Faslodex | Pharmacology Mentor

Generic Name

Faslodex

Selective Estrogen Receptor Degrader (SERD):
Binds irreversibly to the estrogen receptor (ER) on breast cancer cells.
Receptor down‑regulation → complete degradation of the ER protein.
Prevents both estrogen‑dependent and ligand‑independent (cross‑talk) ER signaling that drives tumor growth.
Distinct from SERMs, where the receptor remains but functionally antagonized; with SERDs the receptor is removed, giving a more potent blockade in both early and metastatic disease.

Administration: Intramuscular injection of a 5 mL solution (250 mg/mL).
Absorption: Slow, sustained release; peak serum concentration at ~7 days.
Distribution: Large volume of distribution (~15,000 L).
Metabolism: Hepatic via CYP3A4; minimal CYP-mediated drug–drug interactions.
Half‑life: ~4–5 weeks (terminal phase), enabling monthly dosing after the loading period.
Elimination: Primarily fecal; negligible renal excretion (<5 % unchanged).

Metastatic, hormone‑receptor‑positive (ER+), HER2‑negative breast cancer as first‑line endocrine therapy *or* after failure of aromatase inhibitor, tamoxifen, or ovarian suppression.
May be used for luminal B/grade‑II disease where other endocrine options have failed.
Not approved for adjuvant or early‑stage breast cancer.

Known hypersensitivity to fulvestrant or excipients (polysorbate 80, sodium lauryl sulfate).
Active infection or systemic illness that may be exacerbated by neutropenia.
Severe hepatic impairment (ALT/AST > 3× ULN) – caution; monitor LFTs.
Pregnancy – potential teratogenicity; use effective contraception.
Concurrent strong enzyme inducers (e.g., rifampin) may reduce serum levels.

1. Initial loading period (weeks 0, 2, 4, 6, 8):
• 250 mg IM (5 mL) each dose.

2. Maintenance therapy (starting week 12 onwards):
• 250 mg IM every 28 days.

3. Administration technique:
• Use a 22–24 G needle; inject into the deltoid or gluteal muscle; rotate sites to reduce local reactions.

4. Patient counseling:
• Informed about injection site reactions, fatigue, and hot flushes.

Baseline: CBC with diff, LFTs, electrolytes, pregnancy test (women of childbearing potential).
During therapy:
CBC every 4 weeks for first 3 months, then every 8 weeks.
LFTs monthly for 3 months, then every 3 months or if clinically indicated.
Pain score and injection‑site assessment at each visit.
PSA in patients with pre‑existing endocrine‑sensitive conditions (admittedly, PSA is not typically used for breast cancer; an error to avoid).

SERD vs. SERMs:
SERDs destroy the receptor → better efficacy in resistance to SERMs, especially in luminal B disease.
Loading dose strategy:
The 5‑dose loading ensures therapeutic levels are achieved quickly, which is critical for metastatic disease control.
Intramuscular injection technique matters:
Avoid the gluteal muscle if possible – selvedge area of the gluteus may lead to higher local inflammation; deltoid is safer.
Drug interactions are limited:
Because fulvestrant is largely metabolized by CYP3A4, it is *not* a substrate for CYP2D6 or CYP2C9, reducing typical SERM interaction concerns.
Bone health:
Lack of bisphosphonate co‑therapy is *not recommended* in patients with low bone density; monitor DEXA and consider zoledronic acid or denosumab.
Adverse effect triage:
Mild injection‑site reaction → oral NSAIDs; severe → discontinue if >2 cm induration with systemic symptoms.
Patient adherence:
Because therapy is monthly, patient education on scheduling and follow‑up is essential; missed doses can compromise treatment efficacy.

*(All data drawn from product labeling and peer‑reviewed literature as of 2024.)*