Estrogen more suppressed with letrozole than anastrozole

April 23, 2008

NEW YORK (Reuters Health) - In postmenopausal women with hormone receptor-positive breast cancer, letrozole suppresses plasma estradiol and estrone sulfate more completely than does anastrozole, according to UK researchers.

In the April 1st issue of the Journal of Clinical Oncology, Dr. J. Michael Dixon of Western General Hospital, Edinburgh, and colleagues note that estradiol is the major active hormone binding to the estrogen receptor in breast cancer cells, and even in low concentrations it can stimulate tumor growth.

To investigate how these agents compare in women receiving aromatase inhibitors as part of their adjuvant treatment, the researchers studied 54 patients who participated in a trial of letrozole and anastrozole given orally once daily.

The women were randomized to receive one of the drugs for 3 months and then cross over and use the other drug for a further 3 months.

Their mean plasma estradiol level at baseline was 25.7 pmol/L. This fell to 3 pmol/L or less in all but one of the subjects (98%) after receiving letrozole. However, following anastrozole, only 37% subjects had estradiol levels of 3 pmol/L or less.

The mean estradiol level after letrozole was 1.56 pmol/L compared to 2.71 pmol/L after anastrozole. In addition, letrozole reduced estrone sulfate to a level that was 43% below the level achieved using anastrozole.

"The study," Dr. Dixon told Reuters Health, "has shown that without doubt letrozole is more potent and effective than anastrozole at what it does, that is, suppressing estrogen."

He added that "both drugs lower estrogen so effectively, that whether the greater potency of letrozole matters in clinical practice is still not clear."

More light will be shed on this matter, he and his colleagues conclude, by the results of a large ongoing phase IIIb clinical trial comparing adjuvant therapy with these drugs.