FULVESTRANT
Clinical safety rating: avoid
Contraindicated (not allowed)
Selective estrogen receptor downregulator (SERD). Binds to estrogen receptors (ER) with high affinity, downregulates ER expression, and inhibits estrogen-stimulated growth of breast cancer cells.
| Metabolism | Primarily metabolized via CYP3A4, with minor contributions from other pathways. The major route of elimination is via feces (approximately 90%) and urine (approximately 1%). |
| Excretion | Primarily hepatic metabolism via CYP3A4; <1% excreted renally as unchanged drug; ~90% eliminated in feces as metabolites, <1% in urine. |
| Half-life | Terminal elimination half-life ~40 days (range 30–50 days) due to slow release from intramuscular depot; achieves steady-state after ~3–6 months of monthly dosing. |
| Protein binding | 99% bound to plasma proteins, primarily albumin and to a lesser extent alpha-1-acid glycoprotein. |
| Volume of Distribution | 3–5 L/kg (approximately 210–350 L for a 70 kg adult), indicating extensive tissue distribution and binding to estrogen receptors. |
| Bioavailability | IM administration: ~100% (absolute bioavailability) due to complete absorption from the injection site; oral administration not bioavailable (<5%) due to extensive first-pass metabolism. |
| Onset of Action | IM administration: Clinical effects (estrogen receptor downregulation) detectable within 1–2 weeks; maximal antitumor response typically after 3–6 months. |
| Duration of Action | Duration of estrogen receptor suppression persists for ≥1 month after a single dose; clinical effects last as long as treatment continues (monthly dosing). |
500 mg intramuscularly on days 1, 15, 29, and then once monthly thereafter.
| Dosage form | SOLUTION |
| Renal impairment | No dose adjustment required for mild to moderate renal impairment (creatinine clearance 30-89 mL/min). Not studied in severe renal impairment (creatinine clearance <30 mL/min); use with caution. |
| Liver impairment | Mild hepatic impairment (Child-Pugh A): no dose adjustment. Moderate hepatic impairment (Child-Pugh B): 250 mg intramuscularly on days 1, 15, 29, and then once monthly. Severe hepatic impairment (Child-Pugh C): not recommended (no data). |
| Pediatric use | Not indicated; safety and efficacy not established in pediatric patients. |
| Geriatric use | No specific dose adjustment required; clinical studies included patients up to age 89 years with no overall differences in safety or efficacy compared to younger adults. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
No significant drug interactions Can cause injection site reactions and nausea.
| Breastfeeding | It is unknown if fulvestrant is excreted in human milk. M/P ratio is not available for humans. Due to the potential for serious adverse reactions in breastfed infants from fulvestrant, women should be advised not to breastfeed during treatment and for at least 2 weeks after the last dose. In animal studies, fulvestrant and its metabolites were present in milk of lactating rats. |
| Teratogenic Risk | Fulvestrant is contraindicated in pregnancy. There are no adequate studies in pregnant women. Based on animal studies, fulvestrant can cause fetal harm when administered to a pregnant woman. In reproductive toxicity studies in rats and rabbits, fulvestrant caused embryofetal toxicity and teratogenicity at doses below the recommended human dose. First trimester: High risk of fetal malformations and pregnancy loss due to antiestrogenic effects. Second and third trimesters: Risk of fetal growth restriction and potential adverse effects on fetal reproductive tract development due to continued estrogen receptor blockade. |
■ FDA Black Box Warning
None.
| Common Effects | Nausea |
| Serious Effects |
["Pregnancy","History of hypersensitivity to fulvestrant or any component of the formulation"]
| Precautions | ["Injection site reactions (e.g., sciatica, neuralgia, neuropathy) may occur due to inadvertent injection near the sciatic nerve","Risk of bleeding in patients with thrombocytopenia or anticoagulant use","Potential for increased liver enzyme levels; monitor liver function","Fetal harm if administered to pregnant women; advise effective contraception"] |
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| Fetal Monitoring | Pregnancy status must be verified prior to initiation of fulvestrant. Women of childbearing potential should use effective contraception during treatment and for at least 2 years after the last dose. If pregnancy occurs, immediate discontinuation is required. Monitor for fetal development via ultrasound if inadvertently exposed. No specific maternal monitoring beyond standard oncology care is indicated. |
| Fertility Effects | Fulvestrant may impair fertility in women of childbearing potential. In animal studies, fulvestrant disrupted estrous cycles and reduced fertility. Based on its mechanism of action (estrogen receptor downregulation), reversible disruption of ovarian function and menstrual cycles is expected. Long-term effects on fertility are unknown. |