Aromatase inhibitors are a class of hormonal agents that form part of the therapy for some types of breast cancer. These agents are available as pills and may be started after initial surgery or radiation therapy.

Agents

The aromatase inhibitors include anastrozole (Armidex), letrozole (Femara) and exemastane (Aromasin).

Mechanism of action

The enzyme aromatase works alongside NADPH (nicotinamide adenine dinucleotide phosphate) to convert male hormones or androgens to estrogens. Specifically, androstenedione and testosterone are converted to estrone and estradiol. Aromatase inhibitors prevent the formation of estrogen and thus reduce the estrogen dependent growth of estrogen receptor (ER) positive cancers.

Uses

Aromatase inhibitors are useful in women who have already experienced the menopause and have ER-positive tumors. Since postmenopausal women have small and shrunken ovaries that produce low levels of estrogen, use of aromatase inhibitors may further prevent estrogen production and its stimulation of the tumour growth.

However, among premenopausal who have these ER-positive tumours, the ovaries still produce estrogen at levels which mean aromatase inhibitors will not be able to inhibit tumour growth.

Advantages of using aromatase inhibitors

The advantages of using aromatase inhibitors over other hormonal agents such as tamoxifen include a decrease in vaginal discharge, a reduced risk of blood clotting and less risk of womb cancer.

Side effects

The side effects of taking aromatase inhibitors include:

• Pain in the muscles and joints
• Osteoporosis, brittle bones and fractures resulting from minor traumas
• Confusion and memory impairment

Pharmacokinetics

The aromatase inhibitors anastrozole and letrozole are similar in their pharmacokinetic properties and both have a dosing schedule of once daily due to a half life (time taken for a the amount of a drug present in the body to become halved) of approximately 48 hours for each of them. The half life of exemestane on the other hand, is 24 hours.