aromatase inhibitor
Meaning of aromatase inhibitor: Lowering the estrogen supply is not the same as blocking its receptor, and that difference decides who the drug can help.
Definition of aromatase inhibitor
An aromatase inhibitor is a medicine that blocks aromatase, the enzyme converting androgens such as testosterone into estrogens. Blocking it lowers the amount of estrogen circulating in the body, which starves breast cancers that depend on estrogen to grow. Because the enzyme does its work in fat, muscle and breast tissue rather than the ovaries, the effect is greatest after menopause.
How does it differ from tamoxifen?
| Feature | Aromatase inhibitor | Tamoxifen |
|---|---|---|
| Action | Reduces how much estrogen is made | Blocks estrogen at its receptor |
| Estrogen level in blood | Falls | Largely unchanged |
| Who it suits | After menopause, or alongside ovarian suppression | Before or after menopause |
| Characteristic effects | Joint pain and stiffness, thinning bone | Hot flashes, clot risk, thickening of the uterine lining |
Both are hormone treatments for breast cancer that carries estrogen receptors, and both are taken by mouth for years rather than months, but they attack the same dependence from opposite ends. Because the mechanisms differ, a cancer that stops responding to one can still respond to the other, and the two are sometimes used in sequence.
Why is menopausal status decisive?
Before menopause the ovaries make most of the body's estrogen, and they answer to signals from the pituitary. Lowering estrogen with an aromatase inhibitor releases the brake on those signals, the ovaries are driven harder, and estrogen production can rise rather than fall. That is why these drugs are used after menopause, or in younger women only in combination with treatment that shuts the ovaries down. After menopause the ovaries have stopped and the aromatase working in fat, muscle and skin becomes the main remaining source, which is exactly what the drug removes.
What follows from very low estrogen?
There are two chemical classes. Nonsteroidal drugs such as anastrozole and letrozole occupy the enzyme reversibly, so blocking stops when the drug stops; the steroidal one, exemestane, resembles the enzyme's natural substrate closely enough to disable it permanently, and fresh enzyme must be made. Whichever is used, estrogen falls to very low levels, and the consequences follow from that. Estrogen protects bone, so loss accelerates and fracture risk rises, which is why bone density is measured during treatment. Joint pain and stiffness are the commonest reason treatment is abandoned early. Hot flashes, vaginal dryness and raised cholesterol also occur. A nonsteroidal aromatase inhibitor is separately used at low dose in fertility care, where a brief fall in estrogen prompts the pituitary to trigger ovulation.
Used in a sentence
An aromatase inhibitor was prescribed as hormone therapy following surgery for the patient's postmenopausal, hormone-receptor-positive breast cancer.
Good to know
Aromatase inhibitors are used after menopause or alongside ovarian suppression, because in a woman with working ovaries the fall in estrogen prompts the pituitary to drive the ovaries harder, which can raise estrogen production rather than lower it.