Phe-Gly Dipeptidomimetics Designed for the Di-/Tripeptide Transporters PEPT1 and PEPT2: Synthesis and Biological Investigations

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Potential Development of Tumor-Targeted Oral Anti-Cancer Prodrugs: Amino Acid and Dipeptide Monoester Prodrugs of Gemcitabine

One of the main obstacles for cancer therapies is to deliver medicines effectively to target sites. Since stroma cells are developed around tumors, chemotherapeutic agents have to go through stroma cells in order to reach tumors. As a method to improve drug delivery to the tumor site, a prodrug approach for gemcitabine was adopted. Amino acid and dipeptide monoester prodrugs of gemcitabine were synthesized and their chemical stability in buffers, resistance to thymidine phosphorylase and cytidine deaminase, antiproliferative activity, and uptake/permeability in HFF cells as a surrogate to stroma cells were determined and compared to their parent drug, gemcitabine. The activation of all gemcitabine prodrugs was faster in pancreatic cell homogenates than their hydrolysis in buffer, suggesting enzymatic action. All prodrugs exhibited great stability in HFF cell homogenate, enhanced resistance to glycosidic bond metabolism by thymidine phosphorylase, and deamination by cytidine deaminase compared to their parent drug. All gemcitabine prodrugs exhibited higher uptake in HFF cells and better permeability across HFF monolayers than gemcitabine, suggesting a better delivery to tumor sites. Cell antiproliferative assays in Panc-1 and Capan-2 pancreatic ductal cell lines indicated that the gemcitabine prodrugs were more potent than their parent drug gemcitabine. The transport and enzymatic profiles of gemcitabine prodrugs suggest their potential for delayed enzymatic bioconversion and enhanced resistance to metabolic enzymes, as well as for enhanced drug delivery to tumor sites, and cytotoxic activity in cancer cells. These attributes would facilitate the prolonged systemic circulation and improved therapeutic efficacy of gemcitabine prodrugs.

The development of orally administrable gemcitabine prodrugs with D-enantiomer amino acids: enhanced membrane permeability and enzymatic stability

Gemcitabine prodrugs with D- and L-configuration amino acids were synthesized and their chemical stability in buffers, resistance to glycosidic bond metabolism, enzymatic activation, permeability in Caco-2 cells and mouse intestinal membrane, anti-proliferation activity in cancer cell were determined and compared to that of parent drug, gemcitabine. Prodrugs containing D-configuration amino acids were enzymatically more stable than ones with L-configuration amino acids. The activation of all gemcitabine prodrugs was 1.3-17.6-fold faster in cancer cell homogenate than their hydrolysis in buffer, suggesting enzymatic action. The enzymatic activation of amino acid monoester prodrugs containing D-configuration amino acids in cell homogenates was 2.2-10.9-fold slower than one of amino acid monoester prodrugs with L-configuration amino acids. All prodrugs exhibited enhanced resistance to glycosidic bond metabolism by thymidine phosphorylase compared to parent gemcitabine. Gemcitabine prodrugs showed superior the effective permeability in mouse jejunum to gemcitabine. More importantly, the high plasma concentration of d-amino acid gemcitabine prodrugs was observed more than one of L-amino acid gemcitabine prodrugs. In general, the 5'-mono-amino acid monoester gemcitabine prodrugs exhibited higher permeability and uptake than their parent drug, gemcitabine. Cell proliferation assays in AsPC-1 pancreatic ductal cell line indicated that gemcitabine prodrugs were more potent than their parent drug, gemcitabine. The transport and enzymatic profiles of 5'-D-valyl-gemcitabine and 5'-D-phenylalanyl-gemcitabine suggest their potential for increased oral uptake and delayed enzymatic bioconversion as well as enhanced uptake and cytotoxic activity in cancer cells, would facilitate the development of oral dosage form for anti-cancer agents and, hence, improve the quality of life for the cancer patients.

Allylic Amines as Key Building Blocks in the Synthesis of (E)-Alkene Peptide Isosteres

Nucleophilic imine additions with vinyl organometallics have developed into efficient, high yielding, and robust methodologies to generate structurally diverse allylic amines. We have used the hydrozirconation-transmetalation-imine addition protocol in the synthesis of allylic amine intermediates for peptide bond isosteres, phosphatase inhibitors, and mitochondria-targeted peptide mimetics. The gramicidin S-derived XJB-5-131 and JP4-039 and their analogs have been prepared on up to 160 g scale for preclinical studies. These (E)-alkene peptide isosteres adopt type II' β-turn secondary structures and display impressive biological properties, including selective reactions with reactive oxygen species (ROS) and prevention of apoptosis.

The tandem chain extension aldol reaction used for synthesis of ketomethylene tripeptidomimetics targeting hPEPT1

The rationale for targeting the human di-/tripeptide transporter hPEPT1 for oral drug delivery has been well established by several drug and prodrug cases. The aim of this study was to synthesize novel ketomethylene modified tripeptidomimetics and to investigate their binding affinity for hPEPT1. Three related tripeptidomimetics of the structure H-Phe-ψ[COCH(2)]-Ser(Bz)-X(aa)-OH were synthesized applying the tandem chain extension aldol reaction, where amino acid derived β-keto imides were stereoselectively converted to α-substituted γ-keto imides. In addition, three corresponding tripeptides, composed of amide bonds, were synthesized for comparison of binding affinities. The six investigated compounds were all defined as high affinity ligands (K(i)-values <0.5 mM) for hPEPT1 by measuring the concentration dependent inhibition of apical [(14)C]Gly-Sar uptake in Caco-2 cells. Consequently, the ketomethylene replacement for the natural amide bond and α-side chain modifications appears to offer a promising strategy to modify tripeptidic structures while maintaining a high affinity for hPEPT1.

Potential of amino acid/dipeptide monoester prodrugs of floxuridine in facilitating enhanced delivery of active drug to interior sites of tumors: a two-tier monolayer in vitro study

PURPOSE

To evaluate the advantages of amino acid/dipeptide monoester prodrugs for cancer treatments by assessing the uptake and cytotoxic effects of floxuridine prodrugs in a secondary cancer cell monolayer following permeation across a primary cancer cell monolayer.

METHODS

The first Capan-2 monolayer was grown on membrane transwell inserts; the second monolayer was grown at the bottom of a plate. The permeation of floxuridine and its prodrugs across the first monolayer and the uptake and cell proliferation assay on secondary layer were sequentially determined.

RESULTS

All floxuridine prodrugs exhibited greater permeation across the first Capan-2 monolayer than the parent drug. The correlation between uptake and growth inhibition in the second monolayer with intact prodrug permeating the first monolayer suggests that permeability and enzymatic stability are essential for sustained action of prodrugs in deeper layers of tumors. The correlation of uptake and growth inhibition were vastly superior for dipeptide prodrugs to those obtained with mono amino acid prodrugs.

CONCLUSIONS

Although a tentative general overall correlation between intact prodrug and uptake or cytotoxic action was obtained, it appears that a mixture of floxuridine prodrugs with varying beneficial characteristics may be more effective in treating tumors.

Factors affecting the stability of drugs and drug metabolites in biological matrices

Evaluation of the stability of drugs and drug metabolites in a biological matrix is a critical element to bioanalytical method validation. It is critical to understand the most common factors that affect the stability of such analytes in order to properly develop methods for their detection and measurement. The degradation of drugs and drug metabolites in samples can occur through either reversible or irreversible processes. Common factors that affect this stability include temperature, light, pH, oxidation and enzymatic degradation. Special considerations are also required when dealing with chiral molecules, deuterated internal standards and large biomolecules. Relevant examples of these degradation effects and approaches for dealing with them are presented is this review as taken from the fields of pharmaceutical testing, clinical research and forensic analysis. It is demonstrated through these examples how an understanding of the chemical and physical factors that affect sample stability can be used to avoid stability problems and to create robust and accurate methods for the analysis of drugs and related compounds.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

Peptide-Like Molecules (PLMs): A Journey from Peptide Bond Isosteres to Gramicidin S Mimetics and Mitochondrial Targeting Agents

Peptides are natural ligands and substrates for receptors and enzymes and exhibit broad physiological effects. However, their use as therapeutic agents often suffers from poor bioavailability and insufficient membrane permeability. The success of peptide mimicry hinges on the ability of bioisosteres, in particular peptide bond replacements, to adopt suitable secondary structures relevant to peptide strands and position functional groups in equivalent space. This perspective highlights past and ongoing studies in our group that involve new methods development as well as specific synthetic library preparations and applications in chemical biology, with the goal to enhance the use of alkene and cyclopropane peptide bond isosteres.

Optimal alpha-chymotrypsin-catalyzed synthesis of N-Ac-Phe-Gly-NH(2)

N-Acetyl-phenylalanine-glycinamide (N-Ac-Phe-Gly-NH(2)), a type of dipeptide derivative, was synthesized from N-acetyl phenylalanine ethyl ester and glycinamide and catalyzed by alpha-chymotrypsin, a protease, in a biphasic system. Response surface methodology with a four-factor, five-level central composite rotatable design was employed to evaluate the effects of selected parameters that included incubation time, reaction temperature, enzyme activity, and pH level on the yield of the dipeptide derivative. The results indicated that pH significantly affected the yield of N-Ac-Phe-Gly-NH(2). In a ridge max analysis, the optimum condition for this synthesis included an incubation time of 30.9 min, a reaction temperature of 35.8 degrees C, an enzyme activity of 159.2 U, and a pH of 8.98. The predicted and the actual (experimental) yields were 98.0 and 95.1%, respectively.

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.

Transport of Val-Leu-Pro-Val-Pro in human intestinal epithelial (Caco-2) cell monolayers

Angiotensin converting enzyme (ACE) inhibitory peptides are biologically active peptides that play a very important role in blood pressure regulation. In previous experiments, we obtained an ACE inhibitory peptide Val-Leu-Pro-Val-Pro (VLPVP) by DNA recombinant technology. The purpose of this study was to examine the bidirectional transport of VLPVP by using the human intestinal Caco-2 monolayers. The permeability coefficient ( P app) values of VLPVP over 4-8 mmol/L ranged from 7.44 x 10(-8) to 1.35 x 10(-6) cm/s for apical (AP) to basolateral (BL) transport, while the P app values for BL to AP flux were significantly lower than those for the AP to BL flux, with efflux ratio values of 0.74-0.13 over 4-8 mM. Preincubation of the paracellular transport enhancer (sodium deoxycholate), the inhibitor of multidrug resistant protein (MK-571), or sodium azide stimulated efflux of VLPVP significantly ( p < 0.01); these results indicate that the transport of VLPVP across Caco-2 monolayers was involved in paracellular diffusion and MRP2 transport.

hPEPT1 is responsible for uptake and transport of Gly-Sar in the human bronchial airway epithelial cell-line Calu-3

The purpose of this work was to investigate the apical uptake and transepithelial transport of Gly-Sar along with the expression of the di-/tripeptide transporters hPEPT1 and hPEPT2 in human Calu-3 bronchial epithelial cells. The apical Gly-Sar uptake rate in Calu-3 cells followed Michaelis-Menten kinetics with a Km value of 1.3 +/- 0.3 mM and a Vmax value of 0.60 +/- 0.06 nmol cm(-2) min(-1). Transepithelial apical to basolateral transport of 50 microM [3H]-labelled Gly-Sar across the Calu-3 cell monolayer was pH-dependent. The Gly-Sar flux was significantly reduced in the presence of delta-aminolevulinic acid (2.5 mM), cephalexin (25 mM), and captopril (25 mM; p < 0.05, n = 3). Reverse transcriptase polymerase chain reaction (RT-PCR) revealed the presence of both hPEPT1 and hPEPT2 mRNA in the Calu-3 cells. These findings were confirmed in healthy human bronchial cDNA. Restriction-endonuclease analysis identified hPEPT2 in Calu-3 cells to be the hPEPT2*1 haplotype. Western blotting demonstrated expression of the hPEPT1 protein (approximately 80 kDa), and the immunolabel was mainly localized in the apical membrane as judged by immunolocalization studies using confocal laser scanning microscopy (CLSM). This work presents for the first time hPEPT1 and hPEPT2*1 expression in human Calu-3 cells. Surprisingly, the results indicate that Gly-Sar uptake and transport in Calu-3 cells are hPEPT1-mediated rather than hPEPT2-mediated.

Hydroxyethylene isosteres of selective neuronal nitric oxide synthase inhibitors

Nitric oxide (NO) is an important second messenger molecule for blood pressure homeostasis, as a neurotransmitter, and in the immune defense system. Excessive NO can lead to neurodegeneration and connective tissue damage. Three different isozymes of the enzyme nitric oxide synthase regulate NO production in endothelial (eNOS), neuronal (nNOS), and macrophage (iNOS) cells. Whereas creating a lower level of NO in some cells could be beneficial, it also could be detrimental to the protective effects that NO has on other cells. Therefore, it is essential that therapeutic NOS inhibitors be made that are subtype selective. Previously, we reported a series of nitroarginine-containing dipeptide amides as potent and selective nNOS inhibitors. Here we synthesize peptidomimetic hydroxyethylene isosteres of these dipeptide amides for potential increased bioavailability. None of the compounds is as potent or selective as the dipeptide amides, but they exhibit good inhibition and selectivity. When the terminal amino group was converted to a hydroxyl group, potency and selectivity greatly diminished, supporting the importance of the terminal amino group for binding.

Unequivocal synthesis of (Z)-alkene and (E)-fluoroalkene dipeptide isosteres to probe structural requirements of the peptide transporter PEPT1

[reaction: see text] Described is a novel synthetic route for dipeptide isosteres containing (Z)-alkene and (E)-fluoroalkene units as cis-amide bond equivalents via organocopper-mediated reduction of gamma-acetoxy- or gamma,gamma-difluoro-alpha,beta-unsaturated-delta-lactams. The synthesized isosteres were evaluated in terms of their affinities for the peptide transporter PEPT1. trans-Amide isosteres tended to possess higher affinities for PEPT1 as compared to the corresponding cis-amide bond equivalents.

The renal type H+/peptide symporter PEPT2: structure-affinity relationships

The H(+)/peptide cotransporter PEPT2 is expressed in a variety of organs including kidney, lung, brain, mammary gland, and eye. PEPT2 substrates are di- and tripeptides as well as peptidomimetics, such as beta-lactam antibiotics. Due to the presence of PEPT2 at the bronchial epithelium, the aerosolic administration of peptide-like drugs might play a major role in future treatment of various pulmonary and systemic diseases. Moreover, PEPT2 has a significant influence on the in vivo disposition and half-life time of peptide-like drugs within the body, particularly in kidney and brain. PEPT2 is known to have similar but not identical structural requirements for substrate recognition and transport compared to PEPT1, its intestinal counterpart. In this review we compiled available affinity constants of 352 compounds, measured at different mammalian tissues and expression systems and compare the data whenever possible with those of PEPT1.

In vitro evaluation of N-methyl amide tripeptidomimetics as substrates for the human intestinal di-/tri-peptide transporter hPEPT1

Oral absorption of tripeptides is generally mediated by the human intestinal di-/tri-peptide transporter, hPEPT1. However, the bioavailability of tripeptides is often limited due to degradation in the GI-tract by various peptidases. The aim of the present study was to evaluate the general application of N-methyl amide bioisosteres as peptide bond replacements in tripeptides in order to decrease degradation by peptidases and yet retain affinity for and transport via hPEPT1. Seven structurally diverse N-methyl amide tripeptidomimetics were selected based on a principal component analysis of structural properties of 6859 N-methyl amide tripeptidomimetics. In vitro extracellular degradation of the selected tripeptidomimetics as well as affinity for and transepithelial transport via hPEPT1 were investigated in Caco-2 cells. Decreased apparent degradation was observed for all tripeptidomimetics compared to the corresponding natural tripeptides. However, affinity for and transepithelial transport via hPEPT1 were only seen for Gly-Sar-Sar, AsnPsi[CONCH(3)]PhePsi[CONCH(3)]Trp, and Gly-Sar-Leu. This implies that tripeptidomimetics originating from tripeptides with neutral side chains are more likely to be substrates for hPEPT1 than tripeptidomimetics with charged side chains. The results of the present study indicate that the N-methyl amide peptide bond replacement approach for increasing bioavailability of tripeptidomimetic drug candidates is not generally applicable to all tripeptides. Nevertheless, retained affinity for and transport via hPEPT1 were shown for three of the evaluated N-methyl amide tripeptidomimetics.

Conformational restrictions in ligand binding to the human intestinal di-/tripeptide transporter: implications for design of hPEPT1 targeted prodrugs

The aim of the present study was to develop a computational method aiding the design of dipeptidomimetic pro-moieties targeting the human intestinal di-/tripeptide transporter hPEPT1. First, the conformation in which substrates bind to hPEPT1 (the bioactive conformation) was identified by conformational analysis and 2D dihedral driving analysis of 15 hPEPT1 substrates, which suggested that psi(1) approximately 165 degrees , omega(1) approximately 180 degrees , and phi(2) approximately 280 degrees were descriptive of the bioactive conformation. Subsequently, the conformational energy required to change the peptide backbone conformation (DeltaE(bbone)) from the global energy minimum conformation to the identified bioactive conformation was calculated for 20 hPEPT1 targeted model prodrugs with known K(i) values. Quantitatively, an inverse linear relationship (r(2)=0.81, q(2)=0.80) was obtained between DeltaE(bbone) and log1/K(i), showing that DeltaE(bbone) contributes significantly to the experimentally observed affinity for hPEPT1 ligands. Qualitatively, the results revealed that compounds classified as high affinity ligands (K(i)<0.5 mM) all have a calculated DeltaE(bbone)<1 kcal/mol, whereas medium and low-affinity compounds (0.5 mM<K(i)<15 mM) have DeltaE(bbone) values in the range 1-3 kcal/mol. The findings also shed new light on the basis for the experimentally observed stereoselectivity of hPEPT1.

Delivery aspects of small peptides and substrates for peptide transporters

The present summary highlight chemical strategies applied to improve plasma half-lives and oral bioavailability of peptidic drugs as well as view on intestinal and pancreatic peptidase mediated degradation of peptidic drugs. In general chemical strategies used to increase the oral bioavailability of peptidic drugs consisting of more than three amino acids is disappointing. On the other hand chemical approaches to stabilize peptidic drugs against metabolism seem promising for increasing plasma half-lives of parental peptidic drugs as well as for increasing oral bioavailability of di/tripeptidomimetics and dipeptidyl pro-moieties targeting peptide transporters.

Pyrimidine and nucleoside gamma-esters of L-Glu-Sar: synthesis, stability and interaction with hPEPT1

The aim of the present study was to improve the synthetic pathway of bioreversible dipeptide derivatives as well as evaluate the potential of using l-Glu-Sar as a pro-moiety for delivering three newly synthesised nucleoside and pyrimidine l-Glu-Sar derivatives. l-Glu(trans-2-thymine-1-yl-tetrahydrofuran-3-yl ester)-Sar (I), l-Glu(thymine-1-yl-methyl ester)-Sar (II) and l-Glu(acyclothymidine)-Sar (III) were synthesised and in vitro stability was studied in various aqueous and biological media. Affinity to and translocation via hPEPT1 was investigated in mature Caco-2 cell monolayers, grown on permeable supports. Affinity was estimated in a competition assay, using [14C] labelled Gly-Sar (glycylsarcosine). Translocation was measured as pHi-changes induced by the substrates using the fluorescent probe BCECF and an epifluorescence microscope setup. All dipeptide derivatives released the model drugs quantitatively by specific base-catalysed hydrolysis at pH>6.0. II was labile in aqueous buffer solution, whereas I and III showed appropriate stability for oral administration. In 10% porcine intestinal homogenate, the half-lives of the dipeptide derivatives indicated limited enzyme catalyzed degradation. All compounds showed good affinity to hPEPT1, but the Compounds I and III showed not to be translocated by hPEPT1. The translocation of the l-Glu-Sar derivative of acyclovir, l-Glu(acyclovir)-Sar was also investigated and showed not to take place. Consequently, l-Glu-Sar seems to be a poor pro-moiety for hPEPT1-mediated transport.

Transporter-mediated drug delivery: recent progress and experimental approaches

A comprehensive list of drug transporters has recently become available as a result of extensive genome analysis, as well as membrane physiology and molecular biology studies. This review covers recent progress in identification and characterization of drug transporters, illustrative cases of successful drug delivery to, or exclusion from, target organs via transporters, and novel experimental approaches to therapeutics using heterologously transduced transporters in tissues. We aim to provide clues that could lead to efficient strategies for the use of transporters to deliver drugs and/or to optimize lead compounds.