FOSCARNET SODIUM
Clinical safety rating: safe
Animal studies have demonstrated safety
Foscarnet is an organic analog of inorganic pyrophosphate that selectively inhibits the DNA polymerase activity of herpesviruses, including cytomegalovirus (CMV) and herpes simplex virus (HSV), at the pyrophosphate binding site without requiring activation by thymidine kinase. It also inhibits HIV reverse transcriptase.
| Metabolism | Foscarnet is not metabolized; it is excreted unchanged by the kidney via glomerular filtration and tubular secretion. |
| Excretion | Primarily excreted unchanged by the kidney via glomerular filtration and tubular secretion; >80% of dose recovered in urine within 24 hours; minimal biliary or fecal excretion (<5%). |
| Half-life | Terminal elimination half-life is approximately 48 hours (range 24-88 hours), reflecting prolonged intracellular retention; clinical context necessitates dose adjustment for renal impairment and monitoring of renal function. |
| Protein binding | 14-17% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | 0.3-0.6 L/kg, indicating distribution primarily in extracellular fluid; lower volume in elderly or renally impaired patients due to reduced tissue binding. |
| Bioavailability | Only administered intravenously; oral bioavailability <5% due to poor gastrointestinal absorption. |
| Onset of Action | Intravenous administration: antiviral effect onset within 30 minutes to 1 hour following infusion; time to clinical response varies with viral load and infection site. |
| Duration of Action | Antiviral duration persists for several hours post-infusion; dosing typically requires maintenance infusions every 8-12 hours due to short serum half-life; intracellular drug levels decline slowly. |
| Molecular Weight | 191.99 |
Induction: 60 mg/kg IV every 8 hours for 14–21 days, followed by maintenance: 90–120 mg/kg IV once daily. Infuse at no more than 1 mg/kg/min via central or peripheral line.
| Dosage form | SOLUTION |
| Renal impairment | Adjust based on creatinine clearance (CrCl). For induction: CrCl >=1.6 mL/min/kg: 60 mg/kg q8h; CrCl 1.4–1.59: 45 mg/kg q8h; CrCl 1.2–1.39: 35 mg/kg q8h; CrCl 1.0–1.19: 25 mg/kg q12h; CrCl 0.8–0.99: 20 mg/kg q12h; CrCl 0.6–0.79: 15 mg/kg q12h; CrCl 0.4–0.59: 10 mg/kg q12h; CrCl <0.4: 10 mg/kg q24h. For maintenance: CrCl >=1.6: 90–120 mg/kg q24h; CrCl 1.4–1.59: 70–90 mg/kg q24h; CrCl 1.2–1.39: 55–70 mg/kg q24h; CrCl 1.0–1.19: 45–55 mg/kg q24h; CrCl 0.8–0.99: 35–45 mg/kg q24h; CrCl 0.6–0.79: 25–35 mg/kg q24h; CrCl 0.4–0.59: 20–25 mg/kg q24h; CrCl <0.4: not recommended. |
| Liver impairment | No dosage adjustment required for hepatic impairment; pharmacokinetics not altered. Monitor electrolytes and renal function. |
| Pediatric use | For children (≥3 months): Induction: 60 mg/kg IV every 8 hours for 14–21 days; maintenance: 90–120 mg/kg IV once daily. Administer over at least 1 hour. Adjust dose based on renal function using Schwartz formula for CrCl. Safety and efficacy in neonates not established. |
| Geriatric use | Elderly patients have reduced renal function; base dosing on CrCl. Start at lower end of dosing range and titrate carefully. Monitor serum creatinine and electrolytes frequently. Avoid if CrCl <0.4 mL/min/kg. |
| 1st trimester | Avoid in first trimester; teratogenic in animal studies. |
| 2nd trimester | Use only if benefit outweighs risk; crosses placenta. |
| 3rd trimester | Use only if benefit outweighs risk; crosses placenta. |
Clinical note
Nephrotoxic drugs increase risk of renal impairment Can cause renal impairment and electrolyte imbalances.
| Placental transfer | Crosses placenta; documented in human studies. |
| Breastfeeding | Excreted in breast milk; avoid breastfeeding due to potential toxicity. |
| Lactation Rating | L5 (Contraindicated) |
■ FDA Black Box Warning
Renal impairment: Foscarnet can cause acute renal failure. Renal function must be monitored closely, and dose adjustments are required based on creatinine clearance. Electrolyte disturbances, including hypocalcemia, hypomagnesemia, hypokalemia, and hypophosphatemia, may occur and can be severe. Seizures have been associated with electrolyte abnormalities.
| Common Effects | Nephrotoxicity |
| Serious Effects |
Hypersensitivity to foscarnetCrCl < 0.4 mL/min/kg (relative, but severe renal impairment)
| Precautions | Monitor renal function and electrolyte levels frequently (e.g., serum creatinine, calcium, magnesium, potassium, phosphorus)., Adjust dose according to renal function., Avoid concurrent use with nephrotoxic drugs (e.g., aminoglycosides, amphotericin B, cyclosporine)., May cause seizures, primarily due to electrolyte imbalances., Central nervous system effects: Headache, tremor, paresthesias, and hallucinations., Gastrointestinal effects: Nausea, diarrhea, and vomiting., Hematologic effects: Anemia, leukopenia, and thrombocytopenia. |
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| Teratogenic Risk | Foscarnet is classified as FDA Pregnancy Category C. In animal studies, it has been associated with skeletal abnormalities and embryotoxicity at doses lower than human therapeutic doses. There are no adequate well-controlled studies in pregnant women. The risk is highest during the first trimester; however, potential benefits may warrant use in life-threatening conditions. |
| Fetal Monitoring | Monitor renal function (serum creatinine, BUN), electrolytes (especially calcium, magnesium, phosphate, potassium), and complete blood count. Assess fetal growth and development via ultrasound during pregnancy. Monitor for signs of nephrotoxicity and electrolyte disturbances in the neonate if used near term. |
| Fertility Effects | Foscarnet has been shown to cause testicular atrophy and reduced spermatogenesis in animal studies. In humans, data are limited; however, potential for reversible impairment of fertility in males. No specific data on female fertility. |
| Food/Dietary | No specific food interactions, but maintenance of adequate oral hydration is crucial. Avoid excessive intake of high-sodium or high-phosphate foods that may worsen electrolyte imbalances. |
| Clinical Pearls | Foscarnet is nephrotoxic; ensure adequate hydration (≥2.5 L/day) and monitor serum creatinine at least 2-3 times weekly. Dose adjustment based on creatinine clearance is mandatory. Avoid concurrent use of other nephrotoxic agents (e.g., aminoglycosides, amphotericin B). Penile/vulvar ulcerations may occur; maintain genital hygiene. Monitor electrolytes frequently: hypocalcemia, hypokalemia, hypomagnesemia, and hypophosphatemia are common. Administer via central line to avoid phlebitis. |
| Patient Advice | You must drink at least 8-10 glasses of water daily to reduce kidney damage risk. · This medication may cause sores on the genital area; keep the area clean and dry. · Your doctor will check your kidney function and blood mineral levels regularly. · Avoid taking other medications that can harm your kidneys, like NSAIDs (ibuprofen, naproxen), unless approved by your doctor. · Report symptoms of electrolyte imbalance: muscle cramps, twitching, numbness, or irregular heartbeat. · Do not breastfeed while taking this medication. |