Letrozole
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Product Description
Letrozole is a non-steroidal aromatase inhibitor. Structurally, it features two benzene rings each attached to a cyanide group, connected by a carbon atom linked to a 1,2,4-triazole ring. This unique structure enables Letrozole to potently and selectively inhibit the aromatase enzyme, which is responsible for converting androgens into estrogens. By significantly reducing estrogen levels, Letrozole is primarily used to treat hormone receptor-positive breast cancer in postmenopausal women, both as adjuvant therapy to prevent recurrence and in advanced/metastatic cases. It is also used off-label for ovulation induction in women with fertility issues. Administered orally, Letrozole is highly potent and selective, minimizing side effects by not interfering with other steroid production.
Mechanism of Action
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Inhibits aromatase (CYP19A1), preventing the conversion of androgens (androstenedione, testosterone) into estrogens (estrone, estradiol).
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Reduces estrogen levels by up to 98%, while often increasing testosterone.
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Does not affect mineralocorticoids (aldosterone) or corticosteroids (cortisol, thyroxine).
Uses
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Oncology (Primary Use):
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Hormone receptor–positive breast cancer (postmenopausal women):
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First-line or adjuvant therapy after tamoxifen failure.
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Reduces recurrence risk by suppressing estrogen synthesis.
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Approved by FDA in 1997 for breast cancer treatment.
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Reproductive Medicine:
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Ovulation induction (OI) in PCOS-related infertility:
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Improves live birth and clinical pregnancy rates compared with clomiphene citrate.
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Lower risk of multiple gestation due to higher monofollicular growth.
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Used as first-line treatment in anovulatory PCOS.
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Extended dosing in PCOS: Second course within the same OI/IUI cycle does not significantly affect pregnancy outcomes.
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Fertility Preservation in Cancer Patients:
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Co-administered with FSH during ovarian stimulation in breast cancer patients to keep estrogen levels low.
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Comparable oocyte retrieval, maturation, fertilization, and cryopreservation outcomes to tamoxifen.
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Off-label / Other Reported Uses:
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Bodybuilding / Athletics: Prevents gynecomastia, water retention during anabolic steroid use.
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Pediatric endocrinology: Investigated to delay epiphyseal plate fusion, sometimes used in short stature treatment.
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Properties & Characteristics
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Chemical Name (IUPAC): 4-[(4-cyanophenyl)-(1,2,4-triazol-1-yl)methyl]benzonitrile
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Molecular Formula: C₁₇H₁₁N₅
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Molecular Weight: ~285.3 g/mol
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CAS Number: 112809-51-5
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Identifiers:
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DrugBank: DB01006
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ChEMBL: CHEMBL1444
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KEGG: C08163 / D00964
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UNII: 7LKK855W8I
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Synonyms: Femara®, CGS 20267, 4,4′-(1H-1,2,4-triazol-1-yl-methylene)bis(benzonitrile)
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Structure: Nonsteroidal triazole derivative with benzonitrile groups
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Type: Third-generation, selective, reversible aromatase inhibitor
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Hazards: Classified as Irritant and Health Hazard (GHS07, GHS08)
Safety & Side Effects
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Common Adverse Effects: Hot flashes, fatigue, joint pain, hypoestrogenism-related symptoms.
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Serious Concerns:
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Long-term use may cause osteoporosis; often prescribed with bisphosphonates (e.g., Fosamax).
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Rare hepatotoxicity (low incidence of liver enzyme elevation and rare clinically apparent liver injury).
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High doses (>2.5–5 mg/day for long periods) may impair libido or kidney function.
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