Letrozole suppresses plasma oestradiol (E2) levels more completely than anastrozole in postmenopausal women with breast cancer

Abstract

552 Background: Letrozole (L) is a more potent aromatase inhibitor in vitro than anastrozole (A). One study in 12 patients showed that in patients L provides more complete inhibition of whole body aromatase and suppression of estrone sulphate levels than A but L did not significantly suppress E2 levels more than A possibly because of the small size of the study and the difficulty of measuring E2 in postmenopausal women (Geisler et al, JCO, 20, 751, 2002). The current aim was to conduct a much larger study to determine whether 2.5mg L suppresses E2 significantly more than 1mg A and if so in what proportion of patients this is the case. Methods: 54 postmenopausal women with invasive estrogen receptor positive breast cancer were randomised as part of their adjuvant hormone therapy to receive either: 12 weeks of L followed by 12 weeks of A (L→A) or 12 weeks of A followed by 12 weeks of L (A→L). Blood for hormones were taken at the same time of the day before and after 12 weeks of each drug. E2 was measured by a highly sensitive radioimmunoassay (Dowsett et al, Cancer Res 1987;47:1957–61) with a formal detection limit of 3pmol/l. In this study we also quantified values by extrapolation below this limit. Results: 27 patients had L→A and 27 A→L. Baseline E2 levels varied from 3 to 91 pmol/l with a median of 26 pmol/l. Only 1of 54 (2%) patients had an E2 value ≥ 3pmol/l after L compared with 20 of 54 (37%) after A(p <0.000005). After extrapolation, mean E2 level after A was 2.91 (SEM 0.18) pmol/l with a median of 2.70 pmol/l and after L was 1.76 (SEM 0.10) pmol/l with a median of 1.70 pmol/l (p<0.0001). Mean residual estradiol was 9.2% of baseline with A and 5.6% with L. Conclusions: This study has demonstrated unequivocally that the more complete inhibition of aromatase achieved by 2.5mg of letrozole than 1mg of anastrozole results in a greater degree of suppression of E2, the most bioactive oestrogen. [Table: see text]