Impact of Peptide Transporter 1 on the Intestinal Absorption and Pharmacokinetics of Valacyclovir after Oral Dose Escalation in Wild-Type and PepT1 Knockout Mice

Abstract

The primary objective of this study was to determine the in vivo absorption properties of valacyclovir, including the potential for saturable proton-coupled oligopeptide transporter 1 (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. [ 3 H]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 minutes, and tissue distribution studies were performed 20 minutes after a 25-nmol/ g oral dose of valacyclovir. We found that the C max and area under the curve (AUC) 0-180 of acyclovir were 4-to 6-fold and 2-to 3-fold lower, respectively, in KO mice for all four oral doses of valacyclovir. The time to peak concentration of acyclovir was 3-to 10-fold longer in KO compared with 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 the 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.

In Silico Absorption Analysis of Valacyclovir in Wildtype and Pept1 Knockout Mice Following Oral Dose Escalation

“…The ACAT model implemented in GastroPlus was used to simulate in silico the pharmacokinetics of valacyclovir following escalating oral administration in wildtype and Pept1 knockout mice (11). The intestinal fate of orally dosed valacyclovir solution was governed by three simultaneous kinetic processes in the gastrointestinal (GI) tract including GI transit, intestinal absorption and luminal degradation, based on findings from the previous in situ perfusion studies (10).…”

“…The luminal degradation of valacyclovir was modeled as a first-order process characterized by the degradation rate constant k de , which was estimated by reanalyzing in situ intestinal perfusion results obtained in the jejunum of wildtype mice at pH 5.5, 6, 6.5, 7 and 7.5 (26), as well as those perfusion results obtained in different intestinal segments of wildtype mice at pH 6.5 (10). The derivation of k de is described below.…”

“…Intensive effort has been focused on delineating the quantitative importance of PEPT1 in the intestinal absorption of valacyclovir, which understandably lays a solid basis for promoting the rational design of prodrugs that selectively target intestinal PEPT1. In our laboratory, we evaluated the role and relevance of PEPT1 by performing in situ intestinal permeability and in vivo pharmacokinetic studies of valacyclovir in wildtype and Pept1 knockout mice (10,11). The in situ intestinal perfusion studies demonstrated that valacyclovir was actively transported by intestinal PEPT1, which contributed approximately 90% of the permeability of valacyclovir in mouse small intestine (10).…”

“…The in situ intestinal perfusion studies demonstrated that valacyclovir was actively transported by intestinal PEPT1, which contributed approximately 90% of the permeability of valacyclovir in mouse small intestine (10). Moreover, during the in vivo pharmacokinetic studies of orally administered valacyclovir, 5-fold and 2-fold differences were observed between wildtype and Pept1 knockout mice in acyclovir peak plasma concentrations (C max ) and area under the plasma concentration-time curves from time 0 to 180 min (AUC 0–180 ), respectively (11). Based on the in situ and in vivo results, we concluded that PEPT1 had a major role in mediating the intestinal absorption of valacyclovir.…”

A Minireview: Usefulness of Transporter-Targeted Prodrugs in Enhancing Membrane Permeability

“…50 The impact of PepT1 on the intestinal absorption of valacyclovir was confirmed by using PepT1 knockout (KO) mice, in which Cmax and area under the concentration-time curve (AUC) of ACV after oral administration of valacyclovir were 4-to 6-fold and 2-to 3-fold lower in KO mice, respectively, as compared with those in wildtype mice. 51,52 The usefulness of dipeptide ester prodrugs of ACV was also examined using Val-Val, Gly-Val, Gly-Gly, Gly-Tyr, Val-Try, and Tyr-Val ester moieties. 25 All dipeptide prodrugs except Try-Gly-ACV exhibited a higher affinity toward PEPT1 than cephalexin, a typical substrate for PEPT1, and the affinity and permeability across Caco-2 cell membrane of Gly-Val-ACV was comparable with those of Val-ACV.…”