Isolation and characterization of Caco-2 subclones expressing high levels of multidrug resistance protein efflux transporter

Generation and application of CES1-knockout Tet-Off-regulated CYP3A4 and UGT1A1-expressing Caco-2 cells

Caco-2 cells, a human colorectal adenocarcinoma cell line, are widely used to model small intestinal epithelial cells in the drug development process because they can predict drug absorption with high accuracy. However, Caco-2 cells have several issues. First, Caco-2 cells have little expression of cytochrome P450 3A4 (CYP3A4), which is a major drug-metabolizing enzyme in the human intestine. We previously developed Caco-2 cells whose expression of CYP3A4 can be controlled using doxycycline (Dox) (CYP3A4-Caco-2 cells) (Ichikawa et al., Sci. Rep, 2021). However, since the Tet-On system was used to regulate CYP3A4 expression in these cells, there was concern about drug-drug interactions. The second issue is that in the human small intestine, carboxylesterase 2 (CES2) is more highly expressed than carboxylesterase 1 (CES1), while in Caco-2 cells CES1 is more highly expressed. The third issue is the low level expression of uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), a phase II drug-metabolizing enzyme. In this study, we used genome-editing technology to establish CES1-knockout Caco-2 cells whose CYP3A4 and UGT1A1 expression can be regulated by the Tet-Off system. These cell lines would be useful in pharmaceutical researches, including intestinal toxicological studies, as an in vitro model for orally administered drugs.

Improving the Accuracy of Permeability Data to Gain Predictive Power: Assessing Sources of Variability in Assays Using Cell Monolayers

The ability to predict the rate of permeation of new compounds across biological membranes is of high importance for their success as drugs, as it determines their efficacy, pharmacokinetics, and safety profile. In vitro permeability assays using Caco-2 monolayers are commonly employed to assess permeability across the intestinal epithelium, with an extensive number of apparent permeability coefficient (Papp) values available in the literature and a significant fraction collected in databases. The compilation of these Papp values for large datasets allows for the application of artificial intelligence tools for establishing quantitative structure-permeability relationships (QSPRs) to predict the permeability of new compounds from their structural properties. One of the main challenges that hinders the development of accurate predictions is the existence of multiple Papp values for the same compound, mostly caused by differences in the experimental protocols employed. This review addresses the magnitude of the variability within and between laboratories to interpret its impact on QSPR modelling, systematically and quantitatively assessing the most common sources of variability. This review emphasizes the importance of compiling consistent Papp data and suggests strategies that may be used to obtain such data, contributing to the establishment of robust QSPRs with enhanced predictive power.

Sustainable methods for the carboxymethylation and methylation of ursolic acid with dimethyl carbonate under mild and acidic conditions

Ursolic acid is a triterpene plant extract that exhibits significant potential as an anti-cancer, anti-tumour, and anti-inflammatory agent. Its direct use in the pharmaceutical industry is hampered by poor uptake of ursolic acid in the human body coupled with rapid metabolism causing a decrease in bioactivity. Modification of ursolic acid can overcome such issues, however, use of toxic reagents, unsustainable synthetic routes and poor reaction metrics have limited its potential. Herein, we demonstrate the first reported carboxymethylation and/or methylation of ursolic acid with dimethyl carbonate (DMC) as a green solvent and sustainable reagent under acidic conditions. The reaction of DMC with ursolic acid, in the presence of PTSA, ZnCl2, or H2SO4-SiO2 yielded the carboxymethylation product 3β-[[methoxy]carbonyl]oxyurs-12-en-28-oic acid, the methylation product 3β-methoxyurs-12-en-28-oic acid and the dehydration product urs-2,12-dien-28-oic acid. PTSA demonstrated high conversion and selectivity towards the previously unreported carboxymethylation of ursolic acid, while the application of formic acid in the system led to formylation of ursolic acid (3β-formylurs-12-en-28-oic acid) in quantitative yields via esterification, with DMC acting solely as a solvent. Meanwhile, the methylation product of ursolic acid, 3β-methoxyurs-12-en-28-oic acid, was successfully synthesised with FeCl3, demonstrating exceptional conversion and selectivity, >99% and 99%, respectively. Confirmed with the use of qualitative and quantitative green metrics, this result represents a significant improvement in conversion, selectivity, safety, and sustainability over previously reported methods of ursolic acid modification. It was demonstrated that these methods could be applied to other triterpenoids, including corosolic acid. The study also explored the potential pharmaceutical applications of ursolic acid, corosolic acid, and their derivatives, particularly in anti-inflammatory, anti-cancer, and anti-tumour treatments, using molecular ADMET and docking methods. The methods developed in this work have led to the synthesis of novel molecules, thus creating opportunities for the future investigation of biological activity and the modification of a wide range of triterpenoids applying acidic DMC systems to deliver novel active pharmaceutical intermediates.

Establishment and Characterization of Novel Human Intestinal In Vitro Models for Absorption and First-Pass Metabolism Studies

Commonly used intestinal in vitro models are limited in their potential to predict oral drug absorption. They either lack the capability to form a tight cellular monolayer mimicking the intestinal epithelial barrier or the expression of cytochrome P450 3A4 (CYP3A4). The aim of this study was to establish a platform of colorectal cancer patient-derived cell lines for evaluation of human intestinal drug absorption and metabolism. We characterized ten 2D cell lines out of our collection with confluent outgrowth and long-lasting barrier forming potential as well as suitability for high throughput applications with special emphasis on expression and inducibility of CYP3A4. By assessment of the transepithelial electrical resistance (TEER) the cells barrier function capacity can be quantified. Very high TEER levels were detected for HROC60. A high basal CYP3A4 expression and function was found for HROC32. Eight cell lines showed higher CYP3A4 induction by stimulation via the vitamin D receptor compared to Caco-2 cells (5.1- to 16.8-fold change). Stimulation of the pregnane X receptor led to higher CYP3A4 induction in two cell lines. In sum, we identified the two cell lines HROC183 T0 M2 and HROC217 T1 M2 as useful tools for in vitro drug absorption studies. Due to their high TEER values and inducibility by drug receptor ligands, they may be superior to Caco-2 cells to analyze oral drug absorption and intestinal drug–drug interactions. Significance statement: Selecting appropriate candidates is important in preclinical drug development. Therefore, cell models to predict absorption from the human intestine are of the utmost importance. This study revealed that the human cell lines HROC183 T0 M2 and HROC217 T1 M2 may be better suited models and possess higher predictive power of pregnane X receptor- and vitamin D-mediated drug metabolism than Caco-2 cells. Consequently, they represent useful tools for predicting intestinal absorption and simultaneously enable assessment of membrane permeability and first-pass metabolism.

P-Glycoprotein-Mediated Efflux Using a Rapidly Maturing Caco2 Clone (CLEFF4) in Only 5 Days without Requiring Modified Growth Medium

In drug discovery it is essential that one of the parameters tested for any new chemical entity is its affinity for human efflux systems, most notably P-glycoprotein (P-gp). These efflux systems affect not only rates of oral absorption but also rates of excretion through the liver, blood-brain barrier, and accumulation in potential target cells that upregulate efflux systems. Current methods to determine drugs' P-gp transport potential include in vitro bidirectional transport studies, and the two most common cell lines used are Caco2 and MDR1-transfected MDCK models. Caco2 cells are human but slow growing and require more than 3 weeks to mature, while MDCK cells are canine, but when transfected with human P-gp become a rapid model of P-gp affinity. Our laboratory has generated a Caco2 subclone called CLEFF4 that is fully human, yet now approaches the rapid nature of the MDCK model. No special medium is required. We have shown, in as little as 5 days postseeding, high transepithelial electrical resistance values of more than 1000 Ω·cm² plus P-gp expression more than threefold higher than that of 21-day-old cells. Currently tested drugs included rhodamine 123 (Rh123), vinblastine, and doxorubicin, and all drugs exhibited P-gp-mediated efflux that was inhibited by PSC833. By day 6, bidirectional transport of Rh123 was as potent as that of mature Caco2 cells, for use in comparative P-gp affinity studies. We now have a human P-gp model that is rapid and works without any need for special accelerating medium. We believe this could be a welcome addition to the testing regime of new chemical entities.

Molecular modeling approaches of selective adenosine receptor type 2A agonists as potential anti-inflammatory drugs

Adenosine A_2A receptor (A_2AR) is the predominant receptor in immune cells, where its activation triggers cAMP-mediated immunosuppressive signaling and the underlying inhibition of T cells activation and T cells-induced effects mediated by cAMP-dependent kinase proteins mechanisms. In this study, were used ADME/Tox, molecular docking and molecular dynamics simulations to investigate selective adenosine A_2AR agonists as potential anti-inflammatory drugs. As a result, we obtained two promising compounds (A and B) that have satisfactory pharmacokinetic and toxicological properties and were able to interact with important residues of the A_2AR binding cavity and during the molecular dynamics simulations were able to keep the enzyme complexed.Communicated by Ramaswamy H. Sarma.

Impact of Dynamically Exposed Polarity on Permeability and Solubility of Chameleonic Drugs Beyond the Rule of 5

Conformational flexibility has been proposed to significantly affect drug properties outside rule-of-5 (Ro5) chemical space. Here, we investigated the influence of dynamically exposed polarity on cell permeability and aqueous solubility for a structurally diverse set of drugs and clinical candidates far beyond the Ro5, all of which populated multiple distinct conformations as revealed by X-ray crystallography. Efflux-inhibited (passive) Caco-2 cell permeability correlated strongly with the compounds' minimum solvent-accessible 3D polar surface areas (PSA), whereas aqueous solubility depended less on the specific 3D conformation. Inspection of the crystal structures highlighted flexibly linked aromatic side chains and dynamically forming intramolecular hydrogen bonds as particularly effective in providing "chameleonic" properties that allow compounds to display both high cell permeability and aqueous solubility. These structural features, in combination with permeability predictions based on the correlation to solvent-accessible 3D PSA, should inspire drug design in the challenging chemical space far beyond the Ro5.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Establishment and Characterization of a Novel Caco-2 Subclone with a Similar Low Expression Level of Human Carboxylesterase 1 to Human Small Intestine

Caco-2 cells predominantly express human carboxylesterase 1 (hCE1), unlike the human intestine that predominantly expresses human carboxylesterase 2 (hCE2). Transport experiments using Caco-2 cell monolayers often lead to misestimation of the intestinal absorption of prodrugs because of this difference, as prodrugs designed to increase the bioavailability of parent drugs are made to be resistant to hCE2 in the intestine, so that they can be hydrolyzed by hCE1 in the liver. In the present study, we tried to establish a new Caco-2 subclone, with a similar pattern of carboxylase expression to human intestine, to enable a more accurate estimation of the intestinal absorption of prodrugs. Although no subclone could be identified with high expression levels of only hCE2, two subclones, #45 and #78, with extremely low expression levels of hCE1 were subcloned from parental Caco-2 cells by the limiting dilution technique. Unfortunately, subclone #45 did not form enterocyte-like cell monolayers due to low expression of claudins and β-actin. However, subclone #78 formed polarized cell monolayers over 4 weeks and showed similar paracellular and transcellular transport properties to parental Caco-2 cell monolayers. In addition, the intestinal transport of oseltamivir, a hCE1 substrate, could be evaluated in subclone #78 cell monolayers, including P-glycoprotein-mediated efflux under nonhydrolysis conditions, unlike parental Caco-2 cells. Consequently, it is proposed that subclone #78 may provide a more effective system in which to evaluate the intestinal absorption of prodrugs that are intended to be hydrolyzed by hCE1.

Mechanisms of improvement of intestinal transport of baicalin and puerarin by extracts of Radix Angelicae Dahuricae

Radix Angelicae Dahuricae is the dried root of Angelicae Dahurica (Fisch.ex Hoffm.)Benth.et Hook.f. var.formosana (Boiss.) Shan et Yuan (Fam.Umbelliferae). The total coumarins (Cou) and volatile oil (VO) were main active components that drived from Radix Angelicae Dahuricae. Our previous studies have shown that Cou and VO could increase intestinal absorption for transmucosal drug delivery with unknown mechanism. The aim of this study was to investigate the molecular mechanism of Radix Angelicae Dahuricae for improving drug intestinal transport. Caco-2 cell model was used to study the effect of Radix Angelicae Dahurica on transepithelial electrical resistance. Western blot was used to study its effect on the expression of the actin and ZO-1, tight junction proteins. The effect of Radix Angelicae Dahurica on the expression of P-gp protein was investigated using flow cytometry. VO (0.036-2.88 μL/mL) and Cou (0.027-0.54 mg/mL) caused a reversible, time- and dose-dependent decrease in transepithelial electrical resistance. VO and/or Cou could inhibit the expression of the tight junction protein, ZO-1 and actin. VO and/or Cou also could inhibit the expression of P-gp. These data suggested that Radix Angelicae Dahurica increased cell permeability by affecting the expression of actin, ZO-1 or P-gp, opening the tight junction or inhibiting the efflux induced by P-gp.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Dysfunctions at human intestinal barrier by water-borne protozoan parasites: lessons from cultured human fully differentiated colon cancer cell lines

Some water-borne protozoan parasites induce diseases through their membrane-associated functional structures and virulence factors that hijack the host cellular molecules and signalling pathways leading to structural and functional lesions in the intestinal barrier. In this Microreview we analyse the insights on the mechanisms of pathogenesis of Entamoeba intestinalis, Giardia and Cryptosporidium observed in the human colon carcinoma fully differentiated colon cancer cell lines, cell subpopulations and clones expressing the structural and functional characteristics of highly specialized fully differentiated epithelial cells lining the intestinal epithelium and mimicking structurally and functionally an intestinal barrier.

Computational analysis and predictive modeling of polymorph descriptors

Background

A computation approach based on integrating high throughput binding affinity comparison and binding descriptor classifications was utilized to establish the correlation among substrate properties and their affinity to Breast Cancer Resistant Protein (BCRP). The uptake rates of Mitoxantrone in the presence of various substrates were evaluated as an in vitro screening index for comparison of their binding affinity to BCRP.

The effects of chemical properties of various chemotherapeutics, such as antiviral, antibiotic, calcium channel blockers, anticancer and antifungal agents, on their affinity to BCRP, were evaluated using HEK (human embryonic kidney) cells in which 3 polymorphs, namely 482R (wild type) and two mutants (482G and 482T) of BCRP, have been identified. The quantitative structure activity relationship (QSAR) model was developed using the sequential approaches of Austin Model 1 (AM1), CODESSA program, heuristic method (HM) and multiple linear regression (MLR) to establish the relationship between structural specificity of BCRP substrates and their uptake rates by BCRP polymorphs.

Results

The BCRP mutations may induce conformational changes as manifested by the altered uptake rates of Mitoxantrone by BCRP in the presence of other competitive binding substrates that have a varying degree of affinities toward BCRP efflux. This study also revealed that the binding affinity of test substrates to each polymorph was affected by varying descriptors, such as constitutional, topological, geometrical, electrostatic, thermodynamic, and quantum chemical descriptors.

Conclusion

Descriptors involved with the net surface charge and energy level of substrates seem to be the common integral factors for defining binding specificity of selected substrates to BCRP polymorph. The reproducible outcomes and validation process further supported the accuracy of the computational model in assessing the correlation among descriptors involved with substrate affinity to BCRP polymorph. A quantitative computation approach will provide important structural insight into optimal designing of new chemotherapeutic agents with improved pharmacological efficacies.

Role of 99mTc-mannitol and 99mTc-PEG in the assessment of paracellular integrity of cell monolayers

AIM

To assess the role of 99mTc-mannitol and 99mTc-polyethylene glycol 4000 in the evaluation of paracellular integrity of Caco-2 and Madine-Darby canine kidney (MDCK) cell monolayers, and confirm it in the presence of absorption promoters.

METHODS

Radiolabelling of mannitol and polyethylene glycol was performed by a simple reduction method. Transepithelial electrical resistance values were measured to gain information regarding the integrity of tight junctions of Caco-2 and MDCK cell monolayers. Permeabilities of 99mTc-mannitol/99mTc-polyethylene glycol across cell monolayers were studied in the absence and presence of absorption promoters, namely dimethyl-beta-cyclodextrin, chitosan hydrochloride and sodium lauryl sulfate, and during recovery studies to assess paracellular integrity.

RESULTS

Values for the apparent permeability coefficient (Papp) of Tc-mannitol were found to be 0.286 x 10 cm x s(-1) and 0.507 x 10 cm x s(-1) in Caco-2 and MDCK cell monolayers, respectively, whereas corresponding values for 99mTc-polyethylene glycol were 0.046 x 10 cm x s(-1) and 0.065 x 10 cm x s(-1). The insignificant Papp values of the marker molecules demonstrated the paracellular integrity of the cell monolayers. Significant increases in the Papp values in the presence of absorption promoters and their combinations due to opening of paracellular pathways and a return of Papp values to almost baseline values during recovery studies confirm the role of these marker molecules in the assessment of paracellular integrity of cell monolayers.

CONCLUSION

99mTc-labelled marker molecules can be attractive, useful and viable alternatives to the conventionally used markers in the assessment of paracellular integrity because of the absence of tissue-damaging corpuscular radiation and the ease of production of radiochemically pure and stable molecules at a reasonable cost.

Decrease in intracellular concentration causes the shift in Km value of efflux pump substrates

Passive permeability and active efflux are parallel processes in transcellular flux. Therefore, the observed kinetics of a transporter substrate depends on both of these factors. The transporter expression has been shown to affect both the apparent K(m) and V(max) values. Kinetic parameters can be obtained from various experimental settings, but these do not necessarily reflect the situation in transcellular flux. Kinetic absorption models need reliable estimates of saturable kinetics when accurate in silico predictions are to be made. The effect of increasing P-glycoprotein expression on apparent transport kinetics was studied using quinidine and digoxin as model compounds. The intracellular concentrations of drugs during the transport process were also measured. A dynamic simulation model was constructed to study the observed data. The apparent K(m) and V(max) values increased as the P-glycoprotein expression increased. Simulations reproduced the shift in both kinetic parameters as a function of efflux pump expression. In addition, the apparent K(m) value showed a strong inverse relationship to the passive permeability. In contrast, the apparent V(max) value reached a maximum at intermediate passive permeability and declined above and below this passive permeability. The true V(max) and K(m) values were never reached. The shift in K(m) was assigned to a decrease in intracellular concentration at the P-glycoprotein interaction site with both experimental and simulation data. In conclusion, the apparent kinetic parameters in transcellular permeability assays depend on passive permeability and efflux pump activity. Therefore, parameters that are obtained from in vitro assays should be cautiously applied to in vivo predictions.

Expression, localisation and activity of ATP binding cassette (ABC) family of drug transporters in human amnion membranes

Placental ATP-binding cassette (ABC) transporters limit fetal exposure to xenobiotics by regulating transplacental passage into the fetal circulation; their expression and function in fetal membranes, however, has not been studied. In the present study the expression, localisation and function of ABC transporters in human amnion was examined to explore their potential role in modulating amniotic fluid drug disposition in pregnancy. Single-assay oligo-microarrays were used to profile amnion gene expression, and drug transporters expressed at significant levels were identified and selected for further studies. The expression of ABCG2/breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRP) 1 (ABCC1), 2 (ABCC2) and 5 (ABCC5) was detected on the arrays, and verified by RT-PCR and immunoblotting. On confocal microscopy of fetal membrane cryosections, MRP1 and MRP5 were immunolocalised to both apical and basolateral surfaces of the amniotic epithelium, while MRP2 was expressed at low levels only in the apical membrane. BCRP in contrast showed cytoplasmic staining throughout the amniotic epithelium. In addition to the amnion, MRP1 and BCRP immunostaining was observed in the chorion and the decidua. Cell accumulation studies using selective MRP and BCRP inhibitors showed the transporters to be functionally active in amnion epithelial monolayer cultures. In contrast, transwell transport studies using intact amnion membranes did not show significant vectorial transport. These findings identify the amnion as a novel site of ABC drug transporter expression. Functional studies indicate that they may act primarily to prevent cellular xenobiotic accumulation, rather than to confer fetal protection through reduced accumulation in amniotic fluid.

Effect of cell differentiation and passage number on the expression of efflux proteins in wild type and vinblastine-induced Caco-2 cell lines

The mRNA level expression of MDR1, MRP1-6, BCRP and CYP3A4 was determined by quantitative PCR in wild type (Caco-2WT) and vinblastine-treated (Caco-2VBL) Caco-2 cells at different passage levels (32-53). Differentiation increased the mRNA levels of MDR1, BCRP and all the MRPs except MRP4. Corresponding mRNA levels were observed in Caco-2WT and Caco-2VBL, except that the expression of MRD1 was higher in Caco-2VBL than in Caco-2WT cells. CYP3A4 was barely detected in either cell line. MDR1 functionality was studied using rhodamine123 and verapamil as a substrate-inhibitor pair. Corresponding to the observed differences in mRNA levels, MDR1 activity was higher in the Caco-2VBL cells. In Caco-2WT, MDR1 functionality was elevated at low passage numbers (32-35) compared to higher ones (49-53). Verapamil inhibited MDR1 efflux except at higher passage Caco-2WT cells, where no MDR1 activity could be observed. The results support the use of Caco-2VBL cells in MDR1 screening. The functional expression is higher than in Caco-2WT and remains consistent across the studied passages without major differences in mRNA levels of other efflux proteins. As both the passage number and the level of cell differentiation affect the expression profile of efflux proteins, short-term cell growth protocols should be evaluated accordingly.

Limited P-glycoprotein mediated efflux for anti-epileptic drugs

A variety of anti-epileptic drugs (AEDs) were tested for their ability to be transported by P-glycoprotein (P-gp) through Caco-2 monolayers using bi-directional (apical (Ap) to basolateral (Bas), and Bas to Ap) studies. Transport rates were equivalent in both directions for vigabatrin, gabapentin, phenobarbitone, lamotrigine and carbamazepine, being 0.7 x 10- 6, 0.1 x 10- 6, 34 x 10- 6, 36 x 10- 6 and 55 x 10- 6 cm/s, respectively. Phenytoin displayed a 20% increase in Ap to Bas transport, while topiramate and ethosuximide each had greater transport in the uptake direction, with both drugs showing no efflux. None of the transport rates for these drugs were affected by P-gp inhibitors. However, the efflux rate for acetazolamide was 3-fold higher than its uptake and this was significantly reduced by P-gp inhibitors. Thus, only one anti-epileptic, acetazolamide, was shown to be weak P-gp substrate, suggesting that P-gp efflux may not be a factor in relation to the development of resistance of epilepsy therapy.

Current industrial practices of assessing permeability and P-glycoprotein interaction

Combination of the in vitro models that are high throughput but less predictive and the in vivo models that are low throughput but more predictive is used effectively to evaluate the intestinal permeability and transport characteristics of a large number of drug candidates during lead selection and lead optimization processes. Parallel artificial membrane permeability assay and Caco-2 cells are the most frequently used in vitro models to assess intestinal permeability. The popularity of these models stems from their potential for high throughput, cost effectiveness, and adequate predictability of absorption potential in humans. However, several caveats associated with these models (eg, poor predictability for transporter-mediated and paracellularly absorbed compounds, significant nonspecific binding to cells/devices leading to poor recovery, variability associated with experimental factors) need to be considered carefully to realize their full potential. P-glycoprotein, among other pharmaceutically relevant transporters, has been well demonstrated to be the major determinant of drug disposition. The review article presents an objective analysis of the permeability and transporter models currently being used in the pharmaceutical industry and could help guide the discovery scientists in implementing these models in an optimal fashion.

Cell culture-based models for intestinal permeability: a critique

The model systems that are currently used to determine the intestinal permeability characteristics of discovery compounds often represent a combination of high-throughout, but less predictive, in silico and in vitro models and low-throughput, but more predictive, in vivo models. Cell-based permeability models have been integrated into the discovery paradigm for some time and represent the "method of choice" across the industry. Here, in addition to an objective analysis of the utility of cell culture models for permeability screening, anticipated future trends in the field of cell culture models are discussed.