Exemestane — Steroidal (type I) aromatase inhibitor — FDA-approved for breast cancer; irreversible mechanism distinguishes it from anastrozole/letrozole; used in TRT for estrogen management with unique anabolic properties

Exemestane is a steroidal (androstenedione-based) aromatase inhibitor with an irreversible ('suicide inhibitor') binding mechanism. This is the key pharmacological distinction: (1) Anastrozole and letrozole are non-steroidal (type II) — they bind competitively and reversibly to the heme iron of CYP19A1; (2) Exemestane (type I) — its androstenedione scaffold binds to the active site of aromatase, is converted by the enzyme to a reactive intermediate that covalently inactivates the enzyme permanently. New estradiol synthesis requires synthesis of new aromatase enzyme (3–7 days). Clinical implications of irreversible mechanism: (a) Missing doses has less impact because aromatase is permanently inactivated until new enzyme is made; (b) No drug present = no competitive removal; (c) Androgenic activity — exemestane retains mild androgenic/anabolic activity from its steroid scaffold (elevates IGF-1, maintains bone density better than non-steroidal AIs; does NOT increase estrogen when stopped like anastrozole rebound). ADDITIONAL BENEFIT: Exemestane does not suppress bone density as aggressively as non-steroidal AIs — clinically relevant for long-term TRT management.