Estrogen plus aromatase inhibitor: not a good mix

Last Updated: 2006-01-26 15:58:49 -0400 (Reuters Health)

NEW YORK (Reuters Health) - By increasing levels of estrogen in the body, use of vaginal estrogen products may counter the effects of drugs called aromatase inhibitors and thereby raise the risk of breast cancer recurrence, suggest the results of a small study.

Aromatase inhibitors work to prevent cancer recurrence by dramatically reducing circulating levels of estrogen, a hormone known to stimulate cancer growth, researchers explain in the Annals of Oncology.

Roughly a fifth of women who use aromatase inhibitors have vaginitis resulting from a lack of estrogen. While oral hormone replacement therapy could, in theory, help aromatase inhibitor users with this condition, it is not recommended due to its ability to raise levels of estrogen in the body. Therefore, vaginal estradiol products are often used -- yet it was unclear if they might also increase estrogen levels in the blood.

To investigate, Dr. Ian Smith, from the Royal Marsden Foundation NHS Trust in London, and colleagues assessed serum estradiol levels in seven aromatase inhibitor users before and for several weeks after starting vaginal estradiol therapy. Six of the women used a vaginal estradiol tablet called Vagifem and one used Premarin estradiol cream.

Six of the women displayed a significant increase in serum estradiol levels 2 weeks after starting vaginal estradiol therapy. Serum estradiol levels typically fell after several more weeks of therapy, but only two women had levels that returned to pretreatment levels. Moreover, two women had levels that continued to rise after several weeks of therapy.

"Although the study is small, the magnitude of the effect in such a high proportion of the women strongly indicates it would be reproduced in a larger study population," Smith said in a statement.

"It is particularly important that doctors are aware of the new findings, as increasing uptake of aromatase inhibitor therapy means that it is likely that around 15,000 new women a year in the UK would be receiving aromatase inhibitors, a total of 100,000 at any one time," Smith noted.

SOURCE: Annals of Oncology, online January 25, 2006.