Fulvestrant Injection 250 mg/5 mL (50 mg/mL) – Prefilled Syringe
Fulvestrant (brand: Faslodex) is a selective estrogen receptor downregulator (SERD) used in postmenopausal women with advanced or metastatic hormone receptor–positive (ER+) breast cancer. It binds estrogen receptors and accelerates their degradation, acting as a pure antiestrogen (no agonist effects). By downregulating ER, fulvestrant deprives ER+ tumor cells of estrogen-driven growth.
Indications: Fulvestrant injection is indicated for postmenopausal women with HR+ (ER+), HER2-negative advanced or metastatic breast cancer. It can be used as monotherapy in women who are endocrine therapy–naïve or whose disease has progressed on prior hormonal therapy. It is also approved in combination with CDK4/6 inhibitors (ribociclib, palbociclib, or abemaciclib) for first-line or subsequent endocrine-based therapy in this population.
Dosage and Administration:
The standard dose is 500 mg intramuscularly (IM) on Days 1, 15, 29, and then once every 28 days (monthly). Each 500 mg dose is given as two 5 mL injections (250 mg per syringe); typically one injection is given in each buttock (gluteal muscle) to allow administration of the full 5 mL volume safely. Each injection should be administered deep IM over 1–2 minutes to minimize local irritation. (The solution is a viscous oil-based formulation, requiring slow injection.) For patients with moderate hepatic impairment (Child-Pugh B), the dose is reduced to 250 mg IM on the same schedule (one 5 mL injection). No dose adjustment is needed for renal impairment. Fulvestrant is for healthcare provider use only (injection); patients do not self-administer. If therapy is repeated after disease recurrence, the same schedule may be used as before.
Adverse Reactions:
Common side effects (≥5% of patients) include injection site pain, nausea, bone pain, arthralgia (joint pain), headache, back pain, fatigue, pain in extremities, hot flashes, vomiting, loss of appetite, asthenia (weakness), musculoskeletal pain, cough, difficulty breathing, and constipation. Injection-site reactions (pain, swelling, erythema) are common due to the oil-based solution; patients should be monitored for local reactions. Elevated liver enzymes (ALT, AST, alkaline phosphatase) occurred in >15% of patients so periodic liver function tests are recommended. Rare but serious effects include allergic reactions (anaphylaxis, rash), severe injection-site reactions (including risk of sciatic nerve injury with improper gluteal injection), and interstitial lung disease (very rare). Because fulvestrant is an antiestrogen, it can cause osteoporosis over the long term, so bone density should be monitored in susceptible patients.
Warnings and Precautions:
Before each dose, assess for injection complications. Use caution in patients with coagulopathies or on anticoagulants, as IM injections may cause bleeding or hematoma. Avoid intravascular injection. Fulvestrant is contraindicated in patients with known hypersensitivity to it or its excipients. It is teratogenic: fulvestrant causes fetal harm in animal studies, so it must not be used during pregnancy. Women of childbearing potential should have a negative pregnancy test before starting therapy and use effective contraception during treatment and for one year after the last dose. Advise against breastfeeding during treatment. Because of its estrogen-blocking effect, fulvestrant is not effective in non–hormone-responsive cancers.
Drug Interactions:
No clinically significant drug interactions have been identified. Fulvestrant is metabolized by CYP3A4 in vitro, but coadministration of ketoconazole (inhibitor) or rifampin (inducer) did not produce clinically meaningful changes in fulvestrant levels. Fulvestrant does not inhibit major CYP enzymes, and studies show no significant interactions when used with palbociclib, ribociclib, or abemaciclib. It can be used concurrently with other medications without dose adjustment.
Formulation and Handling:
Fulvestrant injection is supplied as a clear, colorless to yellow viscous solution in prefilled syringes. Each carton contains two glass syringes (Type I) of 5 mL each (250 mg per syringe), 50 mg/mL concentration. Store the syringes refrigerated at 2–8°C (36–46°F), protected from light, in the original carton until use. Do not freeze. Remove only before injection. Allow the syringe to reach room temperature before use to ease injection.
Patient Counseling:
Inform patients that fulvestrant is given by a healthcare professional via monthly intramuscular injection. Emphasize the importance of attending all injection appointments. Advise patients to report any injection site problems (e.g. severe pain, swelling, numbness) immediately. Explain that some side effects (hot flashes, fatigue, nausea) can occur and to notify their physician if severe. Instruct women to avoid pregnancy and to notify the provider if menstruation resumes (indicative of potential pregnancy).
