BSN Tracker

foscarnet

Brand: Foscavir

⚠ BBW Prototype Drug
Drug Class: antiviral — pyrophosphate analog
Drug Family: antiviral
Subclass: non-nucleoside antiviral; direct viral polymerase inhibitor
Organ Systems: infectious-disease

Mechanism of Action

Pyrophosphate analog that directly inhibits viral DNA polymerases and reverse transcriptases at the pyrophosphate binding site; does NOT require intracellular activation by viral kinases, making it active against acyclovir-resistant HSV and ganciclovir-resistant CMV.

viral DNA polymerase, RNA polymerase (HSV/CMV/HIV reverse transcriptase)

Indications

  • CMV retinitis (acyclovir- or ganciclovir-refractory)
  • acyclovir-resistant HSV or VZV infections
  • HHV-6 and HHV-8 infections
  • CMV disease in immunocompromised patients refractory to ganciclovir

Contraindications

  • foscarnet hypersensitivity
  • CrCl <0.4 mL/min/kg (contraindicated)

Adverse Effects

Common

  • nephrotoxicity (dose-limiting, very common)
  • electrolyte abnormalities (hypocalcemia, hypo/hyperphosphatemia, hypomagnesemia, hypokalemia)
  • nausea
  • fever

Serious

  • severe electrolyte abnormalities causing seizures and cardiac arrhythmias
  • acute kidney injury
  • genital ulcers (from crystallization in urine)
  • CNS toxicity

Pharmacokinetics (ADME)

Absorption IV only (poor oral bioavailability)
Distribution 50% deposits in bone; moderate plasma protein binding
Metabolism not metabolized
Excretion renal (92–100% unchanged); dose-critical adjustment
Half-life 3–6 hours (plasma); bone storage may extend effective exposure
Onset immediate (IV)
Peak end of infusion
Duration 8–12 hours
Protein Binding 14–17%
Vd moderate-large (40 L/kg)

Drug Interactions

Drug / Agent Mechanism Severity
nephrotoxic agents (aminoglycosides, amphotericin B, cyclosporine) additive nephrotoxicity major
pentamidine IV additive nephrotoxicity and hypocalcemia risk major
drugs that decrease serum calcium (bisphosphonates, chelating agents) additive hypocalcemia risk major

Nursing Considerations

  1. Mandatory pre-hydration with 750–1000 mL normal saline before each foscarnet dose to reduce nephrotoxicity; maintain hydration throughout infusion.
  2. Monitor electrolytes (Ca2+, Mg2+, PO4-, K+) before each infusion and replace as needed — hypocalcemia can cause perioral numbness, tetany, and cardiac arrhythmias.
  3. Check serum creatinine at least twice weekly during induction; the foscarnet dose MUST be adjusted based on current CrCl using standardized dosing tables.
  4. Ensure infusion via central venous line or at high dilution; foscarnet causes severe phlebitis when given peripherally; use infusion pump to prevent too-rapid infusion.

Clinical Pearls

  • Foscarnet does not require viral kinase activation, making it the treatment of choice for acyclovir-resistant HSV (often caused by TK mutations) and ganciclovir-resistant CMV (often caused by UL97 mutations).
  • The U-shaped electrolyte disturbance of foscarnet — ionized hypocalcemia and hypomagnesemia from chelation — can cause neuromuscular irritability; patients may experience perioral tingling, muscle cramps, or tetany even when total calcium appears normal.

Safety Profile

Pregnancy use-with-caution
Lactation avoid
Renal Adjustment Required
Hepatic Adjustment Not required
TDM Not required

Concordance Terms

Cross-referenced clinical concepts — click any term to see all content where it appears.

Acyclovir-resistant HSV Or VZV Infections Antiviral — Pyrophosphate Analog CMV Disease In Immunocompromised Patients Refractory To Ganciclovir CMV Retinitis (acyclovir- Or Ganciclovir-refractory) HHV-6 And HHV-8 Infections
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