Patterns of use of tamoxifen and aromatase inhibitors: A population-based observational study

Abstract

556 Background: Few data are available outside clinical trials on the use of aromatase inhibitors (AI) as adjuvant treatment for early breast cancer (BC). We therefore used a large population-based database to describe patterns of use of AI over time, in comparison with tamoxifen, as well as switches between these regimens. Methods: We identified 13,479 women treated for BC with tamoxifen, anastrazole, letrozole, or exemestane between 1998 and June 2008 in the UK General Practice Research Database (GPRD). Patients were followed from their first prescription for 5 years or until recurrence, death, switch to another treatment or occurrence of a major thromboembolic or uterine event. Results: Mean age at cohort entry was 62 years (SD=14.0) in the tamoxifen group (n=10,806) and 70.8 (SD = 12.4) in the AI group (n=2,673). Overall, in the first year of treatment 88.8% of patients had prescriptions covering more than 80% of the year (88.3% and 90.8% in tamoxifen and in AI group respectively). Table 1 describes prescriptions covering less than 80% of the days for each year of treatment in 3 subgroups. Among women started on AI therapy diagnosed with BC after 2006, 9.6% switched treatment. Half of them switched from one AI to another AI, the other half switched from AI to tamoxifen. Switches occurred within the first year of treatment in 76% of cases. Among women over 50 years of age who started a tamoxifen therapy after 2000, 31% of women switched to AI in the course of the study, of which 12% within the first year of treatment (11.1% and 13.7% for women diagnosed in 2000–2004 and after 2005 respectively). Conclusions: The real-life patterns of use of tamoxifen and AI therapy demonstrate high rates of adherence. However, the relatively high percentage of switchers among AI users is suggestive of an association with side-effects. [Table: see text] No significant financial relationships to disclose.