The importance of drug-transporting P-glycoproteins in toxicology

Gene therapy and cell therapy for the management of radiation damages to healthy tissues: Rationale and early results

Nowadays, ionizing radiations have numerous applications, especially in medicine for diagnosis and therapy. Pharmacological radioprotection aims at increasing detoxification of free radicals. Radiomitigation aims at improving survival and proliferation of damaged cells. Both strategies are essential research area, as non-contained radiation can lead to harmful effects. Some advances allowing the comprehension of normal tissue injury mechanisms, and the discovery of related predictive biomarkers, have led to developing several highly promising radioprotector or radiomitigator drugs. Next to these drugs, a growing interest does exist for biotherapy in this field, including gene therapy and cell therapy through mesenchymal stem cells. In this review article, we provide an overview of the management of radiation damages to healthy tissues via gene or cell therapy in the context of radiotherapy. The early management aims at preventing the occurrence of these damages before exposure or just after exposure. The late management offers promises in the reversion of constituted late damages following irradiation.

Inhibition of multixenobiotic resistance transporters (MXR) by silver nanoparticles and ions in vitro and in Daphnia magna

The P-glycoprotein (P-gp, ABCB1) and multidrug resistance associated protein 1 (MRP1), important members of the ABC (ATP-binding cassette) transporters, protect cells and organisms via efflux of xenobiotics and are responsible for the phenomenon of multidrug or multixenobiotic resistance (MXR). In this study we first evaluated, in vitro, the interaction of silver nanoparticles (Ag NPs, 20, 23 and 27nm), Ag 200nm particles and Ag ions (AgNO3) with MXR efflux transporters using MDCKII and the P-gp over-expressing MDCKII-MDR1 cells and calcein-AM as a substrate of the transporters. Next the in vivo modulation of MXR activity was studied in Daphnia magna juveniles with the model P-gp and MRP1 inhibitors verapamil-HCl and MK571, respectively. The common environmental contaminants perfluorooctane sulfonate and bisphenol A, previously observed to interfere with the P-gp in vitro, also inhibited the efflux of calcein in vivo. Small-sized Ag NPs (with biomolecules present on the surface) and AgNO3 inhibited the MXR activity in daphnids and MDCKII-MDR1 cells, but abcb1 gene expression remained unchanged. Both Ag NPs and dissolved ions contributed to the effects. This study provides evidence of the interference of Ag NPs and AgNO3 with the MXR activity both in vitro and in D. magna, and should be taken into account when Ag NP toxicity is assessed.

Copper treatment alters the barrier functions of human intestinal Caco-2 cells: involving tight junctions and P-glycoprotein

This study investigated the effects of copper on paracellular permeability and P-glycoprotein (P-gp) in Caco-2 cells. Apical treatment with 100–300,μM CuS04 in Hanks' balanced salt solution (HBSS, up to 3 hours) induced a time-and concentration-dependent increase in permeability of Caco-2 cell monolayers monitored by transepithelial electrical resistance (TEER). Copper treatment also induced a concentration-dependent reduction of F-actin stain, but not of tight junctional protein ZO-1. In addition, without any adverse effects on TEER, apical treatment with 300 μM CuS04 in complete medium (for 24 hours) could reduce basolateral-to-apical transport, and increase apical-to-basolateral transport of rhodamine-123 (Rho-123) and accumulation of Rho-123 in Caco-2 cells. Treatment with 10-100 μM CuS04 in HBSS (up to 3 hours) also induced a time-and concentration-dependent increase in accumulation of Rho-123 in Caco-2 cells. The results indicated that copper treatment increased the paracellular perme-ability probably by perturbing F-actin skeleton, and inhibited P-gp, thus altering the barrier functions of Caco-2 cells.

Upregulation of P-glycoprotein expression by ophthalmic drugs in different corneal in-vitro models

OBJECTIVES

The purpose of this study was to analyse P-glycoprotein (P-gp) expression in different human in-vitro cornea models (HCE-T epithelial model and Hemicornea construct) after stimulation with P-gp substrates (rhodamine 123, levofloxacin and acebutolol).

METHODS

The influence of P-gp substrates on mRNA expression was analysed using reverse transcriptase polymerase chain reaction (PCR) and real-time PCR. The effect of stimulation on the transporter functionality was estimated with a digoxin efflux assay. The Caco-2 cell line was used as positive control.

KEY FINDINGS

The reverse transcriptase PCR results showed an increase in band intensity compared with the control medium for all substrates. The real-time PCR for the Caco-2 and HCE-T epithelial model yielded a similar outcome, in which all tested substrates upregulated P-gp. In contrast, the Hemicornea construct showed no significant increase in the mRNA expression after stimulation. Both in-vitro models possessed similar drug transport profiles after stimulation. A significantly increased efflux of digoxin was measured after 24 and 72 h of stimulation with levofloxacin and acebutolol.

CONCLUSIONS

The expression and functionality of the P-gp in corneal tissue can be influenced through time exposure with specific substrates. However, the exact mechanism still requires further elucidation.

P-gp efflux pump inhibition potential of common environmental contaminants determined in vitro

Across different species, cellular efflux pumps such as P-glycoprotein (P-gp; also termed multidrug resistance protein 1 [MDR1]) serve as a first line of defense by transporting toxic xenobiotics out of the cell. This mechanism is also active in aquatic organisms such as mussels, fish, and their larvae. Modulation of this resistance mechanism by chemical agents occurring in the environment could result in either higher or lower internal concentrations of toxic or endogenous compounds in cells. The aim of the present study was to explore and quantify the inhibition of the P-gp efflux pumps by several ubiquitous aquatic contaminants. The calcein-acetoxymethyl ester (calcein-AM) assay commonly used in pharmacological research was established with P-gp-overexpressing Madin-Darby canine kidney cells (MDCKII-MDR1) in a 96-well plate, avoiding extra washing, centrifugation, and lysis steps. This calcein-AM-based P-gp cellular efflux pump inhibition assay (CEPIA) was used to study the inhibition by commonly occurring environmental contaminants. Among others, the compounds pentachlorophenol, perfluorooctane sulfonate, and perfluorooctanoate strongly inhibited the P-gp-mediated efflux of calcein-AM while the chloninated alkanes did not seem to interact with the transporter. The fact that common pollutants can be potent modulators of the efflux transporters is a motive to further study whether this increases the toxicity of other contaminants present in the same matrices.

Food emulsifier polysorbate 80 increases intestinal absorption of di-(2-ethylhexyl) phthalate in rats

The aim of the present research was to explore whether food emulsifier polysorbate 80 can enhance the absorption of di-(2-ethylhexyl) phthalate (DEHP) and its possible mechanism. We established the high-performance liquid chromatography (HPLC) method for detecting DEHP and its major metabolite, mono-ethylhexyl phthalate (MEHP) in rat plasma, and then examined the toxicokinetic and bioavailability of DEHP with or without polysorbate 80 in rats. The study of its mechanism to increase the absorption of phthalates demonstrated that polysorbate 80 can induce mitochondrial dysfunction in time- and concentration-dependence manners in Caco-2 cells by reducing mitochondrial membrane potential, diminishing the production of the adenosine triphosphate, and decreasing the activity of electron transport chain. Our results indicated that food emulsifier applied in relatively high concentrations in even the most frequently consumed foods can increase the absorption of DEHP, and its role may be related to the structure and function damages of mitochondria in enterocytes.

P-glycoprotein expression in Helicobacter pylori-positive patients: the influence of MDR1 C3435T polymorphism

OBJECTIVE

The aim of this study was to determine whether the presence of Helicobacter pylori (H. pylori) infection and multidrug resistance protein 1 (MDR1) C3435T polymorphism had an influence on P-glycoprotein (P-gp) expression in the upper gastrointestinal tract.

METHODS

A total of 76 patients who underwent upper gastroendoscopy at Sir Charles Gairdner Hospital in Western Australia from October 2010 to July 2011 were enrolled in the study. Antral and duodenal biopsies were collected for P-gp examination. Blood samples were taken and analyzed for MDR1 C3435T polymorphism. H. pylori infection status was confirmed by culture and polymerase chain reaction.

RESULTS

A significant difference was found in P-gp expression between H. pylori-positive and H. pylori-negative patients (P = 0.028). For the MDR1 C3435T polymorphism, the TT genotype had a significantly lower P-gp expression compared with the CC genotype in antral specimens (P = 0.041). The homozygous TT genotype with H. pylori infection was also significantly different in P-gp expression compared with H. pylori-negative patients (P = 0.029).

CONCLUSIONS

P-gp expression in the upper gastrointestinal tract is associated with H. pylori infection, and the TT genotype appeared to be associated with lower P-gp expression than the CC genotype in the stomach.

IL-2-granzyme A chimeric protein overcomes multidrug resistance (MDR) through a caspase 3-independent apoptotic pathway

One of the main problems of conventional anticancer therapy is multidrug resistance (MDR), whereby cells acquire resistance to structurally and functionally unrelated drugs following chemotherapeutic treatment. One of the main causes of MDR is overexpression of the P-glycoprotein transporter. In addition to extruding the chemotherapeutic drugs, it also inhibits apoptosis through the inhibition of caspases. To overcome MDR, we constructed a novel chimeric protein, interleukin (IL)-2 granzyme A (IGA), using IL-2 as a targeting moiety and granzyme A as a killing moiety, fused at the cDNA level. IL-2 binds to the high-affinity IL-2 receptor that is expressed in an array of abnormal cells, including malignant cells. Granzyme A is known to cause caspase 3-independent cell death. We show here that the IGA chimeric protein enters the target sensitive and MDR cancer cells overexpressing IL-2 receptor and induces caspase 3-independent cell death. Specifically, after its entry, IGA causes a decrease in the mitochondrial potential, triggers translocation of nm23-H1, a granzyme A-dependent DNase, from the cytoplasm to the nucleus, where it causes single-strand DNA nicks, thus causing cell death. Moreover, IGA is able to overcome MDR and kill cells resistant to chemotherapeutic drugs. We believe that overcoming MDR with targeted molecules such as IGA chimeric protein that causes caspase-independent apoptotic cell death could be applied to many other resistant types of tumors using the appropriate targeting moiety. Thus, this novel class of targeted molecules could open up new vistas in the fight against human cancer.

Influence of P-glycoprotein on embryotoxicity of the antifouling biocides to sea urchin (Strongylocentrotus intermedius)

P-glycoprotein (P-gp), as an ATP-binding cassette transporter, transports a wide variety of substrates varying from small molecules like steroids to large polypeptides across the cell membrane in human and animals, even in aquatic animals. Although P-gp protein has attracted much attention of research, its effect on the toxicity of environmental toxicants such as antifouling biocides is still poorly understood. The goal of this study is to evaluate whether copper pyrithione (CuPT), Sea-Nine 211, dichlofluanid and tolylfluanid, four widely used antifouling agents, can be transported by P-gp in embryos of sea urchin Strongylocentrotus intermedius in the presence and absence of the P-gp inhibitor verapamil. Cytotoxcicities of Sea-Nine 211 (EC50 = 99 nM, at 4-arm pluteus) and dichlofluanid (EC50 = 144 nM, at multi-cell) are enhanced by the addition of the P-gp inhibitor, indicating that the two biocides are potential P-gp substrates. Tolylfluanid and CuPT are not transported by P-gp out of the cell, since no obvious changes in the cytotoxicities of the two biocides are observed no matter whether verapamil is added or not. In addition, to understand the mechanisms of ligand binding and its interaction with P-gp, a three-dimensional model of the sea urchin P-gp is generated based on the mouse crystal structure by using homology modeling approach. With this model, a flexible docking is performed and the results indicate that Sea-Nine 211 and dichlofluanid share the same binding site with verapamil, composed of key residues Lys677, Lys753, Thr756, Ala780, Met1033 and Phe1037, whereas tolylfluanid and CuPT display totally different binding modes to P-gp. This further demonstrates that Sea-Nine 211 and dichlofluanid are P-gp substrates, which provides us with new insights into the interactions of P-gp with the antifouling contaminants in aquatic invertebrate embryos.

Gamma-linolenic acid modulates the response of multidrug-resistant K562 leukemic cells to anticancer drugs

Gamma-linolenic acid (GLA) is known to have selective tumoricidal action. It is also reported that GLA may increase the cytotoxic activity of cancer chemotherapeutic agents in some cancer cells. The present study examined whether GLA pretreatment could modulate the response of multidrug-resistant K562/ADM leukemic cells to anticancer drugs. The cell viability assay results showed that GLA at 10 microg/ml enhanced cell growth inhibition induced by the MDR-type drugs doxorubicin, etoposide and vincristine, but could not enhance or even attenuated cell growth inhibition induced by the non-MDR-type drug cisplatin, mitomycin and fluorouracil in K562/ADM cells. Further flow cytometry results showed that GLA decreased the expression of P-glycoprotein, enhanced the accumulation of doxorubicin and rhodamine 123 in K562/ADM cells and inhibited the efflux of rhodamine 123 from K562/ADM cells, lowered the efflux rate. These results suggest that GLA could modulate the response to anti-cancer agents in P-gp overexpressing multidrug-resistant cells, and the mechanism may be by decreasing the P-gp expression and inhibiting P-gp function.

Disposition and toxicity of trabectedin (ET-743) in wild-type and mdr1 gene (P-gp) knock-out mice

Trabectedin is a novel anticancer drug active against soft tissue sarcomas. Trabectedin is a substrate for P-glycoprotein (P-gp), which is encoded by mdr1a/1b in rodents. Plasma and tissue distribution, and excretion of [(14)C]-trabectedin were evaluated in wild-type and mdr1a/1b(-/-) mice. In parallel, we investigated the toxicity profile of trabectedin by serial measurements of blood liver enzymes and general pathology. [(14)C]-trabectedin was extensively distributed into tissues, and rapidly converted into a range of unknown metabolic products. The excretion of radioactivity was similar in both genotypes. The plasma clearance of unchanged trabectedin was not reduced when P-gp was absent, but organs under wild type circumstances protected by P-gp showed increased trabectedin concentrations in mdr1a/1b(-/-) mice. Although hepatic trabectedin concentrations were not increased when P-gp was absent, mdr1a/1b(-/-) mice experienced more severe liver toxicity. P-gp plays a role in the in vivo disposition and toxicology of trabectedin.

Effect of the novel antipsychotic drug perospirone on P-glycoprotein function and expression in Caco-2 cells

OBJECTIVE

Perospirone (PER) is a novel atypical antipsychotic drug for the treatment of schizophrenia and other psychotic disorders. The multidrug resistance transporter, P-glycoprotein (Pgp), is involved in the efflux transport of several antipsychotics across the blood-brain barrier (BBB). The aim of the present study was to evaluate the modulating effect of PER on both Pgp activity and expression in Caco-2 cell monolayers.

METHODS

The effects of PER were analyzed by means of rhodamine 123 (Rhd 123) assays, and those of Pgp expression were analyzed by flow cytometry and reverse transcriptase-PCR.

RESULTS

Perospirone at concentrations of 0.01-30 microM, which were found to be non-cytotoxic towards the Caco-2 cells, was observed to inhibit Pgp-mediated efflux transport of Rhd 123 in the cells as well as to down-regulate the cellular Pgp protein and MDR1 mRNA levels in a concentration-dependent manner. In the rhodamine accumulation assays, 30 microM PER produced a 429% increase of the cellular Rhd 123 concentration, which exceeded the inhibitory effect of the well-known Pgp inhibitor verapamil.

CONCLUSION

Our findings provide experimental evidence that PER is an inhibitor of Pgp which interferes directly and indirectly with the function of Pgp.

Apoptotic cell death: A possible key event in mediating the in vitro anti-proliferative effect of a novel copper(II) complex, [Cu(4-Mecdoa)(phen)2] (phen=phenanthroline, 4-Mecdoa=4-methylcoumarin-6,7-dioxactetate), in human malignant cancer cells

The central objective of the current study was to investigate the potential in vitro anti-proliferative effect of the parent ligand, 4-methylcoumarin-6,7-dioxyacyeic acid (4-MecdoaH(2)), and its copper (II) complex, bis(phenanthroline4-methylcoumarin-6,7-dioxacetatocopper(II) ([Cu(4-Mecdoa)(phen)(2)]) using four human model cell lines. In addition, selected mechanistic studies were carried out using the most sensitive of the four cell lines. Results obtained show that the complex could alter proliferation of both human neoplastic renal (A-498) and hepatic (HepG2) cells. Furthermore, non-neoplastic hepatic (CHANG) cells appeared to be less sensitive. However, this effect was not duplicated with non-neoplastic renal (HK-2) cells, a profile shared by cisplatin. The observed anti-proliferative effect appeared to be dose-and time-dependent, and could be attributed to the complex, rather than any of the free components i.e. the 1,10-phenanthroline or coumarin ligand, or the simple metal salt. Furthermore, the complex was shown to decrease DNA synthesis, but did not intercalate with it. Based on IC(50) values, [Cu(4-Mecdoa)(phen)(2)] was shown to be almost 12 times more potent than cisplatin. Moreover, there was no evidence that P-glycoprotein-mediated multi-drug resistance was likely to decrease anti-proliferative activity. Cytological stains, analysis of genomic DNA, and biochemical assays [caspase-3 and -9 and cleaved poly(ADP-ribose)-polymerase protein], showed that cell death could switch between apoptosis and necrosis, and this effect appeared to be concentration-dependent. Additionally, flow cytometric analysis showed that the complex functioned through an alteration in cell cycle progression. Taken together, [Cu(4-Mecdoa)(phen)(2)] has been shown to be a more potent anti-proliferative agent than either the ligand or cisplatin, and is capable of altering key biochemical events leading to the execution of apoptotic and/or necrotic cell death, suggesting that it is worthy of further investigation.

An in vitro investigation of the induction of apoptosis and modulation of cell cycle events in human cancer cells by bisphenanthroline-coumarin-6,7-dioxacetatocopper(II) complex

The central objective of the current study was to investigate the potential in vitro anti-proliferative properties of the parent ligand, coumarin-dioxy-acetic acid (cdoaH(2)), and its copper complex, copper-coumarin-dioxyacetic acetate-phenathroline ([Cu(cdoa)(phen)(2)]) using four human-derived model cell lines, two neoplastic and two non-neoplastic. In addition, selected mechanistic studies were carried out using one of the neoplastic-derived model cell lines, Hep-G2. Results obtained show that the complex, rather than the ligand, could alter the proliferation of both human neoplastic renal (A-498) and hepatic (Hep-G2) cells. Furthermore, hepatic non-neoplastic cells (Chang) appeared to be less sensitive. However, this effect was not mirrored in non-neoplastic renal (HK-2) cells, a profile shared with cisplatin. The observed anti-proliferative effect appeared to be concentration- and time-dependant, and could be attributed to the complex, rather than any of the component parts, i.e. 1,10-phenanthroline, the coumarin ligand, or the simple metal salt. Furthermore, the complex was shown to decrease DNA synthesis, but did not intercalate with it. Based on IC(50) values, [Cu(cdoa)(phen)(2)] was shown to be almost six times more potent than cisplatin. Moreover, there was no evidence to show that P-glycoprotein (P-gp)-mediated multi-drug resistance (MDR) was likely to play a role in decreasing the anti-proliferative activity of the complex. Cytological stains, analysis of genomic DNA, and biochemical assays [caspase-3 and -9 and cleaved poly(ADP-ribose)-polymerase protein], suggested that cell death could switch between apoptosis and necrosis, and this effect appeared to be concentration-dependent. Additionally, flow cytometric analysis showed that the complex functioned through an alteration in cell cycle progression. Taken together, [Cu(cdoa)(phen)(2)] has been shown to be a more potent anti-proliferative agent than either the ligand or cisplatin, and is capable of altering key biochemical events leading to the execution of apoptotic and/or necrotic cell death, suggesting that it is worthy of further investigation.

Proteomic Analysis of the Resistance to Aplidin in Human Cancer Cells

Aplidin (plitidepsin) is an antitumoral agent that induces apoptosis via Rac1-JNK activation. A proteomic approach using 2D-DIGE technology found 52 cytosolic and 39 membrane proteins differentially expressed in wild-type and Aplidin-resistant HeLa cells, of which 39 and 27 were identified by MALDI-TOF mass spectrometry and database interrogation. A number of proteins involved in apoptosis pathways were found to be deregulated. Alterations in Rab geranylgeranyltransferase, protein disulfide isomerase (PDI), cystathionine gamma-lyase, ezrin, and cyclophilin A (CypA) were confirmed by immunoblotting. Moreover, the role of PDI and CypA in Aplidin resistance was functionally confirmed by using the inhibitor bacitracin and overexpression, respectively. These deregulated proteins are candidates to mediate, at least partially, Aplidin action and might provide a route to the cells to escape the induction of apoptosis by this drug.

HIV-1 integrase inhibitors are substrates for the multidrug transporter MDR1-P-glycoprotein

Background

The discovery of diketoacid-containing derivatives as inhibitors of HIV-1 Integrase (IN) (IN inhibitors, IINs) has played a major role in validating this enzyme as an important target for antiretroviral therapy. Since the in vivo efficacy depends on access of these drugs to intracellular sites where HIV-1 replicates, we determined whether the IINs are recognized by the multidrug transporter MDR1-P-glycoprotein (P-gp) thereby reducing their intracellular accumulation. To address the effect of IINs on drug transport, nine quinolonyl diketo acid (DKA) derivatives active on the HIV-1 IN strand transfer (ST) step and with EC50 ranging from 1.83 to >50 μm in cell-based assays were tested for their in vitro interaction with P-gp in the CEM-MDR cell system. IINs were investigated for the inhibition and induction of the P-gp function and expression as well as for multidrug resistance (MDR) reversing ability.

Results

The HIV-1 IINs act as genuine P-gp substrates by inhibiting doxorubicin efflux and inducing P-gp functional conformation changes as evaluated by the modulation of UIC2 mAb epitope. Further, IINs chemosensitize MDR cells to vinblastine and induce P-gp expression in drug sensitive revertants of CEM-MDR cells.

Conclusion

To our knowledge, this is the first demonstration that HIV-1 IINs are P-gp substrates. This biological property may influence the absorption, distribution and elimination of these novels anti HIV-1 compounds.

Apparent active transport of MDMA is not mediated by P-glycoprotein: a comparison with MDCK and Caco-2 monolayers

Amphetamines and their methylenedioxy derivatives generically display similar behavioral, physiologic and toxic effects. Inconsistent pharmacokinetic and toxicity data for methylenedioxymethamphetamine (MDMA) may suggest that active drug transporters are interacting with these compounds, and thus altering drug absorption and tissue distribution. In vitro models of CNS accumulation and intestinal drug transport were used to assess efflux transport of MDMA. Madin-Darby kidney cell epithelial (MDCK) monolayers displayed a 4-fold increase in accumulation in the basolateral to apical orientation relative to the apical to basolateral orientation, although no differential accumulation was noted between MDCK-WT and MDCK-MDR1 monolayers. Caco-2 monolayers demonstrated an approximate 2-fold increase in accumulation of MDMA. Exposure of various inhibitors of active drug transporters demonstrated mixed results; ritonavir, progesterone and indomethacin produced an approximately 50% reduction of MDMA transport, while verapamil, MK-571 and probenecid had no effect. Based on these data, it is concluded that MDMA efflux is mediated via the activity of a transporter distinct from P-glycoprotein. The possible inhibitory effects of amphetamines on rhodamine-123 transport were also assessed. MDMA, methylenedioxyamphetamine, amphetamine and methamphetamine, at physiologically relevant concentrations, did not significantly alter the transport of rhodamine-123 in Caco-2 monolayers or the LS180 cell line, suggesting that these compounds do not alter the function of P-glycoprotein.

Down-regulation of mdr1b mRNA expression in the kidneys of mice following maternal exposure to tributyltin chloride

We investigated the change in renal mdr1b mRNA expression in offspring exposed to tributyltin chloride (TBTC) via the placenta and lactation or via lactation, using the real-time reverse transcription-polymerase chain reaction. Pregnant ICR mice were given water containing TBTC (0, 15, and 50 microg/ml) ad libitum from the start of pregnancy to weaning or from parturition to weaning. Exposure via the placenta and lactation significantly reduced the renal mdr1b level in offspring. Exposure to TBTC through the mother might impair the exclusion system of toxic compounds in offspring.

Effect of organophosphate pesticide diazinon on expression and activity of intestinal P-glycoprotein

Organophosphate insecticide diazinon is widely used in agricultural practices, submitting farmers to repeated exposure. Because efflux pumps, as P-glycoprotein (P-gp), serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs, we analyzed the ability of diazinon to act as efflux modulator. Oral administration of diazinon (2-20 mg/kg, 5 days, or 10 mg/kg, 2-12 days) increased intestinal mdr1a mRNA of rats, in both dose- and time-dependent manner, and increased the expression of intestinal P-gp. Using the intestinal cell-line Caco-2, we found that 100 microM diazinon significantly inhibited digoxin and vinblastine secretive flux through the cell monolayers, whereas digoxin and vinblastine absorptive flux increased. The 25 microM diazinon was transported preferentially in basolateral (BL) to apical (AP) direction, suggesting a net secretion. The efflux rate significantly decreased in the presence of metabolic inhibitors sodium azide and 2-deoxy-d-glucose, P-gp inhibitors cyclosporin A and valspodar, but not in the presence of MRPs inhibitor MK571. Repeated exposure of Caco-2 cells to diazinon increased P-glycoprotein expression and activity. These results suggested the involvement of P-gp in the transfer of diazinon, leading to potential consequences for xenobiotic interactions, and showed that repeated exposure to low doses of pesticide may lead to up-regulated P-gp functions in the intestine of mammals.

Measurement of CYP2D6 and CYP3A4 activity in vivo with dextromethorphan: sources of variability and predictors of adverse effects in 419 healthy subjects

OBJECTIVE

Dextromethorphan (DEM) shares part of the adverse event profile of opioids and is widely used as a probe drug for CYP2D6 phenotyping and for the assessment of CYP2D6 activity. It has also been used to assess CYP3A4 activity. This study examined the influence of anthropometric variables, oral contraceptives, smoking habits, mu-opioid receptor and MDR1 genetic polymorphisms and components of the DEM ratios on the variability of CYP2D6 and CYP3A4 metabolic ratios and on the occurrence of adverse events following DEM administration.

METHODS

This was a retrospective analysis of a database in 419 healthy subjects. CYP2D6 and CYP3A4 metabolic ratios were measured as the log of the ratios of the amount of DEM to the amount of dextrorphan (DOR) and of the amount of DEM to the amount of 3-methoxy-morphinan (MET) excreted in urine during a 12-h time period, respectively, following the oral administration of 80 mg of dextromethorphan hydrobromide. Logistic regression was performed to examine the factors associated with changes in metabolic ratios and with the occurrence of adverse events.

RESULTS

The CYP2D6 metabolic ratio allowed identification of extensive and poor metabolizers of DEM. The CYP2D6 and CYP3A4 metabolic ratios were not strictly independent one from each other. Based on multivariate analysis, the CYP2D6 metabolic ratio was a stronger independent predictor of adverse events (p<0.0001) than the CYP2D6 phenotype (p=0.05). Anthropometric variables, oral contraceptives, smoking habits, mu-opioid receptor and MDR1 genetic polymorphisms did not significantly contribute to changes in metabolic ratios or to the occurrence of adverse events.

CONCLUSIONS

Dextromethorphan can be used for CYP2D6 phenotyping, but the CYP2D6 and CYP3A4 metabolic ratios are not strictly independent one from each other. The CYP2D6 metabolic ratio predicts adverse events to DEM as does CYP2D6 phenotype, and extensive metabolizer subjects are not protected against adverse events.

Identification of a multixenobiotic resistance mechanism in primary cultured epidermal cells from Oncorhynchus mykiss and the effects of environmental complex mixtures on its activity

Multixenobiotic resistance (MXR) is a mechanism analogous to the mammalian multidrug resistance (MDR) phenotype, whereby, simultaneous resistance is conferred against the intracellular accumulation of structurally and functionally diverse, natural, endogenous and environmental toxicants. Expression of P-glycoproteins (P-gp), ATP-dependent transporters encoded for by the mdr1 gene that have been implicated in this xenobiotic efflux mechanism, have previously been detected in normal teleost tissues involved in a secretory, absorption or a barrier function. The presence of these proteins in the epidermis of fish species has not to our knowledge previously been investigated. In the present study, primary cultures of epidermis from the rainbow trout Oncorhynchus mykiss were employed to investigate whether an MXR mechanism is functional in the epidermis of fish. The efflux of the fluorescent mdr1 substrate rhodamine 123 from the cells was significantly inhibited by verapamil, a compound known to interfere with P-gp mediated transport. The cultured epidermal cells were also observed to accumulate this fluorescent dye in a verapamil sensitive manner, thus indicating the presence of an mdr1-like mechanism. Immunocytochemical analysis, using a monoclonal antibody (JSB1) directed against a conserved cytoplasmic P-gp epitope, also demonstrated the presence of P-gp-like proteins. Sediment elutriate extracts were employed as models of environmental complex mixtures to evaluate the potential of the epidermal cultures to discriminate between samples of varying contaminant burden using MXR activity as an endpoint. The induction of P-gp expression was found to be in accordance with the level of contamination detected in the sediments from which the elutriates were extracted. The findings of the functional study also demonstrated that environmental pollutants, which interfere with P-gp function, could be identified using this model.

Mechanisms of resistance to anticancer drugs: the role of the polymorphic ABC transporters ABCB1 and ABCG2

ATP-binding cassette (ABC) genes play a role in the resistance of malignant cells to anticancer agents. The ABC gene products, including ABCB1 (P-glycoprotein) and ABCG2 (breast cancer-resistance protein [BCRP], mitoxantrone-resistance protein [MXR], or ABC transporter in placenta [ABCP]), are also known to influence oral absorption and disposition of a wide variety of drugs. As a result, the expression levels of these proteins in humans have important consequences for an individual’s susceptibility to certain drug-induced side effects, interactions, and treatment efficacy. Naturally occurring variants in ABC transporter genes have been identified that might affect the function and expression of the protein. This review focuses on recent advances in the pharmacogenetics of the ABC transporters ABCB1 and ABCG2, and discusses potential implications of genetic variants for the chemotherapeutic treatment of cancer.

Variable expression of P-glycoprotein in the human placenta and its association with mutations of the multidrug resistance 1 gene (MDR1, ABCB1)

The MDR1 gene product P-glycoprotein in the human placenta is important for protecting the fetus from unintended, harmful drug exposure, but also for limiting the access of therapeutic drugs to the fetus after maternal drug intake. A polymorphism in exon 26 of the MDR1 gene (C3435T) has previously been shown to be associated with reduced P-glycoprotein expression in the small intestine, kidney and lymphocytes. In the present study, we examined systematically whether MDR1 polymorphisms also have an impact on P-glycoprotein expression in the human placenta. MDR1 mRNA and P-glycoprotein were analysed in 73 full-term human placentas of Caucasians, as well as respective MDR1 genotypes/haplotypes, for the C3435T and G2677T/A polymorphisms of mothers and infants. MDR1 mRNA levels were not different between these genotype groups. However, P-glycoprotein expression was significantly lower when both mother and infant were homozygous for the 3435T allele (TT/tt) compared to maternal and fetal homozygotes for the C-allele (0.40 +/- 0.18 a.u. for TT/tt versus 0.66 +/- 0.30 a.u. for CC/cc, P = 0.01). Moreover, placentas from mothers carrying both polymorphisms (3435T and 2677T; TT/TT) also had a significantly lower P-glycoprotein expression (0.31 +/- 0.12 a.u.) compared to placentas of wild-type individuals (CC/GG, 0.71 +/- 0.31 a.u., P = 0.02). Taken together, the MDR1 polymorphisms C3435T and G2677T are associated with altered P-glycoprotein expression in the human placenta, and may have clinical consequences due to genetically determined, variable drug exposure of the fetus.

Physiologically Based Pharmacokinetic/Pharmacodynamic Model for the Organophosphorus Pesticide Diazinon

Diazinon (DZN) is an organophosphorus pesticide with the possibility for widespread exposures. The toxicological effects of DZN are primarily mediated through the effects of its toxic metabolite, DZN-oxon on acetylcholinesterases, which results in accumulation of acetylcholine at neuronal junctions. A physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model was developed to quantitatively assess the kinetics of DZN and its metabolites in blood and the inhibition of cholinesterases in plasma, RBC, brain, and diaphragm. Focused in vivo pharmacokinetic studies were conducted in male Sprague-Dawley rats and the data were used to refine the model. No overt toxicity was noted following doses up to 100mg/kg. However, cholinesterases in plasma, RBC, brain and diaphragm were substantially inhibited at doses of 50 mg/kg. In plasma, total cholinesterase was inhibited to less than 20% of control by 6 h post dosing with 100 mg/kg. Inhibition of brain acetylcholinesterase (AChE) following 100 mg/kg exposures was approximately 30% of control by 6 h. Diaphragm butyrylcholinesterase (BuChE) inhibition following 100 mg/kg dosing was to less than 20% of control by 6 h. The PBPK/PD model was used to describe the concentrations of DZN and its major, inactive metabolite, 2-isopropyl-4-methyl-6-hydroxypyrimidine (IMHP) in plasma and urinary elimination of IMHP. The fit of the model to plasma, RBC, brain, and diaphragm total cholinesterase and BuChE activity was also assessed and the model was further validated by fitting data from the open literature for intraperitoneal, intravenous, and oral exposures to DZN. The model was shown to quantitatively estimate target tissue dosimetry and cholinesterase inhibition following several routes of exposures. This model further confirms the usefulness of the model structure previously validated for chlorpyrifos and shows the potential utility of the model framework for other related organophosphate pesticides.

Characterization of the rates of testosterone metabolism to various products and of glutathione transferase and sulfotransferase activities in rat intestine and comparison to the corresponding hepatic and renal drug-metabolizing enzymes

Metabolism of testosterone to various products (catalyzed by several different CYP isozymes) and the activities of phenol sulfotransferase (pST) and glutathione transferase (GST) in S9 fractions prepared from the mucosa of the duodenum, jejunum, ileum, caecum and upper and lower colon of male Sprague-Dawley rats were determined and compared to the corresponding hepatic and renal activities. Incubation of the S9 fraction prepared from the jejunum with testosterone and NADPH resulted in the formation of 2alpha-, 6alpha-, 6beta- and 16alpha-hydroxytestosterone and androstenedione at rates that were 1.6, 24, 1.3, 0.6 and 1.3%, respectively, of the corresponding hepatic values. The production of 2alpha-hydroxytestosterone was catalyzed only by the preparations from the duodenum and jejunum; whereas 6alpha-, 6beta- and 16alpha-hydroxytestosterone and androstenedione were produced in all regions of the intestine. In the case of the rat kidney, the rates of formation of the different testosterone metabolites were between 0.6 and 35% of the corresponding liver activity. The activity of glutathione transferase was approximately 12-26% of the corresponding hepatic activity throughout the intestine. The highest activity of phenol sulfotransferase was observed in the lower colon (almost 6% of the liver activity) and the lowest activity in the duodenum (1%). The renal activities of GST and pST were 70 and 1%, respectively, of the corresponding liver values. In summary, the metabolism of testosterone and the activities of GST and pST in rat intestine are generally low to very low in comparison to the corresponding activities in rat liver. In most cases, these activities are present throughout the entire intestine and not restricted to a particular portion(s) of this organ.

Role of biokinetics in risk assessment of drugs and chemicals in children

Whether children incur different risks from xenobiotics than adults will depend on the exposure, biokinetics, and dynamics of compound. In this paper, current knowledge on developmental physiology and possible effects on biokinetics are evaluated and the role of biokinetics in risk assessment both for drugs and chemicals is discussed. It is concluded that most dramatic age-related physiological changes that may affect biokinetics occur in the first 6-12 months of age. The difference in internal exposure between children and adults can generally be predicted from already known developmental physiological differences. However, for risk assessment it will also be necessary to determine whether internal exposure is within the drug's therapeutic window or if it will exceed the NOAEL of a chemical. Furthermore, the effects of internal exposure of potentially harmful compounds on developing organ systems is of utmost importance. However, knowledge on this aspect is very limited. Risk assessment in children could be improved by: (1) application of pediatric PBPK-models in order to gain insight into internal exposure in children, (2) studies in juvenile animals for studying effects on developing systems, and (3) extrapolation of knowledge on the relationship between internal exposure and dynamics for drugs to other chemicals.

Daphnetin induced differentiation of human renal carcinoma cells and its mediation by p38 mitogen-activated protein kinase

Daphnetin has been shown to be a potent in vitro anti-proliferative agent to the human renal cell carcinoma (RCC) cell line, A-498. In the present study, we investigated its effects on mitogen-activated protein kinase (MAPK) signalling along with cell cycle events and cellular differentiation. Daphnetin-activated p38, however, higher concentrations were required to inhibit ERK1/ERK2. In addition, it did not activate SAPK or induce apoptosis, but instead inhibited S phase cell cycle transition of A-498 cells at low concentrations and time of exposure. In addition, a late G(1), early S phase inhibition was observed at higher concentrations and time of exposure, indicating that the mechanism of daphnetin-induced differentiation was concentration dependent. Increased expression of the epithelial differentiation markers cytokeratins 8 and 18, correlated with increasing concentrations of daphnetin, while pre-treatment with a specific p38-inhibitor, served to limit this effect. There was no evidence that P-glycoprotein (P-gp) mediated multi-drug resistance (MDR) played a role in the anti-proliferative activity of daphnetin. Consequently, we concluded that p38 MAP kinase is intrinsically involved in mediating the effect of daphnetin in A-498 cells, suggesting that this drug may act by promotion of cellular maturation, and consequently may represent a novel low toxic approach for the treatment of poorly differentiated RCCs.

Increased toxicity of P-glycoprotein-substrate chemotherapeutic agents in a dog with the MDR1 deletion mutation associated with ivermectin sensitivity

Lymphoma was diagnosed in a 4-year-old spayed female Collie, and treatment with a combination chemotherapy protocol incorporating prednisone, L-asparaginase, vincristine, vinblastine, doxorubicin, and cyclophosphamide was initiated. The dog had signs of gastrointestinal tract toxicosis and myelosuppression after treatment with P-glycoprotein-substrate drugs (vincristine, vinblastine, and doxorubicin) even when dosages were reduced, but did not have signs of toxicosis after treatment with cyclophosphamide, a non-P-glycoprotein-substrate drug, even when administered at the full dosage. It was postulated that a deletion mutation in the canine MDR1 gene (deltaMDR1 295-298) could be responsible for the drug toxicoses in this dog. This mutation has been identified as the cause of a functional P-glycoprotein defect in Collies susceptible to the toxic effects of ivermectin, another P-glycoprotein-substrate drug. The MDR1 genotype of this dog consisted of 1 normal and 1 mutant MDR1 allele. Because P-glycoprotein contributes to renal, biliary, and intestinal excretion of P-glycoprotein-substrate drugs, it is possible that drug excretion was delayed in this patient, resulting in clinical signs of toxicosis.

Saquinavir induces stable and functional expression of the multidrug transporter P-glycoprotein in human CD4 T-lymphoblastoid CEMrev cells

BACKGROUND

The multidrug transporter P-glycoprotein (P-gp) is expressed in HIV-1 target cells, in a range of pharmacological barriers and in AIDS-associated tumours. P-gp substrates include HIV-1 protease inhibitors (PIs) and anticancer drugs, which are efficiently effluxed from multidrug-resistant (MDR) cells.

OBJECTIVES

The aim of this study was to investigate the effect on human CD4 T-lymphoblastoid CEMrev cells of saquinavir and other PIs in terms of P-gp expression and to characterize the functional and biochemical patterns of PI-induced P-gp molecules.

METHODS

CEMrev cells no longer expressing detectable amounts of P-gp were cultured for a prolonged period in the presence of 10 microg/mL saquinavir (CEMsaq10) and tested for P-gp expression and function. Subsequently, CEMsaq10 cells were transferred into medium containing 15 microg/mL saquinavir (CEMsaq15) and cultured for several months. These cell lines were continuously monitored for P-gp expression, function and immunochemical patterns. A similar strategy was adopted to determine whether other PIs, such as ritonavir and indinavir, were able to induce P-gp expression in CEMrev cells.

RESULTS

Compared with the drug-diluent control, the exposure of CEMrev cells to 10 microg/mL saquinavir induced, in a consistent fraction of cells (45-50%), de novo expression of functioning P-gp molecules. The transfer of CEMsaq10 cells to 15 microg/mL saquinavir was associated with a dramatic increase in P-gp expression and function (85-90% of CEMsaq15 cells expressed P-gp and effluxed P-gp dye substrates). These saquinavir-induced P-gp molecules included 75-kDa molecules as well as the classical 170-kDa form of P-gp, suggesting induction of a particular isoform of P-gp termed mini-P-glycoprotein. Conversely, ritonavir and indinavir induced transient P-gp expression in a small percentage of the CEMrev cells.

CONCLUSIONS

Treatment of human CD4 T-lymphoblastoid CEMrev cells with saquinavir caused over-expression of functioning P-gp molecules. This de novo acquired MDR phenotype, which differed from that induced by other PIs, was stable, as expression and activity of P-gp were observed in CEMsaq10 and CEMsaq15 cells during prolonged in vitro culturing, even in drug-free conditions.

A role for P-glycoprotein in environmental toxicology

P-Glycoprotein (P-gp) is a transmembrane protein, playing significant roles in the process of drug discovery and development and in pest resistance to pesticides. P-gp affects absorption, disposition, and elimination of different compounds and is mainly expressed in intestines, liver, kidneys, heart, colon, and placenta. The expression of P-gp in the blood-brain barrier (BBB) has been associated with the restricted access of many compounds to the central nervous system. Generated knockout mice by disruption of mdr 1a gene, encoding for P-gp, showed that this protein was expressed in the BBB. The absence or the low levels of P-gp elevated drug concentrations in tissues and decreased drug elimination. P-gp is responsible for resistance of cells to agents, particularly the anticancer drugs, by removing these drugs from cells. Increased expression of P-gp is implicated in decreased HIV drug availability at certain intracellular sites. The role of P-gp in affecting efficacy and toxicity of environmental toxicants such as pesticides and heavy metals has not been adequately investigated. Studies showed that P-gp contributes to resistance to pesticides in certain pest species, and to decrease toxicity by removing compounds from cells in mammals. Placental drug-transporting P-gp plays a significant role in limiting the transport of toxicants such as potential teratogens to the fetus. Several in vitro or in vivo assays, including using P-gp knockout or naturally deficient mice, were described for testing P-gp modulators. The role of P-gp following concurrent exposure to more multiple compounds needs further research. P-gp modulators should be carefully used, since some modulators that reverse P-gp efflux action in vitro may lead to alterations of tissue function and increase toxicity of xenobiotics in normal tissues. Recent reports from the pharmaceutical studies on the significance of P-gp as transporters in altering the efficacy and toxicity clearly highlight the need for further research in interaction with environmental toxicants.

Interactions between doxorubicin and the human iron regulatory system

Anthracyclines are included in clinical treatments against various malignancies, but severe cardiotoxic side-effects and the development of resistance mechanisms limit their usefulness. Many aspects of the cellular response to anthracyclines remain debated. The status of the main regulator of iron homeostasis, namely the RNA-binding activity of iron regulatory proteins (IRPs), has been assessed herein for two types of human tumor cells and their derived doxorubicin-resistant sublines. IRPs were always fully activated in the latter, whereas only partial activation occurred in the former. Doxorubicin exposure reversibly inactivated IRP1 in small cell lung carcinoma (GLC(4)) and myelogenous leukemia (K562) cell lines, but was without effect in their derived doxorubicin-resistant sublines. In contrast, adding doxorubicin to cytosolic fractions of untreated cells or to purified IRPs led to the irreversible alteration of the RNA-binding activity of IRP1. In these different conditions, interaction between doxorubicin and the iron regulatory system disturbs iron metabolism, and cells having developed a resistance mechanism are tuned to maximize the iron supply. The results reported herein may lead the path toward a better therapeutic management of cancer patients receiving doxorubicin by discriminating between the antiproliferative and cardiotoxic properties of this anthracycline.

Drug–phytochemical interactions

Changes in dietary habits favouring diets rich in fruits and vegetables, and a meteoric rise in the consumption of dietary supplements and herbal products have substantially increased human exposure to phytochemicals. It is, therefore, not surprising that diet and herbal remedies can modulate drug-metabolising enzyme systems, such as cytochromes P450, leading to clinically relevant drug-phytochemical interactions. Phytochemicals have the potential to both elevate and suppress cytochrome P450 activity. Such effects are more likely to occur in the intestine, where high concentrations of phytochemicals may be achieved, and alteration in cytochrome P450 activity will influence, in particular, the fate of drugs that are subject to extensive first-pass metabolism as a result of intestinal cytochrome P450-mediated biotransformation. Moreover, it is becoming increasingly apparent that phytochemicals can also influence the pharmacological activity of drugs by modifying their absorption characteristics through interaction with drug transporters. Clearly, phytochemicals have the potential to alter the effectiveness of drugs, either impairing or exaggerating their pharmacological activity.

Differential protein expression in the cytosol fraction of an MCF-7 breast cancer cell line selected for resistance toward melphalan

Analysis of differential protein expression in the cytosol of melphalan-resistant and -susceptible MCF-7 cell lines has been carried out using a combination of two-dimensional gel electrophoresis, mass spectrometry, and bioinformatics. Comparison of multiple digitized gel arrays detected several spots as candidates for differentially expressed proteins in melphalan-resistant MCF-7 cells. The up-regulated proteins included retinoic acid binding protein II, an isoform of the macrophage migration inhibition factor, and other unidentified proteins. The down-regulated proteins included calreticulin, cyclophin A, and an isoform of the 27 kD heat shock protein. Correlation of the differential expression of some of the proteins with acquired resistance of MCF7 cells to melphalan is discussed.