Abstract

The primary objective of this study was to determine the in vivo absorption properties of valacyclovir, including the potential for saturable PepT1-mediated intestinal uptake, after escalating oral doses of prodrug within the clinical dose range. A secondary aim was to characterize the role of PepT1 on the tissue distribution of its active metabolite acyclovir. [³H]Valacyclovir was administered to wild-type (WT) and PepT1 knockout (KO) mice by oral gavage at doses of 10, 25, 50 and 100 nmol/g. Serial blood samples were collected over 180 min, and tissue distribution studies were performed 20 min after a 25 nmol/g oral dose of valacyclovir. We found that the C_max and AUC_0-180 of acyclovir were 4-6 fold and 2-3 fold lower, respectively, in KO mice for all four oral doses of valacyclovir. The T_max of acyclovir was 3-10 fold longer in KO as compared to WT mice. There was dose proportionality in the C_max and AUC_0-180 of acyclovir in WT and KO mice over the valacyclovir oral dose range of 10-100 nmol/g (i.e., linear absorption kinetics). No differences were observed in the peripheral tissue distribution of acyclovir once these tissues were adjusted for differences in perfusing drug concentrations in the systemic circulation. In contrast, some differences were observed between genotypes in the concentrations of acyclovir in distal intestine. Collectively, the findings demonstrate a critical role of intestinal PepT1 in improving the rate and extent of oral absorption for valacyclovir. Moreover, this study provides definitive evidence for the rational development of a PepT1-targeted prodrug strategy.