Makindo Medical Notes

Related Subjects: |Herpes Varicella-Zoster (Shingles) Infection |Chickenpox Varicella Infection |Varicella Cerebral Vasculopathy |Herpes Viruses |Herpes Zoster Ophthalmicus (HZO) Shingles |MonkeyPox |Mumps |Measles |Rubella (German Measles) |Epstein-Barr Virus infection |Cytomegalovirus (CMV) infections |CMV retinitis infections |Toxoplasmosis

⚠️ 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

⚠️ 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.