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⚠️ 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.
