Progress in understanding the structure-activity relationships of P-glycoprotein

Efflux transport proteins of Tetrahymena thermophila play important roles in resistance to perfluorooctane sulfonate exposure

The biotoxicity of perfluorooctane sulfonate (PFOS) has been a concern. However, the effects of PFOS on Tetrahymena thermophila, a unicellular model organism, remain unclear. This study aimed to investigate the toxicity and detoxification mechanism of PFOS in this protozoan. PFOS did not show prominent toxic effects on T. thermophila. Cell viability of T. thermophila can be concentration-dependently increased by PFOS. PFOS also increased the stability of cell membranes and the activity of lysosomes. However, PFOS inhibited efflux transporter activities. Most of the PFOS amount remained in the culture medium during the culture periods. Only a low amount of PFOS was absorbed by cells, where PFOS molecules were mainly combined with membrane proteins. The expressions of four membrane protein genes involved in transporting xenobiotics were analyzed by real time-PCR. The gene abcg25 was significantly up-regulated. The growth of abcg25 gene knockout protozoans under PFOS treatment was slightly inhibited. However, the amount of PFOS adsorbed by the knockout protozoans showed no significant difference from the Wild-type protozoans. We concluded that the ABCG25 protein might play a key role in preventing PFOS from entering the cell or being exported from the cells to protect T. thermophila against PFOS. However, ABCG25 was not the only membrane protein able to bind with PFOS.

Reduced P-glycoprotein recognition of a radioiodine-labeled phosphonium cation by stilbenylation for mitochondrial membrane potential based-imaging

The accumulation of radiolabeled phosphonium cations in cells is dependent on the mitochondrial membrane potential (MMP). However, the efflux of these cations from tumor cells via P-glycoprotein (P-gp) limits their clinical application as MMP-based imaging tracers. In the present study, we designed (E)-diethyl-4-[¹²⁵I]iodobenzyl-4-stilbenylphosphonium ([¹²⁵I]IDESP), which contains a stilbenyl substituent, as a P-gp inhibitor to reduce P-gp recognition, and evaluated its biological properties in comparison with 4-[¹²⁵I]iodobenzyl dipropylphenylphosphonium ([¹²⁵I]IDPP). The in vitro cellular uptake ratio of [¹²⁵I]IDESP in P-gp expressing K562/Vin cells to the parent (P-gp negative) K562 cells was significantly higher than that of [¹²⁵I]IDPP. The efflux rate of [¹²⁵I]IDESP was not significantly different between K562 and K562/Vin, while [¹²⁵I]IDPP was rapidly effluxed from K562/Vin compared with K562, and the efflux of [¹²⁵I]IDPP from K562/Vin was inhibited by the P-gp inhibitor, cyclosporine A. The cellular uptake of [¹²⁵I]IDESP was well correlated with the MMP levels. These results suggested that [¹²⁵I]IDESP was accumulated in cells depending on the MMP levels, without being effluxed via P-gp, while [¹²⁵I]IDPP was rapidly effluxed from the cells via P-gp. Despite having suitable in vitro properties for MMP-based imaging, [¹²⁵I]IDESP showed rapid blood clearance and lower tumor accumulation than [¹²⁵I]IDPP. Improvement in the normal tissue distribution of [¹²⁵I]IDESP is required to develop an agent for use in in vivo MMP-based tumor imaging.

Usnic Acid Isolated from Usnea antarctica (Du Rietz) Reduced In Vitro Angiogenesis in VEGF- and bFGF-Stimulated HUVECs and Ex Ovo in Quail Chorioallantoic Membrane (CAM) Assay

Simple Summary

Anti-angiogenic therapy, a promising strategy against cancer progression, is limited by drug resistance. Natural plants, such as secondary metabolites of lichens, may represent an appropriate strategy to increase the effectiveness of conventional therapies and overcome resistance to anti-angiogenic therapy if combined with existing chemotherapy. Accordingly, our study was designed to determine the potential anti-angiogenic effect of usnic acid, a secondary metabolite of lichens, on VEGF- and bFGF-stimulated HUVECs as well as in quail chorioallantoic membrane assays, which were supplemented by histological sections of CAM-affected layers.

Abstract

Natural products include a diverse set of compounds of drug discovery that are currently being actively used to target tumor angiogenesis. In the present study, we evaluated the anti-angiogenic activities of secondary metabolite usnic acid isolated from Usena antarctica. We investigated the in vitro effects on proliferation, migration, and tube formation of VEGF- and bFGF-stimulated HUVECs. Ex ovo anti-angiogenic activity was evaluated using the CAM assay. Our findings demonstrated that usnic acid in the concentration of 33.57 µM inhibited VEGF (25 ng/mL) and bFGF (30 ng/mL)-induced HUVECs proliferation, migration, and tube formation. The ex ovo CAM model was used to confirm the results obtained from in vitro studies. VEGF- and bFGF-induced vessel formation was inhibited by usnic acid after 72 h in over 2-fold higher concentrations compared to in vitro. Subsequently, histological sections of affected chorioallantoic membranes were stained with hematoxylin–eosin and alcian blue to determine the number and diameter of vessels as well as the thickness of the individual CAM layers (ectoderm, mesoderm, endoderm). Usnic acid was able to suppress the formation of VEGF- and bFGF-induced vessels with a diameter of less than 100 μm, which was demonstrated by the reduction of mesoderm thickness as well.

Effect of Excessive Serotonin on Pharmacokinetics of Cephalexin after Oral Administration: Studies with Serotonin-Excessive Model Rats

PURPOSE

Serotonin (5-HT) is important for gastrointestinal functions, but its role in drug absorption remains to be clarified. Therefore, the pharmacokinetics and oral absorption of cephalexin (CEX) were examined under 5-HT-excessive condition to understand the role of 5-HT.

METHODS

5-HT-excessive rats were prepared by multiple intraperitoneal dosing of 5-HT and clorgyline, an inhibitor for 5-HT metabolism, and utilized to examine the pharmacokinetics, absorption behavior and the intestinal permeability for CEX.

RESULTS

Higher levels of 5-HT in brain, plasma and small intestines were recognized in 5-HT-excessive rats, where the oral bioavailability of CEX was significantly enhanced. The intestinal mucosal transport via passive diffusion of CEX was significantly increased, while its transport via PEPT1 was markedly decreased specifically in the jejunal segment, which was supported by the decrease in PEPT1 expression on brush border membrane (BBM) of intestinal epithelial cells. Since no change in antipyrine permeability and significant increase in FITC dextran-4 permeability were observed in 5-HT-excessive rats, the enhanced permeability for CEX would be attributed to the opening of tight junction, which was supported by the significant decrease in transmucosal electrical resistance. In 5-HT-excessive rats, furthermore, total body clearance of CEX tended to be larger and the decrease in PEPT2 expression on BBM in kidneys was suggested to be one of the reasons for it.

CONCLUSIONS

5-HT-excessive condition enhanced the oral bioavailability of CEX in rats, which would be attributed to the enhanced permeability across the intestinal mucosa via passive diffusion through the paracellular route even though the transport via PEPT1 was decreased.

The Entanglement between Mitochondrial DNA and Tumor Metastasis

Simple Summary

Mitochondrial dysfunction is one of the main features of cancer cells. As genetic material in mitochondria, mitochondrial DNA (mtDNA) variations and dysregulation of mitochondria-encoded genes have been shown to correlate with survival outcomes in cancer patients. Cancer metastasis is often a major cause of treatment failure, which is a multi-step cascade process. With the development of gene sequencing and in vivo modeling technology, the role of mtDNA in cancer metastasis has been continuously explored. Our review systematically provides a summary of the multiple roles of mtDNA in cancer metastasis and presents the broad prospects for mtDNA in cancer prediction and therapy.

Abstract

Mitochondrial DNA, the genetic material in mitochondria, encodes essential oxidative phosphorylation proteins and plays an important role in mitochondrial respiration and energy transfer. With the development of genome sequencing and the emergence of novel in vivo modeling techniques, the role of mtDNA in cancer biology is gaining more attention. Abnormalities of mtDNA result in not only mitochondrial dysfunction of the the cancer cells and malignant behaviors, but regulation of the tumor microenvironment, which becomes more aggressive. Here, we review the recent progress in the regulation of cancer metastasis using mtDNA and the underlying mechanisms, which may identify opportunities for finding novel cancer prediction and therapeutic targets.

Practical Application of Rodent Transporter Knockout Models to assess Brain Penetration in Drug Discovery

BACKGROUND & OBJECTIVE

Compound X is a drug candidate for the treatment of neurodegenerative diseases. Its brain distribution was evaluated as part of the lead identification and optimization of early drug discovery.

METHODS

The brain distribution of compound X was studied in genetic transporter knockout rodent models, in vivo models with a chemical inhibitor and in vitro transporter cell systems.

RESULTS

Compound X was found to be a substrate for human Breast Cancer-Resistance Protein (BCRP) in vitro (efflux ratio 8.1) and rodent Bcrp in vivo (Kp,uuKO/Kp,uuWT = 0.15/0.057 = 2.7, p < 0.05) but not a substrate for human P-glycoprotein (P-gp) in vitro (efflux ratio 1.0) nor rodent P-gp in vivo (Kp,uuKO/Kp,uuWT = 0.056/0.051 = 1.1, p > 0.05). When both transporters were knocked out in vivo, Kp,uu increased to 0.51 ± 0.02. Similar patterns observed across compounds with related chemistry corroborated structure-activity relationship.

CONCLUSION

While in vitro assays showed compound X to be a substrate for human BCRP and not P-gp, in vivo studies indicated a synergistic effect between rodent efflux transporters. However, this only accounted for ~50% of restricted BBB-transport, suggesting involvement from other efflux transporters. Given Kp,uu is a key criterion for assessing technical quality of CNS candidates before progression into clinical development, it is important to identify relevant screening assays for a better understanding of low Kp,uu and brain distribution in pre-clinical models for translation to humans.

Possible Regulation of P-glycoprotein Function by Adrenergic Agonists in a Vascular-luminal Perfused Preparation of Small Intestine

Although the functions of small intestine are largely regulated by enteric nervous system (ENS), an independent intrinsic innervation, as well as central nervous system (CNS), the neural regulation of drug absorption from the small intestine still remains to be clarified. To obtain some information on it, the effect of adrenergic agonists on P-glycoprotein (P-gp) function was investigated by utilizing a vascular-luminal perfused rat small intestine. Adrenaline significantly decreased the secretion of rhodamine-123 (R-123) into the intestinal lumen, but dibutyryl cAMP (DBcAMP) significantly enhanced R-123 secretion. The inhibition study with quinidine clearly indicated that the decrease in secretory clearance of R-123 by adrenaline or the increase by DBcAMP would be attributed to the decrease or increase in P-gp activity, respectively. Expression levels of P-gp in whole mucosal homogenates were not changed at all by any chemicals examined, but those on brush border membrane (BBM) of intestinal epithelial cells were significantly decreased or increased by adrenaline or DBcAMP, respectively. Furthermore, changes in P-gp activity caused by adrenergic agonists and DBcAMP were significantly correlated with changes in expression level of P-gp in BBM, suggesting that the trafficking of P-gp from cytosolic pool to BBM would be regulated by adrenergic agonists and DBcAMP.

Blocking of efflux transporters in rats improves translational validation of brain radioligands

Background

Positron emission tomography (PET) is a molecular imaging technique that can be used to investigate the in vivo pharmacology of drugs. Initial preclinical evaluation of PET tracers is often conducted in rodents due to the accessibility of disease models as well as economic considerations. Compared to larger species, rodents display a higher expression and/or activity of efflux transporters such as the P-glycoprotein (P-gp). Low brain uptake could, therefore, be species-specific and uptake in rodents not be predictive for that in humans. We hypothesized that a better prediction from rodent data could be achieved when a tracer is evaluated under P-gp inhibition. Consequently, we compared the performance of eight neuroreceptor tracers in rats with and without P-gp inhibition including a specific binding blockade. This data set was then used to predict the binding of these eight tracers in pigs.

Methods

PET tracers targeting serotonin 5-HT_2A receptors ([¹⁸F]MH.MZ, [¹⁸F]Altanserin, [¹¹C]Cimbi-36, [¹¹C]Pimavanserin), serotonin 5-HT₇ receptors ([¹¹C]Cimbi-701, [¹¹C]Cimbi-717 and [¹¹C]BA-10) and dopamine D_2/3 receptors ([¹⁸F]Fallypride) were used in the study. The brain uptake and target-specific binding of these PET radiotracers were evaluated in rats with and without inhibition of P-gp. Rat data were subsequently compared to the results obtained in pigs.

Results

Without P-gp inhibition, the amount of target-specific binding in the rat brain was sufficient to justify further translation for three out of eight evaluated tracers. With P-gp inhibition, results for five out of eight tracers justified further translation. The performance in pigs could correctly be predicted for six out of eight tracers when rat data obtained under P-gp inhibition were used, compared to four out of eight tracers without P-gp inhibition.

Conclusions

P-gp strongly affects the uptake of PET tracers in rodents, but false prediction outcomes can be reduced by evaluating a tracer under P-gp inhibition.

Design, synthesis, antifungal activity, and structure-activity relationship studies of chalcones and hybrid dihydrochromane-chalcones

A series of ten chalcones (7a-j) and five new dihydrochromane-chalcone hybrids (7k-o) were synthesized and identified using spectroscopic techniques (IR, NMR, and MS). All compounds were evaluated in vitro against the B. cinerea and M. fructicola phytopathogens that affect a wide range of crops of commercial interest. All compounds were tested against both phytopathogens using the mycelial growth inhibition test, and it was found that two and five compounds had similar activity to that of the positive control for B. cinerea (7a = 43.9, 7c = 45.5, and Captan^®= 24.8 µg/mL) and M. fructicola (7a = 48.5, 7d = 78.2, 7e = 56.1, 7f = 51.8, 7n = 63.2, and Mystic^®= 21.6 µg/mL), respectively. To understand the key chalcone structural features for the antifungal activity on B. cinerea and M. fructicola, we developed structure-activity models with good statistical values (r² and q² higher than 0.8). For B. cinerea, the hydrogen bonding donor and acceptor and the atomic charge on C₅ modulate the mycelial growth inhibition activity. In contrast, dipole moment and atomic charge on C_1' and the carbonyl carbon modify the inhibition activity for M. fructicola. These results allow the design of other compounds with activities superior to those of the compounds obtained in this study.

Molecular Modeling of Drug-Transporter Interactions-An International Transporter Consortium Perspective

Membrane transporters play diverse roles in the pharmacokinetics and pharmacodynamics of small-molecule drugs. Understanding the mechanisms of drug-transporter interactions at the molecular level is, therefore, essential for the design of drugs with optimal therapeutic effects. This white paper examines recent progress, applications, and challenges of molecular modeling of membrane transporters, including modeling techniques that are centered on the structures of transporter ligands, and those focusing on the structures of the transporters. The goals of this article are to illustrate current best practices and future opportunities in using molecular modeling techniques to understand and predict transporter-mediated effects on drug disposition and efficacy.Membrane transporters from the solute carrier (SLC) and ATP-binding cassette (ABC) superfamilies regulate the cellular uptake, efflux, and homeostasis of many essential nutrients and significantly impact the pharmacokinetics of drugs; further, they may provide targets for novel therapeutics as well as facilitate prodrug approaches. Because of their often broad substrate selectivity they are also implicated in many undesirable and sometimes life-threatening drug-drug interactions (DDIs).^5,6.

Cytochrome P450/ABC transporter inhibition simultaneously enhances ivermectin pharmacokinetics in the mammal host and pharmacodynamics in Anopheles gambiae

Mass administration of endectocides, drugs that kill blood-feeding arthropods, has been proposed as a complementary strategy to reduce malaria transmission. Ivermectin is one of the leading candidates given its excellent safety profile. Here we provide proof that the effect of ivermectin can be boosted at two different levels by drugs inhibiting the cytochrome or ABC transporter in the mammal host and the target mosquitoes. Using a mini-pig model, we show that drug-mediated cytochrome P450/ABC transporter inhibition results in a 3-fold increase in the time ivermectin remains above mosquito-killing concentrations. In contrast, P450/ABC transporter induction with rifampicin markedly impaired ivermectin absorption. The same ketoconazole-mediated cytochrome/ABC transporter inhibition also occurs outside the mammal host and enhances the mortality of Anopheles gambiae. This was proven by using the samples from the mini-pig experiments to conduct an ex-vivo synergistic bioassay by membrane-feeding Anopheles mosquitoes. Inhibiting the same cytochrome/xenobiotic pump complex in two different organisms to simultaneously boost the pharmacokinetic and pharmacodynamic activity of a drug is a novel concept that could be applied to other systems. Although the lack of a dose-response effect in the synergistic bioassay warrants further exploration, our study may have broad implications for the control of parasitic and vector-borne diseases.

Preparation, characterization and in vivo studies of amorphous solid dispersion of berberine with hydrogenated phosphatidylcholine

Berberine, a pure crystalline quaternary ammonium salt with the basic structure of isoquinoline alkaloid, has multiple pharmacological bioactivities. But the poor bioavailability of berberine limited its wide clinical applications. In the present study, we aimed to develop an amorphous solid dispersion of berberine with hydrogenated phosphatidylcholine (HPC) in order to improve its bioavailability. The physical characterization studies such as differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), Fourier transform infrared spectrophotometry (FT-IR) and scanning electron microscopy (SEM) were conducted to characterize the formation of amorphous berberine HPC solid dispersion (BHPC-SD). The everted intestinal sac and single-pass intestinal perfusion study proved that permeability and intestinal absorption of amorphous BHPC-SD was improved compared with that of pure crystalline berberine, and the pharmacokinetic study results demonstrated that the extent of bioavailability was significantly increased as well. However, the dissolution study indicated that the aqueous cumulative dissolution percentages of berberine remained unchanged or even lower by means of preparation into solid dispersion with HPC. Therefore, according to the previous mechanistic studies, the present results supported that it is the enhanced molecularly dissolved concentration (supersaturation) of berberine by transformation from crystalline structure into amorphous solid dispersions that triggers the enhanced permeability, and consequently results in the improved intestinal absorption and bioavailability.

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 Toxic Effects of Polychemotherapy onto the Liver Are Accelerated by the Upregulated MDR of Lymphosarcoma

Antitumor therapy of hematological malignancies is impeded due to the high toxicity of polychemotherapy toward liver and increasing multiple drug resistance (MDR) of tumor cells under the pressure of polychemotherapy. These two problems can augment each other and significantly reduce the efficiency of antineoplastic therapy. We studied the combined effect of polychemotherapy and upregulated MDR of lymphosarcoma RLS(40) onto the liver of experimental mice using two treatment schemes. Scheme 1 is artificial: the tumor was subjected to four courses of polychemotherapy while the liver of the tumor-bearing mice was exposed to only one. This was achieved by threefold tumor retransplantation taken from animals subjected to chemotherapy into intact animals. Scheme 2 displays "real-life" status of patients with MDR malignancies: both the tumor and the liver of tumor-bearing mice were subjected to three sequential courses of polychemotherapy. Our data show that the strengthening of MDR phenotype of RLS(40) under polychemotherapy and toxic pressure of polychemotherapy itself has a synergistic damaging effect on the liver that is expressed in the accumulation of destructive changes in the liver tissue, the reduction of the regeneration capacity of the liver, and increasing of Pgp expression on the surface of hepatocytes.

The transport of antiepileptic drugs by P-glycoprotein

Epilepsy is the most common serious chronic neurological disorder. Current data show that one-third of patients do not respond to anti-epileptic drugs (AEDs). Most non-responsive epilepsy patients are resistant to several, often all, AEDs, even though the drugs differ from each other in pharmacokinetics, mechanisms of action, and interaction potential. The mechanisms underlying drug resistance of epilepsy patients are still not clear. In recent years, one of the potential mechanisms interesting researchers is over-expression of P-glycoprotein (P-gp, also known as ABCB1 or MDR1) in endothelial cells of the blood-brain barrier (BBB) in epilepsy patients. P-gp plays a central role in drug absorption and distribution in many organisms. The expression of P-gp is greater in drug-resistant than in drug-responsive patients. Some studies also indicate that several AEDs are substrates or inhibitors of P-gp, implying that P-gp may play an important role in drug resistance in refractory epilepsy. In this article, we review the clinical and laboratory evidence that P-gp expression is increased in epileptic brain tissues and that AEDs are substrates of P-gp in vitro and in vivo. We discuss criteria for identifying the substrate status of AEDs and use structure-activity relationship (SAR) models to predict which AEDs act as P-gp substrates.

Knowledge-Based, Central Nervous System (CNS) Lead Selection and Lead Optimization for CNS Drug Discovery

The central nervous system (CNS) is the major area that is affected by aging. Alzheimer’s disease (AD), Parkinson’s disease (PD), brain cancer, and stroke are the CNS diseases that will cost trillions of dollars for their treatment. Achievement of appropriate blood–brain barrier (BBB) penetration is often considered a significant hurdle in the CNS drug discovery process. On the other hand, BBB penetration may be a liability for many of the non-CNS drug targets, and a clear understanding of the physicochemical and structural differences between CNS and non-CNS drugs may assist both research areas. Because of the numerous and challenging issues in CNS drug discovery and the low success rates, pharmaceutical companies are beginning to deprioritize their drug discovery efforts in the CNS arena. Prompted by these challenges and to aid in the design of high-quality, efficacious CNS compounds, we analyzed the physicochemical property and the chemical structural profiles of 317 CNS and 626 non-CNS oral drugs. The conclusions derived provide an ideal property profile for lead selection and the property modification strategy during the lead optimization process. A list of substructural units that may be useful for CNS drug design was also provided here. A classification tree was also developed to differentiate between CNS drugs and non-CNS oral drugs. The combined analysis provided the following guidelines for designing high-quality CNS drugs: (i) topological molecular polar surface area of <76 Ų (25–60 Ų), (ii) at least one (one or two, including one aliphatic amine) nitrogen, (iii) fewer than seven (two to four) linear chains outside of rings, (iv) fewer than three (zero or one) polar hydrogen atoms, (v) volume of 740–970 ų, (vi) solvent accessible surface area of 460–580 Ų, and (vii) positive QikProp parameter CNS. The ranges within parentheses may be used during lead optimization. One violation to this proposed profile may be acceptable. The chemoinformatics approaches for graphically analyzing multiple properties efficiently are presented.

Discovering natural product modulators to overcome multidrug resistance in cancer chemotherapy

Multidrug resistance caused by the overexpression of ABC drug transporters is a major obstacle in clinical cancer chemotherapy. For several years, it appeared that direct inhibition of ABC transporters would be the cheapest and most efficient way to combat this problem. Unfortunately, progress in finding a potent, selective inhibitor to modulate ABC transporters and restore drug sensitivity in multidrug-resistant cancer cells has been slow and challenging. Candidate drugs should ideally be selective, potent and relatively non-toxic. Many researchers in recent years have turned their attention to utilizing natural products as the building blocks for the development of the next generation of inhibitors, especially after the disappointing results obtained from inhibitors of the first three generations at the clinical trial stage. The first step is to discover natural substances (distinct from the first three generation inhibitors) that are potent, selective and relatively non-toxic in order to be used clinically. Here, we present a brief overview of the prospect of using natural products to modulate the function of ABC drug transporters clinically and their impact on human physiology and pharmacology.

Docking and 3D-QSAR (quantitative structure activity relationship) studies of flavones, the potent inhibitors of p-glycoprotein targeting the nucleotide binding domain

In order to explore the interactions between flavones and P-gp, in silico methodologies such as docking and 3D-QSAR were performed. CoMFA and CoMSIA analyses were done using ligand based and receptor guided alignment schemes. Validation statistics include leave-one-out cross-validated R(2) (q(2)), internal prediction parameter by progressive scrambling (Q(*2)), external prediction with test set. They show that models derived from this study are quite robust. Ligand based CoMFA (q(2) = 0.747, Q(*2) = 0.639, r(pred)(2)=0.802) and CoMSIA model (q(2) = 0.810, Q(*2) = 0.676, r(pred)(2)=0.785) were developed using atom by atom matching. Receptor guided CoMFA (q(2) = 0.712, Q(*2) = 0.497, r(pred)(2) = 0.841) and for CoMSIA (q(2) = 0.805, Q(*2) = 0.589, r(pred)(2) = 0.937) models were developed by docking of highly active flavone into the proposed NBD (nucleotide binding domain) of P-gp. The 3D-QSAR models generated here predicted that hydrophobic and steric parameters are important for activity toward P-gp. Our studies indicate the important amino acid in NBD crucial for binding in accordance with the previous results. This site forms a hydrophobic site. Since flavonoids have potential without toxicity, we propose to inspect this hydrophobic site including Asn1043 and Asp1049 should be considered for future inhibitor design.

P-glycoprotein substrate models using support vector machines based on a comprehensive data set

P-glycoprotein (P-gp) is one of the major ABC transporters and involved in many essential processes such as lipid and steroid transport across cell membranes but also in the uptake of drugs such as HIV protease and reverse transcriptase inhibitors. Despite its importance, reliable models predicting substrates of P-gp are scarce. In this study, we have built several computational models to predict whether or not a compound is a P-gp substrate, based on the largest data set yet published, employing 332 distinct structures. Each molecule is represented by ADRIANA.Code, MOE, and ECFP_4 fingerprint descriptors. The models are computed using a support vector machine based on a training set which includes 131 substrates and 81 nonsubstrates that were evaluated by 5-, 10-fold, and leave-one-out (LOO) cross-validation. The best model gives a Matthews Correlation Coefficient of 0.73 and a prediction accuracy of 0.88 on the test set. Examination of the model based on ECFP_4 fingerprints revealed several substructures which could have significance in separating substrates and nonsubstrates of P-gp, such as the nitrile and sulfoxide functional groups which have a higher frequency in nonsubstrates than in substrates. In addition structural isomerism in sugars was found to result in remarkable differences regarding the likelihood of a compound to be a substrate for P-gp.

Dihydropyridines and multidrug resistance: previous attempts, present state, and future trends

Multidrug resistance is defined as the resistance of a tumor cell to the cytotoxic action of divergent drugs used in chemotherapy. Dihydropyridines are a class of calcium channel antagonists that were discovered to have a multidrug resistance reversing effect and prompted investigations resulting in the synthesis of hundreds of new derivatives. Most of the investigators tried to achieve two goals: a decrease in Ca²(+) channel-blocking activity and an increase in the multidrug resistance reversing effect. Most of the synthesized compounds failed in the later stages of studies especially in clinical trials because of pharmacokinetic or pharmacodynamic limitations. Therefore, it will be necessary to include new methods, such as combinatorial synthesis, and, more importantly, to apply computational methods based on global structure-activity relationship models that consider all problems. Moreover, some compounds should be synthesized that are effective on several multidrug resistance targets.

Spatial expression and functionality of drug transporters in the intact lung: objectives for further research

This commentary provides a background appraising evidence in the intact lung on the spatial expression of drug transporters and, where available, evidence in the intact lung of the impact, or otherwise, that such transporters can have upon pulmonary drug absorption and disposition. Ultimately drug discovery and development scientists will wish to identify in a 'pulmonary' context the effect of disease upon transporter function, the potential for drug transporters to contribute to drug-drug interactions and to inter-individual variation in drug handling and response. The rate and extent of lung epithelial permeation of drugs involve an interplay between the dose and the deposition site of drug within the lung and physiological variables operational at the epithelial-luminal interface. Amongst the latter variables is the potential impact of active transporter processes which may well display regio-selective characteristics along the epithelial tract. In pulmonary tissues the spatial pattern of drug transporter expression is generally poorly defined and the functional significance of transporters within the intact lung is explored in only a limited manner. Active transporters in the lung epithelium may affect airway residence times of drug, modulate access of drug to intracellular targets and to submucosal lung tissue, and potentially influence airway to systemic drug absorption profiles. Transporters in the lung tissue may also have the capacity to mediate uptake of drug from the systemic circulation resulting in drug accumulation in the lung. Transporters have physiological roles and new drug candidates while not necessarily serving as transport substrates may modulate transporter activity and hence physiology. The commentary highlights a series of recommendations for further work in pulmonary drug transporter research.

Genomic evaluation during permeability of indomethacin and its solid dispersion

Drug resistance was first identified in cancer cells that express proteins known as multidrug resistance proteins that extrude the therapeutic agents out of the cells resulting in alteration of pharmacokinetics, tissue distribution, and pharmacodynamics of drugs. To this end studies were carried out to investigate the role of pharmacological inhibitors and pharmaceutical excipients with a primary focus on P-glycoprotein (P-gp). The aim of this study was to investigate holistic changes in transporter gene expression during permeability upon formulation of indomethacin as solid dispersion. Initial characterization studies of solid dispersion of indomethacin showed that the drug was dispersed within the carrier in amorphous form. Analysis of permeability data across Caco-2 monolayers revealed that drug absorption increased by 4-fold when reformulated as solid dispersion. The last phase of the work involved investigation of gene expression changes of transporter genes during permeability. The results showed that there were significant differences in the expression of both ATP-binding cassette (ABC) transporter genes as well as solute carrier transporter (SLC) genes suggesting that the inclusion of polyethylene glycol as well as changes in molecular form of drug from crystalline to amorphous have a significant bearing on the expression of transporter network genes resulting in differences in drug permeability.

Papyriferic acid derivatives as reversal agents of multidrug resistance in cancer cells

Forty-one derivatives of papyriferic acid were prepared based on our previous finding that methyl papyriferate (3) showed potent reversing effect on cytotoxicity of colchicine against multidrug resistance (MDR) human cancer cells (KB-C2), and evaluated for their cytotoxicity and effect on reversing P-gp-mediated MDR against KB-C2 cells. 3-O-(Morpholino-beta-oxopropanoyl)-12beta-acetoxy-3alpha,25-dihydroxy-(20S,24R)-epoxydammarane (37) significantly increased the sensitivity of colchicine against KB-C2 cells by 185-fold at 5microg/mL (7.4microM), and the cytotoxicity of colchicine was recovered to nearly that of sensitive (KB) cells. The other several new amide derivatives also exhibited potent reversal activity comparable to or more potent than methyl papyriferate and verapamil.

Design and synthesis of new symmetrical derivatives of dihydropyridine containing a pyridyl group on the 3, 5-positions and evaluation of their cytotoxic and multidrug resistance reversal activity

Today, chemotherapy is an important part in the treatment of several kinds of cancer; however, the development of drug resistance remains one of the major obstacles in successful chemotherapy. Several types of agents have been recognized as multidrug resistance (MDR) inhibitors, among which the 1,4-dihydropyridines (DHPs) have been investigated the most. P-glycoprotein inhibition has been reported as the main MDR reversal mechanism of DHPs, whilst other mechanisms such as inhibition of topoisomerase II have received less attention. Therefore, in this study new derivatives of DHP have been synthesized. Their cytotoxic activity and their effects in reversing atypical MDR have been evaluated. The results confirmed the appropriate effect of these compounds on atypical MDR. Although it was observed that these compounds had a moderate cytotoxic effect, the cytotoxicity of one compound on the K562 cell line (IC50 = 6.61 μM) was comparable with that of doxorubicin (IC50 = 4.17 μM). Finally, the Ca2+-channel antagonistic activity, an undesired effect for these compounds, was evaluated.

Lack of difference in pulmonary absorption of digoxin, a P-glycoprotein substrate, in mdr1a-deficient and mdr1a-competent mice

Although in-vitro experiments have suggested that P-glycoprotein (P-gp) may have an important influence on the disposition of inhaled drugs, the effect of P-gp on absorption from the lung in-vivo has not been reported previously. The aim of this study was to compare the pulmonary absorption of digoxin, a well-characterised substrate for P-gp, in mdr1a (–/–) (P-gp-deficient) and mdr1a (+/+) (P-gp-competent) mice. Digoxin was administered by intratracheal instillation over 3–4 s, a method demonstrated to result in dispersion of the dose to all regions of the lung. Drug distribution was determined in the lungs, plasma, brain, heart, liver and kidney of individual mice after 5, 10, 30, 60 and 90 min. Digoxin was cleared rapidly from the lung after intratracheal administration. No differences were observed in the maximum serum concentrations between mdr1a (+/+) and mdr1a (–/–) mice (37.8 ± 6.9 and 38.8 ± 15.8 ng mL−1, respectively). The serum concentration versus time profiles were similar in both strains; the area under the drug serum concentration versus time curve (AUC) was 2010 and 1812 ng mL−1 min in mdr1a (–/–) and mdr1a (+/+) mice, respectively. For organs harvested at the end of the experiment (90 min), the only significant difference between the strains was the markedly elevated concentration of digoxin in the brains of mdr1a (–/–) mice. In conclusion, digoxin is rapidly absorbed from the mouse lung following tracheal instillation, with no difference in the rate or extent of absorption between mdr1a-deficient and -competent mice. This suggests that, in contrast to the scenario suggested by in-vitro data, P-gp in the respiratory epithelium may have little influence on the disposition of drugs that are well absorbed from the lung.

Doxorubicin-loaded solid lipid nanoparticles to overcome multidrug resistance in cancer therapy

In the present study we developed doxorubicin-loaded solid lipid nanoparticles (SLN-Dox) using biocompatible compounds, assessed the in vitro hemolytic effect, and examined their in vivo effects on drug retention and apoptosis intensity in P-glycoprotein-overexpressing MCF-7/ADR cells, a representative Dox-resistant breast cancer cell line. Our SLNs did not show hemolytic activity in human erythrocytes. In comparison with Dox, SLN-Dox efficiently enhanced apoptotic cell death through the higher accumulation of Dox in MCF-7/ADR cells. Therefore, SLN-Dox have potential to serve as a useful therapeutic approach to overcome the chemoresistance of adriamycin-resistant breast cancer.

FROM THE CLINICAL EDITOR

Doxorubicin loaded solid lipid nanoparticles (SLN-Dox) were studied in a cell line representative of doxorubicin resistant breast cancer. The nanoparticles did not show hemolytic activity; furthermore, they efficiently enhanced apoptotic cell death through higher accumulation of doxorubicin in cancer cells. This approach may be viable in overcoming the chemoresistance of adriamycin resistant breast cancer.

Lipid nanoparticles: effect on bioavailability and pharmacokinetic changes

The main aim of pharmaceutical technology research is the design of successful formulations for effective therapy, taking into account several issues including therapeutic requirements and patient compliance. In this regard, several achievements have been reported with colloidal carriers, in particular with lipid nanoparticles, due to their unique physicochemical properties. For several years these carriers have been showing potential success for several administration routes, namely oral, dermal, parenteral, and, more recently, for pulmonary and brain targeting. The present chapter provides a review of the use of solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) to modify the release profile and the pharmacokinetic parameters of active pharmaceutical ingredients (APIs) incorporated in these lipid matrices, aiming to modify the API bioavailability, either upwards or downwards depending on the therapeutic requirement. Definitions of the morphological characteristics, surface properties, and polymorphic structures will also be given, emphasizing their influence on the incorporation parameters of the API, such as yield of production, loading capacity, and encapsulation efficiency.

Examination of CYP3A and P-glycoprotein-mediated drug-drug interactions using animal models

With the advent of polytherapy for cancer treatment it has become prudent to minimize, as much as possible, the potential for drug-drug interactions (DDI). Toward this end, the metabolic and transporter pathways involved in the disposition of a drug candidate (phenotyping) and potential for inhibition and induction of drug-metabolizing enzymes and transporters are evaluated in vitro. Such in vitro human data can be made available prior to human dosing and enable in vitro to in vivo-based predictions of clinical outcomes. Despite some success, however, in vitro systems are not dynamic and sometimes fail to predict drug-drug interactions for a variety of reasons. In comparison, relatively less effort has been made to evaluate predictions based on data derived from in vivo animal models. This chapter will attempt to summarize different examples from the literature where animal models have been used to predict cytochrome P450 3A (CYP3A)- and P-glycoprotein-based DDI. When employing data from animal models one needs to be aware of species differences in enzyme- and transporter-activity leading to differences in pharmacokinetics, clearance pathways as well as species differences in selectivity and affinity of probe substrates and inhibitors. Because of these differences, in vivo animal studies alone, cannot be predictive of human DDI. Despite these caveats, the information obtained from validated in vivo animal models may prove useful when used in conjunction with in vitro-in vivo extrapolation methods. Such an integrated data set can be used to select drug candidates with a reduced DDI potential.

Generating inhibitors of P-glycoprotein: where to, now?

The prominent role for the drug efflux pump ABCB1 (P-glycoprotein) in mediating resistance to chemotherapy was first suggested in 1976 and sparked an incredible drive to restore the efficacy of anticancer drugs. Achieving this goal seemed inevitable in 1982 when a series of calcium channel blockers were demonstrated to restore the efficacy of chemotherapy agents. A large number of other compounds have since been demonstrated to restore chemotherapeutic sensitivity in cancer cells or tissues. Where do we stand almost three decades since the first reports of ABCB1 inhibition? Unfortunately, in the aftermath of extensive fundamental and clinical research efforts the situation remains gloomy. Only a small handful of compounds have reached late stage clinical trials and none are in routine clinical usage to circumvent chemoresistance. Why has the translation process been so ineffective? One factor is the multifactorial nature of drug resistance inherent to cancer tissues; ABCB1 is not the sole factor. However, expression of ABCB1 remains a significant negative prognostic indicator and is closely associated with poor response to chemotherapy in many cancer types. The main difficulties with restoration of sensitivity to chemotherapy reside with poor properties of the ABCB1 inhibitors: (1) low selectivity to ABCB1, (2) poor potency to inhibit ABCB1, (3) inherent toxicity and/or (4) adverse pharmacokinetic interactions with anticancer drugs. Despite these difficulties, there is a clear requirement for effective inhibitors and to date the strategies for generating such compounds have involved serendipity or simple chemical syntheses. This chapter outlines more sophisticated approaches making use of bioinformatics, combinatorial chemistry and structure informed drug design. Generating a new arsenal of potent and selective ABCB1 inhibitors offers the promise of restoring the efficacy of a key weapon in cancer treatment--chemotherapy.

New potent P-glycoprotein modulators with the cucurbitane scaffold and their synergistic interaction with doxorubicin on resistant cancer cells

The novel cucurbitacins, balsaminagenin A and B (1-2) and balsaminoside A (3) and the know cucurbitacin karavelagenin C (4), together with five new mono or diacylated derivatives (5-9) of karavelagenin C were evaluated for multidrug resistance reversing activity on human MDR1 gene transfected mouse lymphoma cells. Compounds 2-6 exhibited a strong activity compared with that of the positive control, verapamil. Structure-activity relationships are discussed. Moreover, in the checkerboard model of combination chemotherapy, the interaction between doxorubicin and compounds 2-5 synergistically enhanced the effect of the anticancer drug. Compounds 1-4 were isolated from the aerial parts of Momordica balsamina L. The structures of the compounds were established on the basis of spectroscopic methods including 2D NMR experiments (COSY, HMQC, HMBC and NOESY).

TCP-FA4: a derivative of tranylcypromine showing improved blood-brain permeability

A variety of approaches have been taken to improve the brain penetration of pharmaceutical agents. The amphipathic character of a compound can improve its interaction with the lipid bilayer within cell membranes, and as a result improve permeability. Fatty acid chains or lipoamino acids of various lengths were attached to tranylcypromine (TCP), in an attempt to improve the blood-brain barrier (BBB) permeability by increasing the lipophilicity as well as the amphiphatic character of the drug. TCP-FA4, one of the derivatives containing a four carbon alkyl acid chain, showed the greatest improvement in permeability. This molecule was slightly neuroprotective in a beta-amyloid-induced neurodegeneration assay and may also be capable of upregulating brain derived neurotrophic factor (BDNF), as indicated by cell culture assays using human umbilical vein endothelial cells. Since decreased levels of BDNF are observed in many CNS disorders, and direct injection of BDNF is not a viable option due to its poor permeability across the BBB, small molecules capable of regulating BDNF that also cross the BBB may be an interesting treatment option.

Advancement of structure-activity relationship of multidrug resistance-associated protein 2 interactions

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is mainly expressed in the apical phase of barrier membranes. It functions as a critical efflux pump in the biliary excretion of endogenous substances, such as conjugated bilirubin and bile salts, as well as many structurally diverse xenobiotics and their metabolites. Due to its important role in defining ADME/Tox properties, efforts have emerged to build the structure-activity relationship (SAR) for MRP2/ABCC2 at early stages of drug discovery process. MRP2/ABCC2 is a member of the integral membrane protein family whose high-resolution crystal structure has not been described. To overcome the obstacle of lacking detailed structural depiction, various molecular modeling approaches have been applied to derive the structural requirements for binding interactions with MRP2/ABCC2 protein, including two-dimensional (2D) and three-dimensional (3D) quantitative SAR (QSAR) analysis, pharmacophore models, and homology modeling of the transporter. Here we summarize recent progresses in understanding the SAR of MRP2/ABCC2 recognition of substrates and/or inhibitors, and describe some of the useful in vitro tools for characterizing the interactions with the transporter.

Clinical significance of ABCB1 genotyping in oncology

BACKGROUND

P-glycoprotein (Pgp) is a drug efflux pump that transports natural products, including taxanes and other chemotherapeutic agents, from cells. Several frequent polymorphisms in ATP binding cassette gene B1 (ABCB1) may influence Pgp levels and drug efflux. The purpose of this review was to assess the clinical significance of ABCB1 polymorphisms in oncology.

METHODS

Peer-reviewed studies were identified through a search of PubMed/MEDLINE (1990-2008) and the ASCO abstracts (2003-2008) database. Included studies described clinical trials where ABCB1 genotyping was performed in patients with cancer. Search terms included ABCB1, Pgp, docetaxel, paclitaxel, irinotecan, imatinib, and anticancer agent. Studies were excluded if the manuscript was not available in English.

RESULTS

The influence of polymorphisms in ABCB1 2677G>T/A, 3435C>T, and 1236C>T and progression-free and overall survival in 309 patients from the Australian Ovarian Cancer Study treated with paclitaxel/carboplatin demonstrated that compared to homozygote GG carriers at 2677, women with the minor T/A alleles were significantly less likely to relapse following treatment. Other trials of ABCB1 genotyping in breast and prostate cancer patients receiving taxanes have shown inconsistent results. Pharmacokinetic studies where ABCB1 was genotyped and patients received irinotecan or imatinib have also shown inconsistent results.

CONCLUSION

A number of commercially available drugs are substrates for Pgp, and the ABCB1-variant genotypes are frequent and functionally significant, which may have future implications for drug dosing.

Identification of putative steroid-binding sites in human ABCB1 and ABCG2

Homology modelling was used to generate three-dimensional structures of the nucleotide-binding domains (NBDs) of human ABCB1 and ABCG2. Interactions between a series of steroidal ligands and transporter NBDs were investigated using an in silico docking approach. C-terminal ABCB1 NBD (ABCB1 NBD2) was predicted to bind steroids within a cavity formed partly by the P-Loop, Tyr1044 and Ile1050. The P-Loop within ABCG2 NBD was also predicted to be involved in steroid binding. No overlap between ATP- and RU-486-binding sites was predicted in either NBD, though overlaps between ATP- and steroid-binding sites were predicted in the vicinity of the P-Loop in both nucleotide-binding domains.