Valacyclovir
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Valacyclovir
Valtrex
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Summary
Common uses
Genital herpes (HSV-1/2): episodic treatment and suppression0, Herpes labialis (cold sores)0, Herpes zoster (shingles)0, Varicella (in adolescents/adults)0
Pharmacy
Starting dose
Initial genital herpes 1 g PO BID × 10 days; recurrent 500 mg BID × 3 days; suppression 500 mg-1 g PO daily; zoster 1 g TID × 7 days
Preparations
500 mg, 1 g tablets
US FDA Max
3 g/d (zoster)
Pharmacology
Routes
Oral
Onset
Symptom relief within 24-48 hours
Duration
8 hours per dose
Half-life
~3 hours (acyclovir, the active metabolite); longer in renal impairment[1]
Bioavailability
~55% bioavailability of acyclovir after valacyclovir oral (vs ~20% from oral acyclovir directly)[1]
Pregnancy
Widely used in pregnancy when antiviral indicated; reassuring registry data.[citation needed]
Legal status
Rx-only in US
Purported mechanism
Valacyclovir is the L-valyl ester prodrug of acyclovir, exploiting the intestinal PEPT1 transporter to achieve substantially higher oral bioavailability than acyclovir itself; first-pass hepatic and intestinal hydrolysis releases acyclovir, which is then phosphorylated by viral thymidine kinase and host kinases to acyclovir triphosphate, a chain-terminating inhibitor of viral DNA polymerase selectively in HSV/VZV-infected cells.0 Selectivity comes from the viral-kinase-only initial phosphorylation step, which is why uninfected cells generate minimal active drug[1]. Dose-adjust by renal function; rare crystalline nephropathy with rapid IV acyclovir.
References
- ↑ 1.0 1.1 1.2 FDA Prescribing Information, Valtrex (valacyclovir HCl), GlaxoSmithKline, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2008/020487s014lbl.pdf