Using anastrozole as initial adjuvant treatment prevents early recurrences and reduces adverse events: Updated data from the ATAC (‘Arimidex’, Tamoxifen, Alone or in Combination) trial

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582 Background: Recent interest in the delayed introduction of aromatase inhibitors (AIs) as adjuvant therapy after tamoxifen has highlighted the need to evaluate the relative effectiveness of AIs and tamoxifen over the standard 5-year adjuvant period. Methods: The ATAC trial is a double-blind, randomized trial designed to compare anastrozole (n=3125) with tamoxifen alone (n=3116) or in combination with anastrozole (n=3125), as adjuvant therapy in 9366 postmenopausal women with early breast cancer. The ATAC trial is currently the only trial to have reported long-term direct comparisons of the efficacy and tolerability of an AI with tamoxifen. Results: A total of 1226 first events in the two monotherapy arms were included in the recent ATAC completed treatment analysis (median follow-up 68 months). Anastrozole continued to demonstrate a significantly lower number of first events compared with tamoxifen (575 vs 651, respectively; HR 0.87; 95% CI: 0.78, 0.97; p=0.013), and was associated with a beneficial additional event profile (Table). Conclusions: Initiating tamoxifen treatment with the intention of switching to an AI after 2.5 or 5 years would lose these early benefits of anastrozole over tamoxifen. The higher rates of recurrence, AEs and withdrawals associated with adjuvant tamoxifen at 2.5 and 5 years justify offering adjuvant therapy with anastrozole at the earliest opportunity. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration AstraZeneca AstraZeneca AstraZeneca