VIRA-A
Clinical safety rating: caution
Comprehensive clinical and safety monograph for VIRA-A (VIRA-A).
Vidarabine (VIRA-A) is a purine nucleoside analog that inhibits viral DNA polymerase and incorporates into viral DNA, causing chain termination. It also inhibits ribonucleotide reductase, reducing viral DNA synthesis.
| Metabolism | Vidarabine is rapidly deaminated by adenosine deaminase in the liver and peripheral tissues to arabinosylhypoxanthine (ara-Hx), which is less active but has a longer half-life. Hepatic metabolism predominantly. |
| Excretion | Primarily renal: approximately 40-70% of dose excreted unchanged in urine via glomerular filtration and tubular secretion. A smaller fraction (10-20%) is eliminated as the metabolite hypoxanthine arabinoside. Fecal excretion accounts for <5% of the dose. |
| Half-life | Vidarabine: 0.17-0.25 hours (rapidly deaminated to arabinosylhypoxanthine). Arabinosylhypoxanthine: 3.5-5.5 hours in adults with normal renal function; prolonged to 15-20 hours in severe renal impairment. |
| Protein binding | Vidarabine: 15-30% bound to plasma proteins. Arabinosylhypoxanthine: negligible binding (<10%). |
| Volume of Distribution | Vidarabine: approximately 2.5-4.0 L/kg; extensive tissue distribution including brain and CSF. Arabinosylhypoxanthine: 1.5-2.5 L/kg. |
| Bioavailability | Intravenous: 100% (only approved systemic route). Ophthalmic ointment: negligible systemic absorption (<1% of ocular dose). |
| Onset of Action | Intravenous: within 24-48 hours for virologic response in herpes simplex encephalitis. Ophthalmic: within 2-5 days for reduction of viral shedding in herpes keratitis. |
| Duration of Action | Antiviral effect persists for the duration of detectable plasma levels of arabinosylhypoxanthine (approximately 24 hours after last IV dose). For ophthalmic use, 7-10 days of treatment typical; clinical resolution by 2-3 weeks. |
| Molecular Weight | 285.26 |
15 mg/kg/day IV infused over 12-24 hours for 7 days.
| Dosage form | OINTMENT |
| Renal impairment | CrCl <50 mL/min: 10 mg/kg/day; CrCl <10 mL/min: 7.5 mg/kg/day. |
| Liver impairment | Child-Pugh B or C: reduce dose by 50%. Use with caution in severe hepatic impairment. |
| Pediatric use | 30 mg/kg/day IV divided every 8 hours for 7 days. |
| Geriatric use | Monitor renal function closely; consider dose reduction based on CrCl. |
| 1st trimester | Excreted in human milk; not recommended for use in pregnant women, especially during first trimester, due to potential teratogenicity and embryotoxicity. |
| 2nd trimester | Use only if potential benefit justifies potential risk to the fetus; exhibits embryotoxicity in animal studies. |
| 3rd trimester | Use only if clearly needed; caution near term due to potential for accumulation in amniotic fluid. |
Clinical note
Comprehensive clinical and safety monograph for VIRA-A (VIRA-A).
| Placental transfer | Vidarabine crosses the placenta in humans; measurable levels in cord blood and amniotic fluid. |
| Breastfeeding | Vidarabine is excreted in breast milk. Breastfeeding is not recommended during therapy due to potential for serious adverse reactions in nursing infants. |
■ FDA Black Box Warning
None.
| Serious Effects |
Hypersensitivity to vidarabine or any component of the formulation
| Precautions | May cause dose-related neurotoxicity including tremors, ataxia, confusion, and seizures, especially in renal impairment., Renal function monitoring essential due to accumulation of ara-Hx., Hematologic effects: anemia, leukopenia, thrombocytopenia., Gastrointestinal: nausea, vomiting, diarrhea., Hepatotoxicity: elevated liver enzymes., Carcinogenesis, mutagenesis, impairment of fertility observed in animal studies. |
| Food/Dietary | No specific food interactions are documented. Maintain adequate hydration to prevent crystallization in urine. Avoid concurrent use with alcohol due to potential hepatotoxicity. |
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| Lactation Rating |
| L5 (Contraindicated) |
| Teratogenic Risk | FDA Pregnancy Category C. First trimester: Potential for teratogenicity based on animal studies (cleft palate, exencephaly); human data insufficient. Second/third trimesters: No well-controlled studies; use only if maternal benefit outweighs fetal risk. Risk of kernicterus in third trimester if administered near delivery due to bilirubin displacement. |
| Fetal Monitoring | Monitor maternal CBC, liver function, renal function, and neurological status. Fetal monitoring: serial ultrasound for growth assessment; consider fetal echocardiography if used in first trimester. |
| Fertility Effects | No human data; animal studies show dose-dependent testicular atrophy and impaired spermatogenesis in males, and ovarian suppression in females at high doses. Reversible upon discontinuation in animals. |
| Clinical Pearls | VIRA-A (vidarabine) is an adenine analog used for HSV encephalitis, but is less favored due to toxicity; acyclovir is preferred. Use with caution in renal impairment; dose adjustment needed for CrCl <50 mL/min. Monitor for neurotoxicity (tremor, confusion) and bone marrow suppression. Avoid concomitant use with allopurinol (increases vidarabine toxicity). Administer as slow IV infusion over 12-24 hours; do not use as IV push or bolus due to crystallization risk. |
| Patient Advice | This medication is given intravenously in the hospital, usually for serious viral infections like herpes encephalitis. · You will be monitored closely for side effects including confusion, tremors, or signs of infection (fever, bruising, bleeding). · Report any new or worsening symptoms such as nausea, vomiting, or jaundice immediately. · Avoid taking allopurinol (for gout) while on this treatment, as it can increase the risk of severe side effects. |