Valacyclovir is rapidly and completely converted to acy-clovir by intestinal and hepatic first-pass metabolism. The bioavailability of acyclovir following oral valacy-clovir dosing is three to five times that resulting from oral acyclovir administration and is comparable to that of intravenous acyclovir.
Acyclovir absorption is variable and incomplete following oral administration. It is about 20% bound to plasma protein and is widely distributed throughout body tissues. Significant amounts may be found in amniotic fluid, placenta, and breast milk. Acyclovir is both filtered at the glomeruli and actively secreted. Most of the dose is excreted in the urine as unchanged drug; a small portion is excreted as an oxidized inactive metabolite. The plasma half-life of acyclovir is 3 to 4 hours in patients with normal kidney function and up to 20 hours in patients with renal impairment.
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