⚠️ Prescribing and dosing only by clinicians experienced in antiviral therapy. Used for induction and maintenance therapy of cytomegalovirus (CMV) retinitis in patients with advanced HIV/AIDS, and for acyclovir-resistant herpes simplex (HSV) infections. Requires close renal monitoring and specialist supervision.

💊 About

Foscarnet sodium (Foscavir®) is a broad-spectrum antiviral used when first-line nucleoside analogues fail.
Particularly valuable for CMV retinitis and acyclovir-resistant HSV infections.
Extreme caution in patients with renal impairment due to nephrotoxicity risk.
Always 🔗 check the BNF entry for current guidance.

🧠 Mode of Action

A pyrophosphate analogue that directly inhibits viral DNA polymerase and reverse transcriptase.
Blocks cleavage of pyrophosphate from deoxynucleotide triphosphates, halting viral DNA elongation.
Unlike aciclovir, it does not require phosphorylation by viral thymidine kinase - useful against resistant strains.

📋 Indications & Dosing

CMV Retinitis in AIDS: - 60 mg/kg every 8 hours or 90 mg/kg every 12 hours IV over ≥1 hour for 2–3 weeks (induction). - Maintenance: 60–120 mg/kg/day IV infusion.
HSV (resistant to aciclovir): - 40 mg/kg every 8 hours IV for 2–3 weeks or until lesions heal.
Dosing must be individualised to renal function - see renal dosing chart below.

📊 Dose Range - Specialist Use Only

Indication	Dose	Frequency	Route & Duration
CMV Retinitis (Induction)	60 mg/kg	8-hourly	IV infusion ≥1 hr for 2–3 weeks
CMV Retinitis (Induction – alternate)	90 mg/kg	12-hourly	IV infusion ≥1 hr for 2–3 weeks
CMV Retinitis (Maintenance)	60–120 mg/kg/day	Daily	IV infusion ≥1 hr
HSV (Acyclovir-resistant)	40 mg/kg	8-hourly	IV infusion ≥1 hr for 2–3 weeks

⚠️ Interactions

Concurrent use with other nephrotoxic drugs markedly increases renal toxicity.
Avoid or use with caution alongside:
- Aminoglycosides
- Amphotericin B
- Ciclosporin, Tacrolimus
- Aciclovir, Methotrexate
Monitor renal function (U&E, creatinine) at baseline and frequently during treatment.

🚧 Cautions

Never administer by rapid IV injection - must be diluted and infused slowly over ≥1 hour.
Ensure adequate hydration: infuse 0.5–1 L normal saline before or during each infusion to reduce renal toxicity.
In compliant patients, similar oral hydration regimens can be used.
Correct dehydration prior to initiating therapy.
Monitor for electrolyte imbalance (Ca²⁺, Mg²⁺, K⁺) and adjust therapy as needed.

⛔ Contraindications

Dehydration.
Severe renal impairment unless under specialist supervision.
Hypersensitivity to foscarnet or formulation components.

💥 Side Effects

💧 Nephrotoxicity - dose-related; ensure regular monitoring of renal function.
🩸 Haematological toxicity: pancytopenia, marrow suppression → monitor FBC.
🤒 Sepsis, fever, electrolyte imbalance (hypocalcaemia, hypomagnesaemia).
🧠 Neurological: headache, confusion, tremor, seizures, or ataxia (linked to electrolyte disturbance).
🦠 Rare: diabetes insipidus due to tubular toxicity.

📚 References

BNF: Foscarnet Sodium
BHIVA Opportunistic Infection Guidelines, 2023.
European AIDS Clinical Society (EACS) Guidelines, 2023.

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Foscarnet Sodium 💊