Letrozole Mechanism of Action
Letrozole is a nonsteroidal competitive inhibitor of the aromatase enzyme that blocks the conversion of adrenal androgens (androstenedione and testosterone) to estrogens (estrone and estradiol), thereby suppressing estrogen biosynthesis in peripheral tissues and breast cancer tissue itself. 1
Molecular Mechanism
Letrozole inhibits the aromatase enzyme by competitively binding to the heme of the cytochrome P450 subunit of the enzyme complex, resulting in reduction of estrogen biosynthesis in all tissues 1. This mechanism is highly specific—letrozole selectively inhibits gonadal steroidogenesis without significant effects on adrenal mineralocorticoid or glucocorticoid synthesis 1.
Clinical Pharmacodynamics
The drug achieves profound estrogen suppression in postmenopausal women:
- Suppresses plasma estradiol, estrone, and estrone sulfate by 75-95% from baseline 1
- Maximal suppression occurs within 2-3 days of treatment initiation 1
- At doses ≥0.5 mg, estrone and estrone sulfate levels often fall below assay detection limits 1
- Inhibits peripheral aromatase by over 98% after 2 weeks of treatment 2
Selectivity Profile
A critical advantage of letrozole is its exceptional selectivity for aromatase without affecting other steroidogenic pathways 1, 3:
- No impairment of adrenal steroidogenesis
- No clinically relevant changes in cortisol, aldosterone, 11-deoxycortisol, 17-hydroxyprogesterone, or ACTH levels
- No accumulation of androgenic precursors (androstenedione and testosterone remain unchanged) 1
- No effects on LH, FSH, or thyroid function (TSH, T3, T4) 1
- Glucocorticoid or mineralocorticoid supplementation is not necessary 1
Comparative Potency
Letrozole demonstrates greater potency than other aromatase inhibitors, including anastrozole, exemestane, formestane, and aminoglutethimide, producing near-complete inhibition of aromatase in peripheral tissues with greater estrogen suppression than achieved with other AIs 3.
Clinical Rationale
In postmenopausal women, estrogens are primarily derived from aromatase-mediated conversion of adrenal androgens, as ovarian estrogen production has ceased 1. By specifically targeting this pathway, letrozole effectively deprives hormone-responsive breast cancers of the estrogen needed for tumor growth and progression 4.