Faslodex (Fulvestrant)

What Is Faslodex and What Is It Used For?

Faslodex (fulvestrant) is a selective estrogen receptor degrader (SERD) used to treat estrogen receptor-positive (ER+) breast cancer. It works by binding to and degrading estrogen receptors in breast cancer cells, completely blocking estrogen-driven tumour growth. It is used in postmenopausal women with locally advanced or metastatic breast cancer, either alone or in combination with CDK4/6 inhibitors.

Faslodex contains the active substance fulvestrant, which belongs to a group of medicines known as estrogen receptor antagonists. Estrogens are female sex hormones that, in certain cases, can stimulate the growth of breast cancers that express estrogen receptors (ER-positive breast cancers). Approximately 70–80% of all breast cancers are ER-positive, making endocrine (hormonal) therapy a cornerstone of treatment for the majority of patients with breast cancer.

Fulvestrant was developed by AstraZeneca and first approved by the U.S. Food and Drug Administration (FDA) in 2002 for the treatment of ER-positive metastatic breast cancer in postmenopausal women with disease progression following anti-estrogen therapy. Since then, its approved indications have expanded substantially. In 2017, fulvestrant received FDA approval for use as first-line monotherapy in postmenopausal women with ER-positive, HER2-negative advanced breast cancer who have not previously received endocrine therapy, based on results from the FALCON trial.

Unlike tamoxifen, which is a selective estrogen receptor modulator (SERM) and has partial agonist activity in certain tissues (such as the uterus and bone), fulvestrant has no agonist effects. It acts as a pure antiestrogen by competitively binding to the estrogen receptor with an affinity comparable to that of the natural hormone estradiol. Critically, fulvestrant not only blocks the receptor but also causes its degradation (downregulation), effectively reducing the total number of estrogen receptors on the cancer cell surface. This dual mechanism – blocking and degrading – makes fulvestrant unique among endocrine therapies and is why it is classified as a selective estrogen receptor degrader (SERD).

Faslodex is used in the following clinical settings:

• Monotherapy: For the treatment of postmenopausal women with estrogen receptor-positive, locally advanced or metastatic breast cancer, either as first-line treatment in patients not previously treated with endocrine therapy or after disease progression on prior anti-estrogen therapy (such as tamoxifen or aromatase inhibitors)
• Combination with palbociclib: For the treatment of women with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2−) locally advanced or metastatic breast cancer. Pre- or perimenopausal women receiving this combination will also be treated with a luteinising hormone-releasing hormone (LHRH) agonist to suppress ovarian function
• Combination with other CDK4/6 inhibitors: Fulvestrant is also used in combination with ribociclib or abemaciclib as part of standard-of-care endocrine-based therapy for advanced HR+/HER2− breast cancer

When Faslodex is prescribed in combination with palbociclib (or another CDK4/6 inhibitor), it is important that patients also read the package leaflet for the combination partner. Your oncologist will determine the most appropriate treatment regimen based on the characteristics of your cancer, your prior treatments, and your overall health status.

What Should You Know Before Taking Faslodex?

Before starting Faslodex, inform your doctor if you have liver or kidney problems, a history of blood clots, bleeding disorders, low platelet counts, or bone mineral density loss (osteoporosis). Faslodex must not be used during pregnancy or breastfeeding, and effective contraception is required during treatment and for two years after the last dose.

While fulvestrant is generally well tolerated, there are important medical considerations your doctor will evaluate before starting treatment. Being transparent about your complete medical history, current medications, and any known allergies is essential to ensure that fulvestrant is safe and appropriate for you.

Contraindications

You must not receive Faslodex if any of the following apply to you:

• Allergy to fulvestrant: If you have a known hypersensitivity to fulvestrant or to any of the other ingredients in the formulation, including ethanol (alcohol), benzyl alcohol, benzyl benzoate, or refined castor oil. Allergic reactions can range from skin rashes to severe anaphylactic reactions
• Pregnancy or breastfeeding: Fulvestrant is an antiestrogen and its mechanism of action poses a clear risk of harm to the developing fetus. Animal studies have demonstrated reproductive toxicity. You must not breastfeed while receiving Faslodex
• Severe liver impairment: Patients with severe hepatic impairment (Child-Pugh class C) should not receive fulvestrant, as the drug is extensively metabolised by the liver and accumulation may lead to increased toxicity

Warnings and Precautions

Talk to your doctor, pharmacist, or nurse before receiving Faslodex if any of the following apply to you:

• Kidney or liver problems: Fulvestrant is primarily metabolised in the liver and excreted via the biliary route. Patients with mild to moderate hepatic impairment may require dose reduction. Your doctor will check your liver function before and during treatment
• Low platelet count or bleeding disorders: Faslodex is administered as an intramuscular injection, which can cause bleeding at the injection site. Patients with thrombocytopenia (low platelets) or those taking anticoagulant medications are at increased risk of haematoma formation
• History of blood clots (thromboembolism): Thromboembolic events, including deep vein thrombosis and pulmonary embolism, have been reported in patients receiving fulvestrant. If you have a history of venous thromboembolism, discuss the risks with your oncologist
• Osteoporosis or bone mineral density loss: Estrogen plays a role in maintaining bone density. By blocking estrogen signalling, fulvestrant may contribute to accelerated bone loss. Your doctor may recommend bone density monitoring and consider protective measures such as calcium, vitamin D supplementation, or bisphosphonate therapy
• Alcohol dependence: Each injection of Faslodex contains approximately 500 mg of ethanol (alcohol), equivalent to roughly 12 ml of beer or 5 ml of wine. This may be harmful for people with alcohol dependence and should be considered in patients with liver disease or epilepsy

Children and Adolescents

Faslodex is not indicated for use in children and adolescents under 18 years of age. Its safety and efficacy have not been established in paediatric populations, and there is no relevant clinical indication for its use in this age group.

Pregnancy and Breastfeeding

You must not use Faslodex if you are pregnant. Given its anti-estrogenic mechanism of action, fulvestrant is expected to cause harm to the developing fetus. Animal studies have shown adverse effects on fertility and fetal development at clinically relevant doses. If you are of childbearing potential, you must use effective contraception during treatment and for two years after your last dose of Faslodex.

You must not breastfeed during treatment with Faslodex. It is not known whether fulvestrant or its metabolites are excreted in human breast milk, but given the potential for serious adverse effects in nursing infants, breastfeeding must be discontinued before starting treatment.

If you become pregnant while receiving Faslodex, inform your doctor immediately. Your oncologist will discuss the risks and potential need to discontinue treatment.

Driving and Using Machines

Faslodex is not expected to significantly affect your ability to drive or use machines. However, fatigue and asthenia (weakness) are very common side effects of fulvestrant. If you experience tiredness after treatment, you should avoid driving or operating machinery until you feel well enough to do so safely.

How Does Faslodex Interact with Other Drugs?

Fulvestrant has relatively few significant drug interactions. The most important considerations involve anticoagulants (increased bleeding risk at injection sites), CYP3A4 inhibitors (which may increase fulvestrant exposure), and CDK4/6 inhibitors such as palbociclib (used as combination therapy). Always inform your doctor about all medications you are taking.

Fulvestrant is extensively metabolised by the cytochrome P450 system, primarily via CYP3A4, with minor contributions from CYP1A2, CYP2C9, CYP2C19, and CYP2D6. Despite this extensive metabolism, clinically significant drug interactions are relatively uncommon because fulvestrant is administered intramuscularly and achieves steady-state concentrations gradually. However, there are several interactions that your healthcare team should be aware of.

In vitro studies have shown that fulvestrant does not significantly inhibit CYP450 isoenzymes at clinically relevant concentrations, suggesting a low potential for fulvestrant to affect the metabolism of other drugs. A clinical interaction study with midazolam (a CYP3A4 substrate) demonstrated no clinically relevant effect of fulvestrant on midazolam pharmacokinetics, supporting this finding.

Important Considerations for Drug Interactions

Because fulvestrant is administered intramuscularly rather than orally, it bypasses first-pass hepatic metabolism, which reduces the likelihood of some CYP-mediated interactions. However, since the drug undergoes extensive hepatic biotransformation once in the systemic circulation, clinically relevant interactions with potent CYP3A4 modulators remain theoretically possible. In practice, the slow and sustained release of fulvestrant from the intramuscular depot means that dramatic spikes in drug levels are unlikely, even in the presence of enzyme inhibitors.

The most practically important interaction is with anticoagulants and antiplatelet agents. Because fulvestrant is given by deep intramuscular injection into the gluteal muscle, patients on blood-thinning medications are at increased risk of developing haematomas at the injection site. This does not mean fulvestrant cannot be used in anticoagulated patients, but healthcare professionals should apply prolonged pressure at the injection site and monitor for signs of bleeding or bruising.

What Is the Correct Dosage of Faslodex?

The recommended dose of Faslodex is 500 mg (two 250 mg/5 ml injections) administered intramuscularly once per month, with an additional loading dose of 500 mg given two weeks after the initial dose. The injections are given as slow intramuscular injections, one in each buttock. Faslodex must be administered by a healthcare professional.

Always use this medicine exactly as your doctor, pharmacist, or nurse has instructed. The dosage regimen for fulvestrant has been optimised based on pharmacokinetic modelling and clinical trial data, including the pivotal CONFIRM trial which established the superiority of the 500 mg dose over the previously standard 250 mg dose.

Adult Dosage

The loading dose at day 15 is a critical part of the fulvestrant regimen. Pharmacokinetic studies demonstrated that without this additional early dose, therapeutic steady-state concentrations are not achieved for approximately 3–6 months. The day 15 loading dose allows fulvestrant to reach effective blood levels within the first month of treatment, providing earlier therapeutic benefit.

Each injection should be administered slowly over 1–2 minutes as a deep intramuscular injection into the gluteal muscle (buttock). The injection must be given by a healthcare professional – patients should not attempt to self-administer this medication. The needle bevel should be oriented upwards towards the lever arm of the safety device to facilitate proper needle shielding after use.

Patients with Liver Impairment

No dose adjustment is required for patients with mild hepatic impairment (Child-Pugh class A). However, for patients with moderate hepatic impairment (Child-Pugh class B), the recommended dose is reduced to 250 mg (one pre-filled syringe) once monthly, with a 250 mg loading dose at day 15. Faslodex should not be used in patients with severe hepatic impairment (Child-Pugh class C) due to insufficient safety data and the risk of increased drug exposure.

Patients with Kidney Impairment

No dose adjustment is required for patients with mild to moderate renal impairment (creatinine clearance ≥30 ml/min). There are limited data in patients with severe renal impairment (creatinine clearance <30 ml/min), and caution is advised in this population. Your doctor will assess your kidney function before and during treatment.

Elderly Patients

No dose adjustment is required based on age alone. Clinical trials of fulvestrant included a substantial proportion of patients over 65 years of age, and no overall differences in safety or efficacy were observed compared to younger patients. However, elderly patients may be more susceptible to certain side effects and may have age-related declines in hepatic and renal function that warrant closer monitoring.

Missed Dose

If you miss a scheduled appointment for your Faslodex injection, contact your doctor or treatment centre as soon as possible to reschedule. It is important to maintain the regular monthly dosing schedule to ensure consistent therapeutic drug levels. If a dose is delayed, your doctor will advise on the most appropriate timing for your next injection based on how long it has been since your last dose.

Overdose

There is no clinical experience with accidental overdose of fulvestrant in humans. Animal studies suggest that no effects other than those related to anti-estrogenic activity were observed at very high doses. Should overdose occur, management should be symptomatic and supportive. There is no specific antidote for fulvestrant, and it is unlikely to be removed by dialysis due to its high protein binding (>99%) and large volume of distribution.

What Are the Side Effects of Faslodex?

Like all medicines, Faslodex can cause side effects, though not everyone experiences them. The most common side effects include injection site reactions (pain, inflammation), nausea, fatigue, joint pain, hot flushes, and elevated liver enzymes. Serious but less common side effects include blood clots (thromboembolism), hepatitis, and severe allergic reactions. Contact your doctor immediately if you experience signs of a severe allergic reaction or blood clot.

Fulvestrant is generally well tolerated in clinical trials and post-marketing experience. The side effect profile is largely consistent with the drug's anti-estrogenic mechanism and its intramuscular route of administration. Below is a comprehensive overview of reported adverse reactions organised by frequency according to international regulatory classification.

* Includes side effects for which the exact contribution of Faslodex cannot be determined due to the underlying disease.

Injection Site Reactions

Injection site reactions are the most distinctive side effect of fulvestrant, directly related to its intramuscular administration. Pain at the injection site is very common and may persist for several days after the injection. Some patients experience inflammation, swelling, or bruising. These reactions are generally mild to moderate in severity and tend to decrease over time as patients become accustomed to the injections. Applying ice to the injection area before and after the injection may help reduce discomfort.

The viscosity of the fulvestrant solution (due to the castor oil vehicle) contributes to injection site discomfort. Administering the injection slowly over 1–2 minutes, as recommended, can help minimise pain. Some healthcare professionals warm the pre-filled syringe to room temperature before injection, which slightly reduces the viscosity and may improve patient comfort, although this is not a formal recommendation.

Thromboembolic Events

Venous thromboembolism (VTE), including deep vein thrombosis (DVT) and pulmonary embolism (PE), has been observed in patients receiving fulvestrant. This risk is consistent with the known association between anti-estrogenic therapy and thromboembolic events. Patients with a history of VTE or those with additional risk factors (immobility, recent surgery, obesity) should be closely monitored. Report any sudden leg swelling, chest pain, or shortness of breath to your doctor immediately.

Liver-Related Side Effects

Elevated liver enzymes (transaminases and gamma-GT) are common during fulvestrant treatment and are usually detected on routine blood tests. These elevations are typically mild, transient, and asymptomatic. However, more serious hepatic events, including hepatitis and liver failure, have been reported uncommonly. Your doctor will monitor your liver function regularly through blood tests, particularly during the first few months of treatment. Contact your doctor immediately if you develop jaundice (yellowing of the skin or eyes), dark urine, or severe abdominal pain.

Reporting Side Effects

If you experience any side effects, including those not listed here, tell your doctor, pharmacist, or nurse. You can also report suspected side effects to your national medicines regulatory authority. Reporting helps improve knowledge about the safety of medications and contributes to ongoing benefit-risk assessment.

How Should You Store Faslodex?

Faslodex must be stored and transported refrigerated at 2°C to 8°C (36°F to 46°F). Keep the pre-filled syringes in the original packaging to protect from light. Do not freeze below -20°C. Temporary temperature excursions up to 25°C average are permitted for a maximum of 28 days.

Proper storage of Faslodex is critical because it is a biological product that can be affected by temperature fluctuations. Unlike many oral medications that are stored at room temperature, fulvestrant requires refrigeration to maintain its stability and effectiveness throughout its 4-year shelf life.

Keep Faslodex out of the sight and reach of children. Do not use after the expiry date stated on the carton or syringe label after "EXP". The expiry date refers to the last day of that month. The pre-filled syringes should remain in the original packaging to protect from light exposure.

If Faslodex is temporarily removed from refrigerated storage (for example, during transport to a clinic), the following guidelines apply:

• Temperature excursions outside the 2–8°C range should be minimised
• Storage at temperatures exceeding 30°C must be avoided
• A cumulative maximum of 28 days at an average temperature below 25°C (but above 2–8°C) is permitted during the product's shelf life
• After any temperature excursion, the product must be returned immediately to refrigerated conditions (2–8°C)
• Exposure to temperatures below 2°C will not damage the product, provided it is not stored below −20°C

Healthcare professionals are responsible for ensuring correct storage, use, and disposal of Faslodex. This medication may pose a risk to the aquatic environment. Do not dispose of medicines via wastewater or household waste. Return unused medicines to your pharmacy for environmentally safe disposal.

What Does Faslodex Contain?

Each pre-filled syringe of Faslodex contains 250 mg fulvestrant in 5 ml solution. The inactive ingredients include ethanol (96%), benzyl alcohol, benzyl benzoate, and refined castor oil. Two syringes are required per dose for a total of 500 mg.

Understanding the composition of your medication is important, particularly if you have known allergies or sensitivities to specific excipients. Faslodex has a distinctive formulation designed for slow release from the intramuscular injection site.

Active Ingredient

The active substance is fulvestrant. Each pre-filled syringe contains 250 mg of fulvestrant dissolved in 5 ml of solution, giving a concentration of 50 mg/ml. Two syringes must be administered per dose to achieve the recommended monthly dose of 500 mg. Fulvestrant is a 7α-alkylsulfinyl analogue of 17β-estradiol with the molecular formula C₃₂H₄₇F₅O₃S and a molecular weight of 606.77 g/mol.

Inactive Ingredients (Excipients)

• Ethanol (96%): Acts as a co-solvent. Each injection contains approximately 500 mg of ethanol, equivalent to roughly 12 ml of beer or 5 ml of wine. This may be harmful for individuals with alcohol dependence
• Benzyl alcohol: Used as a preservative. Each injection contains 500 mg of benzyl alcohol (100 mg/ml). Benzyl alcohol may cause allergic reactions in some individuals
• Benzyl benzoate: Acts as a solubiliser. Each injection contains 750 mg of benzyl benzoate (150 mg/ml)
• Refined castor oil: The main vehicle that gives the solution its viscous, oily character. This oil-based formulation creates a depot at the injection site, allowing for slow, sustained release of fulvestrant into the bloodstream over the dosing interval

Physical Description and Packaging

Faslodex is a clear, colourless to yellow, viscous solution supplied in pre-filled glass syringes with tamper-evident closures. Each syringe contains 5 ml of solution for injection. The product is available in two packaging sizes: a pack containing one pre-filled syringe or a pack containing two pre-filled syringes. Safety needles (BD SafetyGlide shielding hypodermic needles) for attachment to each syringe are included in the packaging. Not all pack sizes may be available in all countries.