There is known drug-drug interaction of tamoxifen with anastrozole and letrozole. Concomitant administration of either anastrozole or letrozole with tamoxifen decreases the plasma level of the AI. Concomitant administration of letrozole and tamoxifen decreased the level of letrozole by 38% (90% confidence interval, 32–43%; ). Anastrozole and tamoxifen administrated concomitantly in the ATAC trial lowered the plasma anastrozole level in the combined arm by 27% (90% confidence interval, 20–30%

. Whereas the combined therapy arm of the ATAC trial demonstrated a decrease in the plasma concentration of anastrozole, systemic E2 suppression was similar to that observed in patients treated with anastrozole alone. Based on the observed drug-drug interaction and the lack of clinical data demonstrating the superiority of the combined arm, this arm of the study is being discontinued.
In the ATAC study in early-stage breast cancer, a third combination arm of tamoxifen plus anastrozole was studied. Early results from the trial suggested that women receiving the combination had a comparable outcome in terms of disease recurrence compared with those taking tamoxifen alone. The concept behind this trial was total estrogen blockade in the hope of improving the therapeutic efficacy while exploiting the positive effects of tamoxifen on bone and lipid metabolism to counter the putative negative effects of the aromatase inhibitor. Before the ATAC trial, Brodie
et al.had used a nude mouse model bearing aromatase up-regulated MCF-7 cells to predict that the combination arm would be less effective than the aromatase inhibitor alone. This was true for both anastrozole and letrozole in this preclinical model. The most popular hypothesis for failure of tamoxifen to add to the aromatase inhibitor’s effects is the suggestion that tamoxifen’s agonist effects are provoked or exaggerated in the presence of low estrogen levels.