Anastrozole is a nonsteroidal aromatase inhibitor used mainly in the treatment of hormone receptor–positive breast cancer in postmenopausal women, where it works by blocking the aromatase enzyme and thereby reducing estrogen production, which in turn slows the growth of estrogen-dependent tumors; because of this targeted mechanism, it has become an important alternative to older therapies such as Tamoxifen, offering improved outcomes in many patients. The drug was developed by AstraZeneca in the late 20th century as part of a new class of endocrine therapies, and after successful clinical trials demonstrating its effectiveness and better tolerability, it was approved in 1995; since then, it has played a major role in breast cancer management, especially following large studies like the ATAC trial, which confirmed its superiority in reducing recurrence rates, establishing anastrozole as a cornerstone of modern cancer treatment.
BRAND NAMES
Arimidex – the original and most widely recognized brand
Anastrozole Teva
Anastrozole Accord
Anastrozole Sandoz
MECHANISM OF ACTION
Anastrozole acts by inhibiting the enzyme Aromatase, which is responsible for converting androgens into estrogens in peripheral tissues. In postmenopausal women, estrogen is mainly produced through this pathway rather than by the ovaries. By selectively and reversibly blocking aromatase, anastrozole significantly reduces circulating estrogen levels. This reduction suppresses the growth of estrogen receptor–positive breast cancer cells, as these tumors rely on estrogen for proliferation. Unlike Tamoxifen, which blocks estrogen receptors, anastrozole decreases estrogen production itself, thereby slowing tumor progression and reducing the risk of recurrence.
PHARMACOKINETICS
Absorption
Anastrozole is well absorbed after oral administration with high bioavailability.
Peak plasma concentrations are reached within about 2 hours, and food may slightly delay absorption without affecting the extent. It shows consistent absorption, supporting convenient once-daily dosing.
Distribution
Anastrozole is widely distributed throughout the body and shows moderate plasma protein binding (about 40%). It has a large volume of distribution, indicating extensive tissue uptake. The drug distributes effectively to peripheral tissues where estrogen synthesis occurs.
Metabolism
Anastrozole is extensively metabolized in the liver through processes such as N-dealkylation, hydroxylation, and glucuronidation. These metabolic pathways convert the drug into inactive metabolites, which do not contribute to its therapeutic effect.
Elimination
Anastrozole is primarily eliminated via hepatic metabolism, with metabolites excreted in both urine and feces. The parent drug has a plasma half-life of approximately 50 hours, allowing for once-daily dosing. Less than 10% of anastrozole is excreted unchanged in the urine.
PHARMACODYNAMICS
Anastrozole works by selectively and reversibly inhibiting Aromatase, the enzyme responsible for converting androgens to estrogens in peripheral tissues. This leads to a profound reduction in circulating estrogen levels, particularly in postmenopausal women. The decrease in estrogen suppresses the growth of estrogen receptor–positive breast cancer cells, slowing tumor progression and lowering the risk of recurrence. Anastrozole does not affect adrenal steroid production or estrogen receptor binding, differentiating it from selective estrogen receptor modulators.
ADMINISTRATION
Anastrozole is administered orally in the form of tablets. It is usually taken once daily, with or without food, at the same time each day. Swallowing the tablet whole with water is recommended, and doses should not be doubled if a dose is missed.
DOSAGE AND STRENGTH
Anastrozole is typically prescribed at a standard dose of 1 mg orally once daily.
It is available in 1 mg tablets, which are the most commonly used strength for adults.
Dosage adjustments are generally not required for mild to moderate renal or hepatic impairment, but monitoring is advised in severe cases.
DRUG INTERACTIONS
Anastrozole may interact with drugs affecting CYP3A4, altering its levels. Medications that influence estrogen or bone metabolism, like Tamoxifen, can reduce its effectiveness. Caution and monitoring are advised when used with such drugs.
FOOD INTERACTIONS
Anastrozole can be taken with or without food, as meals have minimal effect on its absorption. There are no significant dietary restrictions associated with anastrozole. Patients should maintain a balanced diet, especially to support bone health during long-term therapy.
CONTRAINDICATIONS
Anastrozole is contraindicated in patients who are hypersensitive to anastrozole or any of its excipients. It should not be used in premenopausal women, as it is ineffective in this population. Use during pregnancy or breastfeeding is also contraindicated due to potential harm to the fetus or infant.
SIDE EFFECTS
Common side effects of Anastrozoleinclude hot flashes, joint pain, fatigue, nausea, and osteoporosis. Less common effects may involve bone fractures, mood changes, headache, and gastrointestinal disturbances. Long-term use may increase the risk of cardiovascular events and decreased bone mineral density, so monitoring is recommended.
TOXICITY
Anastrozole is generally well tolerated, and acute toxicity is rare. Overdose may cause severe nausea, vomiting, dizziness, or fatigue, but serious life-threatening effects are uncommon. Management is usually supportive, as there is no specific antidote, and patients should be monitored until symptoms resolve.
