What Is Fulvestrant?

Fulvestrant is a medication used to treat postmenopausal women who have hormone receptor-positive metastatic breast cancer. AstraZeneca plc, a pharmaceutical company based in London, markets the drug as Faslodex. The United States Food and Drug Administration (FDA), which is in charge of regulating the country's pharmaceuticals, approved fulvestrant on 25 April 2002.

Hormone receptor-positive metastatic breast cancer is defined as breast cancer that relies on hormones to multiply the tumors and let it spread to other parts of the body. In the case of fulvestrant, it works against estrogen, which is the primary female sex hormone. As an estrogen receptor antagonist, it blocks the hormone's activity of growing cancer cells.

Fulvestrant is used in particular for breast cancer patients who have not experienced menstrual bleeding for at least 12 consecutive months. People turn to the drug when other pharmaceuticals have not worked to block estrogen. This includes tamoxifen, brand name Nolvadex, which also acts as an estrogen receptor antagonist.

AstraZeneca also manufactures and markets tamoxifen, which has been the standard anti-estrogen medication for hormone receptor-positive breast cancer in pre-menopausal women. Unlike fulvestrant, however, tamoxifen also acts as an agonist, which means that it triggers responses from cells by binding to their receptors. This is the opposite of the actions of an antagonist, which actually blocks the agonist.

Fulvestrant is manufactured as a 250-milligram solution in a 5-millimeter syringe. Administration of the drug requires intramuscular injection with a 500 mg dose. The process lasts one to two minutes, and involves injecting each side of the buttocks with a syringe. This is done thrice in the first month, with two 14-day intervals, and once in each subsequent month. Its biological half-life—the time the drug has to lose half of its pharmacologic powers—is recorded as 40 days.

This drug is responsible for almost two dozen side effects. The most common ones include appetite loss, constipation, diarrhea, dizziness, headache, nausea, sweating, stomach pain and vomiting. More serious side effects, which demand the immediate attention of a doctor, include chest pain, rashes, breathing problems, and swelling at the upper portion of the body.

The FDA classifies fulvestrant as belonging to pregnancy category D in terms of posing fetal risk to pregnant women. This means that even though physicians can recommend its use, there is significant evidence of human fetal risk from marketing reports, investigations or human-controlled studies. Pregnancy category D is one of the more severe categories of the FDA's fetal risk classification system, second only to Category X.