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PURPOSE: Arimidex, tamoxifen alone, or in combination (ATAC) trial of anastrozole (Arimidex) versus tamoxifen or a combination of the two in 9,366 postmenopausal patients with primary breast cancer found a significant improvement in disease-free survival and time to recurrence (TTR) for anastrozole compared with tamoxifen, that was restricted to patients with hormone receptor-positive (ie, estrogen receptor-positive ER+ and/or progesterone receptor-positive PgR+) disease, the target population for these therapies. We retrospectively tested the hypothesis that this benefit might differ according to PgR status. PATIENTS AND METHODS: TTR was compared between the three treatment groups for subgroups defined by ER and PgR status using Cox's proportional hazards model, with and without adjustment for baseline variables. RESULTS: The unadjusted hazard ratio (HR) for anastrozole versus tamoxifen for TTR was 0.74 (95% CI, 0.64 to 0.87) for women with either ER+ or PgR+ tumors. In the ER+/PgR+ subgroup (n = 3,834) the HR was 0.84 (95% CI, 0.69 to 1.02) compared with 0.43 (95% CI, 0.31 to 0.61) in the ER+/PgR-negative (PgR-) subgroup (n = 880). In the adjusted model the HRs were 0.83 and 0.45, respectively. CONCLUSION: Time to recurrence was longer for anastrozole- than tamoxifen-treated patients in both ER+/PgR+ and ER+/PgR- subgroups, but the benefit was substantially greater in the PgR- subgroup. As this was an "exploratory" analysis, this effect should be considered as hypothesis generating and assessed prospectively in other trials comparing the adjuvant use of an aromatase inhibitor with tamoxifen.
Dowsett et al. (2005) studied this question.