Survey of Pharmaceutical Industry's Best Practices around In Vitro Transporter Assessment and Implications for Drug Development: Considerations from the International Consortium for Innovation and Quality for Pharmaceutical Development Transporter Working Group.

The International Consortium for Innovation and Quality in Pharmaceutical Development Transporter Working Group had a rare opportunity to analyze a crosspharma collation of in vitro data and assay methods for the evaluation of drug transporter substrate and inhibitor potential. Experiments were generally performed in accordance with regulatory guidelines. Discrepancies, such as not considering the impact of preincubation for inhibition and free or measured in vitro drug concentrations, may be due to the retrospective nature of the dataset and analysis. Lipophilicity was a frequent indicator of crosstransport inhibition (P-gp, BCRP, OATP1B, and OCT1), with high molecular weight (MW ≥500 Da) also common for OATP1B and BCRP inhibitors. A high level of overlap in in vitro inhibition across transporters was identified for BCRP, OATP1B1, and MATE1, suggesting that prediction of DDIs for these transporters will be common. In contrast, inhibition of OAT1 did not coincide with inhibition of any other transporter. Neutrals, bases, and compounds with intermediate-high lipophilicity tended to be P-gp and/or BCRP substrates, whereas compounds with MW <500 Da tended to be OAT3 substrates. Interestingly, the majority of in vitro inhibitors were not reported to be followed up with a clinical study by the submitting company, whereas those compounds identified as substrates generally were. Approaches to metabolite testing were generally found to be similar to parent testing, with metabolites generally being equally or less potent than parent compounds. However, examples where metabolites inhibited transporters in vitro were identified, supporting the regulatory requirement for in vitro testing of metabolites to enable integrated clinical DDI risk assessment. SIGNIFICANCE STATEMENT: A diverse dataset showed that transporter inhibition often correlated with lipophilicity and molecular weight (>500 Da). Overlapping transporter inhibition was identified, particularly that inhibition of BCRP, OATP1B1, and MATE1 was frequent if the compound inhibited other transporters. In contrast, inhibition of OAT1 did not correlate with the other drug transporters tested.

Amplifying the impact of kidney microphysiological systems: predicting renal drug clearance using mechanistic modelling based on reconstructed drug secretion.

Accurate prediction of pharmacokinetic parameters, such as renal clearance, is fundamental to the development of effective and safe new treatments for patients. However, conventional renal models have a limited ability to predict renal drug secretion, a process that is dependent on transporters in the proximal tubule. Improvements in microphysiological systems (MPS) have extended our in vitro capabilities to predict pharmacokinetic parameters. In this study a kidney-MPS model was developed that successfully recreated renal drug secretion. Human proximal tubule cells grown in the kidney-MPS, resem­bling an in vivo phenotype, actively secreted the organic cation drug metformin and organic anion drug cidofovir, in contrast to cells cultured in conventional culture formats. Metformin and cidofovir renal secretory clearance were predicted from kid­ney-MPS data within 3.3- and 1.3-fold, respectively, of clinically reported values by employing a semi-mechanistic drug distribution model using kidney-MPS drug transport parameters together with in vitro to in vivo extrapolation. This approach introduces an effective application of a kidney-MPS model coupled with pharmacokinetic modelling tools to evaluate and predict renal drug clearance in humans. Kidney-MPS renal clearance predictions can potentially complement pharma-cokinetic animal studies and contribute to the reduction of pre-clinical species use during drug development.

Development of a hollow fibre-based renal module for active transport studies.

Understanding the active transport of substrates by the kidney in the renal proximal convoluted tubule is crucial for drug development and for studying kidney diseases. Currently, cell-based assays are applied for this this purpose, however, differences between assays and the body are common, indicating the importance of in vitro-in vivo discrepancies. Several studies have suggested that 3D cell cultures expose cells to a more physiological environments, thus, providing more accurate cell function results. To mimic the renal proximal tubule, we have developed a custom-made renal module (RM), containing a single polypropylene hollow fibre (Plasmaphan P1LX, 3M) that serves as a porous scaffold and compared to conventional Transwell cell-based bidirectional transport studies. In addition, a constant flow of media, exposed cells to a physiological shear stress of 0.2 dyne/cm2. MDCK-Mdr1a cells, overexpressing the rat Mdr1a (P-gp) transporter, were seeded onto the HF membrane surface coated with the basement membrane matrix Geltrex which facilitated cell adhesion and tight junction formation. Cells were then seeded into the HF lumen where attachment and tight junction formation were evaluated by fluorescence microscopy while epithelial barrier integrity under shear stress was shown to be achieved by day 7. qPCR results have shown significant changes in gene expression compared to cells grown on Transwells. Kidney injury marker such as KIM-1 and the hypoxia marker CA9 have been downregulated, while the CD133 (Prominin-1) microvilli marker has shown a fivefold upregulation. Furthermore, the renal transporter P-gp expression has been downregulated by 50%. Finally, bidirectional assays have shown that cells grown in the RM were able to reabsorb albumin with a higher efficiency compared to Transwell cell cultures while efflux of the P-gp-specific substrates Hoechst and Rhodamine 123 was decreased. These results further support the effect of the microenvironment and fluidic shear stress on cell function and gene expression. This can serve as the basis for the development of a microphysiological renal model for drug transport studies.

A Novel Method for Preventing Non-specific Binding in Equilibrium Dialysis Assays Using Solutol® as an Additive.

Accurate determination of fraction unbound in plasma is required for the interpretation of pharmacology and toxicology data, in addition to predicting human pharmacokinetics, dose, and drug-drug interaction potential. A trend, largely driven by changing target space and new chemical modalities, has increased the occurrence of compounds beyond the traditional rule of 5 physicochemical property space, meaning many drugs under development have high lipophilicity. This can present challenges for ADME assays, including non-specific binding to labware, low dynamic range and solubility. When determining unbound fraction, low recovery, due to non-specific binding, makes bioanalytical sensitivity limiting and prevents determination of free fraction for highly bound compounds. Here, mitigation of non-specific binding through the addition of 0.01% v/v of the excipient Solutol® to an equilibrium dialysis assay has been explored. Solutol® prevented non-specific binding to the dialysis membrane and showed no significant binding to plasma proteins. A test set of compounds demonstrates that this method gives comparable values of fraction unbound. In conclusion, the use of Solutol® as an additive in equilibrium dialysis formats could provide a method of mitigating non-specific binding, enabling the determination of fraction unbound values for highly lipophilic compounds.

Molecular Mechanisms for Species Differences in Organic Anion Transporter 1, OAT1: Implications for Renal Drug Toxicity.

Species differences in renal drug transporters continue to plague drug development with animal models failing to adequately predict renal drug toxicity. For example, adefovir, a renally excreted antiviral drug, failed clinical studies for human immunodeficiency virus due to pronounced nephrotoxicity in humans. In this study, we demonstrated that there are large species differences in the kinetics of interactions of a key class of antiviral drugs, acyclic nucleoside phosphonates (ANPs), with organic anion transporter 1 [(OAT1) SLC22A6] and identified a key amino acid residue responsible for these differences. In OAT1 stably transfected human embryonic kidney 293 cells, the Km value of tenofovir for human OAT1 (hOAT1) was significantly lower than for OAT1 orthologs from common preclinical animals, including cynomolgus monkey, mouse, rat, and dog. Chimeric and site-directed mutagenesis studies along with comparative structure modeling identified serine at position 203 (S203) in hOAT1 as a determinant of its lower Km value. Furthermore, S203 is conserved in apes, and in contrast alanine at the equivalent position is conserved in preclinical animals and Old World monkeys, the most related primates to apes. Intriguingly, transport efficiencies are significantly higher for OAT1 orthologs from apes with high serum uric acid (SUA) levels than for the orthologs from species with low serum uric acid levels. In conclusion, our data provide a molecular mechanism underlying species differences in renal accumulation of nephrotoxic ANPs and a novel insight into OAT1 transport function in primate evolution.

Current State of In vitro Cell-Based Renal Models.

BACKGROUND: Renal proximal tubule (PT) epithelial cells, expressing uptake and efflux transporters at basolateral and apical membranes, are the location of active renal drug secretion and reabsorption. In addition to singly transfected cells, an in vitro renal cell-based model is a requirement to study the active renal secretion of drugs, drug-drug interactions (DDIs), drug-induced kidney injury, nephrotoxicity holistically and potentially renal replacement therapies. OBJECTIVES: So far, two-dimensional (2D) cell culture of primary and immortalized PT cells has been the only tool to study drugs active secretion, interactions and nephrotoxicity, however a number of in vivo characteristics of cells such as drug transporter expression and function, along with morphological features are lost during in vitro cell culture. Cellular microenvironment, extracellular matrix, cell-cell interactions, microfluidic environment and tubular architecture are the factors lacking in 2D cell culture. Currently, there are a few 3D cell culture platforms mimicking the in vivo conditions of PT cells and thus potentially enabling the necessary factors for the full functional PT cells. CONCLUSION: In this review, we address in vivo physiological and morphological characteristics of PT cells, comparing their available sources and remaining in vivo features. In addition, 2D and 3D cell culture platforms and the influence of cell culture architecture on the physiological characteristics of cells are reviewed. Finally, future perspective of 3D models, kidney and multi organs on a chip, generation of kidney organoids, other ex vivo renal models and their capabilities to study drug disposition and in vitro-in vivo extrapolation are described.

Solitary Inhibition of the Breast Cancer Resistance Protein Efflux Transporter Results in a Clinically Significant Drug-Drug Interaction with Rosuvastatin by Causing up to a 2-Fold Increase in Statin Exposure.

The intestinal efflux transporter breast cancer resistance protein (BCRP) restricts the absorption of rosuvastatin. Of the transporters important to rosuvastatin disposition, fostamatinib inhibited BCRP (IC50 = 50 nM) and organic anion-transporting polypeptide 1B1 (OATP1B1; IC50 > 10 µM), but not organic anion transporter 3, in vitro, predicting a drug-drug interaction (DDI) in vivo through inhibition of BCRP only. Consequently, a clinical interaction study between fostamatinib and rosuvastatin was performed (and reported elsewhere). This confirmed the critical role BCRP plays in statin absorption, as inhibition by fostamatinib resulted in a significant 1.96-fold and 1.88-fold increase in rosuvastatin area under the plasma concentration-time curve (AUC) and Cmax, respectively. An in vitro BCRP inhibition assay, using polarized Caco-2 cells and rosuvastatin as probe substrate, was subsequently validated with literature inhibitors and used to determine BCRP inhibitory potencies (IC50) of the perpetrator drugs eltrombopag, darunavir, lopinavir, clopidogrel, ezetimibe, fenofibrate, and fluconazole. OATP1B1 inhibition was also determined using human embryonic kidney 293-OATP1B1 cells versus estradiol 17ß-glucuronide. Calculated parameters of maximum enterocyte concentration [Igut max], maximum unbound hepatic inlet concentration, transporter fraction excreted value, and determined IC50 value were incorporated into mechanistic static equations to compute theoretical increases in rosuvastatin AUC due to inhibition of BCRP and/or OATP1B1. Calculated theoretical increases in exposure correctly predicted the clinically observed changes in rosuvastatin exposure and suggested intestinal BCRP inhibition (not OATP1B1) to be the mechanism underlying the DDIs with these drugs. In conclusion, solitary inhibition of the intestinal BCRP transporter can result in clinically significant DDIs with rosuvastatin, causing up to a maximum 2-fold increase in exposure, which may warrant statin dose adjustment in clinical practice.

In vitro characterization of axitinib interactions with human efflux and hepatic uptake transporters: implications for disposition and drug interactions.

Axitinib is an inhibitor of tyrosine kinase vascular endothelin growth factor receptors 1, 2, and 3. The ATP-binding cassette (ABC) and solute carrier (SLC) transport properties of axitinib were determined in selected cellular systems. Axitinib exhibited high passive permeability in all cell lines evaluated (Papp ≥ 6 × 10(-6) cm/s). Active efflux was observed in Caco-2 cells, and further evaluation in multidrug resistance gene 1 (MDR1) or breast cancer resistance protein (BCRP) transfected Madin-Darby canine kidney cells type 2 (MDCK) cells indicated that axitinib is at most only a weak substrate for P-glycoprotein (P-gp) but not BCRP. Axitinib showed incomplete inhibition of P-gp-mediated transport of digoxin in Caco-2 cells and BCRP transport of topotecan in BCRP-transfected MDCK cells with IC50 values of 3 µM and 4.4 µM, respectively. Axitinib (10 mg) did not pose a risk for systemic drug interactions with P-gp or BCRP per regulatory guidance. A potential risk for drug interactions through inhibition of P-gp and BCRP in the gastrointestinal tract was identified because an axitinib dose of 10 mg divided by 250 mL was greater than 10-fold the IC50 for each transporter. However, a GastroPlus simulation that considered the low solubility of axitinib resulted in lower intestinal concentrations and suggested a low potential for gastrointestinal interactions with P-gp and BCRP substrates. Organic anion transporting polypeptide 1B1 (OATP1B1) and OATP1B3 transfected human embryonic kidney 293 (HEK293) cells transported axitinib to a minor extent but uptake into suspended hepatocytes was not inhibited by rifamycin SV suggesting that high passive permeability predominates. Mouse whole-body autoradiography revealed that [(14)C]axitinib-equivalents showed rapid absorption and distribution to all tissues except the brain. This suggests that efflux transport of axitinib may occur at the mouse blood-brain barrier.

The use of bioreactors as in vitro models in pharmaceutical research.

Bringing a new drug to market is costly in terms of capital and time investments, and any development issues encountered during late-stage clinical trials can often be the result of in vitro-in vivo extrapolations (IVIVE) not accurately reflecting clinical outcome. In the discipline of drug metabolism and pharmacokinetics (DMPK), current in vitro cellular methods do not provide the 3D structure and function of organs found in vivo; therefore, new dynamic methods need to be established to aid improvement of IVIVE. In this review, we highlight the importance of model progression into dynamic systems for use within drug development, focusing on devices developed currently in the areas of the liver and blood-brain barrier (BBB), and the potential to develop models for other organ systems, such as the kidney. We discuss the development of dynamic 3D bioreactor-based systems as in vitro models for use in DMPK studies.

Mechanistic insights from comparing intrinsic clearance values between human liver microsomes and hepatocytes to guide drug design.

Metabolic stability of drug candidates are often determined in both liver microsome and hepatocyte assays. Comparison of intrinsic clearance values between the two assays provides additional information to guide drug design. Intrinsic clearance values from human liver microsomes and hepatocytes were compared for a set of commercial drugs with known metabolic pathways and transporter characteristics. The results showed that for compounds that were predominately metabolized by CYP mediated mechanisms, the intrinsic clearance values from the two assays were comparable. For compounds with non-CYP pathways, such as UGT and AO, intrinsic clearance was faster in hepatocytes than in microsomes. Substrates of uptake or efflux transporters in this study did not have significant differences of intrinsic clearance between microsomes and hepatocytes, when uptake into the hepatocytes was not the rate-limiting step. When hepatic uptake was rate limiting, intrinsic clearance in microsomes was faster than that in hepatocytes, which was more prevalent for compounds with rapid metabolism. Low passive permeability can limit the exposure to drug molecules to the metabolizing enzymes in the hepatocytes in relationship to the rate of metabolism. The faster the rate of metabolism, the higher permeability is needed for molecule to enter the cells and not becoming rate-limiting. The findings are very useful for drug discovery programs to gain additional insights on mechanistic information to help drug design without added experiments. Follow-up studies can then be designed to address specific questions.

pH-Dependent solubility and permeability criteria for provisional biopharmaceutics classification (BCS and BDDCS) in early drug discovery.

The Biopharmaceutics Classification System (BCS) is a scientific framework that provides a basis for predicting the oral absorption of drugs. These concepts have been extended in the Biopharmaceutics Drug Disposition Classification System (BDDCS) to explain the potential mechanism of drug clearance and understand the effects of uptake and efflux transporters on absorption, distribution, metabolism, and elimination. The objective of present work is to establish criteria for provisional biopharmaceutics classification using pH-dependent passive permeability and aqueous solubility data generated from high throughput screening methodologies in drug discovery settings. The apparent permeability across monolayers of clonal cell line of Madin-Darby canine kidney cells, selected for low endogenous efflux transporter expression, was measured for a set of 105 drugs, with known BCS and BDDCS class. The permeability at apical pH 6.5 for acidic drugs and at pH 7.4 for nonacidic drugs showed a good correlation with the fraction absorbed in human (Fa). Receiver operating characteristic (ROC) curve analysis was utilized to define the permeability class boundary. At permeability ≥ 5 × 10(-6) cm/s, the accuracy of predicting Fa of ≥ 0.90 was 87%. Also, this cutoff showed more than 80% sensitivity and specificity in predicting the literature permeability classes (BCS), and the metabolism classes (BDDCS). The equilibrium solubility of a subset of 49 drugs was measured in pH 1.2 medium, pH 6.5 phosphate buffer, and in FaSSIF medium (pH 6.5). Although dose was not considered, good concordance of the measured solubility with BCS and BDDCS solubility class was achieved, when solubility at pH 1.2 was used for acidic compounds and FaSSIF solubility was used for basic, neutral, and zwitterionic compounds. Using a cutoff of 200 µg/mL, the data set suggested a 93% sensitivity and 86% specificity in predicting both the BCS and BDDCS solubility classes. In conclusion, this study identified pH-dependent permeability and solubility criteria that can be used to assign provisional biopharmaceutics class at early stage of the drug discovery process. Additionally, such a classification system will enable discovery scientists to assess the potential limiting factors to oral absorption, as well as help predict the drug disposition mechanisms and potential drug-drug interactions.

In vitro evaluation of hepatic transporter-mediated clinical drug-drug interactions: hepatocyte model optimization and retrospective investigation.

To assess the feasibility of using sandwich-cultured human hepatocytes (SCHHs) as a model to characterize transport kinetics for in vivo pharmacokinetic prediction, the expression of organic anion-transporting polypeptide (OATP) proteins in SCHHs, along with biliary efflux transporters, was confirmed quantitatively by liquid chromatography-tandem mass spectrometry. Rifamycin SV (Rif SV), which was shown to completely block the function of OATP transporters, was selected as an inhibitor to assess the initial rates of active uptake. The optimized SCHH model was applied in a retrospective investigation of compounds with known clinically significant OATP-mediated uptake and was applied further to explore drug-drug interactions (DDIs). Greater than 50% inhibition of active uptake by Rif SV was found to be associated with clinically significant OATP-mediated DDIs. We propose that the in vitro active uptake value therefore could serve as a cutoff for class 3 and 4 compounds of the Biopharmaceutics Drug Disposition Classification System, which could be integrated into the International Transporter Consortium decision tree recommendations to trigger clinical evaluations for potential DDI risks. Furthermore, the kinetics of in vitro hepatobiliary transport obtained from SCHHs, along with protein expression scaling factors, offer an opportunity to predict complex in vivo processes using mathematical models, such as physiologically based pharmacokinetics models.

Mechanistic pharmacokinetic modeling for the prediction of transporter-mediated disposition in humans from sandwich culture human hepatocyte data.

With efforts to reduce cytochrome P450-mediated clearance (CL) during the early stages of drug discovery, transporter-mediated CL mechanisms are becoming more prevalent. However, the prediction of plasma concentration-time profiles for such compounds using physiologically based pharmacokinetic (PBPK) modeling is far less established in comparison with that for compounds with passively mediated pharmacokinetics (PK). In this study, we have assessed the predictability of human PK for seven organic anion-transporting polypeptide (OATP) substrates (pravastatin, cerivastatin, bosentan, fluvastatin, rosuvastatin, valsartan, and repaglinide) for which clinical intravenous data were available. In vitro data generated from the sandwich culture human hepatocyte system were simultaneously fit to estimate parameters describing both uptake and biliary efflux. Use of scaled active uptake, passive distribution, and biliary efflux parameters as inputs into a PBPK model resulted in the overprediction of exposure for all seven drugs investigated, with the exception of pravastatin. Therefore, fitting of in vivo data for each individual drug in the dataset was performed to establish empirical scaling factors to accurately capture their plasma concentration-time profiles. Overall, active uptake and biliary efflux were under- and overpredicted, leading to average empirical scaling factors of 58 and 0.061, respectively; passive diffusion required no scaling factor. This study illustrates the mechanistic and model-driven application of in vitro uptake and efflux data for human PK prediction for OATP substrates. A particular advantage is the ability to capture the multiphasic plasma concentration-time profiles for such compounds using only preclinical data. A prediction strategy for novel OATP substrates is discussed.

Full efficacy with no CNS side-effects: unachievable panacea or reality? DMPK considerations in design of drugs with limited brain penetration.

Optimising drug properties can be an important strategy to limit penetration into the CNS and offers advantages in reducing the risk of undesirable neurological effects When considering the design of these drugs it is important to consider the relative influx and efflux rates at the relevant biological membranes The highest degree of restriction at the brain is probably achievable by utilising active transport to exclude compounds from the brain Affinity for the efflux transporters Pgp and BCRP has been achieved in two in-house chemistry programmes by increasing polar surface area, which resulted in highly orally bioavailable low CNS penetrant compounds in preclinical species.

Differential modulation of cytochrome P450 activity and the effect of 1-aminobenzotriazole on hepatic transport in sandwich-cultured human hepatocytes.

Sandwich-cultured human hepatocytes (SCHH) have been widely used for in vitro assessments of biliary clearance. However, the modulation of metabolism enzymes has not been fully evaluated in this system. The present study was therefore undertaken to determine the activity of cytochrome P450 (P450) 1A2, 2C8, 2C9, 2C19, 2D6, and 3A and to evaluate the impact of 1-aminobenzotriazole (ABT) on hepatic uptake and biliary excretion in SCHH. The SCHH maintained integrity and viability as determined by lactate dehydrogenase release and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium assays conducted over the culture period. Although all assessed P450 activity decreased in day 2 SCHH, the extent of the decrease and the subsequent rebound in activity varied across the different isoforms. Day 5 CYP1A2 activity was approximately 2.5-fold higher than day 1 activity, whereas the CYP3A and CYP2C9 activities were 90 and 60% of the day 1 levels, respectively. In contrast, the initial CYP2C8, CYP2C19, and CYP2D6 activity losses did not rebound over the 5-day culture period. Furthermore, ABT was not found to have an effect, whether directly or indirectly as a P450 inactivator, with respect to the hepatic transport of rosuvastatin, atrovastatin, and midazolam in SCHH. Taken together, these results suggest that the SCHH model is a reliable tool to characterize hepatic uptake and biliary excretion. Due to the differential modulation of P450 activity, SCHH may not be considered a suitable tool for metabolic stability assessments with compounds predominantly cleared by certain P450 enzymes.

The evolution of the OATP hepatic uptake transport protein family in DMPK sciences: from obscure liver transporters to key determinants of hepatobiliary clearance.

Over the last two decades the impact on drug pharmacokinetics of the organic anion transporting polypeptides (OATPs: OATP-1B1, 1B3 and 2B1), expressed on the sinusoidal membrane of the hepatocyte, has been increasingly recognized. OATP-mediated uptake into the hepatocyte coupled with subsequent excretion into bile via efflux proteins, such as MRP2, is often referred to as hepatobiliary excretion. OATP transporter proteins can impact some drugs in several ways including pharmacokinetic variability, pharmacodynamic response and drug-drug interactions (DDIs). The impact of transporter mediated hepatic clearance is illustrated with case examples, from the literature and also from the Pfizer portfolio. The currently available in vitro techniques to study the hepatic transporter proteins involved in the hepatobiliary clearance of drugs are reviewed herein along with recent advances in using these in vitro data to predict the human clearance of compounds recognized by hepatic uptake transporters.

Absolute immunoquantification of the expression of ABC transporters P-glycoprotein, breast cancer resistance protein and multidrug resistance-associated protein 2 in human liver and duodenum.

The ATP-binding cassette (ABC) transporters breast cancer resistance protein (BCRP), multidrug resistance-associated protein 2 (MRP2), and P-glycoprotein (Pgp) are important in the distribution and elimination of many drugs and endogenous metabolites. Due to their membrane location and hydrophobicity it is difficult to generate purified protein standards to quantify these transporters in human tissues. The present study generated transporter proteins fused with the S-peptide of ribonuclease for use as standards in immunoquantification in human liver and small intestine. Quantification of the S•tag™, a 15 amino acid peptide, is based on the formation of a functional ribonuclease activity upon its high affinity reconstitution with ribonuclease S-protein. S-tagged transporters were used as full-length protein standards in the immunoquantification of endogenous BCRP, MRP2, and Pgp levels in 14 duodenum and 13 liver human tissue samples. Expression levels in the duodenum were 305±248 (BCRP), 66±70 (MRP2), and 275±205 (Pgp) fmoles per cm(2). Hepatic levels were 2.6±0.9 (BCRP), 19.8±10.5 (MRP2), and 26.1±10.1 (total Pgp) pmoles per g of liver. The mean hepatic scaling factor was 35.8mg crude membrane per g of liver, and the mean duodenal scaling factor was 1.3mg crude membrane per cm(2) mucosal lining. Interindividual variability was greater in duodenal samples than liver samples. It is hoped that this innovative method of quantifying these transporters (and other membrane proteins) will improve in vivo-in vitro extrapolation and in silico prediction of drug absorption and elimination, thus supporting drug development.

Development of a new permeability assay using low-efflux MDCKII cells.

Permeability is an important property of drug candidates. The Madin-Darby canine kidney cell line (MDCK) permeability assay is widely used and the primary concern of using MDCK cells is the presence of endogenous transporters of nonhuman origin. The canine P-glycoprotein (Pgp) can interfere with permeability and transporter studies, leading to less reliable data. A new cell line, MDCKII-LE (low efflux), has been developed by selecting a subpopulation of low-efflux cells from MDCKII-WT using an iterative fluorescence-activated cell sorting technique with calcein-AM as a Pgp and efflux substrate. MDCKII-LE cells are a subpopulation of MDCKII cells with over 200-fold lower canine Pgp mRNA level and fivefold lower protein level than MDCKII-WT. MDCKII-LE cells showed less functional efflux activity than MDCKII-WT based on efflux ratios. Notably, MDCKII-MDR1 showed about 1.5-fold decreased expression of endogenous canine Pgp, suggesting that using the net flux ratio might not completely cancel out the background endogenous transporter activities. MDCKII-LE cells offer clear advantages over the MDCKII-WT by providing less efflux transporter background signals and minimizing interference from canine Pgp. The MDCKII-LE apparent permeability values well differentiates compounds from high to medium/low human intestinal absorption and can be used for Biopharmaceutical Classification System. The MDCKII-LE permeability assay (4-in-1 cassette dosing) is high throughput with good precision, reproducibility, robustness, and cost-effective.

A comprehensive non-clinical evaluation of the CNS penetration potential of antimuscarinic agents for the treatment of overactive bladder.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: This study provides antimuscarinic agents for overactive bladder (OAB) display variable association with side effects mediated by the central nervous system (CNS), which may be of particular concern in the elderly. Adverse effects on CNS functioning are related to muscarinic receptor subtype selectivity and the ability of the agent to cross the blood-brain barrier, where P-gp plays a role in limiting permeability. WHAT THIS STUDY ADDS: This study provides a parallel investigation of CNS penetration of antimuscarinic OAB agents in vivo and assessment of physical properties and permeability in cell monolayers in vitro. It adds further understanding of the roles of passive transcellular permeability and P-gp in determining CNS penetration of antimuscarinic OAB agents. It also enables a comparison of CNS side-effect profiles of OAB agents with preclinical CNS penetration data. AIMS: To assess and compare the mechanisms of central nervous system (CNS) penetration of antimuscarinic overactive bladder (OAB) agents. METHODS: Physical properties were computed or compiled from the literature. Rats were administered 5-hydroxymethyl tolterodine (HMT), darifenacin, oxybutynin, solifenacin, tolterodine or trospium subcutaneously. At 1 h postdose, plasma, brain and cerebrospinal fluid (CSF) concentrations were determined using LC-MS/MS assays. Brain and plasma protein binding were determined in vitro. Permeability in the presence and absence of the efflux transporter P-glycoprotein (P-gp) was assessed in RRCK and MDCK-MDR1 transwell assays. RESULTS: Oxybutynin displayed extensive CNS penetration, with brain:plasma ratios (B:P), unbound brain:unbound plasma ratios (Kp,free) and CSF:free plasma ratios each >1. Tolterodine (B:P = 2.95, Kp,free = 0.23 and CSF:free plasma = 0.16) and solifenacin (B:P = 3.04, Kp,free = 0.28 and CSF:free plasma = 1.41) showed significant CNS penetration but with some restriction from CNS as indicated by Kp,free values significantly <1. 5-HMT, darifenacin and trospium displayed much lower B:P (0.03-0.16), Kp,free (0.01-0.04) and CSF:free plasma (0.004-0.06), consistent with poor CNS penetration. Permeability in RRCK cells was low for trospium (0.63 × 10(-6) cm s(-1) ), moderate for 5-HMT (11.7 × 10(-6) cm s(-1) ) and high for darifenacin, solifenacin, tolterodine and oxybutynin (21.5-38.2 × 10(-6) cm s(-1) ). In MDCK-MDR1 cells 5-HMT, darifenacin and trospium, were P-gp substrates, whereas oxybutynin, solifenacin and tolterodine were not P-gp substrates. CONCLUSIONS: Brain penetration was low for antimuscarinics that are P-gp substrates (5-HMT, darifenacin and trospium), and significant for those that are not P-gp substrates (oxybutynin, solifenacin and tolterodine). CNS adverse events reported in randomized controlled clinical trials show general alignment with the preclinical data described in this study.

Targeting intestinal transporters for optimizing oral drug absorption.

While the oral exposure continues to be the major focus, the chemical space of recent drug discovery is apparently trending towards more hydrophilic libraries, due to toxicity and drug-interactions issues usually reported with lipophilic drugs. This trend may bring in challenges in optimizing the membrane permeability and thus the oral absorption of new chemical entities. It is now apparent that the influx transporters such as peptide transporter 1 (PepT1), organic-anion transporting polypeptides (OATPs), monocarboxylate transporters (MCT1) facilitate, while efflux pumps (e.g. P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)) limit oral absorption of drugs. This review will focus on intestinal transporters that may be targeted to achieve optimal clinical oral plasma exposure for hydrophilic and polar drugs. The structure, mechanism, structure-activity relationships and the clinical examples on the functional role of these transporters in the drug absorption was discussed. Physicochemical properties, lipophilicity and hydrogen-bonding ability, show good correlation with transport activity for efflux pumps. Although several attempts were made to describe the structural requirements based on pharmacophore modeling, lack of crystal structure of transporters impeded identification of definite properties for transporter affinity and favorable transport activity. Furthermore, very few substrate drug datasets are currently available for the influx transporters to derive any clear relationships. Unfortunately, gaps also exist in the translation of in vitro end points to the clinical relevance of the transporter(s) involved. However, it may be qualitatively generalized that targeting intestinal transporters are relevant for drugs with high solubility and/or low passive permeability i.e. a class of compounds identified as Class III and Class IV according to the Biopharmaceutic Classification System (BCS) and the Biopharmaceutic Drug Disposition Classification System (BDDCS). A careful considerations to oral dose based on the transporter clearance (V(max)/K(m)) capacity is needed in targeting a particular transporter. For example, low affinity and high capacity uptake transporters such as PEPT1 and MCT1 may be targeted for high oral dose drugs.