(P-231) Association Between Non-adherence to Endocrine Therapy and the Risk of Recurrence Among Breast Cancer Survivors: A Systematic Literature Review

Background: Endocrine therapy is the mainstay of treatment for breast cancer (BC) to reduce the risk of BC recurrence. Treatment-related adverse events (e.g., musculoskeletal/joint pain) may result in treatment non-adherence or discontinuation and, thus, may increase the risk of BC recurrence.

Objectives: We aimed to examine the association between non-adherence to endocrine therapy and recurrence risk among BC survivors by conducting a systematic literature review.

Methods: We conducted a systematic search of PubMed/MEDLINE for randomized controlled trials and observational studies examining associations between adherence to endocrine therapy and recurrence in BC survivors through 10/1/2020. We excluded non-English articles, case reports, reviews, meta-analyses, and editorials/commentaries.

Results: We screened 453 articles and included 6 observational studies (all BC stages [n=2]; non-metastatic [n=3]; not reported [n=1]). Definitions of non-adherence in the studies varied: proportion of days covered (PDC) <80% (n=2), or PDC <90% (n=1), medication possession ratio <80% (n=1), prescription refill gaps >28 days (n=1), and cumulative prescription refill gaps >180 days (n=1). The studies’ reported outcomes included time to recurrence after BC diagnosis (n=3) or BC surgery (n=1) and recurrence within 1 (n=1) or 4 years (n=1) of endocrine therapy. Four out of five European studies found that non-adherence to endocrine therapy was associated with an increased recurrence risk (adjusted hazard ratios range=1.39-1.71), although adherence measurement periods varied within 2.7–5 years. One US study found no association between recurrence risk and non-adherence to endocrine therapy for one year after initiating treatment.

Conclusions: This systematic review suggests that long-term non-adherence to endocrine therapy is associated with an increased recurrence risk. Future observational studies are needed to investigate the impact of early non-adherence to endocrine therapy on recurrence risk, and to identify potential barriers to optimal medication adherence among BC survivors.