Many P-glycoprotein substrates do not inhibit the transport process across cell membranes

Interactions of organophosphate flame retardants with human drug transporters

Organophosphate flame retardants (OPFRs) are environmental pollutants of increasing interest, widely distributed in the environment and exerting possible deleterious effects towards the human health. The present study investigates in vitro their possible interactions with human drug transporters, which are targets for environmental chemicals and actors of their toxicokinetics. Some OPFRs, i.e., tris(2-butoxyethyl) phosphate (TBOEP), tris(1,3-dichloroisopropyl) phosphate (TDCPP), tri-o-cresyl phosphate (TOCP) and triphenyl phosphate (TPHP), were found to inhibit activities of some transporters, such as organic anion transporter 3 (OAT3), organic anion transporting polypeptide (OATP) 1B1, OATP1B3, organic cation transporter 2 (OCT2) or breast cancer resistance protein (BCRP). These effects were concentration-dependent, with IC50 values ranging from 6.1 µM (for TDCPP-mediated inhibition of OCT2) to 51.4 µM (for TOCP-mediated inhibition of BCRP). OPFRs also blocked the transporter-dependent membrane passage of endogenous substrates, notably that of hormones. OAT3 however failed to transport TBOEP and TPHP. OPFRs additionally repressed mRNA expressions of some transporters in cultured human hepatic HepaRG cells, especially those of OAT2 and OCT1 in response to TOCP, with IC50 values of 2.3 µM and 2.5 µM, respectively. These data therefore add OPFRs to the expanding list of pollutants interacting with drug transporters, even if OPFR concentrations required to impact transporters, in the 2-50 µM range, are rather higher than those observed in humans environmentally or dietarily exposed to these chemicals.

In vitro and in silico evaluation of P-glycoprotein inhibition through 99m Tc-methoxyisobutylisonitrile uptake

P-glycoprotein (P-gp) is a multidrug resistance (MDR) transporter with unknown structural details. This macromolecule is normally responsible for extruding xenobiotics from normal cells. Overexpression of P-gp in tumor cells is a major obstacle in cancer chemotherapy. In this study, human 3D model of P-gp was built by homology modeling based on mouse P-gp crystallographic structure and stabilized through 1 ns molecular dynamics (MD) simulation. Stabilized human P-gp structure was used for flexible docking of 80 drugs into the putative active site of P-gp. Accordingly, digoxin, itraconazole, risperidone, ketoconazole, prazosin, verapamil, cyclosporine A, and ranitidine were selected for further in vitro assay. Subsequently, cell-based P-gp inhibition assay was performed on Caco-2 cells while ^99m Tc-methoxyisobutylisonitrile (MIBI) was used as a P-gp efflux substrate for calculating IC50 values. Results of the ^99m Tc-MIBI uptake in drug-treated Caco-2 cells were in agreement with the previously reported activities. This study for the first time described the relation between molecular dynamics and flexible docking with cellular experiments using ^99m Tc-MIBI radiotracer for evaluation of potencies of P-gp inhibitors. Finally, results showed that our radiotracer-cell-based assay is an accurate and fast screening tool for detecting P-gp inhibitors and non-inhibitors in drug development process.

P-glycoprotein (ABCB1) activity decreases raltegravir disposition in primary CD4+P-gphigh cells and correlates with HIV-1 viral load

Objectives

To evaluate the role of P-glycoprotein (P-gp) and multidrug-resistant-protein 1 (MRP1) on raltegravir intracellular drug disposition in CD4+ T cells, investigate the effect of HIV-1 infection on P-gp expression and correlate HIV-1 viraemia with P-gp activity in primary CD4+ T cell subsets.

Methods

The cellular accumulation ratio of [³H]raltegravir was quantified in CD4+ T cell lines overexpressing either P-gp (CEM-P-gp) or MRP1 (CEM-MRP1) and in primary CD3+CD4+ T cells with high (P-gp^high) and low P-gp activity (P-gp^low); inhibition of efflux transporters was confirmed by the intracellular retention of calcein-AM. The correlation of P-gp activity with HIV-1 viraemia was assessed in naive and memory T cell subsets from 21 HIV-1-infected treatment-naive subjects.

Results

[³H]Raltegravir cellular accumulation ratio decreased in CEM-P-gp cells (P < 0.0001). XR9051 (a P-gp inhibitor) and HIV-1 PIs reversed this phenomenon. Primary CD4+P-gp^high cells accumulated less raltegravir (38.4% ± 9.6%) than P-gp^low cells, whereas XR9051 also reversed this effect. In vitro HIV-1 infection of PBMCs and stimulation of CD4+ T cells increased P-gp mRNA and P-gp activity, respectively, while primary CD4+P-gp^high T cells sustained a higher HIV-1 replication than P-gp^low cells. A significant correlation between HIV-1 viraemia and P-gp activity was found in different CD4+ T cell subsets, particularly memory CD4+ T cells (r = 0.792, P < 0.0001).

Conclusions

Raltegravir is a substrate of P-gp in CD4+ T cells. Primary CD4+P-gp^high T cells eliminate intracellular raltegravir more readily than P-gp^low cells and HIV-1 viraemia correlates with P-gp overall activity. Specific CD4+P-gp^high T cell subsets could facilitate the persistence of viral replication in vivo and ultimately promote the appearance of drug resistance.

Insilico studies of organosulfur-functional active compounds in garlic

Garlic has been used medicinally since antiquity because of its antimicrobial activity, anticancer activity, antioxidant activity, ability to reduce cardiovascular diseases, improving immune functions, and antidiabetic activities and also in reducing cardiovascular diseases and improving immune functions. Recent studies identify that the wide variety of medicinal functions are attributed to the sulfur compounds present in garlic. Epidemiological observations and laboratory studies in animal models have also showed anticarcinogenic potential of organosulfur compounds of garlic. In this study, in silico analysis of organosulfur compounds is reported using the methods of theoretical chemistry to elucidate the molecular properties of garlic as it is more time and cost efficient, reduces the number of wet experiments, and offers the possibility of replacing some animal tests with suitable in silico models. The analysis of molecular descriptors defined by Lipinski has been done. The solubility of drug in water has been determined as it is of useful importance in the process of drug discovery from molecular design to pharmaceutical formulation and biopharmacy. All toxicities associated with candidate drug have been calculated. P-Glycoprotein expressed in normal tissues as a cause of drug pharmacokinetics and pharmacodynamics has been examined. Drug-plasma protein binding and volume of distribution have also been calculated. To avoid rejection of drugs, it is becoming more important to determine pK(a), absorption, polar surface area, and other physiochemical properties associated with a drug, before synthetic work is undertaken. The present in silico study is aimed at examining these compounds of garlic to evaluate its possible efficacy and toxicity under conditions of actual use in humans.

Kinetic considerations for the quantitative assessment of efflux activity and inhibition: implications for understanding and predicting the effects of efflux inhibition

PURPOSE

Unexpected and complex experimental observations related to efflux transport have been reported in the literature. This work was conducted to develop relationships for efflux activity (PS(efflux)) as a function of commonly studied kinetic parameters [permeability-surface area product (PS), efflux ratio (ER), degree of efflux inhibition (phi(i)), 50% inhibitory concentration (IC(50)), and Michaelis-Menten constant (K(m))].

METHODS

A three-compartment model (apical, cellular, and basolateral) was used to derive flux equations relating the initial rate of flux and steady-state mass transfer in the presence or absence of active efflux. Various definitions of efflux ratio (ER) were examined in terms of permeability-surface area products. The efflux activity (PS(efflux)) was expressed in terms of ER and PS. The relationships between PS(efflux) and PS, ER, phi(i), IC(50), and K(m) were solved mathematically. Simulations and examples from the literature were used to illustrate the resulting mathematical relationships.

RESULTS

The relationships derived according to a three-compartment model differed fundamentally from commonly accepted approaches for determining PS(efflux), phi(i), IC(50) and K(m). Based on the model assumptions and mathematical derivations, currently used mathematical relationships erroneously imply that efflux activity is proportional to change in PS (i.e., flux or P(app)) and thus underestimate PS(efflux) and phi(i,) and overestimate IC(50) and K(m).

CONCLUSIONS

An understanding of the relationship between efflux inhibition and kinetic parameters is critical for appropriate data interpretation, standardization in calculating and expressing the influence of efflux transport, and predicting the clinical significance of efflux inhibition.

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.

Membrane effects of the antitumor drugs doxorubicin and thaliblastine: comparison to multidrug resistance modulators verapamil and trans-flupentixol

The interactions of the antitumor drugs doxorubicin and thaliblastine with model membranes composed of neutral (phosphatidylcholine) and negatively charged (phosphatidylserine) phospholipids were studied by differential scanning calorimetry and nuclear magnetic resonance. The membrane activities of doxorubicin and thaliblastine were compared to those of the powerful multidrug resistance (MDR) modulators trans-flupentixol and verapamil. The results point out to the potential role of the drug-membrane interactions for the effects of doxorubicin and thaliblastine in resistant tumor cells. They direct also to the artificial membranes as a suitable tool for screening of compounds with potential ability to modulate MDR.

Substrates and inhibitors of efflux proteins interfere with the MTT assay in cells and may lead to underestimation of drug toxicity

The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay is a widely used method in assessment of cytotoxicity and cell viability, and also in anti-cancer drug studies with tumour cells. These cells often express efflux proteins, such as P-glycoprotein (MDR1) or multidrug resistance (MDR) protein 1 (MRP1). MDCKII cells that overexpress these proteins (MDCKII-MDR1 or MDCKII-MRP1) and normal cells (MDCKII-wt) were used to investigate the effects of efflux pump activity on the results of MTT assay. Efflux protein activity was confirmed with calcein-AM efflux assay, and MTT assay was compared to another cytotoxicity test, the LDH release assay. Inhibition of MRP and MDR1 efflux proteins in MDCKII cell lines was associated paradoxically with increased reduction of MTT, implying an apparent increase in cell viability. This effect was seen when MK 571 (MRP1 and MRP2 inhibitor) or verapamil (MRP1 and MDR1 inhibitor) were used to block efflux protein activity. The calcein-AM efflux assay also showed that the MTT reagent inhibits the function of MDR1 in the MDCKII-MDR1 cell line. This study shows that MDR1 and possibly MRP proteins interfere with the MTT assay. Due to wide substrate specificity of efflux proteins and popularity of the MTT assay this interference is not trivial. Presence of any efflux protein substrate may therefore lead to underestimated results in MTT assay, thereby causing potential bias and erroneous conclusions in cytotoxicity studies.

Classification analysis of P-glycoprotein substrate specificity

Prediction of P-glycoprotein substrate specificity (S(PGP)) can be viewed as a constituent part of a compound's "pharmaceutical profiling" in drug design. This task is difficult to achieve due to several factors that raised many contradictory opinions: (i) the disparity between the S(PGP) values obtained in different assays, (ii) the confusion between Pgp substrates and inhibitors, (iii) the confusion between lipophilicity and amphiphilicity of Pgp substrates, and (iv) the dilemma of describing class-specific relationships when Pgp has no binding sites of high ligand specificity. In this work, we compiled S(PGP) data for 1000 compounds. All data were represented in a binary format, assigning S(PGP) = 1 for substrates and S(PGP) = 0 for non-substrates. Each value was ranked according to the reliability of experimental assay. Two data sets were considered. Set 1 included 220 compounds with S(PGP) from polarized transport across MDR1 transfected cell monolayers. Set 2 included the entire list of 1000 compounds, with S(PGP) values of generally lower reliability. Both sets were analysed using a stepwise classification structure-activity relationship (C-SAR) method, leading to derivation of simple rules for crude estimation of S(PGP) values. The obtained rules are based on the following factors: (i) compound's size expressed through molar weight or volume, (ii) H-accepting given by the Abraham's beta (that can be crudely approximated by the sum of O and N atoms), and (iii) ionization given by the acid and base pKa values. Very roughly, S(PGP) can be estimated by the "rule of fours". Compounds with (N + O) > or = 8, MW > 400 and acid pKa > 4 are likely to be Pgp substrates, whereas compounds with (N + O) < or = 4, MW < 400 and base pKa < 8 are likely to be non-substrates. The obtained results support the view that Pgp functioning can be compared to a complex "mini-pharmacokinetic" system with fuzzy specificity. This system can be described by a probabilistic version of Abraham's solvation equation, suggesting a certain similarity between Pgp transport and chromatographic retention. The chromatographic model does not work in the case of "marginal" compounds with properties close to the "global" physicochemical cut-offs. In the latter case various class-specific rules must be considered. These can be associated with the "amphiphilicity" and "biological similarity" of compounds. The definition of class-specific effects entails construction of the knowledge base that can be very useful in ADME profiling of new drugs.

Influence of lipid lowering fibrates on P-glycoprotein activity in vitro

Statin/fibrate combinations are frequently used to treat mixed dyslipidemia. However, these combinations may cause life-threatening drug interactions (e.g. rhabdomyolysis) possibly induced by modifications of cytochrome P450 isozyme activities. Some statins are also transported by P-glycoprotein (Pgp) and may act as inhibitors of this drug efflux pump. So far, nothing is known about possible Pgp modulating effects of fibrates. We tested whether gemfibrozil, fenofibrate, fenofibric acid, and bezafibrate inhibit Pgp in vitro using a calcein acetoxymethylester (calcein-AM) uptake assay and confocal laser scanning microscopy with bodipy-verapamil as substrate in L-MDR1 cells, which overexpress human Pgp. In uptake assays in cells with (L-MDR1) and without (LLC-PK1) human Pgp we also investigated whether these compounds are transported by Pgp. Intracellular concentrations were measured by liquid chromatography tandem mass spectrometry. Of the tested fibrates, only fenofibrate increased calcein-AM uptake into cells indicating an inhibition of Pgp mediated transport by this compound. The potency of fenofibrate (mean+/-SD: 7.1+/-3.2 microM), evaluated by calculating the concentration needed to double baseline fluorescence (f2), was similar to that of simvastatin (5.8+/-1.5 microM), lovastatin (10.1+/-1.0), and verapamil (4.7+/-0.8 microM). For simvastatin and fenofibrate Pgp inhibition was confirmed with confocal laser scanning microscopy. Fenofibrate, fenofibric acid, gemfibrozil, and bezafibrate showed no difference in the cellular uptake between LLC-PK1 and L-MDR1, indicating that the tested fibrates are not Pgp substrates. In conclusion, this study demonstrates that fenofibrate inhibits Pgp in vitro with a potency similar to simvastatin.

Functional Characterization of Human Breast Cancer Resistance Protein (BCRP, ABCG2) Expressed in the Oocytes ofXenopus laevis

To evaluate the function and substrate specificity of human breast cancer resistance protein (BCRP, ABCG2) in the absence of cofactors or heterologous partner proteins, Xenopus laevis oocytes were injected with cRNA of wild-type or mutant (R482T) BCRP. High expression of BCRP was observed on the oocyte surface. Accumulation and efflux assays revealed that oocytes expressing R482T transported daunorubicin (DNR), mitoxantrone (MX), rhodamine 123, and flavopiridol (FLV), whereas wild-type BCRP transported only MX and FLV, in agreement with observations in mammalian and other systems. Transport activity was completely inhibited by fumitremorgin C, a known inhibitor of BCRP. Injection of oocytes with cRNA containing mutations of serine 187 in the ATP-binding cassette signature motif (S187T or S187A) resulted in strong expression of the mutant forms; however, these oocytes were devoid of transporter activity. When oocytes were coinjected with R482T and R482T/S187T, DNR transport was inhibited in a manner dependent on the amount of R482T/S187T cRNA added, consistent with the idea that the active form of BCRP is a homodimer or homomultimer. Substrate interaction studies found that no two substrates reciprocally inhibited the efflux of the other. Although FLV proved to be an effective inhibitor of both MX and DNR transport, and MX inhibited DNR transport, the other substrates tested had only weak or no inhibitory activity, indicating a complex nature of substrate interaction with the BCRP homodimer. We conclude that the X. laevis oocyte heterologous expression system is a valid and effective means of studying BCRP function and substrate specificity.