Initiation of Antiresorptive Drug Treatment during Endocrine Therapy for Breast Cancer-A Retrospective Cohort Study of 161,492 Patients in Germany

Simple Summary Endocrine therapy (ET), which significantly reduces breast cancer (BC) recurrence and mortality rates, is the primary systemic therapy for hormone receptor positive BC. However, in addition to frequently reported TAM- and AI-related side effects, such as hot flashes, arthralgias, and myalgias, treatment with AIs in particular leads to accelerated bone loss (AI-associated bone loss, AIBL) and increased bone fracture risk. To the best of our knowledge, little is known about the prescription spectrum of antiresorptive drugs in BC patients treated with ET in Germany. To explore this further, we conducted a retrospective cohort study that included 161,492 patients under ET to measure the cumulative incidence of antiresorptive drug prescription for TAM and AIs and estimate the relationship between initial drug (AIs versus TAM) and antiresorptive drug prescription. Finally, our study provides an overview of the most frequently prescribed antiresorptive drugs in Germany. Background: The aim of this retrospective cohort study was to measure the proportion of women with an initial prescription of an antiresorptive drug (bisphosphonates or denosumab) during five years of endocrine breast cancer therapy. Methods: The study included women with an initial prescription of tamoxifen (TAM) or aromatase inhibitors (AIs) between January 2016 and December 2020. Kaplan-Meier analyses were performed to show the cumulative incidence of antiresorptive drug prescription for TAM and AIs separately for four age groups. A univariable Cox proportional hazards regression model was also used to estimate the relationship between initial endocrine drug (AIs vs. TAM) and antiresorptive drug prescription. Results: Within 5 years, 14.1% of patients on AI and 6.1% on TAM received their first prescription for an antiresorptive drug (p < 0.001). The difference between AI and TAM was greatest in women <= 50 years (12.9% of AI and 2.8% of patients on TAM), and smallest in women >80 years (14.5% of AI and 10.3% of patients on TAM). The proportion of denosumab was 46.2% among AI patients vs. 29.1% among patients on TAM (p < 0.001) as alendronate was prescribed to 36.9% of AI vs. 50.0% of patients on TAM. Conclusions: Across all age groups, the cumulative incidence of antiresorptive drug prescriptions was higher in patients with BC treated with AI than those receiving TAM. Denosumab was most frequently used as an antiresorptive drug in patients treated with AI, while alendronate was administered more often in patients treated with TAM.