In vitro and in vivo models for assessing drug efflux transporter activity

An approach for an enhanced anticancer activity of ferulic acid-loaded polymeric micelles via MicroRNA-221 mediated activation of TP53INP1 in caco-2 cell line

Ferulic acid (FA) has powerful antioxidant and antitumor activities, but it has low bioavailability owing to its poor water solubility. Our aim is to formulate polymeric mixed micelles loaded with FA to overcome its poor solubility and investigate its potential anticancer activity via miRNA-221/TP53INP1 axis-mediated autophagy in colon cancer. A D-optimal design with three factors was used for the optimization of polymeric mixed micelles by studying the effects of each of total Pluronics mixture (mg), Pluronic P123 percentage (%w/w), and drug amount (mg) on both entrapment efficiency (EE%) and particle size. The anticancer activity of FA and Tocopheryl polyethylene glycol 1000 succinate (TPGS) mixed micelles formula (O2) was assessed by MTT and flow cytometry. O2 showed an EE% of 99.89%, a particle size of 13.86 nm, and a zeta potential of - 6.02 mv. In-vitro drug release studies showed a notable increase in the release rate of FA from O2, as compared to the free FA. The (IC50) values for FA from O2 and free FA were calculated against different cell lines showing a prominent IC50 against Caco-2 (17.1 µg/ml, 191 µg/ml respectively). Flow cytometry showed that FA caused cell cycle arrest at the G2/M phase in Caco-2. RT-PCR showed that O2 significantly increased the mRNA expression level of Bax and CASP-3 (4.72 ± 0.17, 3.67 ± 0.14), respectively when compared to free FA (2.59 ± 0.13, 2.14 ± 0.15), while miRNA 221 levels were decreased by the treatment with O2 (0.58 ± 0.02) when compared to free FA treatment (0.79 ± 0.03). The gene expression of TP53INP1 was increased by the treatment with O2 compared to FA at P < 0.0001. FA-loaded TPGS mixed micelles showed promising results for enhancing the anticancer effect of FA against colorectal cancer, probably due to its enhanced solubility. Thus, FA-loaded TPGS mixed micelles could be a potential therapeutic agent for colorectal cancer by targeting miRNA-221/TP53INP1 axis-mediated autophagy.

Current screening, design, and delivery approaches to address low permeability of chemically synthesized modalities in drug discovery and early clinical development

A drug's permeability across biological membranes is a key property associated with the successful development of an orally absorbed drug candidate. Although a variety of methods are available for predicting and assessing permeability, some are more preferred than others at specific stages of drug discovery and development across the pharmaceutical industry. Permeability measurements may be interpreted differently depending on the chosen method. Herein, we present a refreshed perspective on the screening approaches and philosophy in permeability evaluation, from early drug discovery to early clinical development. Additionally, we review and discuss chemical design and drug delivery technologies that can be leveraged to overcome permeability challenges, which are increasingly being used with emerging modalities.

Importance of ABC Transporters in the Survival of Parasitic Nematodes and the Prospect for the Development of Novel Control Strategies

ABC transporters, a family of ATP-dependent transmembrane proteins, are responsible for the active transport of a wide range of molecules across cell membranes, including drugs, toxins, and nutrients. Nematodes possess a great diversity of ABC transporters; however, only P-glycoproteins have been well-characterized compared to other classes. The ABC transport proteins have been implicated in developing resistance to various classes of anthelmintic drugs in parasitic nematodes; their role in plant and human parasitic nematodes still needs further investigation. Therefore, ABC transport proteins offer a potential opportunity to develop nematode control strategies. Multidrug resistance inhibitors are becoming more attractive for controlling nematodes due to their potential to increase drug efficacy in two ways: (i) by limiting drug efflux from nematodes, thereby increasing the amount of drug that reaches its target site, and (ii) by reducing drug excretion by host animals, thereby enhancing drug bioavailability. This article reviews the role of ABC transporters in the survival of parasitic nematodes, including the genes involved, their regulation and physiological roles, as well as recent developments in their characterization. It also discusses the association of ABC transporters with anthelmintic resistance and the possibility of targeting them with next-generation inhibitors or nutraceuticals (e.g., polyphenols) to control parasitic infections.

The metabolism of valerylfentanyl using human liver microsomes and zebrafish larvae

Valerylfentanyl, a novel synthetic opioid less potent than fentanyl, has been reported in biological samples, but there are limited studies on its pharmacokinetic properties. The goal of this study was to elucidate the metabolism of valerylfentanyl using an in vitro human liver microsome (HLM) model compared with an in vivo zebrafish model. Nineteen metabolites were detected with N-dealkylation-valeryl norfentanyl and hydroxylation as the major metabolic pathways. The major metabolites in HLMs were also detected in 30 day postfertilization zebrafish. An authentic liver specimen that tested positive for valerylfentanyl, among other opioids and stimulants, revealed the presence of a metabolite that shared transitions and retention time as the hydroxylated metabolite of valerylfentanyl but could not be confirmed without an authentic standard. 4-Anilino-N-phenethylpiperidine (4-ANPP), a common metabolite to other fentanyl analogs, was also detected. In this study, we elucidated the metabolic pathway of valerylfentanyl, confirmed two metabolites using standards, and demonstrated that the zebrafish model produced similar metabolites to the HLM model for opioids.

Approaches to minimize the effects of P-glycoprotein in drug transport: A review

P-glycoprotein (P-gp) is a transporter protein that is come under the ATP binding cassette family of proteins. It is situated on the surface of the intestine epithelium, where P-gp substrate binds to the transporter and is pumped into the intestine lumen by the ATP-driven energy-dependent process. In this review, we summarize the role of the P-gp efflux transporter situated on the intestine, the clinical importance of P-gp related drug interactions, and approaches to minimize the effect of P-gp in drug transport. This review also focuses on the impact of P-gp on the bioavailability of the orally administered drug. Many drug's oral bioavailabilities can improve by concomitant use of P-gp inhibitors. Multidrug resistance are reduced by using some naturally occurring compounds obtained from plants and several synthetic P-gp inhibitors. Formulation strategies, one of the most important approaches to mimic the P-gp transporter's action, finally enhancing the oral bioavailability of the drug by inhibiting its P-gp efflux. Vitamin E TPGS, Gelucire 44/14 and other pharmaceutical/formulation excipients inhibit the P-gp efflux. A prodrug approach might be a useful strategy to overcome drug resistance. Prodrug helps to enhance the solubility or alter the pharmacokinetic properties but does not diminish the pharmacological action.

Investigation of the effect of verapamil on the regional absorption of sofosbuvir from rabbit intestine in situ

PURPOSE

Sofosbuvir, a nucleotide antiviral drug, is a Biopharmaceutics Classification System (BCS) class III prodrug suffering from limited intestinal absorption due to its high hydrophilicity and low intestinal permeability. This research aims to investigate the luminal stability of Sofosbuvir, the influence of anatomical site on its intestinal absorption and the effects of verapamil on such absorption.

METHOD

The study utilized in situ rabbit intestinal perfusion technique to examine absorption of Sofosbuvir from duodenum, jejunum, ileum and ascending colon. This was conducted both with and without verapamil.

RESULTS

The luminal stability study showed that Sofosbuvir was subjected to premature degradation with varying fractions degraded from the different intestinal segments. The in situ perfusion data showed incomplete absorption of Sofosbuvir from small and large intestinal segments. The recorded values of the absorptive clearance per unit length (Pe.A/L) of Sofosbuvir were 0.026, 0.0075, 0.0026, & 0.054 ml/min.cm for duodenum, jejunum, ileum, and ascending colon, respectively. The Pe.A/L values were ordered as colon > duodenum > jejunum > ileum. This is the opposite rank of P-gp content in the different intestinal segments. The recorded values of the length required for complete Sofosbuvir absorption (L95%) were 29.58, 128.47, 949.2 and, 13.63 cm for duodenum, jejunum, ileum, and ascending colon, respectively. Co-perfusion with verapamil significantly increased Pe.A/L and reduced the L95% of Sofosbuvir from both jejunum and ileum (P-value < 0.05).

CONCLUSION

The results indicated that the absorptive clearance of Sofosbuvir was site dependent and associated with the content of P-glycoprotein, in addition to the expected drug interactions that can occur in polymedicated hepatitis C virus (HCV) infected patients.

The Intestinal Efflux Transporter Inhibition Activity of Xanthones from Mangosteen Pericarp: An In Silico, In Vitro and Ex Vivo Approach

The capacity of α-mangostin (α-MG) and β-mangostin (β-MG) from mangosteen pericarp on P-glycoprotein (Pgp) in silico, in vitro, and ex vivo was investigated in this study. Screening with the ADMET Predictor™ program predicted the two compounds to be both a Pgp inhibitor and Pgp substrate. The compounds tended to interact with Pgp and inhibit Pgp ATPase activity. Additionally, bidirectional transport on Caco-2 cell monolayers demonstrated a significantly lower efflux ratio than that of the control (α-(44.68) and β-(46.08) MG versus the control (66.26); p < 0.05) indicating an inhibitory effect on Pgp activity. Test compounds additionally revealed a downregulation of MDR1 mRNA expression. Moreover, an ex vivo absorptive transport in everted mouse ileum confirmed the previous results that α-MG had a Pgp affinity inhibitor, leading to an increase in absorption of the Pgp substrate in the serosal side. In conclusion, α- and β-MG have the capability to inhibit Pgp and they also alter Pgp expression, which makes them possible candidates for reducing multidrug resistance. Additionally, they influence the bioavailability and transport of Pgp substrate drugs.

Enhanced oral bioavailability of linagliptin by the influence of gallic acid and ellagic acid in male Wistar albino rats: involvement of p-glycoprotein inhibition

Background Linagliptin is an antidiabetic drug used for the treatment of type-2 diabetes. The oral bioavailability of linagliptin is low (29.5%) due to its first pass metabolism in the intestine and liver. Gallic acid and ellagic acid are natural polyphenols which are widely distributed in fruits and medicinal plants. Gallic acid and ellagic acid have been reported to inhibit p-glycoprotein (p-gp) and enhance the bioavailability of p-gp substrate drugs. Hence, the purpose of the study was to evaluate the effect of gallic acid and ellagic acid on intestinal transport and bioavailability of linagliptin, a p-gp substrate in diabetic rats. Methods The intestinal transport of linagliptin was assessed by conducting an in situ single-pass intestinal perfusion study. The oral pharmacokinetics was evaluated by conducting oral bioavailability study in diabetic rats. Results After pretreatment with gallic acid and ellagic acid, no significant change in effective permeability of linagliptin was observed at the ileum part of the rat intestine. A significant improvement in the peak serum concentration (Cmax) and area under the serum concentration time profile (AUC), AUMC, AUCtotal and decrease in clearance were observed in rats pretreated with gallic acid and ellagic acid. Conclusions This study demonstrates that gallic acid and ellagic acids increase the bioavailability of oral linagliptin in rats due to the inhibition of p-gp. These animal data need to be confirmed in a clinical setting to determine whether linagliptin dosing should be adjusted when given concomitantly with these phytochemicals or gallic acid/ellagic acid-containing dietary supplements.

Different structures of berberine and five other protoberberine alkaloids that affect P-glycoprotein-mediated efflux capacity

Berberine, berberrubine, thalifendine, demethyleneberberine, jatrorrhizine, and columbamine are six natural protoberberine alkaloid (PA) compounds that display extensive pharmacological properties and share the same protoberberine molecular skeleton with only slight substitution differences. The oral delivery of most PAs is hindered by their poor bioavailability, which is largely caused by P-glycoprotein (P-gp)-mediated drug efflux. Meanwhile, P-gp undergoes large-scale conformational changes (from an inward-facing to an outward-facing state) when transporting substrates, and these changes might strongly affect the P-gp-binding specificity. To confirm whether these six compounds are substrates of P-gp, to investigate the differences in efflux capacity caused by their trivial structural differences and to reveal the key to increasing their binding affinity to P-gp, we conducted a series of in vivo, in vitro, and in silico assays. Here, we first confirmed that all six compounds were substrates of P-gp by comparing the drug concentrations in wild-type and P-gp-knockout mice in vivo. The efflux capacity (net efflux) ranked as berberrubine > berberine > columbamine ~ jatrorrhizine > thalifendine > demethyleneberberine based on in vitro transport studies in Caco-2 monolayers. Using molecular dynamics simulation and molecular docking techniques, we determined the transport pathways of the six compounds and their binding affinities to P-gp. The results suggested that at the early binding stage, different hydrophobic and electrostatic interactions collectively differentiate the binding affinities of the compounds to P-gp, whereas electrostatic interactions are the main determinant at the late release stage. In addition to hydrophobic interactions, hydrogen bonds play an important role in discriminating the binding affinities.

Regional difference in intestinal drug absorption as a measure for the potential effect of P-glycoprotein efflux transporters

OBJECTIVES

The aim of this research was to assess regional difference in the intestinal absorption of ranitidine HCl as an indicator for the potential effect of P-glycoprotein (P-gp) efflux transporters.

METHODS

In situ rabbit intestinal perfusion was used to investigate absorption of ranitidine HCl, a substrate for P-gp efflux from duodenum, jejunum, ileum and colon. This was conducted both in the presence and absence of piperine as P-gp inhibitor.

KEY FINDINGS

Ranitidine HCl was incompletely absorbed from rabbit intestine. The length normalized absorptive clearance (PeA/L) of ranitidine HCl was ranked as colon > duodenum > jejunum > ileum. This is the reverse order of the magnitude of P-gp expression. Coperfusion of piperine with ranitidine HCl significantly increased the PeA/L of ranitidine HCl from jejunum and ileum with no significant change on the absorption from duodenum and colon. This was confirmed by significant reduction in the length required for complete ranitidine HCl absorption from jejunum and ileum in presence piperine.

CONCLUSIONS

The results indicate that P-gp transporters play a major role in determining regional difference in intestinal absorption of ranitidine HCl. Thus, the regional absorption of drugs may be taken as an indirect indication for the role of P-gp in intestinal absorption.

Overview of Transporters in Pharmacokinetics and Drug Discovery

Transporters play important roles in absorption, distribution, metabolism, and elimination (ADME) processes, as well as drug pharmacokinetics (PK) and pharmacodynamics (PD). They are also important in maintaining the homeostasis of endogenous compounds and nutrients in the body. Increasing evidences also suggest that they are important in mediating drug-drug interactions (DDIs). While the significance of transporters in drug pharmacodynamics and DDIs are beyond the scope of this overview, the basic concepts of transporters, their contributions in membrane permeation processes, and their roles in influencing drug ADME pathway and PK will be discussed. © 2018 by John Wiley & Sons, Inc.

Theoretical Prediction of the Complex P-Glycoprotein Substrate Efflux Based on the Novel Hierarchical Support Vector Regression Scheme

P-glycoprotein (P-gp), a membrane-bound transporter, can eliminate xenobiotics by transporting them out of the cells or blood⁻brain barrier (BBB) at the expense of ATP hydrolysis. Thus, P-gp mediated efflux plays a pivotal role in altering the absorption and disposition of a wide range of substrates. Nevertheless, the mechanism of P-gp substrate efflux is rather complex since it can take place through active transport and passive permeability in addition to multiple P-gp substrate binding sites. A nonlinear quantitative structure⁻activity relationship (QSAR) model was developed in this study using the novel machine learning-based hierarchical support vector regression (HSVR) scheme to explore the perplexing relationships between descriptors and efflux ratio. The predictions by HSVR were found to be in good agreement with the observed values for the molecules in the training set (n = 50, r² = 0.96, qCV2 = 0.94, RMSE = 0.10, s = 0.10) and test set (n = 13, q² = 0.80⁻0.87, RMSE = 0.21, s = 0.22). When subjected to a variety of statistical validations, the developed HSVR model consistently met the most stringent criteria. A mock test also asserted the predictivity of HSVR. Consequently, this HSVR model can be adopted to facilitate drug discovery and development.

Mdr65 decreases toxicity of multiple insecticides in Drosophila melanogaster

ABC transporters are ubiquitous membrane-bound proteins, present in both prokaryotes and eukaryotes. The major function of eukaryotic ABC transporters is to mediate the efflux of a variety of substrates (including xenobiotics) out of cells. ABC transporters have been widely investigated in humans, particularly for their involvement in multidrug resistance (MDR). Considerably less is known about their roles in transport and/or excretion in insects. ABC transporters are only known to function as exporters in insects. Drosophila melanogaster has 56 ABC transporter genes, including eight which are phylogenetically most similar to the human Mdr genes (ABCB1 clade). We investigated the role of ABC transporters in the ABCB1 clade in modulating the susceptibility to insecticides. We took advantage of the GAL4/UAS system in D. melanogaster to knockdown the expression levels of Mdr65, Mdr50, Mdr49 and ABCB6 using transgenic UAS-RNAi lines and conditional driver lines. The most notable effects were increased sensitivities to nine different insecticides by silencing of Mdr65. Furthermore, a null mutation of Mdr65 decreased the malathion, malaoxon and fipronil LC₅₀ values by a factor of 1.9, 2.1 and 3.9, respectively. Altogether, this data demonstrates the critical role of ABC transporters, particularly Mdr65, in altering the toxicity of specific, structurally diverse, insecticides in D. melanogaster.

Enhanced Oral Bioavailability of Diltiazem by the Influence of Gallic Acid and Ellagic Acid in Male Wistar Rats: Involvement of CYP3A and P-gp Inhibition

The oral bioavailability of diltiazem is very low due to rapid first pass metabolism in liver and intestine. The purpose of the study was to investigate the effect of gallic acid and ellagic acid on intestinal transport and oral bioavailability of diltiazem in rats. The intestinal transport and permeability of diltiazem was evaluated by in vitro non-everted sac method and in situ single pass intestinal perfusion study. The oral pharmacokinetics was evaluated by conducting oral bioavailability study. The intestinal transport and apparent permeability of diltiazem were significantly enhanced in duodenum, jejunum, and ileum of gallic and ellagic acid-treated groups. The effective permeability of diltiazem was significantly enhanced in ileum part of gallic and ellagic acid-treated groups. When compared with control group, the presence of these two phytochemicals significantly enhanced the area under plasma concentration-time curve and the peak plasma concentration of diltiazem (C_max ). Gallic acid and ellagic acid significantly increased the bioavailability of diltiazem due to the inhibition of both CYP3A-mediated metabolism and P-glycoprotein-mediated efflux in the intestine and/or liver. Based on these results, the clinical experiments are warranted for the confirmation to reduce the dose of diltiazem when concomitantly administered with these phytochemicals. Copyright © 2017 John Wiley & Sons, Ltd.

Cellular Models and In Vitro Assays for the Screening of modulators of P-gp, MRP1 and BCRP

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are highly expressed in tumor cells, as well as in organs involved in absorption and secretion processes, mediating the ATP-dependent efflux of compounds, both endogenous substances and xenobiotics, including drugs. Their expression and activity levels are modulated by the presence of inhibitors, inducers and/or activators. In vitro, ex vivo and in vivo studies with both known and newly synthesized P-glycoprotein (P-gp) inducers and/or activators have shown the usefulness of these transport mechanisms in reducing the systemic exposure and specific tissue access of potentially harmful compounds. This article focuses on the main ABC transporters involved in multidrug resistance [P-gp, multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP)] expressed in tissues of toxicological relevance, such as the blood-brain barrier, cardiovascular system, liver, kidney and intestine. Moreover, it provides a review of the available cellular models, in vitro and ex vivo assays for the screening and selection of safe and specific inducers and activators of these membrane transporters. The available cellular models and in vitro assays have been proposed as high throughput and low-cost alternatives to excessive animal testing, allowing the evaluation of a large number of compounds.

Liver and Kidney on Chips: Microphysiological Models to Understand Transporter Function

Because of complex cellular microenvironments of both the liver and kidneys, accurate modeling of transport function has remained a challenge, leaving a dire need for models that can faithfully recapitulate both the architecture and cell-cell interactions observed in vivo. The study of hepatic and renal transport function is a fundamental component of understanding the metabolic fate of drugs and xenobiotics; however, there are few in vitro systems conducive for these types of studies. For both the hepatic and renal systems, we provide an overview of the location and function of the most significant phase I/II/III (transporter) of enzymes, and then review current in vitro systems for the suitability of a transporter function study and provide details on microphysiological systems that lead the field in these investigations. Microphysiological modeling of the liver and kidneys using "organ-on-a-chip" technologies is rapidly advancing in transport function assessment and has emerged as a promising method to evaluate drug and xenobiotic metabolism. Future directions for the field are also discussed along with technical challenges encountered in complex multiple-organs-on-chips development.

In vitro screening of nanomedicines through the blood brain barrier: A critical review

The blood-brain barrier accounts for the high attrition rate of the treatments of most brain disorders, which therefore remain one of the greatest health-care challenges of the twenty first century. Against this background of hindrance to brain delivery, nanomedicine takes advantage of the assembly at the nanoscale of available biomaterials to provide a delivery platform with potential to raising brain levels of either imaging or therapeutic agents. Nevertheless, to prevent later failure due to ineffective drug levels at the target site, researchers have been endeavoring to develop a battery of in vitro screening procedures that can predict earlier in the drug discovery process the ability of these cutting-edge drug delivery platforms to cross the blood-brain barrier for biomedical purposes. This review provides an in-depth analysis of the currently available in vitro blood-brain barrier models (both cell-based and non-cell-based) with the focus on their suitability for understanding the biological brain distribution of forthcoming nanomedicines. The relationship between experimental factors and underlying physiological assumptions that would ultimately lead to a more predictive capacity of their in vivo performance, and those methods already assayed for the evaluation of the brain distribution of nanomedicines are comprehensively discussed.

Temozolomide competes for P-glycoprotein and contributes to chemoresistance in glioblastoma cells

Chemotherapeutic resistance can occur by P-glycoprotein (P-gp), a 12-transmembrane ATP-dependent drug efflux pump. Glioblastoma (GBM) has poor survival rate and uniformly acquired chemoresistance to its frontline agent, Temozolomide (TMZ). Despite much effort, overcoming TMZ resistance remains a challenge. We reported on autonomous induction of TMZ resistance by increased transcription MDR1, the gene for P-gp. This study investigated how P-gp and TMZ interact to gain resistance. Using an experimental model of Adriamycin-resistant DC3F cells (DC3F/Adx), we showed that increased P-gp caused TMZ resistance. Increasing concentrations of TMZ competed with Calcein for P-gp, resulting in reduced efflux in the DC3F/Adx cells. Three different inhibitors of P-gp reversed the resistance to TMZ in two different GBM cell lines, by increasing active Caspase 3. Molecular modeling predicted the binding sites to be the intracellular region of P-gp and also identified specific amino acids and kinetics of energy for the efflux of TMZ. Taken together, we confirmed P-gp targeting of TMZ, a crucial regulator of TMZ resistance in GBM. This study provides insights on the effectiveness by which TMZ competes with other P-gp substrates, thereby opening the door for combined targeted therapies.

Theoretical considerations for thresholds in chemical carcinogenesis

There is increasing evidence for non-linear relationships for gene mutations, chromosomal aberrations and even tumor incidences in response to low doses of genotoxic carcinogens. To attain the biological relevance of such non-linear responses, there is a need to identify the underlying defense mechanisms that allow tolerance to low doses of genotoxicants. This communication discusses presumptive cancer prevention mechanisms that may contribute to thresholds, i.e. points of departure, for each endpoint, from initial DNA lesion to tumor formation. We discuss a sequential order of genome protection during carcinogenesis where genotoxicant scavenging, cellular efflux, DNA repair, elimination of damaged cells by apoptosis, autophagy, silencing by DNA damage-triggered replicative senescence, and finally, elimination of transformed (premalignant) cells by the immune system are thought to be responsible for a threshold in tumor formation. We highlight DNA repair, for which experimental evidence has been recently provided to dictate a role in PoDs. In conclusion, from a theoretical perspective it is reasonable to posit that tolerance to low dose levels exists for each requisite step of tumor formation and these tolerance mechanisms are critical in determining thresholds in chemical carcinogenesis.

Computational classification models for predicting the interaction of drugs with P-glycoprotein and breast cancer resistance protein

P-glycoprotein (P-gp/ABCB1) and breast cancer resistance protein (BCRP/ABCG2) are two members of the adenosine triphosphate (ATP) binding cassette (ABC) family of transporters which function as membrane efflux transporters and display considerable substrate promiscuity. Both are known to significantly influence the absorption, distribution and elimination of drugs, mediate drug-drug interactions and contribute to multiple drug resistance (MDR) of cancer cells. Correspondingly, timely characterization of the interaction of novel leads and drug candidates with these two transporters is of great importance. In this study, several computational classification models for prediction of transport and inhibition of P-gp and BCRP, respectively, were developed based on newly compiled and critically evaluated experimental data. Artificial neural network (ANN) and support vector machine (SVM) ensemble based models were explored, as well as knowledge-based approaches to descriptor selection. The average overall classification accuracy of best performing models was 82% for P-gp transport, 88% for BCRP transport, 89% for P-gp inhibition and 87% for BCRP inhibition, determined across an array of different test sets. An analysis of substrate overlap between P-gp and BCRP was also performed. The accuracy, simplicity and interpretability of the proposed models suggest that they could be of significant utility in the drug discovery and development settings.

The ABC gene family in arthropods: comparative genomics and role in insecticide transport and resistance

About a 100 years ago, the Drosophila white mutant marked the birth of Drosophila genetics. The white gene turned out to encode the first well studied ABC transporter in arthropods. The ABC gene family is now recognized as one of the largest transporter families in all kingdoms of life. The majority of ABC proteins function as primary-active transporters that bind and hydrolyze ATP while transporting a large diversity of substrates across lipid membranes. Although extremely well studied in vertebrates for their role in drug resistance, less is known about the role of this family in the transport of endogenous and exogenous substances in arthropods. The ABC families of five insect species, a crustacean and a chelicerate have been annotated in some detail. We conducted a thorough phylogenetic analysis of the seven arthropod and human ABC protein subfamilies, to infer orthologous relationships that might suggest conserved function. Most orthologous relationships were found in the ABCB half transporter, ABCD, ABCE and ABCF subfamilies, but specific expansions within species and lineages are frequently observed and discussed. We next surveyed the role of ABC transporters in the transport of xenobiotics/plant allelochemicals and their involvement in insecticide resistance. The involvement of ABC transporters in xenobiotic resistance in arthropods is historically not well documented, but an increasing number of studies using unbiased differential gene expression analysis now points to their importance. We give an overview of methods that can be used to link ABC transporters to resistance. ABC proteins have also recently been implicated in the mode of action and resistance to Bt toxins in Lepidoptera. Given the enormous interest in Bt toxicology in transgenic crops, such findings will provide an impetus to further reveal the role of ABC transporters in arthropods.

Investigating permeability related hurdles in oral delivery of 11-keto-β-boswellic acid

11-Keto-β-boswellic acid (KBA) is an important and potent boswellic acids responsible for anti-inflammatory action of Boswellia extract. However, its pharmaceutical development has been severely limited by its poor oral bioavailability. The present work aims to investigate the permeability related hurdles in oral delivery of KBA. Gastrointestinal stability, gastrointestinal metabolism, adsorption-desorption kinetics and Caco-2 permeability studies have been carried out. KBA was found poorly permeable with Papp value of 2.85 ± 0.14 × 10(-6)cm/s. Higher absorptive transport indicated role of carrier mediated transport. Moreover, KBA transport across monolayer showed saturation kinetics at higher concentrations. KBA exposed to 1α,25-(OH)2 vitamin D3 treated cell monolayer showed the lowest Papp value of 2.01×10(-6) ± 0.02 × 10(-6)cm/s indicating role of CYP3A4 mediated metabolism during KBA transport. Metabolic stability experiments in jejunum S9 fractions further confirmed this. KBA was found unstable in simulated gastrointestinal fluids and also got accumulated in the enterocytes. Sorption and desorption kinetic studies using Caco-2 cells further confirmed accumulation of KBA inside the enterocytes. KBA also showed pH dependent permeability with higher flux at gradient pH condition of pH 6.5 at apical and 7.4 at basolateral. Taken as whole, the major permeability related hurdles that hampered oral bioavailability of KBA included its gastrointestinal instability, CYP3A4 mediated intestinal metabolism, accumulation within the enterocytes and saturable kinetics. The present investigation may help in designing novel drug delivery system for KBA.

A long-term three dimensional liver co-culture system for improved prediction of clinically relevant drug-induced hepatotoxicity

Drug-induced liver injury (DILI) is the major cause for liver failure and post-marketing drug withdrawals. Due to species-specific differences in hepatocellular function, animal experiments to assess potential liabilities of drug candidates can predict hepatotoxicity in humans only to a certain extent. In addition to animal experimentation, primary hepatocytes from rat or human are widely used for pre-clinical safety assessment. However, as many toxic responses in vivo are mediated by a complex interplay among different cell types and often require chronic drug exposures, the predictive performance of hepatocytes is very limited. Here, we established and characterized human and rat in vitro three-dimensional (3D) liver co-culture systems containing primary parenchymal and non-parenchymal hepatic cells. Our data demonstrate that cells cultured on a 3D scaffold have a preserved composition of hepatocytes, stellate, Kupffer and endothelial cells and maintain liver function for up to 3months, as measured by the production of albumin, fibrinogen, transferrin and urea. Additionally, 3D liver co-cultures maintain cytochrome P450 inducibility, form bile canaliculi-like structures and respond to inflammatory stimuli. Upon incubation with selected hepatotoxicants including drugs which have been shown to induce idiosyncratic toxicity, we demonstrated that this model better detected in vivo drug-induced toxicity, including species-specific drug effects, when compared to monolayer hepatocyte cultures. In conclusion, our results underline the importance of more complex and long lasting in vitro cell culture models that contain all liver cell types and allow repeated drug-treatments for detection of in vivo-relevant adverse drug effects.

P-glycoprotein at the blood-brain barrier: kinetic modeling of 11C-desmethylloperamide in mice using a 18F-FDG μPET scan to determine the input function

Purpose

The objective of this study is the implementation of a kinetic model for ¹¹C-desmethylloperamide (¹¹C-dLop) and the determination of a typical parameter for P-glycoprotein (P-gp) functionality in mice. Since arterial blood sampling in mice is difficult, an alternative method to obtain the arterial plasma input curve used in the kinetic model is proposed.

Methods

Wild-type (WT) mice (pre-injected with saline or cyclosporine) and P-gp knock-out (KO) mice were injected with 20 MBq of ¹¹C-dLop, and a dynamic μPET scan was initiated. Afterwards, 18.5 MBq of ¹⁸F-FDG was injected, and a static μPET scan was started. An arterial input and brain tissue curve was obtained by delineation of an ROI on the left heart ventricle and the brain, respectively based on the ¹⁸F-FDG scan.

Results

A comparison between the arterial input curves obtained by the alternative and the blood sampling method showed an acceptable agreement. The one-tissue compartment model gives the best results for the brain. In WT mice, the K₁/k₂ ratio was 0.4 ± 0.1, while in KO mice and cyclosporine-pretreated mice the ratio was much higher (2.0 ± 0.4 and 1.9 ± 0.2, respectively). K₁ can be considered as a pseudo value K₁, representing a combination of passive influx of ¹¹C-desmethylloperamide and a rapid washout by P-glycoprotein, while k₂ corresponds to slow passive efflux out of the brain.

Conclusions

An easy to implement kinetic modeling for imaging P-glycoprotein function is presented in mice without arterial blood sampling. The ratio of K₁/k₂ obtained from a one-tissue compartment model can be considered as a good value for P-glycoprotein functionality.

Enniatin B1 is a substrate of intestinal P-glycoprotein, multidrug resistance-associated protein 2 and breast cancer resistance protein

Enniatins are cyclic hexadepsipeptides produced by various fungi, known to have ionophoric, antibiotic and insecticidal activity. The aim of the present study was to evaluate the intestinal absorption characteristics of enniatin B1 (ENN-B1). Using the human intestinal Caco-2 cell line, we found that the permeability of ENN-B1 in the basolateral to apical direction was 6.7× higher as compared to the permeability in the opposite direction, indicating involvement of apically located transporters. Transport of ENN-B1 in the apical to basolateral direction was increased significantly upon treatment of Caco-2 cells with the P-glycoprotein (Pgp) inhibitor verapamil and the multidrug resistance-associated protein 2 (MRP2) inhibitor MK571, but only weakly and not significantly after treatment with the breast cancer resistance protein (BCRP) inhibitor fumitremorgin C. Additionally, MDCK II cells over-expressing Pgp, MRP2 or BCRP, showed reduced sensitivity towards ENN-B1. These data demonstrate for the first time that ENN-B1 is a substrate of MRP2 and suggests that Pgp, MRP2 and possibly BCRP are involved in transport of ENN-B1 across the intestine.

The P-glycoprotein inhibitor GF120918 modulates Ca2+-dependent processes and lipid metabolism in Toxoplasma gondii

Up-regulation of the membrane-bound efflux pump P-glycoprotein (P-gp) is associated with the phenomenon of multidrug-resistance in pathogenic organisms, including protozoan parasites. In addition, P-gp plays a role in normal physiological processes, however our understanding of these P-gp functions remains limited. In this study we investigated the effects of the P-gp inhibitor GF120918 in Toxoplasma gondii, a model apicomplexan parasite and an important human pathogen. We found that GF120918 treatment severely inhibited parasite invasion and replication. Further analyses of the molecular mechanisms involved revealed that the P-gp inhibitor modulated parasite motility, microneme secretion and egress from the host cell, all cellular processes known to depend on Ca²⁺ signaling in the parasite. In support of a potential role of P-gp in Ca²⁺-mediated processes, immunoelectron and fluorescence microscopy showed that T. gondii P-gp was localized in acidocalcisomes, the major Ca²⁺ storage in the parasite, at the plasma membrane, and in the intravacuolar tubular network. In addition, metabolic labeling of extracellular parasites revealed that inhibition or down-regulation of T. gondii P-gp resulted in aberrant lipid synthesis. These results suggest a crucial role of T. gondii P-gp in essential processes of the parasite biology and further validate the potential of P-gp activity as a target for drug development.

Exploring the cellular accumulation of metal complexes

Transition metal complexes offer great potential as diagnostic and therapeutic agents, and a growing number of biological applications have been explored. To be effective, these complexes must reach their intended target inside the cell. Here we review the cellular accumulation of metal complexes, including their uptake, localization, and efflux. Metal complexes are taken up inside cells through various mechanisms, including passive diffusion and entry through organic and metal transporters. Emphasis is placed on the methods used to examine cellular accumulation, to identify the mechanism(s) of uptake, and to monitor possible efflux. Conjugation strategies that have been employed to improve the cellular uptake characteristics of metal complexes are also described.

Novel delivery system enhances efficacy of antiretroviral therapy in animal model for HIV-1 encephalitis

Most potent antiretroviral drugs (e.g., HIV-1 protease inhibitors) poorly penetrate the blood-brain barrier. Brain distribution can be limited by the efflux transporter, P-glycoprotein (P-gp). The ability of a novel drug delivery system (block co-polymer P85) that inhibits P-gp, to increase the efficacy of antiretroviral drugs in brain was examined using a severe combined immunodeficiency (SCID) mouse model of HIV-1 encephalitis (HIVE). Severe combined immunodeficiency mice inoculated with HIV-1 infected human monocyte-derived macrophages (MDM) into the basal ganglia were treated with P85, antiretroviral therapy (ART) (zidovudine, lamivudine and nelfinavir (NEL)), or P85 and ART. Mice were killed on days 7 and 14, and brains were evaluated for levels of viral infection. Antiviral effects of NEL, P85, or their combination were evaluated in vitro using HIV-1 infected MDM and showed antiretroviral effects of P85 alone. In SCID mice injected with virus-infected MDM, the combination of ART-P85 and ART alone showed a significant decrease of HIV-1 p24 expressing MDM (25% and 33% of controls, respectively) at day 7 while P85 alone group was not different from control. At day 14, all treatment groups showed a significant decrease in percentage of HIV-1 infected MDM as compared with control. P85 alone and combined ART-P85 groups showed the most significant reduction in percentage of HIV-1 p24 expressing MDM (8% to 22% of control) that were superior to the ART alone group (38% of control). Our findings indicate major antiretroviral effects of P85 and enhanced in vivo efficacy of antiretroviral drugs when combined with P85 in a SCID mouse model of HIVE.

Transepithelial transport of fusariotoxin nivalenol: mediation of secretion by ABC transporters

Mycotoxin nivalenol (NIV) is a natural contaminant of various cereal crops, animal feed and processed grains throughout the world. Human and animal contamination occurs mainly orally, and the toxin must traverse the intestinal epithelial barrier before inducing potential health effects. In this study, we investigated the mechanisms involved in NIV transepithelial transfer. The human intestinal Caco-2 cell line showed a basal-to-apical polarized transport of NIV. Using metabolic inhibitors and temperature-dependent experiments, we demonstrated that basolateral-apical (BL-AP) transfer of NIV involved an energy-dependent transport whereas apical-basolateral (AP-BL) transfer was governed by passive diffusion. NIV efflux was significantly decreased in the presence of the P-glycoprotein (P-gp) inhibitor valspodar, the multi-drug resistance-associated proteins (MRPs) inhibitor MK571, but was not modified by the breast cancer resistance protein (BCRP) inhibitor Ko143. Intracellular NIV accumulation was investigated using epithelial cell lines transfected with either human P-glycoprotein or MRP2. This accumulation was significantly decreased in LLCPK1/MDR1 and MDCKII/MRP2 cells, compared to wild-type cells, and this effect was reversed by valspodar and MK571, respectively. These in vitro results suggested that NIV was a substrate for both P-glycoprotein and MRP2. This interaction may play a key role in weak intestinal absorption of NIV and the mainly predominant excretion of NIV in faeces in animal studies.

Development of a P-glycoprotein knockout model in rodents to define species differences in its functional effect at the blood-brain barrier

The objective of this study was to establish the optimal blood concentrations of the potent P-glycoprotein (P-gp) inhibitor GF120918 (Elacridar) required to achieve maximal knockout of this efflux transporter in the blood-brain barrier (BBB) of mice, rats, and guinea pigs. Genetic mdr1a/b(-/-) knockout mice and "chemical" P-gp knockout mice, rats, and guinea pigs, generated by 24 h continuous infusion of GF120918, were used to investigate the effects of P-gp modulation on the brain penetration of SB-487946. Genetic mdr1a/b(-/-) knockout mice demonstrated a >70-fold increase in brain:blood ratio of SB-487946 compared to mdr1a/b(+/+) wild-type mice. There was a similar increase in SB-487946 brain:blood ratio in GF120918-treated mice (ca. >67-fold) and rats (ca. 95-fold) but a significantly smaller increase (ca. 10-fold) in guinea pigs treated with GF120918. An appreciable difference was found in the BBB functional effect of P-gp efflux in rodents. GF120918 blood EC90 in mice and rats were similar however, the EC90 in guinea pigs was ca. 10-fold higher, suggesting a species difference in the activity of P-gp at the BBB in some rodents. This study establishes the optimal blood concentrations of GF120918 in relation to SB-487946, to achieve chemically induced P-gp knockout in rodents.

EXPRESSION AND FUNCTIONAL ACTIVITY OF BREAST CANCER RESISTANCE PROTEIN (BCRP, ABCG2) TRANSPORTER IN THE HUMAN CHORIOCARCINOMA CELL LINE BEWO

1. Breast cancer resistance protein (BCRP, ABCG2) is a drug efflux transporter that is believed to affect the drug disposition of several drugs and xenobiotics. In the present study, we evaluated the localization and functional expression of BCRP in the human choriocarcinoma cell line BeWo, an in vitro model of the human trophoblast, and compared it with the expression of P-glycoprotein (MDR1, ABCB1) as the most widely studied placental transporter. In addition, the expression of BCRP at the mRNA level was compared with that of MDR1 in the human term placenta. 2. Western blotting analysis revealed high endogenous expression of BCRP protein in BeWo cells. Using indirect immunofluorescence microscopy, we found that the BCRP transporter appears to be localized predominantly at the apical plasma membrane. Functional studies showed a significant effect of the BCRP inhibitors GF120918 (5 micromol/L) and Ko143 (1 micromol/L) on mitoxantrone accumulation and, thus, confirmed efflux activity of BCRP in BeWo cells. 3. Using absolute mRNA quantification with real-time reverse transcription-polymerase chain reaction, we found high expression of BCRP in BeWo cells, whereas no transcript of MDR1 (P-glycoprotein), the most extensively studied drug transporter, was detected. 4. In the human placenta, BCRP was localized predominantly in the syncytiotrophoblast layer; however, immunopositivity for the BXP-21 antibody was also observed in fetal vessels of the chorionic villi. The number of BCRP transcripts in the human term placenta was found to be more than 10-fold higher compared with the expression of MDR1. 5. In conclusion, we suggest that BeWo cells could serve as a suitable in vitro model to study trans-trophoblast transport of BCRP substrates and that placental BCRP can play an important role in preventing the accumulation of potentially toxic xenobiotics in the trophoblast cells.

P-Glycoprotein Recognition of Substrates and Circumvention through Rational Drug Design

It is now well recognized that membrane efflux transporters, especially P-glycoprotein (P-gp; ABCB1), play a role in determining the absorption, distribution, metabolism, excretion, and toxicology behaviors of some drugs and molecules in development. An investment in screening structure-activity relationship (SAR) is warranted in early discovery when exposure and/or target activity in an in vivo efficacy model is not achieved and P-gp efflux is identified as a rate-limiting factor. However, the amount of investment in SAR must be placed into perspective by assessing the risks associated with the intended therapeutic target, the potency and margin of safety of the compound, the intended patient population(s), and the market competition. The task of rationally designing a chemistry strategy for circumventing a limiting P-gp interaction can be daunting. The necessity of retaining biological potency and metabolic stability places restrictions on what can be done, and the factors for P-gp recognition of substrates are complicated and poorly understood. The parameters within the assays that affect overall pump efficiency or net efflux, such as passive diffusion, membrane partitioning, and molecular interaction between pump and substrate, should be understood when interpreting data sets associated with chemistry around a scaffold. No single, functional group alone is often the cause, but one group can accentuate the recognition points existing within a scaffold. This can be likened to a rheostat, rather than an on/off switch, where addition or removal of a key group can increase or decrease the pumping efficiency. The most practical approach to de-emphasize the limiting effects of P-gp on a particular scaffold is to increase passive diffusion. Efflux pumping efficiency may be overcome when passive diffusion is fast enough. Eliminating, or substituting with fewer, groups that solvate in water, or decreasing their hydrogen bonding capacity, and adding halogen groups can increase passive diffusion. Reducing molecular size, replacing electronegative atoms, blocking or masking H-bond donors with N-alkylation or bulky flanking groups, introducing constrained conformation, or by promoting intramolecular hydrogen bonds are all examples of steps to take. This review discusses our understanding of how P-gp recognizes and pumps compounds as substrates and describes cases where structural changes were made in a chemical scaffold to circumvent the effects of P-gp interactions.

Computational Models for Identifying Potential P-Glycoprotein Substrates and Inhibitors

Multidrug resistance mediated by ATP binding cassette (ABC) transporters such as P-glycoprotein (P-gp) represents a serious problem for the development of effective anticancer drugs. In addition, P-gp has been shown to reduce oral absorption, modulate hepatic, renal, or intestinal elimination, and restrict blood-brain barrier penetration of several drugs. Consequently, there is a great interest in anticipating whether drug candidates are P-gp substrates or inhibitors. In this respect, two different computational models have been developed. A method for discriminating P-gp substrates and nonsubstrates has been set up based on calculated molecular descriptors and multivariate analysis using a training set of 53 diverse drugs. These compounds were previously classified as P-gp substrates or nonsubstrates on the basis of the efflux ratio from Caco-2 permeability measurements. The program Volsurf was used to compute the compounds' molecular descriptors. The descriptors were correlated to the experimental classes using partial least squares discriminant analysis (PLSD). The model was able to predict correctly the behavior of 72% of an external set of 272 proprietary compounds. Thirty of the 53 previously mentioned drugs were also evaluated for P-gp inhibition using a calcein-AM (CAM) assay. On the basis of these additional P-gp functional data, a PLSD analysis using GRIND-pharmacophore-based descriptors was performed to model P-gp substrates having poor or no inhibitory activity versus inhibitors having no evidence of significant transport. The model was able to discriminate between 69 substrates and 56 inhibitors taken from the literature with an average accuracy of 82%. The model allowed also the identification of some key molecular features that differentiate a substrate from an inhibitor, which should be taken into consideration in the design of new candidate drugs. These two models can be implemented in a virtual screening funnel.

pH-dependent functional activity of P-glycoprotein in limiting intestinal absorption of protic drugs: Kinetic analysis of quinidine efflux in situ

The purpose of this investigation is to evaluate the quantitative contribution of pH-dependent passive permeability on the functional activity of P-glycoprotein (P-gp) in limiting intestinal absorption of weakly basic drugs, in order to include this effect in prediction models. pH-dependent octanol/buffer partition coefficient, artificial membrane permeability and in situ rat intestinal permeability of quinidine were determined in the physiological pH range of gastrointestinal tract. In situ permeability, as a function of luminal pH, was also determined in the presence of P-gp inhibitor, verapamil (500 microM). Octanol/buffer partition coefficient, transport across artificial membrane, and rat in situ permeability showed high pH-dependency. Absorption quotient (AQ), calculated from in situ permeability to express the functional activity of P-gp, declined with increase in luminal pH or increase in luminal quinidine concentration because of the increased passive permeability or saturation of P-gp. AQ was 0.57 +/- 0.02 and 0.41 +/- 0.05, while passive permeability was 0.32 +/- 0.01 x 10(-4) cm/sec and 0.43 +/- 0.02 x 10(-4) cm/sec, in jejunum and ileum, respectively, at pH 7.4. Further, apparent Michaelis-Menten constants (K(M), J(P-gp,max)) for the quinidine efflux in jejunum indicated that efflux activity was more at luminal pH 4.5 over pH 7.4. K(M) values for jejunum quinidine efflux at pH 4.5 and pH 7.4 were determined to be 77.63 +/- 10.90 and 22.86 +/- 5.22 microM, with J(P-gp,max) values of 1.47 +/- 0.08 and 0.62 +/- 0.04 nM/cm2/sec, respectively. AQ vs passive permeability showed significant relationship indicating dependency of P-gp-mediated efflux on pH-dependent passive permeability, which is dictated by ionization status for a protic or ampholytic drug. In conclusion, an orally administered drug is absorbed from various segments of intestine, which inherit difference in luminal pH, transcellular permeability and P-gp expression. In situ data suggests that pH-dependency and regional variability in passive permeability of protic substrates significantly influence their P-gp-mediated efflux and may have implications on predictions of the in vivo drug absorption.

Comparison of P-glycoprotein-mediated drug-digoxin interactions in Caco-2 with human and rodent intestine: relevance to in vivo prediction

Inhibition of P-glycoprotein (PGP) resulting from the co-administration of substrate drugs represents a potential source of drug-drug interactions. Although in vitro screens can readily identify such interactions, the accuracy with which they mimic interactions in tissues or their value in predicting interactions in vivo is unresolved. This was addressed for the model PGP substrate digoxin by comparing the modulation of its permeability across Caco-2 cells and ex vivo human and rodent intestine by drugs for which pharmacokinetic data on interactions with digoxin in man is available. All five compounds (talinolol, omeprazole, verapamil, quinidine, cyclosporin) dose-dependently increased absorptive (A-B) digoxin permeability with maximal increases of 2.2-4.5-fold across Caco-2. Quantitatively similar increases were observed in ex vivo human and mouse intestine and studies in mdr1a(-/-) intestine confirmed that these interactions are mediated solely by PGP. In vitro changes in digoxin permeability were qualitative indicators of the increase in digoxin C(max) for these compounds in man, although accounting for the luminal drug concentrations expected for a given oral dose was a critical consideration. Based on a limited dataset these data suggest that Caco-2 accurately mimics intestinal digoxin interactions and may be useful in predicting the threshold dose at which interactions become clinically significant. Further studies across a wider range of drugs are needed to determine the broader applicability of in vitro data for quantitative prediction of clinical drug interactions.