Anastrozole is a potent, non-steroidal, third-generation aromatase inhibitor used primarily in the adjuvant and metastatic treatment of hormone receptor-positive breast cancer in postmenopausal women. It works by selectively inhibiting the aromatase enzyme, which is responsible for the conversion of androgens (androstenedione and testosterone) to estrogens (estrone and estradiol). This leads to a profound reduction in circulating estrogen levels, depriving estrogen-dependent breast cancer cells of their primary growth stimulus. In the Indian context, it is a cornerstone of endocrine therapy and is widely available across multiple brands.
Adult: 1 mg orally, once daily. Can be taken with or without food, at approximately the same time each day.
Note: Tablet should be swallowed whole with a glass of water. Treatment duration for adjuvant therapy is typically 5 years, or as determined by the oncologist based on risk assessment. For metastatic disease, treatment continues until disease progression or unacceptable toxicity.
Anastrozole is a competitive, reversible inhibitor of the aromatase enzyme (cytochrome P450 CYP19). This enzyme complex, located in the endoplasmic reticulum of cells in adipose tissue, liver, breast, and breast cancer tissue, catalyzes the final and rate-limiting step in estrogen biosynthesis: the conversion of androgens (androstenedione and testosterone) to estrogens (estrone and estradiol). By binding to the heme group of the aromatase enzyme, anastrozole prevents the conversion, leading to a profound (≥80%) and sustained reduction in circulating estrogen levels in postmenopausal women.
Pregnancy: Pregnancy Category X. Contraindicated. Can cause fetal harm based on mechanism of action (estrogen deprivation). Exclude pregnancy before initiation. Women of childbearing potential must use effective non-hormonal contraception during and for at least 3 weeks after treatment.
Driving: Anastrozole may cause fatigue, dizziness, and somnolence in some patients. Patients should be cautioned about operating machinery or driving until they know how the drug affects them.
| Tamoxifen | Concomitant use reduces anastrozole plasma levels by 27%. It is not recommended as it may reduce anastrozole efficacy. | Major |
| Estrogen-containing therapies (HRT, oral contraceptives) | May counteract the pharmacological effect of anastrozole. Contraindicated. | Major |
| CYP3A4 Inducers (e.g., Rifampicin, Phenytoin, Carbamazepine, St. John's Wort) | May decrease anastrozole plasma concentrations, potentially reducing efficacy. Monitor clinical response. | Moderate |
| CYP3A4 Inhibitors (e.g., Ketoconazole, Itraconazole, Clarithromycin) | May increase anastrozole plasma concentrations. Clinical significance is likely minimal due to wide therapeutic index, but monitor for increased side effects. | Minor |
| Warfarin | Isolated reports of increased INR and bleeding events. Monitor INR closely when anastrozole is initiated or discontinued. | Moderate |
Same composition (Anastrozole (1mg)), different brands: