Uptake of the cephalosporin, cephalexin, by a dipeptide transport carrier in the human intestinal cell line, Caco-2

Ceftazidime and cefepime antagonize 5-fluorouracil's effect in colon cancer cells

Background

Drug-drug interaction (DDI), which can occur at the pharmacokinetics and/or the pharmacodynamics (PD) levels, can increase or decrease the therapeutic or adverse response of a drug itself or a combination of drugs. Cancer patients often receive, along their antineoplastic agents, antibiotics such as ß-lactams to treat or prevent infection. Despite the narrow therapeutic indices of antibiotics and antineoplastic agents, data about their potential interaction are insufficient. 5-fluorouracil (5-FU), widely used against colon cancer, is known for its toxicity and large intra- and inter- individual variability. Therefore, knowledge about its interaction with antibiotics is crucial.

Methods

In this study, we evaluated at the PD levels, against HCT-116 colon cancer cells, DDI between 5-FU and several ß-lactams (ampicillin, benzypenicillin, piperacillin, meropenem, flucloxacillin, ceftazidime (CFT), and cefepime (CFP)), widely used in intensive care units. All drugs were tested at clinically achieved concentrations. MTT assay was used to measure the metabolic activity of the cells. Cell cycle profile and apoptosis induction were monitored, in HCT-116 and DLD-1 cells, using propidium iodide staining and Caspase-3/7 activity assay. The uptake of CFT and CFP by the cells was measured using LC-MS/MS method.

Results

Our data indicate that despite their limited uptake by the cells, CFT and CFP (two cephalosporins) antagonized significantly 5-FU-induced S-phase arrest (DLD-1 cells) and apoptosis induction (HCT-116 cells). Remarkably, while CFP did not affect the proliferation of colon cancer cells, CFT inhibited, at clinically relevant concentrations, the proliferation of DLD-1 cells via apoptosis induction, as evidenced by an increase in caspase 3/7 activation. Unexpectedly, 5-FU also antagonized CFT’s induced cell death in DLD-1 cells.

Conclusion

This study shows that CFP and CFT have adverse effects on 5-FU’s action while CFT is a potent anticancer agent that inhibits DLD-1 cells by inducing apoptotic cell death. Further studies are needed to decipher the mechanism(s) responsible for CFT’s effects against colon cancer as well as the observed antagonism between CFT, CFP, and 5-FU with the ultimate aim of translating the findings to the clinical settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-09125-4.

Mastering the Gram-negative bacterial barrier - Chemical approaches to increase bacterial bioavailability of antibiotics

To win the battle against resistant, pathogenic bacteria, novel classes of anti-infectives and targets are urgently needed. Bacterial uptake, distribution, metabolic and efflux pathways of antibiotics in Gram-negative bacteria determine what we here refer to as bacterial bioavailability. Understanding these mechanisms from a chemical perspective is essential for anti-infective activity and hence, drug discovery as well as drug delivery. A systematic and critical discussion of in bacterio, in vitro and in silico assays reveals that a sufficiently accurate holistic approach is still missing. We expect new findings based on Gram-negative bacterial bioavailability to guide future anti-infective research.

Self-Nano-Emulsifying Drug-Delivery Systems: From the Development to the Current Applications and Challenges in Oral Drug Delivery

Approximately one third of newly discovered drug molecules show insufficient water solubility and therefore low oral bio-availability. Self-nano-emulsifying drug-delivery systems (SNEDDSs) are one of the emerging strategies developed to tackle the issues associated with their oral delivery. SNEDDSs are composed of an oil phase, surfactant, and cosurfactant or cosolvent. SNEDDSs characteristics, their ability to dissolve a drug, and in vivo considerations are determinant factors in the choice of SNEDDSs excipients. A SNEDDS formulation can be optimized through phase diagram approach or statistical design of experiments. The characterization of SNEDDSs includes multiple orthogonal methods required to fully control SNEDDS manufacture, stability, and biological fate. Encapsulating a drug in SNEDDSs can lead to increased solubilization, stability in the gastro-intestinal tract, and absorption, resulting in enhanced bio-availability. The transformation of liquid SNEDDSs into solid dosage forms has been shown to increase the stability and patient compliance. Supersaturated, mucus-permeating, and targeted SNEDDSs can be developed to increase efficacy and patient compliance. Self-emulsification approach has been successful in oral drug delivery. The present review gives an insight of SNEDDSs for the oral administration of both lipophilic and hydrophilic compounds from the experimental bench to marketed products.

Membrane Chemistry Tunes the Structure of a Peptide Transporter

Membrane proteins require lipid bilayers for function. While lipid compositions reach enormous complexities, high-resolution structures are usually obtained in artificial detergents. To understand whether and how lipids guide membrane protein function, we use single-molecule FRET to probe the dynamics of DtpA, a member of the proton-coupled oligopeptide transporter (POT) family, in various lipid environments. We show that detergents trap DtpA in a dynamic ensemble with cytoplasmic opening. Only reconstitutions in more native environments restore cooperativity, allowing an opening to the extracellular side and a sampling of all relevant states. Bilayer compositions tune the abundance of these states. A novel state with an extreme cytoplasmic opening is accessible in bilayers with anionic head groups. Hence, chemical diversity of membranes translates into structural diversity, with the current POT structures only sampling a portion of the full structural space.

Biowaiver Monographs for Immediate Release Solid Oral Dosage Forms: Cephalexin Monohydrate

Literature data and results of experimental studies relevant to the decision to allow waiver of bioequivalence studies in humans for the approval of immediate release solid oral dosage forms containing cephalexin monohydrate are presented. Solubility studies were performed in accordance with the current biowaiver guidelines of the Food and Drug Administration, World Health Organization and European Medicines Agency, taking the degradation at some pH values into consideration. Together with solubility and permeability data for cephalexin monohydrate from the literature, it was demonstrated to be a Biopharmaceutics Classification System Class 1 drug. The pharmacokinetic behavior, results of bioequivalence studies published in the literature, as well as the therapeutic uses, potential toxicity and potential excipient effects on bioavailability were also assessed. Cephalexin has a wide therapeutic index and no bioequivalence problems have been reported. Dissolution studies were run under Biopharmaceutics Classification System-biowaiver conditions for the pure drug and 2 generic formulations available on the German market. Considering all relevant aspects, it was concluded that a biowaiver-based approval for products containing cephalexin monohydrate as the single active pharmaceutical ingredient is scientifically justified, provided that well-established excipients are used in usual amounts and that both test and reference dosage forms meet the guideline criteria of either "rapidly dissolving" or "very rapidly dissolving."

The Impact of Breast Cancer Resistance Protein (BCRP/ABCG2) on Drug Transport Across Caco-2 Cell Monolayers

Breast cancer resistance protein (BCRP) is expressed on the apical membrane of small intestinal epithelial cells and functions as an efflux pump with broad substrate recognition. Therefore, quantitative evaluation of the contribution of BCRP to the intestinal permeability of new chemical entities is very important in drug research and development. In this study, we assessed the BCRP-mediated efflux of several model drugs in Caco-2 cells using WK-X-34 as a dual inhibitor of P-glycoprotein (P-gp) and BCRP and LY335979 as a selective inhibitor of P-gp. The permeability of daidzein was high with an apparent permeability coefficient for apical-to-basal transport (P _AB) of 20.3 × 10^-6 cm/s. In addition, its efflux ratio (ER) was 1.55, indicating that the contribution of BCRP to its transport is minimal. Estrone-3-sulfate and ciprofloxacin showed relatively higher ER values (>2.0), whereas their BCRP-related absorptive quotient (AQ _BCRP) was 0.21 and 0.3, respectively. These results indicate that BCRP does not play a major role in regulating the permeability of estrone-3-sulfate and ciprofloxacin in Caco-2 cells. Nitrofurantoin showed a P _AB of 1.8 × 10^-6 cm/s, and its ER was 7.6. However, the AQ _BCRP was 0.37, suggesting minimal contribution of BCRP to nitrofurantoin transport in Caco-2 cells. In contrast, topotecan, SN-38, and sulfasalazine had low P _AB values (0.81, 1.13, and 0.19 × 10^-6 cm/s, respectively), and each AQ _BCRP was above 0.6, indicating that BCRP significantly contributes to the transport of these compounds in Caco-2 cells. In conclusion, Caco-2 cells are useful to accurately estimate the contribution of BCRP to intestinal drug absorption. SIGNIFICANCE STATEMENT: We performed an in vitro assessment of the contribution of breast cancer resistance protein (BCRP) to the transport of BCRP and/or P-glycoprotein (P-gp) substrates across Caco-2 cell monolayers using absorptive quotient, which has been proposed to represent the contribution of drug efflux transporters to the net efflux. The present study demonstrates that the combined use of a BCRP/P-gp dual inhibitor and a P-gp selective inhibitor is useful to estimate the impact of BCRP and P-gp on the permeability of tested compounds in Caco-2 cells.

Conductance Changes in Bovine Serum Albumin Caused by Drug-Binding Triggered Structural Transitions

Proteins, due to their binding selectivity, are promising candidates for fabricating nanoscale bio-sensors. However, the influence of structural change on protein conductance caused by specific protein-ligand interactions and disease-induced degeneration still remains unknown. Here, we excavated the relationship between circular dichroism (CD) spectroscopy and conductive atomic force microscopy (CAFM) to reveal the effect of the protein secondary structures changes on conductance. The secondary structure of bovine serum albumin (BSA) was altered by the binding of drugs, like amoxicillin (Amox), cephalexin (Cefa), and azithromycin (Azit). The CD spectroscopy shows that the α-helical and β-sheet content of BSA, which varied according to the molar ratio between the drug and BSA, changed by up to 6%. The conductance of BSA monolayers in varying drug concentrations was further characterized via CAFM. We found that BSA conductance has a monotonic relation with α-helical content. Moreover, BSA conductance seems to be in connection with the binding ability of drugs and proteins. This work elucidates that protein conductance variations caused by secondary structure transitions are triggered by drug-binding and indicate that electrical methods are of potential application in protein secondary structure analysis.

Pathways and progress in improving drug delivery through the intestinal mucosa and blood-brain barriers

One of the major hurdles in developing therapeutic agents is the difficulty in delivering drugs through the intestinal mucosa and blood-brain barriers (BBB). The goal here is to describe the general structures of the biological barriers and the strategies to enhance drug delivery across these barriers. Prodrug methods used to improve drug penetration via the transcellular pathway have been successfully developed, and some prodrugs have been used to treat patients. The use of transporters to improve absorption of some drugs (e.g., antiviral agents) has also been successful in treating patients. Other methods, including blocking the efflux pumps to improve transcellular delivery, and modulation of cell-cell adhesion in the intercellular junctions to improve paracellular delivery across biological barriers, are still in the investigational stage.

Transepithelial Transport ofp-Coumaric Acid and Gallic Acid in Caco-2 Cell Monolayers

The transepithelial transport of such common dietary phenolic acids as p-coumaric acid (CA) and gallic acid (GA) across Caco-2 cell monolayers was examined. CA transport was dependent on pH, and in a vectorial manner in the apical-basolateral direction. The permeation was concentration-dependent and saturable, the Michaelis constant and maximum velocity being 17.5 mM and 82.7 nmol min(-1) (mg of protein)(-1), respectively. Benzoic acid and acetic acid inhibited the permeation of CA. These results indicate that the transepithelial transport of CA was via the monocarboxylic acid transporter (MCT). On the other hand, the permeation of GA was not in a polarized manner, was independent of pH and linearly increased with increasing concentration of GA. The transport rate of GA was about 100 times lower than that of CA, suggesting the transepithelial transport of GA to be via the paracellular pathway. Dietary phenolic acids thus showed diversified characteristics in their intestinal absorption.

Structural Effects of Phenolic Acids on the Transepithelial Transport of Fluorescein in Caco-2 Cell Monolayers

The structural specificity of the monocarboxylic acid transporter (MCT) for the transport of phenolic acids was investigated by measuring the inhibitory effect on the fluorescein transport in Caco-2 cell monolayers. Although most of the monohydroxylated derivatives had an inhibitory effect, the di- and tri-hydroxylated ones did not. The methoxylated derivatives were more inhibitory than the hydroxylated ones in all the meta-substituted derivatives, suggesting that meta-hydroxylation of the substrate would decrease the affinity for MCT.

pH-Responsive poly(itaconic acid-co-N-vinylpyrrolidone) hydrogels with reduced ionic strength loading solutions offer improved oral delivery potential for high isoelectric point-exhibiting therapeutic proteins

pH-Responsive hydrogels comprised of itaconic acid copolymerized with N-vinylpyrrolidone (P(IA-co-NVP)) were synthesized and tested as carriers for the oral delivery of high isoelectric point (pI) exhibiting therapeutic proteins. Swelling studies show that P(IA-co-NVP) hydrogels exhibit significantly greater and faster pH-responsive swelling than previously studied methacrylic acid-based hydrogels, achieving up to 68% greater equilibrium swelling and 10.4 times greater swelling in time-limited experiments. Using salmon calcitonin as a model high pI protein therapeutic, we show that P(IA-co-NVP) hydrogels exhibit significantly greater delivery potential than methacrylic acid-based hydrogels. Additionally, we show that utilizing a lower ionic strength solution during drug loading significantly improves drug delivery potential for high pI therapeutics. By using a 1.5mM PBS buffer rather than the standard 150 mM PBS buffer during loading, up to 83 times as much calcitonin can be delivered in neutral conditions, with up to a 9.6-fold improvement in percent release. Using P(IA-co-NVP) hydrogel microparticles and a low ionic strength loading solution, up to 48 μg calcitonin/mg hydrogel can be delivered in small intestinal conditions. Based on expected absorption in the small intestine, this is sufficient delivery potential for achieving therapeutic dosage via a single, regularly-sized pill taken daily.

Distinguishing between the permeability relationships with absorption and metabolism to improve BCS and BDDCS predictions in early drug discovery

The biopharmaceutics classification system (BCS) and biopharmaceutics drug distribution classification system (BDDCS) are complementary classification systems that can improve, simplify, and accelerate drug discovery, development, and regulatory processes. Drug permeability has been widely accepted as a screening tool for determining intestinal absorption via the BCS during the drug development and regulatory approval processes. Currently, predicting clinically significant drug interactions during drug development is a known challenge for industry and regulatory agencies. The BDDCS, a modification of BCS that utilizes drug metabolism instead of intestinal permeability, predicts drug disposition and potential drug–drug interactions in the intestine, the liver, and most recently the brain. Although correlations between BCS and BDDCS have been observed with drug permeability rates, discrepancies have been noted in drug classifications between the two systems utilizing different permeability models, which are accepted as surrogate models for demonstrating human intestinal permeability by the FDA. Here, we recommend the most applicable permeability models for improving the prediction of BCS and BDDCS classifications. We demonstrate that the passive transcellular permeability rate, characterized by means of permeability models that are deficient in transporter expression and paracellular junctions (e.g., PAMPA and Caco-2), will most accurately predict BDDCS metabolism. These systems will inaccurately predict BCS classifications for drugs that particularly are substrates of highly expressed intestinal transporters. Moreover, in this latter case, a system more representative of complete human intestinal permeability is needed to accurately predict BCS absorption.

AFN-1252 in vitro absorption studies and pharmacokinetics following microdosing in healthy subjects

OBJECTIVES

To investigate the absorption, distribution, metabolism and excretion of AFN-1252, a novel inhibitor of the essential FabI enzyme in Staphylococcus spp., in vitro and following microdosing in healthy adult male subjects following intravenous and oral administration.

METHODS

Three ADME studies, comprising a Caco-2 assay, a rat intestinal perfusion model and a microdosing study in healthy human volunteers, were conducted.

RESULTS

The Caco-2 assay indicated that AFN-1252 in solution is well-absorbed and undergoes insignificant efflux, and its transport across the intestinal wall is probably passive. In the rat intestinal perfusion model, AFN-1252 exhibited high permeability potential across three segments, in the rank order of jejunum=ileum>colon. Taken together with the low aqueous solubility, the data from these studies indicate that AFN-1252 is a BCS Class II molecule with solubility-limited absorption. Analysis of the [(14)C]-AFN-1252 radioactivity concentration-time data indicated similar pharmacokinetics following intravenous and oral administration in the microdosing study in healthy volunteers. These included long terminal half-lives of ∼7 h and 83% bioavailability, indicating that there was little first-pass metabolism following oral dosing. AFN-1252 exhibited good distribution to skin and skin structures where its anti-staphylococcal activity may be required. Urinary and faecal excretion are major elimination routes for [(14)C]-AFN-1252 following intravenous or oral administration.

CONCLUSIONS

AFN-1252 has the potential for both intravenous and oral administration, once- or twice-daily dosing and good tissue distribution in humans. Further safety, efficacy and pharmacokinetic studies in man are required to investigate therapeutically-relevant doses for this novel agent and its targeted selectivity and high potency against Staphylococcus spp.

Synthesis and evaluation of a dipeptide-drug conjugate library as substrates for PEPT1

The oligopeptide transporter PEPT1 is considered as a valuable target for prodrug design, but its 3D structure and substrate specificity of PEPT1 are not fully understood. In this study, we designed a focused dipeptide conjugated azidothymidine (AZT) library and described a convenient and efficient solid phase synthesis scheme based on click chemistry. Over 60 candidate structures containing various dipeptide sequences were obtained with high purity, and screened in a PEPT1 overexpressing cell model for their abilities to compete with the known ligand cephalexin. Some of the compounds selected to have medium or high affinity were tested for their in vivo transport in a single-pass intestinal perfusion experiment. Results showed that the designed library contained some new structure features that have high affinities toward PEPT1 and could be further explored for their application in prodrug design and development.

Genomic evaluation during permeability of indomethacin and its solid dispersion

Drug resistance was first identified in cancer cells that express proteins known as multidrug resistance proteins that extrude the therapeutic agents out of the cells resulting in alteration of pharmacokinetics, tissue distribution, and pharmacodynamics of drugs. To this end studies were carried out to investigate the role of pharmacological inhibitors and pharmaceutical excipients with a primary focus on P-glycoprotein (P-gp). The aim of this study was to investigate holistic changes in transporter gene expression during permeability upon formulation of indomethacin as solid dispersion. Initial characterization studies of solid dispersion of indomethacin showed that the drug was dispersed within the carrier in amorphous form. Analysis of permeability data across Caco-2 monolayers revealed that drug absorption increased by 4-fold when reformulated as solid dispersion. The last phase of the work involved investigation of gene expression changes of transporter genes during permeability. The results showed that there were significant differences in the expression of both ATP-binding cassette (ABC) transporter genes as well as solute carrier transporter (SLC) genes suggesting that the inclusion of polyethylene glycol as well as changes in molecular form of drug from crystalline to amorphous have a significant bearing on the expression of transporter network genes resulting in differences in drug permeability.

Gene ablation for PEPT1 in mice abolishes the effects of dipeptides on small intestinal fluid absorption, short-circuit current, and intracellular pH

PEPT1 function in mouse intestine has not been assessed by means of electrophysiology and methods to assess its role in intracellular pH and fluid homeostasis. Therefore, the effects of the dipeptide glycilsarcosin (Gly-Sar) on jejunal fluid absorption and villous enterocyte intracellular pH (pH(i)) in vivo, as well as on enterocyte[(14)C]Gly-Sar uptake, short-circuit current (I(sc)) response, and enterocyte pH(i) in vitro were determined in wild-type and PEPT1-deficient mice and in mice lacking PEPT1. Immunohistochemistry for PEPT1 failed to detect any protein in enterocyte apical membranes in Slc15a1(-/-) animals. Saturable Gly-Sar uptake in Slc15a1(-/-) everted sac preparations was no longer detectable. Similarly, Gly-Sar-induced jejunal I(sc) response in vitro was abolished. The dipeptide-induced increase in fluid absorption in vivo was also abolished in animals lacking PEPT1. Since PEPT1 acts as an acid loader in enterocytes, enterocyte pH(i) was measured in vivo by two-photon microscopy in SNARF-4-loaded villous enterocytes of exteriorized jejuni in anesthetized mice, as well as in BCECF-loaded enterocytes of microdissected jejunal villi. Gly-Sar-induced pH(i) decrease was no longer observed in the absence of PEPT1. A reversal of the proton gradient across the luminal membrane did not significantly diminish Gly-Sar-induced I(sc) response, whereas a depolarization of the apical membrane potential by high K(+) or via Na(+)-K(+)-ATPase inhibition strongly diminished Gly-Sar-induced I(sc) responses. This study demonstrates for the first time that proton-coupled electrogenic dipeptide uptake in the native small intestine, mediated by PEPT1, relies on the negative apical membrane potential as the major driving force and contributes significantly to intestinal fluid transport.

S-adenosyl-l-methionine: transcellular transport and uptake by Caco-2 cells and hepatocytes

S-adenosyl-l-methionine (SAMe) is an endogenous molecule that is known to be protective against hepatotoxic injury. Although oral SAMe appears to be absorbed across the intestinal mucosa, its systemic bioavailability is low. The reason for this is unknown. Using the Caco-2 cell culture model for enterocyte absorption, we determined the mode by which SAMe is transported across this cell monolayer. We also determined the extent it is taken up by both Caco-2 cells and hepatocytes. In Caco-2 cells transport was observed in both apical to basolateral and basolateral to apical directions. The apparent permeability coefficients (Papp) appeared to be concentration independent and were similar in both directions (0.7 times 10−6 and 0.6 times 10−6 cms−1, respectively), i.e. identical to that of the paracellular transport marker mannitol (0.9 times 10−6 and 0.7 times 10−6 cms−1). This mode of transport was supported by a four-fold increase in the Papp for SAMe transport in Ca++-free buffer. Cellular uptake of SAMe was examined in both Caco-2 cells and cultured rat hepatocytes. Uptake by hepatocytes was not saturable in a concentration range of 0.001–100 μm. Accumulation by both cell types was very low, with a cell:medium ratio at equilibrium of only 0.2–0.5. This low cell accumulation supports the finding of paracellular transport as the only mode of cell membrane transport. Increased hepatocellular protection for SAMe may be accomplished by converting SAMe to a more lipid-soluble prodrug.

Nonlinear absorption of methylprednisolone by absorptive and secretory transporters

The intestinal absorption rate constant of methylprednisolone (MP) evaluated by the loop method increased significantly with increasingly higher concentrations of the drug up to 500 microM in a nonlinear fashion but did not increase further at higher concentrations. Mucosal-to-serosal directed permeation of MP across rat jejunal sheets also increased in a nonlinear fashion in a low concentration range (100-150 microM), followed by a decrease as the concentration increased further, whereas serosal-to-mucosal directed permeation decreased in a concentration-dependent manner. Vectorial transport of MP across Caco-2 cell monolayers was observed, with greater transport in the basolateral-to-apical direction at 37 degrees C. These observations suggest that MP is taken up in the intestinal epithelial cells by a carrier-mediated transport mechanism. The absorptive and secretory clearance of MP increased and decreased with P-glycoprotein (P-gp) inhibitors, respectively. These results strongly suggest that MP is secreted into the intestinal lumen predominantly by P-gp. We conclude that intestinal transport of MP involves P-gp or some other transporters in both the absorptive and secretory directions, and complex nonlinear intestinal absorption characteristics can be ascribed to the existence of multiple transport mechanisms.

Amino acid prodrugs of acyclovir as possible antiviral agents against ocular HSV-1 infections: Interactions with the neutral and cationic amino acid transporter on the corneal epithelium

PURPOSE

The aim of this study was to explore the feasibility of improvement of ocular bioavailability of the antiviral agent acyclovir by designing amino acid prodrugs targeted to the amino acid transporters on the rabbit cornea.

MATERIALS AND METHODS

Transcorneal flux of two water-soluble amino acid ester prodrugs of acyclovir (ACV), gamma-glutamate-ACV (EACV) and L-tyrosine-ACV (YACV), was studied across freshly excised rabbit cornea. Chemical and enzymatic hydrolysis studies of the two prodrugs were also conducted.

RESULTS

EACV inhibited the uptake of [(3)H]L-Arg in rabbit primary corneal epithelial cells (rPCECs). The compound also exhibited longer half-life (t(1/2)) in cornea in comparison to YACV. Transcorneal flux of EACV was observed to be concentration-, energy-, and sodium-dependent and independent of pH within the range studied. EACV transport was inhibited by neutral and cationic amino acids, L-ornithine (specific for cationic amino acids), and BCH (2-aminobicyclo-[2,2,1]-heptane-2-carboxylic-acid) (specific inhibitor for L-type system and B(0,+) system). On the other hand, YACV was not recognized by this amino acid transporter as it failed to inhibit the uptake of [(3)H]Arg, and also its transport across cornea was not inhibited by arginine. YACV and EACV exhibited excellent antiviral activity against HSV-1 and 2 and Varicella-Zoster Virus (VZV) in comparison to ACV.

CONCLUSIONS

Analyses of the inhibition pattern of EACV transport suggests the involvement of a single transport system; namely, B(0,+). Design of amino acid prodrugs seems to be an attractive strategy to enhance the solubility of the otherwise poorly aqueous soluble compounds and also to afford a targeted and possibly enhanced delivery of the active drug.

Pharmacogenetics of OATP (SLC21/SLCO), OAT and OCT (SLC22) and PEPT (SLC15) transporters in the intestine, liver and kidney

The role of carrier-mediated transport in determining the pharmacokinetics of drugs has become increasingly evident with the discovery of genetic variants that affect expression and/or function of a given drug transporter. Drug transporters are expressed at numerous epithelial barriers, such as intestinal epithelial cells, hepatocytes, renal tubular cells and at the blood-brain barrier. Several recent studies have associated alterations in substrate uptake with the presence of SNPs. Here, we summarize the current knowledge on the functional and phenotypic consequences of genetic variation in intestinally, hepatically and renally expressed members of the organic anion-transporting polypeptide family (OATPs; SLC21/SLCO family), the organic anion and organic cation transporters (OATs/OCTs; SLC22 family) and the peptide transporter-1 (PEPT1; SLC15 family).

Pharmacokinetics of amino acid ester prodrugs of acyclovir after oral administration: interaction with the transporters on Caco-2 cells

In vivo systemic absorption of the amino acid prodrugs of acyclovir (ACV) after oral administration was evaluated in rats. Stability of the prodrugs, L-alanine-ACV (AACV), L-serine-ACV (SACV), L-isoleucine-ACV (IACV), gamma-glutamate-ACV (EACV) and L-valine-ACV (VACV) was evaluated in various tissues. Interaction of these prodrugs with the transporters on Caco-2 cells was studied. In vivo systemic bioavailability of these prodrugs upon oral administration was evaluated in jugular vein cannulated rats. The amino acid ester prodrugs showed affinity towards various amino acid transporters as well as the peptide transporter on the Caco-2 cells. In terms of stability, EACV was most enzymatically stable compared to other prodrugs especially in liver homogenate. In oral absorption studies, ACV and AACV showed high terminal elimination rate constants (lambda(z)). SACV and VACV exhibited approximately five-fold increase in area under the curve (AUC) values relative to ACV (p<0.05). C(max(T)) (maximum concentration) of SACV was observed to be 39+/-22 microM in plasma which is 2 times better than VACV and 15 times better than ACV. C(last(T)) (concentration at the last time point) of SACV was observed to be 0.18+/-0.06 microM in plasma which is two times better than VACV and three times better than ACV. Amino acid ester prodrugs of ACV were absorbed at varying amounts (C(max)) and eliminated at varying rates (lambda(z)) thereby leading to varying extents (AUC). The amino acid ester prodrug SACV owing to its enhanced stability, higher AUC and better concentration at last time point seems to be a promising candidate for the oral treatment of herpes infections.

Scintillation proximity assay

Scintillation proximity assay technologies provide a rapid non-separation method to measure common biological interactions using radioactively tagged molecules. This unit identifies potential uses of the technology for the measurement of receptor-ligand binding, cAMP accumulation, GTP binding to heterotrimeric G proteins, protease activity and cellular uptake.

Mechanisms underlying saturable intestinal absorption of metformin

The purpose of the study was to elucidate mechanisms of metformin absorptive transport to explain the dose-dependent absorption observed in humans. Apical (AP) and basolateral (BL) uptake and efflux as well as AP to BL (absorptive) transport across Caco-2 cell monolayers were evaluated over a range of concentrations. Transport was concentration-dependent and consisted of saturable and nonsaturable components (K(m) approximately 0.05 mM, J(max) approximately 1.0 pmol min(-1) cm(-2), and K(d, transport) approximately 10 nl min(-1) cm(-2)). AP uptake data also revealed the presence of saturable and nonsaturable components (K(m) approximately 0.9 mM, V(max) approximately 330 pmol min(-1) mg of protein(-1), and K(d, uptake) approximately 0.04 microl min(-1) mg of protein(-1)). BL efflux was rate-limiting to transcellular transport of metformin; AP efflux was 7-fold greater than BL efflux and was not inhibited by N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GW918), a P-glycoprotein inhibitor. AP efflux was trans-stimulated by metformin and prototypical substrates of organic cation transporters, suggesting that a cation-specific bidirectional transport mechanism mediated the AP efflux of metformin. BL efflux of intracellular metformin was much less efficient in comparison with the overall transport, with BL efflux clearance accounting for approximately 7 and approximately 13% of the overall transport clearance at 0.05 and 10 mM metformin concentrations, respectively. Kinetic modeling of cellular accumulation and transport processes supports the finding that transport occurs almost exclusively via the paracellular route (approximately 90%) and that the paracellular transport is saturable. This report provides strong evidence for a saturable mechanism in the paracellular space and provides insight into possible mechanisms for the dose dependence of metformin absorption in vivo.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Variability in Caco-2 and MDCK Cell-Based Intestinal Permeability Assays

Models predictive of intestinal drug absorption are important in drug development to identify compounds with promising biopharmaceutical properties. Since permeability is a factor affecting absorption, cell culture models (e.g., Caco-2, MDCK) have been developed to predict drug transport from the intestinal lumen into the bloodstream. The differences as to how the assays are performed, along with heterogeneity of the cell lines, have lead to different permeability values for the same drug. Transport and metabolic properties of cultured cells can vary due to culture conditions, seeding density, passage number, confluency, filter support, monolayer age, and stage of differentiation. During the transport experiment, cell absorption properties can change due to the composition and pH of the transport buffer, solute concentration and solubility, temperature, additives and/or cosolvents, agitation, sampling schedule, sink conditions, and analytical methods. Such variability within a laboratory can be avoided by characterizing a cell culture method and setting acceptance criteria in terms of monolayer integrity, passive transport, and active transport. The repeated evaluation of reference compounds will then facilitate intra-laboratory comparisons.

Quantitative prediction of oral absorption of PEPT1 substrates based on in vitro uptake into Caco-2 cells

The method for predicting the fraction absorbed (Fa) of the PEPT1 substrates was established based on the in vitro uptake into Caco-2 cells. Uptake of a drug into Caco-2 cells was measured, and the carrier-mediated initial uptake clearance (DeltaCL uptake) was calculated as the difference between the uptake clearance in the absence of glycyl-sarcosine (Gly-Sar) and that in the presence of 30 mM Gly-Sar. The DeltaCL uptake of each drug was then divided by that of cephradine to obtain DeltaCL*uptake, which was a normalized parameter to correct for inter-day and/or inter-cell variability. Then, cephradine (CED), cefixime (CFIX), and cefotiam (CTM) were selected as marker compounds having excellent, medium and poor absorption, respectively. The DeltaCL*uptake and Fa values for CED, CFIX and CTM were fitted to the equation derived from the complete radial mixing (CRM) model, and the scaling factor (A') was obtained. Using the A' value, Fa was predicted from the DeltaCL*uptake value of each drug. Good correlation was observed between the predicted and reported Fa values, which demonstrated that Fa of PEPT1 substrates can be predicted based on the in vitro uptake in Caco-2 cells.

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.

H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

The H(+)-electrochemical gradient was originally considered as a driving force for solute transport only across cellular membranes of bacteria, plants and yeast. However, in the mammalian small intestine, a H(+)-electrochemical gradient is present at the epithelial brush-border membrane in the form of an acid microclimate. Over recent years, a large number of H(+)-coupled cotransport mechanisms have been identified at the luminal membrane of the mammalian small intestine. These transporters are responsible for the initial stage in absorption of a remarkable variety of essential and non-essential nutrients and micronutrients, including protein digestion products (di/tripeptides and amino acids), vitamins, short-chain fatty acids and divalent metal ions. Proton-coupled cotransporters expressed at the mammalian small intestinal brush-border membrane include: the di/tripeptide transporter PepT1 (SLC15A1); the proton-coupled amino-acid transporter PAT1 (SLC36A1); the divalent metal transporter DMT1 (SLC11A2); the organic anion transporting polypeptide OATP2B1 (SLC02B1); the monocarboxylate transporter MCT1 (SLC16A1); the proton-coupled folate transporter PCFT (SLC46A1); the sodium-glucose linked cotransporter SGLT1 (SLC5A1); and the excitatory amino acid carrier EAAC1 (SLC1A1). Emerging research demonstrates that the optimal intestinal absorptive capacity of certain H(+)-coupled cotransporters (PepT1 and PAT1) is dependent upon function of the brush-border Na(+)-H(+) exchanger NHE3 (SLC9A3). The high oral bioavailability of a large number of pharmaceutical compounds results, in part, from absorptive transport via the same H(+)-coupled cotransporters. Drugs undergoing H(+)-coupled cotransport across the intestinal brush-border membrane include those used to treat bacterial infections, hypercholesterolaemia, hypertension, hyperglycaemia, viral infections, allergies, epilepsy, schizophrenia, rheumatoid arthritis and cancer.

Regulation of intestinal hPepT1 (SLC15A1) activity by phosphodiesterase inhibitors is via inhibition of NHE3 (SLC9A3)

The H⁺-coupled transporter hPepT1 (SLC15A1) mediates the transport of di/tripeptides and many orally-active drugs across the brush-border membrane of the small intestinal epithelium. Incubation of Caco-2 cell monolayers (15 min) with the dietary phosphodiesterase inhibitors caffeine and theophylline inhibited Gly–Sar uptake across the apical membrane. Pentoxifylline, a phosphodiesterase inhibitor given orally to treat intermittent claudication, also decreased Gly–Sar uptake through a reduction in capacity (V_max) without any effect on affinity (K_m). The reduction in dipeptide transport was dependent upon both extracellular Na⁺ and apical pH but was not observed in the presence of the selective Na⁺/H⁺ exchanger NHE3 (SLC9A3) inhibitor S1611. Measurement of intracellular pH confirmed that caffeine was not directly inhibiting hPepT1 but rather having an indirect effect through inhibition of NHE3 activity. NHE3 maintains the H⁺-electrochemical gradient which, in turn, acts as the driving force for H⁺-coupled solute transport. Uptake of β-alanine, a substrate for the H⁺-coupled amino acid transporter hPAT1 (SLC36A1), was also inhibited by caffeine. The regulation of NHE3 by non-nutrient components of diet or orally-delivered drugs may alter the function of any solute carrier dependent upon the H⁺-electrochemical gradient and may, therefore, be a site for both nutrient–drug and drug–drug interactions in the small intestine.

Functional Expression of Stereoselective Metabolism of Cephalexin by Exogenous Transfection of Oligopeptide Transporter PEPT1

Gastrointestinal absorption of the beta-lactam antibiotic cephalexin (CEX) is highly stereoselective: l- and d-CEX are both taken up by intestinal epithelial cells through the brush-border membrane, most likely via oligopeptide transporter PEPT1, but l-CEX is not found in serum or urine after administration p.o. because of its rapid intestinal metabolism, whereas d-CEX is well absorbed in the unchanged form. We examined the contribution of PEPT1 to the stereoselective uptake and metabolism of CEX. We observed stereoselective metabolism of CEX after exogenous transfection of PEPT1 alone into mammalian cell lines: l-CEX, but not d-CEX, was metabolized to 7-aminodesacetoxycephalosporanic acid (7-ADCA) in HeLa and human embryonic kidney 293 cells stably and transiently expressing human PEPT1, respectively, whereas such metabolism was minor in cells expressing the vector alone. The formation rate of 7-ADCA depended on the amount of PEPT1 cDNA transfected. l-CEX metabolism was rapid because only 7-ADCA was found inside and outside the cells during incubation with l-CEX. The characteristics of PEPT1-mediated metabolism of l-CEX were similar, but not identical, to those of PEPT1-mediated transport. PEPT1-mediated metabolism was also observed in permeabilized cells expressing PEPT1, in which PEPT1-mediated intracellular substrate accumulation was negligible, suggesting that the increase in l-CEX metabolism by PEPT1 transfection cannot be fully explained by an increase in uptake and subsequent exposure to intracellular hydrolases. The present findings show that stereoselectivity in CEX absorption can be fully explained in terms of PEPT1, implying that the l-CEX hydrolase is PEPT1 itself or is induced by PEPT1.

Modulation of P-glycoprotein-mediated efflux by prodrug derivatization: an approach involving peptide transporter-mediated influx across rabbit cornea

The aim of this study was to investigate the modulation of efflux mechanisms using transporter- targeted prodrug derivatization of a model P-gp substrate, quinidine. The L-valine, L-valine-valine esters of quinidine, val-quinidine (VQ), and val-val-quinidine (VVQ) were synthesized in our laboratory, respectively. [(14)C] erythromycin was chosen to delineate the affinity of quinidine (Q) toward P-gp. [(3)H] glycylsarcosine (GS, or glysar) was chosen as a model peptide transporter (PEPT) substrate. Uptake studies were performed on rPCEC (rabbit primary corneal epithelial culture) using 12-well plates. Transport studies were conducted with isolated rabbit corneas at 34 degrees C. Efflux of [(14)C] erythromycin was significantly increased in the presence of quinidine, whereas it was unaltered in the presence of VQ and VVQ. VVQ was more stable, both in buffers and tissue homogenate. Transport of VQ and VVQ was inhibited with GS, and their permeability values were 1.5 and 3 times higher than the permeability of quinidine, respectively. Results from this study clearly indicate that prodrug derivatization of quinidine can modulate P-gp-mediated efflux. These prodrugs have a reduced or diminished affinity toward P-gp and were further recognized by the peptide transporter- mediated process. Enhanced permeabilities of the prodrugs indicate that drug derivatization can be a viable strategy for overcoming P-gp-mediated efflux.

Saturable Absorptive Transport of the Hydrophilic Organic Cation Ranitidine in Caco-2 Cells: Role of pH-Dependent Organic Cation Uptake System and P-Glycoprotein

PURPOSE

The purpose of this work was to investigate the involvement of carrier-mediated apical (AP) uptake and efflux mechanisms in the absorptive intestinal transport of the hydrophilic cationic drug ranitidine in Caco-2 cells.

METHODS

Absorptive transport and AP uptake of ranitidine were determined in Caco-2 cells as a function of concentration. Permeability of ranitidine in the absorptive and secretory directions was assessed in the absence or presence of the P-glycoprotein (P-gp) inhibitor, GW918. Characterization of the uptake mechanism was performed with respect to inhibitor specificity, pH, energy, membrane potential, and Na+ dependence. Efflux from preloaded monolayers was evaluated over a range of concentrations and in the absence or presence of high extracellular ranitidine concentrations.

RESULTS

Saturable absorptive transport and AP uptake of ranitidine were observed with Km values of 0.27 and 0.45 mM, respectively. The ranitidine absorptive permeability increased and secretory permeability decreased upon inhibition of P-gp. AP ranitidine uptake was inhibited in a concentration-dependent fashion by a diverse set of organic cations including tetraethylammonium, 1-methyl-4-phenylpyridinium, famotidine, and quinidine. AP ranitidine uptake was pH and membrane potential dependent and reduced under conditions that deplete metabolic energy. Efflux of [3H]ranitidine across the basolateral membrane was neither saturable as a function of concentration nor trans stimulated by unlabeled ranitidine.

CONCLUSIONS

Saturable absorptive transport of ranitidine in Caco-2 cells is partially mediated via a pH-dependent uptake transporter for organic cations and is subject to attenuation by P-gp. Inhibition and driving force studies suggest the uptake carrier exhibits similar properties to cloned human organic cation transporters. The results also imply ranitidine transport is not solely restricted to the paracellular space.

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.

Uptake and metabolism of ginsenoside Rh2 and its aglycon protopanaxadiol by Caco-2 cells

The uptake and metabolism profiles of ginsenoside Rh2 and its aglycon protopanaxadiol (ppd) were studied in the human epithelial Caco-2 cell line. High-performance liquid chromatography-mass spectrometry was applied to determine Rh2 and its aglycon ppd concentration in the cells at different pH, temperature, concentration levels and in the presence or absence of inhibitors. Rh2 uptake was time and concentration dependent, and its uptake rates were reduced by metabolic inhibitors and influenced by low temperature, thus indicating that the absorption process was energy-dependent. Drug uptake was maximal when the extracellular pH was 7.0 for Rh2 and 8.0 for ppd. Rh2 kinetic analysis showed that a non-saturable component (Kd 0.17 nmol x h(-1) x mg(-1) protein) and an active transport system with a Km of 3.95 micromol x l(-1) and a Vmax of 4.78 nmol x h(-1) x mg(-1)protein were responsible for the drug uptake. Kinetic analysis of ppd showed a non-saturable component (Kd 0.78 nmol x h(-1) x mg(-1) protein). It was suggested that active extrusion of P-glycoprotein and drug degradation in the intestine may influence Rh2 bioavailability.

Functional regulation of Na+-dependent neutral amino acid transporter ASCT2 by S-nitrosothiols and nitric oxide in Caco-2 cells

We describe the regulation mechanisms of the Na(+)-dependent neutral amino acid transporter ASCT2 via nitric oxide (NO) in the human intestinal cell line, Caco-2. Exposure of Caco-2 cells to S-nitrosothiol, such as S-nitroso-N-acetyl-DL-penicillamine (SNAP) and S-nitrosoglutathione, and the NO-donor, NOC12, concentration- and time-dependently increased Na(+)-dependent alanine uptake. Kinetic analyses indicated that SNAP increases the maximal velocity (V(max)) of Na(+)-dependent alanine uptake in Caco-2 cells without affecting the Michaelis-Menten constant (K(t)). The stimulatory effect was partially eliminated by actinomycin D and cycloheximide. Increased Na(+)-dependent alanine uptake by SNAP was partially abolished by the NO scavengers, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide sodium salt (carboxy-PTIO) and N-(dithiocarboxy)sarcosine disodium salts (DTCS), as well as the NADPH oxidase inhibitor, diphenyleneiodonium. RT-PCR revealed that Caco-2 cells expressed the Na(+)-dependent neutral amino acid transporter ASCT2, but not the other Na(+)-dependent neutral amino acid transporters ATB(0,+) and B(0)AT1. These results suggested that functional up-regulation of ASCT2 by SNAP might be partially associated with an increase in the density of transporter protein via de novo synthesis.

Functional linkage of H+/peptide transporter PEPT2 and Na+/H+ exchanger in primary cultures of astrocytes from mouse cerebral cortex

In our previous studies, we demonstrated that the high-affinity type peptide transporter PEPT2 is expressed in rat cerebral cortex using synaptosomal membrane study and that the uptake of dipeptide [14C]glycylsarcosine into synaptosomes was stimulated by an inwardly directed H+ gradient (Fujita et al., Brain Res. 972, 52-61, 2004). However, there is no information available for the driving force of PEPT2 function in the nervous system. In the present study, we investigated functional characteristics of PEPT2 mediated transport of Gly-Sar in primary cultured astrocytes from mouse cerebral cortex and examined the effects of Na+/H+ exchanger (NHE) inhibitor on Gly-Sar uptake in mouse astrocytes. In mouse astrocytes, extracellular H+ influenced only the maximal velocity (Vmax) of Gly-Sar uptake without affecting the apparent affinity (Kt). Interestingly, removal of Na+ from uptake buffer significantly reduced Gly-Sar uptake and Gly-Sar uptake was modulated by NHE inhibitors. The treatment of EIPA, an NHE inhibitor, altered the Vmax value of Gly-Sar uptake but had no effect on its Kt value. RT-PCR revealed that NHE1 and NHE2 mRNA are expressed in mouse cerebrocortical astrocytes. These results demonstrated that NHE activity is required to allow optimal uptake of dipeptides mediated by PEPT2 into the astrocytes. This study represents the first description of the functional co-operation of PEPT2 and NHE1 and/or NHE2 in cerebrocortical astrocytes.

New methods to evaluate intestinal drug absorption mediated by oligopeptide transporter from in vitro study using Caco-2 cells

The aim of the present work is to develop a convenient and rapid screening system in vitro for intestinal drug absorption mediated by oligopeptide transporter (PepT1). In this study, (1) Transports of cephalexin (CEX) and L-phenylalanine (L-Phe) across Caco-2 monolayers were measured and compared with those of passively transported drugs, (2) Inhibitory effects of various drugs on the transport of [(14)C]glycylsarcosine (Gly-Sar) across Caco-2 monolayers were measured and correlated with their in vivo permeability to rat small intestine, (3) Intracellular pH-change induced by co-transport of drugs with proton into Caco-2 cells was monitored by using Fluorometric Imaging Plate Reader (FLIPR, Molecular Devices Corp.). Concentration-dependent transport was observed in Caco-2 monolayers for CEX and L-Phe, although their permeability was relatively low compared to those of passively transported drugs. Inhibitory effects of various drugs including beta-lactam antibiotics and angiotensin converting enzyme-inhibitors on the transport of Gly-Sar correlated well with their in vivo permeability to rat small intestine. It was demonstrated that CEX, but not cefazolin, induced gradual decrease in the intracellular pH of Caco-2 cells. The degree of intracellular pH-change caused by various drugs showed a sigmoidal or saturable relationship with their permeability to rat small intestine. These in vitro approaches with Caco-2 cells should be useful to evaluate in vivo intestinal permeability of drugs mediated by PepT1, suggesting a possibility of high throughput screening of drug absorption.

A Randomly Coiled, High-Molecular-Weight Polypeptide Exhibits Increased Paracellular Diffusion in Vitro and in Situ Relative to the Highly Ordered ?-Helix Conformer

PURPOSE

The current investigation was conducted to examine the effect of secondary structure of model polypeptides on their hindered paracellular diffusion.

METHODS

Poly-D-glutamic acid (PDGlu) was selected as one of the model polypeptides because of its ability to form two secondary structures; a negatively charged random coil and an alpha-helix with partial negative charge at pH 7.4 and 4.7, respectively. Poly-D-lysine (PDL) was selected as a positively charged random coil conformation at pH 7.4. Transport experiments were conducted across both a Caco-2 cell monolayer and the intestinal membrane of Sprague-Dawley rats. Additionally, using NMR, an estimation for the diffusion coefficient and the equivalent hydrodynamic radius for each model polypeptide was obtained.

RESULTS

PDGlu in the randomly coiled conformation exhibited greater paracellular transport when compared to either the same polypeptide having an alpha-helix secondary structure or the positively charged, randomly coiled PDL.

CONCLUSIONS

Randomly coiled PDGlu was able to permeate through the negatively charged tight junctions of both biological membranes to a greater extent than PDGlu having an alpha-helix structure and suggests that molecular flexibility associated with the random coil conformation may play a more important role than overall charge and hydrodynamic radius on its hindered paracellular diffusion.