Rhodamine 123 requires carrier-mediated influx for its activity as a P-glycoprotein substrate in Caco-2 cells

Thiomer-coated liposomes harbor permeation enhancing and efflux pump inhibitory properties

An ideal oral drug carrier should facilitate drug delivery to the gastrointestinal tract and its absorption into the systemic circulation. To meet these requirements, we developed a thiomer-coated liposomal delivery system composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a maleimide-functionalized lipid, to which chitosan-thioglycolic acid (CS-TGA) was covalently coupled. In addition to conventional 77 kDa CS-TGA (CS-TGA77), we tested the 150 kDa homologue (CS-TGA150) as well as an S-protected version of this polymer (CS-TGA150-MNA), in which some of the free SH-groups are conjugated with 6-mercaptonicotinamide to protect them from oxidation. Coupling of CS-TGA to the liposomal surface led to an increase in the particle size of at least 150 nm and an increase in the zeta potential from approximately − 33 mV to a maximum of about + 36 mV, depending on the polymer. As revealed by fluorescence dequenching the formulations have a storage stability of at least two weeks without releasing any encapsulated compounds. In simulated gastric fluid, the system was shown to be stable over 24 h, while in simulated intestinal fluid, a slow, sustained release of encapsulated compounds was observed. According to our experiments, thiomer-coated liposomes did not induce immunogenic reactions after an oral administration to mice. To evaluate the permeation enhancing and efflux pump inhibiting properties of CS-TGA coated liposomes we monitored the transport of fluoresceinisothiocyanate-dextran (FD₄) and rhodamine-123 (Rho-123), respectively, through rat small intestine. Permeation studies showed a 2.8-fold higher permeation of FD₄ in the presence of CS-TGA77 coated liposomes and an even 4-fold higher permeation in the presence of CSA-TGA150-MNA coated liposomes. The latter also performed best when we evaluated P-glycoprotein inhibiting properties by monitoring the transport of Rho-123, revealing a 4.2-fold enhancement respective to the buffer control. Taken together, thiomer-coated liposomes were shown to protect encapsulated drugs in the stomach, slowly release them in the small intestine and enhance their absorption through the intestinal tissue by opening tight junctions and inhibiting efflux pumps.

Thiorhodamines containing amide and thioamide functionality as inhibitors of the ATP-binding cassette drug transporter P-glycoprotein (ABCB1)

Twelve thiorhodamine derivatives have been examined for their ability to stimulate the ATPase activity of purified human P-glycoprotein (P-gp)-His(10), to promote uptake of calcein AM and vinblastine into multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells, and for their rates of transport in monolayers of multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells. The thiorhodamine derivatives have structural diversity from amide and thioamide functionality (N,N-diethyl and N-piperidyl) at the 5-position of a 2-thienyl substituent on the thiorhodamine core and from diversity at the 3-amino substituent with N,N-dimethylamino, fused azadecalin (julolidyl), and fused N-methylcyclohexylamine (half-julolidyl) substituents. The julolidyl and half-julolidyl derivatives were more effective inhibitors of P-gp than the dimethylamino analogues. Amide-containing derivatives were transported much more rapidly than thioamide-containing derivatives.

S-protected thiolated chitosan for oral delivery of hydrophilic macromolecules: evaluation of permeation enhancing and efflux pump inhibitory properties

The objective of this study was the investigation of permeation enhancing and P-glycoprotein (P-gp) inhibition effects of a novel thiolated chitosan, the so-named S-protected thiolated chitosan. Mediated by a carbodiimide, increasing amounts of thioglycolic acid (TGA) were covalently bound to chitosan (CS) in the first step of modification. In the second step, these thiol groups of thiolated chitosan were protected by disulfide bond formation with the thiolated aromatic residue 6-mercaptonicotinamide (6-MNA). Mucoadhesive properties of all conjugates were evaluated in vitro on porcine intestinal mucosa based on tensile strength investigations. Permeation enhancing effects were evaluated ex vivo using rat intestinal mucosa and in vitro via Caco-2 cells using the hydrophilic macromolecule FD(4) as the model drug. Caco-2 cells were further used to show P-gp inhibition effects by using Rho-123 as P-gp substrate. Apparent permeability coefficients (P(app)) were calculated and compared to values obtained from each buffer control. Three different thiolated chitosans were generated in the first step of modification, which displayed increasing amounts of covalently attached free thiol groups on the polymer backbone. In the second modification step, more than 50% of these free thiol groups were covalently linked with 6-MNA. Within 3 h of permeation studies on excised rat intestine, P(app) values of all S-protected chitosans were at least 1.3-fold higher compared to those of corresponding thiomers and more than twice as high as that of unmodified chitosan. Additional permeation studies on Caco-2 cells confirmed these results. Because of the chemical modification and higher amount of reactive thiol groups, all S-protected thiolated chitosans exhibit at least 1.4-fold pronounced P-gp inhibition effects in contrast to their corresponding thiomers. These features approve S-protected thiolated chitosan as a promising excipient for various drug delivery systems providing improved permeation enhancing and efflux inhibition effects.

Characterization of rhodamine-123 as a tracer dye for use in in vitro drug transport assays

Fluorescent tracer dyes represent an important class of sub-cellular probes and allow the examination of cellular processes in real-time with minimal impact upon these processes. Such tracer dyes are becoming increasingly used for the examination of membrane transport processes, as they are easy-to-use, cost effective probe substrates for a number of membrane protein transporters. Rhodamine 123, a member of the rhodamine family of flurone dyes, has been used to examine membrane transport by the ABCB1 gene product, MDR1. MDR1 is viewed as the archetypal drug transport protein, and is able to efflux a large number of clinically relevant drugs. In addition, ectopic activity of MDR1 has been associated with the development of multiple drug resistance phenotype, which results in a poor patient response to therapeutic intervention. It is thus important to be able to examine the potential for novel compounds to be MDR1 substrates. Given the increasing use rhodamine 123 as a tracer dye for MDR1, a full characterisation of its spectral properties in a range of in vitro assay-relevant media is warranted. Herein, we determine λmax for excitation and emission or rhodamine 123 and its metabolite rhodamine 110 in commonly used solvents and extraction buffers, demonstrating that fluorescence is highly dependent on the chemical environment: Optimal parameters are 1% (v/v) methanol in HBSS, with λex = 505 nm, λem = 525 nm. We characterise the uptake of rhodamine 123 into cells, via both passive and active processes, and demonstrate that this occurs primarily through OATP1A2-mediated facilitated transport at concentrations below 2 µM, and via micelle-mediated passive diffusion above this. Finally, we quantify the intracellular sequestration and metabolism of rhodamine 123, demonstrating that these are both cell line-dependent factors that may influence the interpretation of transport assays.

Effect of pluronic P123 and F127 block copolymer on P-glycoprotein transport and CYP3A metabolism

The aim of the present study was to evaluate the effect of pluronic P123 (P123) and pluronic F127 (F127) on intestinal P-glycoprotein (P-gp) and cytochrome P450 3A using the specific substrates rhodamine-123 (R-123) and midazolam, respectively. Caco-2 cells and everted gut sacs were used as models of intestinal mucosa to assess intestinal absorption of R-123, while rat intestinal microsomes were utilized to examine the effect of P123 and F127 on in vitro midazolam metabolism. P123 and F127 were observed to increase the intracellular accumulation of R-123 in Caco-2 cells in a dose-dependent manner. P123 significantly lowered the efflux ratio of R-123 at two concentrations in Caco-2 monolayers, whereas F127 lowered the efflux ratio only at 1%. Moreover, both pluronics markedly enhanced mucosal to serosal absorption of R-123 in excised ileum of rats. However, no significant difference in relative enzyme activity were observed between P123- or F127-treated and control groups, regardless of the concentrations of P123 and F127 studied. Collectively, these results obtained from the present study demonstrated that P123 and F127 were capable of inhibiting the intestinal P-gp activity, but had little or no effect on intestinal cytochrome P450 3A activity, indicating that P123 and F127 can potentially be used as pharmaceutical ingredients to improve the oral bioavailability of coadministered P-gp substrates via P-gp efflux pump inhibition.

Interactions of dietary phytochemicals with ABC transporters: possible implications for drug disposition and multidrug resistance in cancer

Common foods, such as fruits and vegetables, contain a large variety of secondary metabolites known as phytochemicals, many of which have been associated with health benefits. However, there is a limited knowledge of the processes by which these, mainly charged, phytochemicals (and/or their metabolites) are absorbed into the body, reach their biological target, and how they are eliminated. Recent studies have indicated that some of these phytochemicals are substrates and modulators of specific members of the superfamily of ABC transporting proteins. In this review, we present the reported interactions between the different classes of phytochemicals and ABC transporters and the mechanism by which they modulate the activity of these transporters. We also discuss the implications that such interactions may have on the pharmacokinetics of xenobiotics and the possible role of phytochemicals in the reversal of multidrug resistance in cancer chemotherapy.

The physiological performance of a three-dimensional model that mimics the microenvironment of the small intestine

Our focus was to develop a three-dimensional (3D) human dynamic in vitro tissue model that mimics the natural microenvironment of the small intestine. We co-cultured human Caco-2 cells with primary-isolated human microvascular endothelial cells (hMECs) on decellularized porcine jejunal segments within a custom-made dynamic bioreactor system that resembles the apical and basolateral side of the intestine for up to 14 days. The obtained data were compared to results generated using routine static Caco-2 assays. We performed histology and immunohistochemistry. Permeability was measured using directed transport studies. Histological analyses revealed that in tissue-engineered segments, which had been cultured under dynamic conditions, the Caco-2 cells showed a high-prismatic morphology, resembling normal primary enterocytes within their native environment. We further identified that the transport of low permeable substances, such as fluorescein and desmopressin increased within the dynamic bioreactor cultures. Immunohistochemical staining showed a significantly higher expression of the efflux transport p-glycoprotein (p-gp) under dynamic culture conditions when compared to the static cultures. We conclude that the integration of physiological parameters is crucial for the establishment of a reliable 3D intestinal in vitro model, which enables the simulation of drug transport over the gut-blood-barrier in a simplified way.

Expression and functional activity of P-glycoprotein in passaged primary human nasal epithelial cell monolayers cultured by the air-liquid interface method for nasal drug transport study

OBJECTIVES

P-glycoprotein (P-gp) is an efflux transporter encoded by the multidrug resistance gene (MDR1), which is also known as the human ABCB1 gene (ATP-binding cassette, subfamily-B). The objectives of this study were to investigate the expression of P-gp in passaged primary human nasal epithelial (HNE) cell monolayer, cultured by the air-liquid interface (ALI) method, and to evaluate its feasibility as an in-vitro model for cellular uptake and transport studies of P-gp substrates.

METHODS

Reverse transcriptase-polymerase chain reaction (RT-PCR) was performed to verify the expression of the MDR1 gene. Transport and cellular uptake studies with P-gp substrate (rhodamine123) and P-gp inhibitors (verapamil and cyclosporin A) were conducted to assess the functional activity of P-gp in HNE cell monolayers cultured by the ALI method.

KEY FINDINGS

MDR1 gene expression in primary HNE cell monolayers cultured by ALI method was confirmed by RT-PCR. The apparent permeability coefficient (P(app) ) of the P-gp substrate (rhodamine123) in the basolateral to apical (B to A) direction was 6.9 times higher than that in the apical to basolateral (A to B) direction. B to A transport was saturated at high rhodamine123 concentration, and the treatment of P-gp inhibitors increased cellular uptake of rhodamine123 in a time- and concentration-dependent manner.

CONCLUSIONS

These results support the MDR1 gene expression and the functional activity of P-gp in primary HNE cell monolayers cultured by the ALI method.

Positively-charged microemulsion for improving the oral bioavailability of alendronate: in-vitro and in-vivo assessment

Objectives

Alendronate is a poorly absorbed bisphosphonate with an oral bioavailability of 0.7%. In this study, a positively-charged microemulsion was prepared with the aim of improving the bioavailability of alendronate.

Methods

The positively-charged microemulsion was evaluated for physical stability, cellular uptake and permeability enhancement on Caco-2 monolayers. The bioavailability of alendronate from the microemulsion was compared with the commercially available tablet (Fosmax) for beagle dogs.

Key findings

The 2.0, 0.4 and 0.2% positively-charged microemulsion, stable for 4 h after preparation, promoted alendronate transport across the Caco-2 cells by a factor of 194, 146,and 45.1, respectively, compared with the alendronate solution, though no significant cellular uptake enhancement of alendronate was observed. The permeability enhancement was parallel to the reduction in transendothelial electrical resistance, which indicated the microemulsion modulated the tight junctions and widened the paracellular pathway. In-vivo results showed that the microemulsion gave the highest alendronate plasma concentration at 502 ng/ml (Cmax) after 0.563 h (Tmax), while tablets gave a Cmax of 152 ng/ml after 0.750 h (Tmax). Furthermore, the AUC0-∞ of alendronate from the microemulsion increased by 2.82-fold when compared with the tablets.

Conclusions

Based on the results, the oral bioavailability of alendronate could be significantly improved by the positively-charged microemulsion, which opened the tight junctions and thus increased absorption through the paracellular route.

Vitamin E TPGS P-glycoprotein inhibition mechanism: influence on conformational flexibility, intracellular ATP levels, and role of time and site of access

Previous work conducted in our laboratories established the notion that TPGS 1000 (d-alpha-tocopheryl polyethylene glycol 1000 succinate), a nonionic surfactant, modulates P-glycoprotein (P-gp) efflux transport via P-gp ATPase inhibition. The current in vitro research using Caco-2 cells was conducted to further explore the P-gp ATPase inhibition mechanism. Using a monoclonal CD243 P-gp antibody shift assay (UIC2), we probed P-gp conformational changes induced via TPGS 1000. In the presence of TPGS 1000, UIC2 binding was slightly decreased. TPGS 1000 does not appear to be a P-gp substrate, nor does it function as a competitive inhibitor in P-gp substrate efflux transport. The reduction in UIC2 binding with TPGS 1000 was markedly weaker than with orthovanadate, data ruling out trapping P-gp in a transition state by direct interaction with one or both of the P-gp ATP nucleotide binding domains. An intracellular ATP depletion mechanism could be ruled out in the UIC2 assay, and by monitoring intracellular ATP levels in the presence of TPGS 1000. Indicating slow distribution of TPGS 1000 into the membrane, and in agreement with an intramembranal or intracellular side of action, Caco-2 cell monolayer experiments preincubated with TPGS 1000 produce stronger substrate inhibitory activity than those conducted by direct substrate and surfactant coapplication.

Effect of HEPES buffer on the uptake and transport of P-glycoprotein substrates and large neutral amino acids

HEPES has been widely employed as an organic buffer agent in cell culture medium as well as uptake and transport experiments in vitro. However, concentrations of HEPES used in such studies vary from one laboratory to another. In this study, we investigated the effect of HEPES on the uptake and bidirectional transport of P-gp substrates employing both Caco-2 and MDCK-MDR1 cells. ATP-dependent uptake of glutamic acid was also examined. ATP production was further quantified applying ATP Determination Kit. An addition of HEPES to the growth and incubation media significantly altered the uptake and transport of P-gp substrates in both Caco-2 and MDCK-MDR1 cells. Uptake of P-gp substrates substantially diminished as the HEPES concentration was raised to 25 mM. Bidirectional (A-B and B-A) transport studies revealed that permeability ratio of P(appB-A) to P(appA-B) in the presence of 25 mM HEPES was significantly higher than control. The uptake of phenylalanine is an ATP-independent process, whereas the accumulation of glutamic acid is ATP-dependent. While phenylalanine uptake remained unchanged, glutamic acid uptake was elevated with the addition of HEPES. Verapamil is an inhibitor of P-gp mediated uptake; elevation of cyclosporine uptake in the presence of 5 muM verapamil was compromised by the presence of 25 mM HEPES. The results of ATP assay indicated that HEPES stimulated the production of ATP. This study suggests that the addition of HEPES in the medium modulated the energy dependent efflux and uptake processes. The effect of HEPES on P-gp mediated drug efflux and transport may provide some mechanistic insight into possible reasons for inconsistencies in the results reported from various laboratories.

Refining the in vitro and in vivo critical parameters for P-glycoprotein, [I]/IC50 and [I2]/IC50, that allow for the exclusion of drug candidates from clinical digoxin interaction studies

The objective of this work was to further investigate the reasons for disconcordant clinical digoxin drug interactions (DDIs) particularly for false negative where in vitro data suggests no P-glycoprotein (P-gp) related DDI but a clinically relevant DDI is evident. Applying statistical analyses of binary classification and receiver operating characteristic (ROC), revised cutoff values for ratio of [I]/IC(50) < 0.1 and [I(2)]/IC(50) < 5 were identified to minimize the error rate, a reduction of false negative rate to 9% from 36% (based on individual ratios). The steady state total C(max) at highest dose of the inhibitor is defined as [I] and the ratio of the nominal maximal gastrointestinal concentration determined for highest dose per 250 mL volume defined [I(2)](.) We also investigated the reliability of the clinical data to see if recommendations can be made on values that would allow predictions of 25% change in digoxin exposure. The literature derived clinical digoxin interaction studies were statistically powered to detect relevant changes in exposure associated with digitalis toxicities. Our analysis identified that many co-meds administered with digoxin are cardiovascular (CV) agents. Moreover, our investigations also suggest that the presence of CV agents may alter cardiac output and/or kidney function that may act alone or are additional components to enhance digoxin exposure along with P-gp interaction. While we recommend digoxin as the probe substrate to define P-gp inhibitory potency for clinical assessment, we observed high concordance in P-gp inhibitory potency for calcein AM as a probe substrate.

Modulating effect of polyethylene glycol on the intestinal transport and absorption of prednisolone, methylprednisolone and quinidine in rats by in-vitro and in-situ absorption studies

The effects of polyethylene glycol 20000 (PEG 20000) on the intestinal absorption of prednisolone, methylprednisolone and quinidine, three P-glycoprotein (P-gp) substrates, across the isolated rat intestinal membranes were examined by an in-vitro diffusion chamber system. The serosal-to-mucosal (secretory) transport of these P-gp substrates was greater than their mucosal-to-serosal (absorptive) transport, indicating that their net movement across the intestinal membranes was preferentially in the secretory direction. The polarized secretory transport of these drugs was remarkably diminished and their efflux ratios decreased in the presence of PEG 20000. In addition, PEG 20000 did not affect the transport of Lucifer yellow, a non-P-gp substrate. The intestinal membrane toxicity of PEG 20000 was evaluated by measuring the release of alkaline phosphatase (ALP) and protein from the intestinal membranes. The release of ALP and protein was enhanced in the presence of 20 mM sodium deoxycholate (NaDC), a positive control, while these biological parameters did not change in the presence of 0.1–5% (w/v) PEG 20000. These findings indicated that the intestinal membrane damage caused by PEG 20000 was not a main reason for the enhanced absorptive transport of these P-gp substrates in the presence of PEG 20000. Furthermore, the transepithelial electrical resistance (TEER) of rat jejunal membranes in the presence or absence of PEG 20000 was measured by a diffusion chamber method. PEG 20000 (0.1–5.0 % w/v) did not change the TEER values of the rat jejunal membranes, indicating that the increase in the absorptive transport of these P-gp substrates might not be due to the increased transport of these P-gp substrates via a paracellular pathway caused by PEG 20000. Finally, the effect of PEG 20000 on the intestinal absorption of quinidine was examined by an in-situ closed-loop method. The intestinal absorption of quinidine was significantly enhanced in the presence of 0.1-1.0% (w/v) PEG 20000. These findings suggest that PEG 20000 might be a useful excipient to improve the intestinal absorption of quinidine, which is mainly secreted by a P-gp-mediated efflux system in the intestine.

Rhodamine inhibitors of P-glycoprotein: an amide/thioamide "switch" for ATPase activity

We have examined 46 tetramethylrosamine/rhodamine derivatives with structural diversity in the heteroatom of the xanthylium core, the amino substituents of the 3- and 6-positions, and the alkyl, aryl, or heteroaryl group at the 9-substituent. These compounds were examined for affinity and ATPase stimulation in isolated MDR3 CL P-gp and human P-gp-His(10), for their ability to promote uptake of calcein AM and vinblastine in multidrug-resistant MDCKII-MDR1 cells, and for transport in monolayers of MDCKII-MDR1 cells. Thioamide 31-S gave K(M) of 0.087 microM in human P-gp. Small changes in structure among this set of compounds affected affinity as well as transport rate (or flux) even though all derivatives examined were substrates for P-gp. With isolated protein, tertiary amide groups dictate high affinity and high stimulation while tertiary thioamide groups give high affinity and inhibition of ATPase activity. In MDCKII-MDR1 cells, the tertiary thioamide-containing derivatives promote uptake of calcein AM and have very slow passive, absorptive, and secretory rates of transport relative to transport rates for tertiary amide-containing derivatives. Thioamide 31-S promoted uptake of calcein AM and inhibited efflux of vinblastine with IC(50)'s of approximately 2 microM in MDCKII-MDR1 cells.

Population-based mechanistic prediction of oral drug absorption

The bioavailability of drugs from oral formulations is influenced by many physiological factors including gastrointestinal fluid composition, pH and dynamics, transit and motility, and metabolism and transport, each of which may vary with age, gender, race, food, and disease. Therefore, oral bioavailability, particularly of poorly soluble and/or poorly permeable compounds and those that are extensively metabolized, often exhibits a high degree of inter- and intra-individual variability. While several models and algorithms have been developed to predict bioavailability in an average person, efforts to accommodate intrinsic variability in the component processes are less common. An approach that incorporates such variability for human populations within a mechanistic framework is described together with examples of its application to drug and formulation development.

Increased paracellular absorption by bile salts and P-glycoprotein stimulated efflux of otilonium bromide in Caco-2 cells monolayers as a model of intestinal barrier

The present study investigates the intestinal permeability of otilonium bromide, a spasmolytic drug used to treat irritable bowel syndrome, across Caco-2 cell monolayers. The amount of otilonium bromide transported was determined by high-performance liquid chromatography-mass spectrometry. Epithelial barrier integrity was estimated by measuring transepithelial electrical resistance and the transport of reference compounds, P-glycoprotein activity by measuring rhodamine 123 efflux. Results showed that the apparent permeability of otilonium bromide was comparable to that of our zero permeability marker, inulin, in the apical-to-basal direction and similar to that of rhodamine 123 in the basal-to-apical direction. The P-glycoprotein substrate, verapamil, prevented otilonium bromide efflux and, conversely, otilonium bromide inhibited P-glycoprotein activity. Bile salts induced a transient opening of tight junctions, as measured by selective increase of paracellular transport, and significantly enhanced the absorption of otilonium bromide. In turn otilonium bromide potentiates the effect of bile salts on tight junctions without modifying their critical micellar concentration or altering cell viability. In conclusion, otilonium bromide is a paracellularly transported drug whose absorption, in amounts sufficient to exert a spasmolytic effect, is favoured by bile salts. P-glycoprotein, by stimulating efflux, contributes to remove excess compound, restraining its distribution and site of action to the intestinal wall.

Enhanced transport of P-glycoprotein substrate saquinavir in presence of thiolated chitosan

It was the aim of this study to investigate the effect of chitosan-4-thiobutylamidine (Ch-TBA) and reduced glutathione (GSH) on the absorption of P-glycoprotein (P-gp) and multidrug resistance protein (MRP) substrate saquinavir in vitro and in vivo. Bidirectional transport studies were performed with Caco-2 cell monolayers and additionally with freshly excised rat small intestinal mucosa mounted in Ussing type chambers. Furthermore, a delivery system based on Ch-TBA and GSH was evaluated in vivo in rats. The functional activity of the efflux pumps in Caco-2 cells and rat intestinal mucosa during the experiment was proven by the efflux ratio of saquinavir, which was 6.4 for Caco-2 cells and 2.1 for rat intestinal mucosa, respectively. Ch-TBA and particularly the combination of Ch-TBA with GSH enhanced apical (AP) absorption and decreased the secretory transport of saquinavir. In presence of 0.5% Ch-TBA and 0.5% GSH, the uptake of saquinavir was 1.6-fold improved in Caco-2 monolayer and 2.1-fold improved in rat intestinal mucosa. In vivo, the area under the plasma concentration time curve (AUC) of saquinavir was 1.4-fold and Cmax 1.6-fold increased, in comparison with control. Results of this study showed that Ch-TBA in combination with GSH can be an interesting tool for increasing the oral bioavailability of actively secreted compounds.

Chalcogenopyrylium dyes as inhibitors/modulators of P-glycoprotein in multidrug-resistant cells

A series of chalcogenopyrylium dyes were evaluated as modulators/inhibitors of P-glycoprotein (Pgp). Their ability to inhibit verapamil (VER)-dependent ATPase activity (IC(50) values) in lipid-activated, mouse Cys-less mdr3 Pgp was determined. Their ability to promote calcein-AM (CAM) uptake in MDCKII-MDR1 cells and their capacity to be transported by Pgp in monolayers of MDCKII-MDR1 cells were also evaluated. The chalcogenopyrylium dyes promoted CAM uptake with values of EC(50) between 5 x 10(-6) and 3.5 x 10(-5)M and 7 of the 9 dyes examined in transport studies were substrates for Pgp with efflux ratios (P(BA/AB)) between 14 and 390. Binding of three compounds (1-S, 3-S, and 4-S) to Pgp was also assessed by fluorescence. These three thiopyrylium dyes showed increased fluorescence upon binding to Pgp, giving apparent binding constants, K(app), on the order of 10(-7) to 10(-6)M. Compound 8-Te was particularly intriguing since it appeared to influence Pgp at low micromolar concentrations as evidenced by its influence on VER-stimulated ATPase activity (IC(50) of 1.2 x 10(-6)M), CAM uptake (EC(50) of 5.4 x 10(-6)M), as well as [(3)H]-vinblastine transport by Pgp in cells (IC(50) of 4.3 x 10(-6)M) and within inside-out membrane vesicles (IC(50) of 9.6 x 10(-6)M). Yet, Pgp did not influence the distribution of 8-Te in MDCKII-MDR1 monolayers suggesting that 8-Te may bind to an allosteric site.

Salmonella enterica serovar Typhimurium modulates P-glycoprotein in the intestinal epithelium

Studies over the last decade have shown that Salmonella enterica serovar Typhimurium (S. typhimurium) is able to preferentially locate to sites of tumor growth and modulate (shrink) the growth of many cancers. Given this unique association between S. typhimurium and cancer cells, the objective of this study was to investigate the capacity of this microorganism to modulate the plasma membrane multidrug resistance (MDR) protein P-glycoprotein (P-gp), an ATP-binding cassette transporter responsible for effluxing many cancer drugs. Using an in vitro model of S. typhimurium infection of polarized human cancer intestinal cell lines, we have found that this enteric pathogen functionally downregulates the efflux capabilities of P-gp. Specifically, we show that S. typhimurium infection of human intestinal cancer cells results in the enhanced intracellular accumulation of a number of P-gp substrates that corresponds to the posttranscriptional downregulation of P-gp expression. Furthermore, cells expressing small interfering RNAs against MDR1, the gene encoding P-gp, were significantly more susceptible to the cytotoxic effects of bacterial infection. This result is consistent with our observation that S. typhimurium was significantly less able to invade cells overexpressing MDR1. Taken together, these results reveal a novel role for P-gp in the maintenance of homeostasis in the gastrointestinal tract in regard to bacterial infection. Thus the regulation of P-gp by S. typhimurium has important implications not only for the development of new cancer therapeutics aimed at reversing drug resistance but also in the understanding of how microbes have evolved diverse strategies to interact with their host.

An integrated microfluidic system for long-term perfusion culture and on-line monitoring of intestinal tissue models

Conventional cell-based assays in life science and medical applications can be difficult to maintain functionally over long periods. Microfluidics is an emerging technology with potential to provide integrated environments for cell maintenance, continuous perfusion, and monitoring. In this study, we developed an integrated microfluidic device with on-chip pumping and detection functionalities. The microfluidic structure in the device is divided into two independent channels separated by a semipermeable membrane on which cells are inoculated and cultured. Perfusion and fluorescence measurements of culture media for each channel can be conducted by the on-chip pumping system and optical fiber detection system. Performance of the device was examined through long-term culture and monitoring of polarized transport activity of intestinal tissue models (Caco-2 cells). The cells could be cultured for more than two weeks, and monolayer transport of rhodamine 123 was successfully monitored by on-line fluorescent measurement. This device may have applications in toxicity testing and drug screening.

ATP occlusion by P-glycoprotein as a surrogate measure for drug coupling

The multidrug efflux pump P-glycoprotein (Pgp) couples drug transport to ATP hydrolysis. Previously, using a synthetic library of tetramethylrosamine ( TMR) analogues, we observed significant variation in ATPase stimulation ( V m (D)). Concentrations required for half-maximal ATPase stimulation ( K m (D)) correlated with ATP hydrolysis transition-state stabilization and ATP occlusion (EC 50 (D)) at a single site. Herein, we characterize several TMR analogues that elicit modest turnover ( k cat <or= 1-2 s (-1)) compared to verapamil (VER) ( k cat approximately 10 s (-1)). Apparent ATPase activities manifest as nearly equivalent to basal values. In some cases, K m (D) parameters for drug stimulation of ATPase could not be accurately determined, yet these same TMR analogues promoted ATP occlusion at relatively low concentrations ( approximately 0.4-40 microM). Moreover, the TMR analogues competitively inhibited VER-dependent ATPase activity at concentrations similar to those required for ATP occlusion. Finally, the TMR analogues facilitated uptake of calcein-AM into CR1R12 and MDCK-MDR1 cells and are actively transported by Pgp in monolayers of MDCK-MDR1 cells at similarly low concentrations ( approximately 1-20 microM). ADP.V i release kinetics were identical in the presence of the TMR derivatives, VER, or in the absence of drug, suggesting that slow turnover is not likely due to slow release of the ATP hydrolysis products ADP and P i. These data support the partition model in which drug site occupancy converts residual basal ATPase activity to a drug-dependent mechanism even in cases where stimulation appears to be exactly compensatory to basal values. It is noteworthy that when compared to previously reported TMR analogues, subtle modification of the TMR scaffold can confer large differences in ATP turnover.

Permeability, transport, and metabolism of solutes in Caco-2 cell monolayers: a theoretical study

We explored the properties of a catenary model that includes the basolateral (B), apical (A), and cellular compartments via simulations under linear and nonlinear conditions to understand the asymmetric observations arising from transporters, enzymes, and permeability in Caco-2 cells. The efflux ratio (EfR; P(app,B-->A)/P(app,A-->B)), obtained from the effective permeability from the A-->B and B-->A direction under linear conditions, was unity for passively permeable drugs whose transport does not involve transporters; the value was unaffected by cellular binding or metabolism, but increased with apical efflux. Metabolism was asymmetric, showing lesser metabolite accrual for the B-->A than A-->B direction because of inherent differences in the volumes for A and B. Moreover, the net flux (total - passive permeation) due to saturable apical efflux, absorption, or metabolism showed nonconformity to simple Michaelis-Menten kinetics against C(D,0), the loading donor concentration. EfR values differed with saturable apical efflux and metabolism (>1), as well as apical absorption (EfRs <1), but approached unity with high passive diffusive clearance (CL(d)) and increasing C(D,0) at a higher degree of saturation of the process. The J(max) (apparent V(max) estimated for the carrier system) and K(m)(') [or the K(m)('') based on a modified equation with the Hill coefficient (beta)] estimates from the Eadie-Hofstee plot revealed spurious correlations with the assigned V(max) and K(m). The sampling time, CL(d), and parameter space of K(m) and V(max) strongly influenced both the correlation and accuracy of estimates. Improved correlation was found for compounds with high CL(d). These observations showed that the catenary model is appropriate in the description of transport and metabolic data in Caco-2 cells.

Polyoxyethylene 40 stearate modulates multidrug resistance and enhances antitumor activity of vinblastine sulfate

Multidrug resistance (MDR) is one of the major obstacles limiting the efficacy of cancer chemotherapy. Identification of new and effective MDR reversal agents is needed. In this study, the effects of polyoxyethylene 40 stearate (PS40) on MDR were evaluated via the transport of the P-glycoprotein (P-gp) substrate vinblastine sulfate (VBL) through Caco-2 cell monolayers and rat intestine tissue. The effects of PS40 on the antitumor activity of VBL were examined through 3-(4,5)-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cytotoxicity assay and multidrug-resistant tumor-bearing mice. Results of the transport experiments showed that PS40 reduced VBL efflux. The cytotoxicity of vinblastine to K562/ADR cells was significantly enhanced when the cells were cotreated with 100 or 150 microg/mL PS40. In vivo data revealed that average tumor volume and average tumor weight were significantly less in the VBL+PS40 group than in the VBL group. The inhibition rate for tumor growth was increased from 0.06 (VBL group) to 0.84 (VBL+PS40 group). These results suggest that PS40 may be a potentially useful adjuvant to enhance the therapeutic effects of P-gp substrates.

Correlation of in vitro and in vivo models for the oral absorption of peptide drugs

The aim of this study was to evaluate two in vitro models, Caco-2 monolayer and rat intestinal mucosa, regarding their linear correlation with in vivo bioavailability data of therapeutic peptide drugs after oral administration in rat and human. Furthermore the impact of molecular mass (Mm) of the according peptides on their permeability was evaluated. Transport experiments with commercially available water soluble peptide drugs were conducted using Caco-2 cell monolayer grown on transwell filter membranes and with freshly excised rat intestinal mucosa mounted in Using type chambers. Apparent permeability coefficients (P (app)) were calculated and compared with in vivo data derived from the literature. It was shown that, besides a few exceptions, the Mm of peptides linearly correlates with permeability across rat intestinal mucosa (R (2) = 0.86; y = -196.22x + 1354.24), with rat oral bioavailability (R (2) = 0.64; y = -401.90x + 1268.86) as well as with human oral bioavailability (R (2) = 0.91; y = -359.43x + 1103.83). Furthermore it was shown that P (app) values of investigated hydrophilic peptides across Caco-2 monolayer displayed lower permeability than across rat intestinal mucosa. A correlation between P (app) values across rat intestinal mucosa and in vivo oral bioavailability in human (R (2) = 0.98; y = 2.11x + 0.34) attests the rat in vitro model to be a very useful prediction model for human oral bioavailability of hydrophilic peptide drugs. Presented correlations encourage the use of the rat in vitro model for the prediction of human oral bioavailabilities of hydrophilic peptide drugs.

Air-liquid interface (ALI) culture of human bronchial epithelial cell monolayers as an in vitro model for airway drug transport studies

Serially passaged normal human bronchial epithelial (NHBE) cell monolayers were established on Transwell inserts via an air-liquid interface (ALI) culture method. NHBE cells were seeded on polyester Transwell inserts, followed by an ALI culture from day 3, which resulted in peak TEER value of 766+/-154 Omegaxcm2 on the 8th day. Morphological characteristics were observed by light microscopy and SEM, while the formation of tight junctions was visualized by actin staining, and confirmed successful formation of a tight monolayer. The transepithelial permeability (Papp) of model drugs significantly increased with the increase of lipophilicity and showed a good linear relationship, which indicated that lipophilicity is an important factor in determining the Papp value. The expression of P-gp transporter in NHBE cell monolayers was confirmed by the significantly higher basolateral to apical permeability of rhodamine123 than that of reverse direction and RT-PCR of MDR1 mRNA. However, the symmetric transport of fexofenadine.HCl in this NHBE cell monolayers study seems to be due to the low expression of P-gp transporter and/or to its saturation with high concentration of fexofenadine.HCl. Thus, the development of tight junction and the expression of P-gp in the NHBE cell monolayers in this study imply that they could be a suitable in vitro model for evaluation of systemic drug absorption via airway delivery, and that they reflect in vivo condition better than P-gp over-expressed cell line models.

Impact of Cremophor-EL and Polysorbate-80 on Digoxin Permeability across Rat Jejunum: Delineation of Thermodynamic and Transporter Related Events Using the Reciprocal Permeability Approach

The effect of Cremophor-EL (Cr-EL) and polysorbate-80 (PS-80) on the transepithelial permeability of digoxin (DIG) has been evaluated using the reciprocal permeability approach to delineate thermodynamic and transporter related events. Permeability data were corrected for solubilization using the micellar association constant (Ka) obtained from Papp data generated in the presence of the nonspecific ATPase inhibitor sodium orthovanadate. In the presence of mucosal Cr-EL, a concentration dependent decrease in serosal-mucosal (S-M) and increase in M-S transport was observed. Whilst serosal Cr-EL resulted in a reduction in S-M DIG transport, no impact on M-S transport was apparent. For PS-80, the presence of either serosal or mucosal surfactant led to a decrease in secretory (S-M) DIG transport, however no effect on absorptive transport was evident. The data confirm the potential P-gp inhibitory effects of Cr-EL, but suggest that in contrast to Cr-EL, PS-80 is not a potent inhibitor of P-gp and is incapable of increasing absorptive drug transport, at least in excised rat intestinal tissue and at the concentrations tested. The data are also consistent with the involvement of additional transporters (both apical and basolateral) in the intestinal permeability of DIG, although more definitive data is required to confirm this possibility.

Kinetic considerations for the quantitative assessment of efflux activity and inhibition: implications for understanding and predicting the effects of efflux inhibition

PURPOSE

Unexpected and complex experimental observations related to efflux transport have been reported in the literature. This work was conducted to develop relationships for efflux activity (PS(efflux)) as a function of commonly studied kinetic parameters [permeability-surface area product (PS), efflux ratio (ER), degree of efflux inhibition (phi(i)), 50% inhibitory concentration (IC(50)), and Michaelis-Menten constant (K(m))].

METHODS

A three-compartment model (apical, cellular, and basolateral) was used to derive flux equations relating the initial rate of flux and steady-state mass transfer in the presence or absence of active efflux. Various definitions of efflux ratio (ER) were examined in terms of permeability-surface area products. The efflux activity (PS(efflux)) was expressed in terms of ER and PS. The relationships between PS(efflux) and PS, ER, phi(i), IC(50), and K(m) were solved mathematically. Simulations and examples from the literature were used to illustrate the resulting mathematical relationships.

RESULTS

The relationships derived according to a three-compartment model differed fundamentally from commonly accepted approaches for determining PS(efflux), phi(i), IC(50) and K(m). Based on the model assumptions and mathematical derivations, currently used mathematical relationships erroneously imply that efflux activity is proportional to change in PS (i.e., flux or P(app)) and thus underestimate PS(efflux) and phi(i,) and overestimate IC(50) and K(m).

CONCLUSIONS

An understanding of the relationship between efflux inhibition and kinetic parameters is critical for appropriate data interpretation, standardization in calculating and expressing the influence of efflux transport, and predicting the clinical significance of efflux inhibition.

In vivo comparison of various polymeric and low molecular mass inhibitors of intestinal P-glycoprotein

Several polymers have been reported to modulate drug absorption by inhibition of intestinal P-glycoprotein (P-gp). The aim of the present study was to provide a direct in vivo comparison of delivery systems based on Pluronic P85, Myrj 52 and chitosan-4-thiobutylamidine (Ch-TBA) in vivo in rats, using rhodamine-123 (Rho-123) as representative P-gp substrate. Furthermore, the postulated low molecular mass P-gp inhibitors 6-mercaptopurine and reduced glutathione (GSH) were evaluated in vitro and in vivo. In vitro, the permeation enhancing effect of 6-mercaptopurine, GSH, Pluronic P85, Myrj 52, and the combination of Ch-TBA with GSH was evaluated by using freshly excised rat intestinal mucosa mounted in Ussing-type diffusion chambers. In comparison to buffer only, Rho-123 transport in presence of 100 microm 6-mercaptopurine, 0.5% (w/v) GSH, 0.5% (w/v) Pluronic P85, 0.5% (w/v) Myrj 52 and the combination of 0.5% (w/v) Ch-TBA/ 0.5% (w/v) GSH, was 2.1, 1.6, 1.9, 1.8, 3.0-fold improved, respectively. In vivo in rat, enteric-coated tablets based on Pluronic P85, Myrj 52 or Ch-TBA/GSH increased the area under the plasma concentration time curve (AUC(0-12)) of Rho-123 1.6-fold, 2.4-fold, 4.3-fold, respectively, in comparison to control only. Contrariwise, the low molecular mass excipients 6-mercaptopurine and GSH showed no significant effect in vivo at all. This in vivo study showed that polymeric P-gp inhibitors and especially the delivery system based on thiolated chitosan significantly increased the oral bioavailability of P-gp substrate Rho-123.

In vivo evaluation of an oral delivery system for P-gp substrates based on thiolated chitosan

Recently, thiolated polymers, so called thiomers, have been reported to modulate drug absorption by inhibition of intestinal P-glycoprotein (P-gp). The aim of the present study was to provide a proof-of-principle for a delivery system based on thiolated chitosan in vivo in rats, using rhodamine-123 (Rho-123) as representative P-gp substrate. In vitro, the permeation enhancing effect of unmodified chitosan, chitosan-4 thiobutylamidine (Ch-TBA) and the combination of Ch-TBA with reduced glutathione (GSH) was evaluated by using freshly excised rat intestinal mucosa mounted in Ussing-type chambers. In comparison to buffer only, Rho-123 transport in presence of 0.5% (w/v) chitosan, 0.5% (w/v) Ch-TBA and the combination of 0.5% (w/v) Ch-TBA/0.5% (w/v) GSH, was 1.8-fold, 2.6-fold, 3.8-fold improved, respectively. Furthermore, enteric-coated tablets based on unmodified chitosan or Ch-TBA/GSH, were investigated in vivo. In rats, the Ch-TBA/GSH tablets increased the area under the plasma concentration time curve (AUC0-12) of Rho-123 by 217% in comparison to buffer control and by 58% in comparison to unmodified chitosan. This in vivo study showed that a delivery system based on thiolated chitosan significantly increased the oral bioavailability of P-gp substrate Rho-123.

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.

Modulation of intestinal P-glycoprotein function by polyethylene glycols and their derivatives by in vitro transport and in situ absorption studies

We examined the effect of polyethylene glycols (PEGs) with different molecular weights and their derivatives on the intestinal absorption of rhodamine123, a P-glycoprotein (P-gp) substrate, across the isolated rat intestinal membranes by an in vitro diffusion chamber system. The serosal to mucosal (secretory) transport of rhodamine123 was greater than its mucosal to serosal (absorptive) transport, indicating that the net movement of rhodamine123 across the intestinal membranes was preferentially secretory direction. The secretory transport of rhodamine123 was inhibited by the addition of PEGs with average molecular weights of 400, 2000 and 20,000, irrespective of its molecular weight. The inhibitory effects of these PEGs for the intestinal P-gp function were concentration dependent over the range 0.1-20% (v/v or w/v). Similar inhibitory effect for the intestinal P-gp function was observed when PEG derivatives including PEG monolaurate, PEG monooleate and PEG monostearate were added to the mucosal site of the chambers. Furthermore, we also examined effect of PEG20,000 on the intestinal absorption of rhodamine123 by an in situ closed loop method. The intestinal absorption of rhodamine123 was enhanced in the presence of PEG20,000. These findings suggest that PEGs and their derivatives are useful excipients to inhibit the function of intestinal P-gp, thereby improving the intestinal absorption of P-gp substrates, which are secreted by a P-gp-mediated efflux system.

Ischemia/Reperfusion Injury in the Monolayers of Human Intestinal Epithelial Cell Line Caco-2 and Its Recovery by Antioxidants

We previously established a in vitro system for assessing early ischemia/reperfusion injury using monolayers of human intestinal epithelial cell line Caco-2, in which lipid peroxidation caused by tertiary-butylhydroperoxide (t-BuOOH), a lipid peroxidation inducer, acts as a trigger of the injury. By now, we have shown that superoxide anion participates in the opening of tight junctions (TJ) induced by reoxygenation following the induction of lipid peroxidation by t-BuOOH at a low concentration. The present objectives are to elucidate the dysfunction of P-glycoprotein (P-gp) in addition to the opening of TJ by t-BuOOH at a high concentration condition using rhodamine123 (Rho123) as a P-gp substrate and cyclosporine A (CyA) as a P-gp inhibitor. Also, we compared the inhibition effect of lutein and other compounds such as biliverdin as a radical scavenger on the opening of TJ and the dysfunction of P-gp. t-BuOOH at a high concentration increased the permeability of Rho123 in the apical to basal direction and decreased basal to apical direction when compared with control conditions. t-BuOOH at a high concentration showed no significant difference between directional transport of Rho123 and no inhibition was observed in the permeability of both directions by CyA. The staining intensity of Western blot was decreased by t-BuOOH at a high concentration. Although lutein and the other compounds had recovery effects on the opening of TJ and P-gp dysfunction induced by t-BuOOH, lutein is more advantageous than other compounds since it has effective effects at the lower concentration. In conclusion, the barrier dysfunction such as the inhibition of P-gp in addition to the opening of TJ was induced by t-BuOOH at a high concentration condition. The above two barrier dysfunctions was ameliorated by antioxidant such as lutein and biliverdin.

Improvement of Intestinal Absorption of P­glycoprotein Substrate by D­tartaric Acid

The purpose of the present experiment was to examine the effects of D-tartaric acid (TA) on intestinal drug absorption under both in situ and in vitro experimental conditions. In the in vitro diffusion chamber experiments, TA (10 mM) added to the mucosal side of rat colon significantly decreased rhodamine123 (Rho 123) transport from the serosal to mucosal side. Since TA has been shown to change the integrity of the epithelial tight junctions in rat colon at low pH conditions, resulting in improved paracellular drug transport, the effect of TA on membrane resistance was examined at pH 7.4 in the present study. It was found that membrane resistance, an indicator of paracellular integrity, did not change at pH 7.4. In the in situ loop method, TA (20 mM) increased the absorption of Rho123 in both ileum and colon but not in jejunum. TA (20 mM) also increased the absorption of daunorubicin in the ileum, but TA (20 mM) did not change the expression level of P-glycoprotein (P-gp). TA (20 mM) significantly inhibited excretion of i.v.-administered Rho123 and daunorubicin into the ileal lumen. In conclusion, for the first time we demonstrated that TA increases the intestinal absorption of P-gp substrates Rho123 and daunorubicin, possibly by modulating the P-gp function without changing the expression level of P-gp in the rat intestine.

Absorption properties and P-glycoprotein activity of modified Caco-2 cell lines

Caco-2 cell line is extensively used as an in vitro model in studying small intestinal absorption but it lacks proper expression of efflux pumps and cytochrome P450 enzymes that are involved in absorption and first pass metabolism of drugs. We created two novel Caco-2 cell lines expressing orphan nuclear receptors pregnane X receptor and constitutive androstane receptor that regulate many genes involved in xenobiotic metabolism. We conducted a systematic study on expression of some metabolic genes, P-glycoprotein activity and absorption properties of several drugs with these new cell lines and previously described modified Caco-2 cell lines (MDR1 transfection, vincristine treatment and 1alpha,25-dihydroxyvitamin D3 treatment). A short culture time medium was also included in the study. Most modified cell lines formed tight differentiated monolayers. MDR1, CYP2C9 and CYP3A4 genes were upregulated in some cell lines. Elevated P-glycoprotein activities were observed by calcein-AM uptake experiments but this did not affect significantly the permeability of selected P-glycoprotein substrates. Some cell lines had similar passive and active permeability properties to Caco/WT cells while in few cell lines these were altered. Passive transcellular permeability was modestly elevated in all modified cell lines. In addition, several compounds showed pH-dependent permeability properties.

Selamectin is a potent substrate and inhibitor of human and canine P-glycoprotein

The transport of the antiparasitic agents, ivermectin, selamectin and moxidectin was studied in human intestinal epithelial cell monolayers (Caco-2) and canine peripheral blood lymphocytes (PBL). Both models expressed the mdr1-coded 170 kDa ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp). Fluxes of the P-gp substrate rhodamine-123 (Rh-123) across Caco-2 monolayers showed that ivermectin and selamectin acted as potent P-gp inhibitors with IC50 values of 0.1 microm. In contrast, moxidectin was a weaker P-gp inhibitor with an IC50 of 10 microm. The transport of radiolabelled ivermectin, selamectin and moxidectin through Caco-2 monolayers showed that ivermectin, selamectin and moxidectin were P-gp substrates with secretory/absorptive ratios of 7.5, 4.7 and 2.6 respectively. Secretory transport of [3H]-ivermectin and [3H]-selamectin was blocked by the P-gp inhibitor, verapamil. Ivermectin and selamectin inhibited the efflux of Rh-123 from PBL and the concentration of inhibition was similar to that of verapamil. In contrast, moxidectin did not have a significant effect on Rh-123 efflux from PBL. The data suggest that ivermectin and selamectin are potent P-gp substrates, while moxidectin is a weak one.

ASSESSMENT OF DRUG INTERACTIONS IN HEPATOBILIARY TRANSPORT USING RHODAMINE 123 IN SANDWICH-CULTURED RAT HEPATOCYTES

The purpose of the present study was to explore the utility of sandwich-cultured rat hepatocytes as an in vitro tool to examine drug interactions at the hepatic transport level. Rhodamine 123 was used as a model substrate for P-glycoprotein-mediated biliary excretion. Effects of various types of P-glycoprotein modulation on the biliary excretion index (BEI; a relative measure of the extent of biliary excretion) and the in vitro biliary clearance (CL(bile)) were determined. Significant reductions in rhodamine 123 BEI and CL(bile) were noted in the presence of the P-glycoprotein inhibitors verapamil (30-100 microM) and progesterone (100 microM). The P-glycoprotein activator quercetin (10-100 microM) enhanced rhodamine 123 CL(bile) by approximately 4-fold, with only a minor effect on BEI, suggesting that quercetin had a more pronounced effect on uptake at the basolateral membrane rather than excretion across the canalicular membrane. Treatment of hepatocytes for 48 h with dexamethasone (10 microM) resulted in significant enhancement of CL(bile), whereas rifampin (5-50 microM) increased both BEI and CL(bile), indicating that the inducing effects of dexamethasone and rifampin were occurring at the basolateral and canalicular membranes, respectively. Total rhodamine 123 uptake in sandwich-cultured rat hepatocytes was partly saturable and was affected by the presence of typical Oatp1a4 substrates (digoxin, quinine, d-verapamil, 17beta-estradiol-d-17beta-glucuronide). In summary, sandwich-cultured rat hepatocytes are a useful tool to study mechanisms of hepatobiliary drug disposition and to predict the potential for drug interactions in hepatic transport.

Advances in screening for membrane permeability: high-resolution PAMPA for medicinal chemists

The majority of orally administered drugs are described to be passively transported across the lipophilic cell membranes [Lennernäs, H. et al. (1994) Intestinal drug absorption during induced net water absorption in human; a mechanistic study using antipyrine, atenolol and enalaprilat. Br. J. Clin. Pharmacol. 37, 589-596; [1] Artursson, P. Application of physicochemical properties of molecules to predict intestinal permeability. Proceedings of the AAPS Workshop on Permeability Definitions and Regulatory Standards, Arlington, VA, 17-19 August 1998] [2]. Parallel artificial membrane permeability assay (PAMPA), as a passive-permeability screen with focus on the simulation of transcellular processes, is an excellent compliment to cellular models in absorption, distribution, metabolism, excretion (ADME) screening of research compounds. Being fast, versatile, and low-cost, PAMPA is a compelling and biologically relevant model of transport. The problem of low solubility of research compounds has been largely eliminated in the PAMPA method. This review will emphasize how high-resolution PAMPA can help in the design of structural features into molecules to improve their absorption-related properties.:

In Vitro absorption and secretory quotients: Practical criteria derived from a study of 331 compounds to assess for the impact of P-glycoprotein-mediated efflux on drug candidates

The absorptive (AQ) and secretory (SQ) quotients have been proposed as a novel experimental approach to quantify the modulation of intestinal absorption and secretion by P-glycoprotein (Pgp). Because these unidirectional assays inherently assess for the impact of Pgp, conclusions as to whether a compound is a Pgp substrate will be made from the data. Therefore, the objective of this study was to establish the relationship between AQ/SQ and the bidirectional efflux assay and to derive criteria to classify a compound as a Pgp substrate. AQ and SQ parameters were calculated for 331 compounds that had previously been evaluated in the bidirectional assay and the concordance of Pgp substrate classification between these methods assessed by establishing AQ/SQ criteria of increasing magnitude. The AQ and SQ values correctly identified 80 and 85% of the compounds as Pgp substrates/nonsubstrates relative to the bidirectional efflux assay. This study demonstrates that the optimal AQ and SQ value to classify compounds as Pgp substrates was 0.3 and provides a basis to deploy unidirectional efflux assays in the early stages of drug discovery, which would benefit from the twofold increase in throughput over current bidirectional transport assays.