Pharmacology · Class
Aromatase inhibitors !
Breast cancer medicines that block aromatase, the enzyme that makes oestrogen from androgens in fat and other tissues after menopause.
Updated 2026-11-01
What they are
Oral hormonal therapy for hormone receptor-positive breast cancer in postmenopausal women, used after surgery to lower the risk of recurrence and for advanced disease. Two groups: non-steroidal reversible inhibitors (anastrozole, letrozole) and the steroidal irreversible inactivator exemestane.
Mechanism of action
After menopause the ovaries make little oestrogen; most of it comes from aromatase (CYP19A1), which converts androstenedione and testosterone into estrone and estradiol in fat, muscle and the tumour itself. Blocking aromatase lowers circulating oestrogen and starves oestrogen-dependent tumour cells. In premenopausal women the ovary keeps producing oestrogen, so these drugs alone do not suppress it.
Target: Aromatase (CYP19A1), a cytochrome P450 enzyme
Drugs in this class
- AnastrozoleA breast cancer tablet for postmenopausal women that blocks the enzyme making oestrogen. It can thin the bones and…Aromatase (CYP19A1)
- LetrozoleA tablet that blocks oestrogen production, used for breast cancer after menopause. Like the other aromatase…Aromatase (CYP19A1)
- ExemestaneA steroidal breast cancer tablet that permanently disables the enzyme making oestrogen after menopause. Bone loss is…Aromatase (CYP19A1)
Test yourself
Think of the answer, then tap the question to see it.
Why are aromatase inhibitors used on their own only after menopause?
Before menopause the ovaries make most of the oestrogen, and lowering oestrogen raises gonadotropins that drive the ovary further. After menopause oestrogen comes mainly from peripheral aromatase, which these drugs block.
Which bone effect separates aromatase inhibitors from tamoxifen in postmenopausal women?
Aromatase inhibitors lower bone mineral density and raise fracture risk, while tamoxifen acts as an oestrogen agonist on bone and tends to preserve it.
How does exemestane differ from anastrozole and letrozole at the enzyme?
Exemestane is a steroid that looks like androstenedione; aromatase processes it into an intermediate that binds irreversibly and inactivates the enzyme. Anastrozole and letrozole are non-steroidal and bind reversibly.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Anastrozole: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2018); DailyMed - EXEMESTANE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2024).
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Representative model, for educational purposes only.