Fulvestrant

Fulvestrant injection is approved to treat hormone receptor (HR)-positive advanced or metastatic breast cancer in postmenopausal women. The specific approvals vary slightly by product:

• Faslodex (intramuscular injection): HR-positive, HER2-negative advanced breast cancer in postmenopausal women with no prior endocrine therapy; or HR-positive advanced breast cancer that has progressed on endocrine therapy — alone or combined with ribociclib (as initial endocrine therapy or after progression), or with palbociclib or abemaciclib after endocrine therapy progression.
• Cligavyx (intramuscular injection): Same HR-positive advanced and metastatic breast cancer indications as Faslodex, alone or in combination with ribociclib, palbociclib, or abemaciclib.
• Fulvestrant injection (intramuscular and intravenous): HR-positive, HER2-negative advanced breast cancer in postmenopausal women not previously treated with endocrine therapy; or HR-positive advanced breast cancer with disease progression on endocrine therapy — alone or combined with ribociclib, palbociclib, or abemaciclib.

Many breast cancers have estrogen receptors (ER) — proteins on the cancer cell surface that estrogen locks onto to trigger growth. Fulvestrant binds to those receptors very tightly (with affinity similar to estrogen itself), blocks estrogen from attaching, and also causes the receptors to break down so there are fewer of them on the cell. The result: the cancer cell loses one of its main growth signals. Unlike tamoxifen, fulvestrant has no partial estrogen-like activity — it is a pure blocker.

In studies, fulvestrant also inhibited the growth of tamoxifen-resistant breast cancer cells, meaning it can work even when some other hormonal therapies have stopped being effective.

Fulvestrant is given as an intramuscular injection into the buttock by a healthcare professional in a clinical setting — it is not a pill and is not self-administered at home. The typical schedule is an injection on Day 1, Day 15, Day 29, and then once every month afterward. Patients with moderate liver disease are given a lower dose on the same schedule. There are no specific food restrictions tied to fulvestrant itself.

• Injections are given slowly — over 1 to 2 minutes — to reduce discomfort.
• If you are also taking a combination partner medicine (such as ribociclib, palbociclib, or abemaciclib), follow the separate dosing schedule for that drug as directed by your prescriber.
• Never skip or delay an injection without talking to your care team first.

The most common side effects reported with fulvestrant injection include:

• Injection site pain
• Nausea and vomiting
• Bone pain and joint pain (arthralgia)
• Headache and back pain
• Fatigue and weakness (asthenia)
• Hot flashes
• Loss of appetite
• Cough or shortness of breath
• Constipation
• Elevated liver enzymes (seen in lab tests in more than 15% of patients — usually mild)

More serious reactions to watch for include signs of a severe allergic reaction (hives, facial swelling, trouble breathing), nerve pain or numbness shooting down the leg after an injection, and signs of significant liver problems. Contact your care team promptly if any of these occur.

• Embryo-fetal toxicity (harm to an unborn baby): Fulvestrant can cause birth defects or pregnancy loss. Women who could become pregnant must use effective contraception during treatment and for one full year after the last dose. A pregnancy test is recommended before starting. Do not use during pregnancy.
• Bleeding risk: Because fulvestrant is injected into the muscle, it carries a risk of bleeding at the injection site — particularly for people taking blood thinners or with low platelet counts or bleeding disorders.
• Injection site nerve reactions: Sciatic nerve irritation, nerve pain, and peripheral neuropathy (numbness or tingling) have been reported. Healthcare providers take precautions with injection placement.
• Liver impairment: Fulvestrant levels are significantly higher in people with moderate liver disease — a lower dose is used in that setting. It has not been studied in severe liver disease.
• Lab test interference: Fulvestrant can cause estradiol (estrogen) blood tests to read falsely high. Tell any lab or doctor ordering hormone tests that you are taking this medication.
• Breastfeeding: Do not breastfeed during treatment or for one year after stopping — the drug may pass into breast milk and harm a nursing infant.

Fulvestrant has no known clinically significant drug-drug interactions, which is reassuring for patients on multiple medications. Key points:

• CYP 3A4 inhibitors (e.g., ketoconazole): Although fulvestrant is partly broken down by the liver enzyme CYP 3A4, studies show that strong inhibitors of this enzyme do not meaningfully raise fulvestrant levels — no dose change is needed.
• CYP 3A4 inducers (e.g., rifampin): Similarly, drugs that speed up CYP 3A4 did not reduce fulvestrant exposure in studies — no dose adjustment required.
• Combination cancer medicines (palbociclib, abemaciclib, ribociclib): Clinical trials found no meaningful pharmacokinetic interaction when fulvestrant was given alongside these agents.

This is not a complete list of interactions. Always talk with your doctor or pharmacist before starting, stopping, or changing any medication.

• Allergy: Do not take fulvestrant if you have had a serious allergic reaction (such as hives or swelling) to it or any of its ingredients.
• Pregnancy: Fulvestrant causes serious harm to unborn babies. Use effective contraception during treatment and for one year after. Get a pregnancy test before starting.
• Breastfeeding: Do not breastfeed during treatment or for one year after the last dose.
• Liver disease: Tell your doctor if you have liver problems — a dose adjustment is needed for moderate liver impairment, and the drug has not been studied in severe liver impairment.
• Bleeding disorders or blood thinners: Tell your doctor if you take anticoagulants (blood thinners), have low platelets (thrombocytopenia), or have any bleeding disorder, because of the intramuscular injection route.
• Children: Fulvestrant has not been shown to be safe or effective for breast cancer in pediatric patients.
• Older adults: No overall difference in how the drug behaves has been seen across adult age ranges, but clinical response rates vary — your doctor will monitor you closely.
• Kidney disease: Because very little fulvestrant is cleared through the kidneys, kidney impairment is not expected to require a dose change.
• Fertility: Fulvestrant may affect fertility in women and men — discuss this with your doctor before starting if it is a concern.

There is very limited information about fulvestrant overdose in humans. In healthy volunteer studies using intravenous fulvestrant at much higher concentrations than normal intramuscular doses, no serious adverse reactions were observed — but the risk in cancer patients with other health conditions is not fully known. There is no specific antidote; if an overdose is suspected, healthcare providers will provide supportive care and treat any symptoms that arise.

In clinical studies, fulvestrant progressively reduced the number of estrogen receptors (ER) on breast cancer cells in a dose-dependent way — the higher the dose, the greater the reduction. This was accompanied by a decrease in the progesterone receptor (an estrogen-regulated protein), confirming the drug was cutting off estrogen signaling. It was also associated with lower levels of Ki67, a protein that marks how fast cancer cells are dividing, suggesting fulvestrant actively slows tumor cell proliferation.

After intramuscular injection, fulvestrant is absorbed slowly and spreads widely through body tissues — about 99% of the drug is bound to proteins in the blood. Steady-state blood levels are typically reached within the first month of dosing when an extra dose is given on Day 15. The drug is broken down mainly in the liver (primarily via the CYP 3A4 enzyme) and cleared mostly through the feces (about 90%); very little leaves the body through urine. Fulvestrant has a long half-life of roughly 40 days, which is why monthly dosing maintains consistent levels. No meaningful differences in drug behavior have been seen across age groups, racial groups, or in people with mild kidney impairment.

REFRIGERATE, 2° to 8°C (36° to 46°F). PROTECT FROM LIGHT, STORE IN THE ORIGINAL CARTON UNTIL TIME OF USE.