Letrozole Mechanism of Action
Letrozole is a nonsteroidal competitive inhibitor that blocks the aromatase enzyme by binding to the heme group of its cytochrome P450 subunit, thereby preventing the conversion of adrenal androgens (androstenedione and testosterone) to estrogens (estrone and estradiol) in peripheral tissues. 1
Primary Molecular Mechanism
Letrozole competitively binds to the heme of the cytochrome P450 subunit of the aromatase enzyme, resulting in near-complete inhibition of estrogen biosynthesis in all tissues. 1
This binding is reversible, distinguishing letrozole from steroidal aromatase inhibitors like exemestane that bind irreversibly. 2
The drug achieves greater than 98% inhibition of peripheral aromatase activity and suppresses blood and urinary estrogen levels by over 95% within 2 weeks of treatment. 3
Estrogen Suppression Effects
Letrozole produces 75-95% suppression of plasma estradiol, estrone, and estrone sulfate from baseline, with maximal suppression achieved within 2-3 days of initiating therapy. 1
The drug markedly suppresses both circulating and intratumoral estrogen levels, causing decreased immunoexpression of proliferation markers in hormone receptor-positive breast cancer. 2
Estrogen suppression is dose-related, with doses of 0.5 mg and higher producing estrone and estrone sulfate levels below the limit of detection in assays. 1
Selectivity Profile
Letrozole is highly specific for aromatase inhibition and does not significantly affect other steroidogenic pathways. 1, 4
There is no impairment of adrenal steroidogenesis—plasma concentrations of cortisol, aldosterone, 11-deoxycortisol, 17-hydroxyprogesterone, and ACTH remain unchanged. 1
Glucocorticoid or mineralocorticoid supplementation is not necessary because ACTH stimulation testing shows no attenuation of aldosterone or cortisol production. 1
Plasma levels of androgens (androstenedione and testosterone), LH, FSH, and thyroid function (TSH, T3, T4) are not affected by letrozole. 1
Clinical Implications of Mechanism
Anti-Tumor Effects
By depleting estrogen, letrozole causes regression of estrogen-dependent tumors and is as effective as ovariectomy in reducing uterine weight in animal models. 1
The drug produces clear anti-estrogenic and anti-proliferative effects in estrogen receptor-rich tumors, with significant reductions in progesterone receptor expression and the cell-cycle marker Ki67. 5
Postmenopausal-Specific Action
Letrozole only works effectively in postmenopausal women because in this population, estrogens are mainly derived from peripheral aromatization of adrenal androgens rather than ovarian production. 1, 2
The drug blocks aromatase-dependent synthesis of estradiol but cannot adequately suppress ovarian estrogen synthesis in premenopausal women with functioning ovaries. 6
Adverse Effects Related to Mechanism
The profound estrogen suppression accounts for adverse effects on bone metabolism, leading to increased bone resorption (25% increase in C-telopeptide crosslinks) and fracture risk. 2, 7
Letrozole causes impaired conversion of androgen to estradiol in the brain and decreased density of dendritic spines on synapses, potentially contributing to cognitive impairment. 6
Comparative Potency
Letrozole demonstrates greater potency than other aromatase inhibitors, including anastrozole, exemestane, formestane, and aminoglutethimide. 4
The drug produces near-complete inhibition of aromatase in peripheral tissues and achieves greater estrogen suppression than other third-generation aromatase inhibitors. 4