Arimidex (Anastrozole): What It Is and What to Expect

Anastrozole is a hormone therapy. NCI describes it as an aromatase inhibitor.

Aromatase is an enzyme. The body uses it to make estrogen, in the ovaries and in other tissues. Anastrozole blocks that enzyme, and the block is reversible. Less enzyme activity means less estrogen in circulation.

That matters because some breast cancers need estrogen to grow. Starve the supply and the growth slows or stops.

After menopause the ovaries are no longer the main source of estrogen. Other tissues take over, using aromatase. That is why this class is used after menopause and not before.

The brand name is Arimidex. The generic name is anastrozole. They are the same drug.

It is worth being clear about what this is not. It is not chemotherapy. It does not work by killing dividing cells. It is a tablet that changes the hormone environment the tumor sits in. That is also why the side effects look more like menopause than like chemotherapy.

This is the part most summaries flatten. The FDA-approved label lists three separate settings, not one.

First, adjuvant treatment of postmenopausal women with hormone receptor-positive early breast cancer. Adjuvant means after surgery, to lower the chance of return.

Second, first-line treatment of postmenopausal women with locally advanced or metastatic breast cancer. Here the label covers hormone receptor-positive disease and hormone receptor-unknown disease.

Third, treatment of advanced breast cancer in postmenopausal women whose disease progressed on tamoxifen.

The label adds a limit to that third setting. Women with ER-negative disease, and women who did not respond to earlier tamoxifen, rarely responded to anastrozole.

The dose is one 1 mg tablet once a day. It can be taken with food or without.

There is only the one tablet strength, but that still does not make this page your prescription. Take anastrozole exactly as your own breast team wrote it, and tell them before you stop or skip doses.

Duration depends on the setting. For advanced breast cancer, the label says to continue until the tumor progresses. For early breast cancer, it says the optimal duration is unknown. In the ATAC trial the drug was given for five years.

No dose change is needed for kidney impairment or for older patients. One drug interaction is stated flatly: tamoxifen should not be given alongside it.

ATAC is the trial behind the adjuvant indication, and its numbers sit in the label.

It was a multicenter, double-blind trial. It randomized 9,366 postmenopausal women with operable breast cancer. Three arms: anastrozole 1 mg daily, tamoxifen 20 mg daily, or both together, for five years or until recurrence.

The combination arm was dropped. At 33 months of median follow-up it showed no benefit over tamoxifen alone.

The two remaining arms were treated for a median of 60 months and followed for a median of 68 months. Disease-free survival favored anastrozole, with a hazard ratio of 0.87. In the hormone receptor-positive group, about 84% of the trial, the hazard ratio was 0.83.

One subgroup detail is worth knowing. The benefit was smaller in women aged 65 and over, where the hazard ratio was 0.93.

Lowering estrogen has consequences beyond the tumor, and the label names three.

Bone comes first. In the ATAC bone substudy at 12 and 24 months, women on anastrozole lost bone mineral density at the lumbar spine and total hip compared with their own baseline. Women on tamoxifen gained it. The label asks prescribers to consider bone density monitoring.

Cholesterol is second. More women on anastrozole had raised serum cholesterol than on tamoxifen, 9% against 3.5%. The label asks prescribers to consider cholesterol monitoring.

The heart is third, and it is narrower than it sounds. Among women who already had ischemic heart disease, ischemic cardiovascular events occurred in 17% on anastrozole and 10% on tamoxifen. The label asks that risks and benefits be weighed in that specific group.

Osteoporosis and fractures both appear on the list of reactions reported in at least 10% of women, alongside hot flashes, joint pain, arthritis, back pain and bone pain.

Four drug interactions get their own subsections: tamoxifen, estrogen, warfarin and the cytochrome P450 enzymes.

Two are simple to state. Tamoxifen should not be given at the same time. Estrogen-containing therapy would work against the whole point of the drug, and the label gives it its own entry.

Pregnancy has a section as well. Based on animal studies and how the drug works, anastrozole can harm a fetus. The label advises effective contraception during therapy and for at least 3 weeks after the last dose, in females of reproductive potential.

There is a pediatric section too, which surprises people. It covers a single-arm study in 28 girls with McCune-Albright syndrome and precocious puberty. That is not a cancer use, and it is not an approval.

The Contraindications section is short. So is the list of things needing a dose change: none for kidney impairment, none for older age.

This page does not replace the label, and it is not trying to. The label is written by the reviewers who cleared the drug, and it is kept current.

The full text sits on DailyMed, the National Library of Medicine's official label database. Three parts are worth finding: Warnings and Precautions, Adverse Reactions, and the patient leaflet.

Which of those risks apply to one person depends on dose, other medicines and medical history. A pharmacist or oncology team can answer that. So can a question about which symptoms warrant a call rather than a wait, and what number covers nights and weekends.

For how this drug sits beside others in its class, see letrozole and tamoxifen. For the wider picture, see hormone therapy.

• Call 911 or go to an emergency department if you get chest pain or chest tightness, or you become suddenly breathless. The label singles out ischemic cardiovascular events in women who already have ischemic heart disease. Sudden swelling of the face, lips or tongue, or a widespread blistering rash, also needs emergency care.
• Call your care team the same day if you fall and then cannot put weight on a limb, or sudden back pain starts after a minor knock. Thinning bone makes a fracture more likely on this drug.
• Call your care team within a day or two if joint pain or stiffness has reached the point where you are thinking of stopping the tablets. There are other options in this class, and stopping without telling anyone is the worse route.

• DailyMed — ARIMIDEX (anastrozole) tablet, FDA-approved label
• NCI — Anastrozole