Endometriosis occurs when tissue that usually lines the inside of the uterus instead grows outside on the ovaries, bowel, or bladder or behind the uterus. This can lead to pain, heavy periods, or infertility, among other problems.
The female hormone estrogen has been linked to the growth of endometriosis patches. Letrozole, one of a class of drugs known as aromatase inhibitors, reduces the body’s production of estrogen by blocking the activity of aromatase, the enzyme that produces estrogen. Letrozole is sometimes used to treat endometriosis, although it has not been approved by the Food and Drug Administration for this use.
To examine the effect of letrozole on the growth of endometriosis and the production of aromatase, scientists funded by the Fertility and Infertility Branch studied 16 baboons with endometriosis. Ten of the baboons were treated with letrozole. The remaining six baboons were given a placebo.
The researchers found that the 10 treated baboons had much smaller patches of endometriosis after treatment, while the patches in the placebo-treated baboons had grown. The letrozole-treated baboons produced less aromatase, suggesting that aromatase and the estrogen it produces play an important role in endometriosis.
Previous studies of aromatase inhibitors in women have suggested that these drugs work to treat endometriosis, and the results of this study illustrate the details of how they do so (PMID: 23257603).