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Anastrozole

Anastrozole

Arimidex Anastra Anazolate

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Updated: December 06, 2025

Overview

Anastrozole is a potent, nonsteroidal aromatase inhibitor used primarily in the treatment of hormone receptor-positive breast cancer in postmenopausal women. It works by reducing estrogen production, which slows or stops the growth of estrogen-dependent tumors. Anastrozole is considered a first-line agent for adjuvant treatment of early breast cancer and management of advanced metastatic disease. It has shown superior efficacy compared to tamoxifen in certain clinical settings with a different side effect profile, particularly regarding thromboembolic risks.

Drug Class

Nonsteroidal aromatase inhibitor Antineoplastic agent Hormone therapy

Mechanism of Action

Anastrozole selectively inhibits the aromatase enzyme (cytochrome P450 CYP19) responsible for converting androgens (androstenedione and testosterone) into estrogens (estrone and estradiol). This inhibition leads to: • Significant reduction of circulating estrogen levels (up to 80%) • Deprivation of estrogen stimulation in hormone-sensitive breast tumors • No effect on adrenal corticosteroid or aldosterone synthesis

Pharmacokinetics

Absorption: Well absorbed orally (83.1% bioavailability), not affected by food Time to peak plasma concentration: 2-5 hours Distribution: Volume of distribution: 83-87 L Protein binding: 40% Metabolism: Extensive hepatic metabolism via N-dealkylation, hydroxylation, and glucuronidation Primary isoenzyme: CYP3A4/5 Excretion: Urine (85%, mostly as metabolites) Feces (11%) Half-life: Terminal elimination half-life: 50 hours

Pharmacodynamics

• Estrogen suppression begins within 24 hours • Maximal estrogen suppression achieved in 3-7 days • Causes tumor regression in estrogen receptor-positive breast cancers • May decrease bone mineral density due to estrogen suppression • Increases serum gonadotropins (LH and FSH) through feedback mechanism

Indications

• Adjuvant treatment of postmenopausal women with hormone receptor-positive early breast cancer • First-line treatment of postmenopausal women with hormone receptor-positive or hormone receptor-unknown locally advanced or metastatic breast cancer • Treatment of advanced breast cancer in postmenopausal women with disease progression following tamoxifen therapy

Contraindications

Absolute: • Pregnancy • Premenopausal women • Hypersensitivity to anastrozole or its components Relative: • Severe hepatic impairment • Osteoporosis or high fracture risk • Ischemic heart disease

Dosage & Administration

Adjuvant treatment of early breast cancer: 1 mg orally once daily Advanced or metastatic breast cancer: 1 mg orally once daily Dose adjustments: • No routine dose adjustment needed for mild-moderate hepatic/renal impairment • Consider discontinuation if severe reactions occur

Special Populations

Pediatric: Not indicated for use in children Geriatric: No dose adjustment required Renal impairment: No dose adjustment needed for mild-moderate impairment Use with caution in severe impairment Hepatic impairment: No dose adjustment needed for mild impairment Use with caution in moderate-severe impairment

Adverse Effects

Common (>10%): • Hot flashes (35%) • Arthralgia (25-30%) • Asthenia (15-20%) • Nausea (10-15%) • Headache (10-15%) Serious (<1%): • Osteoporosis/fractures • Cardiovascular events (MI, angina) • Thromboembolism • Hepatic enzyme elevations • Severe cutaneous reactions

Drug Interactions

• Estrogen-containing products: May interfere with pharmacological action • Tamoxifen: Reduces anastrozole plasma concentration (avoid concomitant use) • CYP3A4 inducers: May decrease anastrozole effectiveness • Warfarin: Increased INR reported in some patients

Warnings & Precautions

• Reductions in bone mineral density may occur - monitor bone health • Increased serum cholesterol levels reported in some patients • Not effective in patients with ER-negative tumors • Embryo-fetal toxicity - contraindicated in pregnancy • Use caution in patients with ischemic heart disease

Pregnancy & Lactation

Pregnancy: FDA Category X Contraindicated in pregnancy due to potential fetal harm May cause fetal malformations and pregnancy loss Lactation: Not indicated in premenopausal women Unknown if excreted in human milk Breastfeeding not recommended

Monitoring Parameters

• Bone mineral density (baseline and periodic) • Serum lipid profile • Liver function tests • Tumor response markers • Signs of thromboembolism • Vitamin D and calcium levels • Regular breast exams and mammograms

Patient Counseling

• Take exactly as prescribed, usually once daily • May take with or without food • Report any new bone pain or mobility issues • Use nonhormonal contraception if premenopausal • Maintain adequate calcium and vitamin D intake • Report visual changes or severe headache • Regular follow-up appointments are essential

Storage & Stability

Store at 20-25°C (68-77°F) Protect from light and moisture Keep in original container Discard expired medication

Clinical Pearls

• More effective than tamoxifen in ER+/PR+ tumors • Does not require corticosteroid replacement • Consider baseline bone density scan before starting • May cause false-positive LH surge measurements • Can be used sequentially after tamoxifen therapy • Less thrombogenic than tamoxifen

References

• FDA Prescribing Information: Arimidex (anastrozole) tablets • Early Breast Cancer Trialists' Collaborative Group. Lancet. 2015 • Baum M, et al. Lancet. 2002;359(9324):2131-2139 • NCCN Clinical Practice Guidelines in Oncology: Breast Cancer