Intestinal absorptive transport of the hydrophilic cation ranitidine: a kinetic modeling approach to elucidate the role of uptake and efflux transporters and paracellular vs. transcellular transport in Caco-2 cells

Experimental and computational models to investigate intestinal drug permeability and metabolism

Oral administration is the preferred route for drug administration that leads to better therapy compliance. The intestine plays a key role in the absorption and metabolism of oral drugs, therefore, new intestinal models are being continuously proposed, which contribute to the study of intestinal physiology, drug screening, drug side effects, and drug-drug interactions.Advances in pharmaceutical processes have produced more drug formulations, causing challenges for intestinal models. To adapt to the rapid evolution of pharmaceuticals, more intestinal models have been created. However, because of the complexity of the intestine, few models can take all aspects of the intestine into account, and some functions must be sacrificed to investigate other areas. Therefore, investigators need to choose appropriate models according to the experimental stage and other requirements to obtain the desired results.To help researchers achieve this goal, this review summarised the advantages and disadvantages of current commonly used intestinal models and discusses possible future directions, providing a better understanding of intestinal models.

Recent Advancement in Chitosan-Based Nanoparticles for Improved Oral Bioavailability and Bioactivity of Phytochemicals: Challenges and Perspectives

The excellent therapeutic potential of a variety of phytochemicals in different diseases has been proven by extensive studies throughout history. However, most phytochemicals are characterized by a high molecular weight, poor aqueous solubility, limited gastrointestinal permeability, extensive pre-systemic metabolism, and poor stability in the harsh gastrointestinal milieu. Therefore, loading of these phytochemicals in biodegradable and biocompatible nanoparticles (NPs) might be an effective approach to improve their bioactivity. Different nanocarrier systems have been developed in recent decades to deliver phytochemicals. Among them, NPs based on chitosan (CS) (CS-NPs), a mucoadhesive, non-toxic, and biodegradable polysaccharide, are considered the best nanoplatform for the oral delivery of phytochemicals. This review highlights the oral delivery of natural products, i.e., phytochemicals, encapsulated in NPs prepared from a natural polymer, i.e., CS, for improved bioavailability and bioactivity. The unique properties of CS for oral delivery such as its mucoadhesiveness, non-toxicity, excellent stability in the harsh environment of the GIT, good solubility in slightly acidic and alkaline conditions, and ability to enhance intestinal permeability are discussed first, and then the outcomes of various phytochemical-loaded CS-NPs after oral administration are discussed in detail. Furthermore, different challenges associated with the oral delivery of phytochemicals with CS-NPs and future directions are also discussed.

Intestinal Fish Cell Barrier Model to Assess Transfer of Organic Chemicals in Vitro: An Experimental and Computational Study

We studied the role of the fish intestine as a barrier for organic chemicals using the epithelial barrier model built on the rainbow trout (Oncorhynchus mykiss) intestinal cell line, RTgutGC and the newly developed exposure chamber, TransFEr, specifically designed to work with hydrophobic and volatile chemicals. Testing 11 chemicals with a range of physicochemical properties (logK_OW: 2.2 to 6.3, logHLC: 6.1 to 2.3) and combining the data with a mechanistic kinetic model enabled the determination of dominant processes underlying the transfer experiments and the derivation of robust transfer rates. Against the current assumption in chemical uptake modeling, chemical transfer did not strictly depend on the logK_OW but resulted from chemical-specific intracellular accumulation and biotransformation combined with paracellular and active transport. Modeling also identified that conducting elaborate measurements of the plastic parts, including the polystyrene insert and the PET filter, is unnecessary and that stirring in the TransFEr chamber reduced the stagnant water layers compared to theoretical predictions. Aside from providing insights into chemical uptake via the intestinal epithelium, this system can easily be transferred to other cell-based barrier systems, such as the fish gill or mammalian intestinal models and may improve in vitro-in vivo extrapolation and prediction of chemical bioaccumulation into organisms.

Pharmacokinetic Drug-Drug Interactions Between Trospium Chloride and Ranitidine Substrates of Organic Cation Transporters in Healthy Human Subjects

Trospium chloride, a muscarinic receptor blocker, is poorly absorbed with different rates from areas in the jejunum and the cecum/ascending colon. To evaluate whether organic cation transporter (OCT) 1, OCT2 and multidrug and toxin extrusion (MATE) 1 and MATE2-K are involved in pharmacokinetics, competitions with ranitidine, a probe inhibitor of the cation transporters, were evaluated in transfected HEK293 cells. Furthermore, a drug interaction study with trospium chloride after intravenous (2 mg) and oral dosing (30 mg) plus ranitidine (300 mg) was performed in 12 healthy subjects and evaluated by noncompartmental analysis and population pharmacokinetic modeling. Ranitidine inhibited OCT1, OCT2, MATE1, and MATE2-K with half maximal inhibitory concentration values of 186 ± 25 µM, 482 ± 105 µM, 134 ± 37 µM, and 35 ± 11 µM, respectively. In contrast to our hypothesis, coadministration of ranitidine did not significantly decrease oral absorption of trospium. Instead, renal clearance was lowered by ∼15% (530 ± 99 vs 460 ± 120 mL/min; P < .05). It is possible that ranitidine was not available in competitive concentrations at the major colonic absorption site, as the inhibitor is absorbed in the small intestine and undergoes degradation by microbiota. The renal effects apparently result from inhibition of MATE1 and/or MATE2-K by ranitidine as predicted by in vitro to in vivo extrapolation. However, all pharmacokinetic changes were not of clinical relevance for the drug with highly variable pharmacokinetics. Intravenous trospium significantly lowered mean absorption time and relative bioavailability of ranitidine, which was most likely caused by muscarinic receptor blocking effects on intestinal motility and water turnover.

Advance in oral delivery systems for therapeutic protein

Oral delivery is the most common method of drug administration with high safety and good compliance for patients. However, delivering therapeutic proteins to the target site via oral route involves tremendous challenge due to unfavourable conditions like biochemical barrier, mucus barrier and epithelial barriers. According to the functional differences of various protein drug delivery systems, the recent advances in pH responsive polymer-based drug delivery system, mucoadhesive polymer-based drug delivery system, absorption enhancers-based drug delivery system and composite polymer-based delivery system all were briefly summarised in this review, which not only clarified the clinic potential of these novel drug delivery systems, but also described the way for increasing oral bioavailability of therapeutic protein.

Clinical Implications of P-Glycoprotein Modulation in Drug-Drug Interactions

Drug-drug interactions (DDIs) occur commonly and may lead to severe adverse drug reactions if not handled appropriately. Considerable information to support clinical decision making regarding potential DDIs is available in the literature and through various systems providing electronic decision support for healthcare providers. The challenge for the prescribing physician lies in sorting out the evidence and identifying those drugs for which potential interactions are likely to become clinically manifest. P-glycoprotein (P-gp) is a drug transporting protein that is found in the plasma membranes in cells of barrier and elimination organs, and plays a role in drug absorption and excretion. Increasingly, P-gp has been acknowledged as an important player in potential DDIs and a growing body of information on the role of this transporter in DDIs has become available from research and from the drug approval process. This has led to a clear need for a comprehensive review of P-gp-mediated DDIs with a focus on highlighting the drugs that are likely to lead to clinically relevant DDIs. The objective of this review is to provide information for identifying and interpreting evidence of P-gp-mediated DDIs and to suggest a classification for individual drugs based on both in vitro and in vivo evidence (substrates, inhibitors and inducers). Further, various ways of handling potential DDIs in clinical practice are described and exemplified in relation to drugs interfering with P-gp.

Development of In Vitro Pharmacokinetic Screens Using Caco-2, Human Hepatocyte, and Caco-2/Human Hepatocyte Hybrid Systems for the Prediction of Oral Bioavailability in Humans

In this study, in vitro systems were used to build 2 pharmacokinetic models that predict human oral bioavailability: the Caco-2/hepatocyte combination model and the Caco-2/hepatocyte hybrid model. Data obtained in vitro on Caco-2 cell permeability and hepatocyte clearance are routinely used to predict the fraction of absorption after oral administration and the extent of first-pass metabolism, respectively. In the Caco-2/hepatocyte combination model, results from a Caco-2 cell permeability assay and a hepatocyte clearance assay were combined to project oral bioavailability. Comparison of oral bioavailabilities predicted by the combination model and reported oral bioavailabilities in humans for 30 marketed compounds resulted in a modest correlation ( r2 = 0.66). The Caco-2/hepatocyte hybrid model, as previously reported, joins the Caco-2 and hepatocyte clearance systems into 1 assay. Improvements to the previous model were made by incorporating an elimination phase into the Caco-2/hepatocyte hybrid model. In the new hybrid model, the compound was added to a Caco-2-containing donor compartment and allowed to permeate for 2 h to a hepatocyte-containing receiver compartment. Subsequently, to mimic an elimination phase, the donor compartment was removed, and permeated compound was incubated with hepatocytes alone for an additional 3 h. The area under the concentration versus time curve (AUC) was determined for each of the same 30 marketed compounds assessed by the combination model. A linear regression analysis comparing the in vitro AUCs and reported oral bioavailabilities in humans showed a reasonable correlation ( r 2 = 0.73). This study demonstrates that the Caco-2/hepatocyte hybrid model is more favorable and further proves the potential and feasibility of using in vitro screenings for the prediction of in vivo pharmacokinetics in humans. ( Journal of Biomolecular Screening 2007:1084-1091)

Oral delivery of insulin for treatment of diabetes: status quo, challenges and opportunities

Objectives

Diabetes mellitus is characterised by progressive β-cell destruction and loss of function, or loss of ability of tissues to respond to insulin. Daily subcutaneous insulin injection is standard management for people with diabetes, although patient compliance is hard to achieve due to the inconvenience of injections, so other forms of delivery are being tested, including oral administration. This review summarises the developments in oral insulin administration.

Methods

The PubMed database was consulted to compile this review comparing conventional subcutaneous injection of insulin to the desired oral delivery.

Key findings

Oral administration of insulin has potential benefits in reducing pain and chances of skin infection, improving the portal levels of insulin and avoiding side effects such as hyperinsulinemia, weight gain and hypoglycaemia. Although oral delivery of insulin is an ideal administration route for patients with diabetes, several physiological barriers have to be overcome. An expected low oral bioavailability can be attributed to its high molecular weight, susceptibility to enzymatic proteolysis and low diffusion rate across the mucin barrier.

Conclusions

Strategies for increasing the bioavailability of oral insulin include the use of enzyme inhibitors, absorption enhancers, mucoadhesive polymers and chemical modification for endogenous receptor-mediated absorption. These may help significantly increase patient compliance and disease management.

In vitro transport characteristics of EFdA, a novel nucleoside reverse transcriptase inhibitor using Caco-2 and MDCKII cell monolayers

4'-Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) is a novel nucleoside reverse transcriptase inhibitor with a unique mechanism of action and highly potent activity against both wild-type and clinically relevant drug resistant HIV-1 variants. Furthermore, in vivo efficacy and safety evaluations have shown EFdA to be a promising therapeutic candidate for use in the treatment of HIV infection. However, little is known about the pharmacokinetic and biopharmaceutical properties of EFdA. In this study, we evaluated cellular EFdA transport using Caco-2 and Madin-Darby Canine Kidney II (MDCKII) in vitro cell models. Studies using Caco-2 cell monolayers showed that EFdA efflux ratios were >2.0, suggesting that active drug transport mechanisms may play a role in EFdA flux. ABCB1 transporter (PGP1) inhibition was assessed using the acetomethoxy derivate of calcein (calcein-AM) as a fluorescent probe in both wild-type MDCKII and PGP1 overexpressing MDCKII cells. Nonetheless, our data showed that EFdA is not a substrate of PGP1. Additionally, comparative bidirectional flux of EFdA and Lucifer yellow (LY, a well-known paracellular marker) was studied over a range of EFdA concentrations. In MDCKII monolayers, EFdA had an apparent permeability coefficient (Papp) (a-b) of <1×10(-6)cm/s. The Papp values significantly increased in the presence of the paracellular permeability enhancer, indicating that EFdA primarily permeates via the paracellular route.

Computational approaches to analyse and predict small molecule transport and distribution at cellular and subcellular levels

Quantitative structure-activity relationship (QSAR) studies and mechanistic mathematical modeling approaches have been independently employed for analysing and predicting the transport and distribution of small molecule chemical agents in living organisms. Both of these computational approaches have been useful for interpreting experiments measuring the transport properties of small molecule chemical agents, in vitro and in vivo. Nevertheless, mechanistic cell-based pharmacokinetic models have been especially useful to guide the design of experiments probing the molecular pathways underlying small molecule transport phenomena. Unlike QSAR models, mechanistic models can be integrated from microscopic to macroscopic levels, to analyse the spatiotemporal dynamics of small molecule chemical agents from intracellular organelles to whole organs, well beyond the experiments and training data sets upon which the models are based. Based on differential equations, mechanistic models can also be integrated with other differential equations-based systems biology models of biochemical networks or signaling pathways. Although the origin and evolution of mathematical modeling approaches aimed at predicting drug transport and distribution has occurred independently from systems biology, we propose that the incorporation of mechanistic cell-based computational models of drug transport and distribution into a systems biology modeling framework is a logical next step for the advancement of systems pharmacology research.

Recent advances in chitosan-based nanoparticles for oral delivery of macromolecules

Chitosan (CS), a cationic polysaccharide, is widely regarded as a safe and efficient intestinal absorption enhancer of therapeutic macromolecules, owing to its inherent mucoadhesive feature and ability to modulate the integrity of epithelial tight junctions reversibly. By using CS-based nanoparticles, many studies have attempted to protect the loaded macromolecules against acidic denaturation and enzymatic degradation, prolong their intestinal residence time, and increase their absorption by the intestinal epithelium. Derivatives of CS such as quaternized CS, thiolated CS and carboxylated CS have also been examined to further enhance its effectiveness in oral absorption of macromolecular drugs. This review article describes the synthesis of these CS derivatives and their characteristics, as well as their potential transport mechanisms of macromolecular therapeutics across the intestinal biological membrane. Recent advances in using CS and its derivatives as carriers for oral delivery of hydrophilic macromolecules and their effects on drug transport are also reviewed.

Organic cation transporter 1 (OCT1/mOct1) is localized in the apical membrane of Caco-2 cell monolayers and enterocytes

Organic cation transporters (OCTs) are members of the solute carrier 22 family of transporter proteins that are involved in absorption, distribution, and excretion of organic cations. OCT3 is localized in the apical (AP) membrane of enterocytes, but the literature is ambiguous about OCT1 (mOct1) localization, with some evidence suggesting a basolateral (BL) localization in human and mouse enterocytes. This is contrary to our preliminary findings showing AP localization of OCT1 in Caco-2 cell monolayers, an established model of human intestinal epithelium. Therefore, this study aims at determining the localization of OCT1 (mOct1) in Caco-2 cells, and human and mouse enterocytes. Functional studies using OCT1-specific substrate pentamidine showed transporter-mediated AP but not BL uptake in Caco-2 cells and human and mouse intestinal tissues. OCT1 inhibition decreased AP uptake of pentamidine by ∼50% in all three systems with no effect on BL uptake. A short hairpin RNA-mediated OCT1 knockdown in Caco-2 cells decreased AP uptake of pentamidine by ∼50% but did not alter BL uptake. Immunostaining and confocal microscopy in all three systems confirmed AP localization of OCT1 (mOct1). Our studies unequivocally show AP membrane localization of OCT1 (mOct1) in Caco-2 cells and human and mouse intestine. These results are highly significant as they will require reinterpretation of previous drug disposition and drug-drug interaction studies where conclusions were drawn assuming BL localization of OCT1 in enterocytes. Most importantly, these results will require revision of the regulatory guidance for industry in the United States and elsewhere because it has stated that OCT1 is basolaterally localized in enterocytes.

A permeation enhancer for increasing transport of therapeutic macromolecules across the intestine

Delivery of therapeutic macromolecules is limited by the physiological limitations of the gastrointestinal tract including poor intestinal permeability, low pH and enzymatic activity. Several permeation enhancers have been proposed to enhance intestinal permeability of macromolecules; however their utility is often hindered by toxicity and limited potency. Here, we report on a novel permeation enhancer, Dimethyl palmitoyl ammonio propanesulfonate (PPS), with excellent enhancement potential and minimal toxicity. PPS was tested for dose- and time-dependent cytotoxicity, delivery of two model fluorescent molecules, sulforhodamine-B and FITC-insulin in vitro, and absorption enhancement of salmon calcitonin (sCT) in vivo. Caco-2 studies revealed that PPS is an effective enhancer of macromolecular transport while being minimally toxic. TEER measurements in Caco-2 monolayers confirmed the reversibility of the effect of PPS. Confocal microscopy studies revealed that molecules permeate via both paracellular and transcellular pathways in the presence of PPS. In vivo studies in rats showed that PPS enhanced relative bioavailability of sCT by 45-fold after intestinal administration. Histological studies showed that PPS does not induce damage to the intestine. PPS is an excellent permeation enhancer which provides new opportunities for developing efficacious oral/intestinal delivery systems for therapeutic macromolecules.

Drug discovery and regulatory considerations for improving in silico and in vitro predictions that use Caco-2 as a surrogate for human intestinal permeability measurements

There is a growing need for highly accurate in silico and in vitro predictive models to facilitate drug discovery and development. Results from in vitro permeation studies across the Caco-2 cell monolayer are commonly used for drug permeability screening in industry and are also accepted as a surrogate for human intestinal permeability measurements by the US FDA to support new drug applications. Countless studies carried out in this cell line with published permeability measurements have enabled the development of many in silico prediction models. We identify several common cases that illustrate how using Caco-2 permeability measurements in these in silico and in vitro predictive models will not correlate with human intestinal permeability and will further lead to inaccuracies in these models. We provide guidelines and recommendations for improving these models to more accurately predict clinically relevant information, thereby enhancing the drug discovery, development, and regulatory approval processes.

Development and evaluation of an in vitro vaginal model for assessment of drug's biopharmaceutical properties: curcumin

Vaginal administration is a promising alternative to the per-oral route in achieving systemic or local therapeutic effects, when intestinal drug absorption is hindered by problematic biopharmaceutical drug properties. The aim of this study was to establish an in vitro vaginal model and use it to characterize biopharmaceutical properties of liposomally associated curcumin destined for vaginal delivery. The in vitro permeability, metabolism, and tissue retention of high/low permeable compounds were assessed on cow vaginal mucosa and compared to the permeabilities determined through Caco-2 cells and rat jejunum in vitro. The results showed that the intestinal mucosa was superior to the vaginal one in categorizing drugs based on their permeabilities in high/low permeable classes. Passive diffusion was found to be the main mechanism of drug penetration through vaginal mucosa and it was not affected by transporter-enzyme alliance, as their expression/activity was significantly reduced compared to the intestinal tract. Curcumin permeability from the solution form was the lowest of all tested substances due to its significant tissue retention and curcumin-mucus interactions. The permeability of liposomally associated curcumin was even lower but the binding of liposomally associated curcumin to the vaginal tissue was significantly higher. The permeability and tissue retention of liposomal curcumin were vesicle size dependent. Vaginal application of liposomally associated curcumin provides relatively high levels of curcumin in vaginal tissue, with limited systemic absorption.

Specific microbicides in the prevention of HIV infection

Microbicides are products that are designed for application at vaginal or rectal mucosae to inhibit or block early events in HIV infection and thereby prevent transmission of HIV. Currently, the most advanced microbicides in the development pipeline are based on highly active anti-retroviral drugs (ARVs). Significant protection of women by vaginally applied tenofovir gel, demonstrated in the CAPRISA 004 trial, has provided proof-of-concept that microbicides can be effective. The rationale for investigating ARVs and other compounds as vaginal or rectal microbicides is discussed together with approaches to improve efficacy by the development of combination microbicides and by new formulations that may increase user acceptance.

Analysis of unstirred water layer in in vitro permeability experiments

In vitro permeability experiments are used widely in drug discovery and other areas of pharmaceutical research. Much effort has been expended in developing novel epithelial models but generally much less attention has been paid to the hydrodynamic barrier in the actual experiments. The restricted liquid flow in the vicinity of solid surfaces leads to a zone where the diffusional movement of molecules exceeds the convection. This leads to formation of a concentration gradient between the bulk solution and the surface. The formed unstirred water layer (UWL) reduces the apparent permeability (P(app)) of compounds that rapidly pass through the actual epithelial layer. This lowers the resolution of P(app) versus fraction-absorbed assay, complicates the structure-permeability analysis and skews apparent kinetic parameters of transporter substrates. This review describes the techniques that can be used to determine the UWL thickness in permeability experiments and apparatuses described in the literature to control the in vitro hydrodynamics.

The Caco-2 cell monolayer: usefulness and limitations

BACKGROUND

The Caco-2 monolayer has been used extensively for the high-throughput screening of drug permeability and identification of substrates, inhibitors, and inducers of intestinal transporters, especially P-glycoprotein (P-gp). Traditionally, the Caco-2 monolayer is viewed as a single barrier rather than a polarized cell monolayer consisting of metabolic enzymes that are sandwiched between two membrane barriers with distinctly different transporters.

OBJECTIVE

This review addressed the usefulness and limitations of the Caco-2 cell monolayer in drug discovery and mechanistic studies.

METHODS

This mini-review covered applications of the Caco-2 monolayer, clarified misconceptions, and critically addressed issues on data interpretation.

CONCLUSION

The catenary model extends the usefulness of Caco-2 monolayer and provides proper mechanistic insight and data interpretation.

Polyethylene glycol 400 enhances the bioavailability of a BCS class III drug (ranitidine) in male subjects but not females

PURPOSE

The aim of this study was to investigate the effects of different doses of polyethylene glycol 400 (PEG 400) on the bioavailability of ranitidine in male and female subjects.

METHOD

Ranitidine (150 mg) was dissolved in 150 ml water with 0 (control), 0.5, 0.75, 1, 1.25 or 1.5 g PEG 400 and administered to 12 healthy human volunteers (six males and six females) in a randomized order. The cumulative amount of ranitidine and its metabolites excreted in urine over 24 h was determined for each treatment using a validated HPLC method.

RESULTS

In the male volunteers, the mean cumulative amount of ranitidine excreted in the presence of 0, 0.5, 0.75, 1, 1.25 and 1.5 g PEG 400 were 35, 47, 57, 52, 50 and 37 mg respectively. These correspond to increases in bioavailability of 34%, 63%, 49%, 43% and 6% over the control treatment. In the female subjects, the mean cumulative quantity of ranitidine excretion in the absence and presence of increasing amounts of PEG 400 were 38, 29, 35, 33, 33 and 33 mg, corresponding to decreases in bioavailability of 24%, 8%, 13%, 13% and 13% compared to the control. The metabolite excretion profiles followed a similar trend to the parent drug at all concentrations of PEG 400.

CONCLUSIONS

All doses of PEG 400 enhanced the bioavailability of ranitidine in male subjects but not females, with the most pronounced effect in males noted with the 0.75 g dose of PEG 400 (63% increase in bioavailability compared to control, p < 0.05). These findings have significant implications for the use of PEG 400 in drug development and also highlight the importance of gender studies in pharmacokinetics.

Mechanisms underlying saturable intestinal absorption of metformin

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

A catenary model to study transport and conjugation of baicalein, a bioactive flavonoid, in the Caco-2 cell monolayer: demonstration of substrate inhibition

The transport and metabolism of baicalein (Ba) was studied in vitro and in Caco-2 cells. Protein binding of Ba with Caco-2 lysate showed that Ba was bound to two classes of sites: a higher affinity, lower capacity site (K(A1) = 27.6 +/- 4.7 microM(-1), n(1) = 10.6 +/- 0.6 nmol/mg) and lower affinity, higher capacity site (K(A2) = 0.015 +/- 0.0013 microM(-1), n(2) = 413 +/- 21 nmol/mg). Incubation studies of Ba with Caco-2 lysate showed substrate inhibition of both glucuronidation and sulfation, with K(m) values of 0.14 +/- 0.034 and 0.015 +/- 0.0053 microM, and K(I) values of 6.75 +/- 1.70 and 0.37 +/- 0.16 microM, respectively. In the Caco-2 monolayer, Ba (8-47 microM) displayed good apparent permeabilities (P(app)) across the membrane; P(app) was found to be increased with elevated loading concentration in both the absorptive and secretory directions. However, the efflux ratio was less than unity, negating the involvement of apical efflux transporters. The concentration ratios of Ba sulfate (BS) and glucuronide (BG) decreased with increased loading Ba concentration, suggesting that BS and BG are apically excreted via transporters, likely breast cancer resistance protein and multidrug resistance-associated protein 2, respectively. Data fit to the catenary model, composed of basolateral, cellular, and apical compartments, showed a low cellular unbound fraction (0.0019 +/- 0.00018), a high passive diffusion clearance (0.012 +/- 0.00029 ml/min/mg), and substrate inhibition, with sulfation being more readily saturated and inhibited than glucuronidation, as evidenced by smaller K(m) value (0.35 +/- 0.078 versus 1.95 +/- 0.57 microM) and K(I) value (0.58 +/- 0.20 versus 7.90 +/- 1.10 microM); these patterns paralleled those observed in the lysate incubation studies. The results showed that the catenary model aptly predicts substrate inhibition kinetics and offers significant and mechanistic insight into the transport and atypical metabolism of drugs in the Caco-2 monolayer.

Predicting drug disposition, absorption/elimination/transporter interplay and the role of food on drug absorption

The ability to predict drug disposition involves concurrent consideration of many chemical and physiological variables and the effect of food on the rate and extent of availability adds further complexity due to postprandial changes in the gastrointestinal (GI) tract. A system that allows for the assessment of the multivariate interplay occurring following administration of an oral dose, in the presence or absence of meal, would greatly benefit the early stages of drug development. This is particularly true in an era when the majority of new molecular entities are highly permeable, poorly soluble, extensively metabolized compounds (BDDCS Class 2), which present the most complicated relationship in defining the impact of transporters due to the marked effects of transporter-enzyme interplay. This review evaluates the GI luminal environment by taking into account the absorption/transport/elimination interplay and evaluates the physiochemical property issues by taking into account the importance of solubility, permeability and metabolism. We concentrate on the BDDCS and its utility in predicting drug disposition. Furthermore, we focus on the effect of food on the extent of drug availability (F), which appears to follow closely what might be expected if a significant effect of high fat meals is inhibition of transporters. That is, high fat meals and lipidic excipients would be expected to have little effect on F for Class 1 drugs; they would increase F of Class 2 drugs, while decreasing F for Class 3 drugs.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

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.