Cefaclor uptake by the proton-dependent dipeptide transport carrier of human intestinal Caco-2 cells and comparison to cephalexin uptake

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.

Characterization of a transport activity for long-chain peptides in barley mesophyll vacuoles

The plant vacuole is the largest compartment in a fully expanded plant cell. While only very limited metabolic activity can be observed within the vacuole, the majority of the hydrolytic activities, including proteolytic activities reside in this organelle. Since it is assumed that protein degradation by the proteasome results in the production of peptides with a size of 3-30 amino acids, we were interested to show whether the tonoplast exhibits a transport activity, which could deliver these peptides into the vacuole for final degradation. It is shown here that isolated barley mesophyll vacuoles take up peptides of 9-27 amino acids in a strictly ATP-dependent manner. Uptake is inhibited by vanadate, but not by NH(+)(4), while GTP could partially substitute for ATP. The apparent affinity for the 9 amino acid peptide was 15 μM, suggesting that peptides are efficiently transferred to the vacuole in vivo. Inhibition experiments showed that peptides with a chain length below 10 amino acids did not compete as efficiently as longer peptides for the uptake of the 9 amino acid peptide. Our results suggest that vacuoles contain at least one peptide transporter that belongs to the ABC-type transporters, which efficiently exports long-chain peptides from the cytosol into the vacuole for final degradation.

Transport of hop bitter acids across intestinal Caco-2 cell monolayers

Several health-beneficial properties of hop bitter acids have been reported (inhibition of bone resorption and anticarcinogenic and anti-inflammatory activities); however, scientific data on the bioavailability of these compounds are lacking. As a first approach to study the bioavailability, the epithelial transport of hop alpha- and beta-acids across Caco-2 monolayers was investigated. Hop acids were added either to the apical or to the basolateral chamber and, at various time points, amounts transported to the receiving compartment were determined. The monolayer integrity control was performed by using marker compounds (atenolol and propranolol), transepithelial electrical resistance (TEER) measurement, and determination of the fluorescein efflux. The TEER and fluorescein efflux confirmed the preservation of the monolayer integrity. The membrane permeability of the alpha-acids (apparent permeability coefficients for apical to basolateral transport (P(appAB)) ranged from 14 x 10(-6) to 41 x 10(-6) cm/s) was determined to be substantially higher than that of the beta-acids (P(appAB) values ranging from 0.9 x 10(-6) to 2.1 x 10(-6) cm/s). Notably, the beta-acids exhibited significantly different bidirectional P(app) values with efflux ratios around 10. The involvement of carrier-mediated transport for beta-acids (active efflux pathway by P-gp, BCRP, and/or MRP-2 type efflux pumps) could be confirmed by transport experiments with specific inhibitors (verapamil and indomethacin). It appears that alpha-acids are efficiently absorbed, whereas the permeability of beta-acids is low. Limiting factors in the absorption of beta-acids could involve P-gp and MRP-2 type efflux transporters and phase II metabolism.

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.

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.

Prediction of oral drug absorption in humans by theoretical passive absorption model

The purpose of the present study was to examine the oral drug absorption predictability of the theoretical passive absorption model (TPAM). As chemical descriptors of drugs, the octanol/buffer distribution coefficient at pH 6.0 (D(ow)), intrinsic octanol-water partition coefficient (P(ow)), pK(a), and molecular weight (MW) were calculated from the chemical structure. Total passive intestinal membrane permeation consists of transcellular, paracellular and unstirred water layer (UWL) permeation. Transcellular permeation was modeled based on the pH-partition hypothesis with correction for cationic species permeation, and the independent variables were D(ow), P(ow), and pK(a). Paracellular permeation was modeled as a size-restricted diffusion within a negative electrostatic field-of-force, and the independent variables were MW and pK(a). UWL permeation was modeled as diffusion across a water layer, and the independent variable was MW. Cationic species permeation in the transcellular permeation model and the effect of a negative electric field-of-force in the paracellular permeation model were the extensions to the previous TPAM. The coefficients of the paracellular and UWL permeation models were taken from the literature. A data set of 258 compounds with observed values of Fa% (the fraction of a dose absorbed in humans) taken from the literature was employed to optimize four fitting coefficients in the transcellular permeation model. The TPAM predicted Fa%, with root mean square errors of 15-21% and a correlation coefficient (CC) of 0.78-0.88. In addition, the TPAM predicted the effective human intestinal membrane permeability with a CC of 0.67-0.77, as well as the contribution of paracellular permeation. The TPAM was found to predict oral absorption from the chemical structure of drugs with adequate predictability for usage in drug discovery.

AtOPT6 transports glutathione derivatives and is induced by primisulfuron

The oligopeptide transporter (OPT) family contains nine members in Arabidopsis. While there is some evidence that AtOPTs mediate the uptake of tetra- and pentapeptides, OPT homologs in rice (Oryza sativa; OsGT1) and Indian mustard (Brassica juncea; BjGT1) have been described as transporters of glutathione derivatives. This study investigates the possibility that two members of the AtOPT family, AtOPT6 and AtOPT7, may also transport glutathione and its conjugates. Complementation of the hgt1met1 yeast double mutant by plant homologs of the yeast glutathione transporter HGT1 (AtOPT6, AtOPT7, OsGT1, BjGT1) did not restore the growth phenotype, unlike complementation by HGT1. By contrast, complementation by AtOPT6 restored growth of the hgt1 yeast mutant on a medium containing reduced (GSH) or oxidized glutathione as the sole sulfur source and induced uptake of [3H]GSH, whereas complementation by AtOPT7 did not. In these conditions, AtOPT6-dependent GSH uptake in yeast was mediated by a high affinity (Km = 400 microm) and a low affinity (Km = 5 mm) phase. It was strongly competed for by an excess oxidized glutathione and glutathione-N-ethylmaleimide conjugate. Growth assays of yeasts in the presence of cadmium (Cd) suggested that AtOPT6 may transport Cd and Cd/GSH conjugate. Reporter gene experiments showed that AtOPT6 is mainly expressed in dividing areas of the plant (cambium, areas of lateral root initiation). RNA blots on cell suspensions and real-time reverse transcription-PCR on Arabidopsis plants indicated that AtOPT6 expression is strongly induced by primisulfuron and, to a lesser extent, by abscisic acid but not by Cd. Altogether, the data show that the substrate specificity and the physiological functions of AtOPT members may be diverse. In addition to peptide transport, AtOPT6 is able to transport glutathione derivatives and metal complexes, and may be involved in stress resistance.

Comparison of ceftibuten transport across Caco-2 cells and rat jejunum mounted on modified Ussing chambers

Ceftibuten uptake into Caco-2 cells and intestinal brush border membrane vesicles is mediated by the dipeptide transport system (PEPT1). The apical to basolateral transport characteristics of ceftibuten across Caco-2 cells and rat jejunum mounted on a modified Ussing chamber was examined. Mannitol was used as a paracellular marker along with trans-epithelial electrical resistance (TEER) for monitoring tight junction permeability. Transport across Caco-2 cells and rat jejunum mounted on a modified Ussing chamber was linear across the concentration range 0.25-10 mM. The net flux of mannitol and ceftibuten was higher across rat jejunum compared with Caco-2 cells. At a donor concentration of 0.25 mM, ceftibuten transport across Caco-2 cells was found to be pH dependent. Glycyl proline, a dipeptide, and 2,4- dinitrophenol, an energy poison, caused a reduction in the permeability of 0.25 mM ceftibuten across Caco-2 cells. Benzoic acid and adipic acid also inhibited transcellular transport of ceftibuten. At a donor concentration of 0.25 mM, passive paracellular transport accounts for about 60% and the active carrier mediated mechanism accounts for about 40% of ceftibuten transport across Caco-2 cells. None of the inhibitors however, had a significant effect on ceftibuten transport across rat jejunum mounted on a modified Ussing chamber at a donor concentration of 0.25 mM. In the concentration range 0.25-10 mM, ceftibuten is predominantly transported by paracellular mechanisms across rat jejunum and a mixture of active and passive transport across Caco-2 cells.

Transporters involved in apical and basolateral uptake of ceftibuten into Caco-2 cells

Ceftibuten uptake from the apical and basolateral side of Caco-2 cells grown on transwells was studied. Uptake into the cells showed concentration dependent saturation. The apical transporter(s) showed a higher capacity and lower affinity for ceftibuten than the basolateral transporter(s). Uptake was inhibited in the presence of higher pH and in the presence of 2,4-dinitro phenol (DNP). A proton gradient had a greater effect on the apical than on the basolateral transporter. Glycyl proline, a dipeptide transport system (PEPT1) substrate, inhibited ceftibuten uptake into Caco-2 cells. Benzoic acid, a monocarboxylic acid (MCT) transporter substrate also exhibited a strong inhibition of ceftibuten uptake, but acetic acid had no effect. Adipic acid inhibited apical uptake of ceftibuten but had no effect on the basolateral uptake. None of the inhibitors had a significant effect on ceftibuten uptake in absence of a pH gradient. Addition of inhibitors in presence of DNP led to a greater decrease in ceftibuten uptake, when compared to the effect of DNP alone, indicating a facilitated diffusion process. These results indicate that ceftibuten uptake in Caco-2 cells involve multiple transport pathways. Apical uptake is mediated by an energy dependent carrier-mediated process and an energy independent facilitated diffusion process. The apical transport system is different from the basolateral transporter.

Comparative target site pharmacokinetics of immediate- and modified-release formulations of cefaclor in humans

Optimal dosing of beta-lactam antibiotics aims at maximizing the time at which drug levels in the interstitial space fluid (ISF)--the fluid that surrounds the causative microorganisms at the target site--exceed the minimal inhibitory concentration (MIC). One potentially attractive strategy to achieve this goal is to administer antibiotics as oral sustained-release formulations. The present study was designed to test the hypothesis that sustained-release formulations could lead to a more suitable pharmacokinetic profile in the ISF at the relevant target site. For this purpose, time versus cefaclor concentration profiles attained in the ISF were measured following administration of two formulations, an immediate- (500 mg IR) and a modified-release formulation in two different doses (500 mg MR and 750 mgMR) in a three-way crossover study of healthy male volunteers (n = 12). For the measurement of unbound cefaclor concentrations in the ISF of human skeletal muscle, the in vivo microdialysis technique was employed. For all three formulations, unbound cefaclor concentration in the ISF closely followed individual plasma concentration profiles in a dose-dependent pattern, with ISF to unbound plasma ratios ranging from 0.67 to 0.73. The mean residence time was found to be significantly longer for the MR formulations versus the IR formulation. The data of the present study indicate that time above MIC values at the target site can be substantially prolonged if an antibiotic is administered as a sustained-release product.

Ocular pharmacokinetics of cephalosporins using microdialysis

The purpose of this study was to delineate the ocular pharmacokinetics of cephalosporins and investigate the presence of peptide transporters in the retina. New Zealand albino rabbits were kept under anesthesia. A concentric microdialysis probe was implanted in the vitreous chamber and linear probe across the cornea in the aqueous humor. Isotonic phosphate buffer saline was perfused through the probes, and samples were collected every 20 min over a period of 10 hr. A 500 microg dose of cephalexin, cephazolin, and cephalothin was administered intravitreally. Inhibition experiments were carried out in vivo, using gly-pro and gly-sar. The vitreal half-lives of cephalexin, cefazolin, and cephalothin were 185.38 +/- 27.25 min, 111.40 +/- 17.17 min, and 146.68 +/- 47.52 min, respectively. Cephalexin generated higher aqueous humor concentrations compared to cefazolin. The pharmacokinetic parameters of cephalexin in the presence of gly-pro, i.e., AUC (44452.06 +/- 3326.55 microg x min/ml), clearance (0.0013 +/- 0.0004 ml/min) and vitreal half-life (825.12 +/- 499.95 min) were different from that of the control (14612.83 +/- 4036.47 microg x min/ml, 0.0036 +/- 0.0011 ml/min, and 187.96 +/- 65.12 min, respectively). Gly-pro did not inhibit cefazolin, and gly-sar showed no effect on the pharmacokinetics of both drugs. These studies indicate the involvement of a peptide carrier in the transport of cephalosporins across the retina. Although gly-pro inhibited the elimination of cephalexin from the vitreous, the effect of an alpha-amino group on peptide carriers was not clearly evident.

A rapid screening system to determine drug affinities for the intestinal dipeptide transporter 2: affinities of ACE inhibitors

PURPOSE

To assess the affinities of a series of ACE inhibitors for the di/tri/oligopeptide transport system (DTS) using a rapid in vitro system.

METHODS

Monolayers of Caco-2 cells were cultured in plastic wells for 7-9 days and the uptake of Gly-[3H]L-Pro was used as an affinity probe. Gly-[3H]L-Pro (50 nM), together with excess L-Pro (10 mM), to suppress uptake of any [3H]L-Pro produced by degradation of the probe, was incubated with the test compound (usually 1 mM) at pH 6 for 3-mins. The uptake of radiolabel was determined by liquid scintillation counting.

RESULTS

A 2-dimensional six-domain model of the transporter based on the structure of a phosphinate ACE inhibitor (SQ-29852) was constructed to facilitate interpretation of the competitor affinities. The SQ-29852 molecule was divided into six binding domains (A-F) based on functional groups within these regions and the effects of structural variation in four of these domains (A, C-E) were explored. A series of dipeptide-like compounds varying within specific domains were selected from a large number of commercially available ACE inhibitors and SQ-29852 analogues. Domain A had a preference for an uncharged group, with bulky hydrophobic groups reducing affinity. Domain C exhibited a preference for a positive charge over a neutral function, with the space this functional group occupies contributing to affinity. Domain D favoured lipophilic residues and domain E retained activity when the carboxylic acid was esterified.

CONCLUSION

The test system is able to reveal structure-activity relationships of peptidomimetic agents and may well serve as a design tool to optimise affinity for the DTS.

Intestinal peptide transport systems and oral drug availability

The intestinal peptide transport system has broad substrate specificities. In addition to its physiological function of absorbing di- and tripeptides resulting from the digestion of dietary proteins, this transport system also absorbs some orally administered peptidomimetic drugs, including beta-lactam antibiotics, angiotensin converting enzyme inhibitors, renin inhibitors, bestatin, thrombin inhibitors, and thyrotropin-releasing hormone and its analogues. There have been several studies on the mechanism and substrate structure-affinity relationship for this transport system. Rapid progress has been made recently in studies on the molecular basis of the intestinal peptide transport system. A protein apparently involved in peptide transport has been isolated from rabbit small intestines, and genes for human intestinal peptide transporters have been cloned, sequenced and functionally expressed. This review summarizes these studies and addresses the pharmaceutical potential of the intestinal peptide transport system.

Saturable transport of H2-antagonists ranitidine and famotidine across Caco-2 cell monolayers

The purpose of this study was to investigate the mechanism by which the H2-antagonists ranitidine and famotidine interacted with the paracellular space during their transport across Caco-2 cell monolayers. Transport experiments with ranitidine and famotidine across Caco-2 cell monolayers were performed to determine the apical-to-basolateral flux at various concentrations. Kinetic analysis of the transport data showed that ranitidine and famotidine were transported by both saturable and nonsaturable processes. Na+, K+-ATPase inhibitor ouabain and metabolic inhibitors sodium azide + 2-deoxy-D-glucose did not affect ranitidine transport, suggesting that the active transport was not involved. Famotidine and some other guanidine-containing compounds, e.g., guanethidine, Arg-Gly, L-arginine methyl ester, and L-argininamide, inhibited the transport of ranitidine, whereas other guanidine-containing compounds with an additional negative charge, e.g., L-arginine, did not. 2,4, 6-Triaminopyrimidine (TAP), an inhibitor of paracelluar cationic conductance, also inhibited the transport of both ranitidine and famotidine. On the basis of these results, it is proposed that the saturable transport of ranitidine and famotidine across Caco-2 cell monolayers appears to be via a facilitated diffusion process mediated by the paracellular anionic sites. This mechanism is consistent with the observation that ranitidine and famotidine caused a concentration-dependent increase in transepithelial electrical resistance (TEER) across Caco-2 cell monolayers, presumably by blocking the paracellular anionic sites and thus inhibiting the flux of cations (e.g., Na+).

The role of an alpha-amino group on H+ -dependent transepithelial transport of cephalosporins in Caco-2 cells

The role of an alpha-amino group on interaction with the intestinal and renal peptide carriers (PEPT 1 and PEPT 2, respectively) has been the subject of much investigation. Studies have differed in their conclusions about the role of an alpha-amino group on carrier-mediated absorption. Most studies have used brush-border membrane vesicles or perfused intestinal segments. These techniques enable the determination of membrane uptake and luminal disappearance, respectively, but not transepithelial transport. Transepithelial transport should be more predictive of absorption because it includes basolateral efflux, which could be the rate-limiting process in drug absorption. The objective of this study was to evaluate the influence of an alpha-amino group on PEPT 1-mediated transepithelial transport in Caco-2 cells. The apical-to-basolateral permeability coefficients of cephalosporins with or without a free alpha-amino group were determined in the presence and absence of a pH gradient. Permeability coefficients obtained under these conditions were used to calculate a permeability ratio (i.e. P(app) (pH 6.0)/P(app) (pH 7.4)), which should indicate whether PEPT 1 is involved in transport. For cephalosporins with an alpha-amino group (cephalexin, cefaclor, cefadroxil, cephradine, cephaloglycin) the permeability ratios ranged between 1.77 and 2.77. In contrast, the permeability ratios for cephalosporins without an alpha-amino group were 1 (approx.; range = 0.74-1.26). These data suggest that the presence of an alpha-amino group on cephalosporins increases their PEPT 1-mediated transepithelial transport in Caco-2 monolayers.

Uptake of N,N-dimethyl-2-phenylethylene HCl into acini cells removed from rabbit lacrimal glands

Acini cells were obtained from the lacrimal gland of the white New Zealand rabbit. Following isolation and purification, the cells were used to study the uptake of N,N'-dimethyl-2-phenylethylamine HCl (AF2975), which was found to be sodium- and proton-independent, but energy-dependent. Uptake was mainly accomplished via a carrier-mediated transport system for which a Km of 8.72+/-0.96 mM, a Vmax of 602.6+/-41.3 nmol/mg of protein/min, and an exponential coefficient of 2.55+/-0.46 were obtained following a least squares nonlinear fit to the Hill equation. With the addition of the metabolic inhibitors, sodium azide or 2,4-dinitrophenol, the initial uptake rates were reduced from the control experiments by 35.7% and 26.2%, respectively.

Protection against ethanol injury by prostaglandin in a human intestinal cell line: role of microtubules

Prostaglandins have been shown to protect the gastrointestinal (GI) epithelium from injury induced by various luminal insults independent of their known acid-inhibitory effects, a process termed "cytoprotection." The mechanism of this protective action remains unknown. The present investigation determined the role of microtubules (a major cytoskeletal component) in GI injury induced by ethanol (EtOH) and its prevention by 16,16-dimethylprostaglandin E2 (dmPGE2) using cells from a human colonic cell line known as Caco-2 cells. These cells were preincubated in Eagle's minimum essential medium with and without dmPGE2 (2.6 microM) for 15 min and subsequently incubated in media containing 1, 2.5, 5, 7.5, and 10% EtOH. The effects on cell viability and tubulin (the major protein backbone of microtubules) were then determined. EtOH concentrations > or = 2.5% extensively disrupted the microtubules as demonstrated by fragmentation, kinking, and perturbation of the microtubule organizer center. EtOH treatment also led to a significant decrease in the S2 (polymerized) fraction and an increase in the S1 (monomeric) pool of tubulin. Concomitant with these effects were marked decreases in cellular viability. DmPGE2 pretreatment abolished the disruption of microtubules, significantly increased the S2 fraction of tubulin, and increased cellular viability in cultures exposed to EtOH. Furthermore, pretreatment with colchicine, an inhibitor of microtubule assembly, prevented the cytoprotective action of dmPGE2. Taxol, a microtubule stabilizing agent, mimicked the effects of dmPGE2 by also enhancing microtubule integrity and increasing cellular viability in cells exposed to EtOH. Our data indicate that organization and stabilization of microtubules may play an essential role in the mechanism of prostaglandin-induced protection.

Synthesis and structure-activity relationships of some pesticides with an alpha-amino acid function

The synthesis of several derivatives of a xenobiotic (phenoxyalkanecarboxylic acid) and an alpha-amino acid (L-lysine) is described. Various substituents were introduced in the aromatic ring of the phenoxyalkanecarboxylic acid and the side-chain was modified. They were tested for their effect on the transport of a neutral (L-threonine), an acidic (L-glutamic acid) and a basic (L-lysine) amino acid, and a sugar (sucrose). All compounds markedly inhibited threonine uptake by leaf tissues of Vicia faba L.--and more specifically phloem loading--with two exceptions, when the aromatic ring was substituted in the 4-position by a primary amino group or when D-lysine was used instead of L-lysine. By contrast, the addition of a chlorine atom in the 4-position of the aromatic ring enhanced the inhibitory activity. Similar results were obtained for inhibition of glutamate uptake and, to a lesser extent, for lysine uptake. pH dependence of the inhibitory activity as well as electrophysiological data indicate that permeases mediating active transport of amino acids are the target of these conjugates. These, in addition to other data obtained with other xenobiotics, show that the amino acid carrier system is capable of recognizing a wide range of conjugates of various sizes, structures and octanol/water partition coefficients.

Structure-activity relationship of carbacephalosporins and cephalosporins: antibacterial activity and interaction with the intestinal proton-dependent dipeptide transport carrier of Caco-2 cells

An intestinal proton-dependent peptide transporter located on the lumenal surface of the enterocyte is responsible for the uptake of many orally absorbed beta-lactam antibiotics. Both cephalexin and loracarbef are transported by this mechanism into the human intestinal Caco-2 cell line. Forty-seven analogs of the carbacephalosporin loracarbef and the cephalosporin cephalexin were prepared to evaluate the structural features necessary for uptake by this transport carrier. Compounds were evaluated for their antibacterial activities and for their ability to inhibit 1 mM cephalexin uptake and, subsequently, uptake into Caco-2 cells. Three clinically evaluated orally absorbed carbacephems were taken up by Caco-2 cells, consistent with their excellent bioavailability in humans. Although the carrier preferred the L stereoisomer, these compounds lacked antibacterial activity and were hydrolyzed intracellularly in Caco-2 cells. Compounds modified at the 3 position of cephalexin and loracarbef with a cyclopropyl or a trifluoromethyl group inhibited cephalexin uptake. Analogs with lipophilic groups on the primary amine of the side chain inhibited cephalexin uptake, retained activity against gram-positive bacteria but lost activity against gram-negative bacteria. Substitution of the phenylglycl side chain with phenylacetyl side chains gave similar results. Compounds which lacked an aromatic ring in the side chain inhibited cephalexin uptake but lost all antibacterial activity. Thus, the phenylglycl side chain is not absolutely required for uptake. Different structural features are required for antibacterial activity and for being a substrate of the transporter. Competition studies with cephalexin indicate that human intestinal Caco-2 cells may be a useful model system for initially guiding structure-activity relationships for the rational design of new oral agents.

Antibacterial effect of the adhering human Lactobacillus acidophilus strain LB

The spent culture supernatant of the human Lactobacillus acidophilus strain LB produces an antibacterial activity against a wide range of gram-negative and gram-positive pathogens. It decreased the in vitro viability of Staphylococcus aureus, Listeria monocytogenes, Salmonella typhimurium, Shigella flexneri, Escherichia coli, Klebsiella pneumoniae, Bacillus cereus, Pseudomonas aeruginosa, and Enterobacter spp. In contrast, it did not inhibit lactobacilli and bifidobacteria. The activity was heat stable and relatively sensitive to enzymatic treatments and developed under acidic conditions. The antimicrobial activity was independent of lactic acid production. Activity against S. typhimurium SL1344 infecting human cultured intestinal Caco-2 cells was observed as it was in the conventional C3H/He/oujco mouse model with S. typhimurium C5 infection and oral treatment with the LB spent culture supernatant.

Derivatives of melphalan designed to enhance drug accumulation in cancer cells

The objective of this study was to develop chemical strategies to improve the uptake and accumulation of melphalan (L-Mel and D-Mel), a cytotoxic agent, into cancer cells. Dipeptides synthesized from L- (or D-) Mel and L-glutamic acid (L-Glu) or L-valine (L-Val) and their methyl or ethyl esters (all compounds were trifluoroacetic acid salts) were evaluated for cytotoxicity and cellular uptake using Caco-2 cells, a human colon carcinoma cell line, and RT-2 cells, a rat brain glioma cell line. Treatment of Caco-2 cells with L-Mel or D-Mel (0.5 mg/ml equivalent of melphalan) for 48 h resulted in approximately 50% cell survival. Treatment of the Caco-2 cells with dipeptide derivatives of L-Mel (or D-Mel) (11c-d, 12c-d and 13) caused similar cytotoxicity effects (approximately 50-70% of cell survival). When the cytotoxicities of the esters of L-Mel, D-Mel and their dipeptide derivatives (11a-b, 12a-b and 14) in Caco-2 cells were determined, less than 10% cell survival was observed. Similar results were observed in RT-2 cells. When the cellular uptake properties of these compounds were determined in Caco-2 cell monolayers, L-Glu-L-Mel (12c), L-Glu-D-Mel (12d), and L-Mel-L-Glu (11c) generated slightly lower intracellular levels of L-Mel or D-Mel than when the cell monolayer was treated with the amino acids (L-Mel or D-Mel). In Caco-2 cells treated with 11c, 12c or 12d, low levels of the dipeptides were also detected. Caco-2 cell monolayers treated with D-Mel-L-Glu (11d) or D-Mel-L-Val (13) showed very low levels of the amino acids (L-Mel or D-Mel), but generally higher levels of the dipeptides. In contrast to the amino acids (L-Mel, D-Mel) or the dipeptide derivatives (11c-d, 12c-d and 13), the ester derivatives of the amino acids [L-Mel(OEt), D-Mel(OEt)] or the dipeptides (11a-b, 12a-b and 14) produced 5-20 times higher intracellular concentrations of potentially cytotoxic metabolites (e.g., L-Mel, D-Mel, Mel-containing dipeptides or Mel-containing dipeptide monoesters). L-Mel(OEt), D-Mel(OEt), L-Glu(OEt)-L-Mel(OEt) (12a), L-Glu(OEt)-D-Mel(OEt) (12b), and L-Mel-L-Glu(OEt)2 (11a) accumulated mainly as either L-Mel or D-Mel, and the percentages of L-Mel or D-Mel were 99%, 99%, 90%, 75% and 98% of the total intracellular concentration of potentially cytotoxic agents, respectively. D-Mel-L-Glu(OEt)2 (11b) accumulated as its monoester (> 95%) and D-Mel-L-Val(OMe) (14) accumulated as its dipeptide metabolite (> 98%). Inclusion of Gly-Pro, carnosine, L-Phe or L-Glu did not inhibit uptake of the dipeptide derivatives of L-Mel (or D-Mel) or their esters. These results suggest that the cellular uptake of the dipeptide derivatives of melphalan and their esters is probably via passive diffusion rather than being facilitated by an amino acid transporter or a di/tripeptide transporter. The higher intracellular levels of cytotoxic agents generated from the ester derivatives of the amino acids and the dipeptides are probably due to their higher lipophilicity and the overall neutral charge of the esters and subsequent intracellular formation of the more polar amino acids (L- or D-Mel) and/or Mel-containing dipeptides. Finally, these studies suggest that dipeptides of D-Mel [11b, 11d, 13] have inherent cytotoxicity properties.

HT29-MTX/Caco-2 cocultures as an in vitro model for the intestinal epithelium: in vitro-in vivo correlation with permeability data from rats and humans

The diverse secretory and absorptive functions of the intestinal epithelium are conducted by a mixed population of absorptive cells and mucus-producing goblet cells as the major cell types. In order to approach the main characteristics in an in vitro model, a coculture system of absorptive Caco-2 cells and mucus-secreting HT29-MTX cells was developed and the permeability of a range of different drugs was tested. Variable goblet cell frequency can be achieved, preserving a significant barrier to drug transport and maintaining the differentiated features of both cell types. Absorption rates for actively transported drugs are rather underestimated in the cell culture model when compared to in vivo data. However, a good correlation with fraction absorbed in humans was attained separating the range of passively transported drugs into two groups of well-absorbable compounds with Peff > or = 10 x 10(-6) cm/s and drugs that are absorbed 40-70% with Peff = 0.1-1 x 10(-5) cm/s. A permeability of Peff < 0.1 x 10(-5) cm/s is suggested for low absorbable drugs.

Interactions of cephradine and cefaclor with the intestinal absorption of D-galactose

Oral cephalosporins are frequently prescribed beta-lactam antibiotics. Although it has been well established that cephalosporins compete with dipeptides for absorption in the intestine, using the same transport mechanism, little is known about the action of the drugs on the absorption of other nutrients. In this work the effect of cephradine and cefaclor on the absorption of D-galactose has been studied. Intestinal sugar uptake was measured in-vitro in pieces of intestine (50 mg) and brush-border membrane vesicles, and in-vivo in intestinal loops. Galactose uptake was inhibited by cephalosporins in a dose-related, time-dependent manner. In-vivo the inhibition appeared when the antibiotics were on the luminal side of the enterocyte and when they reached the gut from the basolateral side. Only the active transport of the sugar was modified; passive transfer did not change in the presence of cephalosporins. In brush-border membrane vesicles, cephradine and cefaclor did not alter sugar uptake in either sodium or potassium gradients. Both antibiotics non-competitively inhibited basolateral Na+,K(+)-ATPase activity. These findings show that cephradine and cefaclor inhibit the active-transport component of galactose absorption because they reduce the activity of the basolateral Na+,K(+)-ATPase.

Molecular modeling study of structural requirements for the oligopeptide transporter

The intestinal oligopeptide transporter (OPT) mediates the absorption of di-/tripeptides, beta-lactam antibiotics, angiotensin converting enzyme (ACE) inhibitors and renin inhibitors. This suggests that the targeting of molecules to this transporter could result in orally-absorbed drugs. Results from a recent study with renal brush border membrane vesicles (BBMV) suggested that an alpha-NH2 group is required for interaction with the renal OPT. In general, structural requirements for interaction with the renal and intestinal OPT are similar. However, these recent findings do not agree with earlier studies, which showed that an alpha-NH2 group is not essential for interaction with the intestinal OPT. Thus, it appears that the renal and intestinal OPT may differ in their recognition of compounds containing an alpha-NH2 group. In this study, molecular modeling was used to determine the tridimensional structures of various cephalosporins for which Ki values had been determined using renal BBMV. All cephalosporins which interact with the OPT have two, energetically equivalent, conformations. Most compounds which do not interact with the OPT cannot adopt the two conformations. A key factor which influences the conformation seems to be the substituent group at the alpha position; an electron drawing group at that position alters the common conformations. For the OPT substrates, the distances between the -NH2 and -COOH groups are comparable to those of the tripeptide, GlyGlyGly; and the distances between -NH2 and carbonyl group in the beta-lactam ring are close to the distance between N-terminal and C-terminal in the dipeptide, GlyGly. The corresponding distances in cephamycin C (in which a -NH2 group is located in a different position) and the tetrapeptide, GlyGlyGlyGly, are longer than those in alpha-NH2 cephalosporins and GlyGlyGly. Cephamycin C and the tetrapeptide have low affinity for the renal OPT, suggesting that the distances between functional groups are critical for affinity. The alpha-NH2 group had no effect on the conformations of the molecules. We concluded that the alpha-NH2 group may interact directly with the renal oligopeptide transporter. Whether this is unique to the renal transporter or could be applied to the intestinal transporter will require further investigation.

Stereoselective uptake of beta-lactam antibiotics by the intestinal peptide transporter

1. The stereoselective transport of beta-lactam antibiotics has been investigated in the human intestinal epithelial cell line, Caco-2, by use of D- and L-enantiomers of cephalexin and loracarbef as substrates. 2. The L-isomers of cephalexin, loracarbef and dipeptides displayed a higher affinity for the oligopeptide/H(+)-symporter in Caco-2 cells than the D-isomers. This was demonstrated by inhibition of the influx of the beta-lactam, [3H]-cefadroxil. 3. By measurement of the substrate-induced intracellular acidification in Caco-2 cells loaded with the pH-sensitive fluorescent dye BCECF (2',7'-bis(2-carboxyethyl)-5-(6)-carboxy-fluorescein), it was demonstrated for the first time that L-isomers of beta-lactams not only bind to the peptide transporter with high affinity but are indeed transported. 4. Efficient proton-coupled transport of L-beta-lactam antibiotics was also shown to occur in Xenopus laevis oocytes expressing the cloned peptide transporter PepT1 from rabbit small intestine. 5. Both cell systems therefore express a stereoselective transport pathway for beta-lactam antibiotics with very similar characteristics and may prove useful for screening rapidly the oral availability of peptide-derived drugs.

The influence of uptake from the gastrointestinal tract and first-pass effect on oral bioavailability of (Z)-alkyloxyimino penicillins

We have investigated the contribution of uptake from the gastrointestinal tract and first-pass effect to the poor oral bioavailability of a series of (Z)-alkyloxyimino penicillins in mice. Investigative studies in gut sacs and perfused small intestine demonstrated that these penicillins were able to pass across the mucosal epithelium although to a lesser extent than amoxycillin and cyclacillin, both of which exhibit excellent oral bioavailability in man and animals. In the jejunal gut sacs the mucosal to serosal flux for BRL 44154 was approximately half that of amoxycillin and four times less than that of cyclacillin, and for all, uptake was pH dependent. The serosal to mucosal fluxes were however similar for these compounds and significantly lower than mucosal to serosal fluxes, suggesting involvement of carrier mechanisms in uptake from the mucosal surface. The order of results for the alkyloxyimino penicillins paralleled that observed for oral bioavailability in the mouse. For the alkyloxyimino penicillins, between 5.5 and 9.9% was taken up from the perfused intestine, values which were significantly less than those for amoxycillin (13.2%) and cyclacillin (33.3%). However, uptake was concentration-dependent for BRL 44154 as it was for amoxycillin, thus confirming the possible use of carrier mechanisms in absorption. These observations suggest that the poor peripheral blood concentrations of the alkyloxyimino penicillins achieved after oral dosing were not a consequence of the inability of the compounds to cross the mucosal epithelium. The biliary clearance of the alkyloxyimino penicillins was, however, considerably greater than for amoxycillin and cyclacillin, a finding which may well have been a contributory factor to the comparatively low peripheral concentrations of BRL 44154 and its analogues achieved after oral administration.

Mechanisms of transport of quinapril in Caco-2 cell monolayers: comparison with cephalexin

PURPOSE

To determine the transport mechanisms of quinapril and cephalexin in Caco-2 cell monolayers, a cell culture model of the human small intestinal epithelium.

METHODS

Uptake, transepithelial transport and intracellular accumulations of these two drugs were measured using Caco-2 cell monolayers grown onto Millicells and magnetically stirred diffusion chambers.

RESULTS

Transepithelial transport, apical (AP)4 uptake and intracellular accumulation of both drugs depended on the maintenance of a transepithelial proton gradient and temperature of the medium. However, quinapril transport and accumulation, which did not display a maximum at approximately pH 6, was more sensitive to proton gradient change, whereas cephalexin transport was more sensitive to concentration change (range 0.5-5 mM). In addition, quinapril (1 mM) transport was decreased significantly (p < 0.05) by 10 mM cephalexin, loracarbef, Gly-Pro and Phe-Pro, but not by enalapril; whereas cephalexin (0.1 mM) transport was decreased significantly (p < 0.05) by all four compounds. Similarly, AP quinapril (1 mM) uptake was also decreased by 10 mM loracarbef, Gly-Pro, cephalexin, and enalapril, but these inhibitory effects (20-50%) were quantitatively less than their inhibitory effects on cephalexin uptake (50-90%). Finally, the AP uptake of quinapril was also significantly (p < 0.05) inhibited by FCCP (10 micrograms/ml), amiloride (0.5 mM), DEP (0.5 mM), and staurosporine (5 nM).

CONCLUSIONS

The transport of quinapril in the Caco-2 cells is via a combination of the carrier-mediated proton gradient-dependent peptide transporter and passive diffusion.

The human intestinal epithelial cell line Caco-2; pharmacological and pharmacokinetic applications

The gastrointestinal tract remains the most popular and acceptable route of administration for drugs. It offers the great advantage of convenience and many compounds are well absorbed and thereby provide acceptable plasma concentration-time profiles. Currently there is considerable interest from the pharmaceutical industry in development of cell culture systems that would mimic the intestinal mucosa in order to evaluate strategies for investigating and/or enhancing drug absorption. The intestinal epithelial cells of primary interest, from the standpoint of drug absorption and metabolism, are the villus cells, which are fully differentiated cells. An in vitro cell culture system consisting of a monolayer of viable, polarized and fully differentiated villus cells, similar to that found in the small intestine, would be a valuable tool in the study of drug and nutrient transport and metabolism. The Caco-2 cell line, which exhibits a well-differentiated brush border on the apical surface and tight junctions, and expresses typical small-intestinal microvillus hydrolases and nutrient transporters, has proven to be the most popular in vitro model (a) to rapidly assess the cellular permeability of potential drug candidates, (b) to elucidate pathways of drug transport (e.g., passive versus carrier mediated), (c) to assess formulation strategies designed to enhance membrane permeability, (d) to determine the optimal physicochemical characteristics for passive diffusion of drugs, and (e) to assess potential toxic effects of drug candidates or formulation components on this biological barrier. Since differentiated Caco-2 cells express various cytochrome P450 isoforms and phase II enzymes such as UDP-glucuronosyltransferases, sulfotransferases and glutathione-S-transferases, this model could also allow the study of presystemic drug metabolism.

Molecular mechanism for the relative binding affinity to the intestinal peptide carrier. Comparison of three ACE-inhibitors: enalapril, enalaprilat, and lisinopril

The affinity of three substrates for the intestinal peptide carrier is explained based on their three-dimensional (3D) structural data. The kinetic transport parameters of three ACE-inhibitors, enalapril, enalaprilat, and lisinopril, have been determined in an in vivo system using rat intestine. The observed kinetic transport parameters (+/- asymptotic standard error) of enalapril are: 0.81 (+/- 0.23) mM, 0.58 (+/- 0.37) mumol/h per cm2, and 0.56 (+/- 0.04) cm/h for the half-maximal transport concentration (KT), the maximal transport flux (Jmax) and the passive permeability constant (Pm). Enalaprilat was transported by passive diffusional with a Pm of 0.51 (+/- 0.04) cm/h. For lisinopril the kinetic transport parameters were 0.38 (+/- 0.19) mM, 0.12 (+/- 0.07) mumol/h per cm2, and 0.18 (+/- 0.02) cm/h for KT, Jmax, and Pm, respectively. The affinity of the ACE-inhibitors for the intestinal peptide carrier has been evaluated based on their ability to inhibit the transport rate of cephalexin. The inhibition constants (Ki) of enalapril, enalaprilat and lisinopril were 0.15, 0.28 and 0.39 mM, respectively. 3D structural analysis of lisinopril using molecular modelling techniques reveals that intramolecular hydrogen bond formation is responsible for decreased carrier affinity.

Diastereomeric beta-lactam antibiotics. Analytical methods, isomerization and stereoselective pharmacokinetics

Stereospecific HPLC methods for the determination of various diastereomeric beta-lactam antibiotics are reviewed. Stereoselectivity in the absorption, distribution and excretion of several diastereomeric beta-lactams is summarized. The isomerization of beta-lactam isomers and its influence on the pharmacokinetics and pharmacodynamics are discussed.

Characteristics of ceftibuten uptake into Caco-2 cells

The characteristics of ceftibuten uptake into Caco-2 cells grown in a collagen-coated dish were examined. Ceftibuten showed stereoselective and pH-dependent uptake. The pH-dependency of ceftibuten was more marked than that of cefaclor or cephalexin, but all three antibiotics showed maximal uptake at pH 5.5. Ceftibuten uptake was linear for the initial 1 hr and then reached a plateau. The initial uptake (15 min) was markedly reduced by the addition of 2,4-dinitrophenol or FCCP (a protonophore), or by lowering the incubation temperature. The uptake of ceftibuten into the brush-border membrane vesicles prepared from cultured Caco-2 cells showed an overshoot in the presence of an H(+)-gradient. These findings indicated that the uptake of ceftibuten was energy-dependent, especially H(+)-gradient-dependent. Uptake inhibition by various compounds was compared using Caco-2 cells. Amino acids and a tetrapeptide did not inhibit uptake, whereas di- or tri-peptides were effective inhibitors. These observations suggest that ceftibuten is taken up by a carrier-mediated transport system(s) for dipeptides. Various antibiotics differed in their ability to inhibit uptake, with cyclacillin showing maximum inhibition. Differences in the inhibitory effect may be accounted for by the heterogeneity (multiplicity) of the transport systems.

Expression cloning of a cDNA from rabbit small intestine related to proton-coupled transport of peptides, beta-lactam antibiotics and ACE-inhibitors

Injection of poly(A)+ RNA from rabbit small intestine into Xenopus laevis oocytes resulted in expression of pH dependent transport of the aminocephalosporin cefadroxil. A cDNA library constructed from a 2.2 to 5 kb fraction was screened for expression of cefadroxil transport after injection of the corresponding cRNA synthetized in vitro from different pools of clones. The single clone identified stimulated uptake of cefadroxil into oocytes about 50-fold at pH 6.5. Kinetic analysis of expressed transport activity revealed a saturable transport system shared by amino beta-lactam antibiotics, dipeptides and selected angiotensin converting enzyme inhibitors. Evidence for rheogenic cefadroxil/H(+)-cotransport was obtained by a) The demonstration that cefadroxil influx increased the inward current in oocytes clamped at a holding potential of -60 mV in sodium-free medium and b) A decrease of intracellular pH in oocytes caused by cefadroxil uptake. Current-voltage relationships in the presence of glycylsarcosine or cefadroxil showed that transport activity is dependent on the membrane potential. Sequencing of the cDNA revealed its identity with the recently cloned peptide transporter from rabbit small intestine designated PepT1.

D-cycloserine uses an active transport mechanism in the human intestinal cell line Caco 2

In a previous study we have shown that cultured epithelial cell lines can be used to measure the transepithelial passage of antimicrobial agents across the intestine and to obtain information on the mechanisms of transport utilized and predict the bioavailability of the antimicrobial agents after oral administration. In particular, among the drugs investigated, D-cycloserine had been shown to be transported in a polarized manner only in the intestinal cells. In the present work, further characterization of the transport of D-cycloserine in the human intestinal cell line Caco 2 has shown that this occurs in the apical-to-basolateral direction by an active mechanism which is energy dependent but only partially sodium dependent. Competition studies have also indicated that the transport of D-cycloserine occurs via a carrier for imino acids, amino acids with aliphatic side chains (L-Ala, D-Ala, and beta Ala), and L-Trp, L-Tyr, L-Cys, and alpha-amino isobutyric acid. This system may correspond to a proton-dependent system for L-proline and beta-alanine recently described for Caco 2 cells. In contrast with the cephalosporins, which are taken up by the Caco 2 cells via a dipeptide carrier, D-cycloserine transport cannot be inhibited by either cephalexin (a member of the class of cephalosporins) or dipeptides.

Transport mechanisms responsible for the absorption of loracarbef, cefixime, and cefuroxime axetil into human intestinal Caco-2 cells

Loracarbef, cefixime and cefuroxime axetil are beta-lactam antibiotics that are administered orally. Oral absorption of loracarbef is nearly complete, while that of cefixime and cefuroxime axetil is 30-50%. To investigate this we used the human intestinal cell line Caco-2 that possesses the proton-dependent peptide transporter that takes up cephalexin and cefaclor. Drug uptake was measured at pH 6 by high performance liquid chromatography or with radioactively labelled drug. The initial uptake rate of 1 mM cefixime was lower than that of 1 mM loracarbef. By 2 h both drugs were concentrated intracellularly against a gradient; however, the accumulation of cefixime was only 40% of that of loracarbef. The uptake rate of both drugs was sodium-independent, temperature- and energy-dependent, and was inhibited by dipeptides, cephalexin, cefaclor, but not by amino acids. Kinetic analysis of the concentration-dependence of the uptake rates for loracarbef and cefixime indicated that diffusion and a single transport system were responsible for uptake. The kinetic parameters for loracarbef and cefixime, respectively, were: Km values of 8 and 17 mM and Vmax values of 6.5 and 2 nmol/min per mg protein. Loracarbef and cefixime were competitive inhibitors of each other's uptake. By contrast, cefuroxime axetil was taken up and rapidly hydrolyzed to cefuroxime by Caco-2 cells. Cefuroxime axetil uptake was not dependent on energy and was not affected by dipeptides. Thus, cefuroxime axetil apparently enters Caco-2 cells by simple diffusion. By contrast, loracarbef and cefixime share a common transport mechanism, the proton-dependent dipeptide transporter. Cefixime was taken up less well than loracarbef due to a substantial reduction in the turnover rate and decreased affinity of the transporter for cefixime.

Association of intestinal peptide transport with a protein related to the cadherin superfamily

The first step in oral absorption of many medically important peptide-based drugs is mediated by an intestinal proton-dependent peptide transporter. This transporter facilitates the oral absorption of beta-lactam antibiotics and angiotensin-converting enzyme inhibitors from the intestine into enterocytes lining the luminal wall. A monoclonal antibody that blocked uptake of cephalexin was used to identify and clone a gene that encodes an approximately 92-kilodalton membrane protein that was associated with the acquisition of peptide transport activity by transport-deficient cells. The amino acid sequence deduced from the complementary DNA sequence of the cloned gene indicated that this transport-associated protein shares several conserved structural elements with the cadherin superfamily of calcium-dependent, cell-cell adhesion proteins.

Expression and protein kinase C-dependent regulation of peptide/H+ co-transport system in the Caco-2 human colon carcinoma cell line

The characteristics of the transport of the dipeptide glycylsarcosine were studied in the human colon carcinoma cell line Caco-2 grown as a monolayer on impermeable plastic support. Transport of glycylsarcosine in these cells was found to be Na(+)-independent, but was stimulated by an inwardly directed H+ gradient. This H(+)-dependent transport of glycylsarcosine was inhibited by di- and tri-peptides and also by the beta-lactam antibiotic cephalexin, but was unaffected by the amino acids glycine and leucine. The transport system exhibited a Michaelis-Menten constant (Kt) of 1.1 +/- 0.1 mM for glycylsarcosine. The specific activity of the transport system in this cell line was found to be maximal when the cultures were confluent. Treatment of the cells with phorbol esters which activate protein kinase C resulted in a significant inhibition of the transport system. This inhibition was specific and could be blocked if treatment was done in the presence of staurosporine, an inhibitor of protein kinase C. Kinetic analysis revealed that the inhibition was associated with a decrease in the maximal velocity, the Kt remaining unaffected. The phorbol-ester-induced inhibition of the peptide-transport system was not prevented by co-treatment with cycloheximide, an inhibitor of cellular protein synthesis. In addition, there was no change in the intracellular pH following treatment with the phorbol ester, suggesting that the effect was not due to alterations in the transmembrane pH gradient. It is concluded that the peptide/H+ co-transport system, which is known to exist in the normal intestine, is expressed in Caco-2 cells and that the function of the transport system is under the regulatory control of protein kinase C.