Zovirax
Zovirax
Generic Name
Zovirax
Mechanism
- Selective viral targeting: Acyclovir is a guanine nucleoside analog that requires phosphorylation by viral thymidine kinase to become active.
- Phosphorylation cascade:
- Monophosphate → Diphosphate → Triphosphate (most active form).
- DNA chain termination:
- The acyclovir‑triphosphate competitively inhibits viral DNA polymerase.
- Incorporation into the viral DNA strand causes premature chain termination, halting viral replication.
- Low host toxicity: Because activation is primarily viral, normal host cells are spared, explaining the drug’s favorable safety profile.
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Pharmacokinetics
| Parameter | Typical Value | Notes |
| Absorption | 15–30 % oral bioavailability | Peak plasma concentration 0.5–1 h after dosing. |
| Distribution | Minimal protein binding (~10 %) | Vd ≈ 0.2–0.4 L/kg |
| Metabolism | Primarily renal excretion | No significant hepatic biotransformation. |
| Half‑life | 2–3 h (oral) | Shorter in patients with impaired renal function. |
| Clearance | 200–300 mL/min | Primarily glomerular filtration; tubular reabsorption accounts for the rest. |
| Renal handling | 60–70 % filtered, ~20 % reabsorbed, 10 % secreted | Adjustments for GFR < 30 mL/min necessary. |
| Special populations | Elevated peak concentrations in pregnancy (30 % increase) | No dose adjustment required. |
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Indications
- Herpes Simplex Virus (HSV)
- Oral and genital herpes outbreaks
- Primary infection, recurrent lesions, and neonatal herpes prophylaxis
- Varicella Zoster Virus (VZV)
- Shingles (herpes zoster) – early‑onset treatment improves pain control
- Varicella (chickenpox) – especially in immunocompromised hosts
- Prophylaxis
- Post‑herpes ophthalmicus (for ocular lesions)
- Chronic suppressive therapy in pregnant women born with congenital herpes
- Special Situations
- Early treatment of ocular HSV in patients on immunosuppressive therapy
- Adjunctive therapy in HIV‑positive patients with HSV disease
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Contraindications
| Category | Key Points |
| Allergy | Hypersensitivity to acyclovir or other nucleoside analogues |
| Renal impairment | Contraindicated when GFR < 30 mL/min unless dose is adjusted; monitor serum creatinine |
| Pregnancy | Category B; safe in all trimesters but dose slows if renal function is affected |
| Drug interactions | Concurrent use with nephrotoxic agents (e.g., amphotericin B, aminoglycosides) increases risk of acute tubular necrosis |
| Toxicity | Nephrotoxicity, nephrolithiasis, neurotoxicity (e.g., encephalopathy) if not monitored |
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Adverse Effects
| Category | Frequency | Notable Examples |
| Common | 5–10 % | Nausea, vomiting, diarrhea, headache, mild skin rash |
| Rare | < 1 % | Anemia, leukopenia, thrombocytopenia |
| Serious | < 0.1 % | Nephrotoxicity (acute tubular necrosis, interstitial nephritis), nephrolithiasis, neurotoxicity (encephalopathy, seizures), hypersensitivity reactions |
Monitoring
- Renal function: Serum creatinine and BUN before and after IV initiation; every 3 days for long‑term therapy.
- Urine output: (≥ 1 mL/kg/h) if IV; check for stringy, cloudy urine.
- Neurological symptoms: Any new confusion or seizures warrants immediate assessment.
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Clinical Pearls
- Early Start Beats Neuron: Initiate treatment within *48 h* of lesion appearance; delays > 48 h reduce efficacy, particularly for shingles.
- Renal Dose is a Lifeline: In patients with chronic kidney disease (CKD stage 3–4), dosing based on GFR reduces odds of nephrotoxicity; always verify GFR before administering IV.
- Hydration Is Your Best Friend: Adequate fluid intake prevents crystalline nephropathy; for IV therapy, maintain a fluid rate of at least 0.5 mL/kg/h.
- Cryogenic Nephrolithiasis Alert: At GFR 10 mg/kg) can precipitate encephalopathy—watchful monitoring of mental status is advised.
- Pregnancy Safe, but Wash Out: Category B, but still avoid during early gestational rise of renal clearance; maintain hydration and monitor for teratogenicity literature updates.
- Drug Interactions Matter: Avoid administering acyclovir at the same time as nephrotoxic drugs to mitigate cumulative renal risk.
These concise, high‑yield points will help healthcare professionals recall the most critical aspects of acyclovir therapy when making evidence‑based clinical decisions.