Identification of a human valacyclovirase: biphenyl hydrolase-like protein as valacyclovir hydrolase

Specificity of a prodrug-activating enzyme hVACVase: the leaving group effect

Human valacyclovirase (hVACVase) is a prodrug-activating enzyme for amino acid prodrugs including the antiviral drugs valacyclovir and valganciclovir. In hVACVase-catalyzed reactions, the leaving group of the substrate corresponds to the drug moiety of the prodrug, making the leaving group effect essential for the rational design of new prodrugs targeting hVACVase activation. In this study, a series of valine esters, phenylalanine esters, and a valine amide were characterized for the effect of the leaving group on the efficiency of hVACVase-mediated prodrug activation. Except for phenylalanine methyl and ethyl esters, all of the ester substrates exhibited a relatively high specificity constant (k(cat)/K(m)), ranging from 850 to 9490 mM(-1)·s(-1). The valine amide Val-3-APG exhibited significantly higher K(m) and lower k(cat) values compared to the corresponding ester Val-3-HPG, indicating poor specificity for hVACVase. In conclusion, the substrate leaving group has been shown to affect both binding and specific activity of hVACVase-catalyzed activation. It is proposed that hVACVase is an ideal target for α-amino acid ester prodrugs with relatively labile leaving groups while it is relatively inactivate toward amide prodrugs.

Enhancing the intestinal absorption of molecules containing the polar guanidino functionality: a double-targeted prodrug approach

A prodrug strategy was applied to guanidino-containing analogues to increase oral absorption via hPEPT1 and hVACVase. l-Valine, l-isoleucine, and l-phenylalanine esters of [3-(hydroxymethyl)phenyl]guanidine (3-HPG) were synthesized and evaluated for transport and activation. In HeLa/hPEPT1 cells, Val-3-HPG and Ile-3-HPG exhibited high affinity to hPEPT1 (IC(50): 0.65 and 0.63 mM, respectively), and all three l-amino acid esters showed higher uptake (2.6- to 9-fold) than the parent compound 3-HPG. Val-3-HPG and Ile-3-HPG demonstrated remarkable Caco-2 permeability enhancement, and Val-3-HPG exhibited comparable permeability to valacyclovir. In rat perfusion studies, Val-3-HPG and Ile-3-HPG permeabilities were significantly higher than 3-HPG and exceeded/matched the high-permeability standard metoprolol, respectively. All the l-amino acid 3-HPG esters were effectively activated in HeLa and Caco-2 cell homogenates and were found to be good substrates of hVACVase (k(cat)/K(m) in mM(-1) x s(-1): Val-3-HPG, 3370; Ile-3-HPG, 1580; Phe-3-HPG, 1660). In conclusion, a prodrug strategy is effective at increasing the intestinal permeability of polar guanidino analogues via targeting hPEPT1 for transport and hVACVase for activation.

Human carboxymethylenebutenolidase as a bioactivating hydrolase of olmesartan medoxomil in liver and intestine

Olmesartan medoxomil (OM) is a prodrug type angiotensin II type 1 receptor antagonist widely prescribed as an antihypertensive agent. Herein, we describe the identification and characterization of the OM bioactivating enzyme that hydrolyzes the prodrug and converts to its pharmacologically active metabolite olmesartan in human liver and intestine. The protein was purified from human liver cytosol by successive column chromatography and was identified by mass spectrometry to be a carboxymethylenebutenolidase (CMBL) homolog. Human CMBL, whose endogenous function has still not been reported, is a human homolog of Pseudomonas dienelactone hydrolase involved in the bacterial halocatechol degradation pathway. The ubiquitous expression of human CMBL gene transcript in various tissues was observed. The recombinant human CMBL expressed in mammalian cells was clearly shown to activate OM. By comparing the enzyme kinetics and chemical inhibition properties between the recombinant protein and human tissue preparations, CMBL was demonstrated to be the primary OM bioactivating enzyme in the liver and intestine. The recombinant CMBL also converted other prodrugs having the same ester structure as OM, faropenem medoxomil and lenampicillin, to their active metabolites. CMBL exhibited a unique sensitivity to chemical inhibitors, thus, being distinguishable from other known esterases. Site-directed mutagenesis on the putative active residue Cys¹³² of the recombinant CMBL caused a drastic reduction of the OM-hydrolyzing activity. We report for the first time that CMBL serves as a key enzyme in the bioactivation of OM, hydrolyzing the ester bond of the prodrug type xenobiotics.

Chemical and enzymatic stability of amino acid prodrugs containing methoxy, ethoxy and propylene glycol linkers

We evaluated the chemical and enzymatic stabilities of prodrugs containing methoxy, ethoxy and propylene glycol linkers in order to find a suitable linker for prodrugs of carboxylic acids with amino acids. l-Valine and l-phenylalanine prodrugs of model compounds (benzoic acid and phenyl acetic acid) containing methoxy, ethoxy and propylene glycol linkers were synthesized. The hydrolysis rate profile of each compound was studied at physiologically relevant pHs (1.2, 4, 6 and 7.4). Enzymatic hydrolysis of propylene glycol containing compounds was studied using Caco-2 homogenate as well as purified enzyme valacyclovirase. It was observed that the stability of the prodrugs increases with the linker length (propyl > ethyl > methyl). The model prodrugs were stable at acidic pH as compared to basic pH. It was observed that the prodrug with the aliphatic amino acid promoiety was more stable compared to its aromatic counterpart. The comparison between benzyl and the phenyl model compounds revealed that the amino acid side chain is significant in determining the stability of the prodrug whereas the benzyl or phenyl carboxylic acid had little or no effect on the stability. The enzymatic activation studies of propylene glycol linker prodrug in the presence of valacyclovirase and cell homogenate showed faster generation of the parent drug at pH 7.4. The half-life of prodrugs at pH 7.4 was more than 12 h, whereas in the presence of cell homogenate the half-lives were less than 1 h. Hydrolysis by Caco-2 homogenate generated the parent compound in two steps, where the prodrug was first converted to the intermediate, propylene glycol benzoate, which was then converted to the parent compound (benzoic acid). Enzymatic hydrolysis of propylene glycol containing prodrugs by valacyclovirase showed hydrolysis of the amino acid ester part to generate the propylene glycol ester of model compound (propylene glycol benzoate) as the major product. The amino acid prodrugs containing methoxy linker were the least stable while prodrugs containing propylene glycol linker were most stable. This work suggests that the propylene glycol linker is an optimal linker for amino acid prodrugs since it has good chemical stability and is enzymatically hydrolyzed to yield the parent drug. This approach can be further extended to other non-amino acid prodrugs and to provide a chemical handle to modify lead molecules containing carboxylic group(s).

Puromycin-sensitive aminopeptidase: an antiviral prodrug activating enzyme

Cidofovir (HPMPC) is a broad-spectrum antiviral agent, currently used to treat AIDS-related human cytomegalovirus retinitis. Cidofovir has recognized therapeutic potential for orthopox virus infections, although its use is hampered by its inherent low oral bioavailability. Val-Ser-cyclic HPMPC (Val-Ser-cHPMPC) is a promising peptide prodrug which has previously been shown by us to improve the permeability and bioavailability of the parent compound in rodent models (Eriksson et al., 2008. Molecular Pharmaceutics 5, 598-609). Puromycin-sensitive aminopeptidase was partially purified from Caco-2 cell homogenates and identified as a prodrug activating enzyme for Val-Ser-cHPMPC. The prodrug activation process initially involves an enzymatic step where the l-Valine residue is removed by puromycin-sensitive aminopeptidase, a step that is bestatin-sensitive. Subsequent chemical hydrolysis results in the generation of cHPMPC. A recombinant puromycin-sensitive aminopeptidase was generated and its substrate specificity investigated. The k(cat) for Val-pNA was significantly lower than that for Ala-pNA, suggesting that some amino acids are preferred over others. Furthermore, the three-fold higher k(cat) for Val-Ser-cHPMPC as compared to Val-pNA suggests that the leaving group may play an important role in determining hydrolytic activity. In addition to its ability to hydrolyze a variety of substrates, these observations strongly suggest that puromycin-sensitive aminopeptidase is an important enzyme for activating Val-Ser-cHPMPC in vivo. Taken together, our data suggest that puromycin-sensitive aminopeptidase makes an attractive target for future prodrug design.

Influence of ALDH2 genetic polymorphisms on aciclovir pharmacokinetics following oral administration of valaciclovir in Japanese end-stage renal disease patients

This study was performed to investigate the pharmacokinetics of valaciclovir (VACV), aciclovir (ACV) and 9-(carboxymethoxy)methylguanine (CMMG) in Japanese chronic hemodialysis patients following a single oral administration of 1000 mg VACV and the influence of genetic polymorphisms of aldehyde dehydrogenase-2 (ALDH2) on their pharmacokinetics. A total of eighteen individuals genotyped as ALDH2*1/*1, ALDH2*1/*2 or ALDH2*2/*2 were enrolled in this study. Blood samples were obtained pre-dose and up to 48 hour post-dose. ACV t(1/2) was significantly affected by ALDH2 genotype and prolonged in the order of ALDH2*1/*1 (18.1 hr)<ALDH2*1/*2 (21.9 hr)<ALDH2*2/*2 (26.7 hr). ACV AUC from zero to infinity (AUC(0-infinity)) increased with prolonged ACV t(1/2). ACV C(max) was similar across the three ALDH2 genotype groups. There was no apparent relationship between ALDH2 genotype and VACV or CMMG pharmacokinetics. This is the first study to show an association between ALDH2 genetic polymorphisms and ACV elimination rate (t(1/2)) in Japanese end-stage renal disease patients.

Enhanced cancer cell growth inhibition by dipeptide prodrugs of floxuridine: increased transporter affinity and metabolic stability

Dipeptide monoester prodrugs of floxuridine were synthesized, and their chemical stability in buffers, resistance to glycosidic bond metabolism, affinity for PEPT1, enzymatic activation and permeability in cancer cells were determined and compared to those of mono amino acid monoester floxuridine prodrugs. Prodrugs containing glycyl moieties were the least stable in pH 7.4 buffer ( t 1/2 < 100 min). The activation of all floxuridine prodrugs was 2- to 30-fold faster in cell homogenates than their hydrolysis in buffer, suggesting enzymatic action. The enzymatic activation of dipeptide monoester prodrugs containing aromatic promoieties in cell homogenates was 5- to 20-fold slower than that of other dipeptide and most mono amino acid monoester prodrugs ( t 1/2 approximately 40 to 100 min). All prodrugs exhibited enhanced resistance to glycosidic bond metabolism by thymidine phosphorylase compared to parent floxuridine. In general, the 5'-O-dipeptide monoester floxuridine prodrugs exhibited higher affinity for PEPT1 than the corresponding 5'-O-mono amino acid ester prodrugs. The permeability of dipeptide monoester prodrugs across Caco-2 and Capan-2 monolayers was 2- to 4-fold higher than the corresponding mono amino acid ester prodrug. Cell proliferation assays in AsPC-1 and Capan-2 pancreatic ductal cell lines indicated that the dipeptide monoester prodrugs were equally as potent as mono amino acid prodrugs. The transport and enzymatic profiles of 5'- l-phenylalanyl- l-tyrosyl-floxuridine, 5'- l-phenylalanyl- l-glycyl-floxuridine, and 5'- l-isoleucyl- l-glycyl-floxuridine suggest their potential for increased oral uptake, delayed enzymatic bioconversion and enhanced resistance to metabolism to 5-fluorouracil, as well as enhanced uptake and cytotoxic activity in cancer cells, attributes that would facilitate prolonged systemic circulation for enhanced therapeutic action.

Enhanced absorption and growth inhibition with amino acid monoester prodrugs of floxuridine by targeting hPEPT1 transporters

A series of amino acid monoester prodrugs of floxuridine was synthesized and evaluated for the improvement of oral bioavailability and the feasibility of target drug delivery via oligopeptide transporters. All floxuridine 5'-amino acid monoester prodrugs exhibited PEPT1 affinity, with inhibition coefficients of Gly-Sar uptake (IC50) ranging from 0.7 - 2.3 mM in Caco-2 and 2.0 - 4.8 mM in AsPC-1 cells, while that of floxuridine was 7.3 mM and 6.3 mM, respectively. Caco-2 membrane permeabilities of floxuridine prodrugs (1.01 - 5.31 x 10(-6 )cm/sec) and floxuridine (0.48 x 10(-6 )cm/sec) were much higher than that of 5-FU (0.038 x 10(-6) cm/sec). MDCK cells stably transfected with the human oligopeptide transporter PEPT1 (MDCK/hPEPT1) exhibited enhanced cell growth inhibition in the presence of the prodrugs. This prodrug strategy offers great potential, not only for increased drug absorption but also for improved tumor selectivity and drug efficacy.

Prodrugs: design and clinical applications

Prodrugs are bioreversible derivatives of drug molecules that undergo an enzymatic and/or chemical transformation in vivo to release the active parent drug, which can then exert the desired pharmacological effect. In both drug discovery and development, prodrugs have become an established tool for improving physicochemical, biopharmaceutical or pharmacokinetic properties of pharmacologically active agents. About 5-7% of drugs approved worldwide can be classified as prodrugs, and the implementation of a prodrug approach in the early stages of drug discovery is a growing trend. To illustrate the applicability of the prodrug strategy, this article describes the most common functional groups that are amenable to prodrug design, and highlights examples of prodrugs that are either launched or are undergoing human trials.

Molecular basis of prodrug activation by human valacyclovirase, an alpha-amino acid ester hydrolase

Chemical modification to improve biopharmaceutical properties, especially oral absorption and bioavailability, is a common strategy employed by pharmaceutical chemists. The approach often employs a simple structural modification and utilizes ubiquitous endogenous esterases as activation enzymes, although such enzymes are often unidentified. This report describes the crystal structure and specificity of a novel activating enzyme for valacyclovir and valganciclovir. Our structural insights show that human valacyclovirase has a unique binding mode and specificity for amino acid esters. Biochemical data demonstrate that the enzyme hydrolyzes esters of alpha-amino acids exclusively and displays a broad specificity spectrum for the aminoacyl moiety similar to tricorn-interacting aminopeptidase F1. Crystal structures of the enzyme, two mechanistic mutants, and a complex with a product analogue, when combined with biochemical analysis, reveal the key determinants for substrate recognition; that is, a flexible and mostly hydrophobic acyl pocket, a localized negative electrostatic potential, a large open leaving group-accommodating groove, and a pivotal acidic residue, Asp-123, after the nucleophile Ser-122. This is the first time that a residue immediately after the nucleophile has been found to have its side chain directed into the substrate binding pocket and play an essential role in substrate discrimination in serine hydrolases. These results as well as a phylogenetic analysis establish that the enzyme functions as a specific alpha-amino acid ester hydrolase. Valacyclovirase is a valuable target for amino acid ester prodrug-based oral drug delivery enhancement strategies.

Regulation of the oligopeptide transporter, PEPT-1, in DSS-induced rat colitis

The effect of colitis induced with dextran sodium sulfate (DSS) in rats on the bioavailability of drugs transported by the oligopeptide transporter PepT-1 was analyzed by studying the pharmacokinetics of PepT-1 substrates: cephalexin and valacyclovir, the prodrug of antiviral acyclovir. Western blot, immunohistochemistry, and real-time PCR were used to determine the PepT-1 protein and gene expression. We observed (1) no significant modification of PepT-1 expression in the duodenum and jejunum; (2) a slight decrease in both PepT-1 mRNA (50%) and protein expression (25%) in the ileum following DSS challenge; and (3) ectopic PepT-1 immunostaining in regenerative hyperplasia segments in the distal colon from DSS-treated rats where focal inflammation is localized. However, no modification of pharmacokinetic parameters (C (max), T (max), AUC) of cephalexin or acyclovir was detected. In conclusion, DSS-induced rat colitis did not alter PepT-1 substrate bioavailability despite certain modifications in PepT-1 expression profile.

Antiviral prodrugs - the development of successful prodrug strategies for antiviral chemotherapy

Following the discovery of the first effective antiviral compound (idoxuridine) in 1959, nucleoside analogues, especially acyclovir (ACV) for the treatment of herpesvirus infections, have dominated antiviral therapy for several decades. However, ACV and similar acyclic nucleosides suffer from low aqueous solubility and low bioavailability following oral administration. Derivatives of acyclic nucleosides, typically esters, were developed to overcome this problem and valaciclovir, the valine ester of ACV, was among the first of a new series of compounds that were readily metabolized upon oral administration to produce the antiviral nucleoside in vivo, thus increasing the bioavailility by several fold. Concurrently, famciclovir was developed as an oral formulation of penciclovir. These antiviral 'prodrugs' thus established a principle that has led to many successful drugs including both nucleoside and nucleotide analogues for the control of several virus infections, notably those caused by herpes-, retro- and hepatitisviruses. This review will chart the origins and development of the most important of the antiviral prodrugs to date.

Enzymes involved in the bioconversion of ester-based prodrugs

Enzymes are essential for the activation of many prodrugs. In this review, the most important enzymes (e.g., paraoxonase, carboxylesterase, acetylcholinesterase, cholinesterase) involved in the bioconversion of ester-based prodrugs will be discussed in terms of their biology and biochemistry. Most of these enzymes fall into the category of hydrolytic enzymes. However, nonhydrolytic enzymes, including cytochrome P450s, can also catalyze the bioconversion of ester prodrugs and thus will be discussed here. Other factors influencing the ability of these enzymes to catalyze the bioconversion of ester-based prodrugs, particularly species and interindividual differences and stereochemical and structural features of the prodrugs, will be discussed.

Interaction of intestinal nucleoside transporter hCNT2 with amino acid ester prodrugs of floxuridine and 2-bromo-5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole

Amino acid ester prodrugs of antiviral and anticancer nucleoside drugs were developed to improve oral bioavailability or to reduce systemic toxicity. We studied the interaction of human concentrative nucleoside transporter (hCNT2) cloned from intestine with various amino acid ester prodrugs of floxuridine (FUdR) and 5,6-dichloro-2-bromo-1-beta-D-ribofuranosylbenzimidazole (BDCRB). Na(+)-dependent uptakes of [(3)H]-inosine and [(3)H]-adenosine were measured in U251 cells transiently expressing intestinal hCNT2. FUdR significantly inhibited the uptake of both [(3)H]-inosine and [(3)H]-adenosine (60-70% of control), while its amino acid ester prodrugs including Val, Phe, Pro, Asp, and Lys esters exhibited markedly decreased inhibition potency (10-30% of control). On the other hand, BDCRB and its amino acid prodrugs markedly inhibited the uptake of both [(3)H]-inosine and [(3)H]-adenosine. Val, Phe, and Pro ester prodrugs of BDCRB showed similar inhibition capacities as parent compound BDCRB (80-90% for adenosine and 60-80% for inosine). The amino acid site of attachment (3'- and 5'-monoesters) and stereochemistry (L- and D-amino acid esters), did not significantly affect the uptake of [(3)H]-inosine and [(3)H]-adenosine. These results demonstrate that the hCNT2 favorably interacts with BDCRB and its amino acid prodrugs, compared to those of FUdR, and that neutral amino acid esters of BDCRB have a high affinity toward this transporter. Therefore, the intestinal hCNT2 may be a target transporter as a factor for modulating oral pharmacokinetics of BDCRB prodrugs.

Nucleoside Ester Prodrug Substrate Specificity of Liver Carboxylesterase

Carboxylesterases are among the best characterized prodrug-hydrolyzing enzymes involved in the activation of several therapeutic carbamate and ester prodrugs. The broad specificity of these enzymes makes them amenable for designing prodrugs. Porcine liver carboxylesterase 1 specificity for amino acid esters of three nucleoside analogs [floxuridine, gemcitabine, and 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl) benzimidazole] was evaluated to assess optimal structural preferences for prodrug design. The amino acid promoiety and the esterification site influenced carboxylesterase hydrolysis rates up to 1164-fold and the binding affinity up to 26-fold. Carboxylesterase (CES) 1 exhibited high-catalytic efficiency hydrolyzing prodrugs containing a phenylalanyl moiety but was over 100-fold less efficient with valyl or isoleucyl prodrugs, regardless of the nucleoside or esterification site. CES1 catalytic efficiency was 2-fold higher with 5' phenylalanyl monoesters than the corresponding 3' esters of floxuridine. This preference was reversed with phenylalanyl gemcitabine prodrugs, evident from a 2-fold preference for 3' monoesters over 5' esters. The newly characterized esterase valacyclovirase was several hundred-fold more efficient (up to 19,000-fold) than carboxylesterase in hydrolyzing amino acid esters but similar in apparent binding affinity. The specific activities of the two enzymes with several amino acid ester prodrugs clearly suggest that initial hydrolysis rates are relatively low for prodrugs with isoleucyl, aspartyl, and lysyl promoieties for both enzymes compared with those with phenylalanyl, valyl, prolyl, and leucyl progroups. The low relative hydrolysis rates of isoleucyl, aspartyl, and lysyl prodrugs may facilitate prolonged systemic disposition of the nucleoside analogs for improved therapeutic action.

Targeted delivery to PEPT1-overexpressing cells: acidic, basic, and secondary floxuridine amino acid ester prodrugs

Floxuridine is a clinically proven anticancer agent in the treatment of metastatic colon carcinomas and hepatic metastases. However, prodrug strategies may be necessary to improve its physiochemical properties and selectivity and to reduce undesirable toxicity effects. Previous studies with amino acid ester prodrugs of nucleoside drugs targeted to the PEPT1 transporter coupled with recent findings of the functional expression of the PEPT1 oligopeptide transporter in pancreatic adenocarcinoma cell lines suggest the potential of PEPT1 as therapeutic targets for cancer treatment. In this report, we show the feasibility of achieving enhanced transport and selective antiproliferative action of amino acid ester prodrugs of floxuridine in cell systems overexpressing PEPT1. All prodrugs exhibited affinity for PEPT1 (IC50, 1.1-2.3 mmol/L). However, only the prolyl and lysyl prodrugs exhibited enhanced uptake (2- to 8-fold) with HeLa/PEPT1 cells compared with HeLa cells, suggesting that the aspartyl prodrugs are PEPT1 inhibitors. The selective growth inhibition of Madine-Darby canine kidney (MDCK)/PEPT1 cells over MDCK cells by the prodrugs was consistent with the extent of their PEPT1-mediated transport. All ester prodrugs hydrolyzed to floxuridine fastest in Caco-2 cell and MDCK homogenates and slower in human plasma and were most chemically stable in pH 6.0 buffer. Prolyl and lysyl prodrugs were relatively less stable compared with aspartyl prodrugs in buffers and in cell homogenates. The results suggest that optimal design for targeted delivery would be possible by combining both stability and transport characteristics afforded by the promoiety.

Amino acid ester prodrugs of 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole enhance metabolic stability in vitro and in vivo

2-Bromo-5,6-dichloro-1-(beta-d-ribofuranosyl)benzimidazole (BDCRB) is a potent and selective inhibitor of human cytomegalovirus (HCMV), but it lacks clinical utility due to rapid in vivo metabolism. We hypothesized that amino acid ester prodrugs of BDCRB may enhance both in vitro potency and systemic exposure of BDCRB through evasion of BDCRB-metabolizing enzymes. To this end, eight different amino acid prodrugs of BDCRB were tested for N-glycosidic bond stability, ester bond stability, Caco-2 cell uptake, antiviral activity, and cytotoxicity. The prodrugs were resistant to metabolism by BDCRB-metabolizing enzymes, and ester bond cleavage was rate-limiting in metabolite formation from prodrug. Thus, BDCRB metabolism could be controlled by the selection of promoiety. In HCMV plaque-formation assays, l-Asp-BDCRB exhibited 3-fold greater selectivity than BDCRB for inhibition of HCMV replication. This potent and selective antiviral activity in addition to favorable stability profile made l-Asp-BDCRB an excellent candidate for in vivo assessment and pharmacokinetic comparison with BDCRB. In addition to rapid absorption and sufficient prodrug activation after oral administration to mice, l-Asp-BDCRB exhibited a 5-fold greater half-life than BDCRB. Furthermore, the sum of area under the concentration-time profile (AUC)(BDCRB) and AUC(prodrug) after l-Asp-BDCRB administration was roughly 3-fold greater than AUC(BDCRB) after BDCRB administration, suggesting that a reservoir of prodrug was delivered in addition to parent drug. Overall, these findings demonstrate that amino acid prodrugs of BDCRB exhibit evasion of metabolizing enzymes (i.e., bioevasion) in vitro and provide a modular approach for translating this in vitro stability into enhanced in vivo delivery of BDCRB.

Pharmacokinetics of Novel Dipeptide Ester Prodrugs of Acyclovir after Oral Administration: Intestinal Absorption and Liver Metabolism

The amino acid prodrug of acyclovir (ACV), valacyclovir (VACV), is an effective antiherpetic drug. Systemic availability of ACV in humans is 3 to 5 times higher after oral administration of VACV. Enhanced bioavailability of VACV has been attributed to its carrier-mediated intestinal absorption via hPEPT1 peptide transporter followed by rapid and complete conversion to ACV. An earlier report suggested that the dipeptide ester prodrugs of ACV possess high affinity toward the intestinal oligopeptide transporter hPEPT1 and therefore seem to be promising candidates in the treatment of oral herpes virus infections. In the present study, we have examined the bioavailability of a series of dipeptide prodrugs of ACV after oral administration in Sprague-Dawley rats with cannulated jugular and portal veins. The area under plasma-concentration time curves expressed as minutes microgram milliliter(-1) for total concentration of VACV (208.4 +/- 41.2), and the dipeptide prodrugs Gly-Val-ACV (GVACV) (416.1 +/- 140.9), Val-Val-ACV (VVACV) (147.7 +/- 89.3), and Val-Tyr-ACV (VYACV) (180.7 +/- 81.2) were significantly higher than that of ACV (21.2 +/- 5.2) upon intestinal absorption. Interestingly, the bioavailability of ACV after administration of GVACV was approximately 2-fold higher than VACV. There was significant metabolism by hepatic first pass effect of the dipeptide prodrugs as evident by the higher levels of ACV obtained after systemic absorption compared with intestinal absorption of GVACV and VVACV. The dipeptide prodrugs of ACV exhibited higher systemic availability of regenerated ACV upon oral administration and thus seem to be promising drug candidates in treatment of genital herpes infections.