Foscarnet

FOSCAVIR

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Pronunciation: fos-KAR-net

Foscarnet ballicule: receptor binding, kinetics and half-life diagram — Mechanism of action: Foscarnet ballicule

What it isPyrophosphate analog directly inhibiting viral DNA polymerase; resistant CMV and acyclovir-resistant HSV.

Why this oneBecause it needs no viral kinase activation, it works when kinase mutations defeat nucleoside analogs.

What limits itNephrotoxicity and severe electrolyte derangements, especially hypocalcemia, require saline hydration and close monitoring.

Foscarnet molecular structure (skeletal formula) — Molecular structure: Foscarnet

FDA label

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BOXED WARNING Renal impairment is the major toxicity — monitor serum creatinine frequently, adjust dose, hydrate. Seizures from mineral/electrolyte changes — monitor. Indicated ONLY in immunocompromised patients (CMV retinitis, acyclovir-resistant HSV).

FDA dosing	Population	Start	Target	Max
CMV retinitis — induction	Immunocompromised adults, normal renal function	90 mg/kg IV Q12H (1.5-2 h infusion) or 60 mg/kg IV Q8H (>=1 h infusion) x 2-3 wk	2-3 wk induction per clinical response	180 mg/kg/day; individualize for renal function
CMV retinitis — maintenance	Immunocompromised adults	90 mg/kg IV QD over 2 h	90-120 mg/kg IV QD (escalate to 120 for retinitis progression/good tolerance)	120 mg/kg/day
Acyclovir-resistant mucocutaneous HSV	Immunocompromised adults, normal renal function	40 mg/kg IV Q8H or Q12H (>=1 h infusion)	2-3 wk or until healed	120 mg/kg/day

Renal Dose-adjust per CrCl (mL/min/kg) nomogram — required even with normal serum creatinine; discontinue if CrCl <0.4 mL/min/kg; not recommended on hemodialysis (no guidelines)

Hepatic Undefined

Do not exceed recommended dose, frequency, or infusion rate; all doses individualized for renal function

Instructions

NEVER rapid or bolus IV — infusion pump required; max rate 1 mg/kg/min
Central line: 24 mg/mL undiluted; peripheral vein: dilute to 12 mg/mL with D5W or NS; use within 24 h
Hydrate: 750-1,000 mL NS/D5W before first infusion; then 750-1,000 mL with each 90-120 mg/kg dose, 500 mL with 40-60 mg/kg dose
Administer only with NS or D5W; no other drug in same catheter; incompatible with calcium-containing solutions (LR, TPN), ganciclovir, many IV drugs
Monitor CrCl at baseline, 2-3x/wk during induction, weekly during maintenance; monitor Ca, Mg, K, phosphate on same schedule

Available strengths

inj 24 mg/mL

Cautions

Nephrotoxicity — most patients have some renal function decline; hydration reduces risk
Seizures assoc. with electrolyte/mineral changes (esp. hypocalcemia); caution in low baseline Ca, neurologic/cardiac disease, seizure-predisposing conditions
Electrolyte disturbances: hypocalcemia, hypomagnesemia, hypokalemia, hypo-/hyperphosphatemia — supplement as needed
Limited data if baseline SCr >2.8 mg/dL or CrCl <50 mL/min
Accidental skin/eye contact: flush with water (local irritation/burning)

Adverse reactions

Fever (65%), nausea (47%), anemia (33%), diarrhea (30%), vomiting, headache (26% ea)
Abnormal renal function (27%); renal impairment ~12% even with hydration
Seizures (10%)
Electrolytes: hypokalemia (16-48%), hypocalcemia (15-30%), hypomagnesemia (15-30%), hypophosphatemia (8-26%), hyperphosphatemia (6%)
Marrow suppression, granulocytopenia; injection-site pain/inflammation

Pregnancy No adequate human studies — use only if clearly needed; skeletal anomalies in animals at sub-human exposures

Lactation Undefined

Cost

Foscarnet (FOSCAVIR)

Strength		#/mo	generic $/unit	CostPlus
24 mg/mL	inj	per unit		not offered

NADAC is a voluntary CMS survey of what retail community pharmacies pay for the drugs they stock. Foscarnet is administered under clinical supervision and dispensed outside retail pharmacies, so it rarely appears on surveyed invoices.

⚠ BOXED WARNINGS

Renal impairment
Seizures related to mineral and electrolyte changes

Classes and tags

Clinical profile

Therapeutic safety burden4 / 4

Overdose danger3 / 4

Weight-gain liability0 / 4

Sedation liability1 / 4

QT / Torsades1 / 4

Direct muscarinic antagonism0 / 4

Embryofetal risk2 / 4

Defining liabilityNephrotoxicity, electrolyte abnormalities and seizures

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