 50 years or older, 61% were white and 76% had an Elixhauser Comorbidity Score of 2 or less. Most women had Stage I or II breast cancer (90.9%) that was progesterone receptor positive (79%) and HER2 negative (84%). We observed a gradual decline in adherence to AET each year from 79% in year 1 to 60% in year 5, followed by a dramatic drop to 23% in year 6. After year 6, annual adherence again dropped gradually and was 10% in year 10. Median AET continuation time was 5.1 years. Similarly, there was also a striking decline in AET continuation between year 5 and 6, with 52% continuing AET to the end of year 5 and 20% continuing to the end of year 6. Only 4.5% of the cohort continued to the end of year 10. Conclusions: We observed a substantial drop in both adherence to and continuation of AET following year 5 into year 6 with only 4.5% of the cohort continuing to the end of year 10. This period may represent an opportunity for intervention to improve use of extended AET. Future studies are needed to identify factors affecting adherence to and continuation with extended AET.