Anastrozole vs Exemestane vs Letrozole — Aromatase Inhibitor Comparison
QSC Research Guide
Anastrozole vs Exemestane vs Letrozole — Aromatase Inhibitor Comparison
Three aromatase inhibitors (AIs) available in QSC’s catalog differ fundamentally in mechanism of CYP19A1 inhibition — reversible non-steroidal (Anastrozole, Letrozole) vs. irreversible steroidal suicide inhibitor (Exemestane) — with different implications for oestrogen suppression research.
Side-by-Side Comparison
Attribute
Anastrozole (Arimidex)
Exemestane (Aromasin)
Letrozole (Femara)
Mechanism class
Non-steroidal reversible AI
Steroidal suicide AI
Non-steroidal reversible AI
Receptor targets
CYP19A1 (reversible competitive)
CYP19A1 (irreversible covalent)
CYP19A1 (reversible, potent)
Half-life
~50 hours
~27 hours (but effect permanent)
~48 hours
Key advantage
Reversible binding — oestrogen rebounds fully upon discontinuation. Standard reference AI in clinical research
Permanent CYP19A1 inactivation — oestrogen suppression maintained even after drug clearance. Partial AR agonist activity
Most potent reversible AI — produces near-complete oestrogen suppression. Highest potency of the three
Anastrozole and Letrozole are both reversible non-steroidal AIs — Letrozole is more potent and produces near-complete oestrogen suppression while Anastrozole is the standard clinical reference. Exemestane is fundamentally different — a steroidal suicide inhibitor that permanently destroys CYP19A1 enzyme rather than competitively blocking it. For reversible/washout designs, use Anastrozole or Letrozole; for permanent suppression studies, Exemestane.
Frequently Asked Questions
What is the key difference between Anastrozole and Exemestane?
Anastrozole competitively and reversibly inhibits CYP19A1 — oestrogen returns to baseline after drug clearance. Exemestane covalently and irreversibly destroys CYP19A1 — the enzyme is inactivated until new CYP19A1 is synthesised (weeks). This makes Exemestane significantly more persistent in its oestrogen-suppressive effects.
Which aromatase inhibitor produces the most oestrogen suppression?
In terms of % oestrogen reduction, Letrozole produces the most complete suppression (~98%+ in some studies), followed closely by Exemestane (permanent mechanism) and Anastrozole (~85-95%). Potency ranking: Letrozole ≥ Exemestane > Anastrozole.
Does Exemestane have androgen activity?
Yes. Exemestane is a steroidal AI derived from androstenedione — it has weak partial AR agonist activity. This distinguishes it from non-steroidal AIs and is relevant in research models where androgen receptor activity is a confound.
Research Use Only: All QSC compounds are sold strictly for laboratory research purposes only. Not for human consumption, veterinary use, or any other application. Researchers are responsible for compliance with local regulations.
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