VIRAZOLE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for VIRAZOLE (VIRAZOLE).
Ribavirin, the active ingredient in Virazole, is a synthetic nucleoside analog that inhibits viral RNA synthesis by competing with endogenous nucleotides for incorporation into viral RNA, leading to chain termination and inhibition of viral replication. It also has immunomodulatory effects.
| Metabolism | Ribavirin is metabolized via reversible phosphorylation and degradation pathways. It undergoes deribosylation and hydrolysis to form a triazole carboxamide metabolite. Metabolism involves enzymes including adenosine deaminase and possibly cytochrome P450 (CYP) enzymes, but specific CYP pathways are not well characterized. Elimination is primarily renal. |
| Excretion | Renal excretion as unchanged drug and metabolites; approximately 40% of an oral dose is excreted unchanged in urine, with 30-40% as metabolites; fecal elimination is minor (<10%). |
| Half-life | Terminal elimination half-life: oral administration ~24-36 hours; intravenous ~24 hours; inhalation systemic half-life ~24 hours. Clinical context: Prolonged half-life allows once-daily or twice-daily dosing, but accumulation may occur in renal impairment. |
| Protein binding | Protein binding: approximately 10-20% (low); primarily binds to albumin. |
| Volume of Distribution | Volume of distribution: 7 L/kg (range 6-8 L/kg) indicating extensive tissue distribution and intracellular penetration. |
| Bioavailability | Bioavailability: Oral: ~45% (range 35-50%); Inhalation: variable but sufficient for systemic effects (not quantified); Intravenous: 100%. |
| Onset of Action | Inhalation (aerosol): clinical improvement in respiratory symptoms usually within 3-7 days of continuous therapy. Intravenous: time to antiviral effect not well-defined; oral: onset variable. |
| Duration of Action | Duration of antiviral effect persists as long as drug is present due to prolonged intracellular metabolism; clinical duration depends on therapy length (typically 3-7 days for inhalation in RSV). Residual effects may last several days after discontinuation. |
| Molecular Weight | 244.21 |
Inhalation: 20 mg/mL solution via small particle aerosol generator (SPAG-2) for 12-18 hours per day for 3-7 days. Oral: Not FDA-approved for oral use; investigational oral dose 600-2400 mg/day in divided doses.
| Dosage form | FOR SOLUTION |
| Renal impairment | No specific guidelines; ribavirin is renally eliminated; dose reduction recommended in renal impairment (CrCl <50 mL/min) but no formal criteria exist; avoid if CrCl <30 mL/min. |
| Liver impairment | No formal recommendations; use caution in hepatic impairment; no Child-Pugh based modifications established. |
| Pediatric use | Inhalation (RSV): Same as adult via SPAG-2; 20 mg/mL solution for 12-18 hours/day for 3-7 days; weight-based oral not established. |
| Geriatric use | No specific geriatric dose adjustments; may be more sensitive to hematologic toxicity; monitor renal function and hemoglobin closely. |
| 1st trimester | Contraindicated due to teratogenicity in animal studies; ribavirin is an FDA Pregnancy Category X drug. |
| 2nd trimester | Contraindicated; ribavirin can cause fetal harm when administered to a pregnant woman. |
| 3rd trimester | Contraindicated; ribavirin is associated with fetal toxicity and should not be used during pregnancy. |
Clinical note
Comprehensive clinical and safety monograph for VIRAZOLE (VIRAZOLE).
| Placental transfer | Ribavirin crosses the placenta extensively; accumulation in fetal tissues has been observed with potential for teratogenic and embryocidal effects. |
| Breastfeeding | Ribavirin is excreted into human milk; due to the potential for serious adverse reactions in nursing infants, breastfeeding is not recommended during treatment. |
■ FDA Black Box Warning
WARNING: HEMOLYTIC ANEMIA. Ribavirin causes hemolytic anemia, which may worsen cardiac disease and lead to fatal myocardial infarction. Patients with pre-existing cardiac disease should be monitored closely. Discontinue if signs of cardiac disease worsen.
| Serious Effects |
PregnancyWomen of childbearing potential not using effective contraceptionBreastfeedingSevere hepatic impairmentHemoglobinopathies (e.g., thalassemia major, sickle cell anemia)Autoimmune hepatitisConcomitant use with didanosine
| Precautions | Hemolytic anemia: Monitor hemoglobin and reticulocyte counts; dose reduction or discontinuation may be necessary., Cardiac disease: Worsening of pre-existing cardiac disease; monitor ECG and cardiac function., Teratogenicity: Pregnancy category X; avoid use in pregnant women and their male partners during treatment and for 6 months after., Pancreatitis and hypertriglyceridemia: Discontinue if signs of pancreatitis., Pulmonary symptoms: May cause cough, dyspnea, or pulmonary infiltrates., Lactic acidosis: Reported with oral ribavirin in combination with interferon., Ophthalmic effects: Retinopathy, retinal hemorrhages; discontinue if visual disturbances occur. |
| Food/Dietary |
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| Lactation Rating |
| L5 (Contraindicated) |
| Teratogenic Risk | FDA Pregnancy Category X. Ribavirin (VIRAZOLE) is contraindicated in pregnancy. Animal studies have shown significant teratogenic and embryocidal effects at doses well below the recommended human dose. In humans, ribavirin is known to cause birth defects and fetal death. Negative pregnancy test must be obtained immediately prior to initiation of therapy and monthly thereafter. Two reliable forms of contraception must be used during treatment and for 6 months after completion in both female patients and female partners of male patients. |
| Fetal Monitoring | Monitor pregnancy status: serum pregnancy test immediately before initiation, monthly during treatment, and for 6 months after discontinuation. Assess for fetal anomalies via ultrasound if exposure occurs. Monitor hepatic function, bilirubin, complete blood count (CBC), and reticulocyte count regularly due to hemolytic anemia risk. Monitor serum ribavirin levels if renal impairment present. Thyroid function tests should be monitored as ribavirin may exacerbate thyroid abnormalities. |
| Fertility Effects | Ribavirin is genotoxic and may impair fertility. Animal studies demonstrated reduced sperm counts and testicular atrophy at clinically relevant doses. In men, reversible oligospermia has been reported. Women of childbearing potential must use effective contraception during and for 6 months after treatment. Male patients and their female partners must also use contraception during and for 6 months after therapy. Fertility may be impaired during treatment; recovery of sperm parameters may occur post-treatment. |
| No significant food interactions. May be taken with or without food. However, in oral form, taking with fatty meals may increase absorption. Avoid alcohol due to potential hepatotoxicity. |
| Clinical Pearls | Ribavirin (Virazole) is FDA-approved for inhalation therapy in severe RSV infections, especially in high-risk infants. It is also used orally in combination with peginterferon alfa for hepatitis C. Teratogenic: contraindicated in pregnancy and in partners of women of childbearing age. Hemolytic anemia is a dose-limiting toxicity; monitor hemoglobin frequently. Ribavirin accumulates in erythrocytes; half-life in plasma is ~44 hours but in RBCs it can be weeks. Inhaled ribavirin may precipitate in ventilator valves. Avoid use in patients with unstable cardiac disease due to anemia risk. |
| Patient Advice | Take ribavirin exactly as prescribed; do not alter dose or stop without consulting your doctor. · Ribavirin can cause birth defects; you must not become pregnant or father a child during treatment and for 6 months after therapy. Use two effective forms of contraception. · This drug may decrease hemoglobin; you may experience fatigue, shortness of breath, or pale skin. Report these symptoms immediately. · Inhaled ribavirin is given via a special nebulizer; follow the respiratory therapist's instructions carefully. · Avoid alcohol while on ribavirin therapy, especially for hepatitis C, as it can worsen liver damage. |