Exemestane (Aromasin) - Irreversible Steroidal Aromatase Inhibitor — Anabolic Steroids
Irreversible (suicidal) steroidal aromatase inhibitor with the unique advantage of not negatively impacting lipid profiles, FDA-approved for breast cancer and widely used in bodybuilding for estrogen management.
Overview
Exemestane (Aromasin) is a third-generation irreversible steroidal aromatase inhibitor (also called a suicidal inhibitor) that permanently inactivates the CYP19 aromatase enzyme by binding as a false substrate. Unlike anastrozole and letrozole (reversible, non-steroidal AIs), exemestane's binding is covalent, meaning estradiol production only resumes when new aromatase enzyme is synthesized (approximately 2-3 days). The IES trial (Intergroup Exemestane Study; n=4,724) demonstrated a 32% reduction in risk of breast cancer recurrence when switching from tamoxifen to exemestane after 2-3 years (HR 0.68). Uniquely among AIs, exemestane has a neutral-to-favorable lipid profile due to its androgenic steroidal backbone: it does not significantly suppress HDL cholesterol, making it preferred in patients with lipid concerns. The MAP.3 trial (n=4,560) also demonstrated 65% breast cancer risk reduction in high-risk postmenopausal women for chemoprevention. In bodybuilding, exemestane is favored for on-cycle estrogen management due to its lipid-friendly profile and irreversible mechanism. PMID: 15466823, 21145607, 14614199.
Indications
- Adjuvant treatment of postmenopausal HR+ breast cancer after 2-3 years of tamoxifen (FDA-approved; IES trial)
- Advanced/metastatic breast cancer in postmenopausal women after tamoxifen failure (FDA-approved)
- Breast cancer chemoprevention in high-risk postmenopausal women (MAP.3 trial; off-label in some regions)
- Estrogen management during anabolic steroid cycles (off-label, bodybuilding use; preferred for lipid profile)
- Male hypogonadism estrogen management (off-label; alternative to non-steroidal AIs)
Mechanism of Action
Exemestane's structure mimics androstenedione (the natural aromatase substrate), allowing it to enter the aromatase enzyme active site as a false substrate
Dosing
| Compound | Dose | Frequency | Notes |
|---|---|---|---|
| Exemestane (Aromasin) | 25 mg | Once daily after a meal | FDA-approved dose for breast cancer; MUST be taken with food (fatty meal increases absorption by 60%) |
| Exemestane | 12.5 mg | Every other day (EOD) | Common off-label dose for bodybuilding estrogen management; good balance of E2 control and lipid preservation |
| Exemestane | 12.5 mg | Once daily | Higher off-label dose for heavy aromatizing cycles; monitor E2 closely |
| Exemestane | 25 mg | Every other day (EOD) | Aggressive dose; risk of estradiol over-suppression; typically unnecessary |
Evidence Grade
GRADE B
Safety & Contraindications
- Irreversible mechanism means effects persist until new aromatase is synthesized (2-3 days); cannot be rapidly reversed by discontinuation
- Musculoskeletal effects: arthralgia, myalgia in 20-25% of patients; generally milder than with non-steroidal AIs
- Bone mineral density loss occurs but may be less severe than with non-steroidal AIs; still requires monitoring
- Must be taken with a fatty meal for adequate absorption (60% increase in bioavailability with food)
- Hot flashes, fatigue, and headache are common (15-25%); usually mild and manageable
- Mild androgenic steroidal structure: rare reports of acne and hair thinning at standard doses