Valaciclovir ( INN ) or valacyclovir ( USAN ) is an antiviral drug used in the management of herpes simplex and herpes zoster (shingles). It is a prodrug , being converted in vivo to aciclovir . more

Valaciclovir ( INN ) or valacyclovir ( USAN ) is an antiviral drug used in the management of herpes simplex and herpes zoster (shingles). It is a prodrug , being converted in vivo to aciclovir . It is marketed by GlaxoSmithKline under the trade name Valtrex or Zelitrex .

Valaciclovir is a prodrug, an esterified version of aciclovir that has greater oral bioavailability (about 55%) than aciclovir (10-20%). It is converted by esterases to the active drug aciclovir via hepatic first-pass metabolism . Acyclovir is selectively converted into a monophosphate form by viral thymidine kinase , which is far more effective (3000 times) in phosphorylation than cellular thymidine kinase. Subsequently, the monophosphate form is further phosphorylated into the active triphosphate form, aciclo-GTP, by cellular kinases . Aciclo-GTP is a very potent inhibitor of viral DNA polymerase ; it has approximately 100 times higher affinity to viral than cellular polymerase. Its monophosphate form also incorporates into the viral DNA, resulting in chain termination . It has also been shown that the viral enzymes cannot remove aciclo-GMP from the chain, which results in inhibition of further activity of DNA polymerase. Aciclo-GTP is fairly rapidly metabolised within the cell, possibly by cellular phosphatases . less