Pharmacology · Aromatase inhibitors
Exemestane !
A steroidal breast cancer tablet that permanently disables the enzyme making oestrogen after menopause. Bone loss is its main long-term risk.
Updated 2026-11-01
The figure shows exemestane bound in the active site of human placental aromatase, above the heme.
Mechanism of action
Exemestane is a steroid that resembles androstenedione, the natural substrate of aromatase. The enzyme takes it as a false substrate and converts it into an intermediate that binds irreversibly to the active site, inactivating the enzyme (suicide inhibition). New enzyme must be made before oestrogen synthesis recovers.
- Enters the aromatase active site as a false substrate
- The enzyme turns it into a reactive intermediate
- The intermediate binds irreversibly: aromatase is inactivated
- Oestrogen falls; receptor-positive tumours stop growing
Pharmacokinetics
- Absorption
- Taken by mouth after a meal
- Metabolism
- In the liver, by CYP3A4 among other pathways
- Elimination
- About equally in urine and faeces, as metabolites
- Half-life
- About 24 hours
Indications
- Adjuvant treatment of oestrogen receptor-positive early breast cancer in postmenopausal women after several years of tamoxifen Breast cancer →
- Advanced breast cancer in postmenopausal women that progressed on tamoxifen Breast cancer →
Adverse effects
Common
- Hot flashes
- Joint pain
- Fatigue
- Sweating
- Insomnia and headache
Contraindications
- Hypersensitivity to exemestane or any excipient
Key interactions
- Strong CYP3A4 inducers (rifampicin, phenytoin, carbamazepine)
- Lower exemestane exposure.
- Systemic oestrogen-containing products
- Interfere with its action; not given together.
What is monitored
- Bone mineral density
- Vitamin D (25-hydroxyvitamin D) before starting
- Joint symptoms
Administration and safety
- Oral tablet once a day after a meal.
- Hazardous drug: people who are pregnant avoid handling broken tablets.
Nursing care
- Check that the patient is postmenopausal; it is not used for breast cancer before menopause.
- Ask about rifampicin, phenytoin, carbamazepine and other inducers.
- Support bone health: vitamin D, calcium, exercise and bone density checks.
- Teach about hot flashes and joint pain.
Pregnancy and breastfeeding
Can cause fetal harm; women who could become pregnant use effective contraception.
Test yourself
Think of the answer, then tap the question to see it.
Why is exemestane called a suicide inhibitor of aromatase?
The enzyme processes it like its own substrate, and the product binds irreversibly to the active site, so the enzyme destroys itself.
What do rifampicin and phenytoin do to exemestane?
As strong CYP3A4 inducers they lower exemestane exposure, which can weaken its effect.
Which laboratory value is checked before starting exemestane, and why?
25-hydroxyvitamin D, because aromatase inhibitors reduce bone density and vitamin D deficiency adds to the risk.
Related drugs
Related conditions
← Back to the class: Aromatase inhibitors
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Exemestane: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2022); DailyMed - EXEMESTANE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2024); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Crystal structure of human placental aromatase complexed with breast cancer drug exemestane (RCSB Protein Data Bank, 2012).
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Representative model, for educational purposes only.