129P Adherence to adjuvant endocrine therapy in the era of combination treatments: A population-based study

Addition of gonadotropin-releasing hormone-analogues (GnRH) to either tamoxifen (TAM) or aromatase inhibitors (AI) improves survival in premenopausal women with estrogen receptor-positive (ER+) breast cancer (BC) compared to TAM alone and is well-tolerated according to randomized control trials (RCTs). However, study participants of RCTs differ from non-participants in ways that may affect adherence. We therefore wished to assess adherence in a population-based cohort with real-world data. All individuals (n=21898) diagnosed with invasive BC, 2007-2020, in the region of Stockholm-Gotland, Sweden, were identified through the National Quality Register for Breast Cancer. Using the Personal Identity Number, linkages were done to several national healthcare registers including the Prescribed Drug Register. Adherence was computed using the medication possession ratio (MPR) and defined as an MPR of >=80%. Follow-up of adherence started at the first date of dispensation and ended after five years, recurrence, death, emigration, or end of follow-up (January 14, 2022). For combination treatments, adherence to GnRH was added and based on a two-year follow-up. 4118 premenopausal women had early, ER+ BC, and were prescribed adjuvant endocrine treatment. As first treatment, 2648 women retrieved TAM only, 535 retrieved TAM and GnRH, and 352 retrieved AI and GnRH. 78% were adherent to adjuvant endocrine treatment. However, adherence was lower in combination groups: 80% for the TAM-only group, 74% for TAM + GnRH, and 66% for AI + GnRH (chi-squared p<0.001). Multivariate logistic regression yielded an odds ratio (OR) of non-adherence of 1.47 (95% CI 1.16-1.87) for TAM + GnRH and 2.25 (95% CI 1.74-2.90) for AI + GnRH, respectively, compared to TAM only. Results remained unchanged after excluding cases with first dispensation < 5 years before end of follow-up. Adherence was negatively associated with comorbidity (p=0.034) and positively associated with age (p=0.001) and chemotherapy (p<0.001). In this population-based cohort, we found a lower adherence to combination regimes compared to results from RCTs. Future studies should focus on reasons for non-adherence.