Patterns of endocrine therapy in a national cohort of early stage HER2-positive breast cancer patients

Purpose: Guidelines recommend ≥5 years of endocrine therapy for hormone receptor-positive breast cancer patients, but nonadherence and treatment discontinuation are common. We examined adherence trajectories and early discontinuation of endocrine therapy over 5 years from treatment initiation. Methods: Our retrospective cohort study used a national sample of Australian women dispensed publicly subsidised trastuzumab for early HER2-positive breast cancer. We included women initiating endocrine therapy between April 2007 and June 2011, followed until June 2016 (n = 2656). We used group-based trajectory modelling and Kaplan-Meier analysis to examine patterns of adherence and time to discontinuation and restarting. Results: We identified five adherence trajectories: quick decline (10.4%), moderate decline (8.6%), quick decline then stable (10.3%), stable with late decline (23.6%), and high and stable (47.2%). Women in the high and stable trajectory group were older and more likely to initiate therapy with anastrozole than women in other groups. Time periods after first 6 months, 1.5, and 4 years from initiation seemed critical in terms of remaining adherent on endocrine therapy; 45.8% of the cohort discontinued endocrine therapy with a median time to discontinuation of 2.6 years (interquartile range 1.0-4.4), and 45.8% of the women discontinuing restarted treatment (median time 182.0, interquartile range 133.0-279.0 days). Conclusions: Our study highlights evidence-practice gaps in the use of endocrine therapy, with half of our sample experiencing suboptimal adherence or persistence. Trajectory modelling provided detailed information about patterns of nonadherence and critical time periods for adherence to endocrine therapy. This information is important for developing targeted interventions to improve adherence.