What have we learnt thus far from mice with disrupted P-glycoprotein genes?

P2X4 receptors (P2X4Rs) represent a novel target for the development of drugs to prevent and/or treat alcohol use disorders

Alcohol use disorders (AUDs) have a staggering socioeconomic impact. Few therapeutic options are available, and they are largely inadequate. These shortcomings highlight the urgent need to develop effective medications to prevent and/or treat AUDs. A critical barrier is the lack of information regarding the molecular target(s) by which ethanol (EtOH) exerts its pharmacological activity. This review highlights findings implicating P2X4 receptors (P2X4Rs) as a target for the development of therapeutics to treat AUDs and discusses the use of ivermectin (IVM) as a potential clinical tool for treatment of AUDs. P2XRs are a family of ligand-gated ion channels (LGICs) activated by extracellular ATP. Of the P2XR subtypes, P2X4Rs are expressed the most abundantly in the CNS. Converging evidence suggests that P2X4Rs are involved in the development and progression of AUDs. First, in vitro studies report that pharmacologically relevant EtOH concentrations can negatively modulate ATP-activated currents. Second, P2X4Rs in the mesocorticolimbic dopamine system are thought to play a role in synaptic plasticity and are located ideally to modulate brain reward systems. Third, alcohol-preferring (P) rats have lower functional expression of the p2rx4 gene than alcohol-non-preferring (NP) rats suggesting an inverse relationship between alcohol intake and P2X4R expression. Similarly, whole brain p2rx4 expression has been shown to relate inversely to innate 24 h alcohol preference across 28 strains of rats. Fourth, mice lacking the p2rx4 gene drink more EtOH than wildtype controls. Fifth, IVM, a positive modulator of P2X4Rs, antagonizes EtOH-mediated inhibition of P2X4Rs in vitro and reduces EtOH intake and preference in vivo. These findings suggest that P2X4Rs contribute to EtOH intake. The present review summarizes recent findings focusing on the P2X4R as a molecular target of EtOH action, its role in EtOH drinking behavior and modulation of its activity by IVM as a potential therapy for AUDs.

Oral delivery of paclitaxel nanocrystal (PNC) with a dual Pgp-CYP3A4 inhibitor: preparation, characterization and antitumor activity

Several molecular inheritances have severely restrained the peroral delivery of taxanes. The main objective of the present investigation was to develop a paclitaxel (PTX) formulation which can circumvent the hurdles of its extremely poor solubility and permeability, Pgp efflux and high pre-systemic metabolism. Positively charged PTX nanocrystals of 209 nm were prepared by sonoprecipitation with high pressure homogenization technique, wherein an arginine based surfactant was explored as a stabilizer. The BET surface area analysis revealed that the surface area of PNC was 8.53 m(2)/gm, reflecting significant rise in surface area with nanonization of PTX. The DSC and XRD pattern suggested that the PTX is in the form of the most stable dihydrate crystal. The PNC showed very rapid dissolution profile compared to plain PTX in both sinks and non-sink conditions. Clarithromycin (CLM) was evaluated as a better alternative to cyclosporin A in improving PTX permeability. The PNC-CLM showed remarkable enhancement of 453% in relative bioavailability along with maintaining the therapeutic concentration of PTX for 8h. Efficacy data in B16 F10 melanoma tumor bearing mice showed substantial reduction in tumor volume and improvement in percentage survival compared to the control group.

Icaritin reverses multidrug resistance of HepG2/ADR human hepatoma cells via downregulation of MDR1 and P‑glycoprotein expression

Multidrug resistance (MDR) of tumor cells is a serious obstacle encountered in cancer treatment. In the current study a multiple drug‑resistant HepG2/adriamycin (HepG2/ADR) cell line was established and its MDR was characterized. Icaritin, an active ingredient isolated from the medical plant Herba Epimedium, was observed to reverse MDR in the present model. Icaritin significantly increased the intracellular accumulation of ADR and decreased the expression of the MDR1 gene in HepG2/ADR cells compared with drug‑sensitive HepG2 cells. In addition, the present results showed that icaritin may significantly downregulate the expression of P‑glycoprotein. These results indicate that icaritin is a novel and potent MDR reversal agent and may be a promising drug for tumor chemotherapy.

Altered expression of the xenobiotic transporter P-glycoprotein in liver and liver tumours of mummichog Fundulus heteroclitus from a creosote-contaminated environment

P-glycoproteins (Pgps) are involved in efflux of xenobiotics from drug-resistant cell lines and tumours, and in excretion of toxicants from normal tissues. Recently, investigators have proposed that Pgp activity contributes to resistance or tolerance of certain aquatic species to pollutants. In the present study using immunoblot and immunohistochemical techniques, we found elevation of Pgp in liver and liver tumours of creosote-resistant mummichog from a contaminated site in the Elizabeth River, Virginia. Immunoblots of mummichog liver extracts showed an immunoreactive band at 170 kDa and indicated two- to three-fold elevation of Pgp in livers of resistant fish relative to those from a reference site. Laboratory exposures of reference site fish to a model PAH (3-methylcholanthrene), however, produced no increase in liver Pgp levels as measured by immunoblot. Normal mummichog liver sections showed specific immunohistochemical staining for Pgp on the canalicular surface of hepatocytes. In the majority of hepatic neoplasms we observed a high level of over-expression and altered patterns of Pgp expression. However we did not observe Pgp over-expression in early proliferative lesions. Elevation of Pgp in livers and liver tumoursof these resistant mummichog may contribute to their survival in a heavily contaminated environment.

Neurotoxic effects of ivermectin administration in genetically engineered mice with targeted insertion of the mutated canine ABCB1 gene

OBJECTIVE

To develop in genetically engineered mice an alternative screening method for evaluation of P-glycoprotein substrate toxicosis in ivermectin-sensitive Collies.

ANIMALS

14 wild-type C57BL/6J mice (controls) and 21 genetically engineered mice in which the abcb1a and abcb1b genes were disrupted and the mutated canine ABCB1 gene was inserted.

PROCEDURES

Mice were allocated to receive 10 mg of ivermectin/kg via SC injection (n = 30) or a vehicle-only formulation of propylene glycol and glycerol formal (5). Each was observed for clinical signs of toxic effects from 0 to 7 hours following drug administration.

RESULTS

After ivermectin administration, considerable differences were observed in drug sensitivity between the 2 types of mice. The genetically engineered mice with the mutated canine ABCB1 gene had signs of severe sensitivity to ivermectin, characterized by progressive lethargy, ataxia, and tremors, whereas the wild-type control mice developed no remarkable effects related to the ivermectin.

CONCLUSIONS AND CLINICAL RELEVANCE

The ivermectin sensitivity modeled in the transgenic mice closely resembled the lethargy, stupor, disorientation, and loss of coordination observed in ivermectin-sensitive Collies with the ABCB1-1Δ mutation. As such, the model has the potential to facilitate toxicity assessments of certain drugs for dogs that are P-glycoprotein substrates, and it may serve to reduce the use of dogs in avermectin derivative safety studies that are part of the new animal drug approval process.

Changes in the brain accumulation of glucocorticoids in abcb1a-deficient CF-1 mice

The multidrug resistance transporter, P-glycoprotein (P-gp), contributes to highly lipophilic molecules penetrating the brain from the blood at a much lower rate than expected, and has numerous substrates, inhibitors and modulators. The drug-transporting isoform of P-gp is coded by a single human gene, ABCB1, and shares 80% homology with the murine drug-transporting isoforms, abcb1a and abcb1b, which share 92% homology with each other. Although these murine isoforms are highly similar, there are known affinity differences between the isoforms, and the localisation of the two isoforms in the brain is also disputed. Studies using mice genetically modified to be deficient in one or both isoforms of P-gp have also resulted in conflicting data. The contribution of the abcb1a isoform, which is considered to contribute most to the central nervous system (CNS)-protective role of P-gp, is investigated in the present study using CF-1-abcb1a(-/-) mice and the well-established brain/choroid plexus perfusion technique. Twenty-minute in situ brain/choroid plexus perfusions in CF-1-abcb1a(-/-) mice indicated the increased accumulation of [(3) H]cortisol, [(3) H]corticosterone and [(3) H]dexamethasone in most of the brain regions examined compared to CF-1-abcb1a(+/+) mice. Taken together with our earlier published studies in abcb1a/b(-/-) mice, these data strongly suggest that the in vivo CNS accumulation of glucocorticoids obtained using single knockout strains [e.g. abcb1a(-/-)] cannot be directly compared with those obtained in double knockout strains [e.g. abcb1a/b(-/-)].

Molecular analysis of the multidrug transporter, P-glycoprotein

Inherent or acquired resistance of tumor cells to cytotoxic drugs represents a major limitation to the successful chemotherapeutic treatment of cancer. During the past three decades dramatic progress has been made in the understanding of the molecular basis of this phenomenon. Analyses of drug-selected tumor cells which exhibit simultaneous resistance to structurally unrelated anti-cancer drugs have led to the discovery of the human MDR1 gene product, P-glycoprotein, as one of the mechanisms responsible for multidrug resistance. Overexpression of this 170 kDa N-glycosylated plasma membrane protein in mammalian cells has been associated with ATP-dependent reduced drug accumulation, suggesting that P-glycoprotein may act as an energy-dependent drug efflux pump. P-glycoprotein consists of two highly homologous halves each of which contains a transmembrane domain and an ATP binding fold. This overall architecture is characteristic for members of the ATP-binding cassette or ABC superfamily of transporters. Cell biological, molecular genetic and biochemical approaches have been used for structure-function studies of P-glycoprotein and analysis of its mechanism of action. This review summarizes the current status of knowledge on the domain organization, topology and higher order structure of P-glycoprotein, the location of drug- and ATP binding sites within P-glycoprotein, its ATPase and drug transport activities, its possible functions as an ion channel, ATP channel and lipid transporter, its potential role in cholesterol biosynthesis, and the effects of phosphorylation on P-glycoprotein activity.

Behavioural consequences of p-glycoprotein deficiency in mice, with special focus on stress-related mechanisms

P-glycoprotein (P-gp), an efflux transporter localised in the blood-brain barrier, limits the access of multiple xenobiotics to the central nervous system. Whether it is also implemented in the transport of the endogenous glucocorticoid corticosterone is a matter of debate. The P-gp knockout mouse model [abcb1a/b (-/-)] has been shown to differ in the functioning of the hypothalamic-pituitary adrenal (HPA) axis. In the present study, we investigated the behaviour of abcb1a/b (-/-) and wild-type mice with respect to stress-related tests and the effects of corticosterone. Behavioural activities were assessed in the open field (OF) test for 4 days, and in the forced swimming test (FST) and tail suspension test (TST) under naïve and stressed conditions. The FST was also conducted after exogenous corticosterone injection (0.25 and 2.5 mg/kg). Moreover, the elevated plus maze test and the RotaRod test (RotaRod Advanced; TSE Systems, Bad Homburg, Germany) were assessed. Brain corticosterone levels were determined by an immunoassay and expression of glucocorticoid receptors by western blot analysis. Abcb1a/1b (-/-) mice showed significantly decreased brain corticosterone levels and elevated glucocorticoid receptor expression. Behavioural analysis revealed a significantly decreased activity in the OF test on the first 2 days in abcb1a/1b (-/-) mice compared to wild-type mice, although the differences disappeared under habituation. Immobility time in the FST was significantly decreased in abcb1a/1b (-/-) mice under basal and under stressed conditions, whereas immobility in the TST was significantly elevated in these mice under all conditions. Injection of exogenous corticosterone resulted in significant reductions of immobility in the FST in abcb1a/1b (-/-) mice, whereas wild-type mice did not respond to the same doses. There were no differences in the elevated plus maze test and RotaRod test. The results obtained in the present study demonstrate that a P-gp deficiency has an impact on the stress-related behaviour, possibly as a result of differences in HPA axis-feedback regulation.

Desipramine treatment has minimal effects on the brain accumulation of glucocorticoids in P-gp-deficient and wild-type mice

Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis in patients with depression can be reduced by antidepressants, which are thought to improve endogenous glucocorticoid-mediated negative feedback. A proportion of peripherally released glucocorticoids need to enter brain tissue, protected by the blood-brain barrier (BBB), in order to achieve this negative feedback effect at the level of the central nervous systems (CNS). The multidrug resistance transporter P-glycoprotein (P-gp) has been shown to actively transport glucocorticoid hormones and has been implicated in the regulation of glucocorticoid access to the CNS. Using an in situ brain/choroid plexus perfusion method, we tested the hypothesis that the antidepressant desipramine increases glucocorticoid accumulation in the mouse brain by inhibiting P-gp, following either chronic treatment (8 days, 20 mg/kg/day, IP) or acute administration (20 min brain perfusion in the presence of either 0.9 μM or 10 μM desipramine). Contrary to our hypothesis, chronic treatment with desipramine did not affect the accumulation of [³H]dexamethasone in any sample compared to saline-treated mice. Acute desipramine had limited and variable effects on glucocorticoid accumulation in the CNS, with accumulation of [³H]dexamethasone increased in the cerebellum, accumulation of [³H]cortisol reduced in the frontal cortex, hypothalamus, and cerebellum, and accumulation of [³H]corticosterone (the endogenous glucocorticoid in rodents) not affected. Overall, under the conditions tested, these results do not support the hypothesis that treatment with desipramine can inhibit P-gp at the BBB and subsequently increase the accumulation of glucocorticoids in the brain.

Synthesis and antimultidrug resistance evaluation of icariin and its derivatives

A series of icariin derivatives were synthesized. Their multidrug resistance (MDR) reversal activities were evaluated by MTT assay and the results indicated that the derivatives were the potent modulators of MDR. It was showed that the derivatives significantly increased the intracellular accumulation of ADR in MCF-7/ADR cells compared with drug sensitive MCF-7 cells. The results of bi-directional assay and reverse transcription polymerase chain reaction (RT-PCR) assay showed that the derivatives had high inhibitory activity against P-gp efflux function and significantly down-regulated on the expression of P-gp.

Positron emission tomography studies on binding of central nervous system drugs and P-glycoprotein function in the rodent brain

The permeability of the blood-brain barrier (BBB) is one of the factors determining the bioavailability of drugs in the brain. The BBB only allows passage of lipophilic drugs by passive diffusion. However, some lipophilic drugs hardly enter the brain. The transmembrane protein P-glycoprotein (P-gp) is one of the carrier systems that is responsible for transportation of drugs out of the brain. P-Glycoprotein affects the pharmacokinetics of many drugs and can be inhibited by administration of modulators or competitive substrates. Identification and classification of central nervous system (CNS) drugs as P-gp substrates or inhibitors are of crucial importance in drug development. Positron emission tomography (PET) studies can play an important role in the screening process as a follow-up of high-throughput in vitro assays. Several rodent studies have shown the potential value of PET to measure the effect of P-gp on the pharmacokinetics and brain uptake of radiolabeled compounds. P-Glycoprotein-mediated effects were observed for two 5-HT(1a) receptor ligands, [(18)F]MPPF vs. [carbonyl-(11)C]WAY100635. Under control conditions, the specific brain uptake of [(18)F]MPPF is five- to eightfold lower than that of [(11)C]WAY100635. After cyclosporin A (CsA) modulation, [(18)F]MPPF uptake in the rat brain increased five- to tenfold. Cerebral uptake of [carbonyl-(11)C]WAY100635 was also increased by modulation, but in general the increase was lower than that observed for [(18)F]MPPF (two- to threefold). Brain uptake of the beta-adrenergic receptor ligands [(11)C]carazolol and [(18)F]fluorocarazolol was increased in P-gp knockout mice and CsA-treated rats. Both the specific and nonspecific binding of [(18)F]fluorocarazolol were doubled by CsA. Cerebral uptake of [(11)C]carazolol in rats was much lower than that of [(18)F]fluorocarazolol and no specific binding was measured. After CsA modulation, the uptake of [(11)C]carazolol increased five- to sixfold, but this uptake was not receptor-mediated. Quantitative PET studies in rodents on P-gp functionality demonstrated a dose-dependent increase of radioligands after administration of CsA. Studies with [(11)C]verapamil and [(11)C]carvedilol showed that complete modulation was achieved at 50 mg/kg CsA. The distribution volume of [(11)C]carvedilol increased from 0.25 in the control study to 1.0 after full modulation with CsA. By quantitative PET measurement of P-gp function, the dose of modulators required to increase the concentration of CNS drugs may be determined, which may result in improved drug therapy.

A revised role for P-glycoprotein in the brain distribution of dexamethasone, cortisol, and corticosterone in wild-type and ABCB1A/B-deficient mice

The ABCB1-type multidrug resistance efflux transporter P-glycoprotein (P-gp) has been hypothesized to regulate hypothalamic-pituitary-adrenal axis activity by limiting the access of glucocorticoids to the brain. In vivo systemic administration studies using P-gp-deficient mice have shown increased glucocorticoid entry to the brain compared with wild-type controls. However, these studies did not control for the presence of radiolabeled drug in the capillaries, verify an intact blood-brain barrier, or confirm stability of the glucocorticoids used. In the present study, an in situ brain perfusion method, coupled with capillary depletion and HPLC analyses, was used to quantify brain uptake of [3H]dexa-methasone, [3H]cortisol, and [3H]corticosterone in P-gp-deficient and control mice. A vascular marker was included in these experiments. The results show that brain uptake of [3H]dexamethasone was increased in the frontal cortex, hippocampus, hypothalamus, and cerebellum of P-gp-deficient mice compared with wild-type controls. Brain uptake of [3H]cortisol was increased in the hypothalamus of P-gp-deficient mice compared with wild-type controls, but no differences were detected in other regions. Brain uptake of [3H]corticosterone was not increased in P-gp-deficient mice compared with wild-type controls in any brain areas. After our systemic administration of the same radiolabeled glucocorticoids, HPLC analysis of plasma samples identified additional radiolabeled components, likely to be metabolites. This could explain previous findings from systemic administration studies, showing an effect of P-gp not only for dexamethasone and cortisol, but also for corticosterone. This in situ study highlights the different affinities of dexamethasone, cortisol, and corticosterone for P-gp, and suggests that the entry of the endogenous glucocorticoids into the mouse brain is not tightly regulated by P-gp. Therefore, our current understanding of the role of P-gp in hypothalamic-pituitary-adrenal regulation in mice requires revision.

Doxorubicin conjugated to D-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS): conjugation chemistry, characterization, in vitro and in vivo evaluation

To develop a polymer-anticancer drug conjugate, D-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS) was employed as a carrier of doxorubicin (DOX) to enhance its therapeutic effects and reduce its side effects. Doxorubicin was chemically conjugated to TPGS. The molecular structure, drug loading efficiency, drug release kinetics and stability of the conjugate were characterized. The cellular uptake, intracellular distribution, and cytotoxicity were accessed by using MCF-7 breast cancer cells and C6 glioma cells as in vitro cell model. The conjugate showed higher cellular uptake efficiency and broader distribution within the cells. Judged by IC(50), the conjugate was found 31.8, 69.6, 84.1% more effective with MCF-7 cells and 43.9, 87.7, 42.2% more effective with C6 cells than the parent drug after 24, 48, 72 h culture, respectively. The in vivo pharmacokinetics and biodistribution were investigated after an i.v. administration at 5 mg DOX/kg body weight in rats. Promisingly, 4.5-fold increase in the half-life and 24-fold increase in the area-under-the-curve (AUC) of DOX were achieved for the TPGS-DOX conjugate compared with the free DOX. The drug level in heart, gastric and intestine was significantly reduced, which is an indication of reduced side effects. Our TPGS-DOX conjugate showed great potential to be a prodrug of higher therapeutic effects and fewer side effects than DOX itself.

Substituted tetrahydroisoquinoline compound B3 inhibited P-glycoprotein-mediated multidrug resistance in-vitro and in-vivo

P-glycoprotein (P-gp) mediated multidrug resistance (MDR) is one of the main obstacles in tumour chemotherapy. A promising approach to reverse MDR is the combined use of nontoxic and potent P-gp inhibitor with conventional anticancer drugs. We have examined the potential of a newly synthesized tetrahydroisoquinoline derivative B3 as a MDR-reversing agent. The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used to examine the effect of B3 on the cytotoxicity in K562/A02 and MCF-7/ADM cells caused by doxorubicin (adriamycin). Accumulation and efflux of P-gp substrate rhodamine123 in K562/A02 and primary cultured rat brain microvessel endothelial cells (RBMECs) were measured to evaluate the inhibitory effect of B3 on P-gp. The K562/A02 xenograft model in nude mice was established to examine MDR-reversing efficacy of B3 in-vivo. The results indicated that co-administration of B3 resulted in an increase on chemosensitivity of K562/A02 and MCF-7/ADM cells to doxorubicin in a dose-dependent manner. Rhodamine123 accumulation in K562/A02 cells and RBMECs were significantly enhanced after the incubation with various concentrations of B3. Furthermore, B3 inhibited the efflux of rhodamine123 from RBMECs. Co-administration of B3 with doxorubicin significantly decreased weight and volume of tumour in nude mice. In conclusion, B3 is a novel and potent MDR reversal agent with the potential to be an adjunctive agent for tumour chemotherapy.

ABC multidrug transporters: structure, function and role in chemoresistance

Three ATP-binding cassette (ABC)-superfamily multidrug efflux pumps are known to be responsible for chemoresistance; P-glycoprotein (ABCB1), MRP1 (ABCC1) and ABCG2 (BCRP). These transporters play an important role in normal physiology by protecting tissues from toxic xenobiotics and endogenous metabolites. Hydrophobic amphipathic compounds, including many clinically used drugs, interact with the substrate-binding pocket of these proteins via flexible hydrophobic and H-bonding interactions. These efflux pumps are expressed in many human tumors, where they likely contribute to resistance to chemotherapy treatment. However, the use of efflux-pump modulators in clinical cancer treatment has proved disappointing. Single nucleotide polymorphisms in ABC drug-efflux pumps may play a role in responses to drug therapy and disease susceptibility. The effect of various genotypes and haplotypes on the expression and function of these proteins is not yet clear, and their true impact remains controversial.

Association of MDR1 (C3435T) polymorphism and resistance to carbamazepine in epileptic patients from Turkey

BACKGROUND AND AIMS

We investigated the prevalence of this multidrug resistance 1 gene (MDR1) polymorphism in drug-responsive versus drug-resistant epilepsy patients treated with carbamazepine (CBZ), which is a substrate of this protein.

METHODS

We genotyped the C3435T variant of MDR1 in 97 patients treated with CBZ monotherapy who had been on stable doses for more than 1 month. Our control group included 174 healthy individuals. Plasma CBZ concentrations were also measured using fluorescence polarization immunoassay.

RESULTS

We could not demonstrate any statistically significant relationship with the genotypes among drug-resistant patients (n = 44). The frequency of the homozygous mutant (TT) genotype was 15% in drug-responsive patients, 11.3% in drug-resistant patients and 25.8% in the control group. We also did not observe any significant correlation between the presence of a specific allele and CBZ plasma level/dose index.

CONCLUSION

Our study did not support any significant association between the MDR1 (C3435T) polymorphism and resistance to CBZ in epilepsy patients from Turkey.

Improved therapeutic efficacy of doxorubicin through conjugation with a novel peptide drug delivery technology (Vectocell)

Improvement in the therapeutic index of doxorubicin, a cytotoxic molecule, has been sought through its chemical conjugation to short (15-23 amino acid) peptide sequences called Vectocell peptides. Vectocell peptides are highly charged drug delivery peptides and display a number of characteristics that make them attractive candidates to minimize many of the limitations observed for a broad range of cytotoxic molecules. The studies reported here characterized the in vitro and in vivo efficacy of a range of Vectocell peptides conjugated to doxorubicin through different linkers. These studies show that the in vivo therapeutic index of doxorubicin can be improved by conjugation with a specific Vectocell peptide (DPV1047) through an ester linker to C14 of doxorubicin, in both colon and breast tumor models. This conjugate was also shown to have significant in vivo antitumoral activity in a model resistant to doxorubicin, suggesting that this conjugate is able to circumvent the multidrug resistance (MDR) phenotype. These experiments therefore provide support for the use of the Vectocell technology with other cytotoxic agents.

Adventures with ABC-proteins: highly conserved ATP-dependent transporters

The general properties of ABC transporters, from bacteria to humans, including a brief history of their initial discovery, are considered. ABC transporters, one of the largest protein super families and vital for human health, are in toto responsible for the transport of an enormous range of molecules from ions (CFTR) or anti-tumour drugs (Pgp/MDR) to large polypeptides. Nevertheless, all ABC transporters are powered by a conserved ATPase the ABC or NBD domain, using in all probability the same basic mechanism of action for the hydrolysis of ATP and its coupling to the transport process. Based on recent high resolution structures of several NBDs and an intact transporter, a model of how dimers of these important proteins function will be discussed, with particular attention to HlyB, the ABC transporter from E. coli.

In vitro andin vivo evaluation of WK-X-34, a novel inhibitor of P-glycoprotein and BCRP, using radio imaging techniques

Overexpression of the multidrug resistance proteins P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) results in treatment failure of many malignancies including ovarian cancer. Dual inhibition of Pgp and BCRP may restore the sensitivity of resistant cells to anticancer drugs. We report the synthesis and characterization of a novel anthranilic-acid based Pgp and BCRP modulator, WK-X-34. In vitro inhibition of Pgp activity was evaluated using 99mTc-Sestamibi and daunorubicin accumulation in Pgp overexpressing human ovarian cancer cells (A2780/Adr) and its sensitive counterpart (A2780/wt). Interaction with BCRP was examined with a mitoxantrone-efflux assay in BCRP-overexpressing MCF7/mx cells, with flow cytometry. Interactions with the multidrug resistance associated proteins (MRP) were evaluated in transfected MRP1, MRP2 and MRP3 cell lines, using a 5-CFDA efflux assay. In vivo 99mTc-Sestamibi imaging of human ovarian cancer xenografts was used to evaluate the in vivo efficacy of WK-X-34 in mice. Daunorubicin accumulation in A2780/Adr cells was inhibited by WK-X-34 at nanomolar concentrations (IC50: 82.1 +/- 6 nM). WK-X-34 inhibited mitoxantrone accumulation in BCRP-overexpressing cells at micromolar concentrations (IC50 = 26.5 +/- 4.6 microM), whereas WK-X-34 did not significantly alter 5-CFDA accumulation in MRP transfected cells. In vivo, uptake of 99mTc-Sestamibi was significantly increased in A2780/Adr xenograft tumors, brain and intestine (AUCs(0-4h) 136%, 147% and 138%; p < 0.05) in mice dosed with WK-X-34 (20 mg/kg i.p.). WK-X-34 selectively modulates Pgp and BCRP in vitro and in vivo in multidrug resistant ovarian cancer cells, and thus may have potential utility in the treatment of multidrug resistant tumors.

Caspase-dependent cleavage of 170-kDa P-glycoprotein during apoptosis of human T-lymphoblastoid CEM cells

Multidrug resistance (MDR) mediated by the drug efflux protein, 170-kDa P-glycoprotein (P-gp), is one mechanism that tumor cells use to escape cell death induced by chemotherapeutic drugs. Moreover, evidence suggests that cell lines expressing high levels of 170-kDa P-gp are less sensitive to caspase-mediated apoptosis induced by a wide range of death stimuli, including Fas ligand, tumor necrosis factor, and ultraviolet irradiation. However, the fate of 170-kDa P-gp during apoptosis is unknown. In this study, we demonstrate for the first time that 170-kDa P-gp is cleaved during apoptosis of VBL100 human T-lymphoblastoid CEM cells. Apoptotic cell death was induced by LY294002 (a pharmacological inhibitor of the phosphoinositide 3-kinase/Akt survival pathway), H2O2, and Z-LEHD-FMK (a caspase-9 inhibitor which has been recently reported to induce apoptosis in CEM cells). Using an antibody to a common epitope present in both the third and the sixth extracellular loop of P-gp, two cleavage products were detected, with an apparent molecular weight of 80 and 85 kDa. DEVD-FMK (a caspase-3 inhibitor), but not VEID-CHO (a caspase-6 inhibitor), blocked 170-kDa P-gp cleavage. Recombinant caspase-3 was able to cleave in vitro 170-kDa P-gp yielding two fragments of equal size to those generated in vivo. Considering the size of the cleaved fragments and their reactivity with antibodies, which recognize either the N-half or the C-half region of the protein, it is conceivable that the cleavage occurs intracytoplasmically. Since 170-kDa P-gp has been reported to counteract apoptosis, its cleavage may be a mechanism aimed at blocking an important cell survival component.

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.

Increased anti-P-glycoprotein activity of baicalein by alkylation on the A ring

The aqueous extract of Scutellariae baicalensis Georgi has inhibitory activity against P-gp 170, a multiple drug resistant gene product. Baicalein, one of the major flavones, was found to be responsible for this activity. The hydroxyl groups of the A ring of baicalein were systematically alkylated in order to assess the effect of such modifications on the activity against P-gp 170. The impact of the baicalein modifications on activity against the growth of a human nasopharyngeal cancer cell line KB and its P-gp 170 overexpressing cell line KB/MDR were also examined. The results indicate that alkylation of R5 of baicalein does not have a major impact on the interaction with P-gp 170, whereas alkylation of R6 or R7 alone or both, could enhance the interaction of baicalein with P-gp 170 as well as the amount of intracellular accumulation of vinblastine, a surrogate marker for the activity of P-gp 170 pump of KB/MDR cells. In this case, the optimal linear alkyl functionality is a propyl side chain. These modifications could also alter the activity of compounds inhibiting cell growth. Among the different compounds synthesized, the most potent molecule against P-gp 170 is 5-methoxy-6,7-dipropyloxyflavone (23). Its inhibitory activity against P-gp 170 is approximately 40 times better, based on EC50 (concentration of the compound enhancing 50% of the intracellular vinblastine accumulation in the KB/MDR cells) and 3 times higher, based on Amax (the intracellular vinblastine accumulation of the KB/MDR cells caused by the compound) as compared to baicalein. Compound 23 is also a more selective inhibitor than baicalein against P-gp 170, because its cytotoxicity is less than that observed for baicalein. The growth inhibitory IC50 of compound 23 against KB and KB/MDR cells are about the same, suggesting that compound 23 is unlikely to be a substrate of P-gp 170 pump. Acetylation of R6, R7 or both could also decrease EC50 and increase Amax. Acetylated compounds are more toxic than baicalein, and their potency against cell growth is compromised by the presence of P-gp 170, suggesting that these compounds are substrates of P-gp 170. Benzylation of R6 or R7 but not both also enhanced anti-P-gp170 activity and potency against cell growth; however, the presence of P-gp 170 in cells did not have an impact on their sensitivity to these molecules, suggesting that the benzylated compounds are inhibitors but not substrates of P-gp 170, and perhaps have a different mechanism of action. In conclusion, the substitutions of R6 and R7 hydroxyl groups by alkoxy groups, acetoxy groups, or benzyloxy groups could yield compounds with different modes of action against P-gp 170 with different mechanisms of action against cell growth.

Lack of strain-related differences in drug metabolism and efflux transporter characteristics between CD-1 and athymic nude mice

CD-1 mice are commonly used in oncology metabolism and toxicity to support drug discovery and development and to examine drug metabolism and toxicity properties of new chemical entities. On the other hand, athymic nude mice are the preferred animals to investigate tumor growth inhibition. Therefore, a frequently asked question is: are the metabolic and pharmacokinetic characteristics of xenobiotics in these two mouse strains comparable or not? To address this issue, we characterized drug metabolism and efflux transporter properties in both strains and in different organs. The metabolic stability of a set of 20 compounds and metabolite formation of cytochrome P450 (CYP) marker substrates (testosterone, ethoxyresorufin and pentoxyresorufin) were measured in liver microsomes. Drug conjugation was studied by following the disappearance of 7-hydroxycoumarin and the formation of its glucuronide and sulfate conjugates in freshly prepared liver slices. In addition, mRNA expression levels of the main cyp genes and drug efflux transporters were investigated by real-time RT-PCR in the liver, kidney, intestine and adrenal glands. No significant differences in enzymatic activities and metabolite formation were observed between the two strains. Also mRNA expression profiles of cyp and drug transporter genes were similar between CD-1 and nude mice.

Characterization of YvcC (BmrA), a multidrug ABC transporter constitutively expressed in Bacillus subtilis

The involvement of transporters in multidrug resistance of bacteria is an increasingly challenging problem, and most of the pumps identified so far use the protonmotive gradient as the energy source. A new member of the ATP-binding cassette (ABC) family, known in Bacillus subtilis as YvcC and homologous to each half of mammalian P-glycoprotein and to LmrA of Lactococcus lactis, has been studied here. The yvcC gene was constitutively expressed in B. subtilis throughout its growth, and a knockout mutant showed a lower rate of ethidium efflux than the wild-type strain. Overexpression of yvcC in Escherichia coli allowed the preparation of highly enriched inverted-membrane vesicles that exhibited high transport activities of three fluorescent drugs, namely, Hoechst 33342, doxorubicin, and 7-aminoactinomycin D. After solubilization with n-dodecyl beta-D-maltoside, the hexahistidine-tagged YvcC was purified by a one-step affinity chromatography, and its ability to bind many P-glycoprotein effectors was evidenced by fluorescence spectroscopy experiments. Collectively, these results showed that YvcC is a multidrug ABC transporter functionally active in wild-type B. subtilis, and YvcC was therefore renamed BmrA for Bacillus multidrug resistance ATP. Besides, reconstitution of YvcC into liposomes led to the highest, vanadate-sensitive, ATPase activity reported so far for an ABC transporter. Interestingly, such a high ATP hydrolysis proceeds with a positive cooperativity mechanism, a property only found so far with ABC importers.

Genomic evidence for recent positive selection at the human MDR1 gene locus

The MDR1 multidrug transporter regulates the traffic of drugs, peptides and xenobiotics into the body as well as sensitive tissues like the brain, germ cells and the developing fetus. Hence, it may influence an individual's response to drugs as well as his/her susceptibility to complex diseases in which environmental factors, especially xenobiotics, play a role. Polymorphisms within this gene, especially single-nucleotide polymorphism e26/3435(C/T), have been variously associated with differences in MDR1 expression, function, drug response and disease susceptibility. Here, we report the detailed characterization of the haplotype and linkage disequilibrium architecture of the entire 200 kb of the MDR1 gene in five world populations, namely, Chinese, Malays, Indians, Caucasians and African-Americans. We observed varied haplotype diversity across the entire gene in the different populations. The major haplotype mh5, which contains the subhaplotype e12/1236T-e21/2677T-e26/3435T, is highly represented among the four non-African populations, while mh7, which contains the subhaplotype e12/1236C-e21/2677G-e26/3435C, accounts for over a third of African-American chromosomes. These observations are inconsistent with a simple population evolution model, but instead are suggestive of recent historical events that have maintained such long range linkage disequilibrium. Using a modified long-range haplotype test, we found statistically significant evidence of recent positive selection for the e21/2677T and e26/3435T alleles in the Chinese population, and for the e26/3435T allele in the Malay population. Interestingly, we also detected evidence for positive selection of the alternative allele e26/3435C in the African-American population. These data suggest that independent mutational events may have occurred on the mh5 and mh7 haplotypes of the MDR1 gene to confer positive selection in the non-African and African-American populations, respectively.

P-glycoprotein and multidrug resistance-associated protein mediate specific patterns of multidrug resistance in malignant glioma cell lines, but not in primary glioma cells

Understanding and overcoming multidrug resistance (MDR) may be a promising strategy to develop more effective pharmacotherapies for malignant gliomas. In the present study, human malignant glioma cell lines (n=12) exhibited heterogeneous mRNA and protein expression and functional activity of the mdr gene-encoded P-glycoprotein (PGP) and MDR-associated protein (MRP). Correlation between mRNA expression, protein levels and functional activity was strong. Inhibition of PGP activity by verapamil or PSC 833 enhanced the cytotoxic effects of vincristine, doxorubicin, teniposide and taxol. Inhibition of MRP activity by indomethacin or probenecid enhanced the cytotoxic effects of vincristine, doxorubicin and teniposide. The human cerebral endothelial cell line, SV-HCEC, exhibited the strongest PGP activity of all cell lines. Five primary human glioblastomas and one anaplastic astrocytoma displayed heterogenous protein levels of PGP and MRP-1 in tumor cells and of PGP in biopsy specimens in vivo, but no functional activity of these proteins upon ex vivo culturing. These data suggest that the glioma cell line-associated MDR-type drug resistance is a result of long-term culturing and that cerebral endothelial, but not glioma cells, may contribute to MDR-type drug resistance of gliomas in vivo.

The role of drug transporters at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic interface between the blood and the brain. It eliminates (toxic) substances from the endothelial compartment and supplies the brain with nutrients and other (endogenous) compounds. It can be considered as an organ protecting the brain and regulating its homeostasis. Until now, many transport systems have been discovered that play an important role in maintaining BBB integrity and brain homeostasis. In this review, we focus on the role of carrier- and receptor-mediated transport systems (CMT, RMT) at the BBB. These include CMT systems, such as P-glycoprotein, multidrug-resistance proteins 1-7, nucleoside transporters, organic anion transporters, and large amino-acid transporters; RMT systems, such as the transferrin-1 and -2 receptors; and the scavenger receptors SB-AI and SB-BI.

Rhodamine 123 requires carrier-mediated influx for its activity as a P-glycoprotein substrate in Caco-2 cells

PURPOSE

The purpose of this work was to elucidate transport pathways of the P-glycoprotein (P-gp) substrates rhodamine 123 (R123) and doxorubicin across Caco-2 cells.

METHODS

Experiments were designed to identify saturable and nonsaturable transport processes and transport barriers for R123 and doxorubicin transport across Caco-2 cells. Confocal laser scanning microscopy (CLSM) imaged R123 transport under normal conditions and in the presence of the P-gp inhibitor, GW918 (used to abolish P-gp-mediated efflux activity).

RESULTS

R123 secretory P(app) (P(app,BA)) showed concentration dependence, whereas R123 absorptive P(app) (P(app,AB)) did not. Inhibition of P-gp efflux revealed that P-gp-mediated efflux had no effect on R123 or doxorubicin P(app,AB), but enhanced R123 and doxorubicin P(app,BA). In calcium-free medium, R123 P(app,AB) increased 15-fold, indicating intercellular junctions are a barrier to R123 absorption. CLSM of R123 fluorescence during absorptive transport under normal conditions and in the presence of GW918 was identical, and was limited to paracellular space, confirming that P-gp is not a barrier to R123 absorption. CLSM revealed that R123 fluorescence during secretory transport under normal conditions and in the presence of GW918 was localized intracellularly and in paracellular space. R123 and doxorubicin uptake across Caco-2 cells basolateral membrane was saturable.

CONCLUSIONS

R123 absorptive transport occurs primarily by paracellular route, whereas R123 secretory transport involves influx across BL membrane mediated solely by a saturable process followed by apically directed efflux via P-gp. Doxorubicin utilizes similar transport pathways to cross Caco-2 cells.

Improvement of oral drug treatment by temporary inhibition of drug transporters and/or cytochrome P450 in the gastrointestinal tract and liver: an overview

The oral bioavailability of many cytotoxic drugs is low and/or highly variable. This can be caused by high affinity for drug transporters and activity of metabolic enzymes in the gastrointestinal tract and liver. In this review, we will describe the main involved drug transporters and metabolic enzymes and discuss novel methods to improve oral treatment of affected substrate drugs. Results of preclinical and clinical phase I and II studies will be discussed in which affected substrate drugs, such as paclitaxel, docetaxel, and topotecan, are given orally in combination with an inhibitor of drug transport or drug metabolism. Future randomized studies will, hopefully, confirm that this strategy for oral treatment is at least as equally effective and safe as standard intravenous administration of these drugs.

MDR1 gene polymorphisms and disposition of the P-glycoprotein substrate fexofenadine

AIMS

The C3435T polymorphism in the human MDR1 gene is associated with lower intestinal P-glycoprotein expression, reduced protein function in peripheral blood cells and higher plasma concentrations of the P-glycoprotein substrate digoxin. Using fexofenadine, a known P-glycoprotein substrate, the hypothesis was tested whether this polymorphism also affects the disposition of other drugs in humans.

METHODS

Ten Caucasian subjects homozygous for the wild-type allele at position 3435 (CC) and 10 individuals homozygous for T at position 3435 participated in this study. A single oral dose of 180 mg fexofenadine HCl was administered. Plasma and urine concentrations of fexofenadine were measured up to 72 h using a sensitive LC/MS method. In addition, P-glycoprotein function was assessed using efflux of the P-glycoprotein substrate rhodamine 123 from CD56+ cells. Results Fexofenadine plasma concentrations varied considerably among the study population. However, fexofenadine disposition was not significantly different between the CC and TT groups (e.g. AUC(0,infinity) CC vs TT: 3567.1+/-1535.5 vs 3910.1+/-1894.8 ng ml-1 h, NS; 95% CI on the difference -1364.9, 2050.9). In contrast, P-glycoprotein function was significantly decreased in CD56+ cells of the TT compared with the CC group (rhodamine fluorescence CC vs TT: 45.6+/-7.2% vs 61.1+/-12.3%, P<0.05; 95% CI on the difference 5.6, 25.5). Conclusions In spite of MDR1 genotype-dependent differences in P-glycoprotein function in peripheral blood cells, there was no association of the C3435T polymorphism with the disposition of the P-glycoprotein substrate fexofenadine in this German Caucasian study population. These data indicate that other mechanisms including uptake transporter function are likely to play a role in fexofenadine disposition.

The mechanism of action of multidrug-resistance-linked P-glycoprotein

P-glycoprotein (Pgp), the ATP-binding cassette (ABC) transporter, confers multidrug resistance to cancer cells by extruding cytotoxic natural product amphipathic drugs using the energy of ATP hydrolysis. Our studies are directed toward understanding the mechanism of action of Pgp and recent work deals with the assessment of interaction between substrate and ATP sites and elucidation of the catalytic cycle of ATP hydrolysis. The kinetic analyses of ATP hydrolysis by reconstituted purified Pgp suggest that ADP release is the rate-limiting step in the catalytic cycle and the substrates exert their effect by modulating ADP release. In addition, we provide evidence for two distinct roles for ATP hydrolysis in a single turnover of Pgp, one in the transport of drug and the other in effecting conformational changes so as to reset the transporter for the next catalytic cycle. Detailed kinetic measurements determined that both nucleotide-binding domains behave symmetrically and during individual hydrolysis events the ATP sites are recruited in a random manner. Furthermore, only one nucleotide site hydrolyzes ATP at any given time, causing (in this site) a conformational change that drastically decreases (>30-fold) the affinity of the second site for ATP-binding. Thus, the blocking of ATP-binding to the second site while the first one is in catalytic conformation appears to be the basis for the alternate catalytic cycle of ATP hydrolysis by Pgp, and this may be applicable as well to other ABC transporters linked with the development of multidrug resistance.

Polymorphisms of metabolizing enzymes and transporter proteins involved in the clearance of anticancer agents

BACKGROUND

The efficacies and toxicities of anticancer agents vary greatly among patients. This is attributable to the activities of drug-metabolizing enzymes and membrane transporters, primarily determined by polymorphisms of the functions of genes encoding these proteins.

DESIGN

We reviewed the available literature on drug-metabolizing enzymes and membrane transporters, especially their physiological functions, genetic and functional polymorphisms, and involvement in metabolism, pharmacokinetics and toxicity of anticancer agents.

RESULTS

Nine enzymes metabolizing anticancer agents have been shown to have genetic polymorphisms: dihydropyrimidine dehydrogenase, cytochrome P450, NAD(P)H:quinone oxidoreductase 1, N-acetyltransferase 2, thiopurine methyltransferase, glutathione S-transferase, and uridine diphosphate glucuronosyltransferase. Decreased activities of these proteins can cause not only inherited metabolic disorders, but also extraordinarily severe toxicity in cancer patients given chemothearpy. Transporter proteins mediate cellular uptake and secretion of organic anions and cations. These proteins have recently been shown to play critical roles in the clearance of anticancer agents, although relations between patients' genetics backgrounds and the clinical significance of drug actions are poorly understood.

CONCLUSIONS

Further studies should be focused on dosing and selection of anticancer agents, based on the type and extent of metabolic variation among individuals, in order to avoid adverse reactions and therapeutic failure.

Prochloraz and nonylphenol diethoxylate inhibit an mdr1-like activity in vitro, but do not alter hepatic levels of P-glycoprotein in trout exposed in vivo

P-glycoproteins (P-gps) encoded by multidrug resistance 1 (mdr1) genes are ATP-dependent transporters located in the cytoplasmic membrane which mediate the efflux of a broad spectrum of hydrophobic compounds from the cell. The tissue distribution of P-gps suggests their role in the organismal defense against xenobiotics by effecting xenobiotic excretion and reducing xenobiotic uptake. In the present work, the interaction of P-gp(s) in the liver and in primary cultured hepatocytes of rainbow trout with two model pollutants was studied - the imidazole fungicide prochloraz and the alkylphenolic surfactant nonylphenol diethoxylate (NP2EO). Using a monoclonal antibody (mAB C219) directed against a conserved P-gp epitope, an immunoreactive protein of 160 kDa was detected in immunoblots of liver extracts from control trout. In sections of control trout livers, immunohistochemistry with the mAB C219 resulted in specific staining of bile canaliculi. In juvenile trout exposed for 7 days to sublethal concentrations of prochloraz (0.027 microM; 0.27 microM) or NP2EO (0.32 microM; 1.30 microM), no changes in levels of hepatic P-gp(s) were found in immunoblot and immunochemical investigations. The efflux of the fluorescent mdr 1 substrate rhodamine 123 (Rh123) from cultured isolated trout hepatocytes was partly inhibited by verapamil and vinblastine, compounds known to interfere with mdr 1-dependent transport. This demonstrates the presence of a mdr1-like mechanism in trout liver which is probably involved in the biliary excretion of hydrophobic xenobiotics. Non-cytotoxic concentrations of prochloraz and NP2EO were tested for effects on the efflux of Rh123 from trout hepatocytes. Prochloraz was a potent inhibitor of the mdr1-like mechanism, being effective at 0.3 microM and above. NP2EO inhibited Rh123 efflux only at the highest concentration tested (31.6 microM). The accumulation and elimination of 14C-prochloraz by cultured trout hepatocytes was not affected by mdr 1-type substrates (Rh123, vinblastine) and a mdr 1 inhibitor (verapamil). This shows that prochloraz is, despite its inhibitory potency, not a substrate of the mdr1-like mechanism in trout liver. The inhibition by prochloraz and NP2EO of the md r1-like mechanism in trout hepatocytes suggests that water pollutants can interfere with P-gp-function in fish and thus may impair the organismal defense against xenobiotics.

The C3435T mutation in the human MDR1 gene is associated with altered efflux of the P-glycoprotein substrate rhodamine 123 from CD56+ natural killer cells

P-glycoprotein (PGP) is a membrane protein which determines drug disposition in humans (e.g. digoxin). It is also expressed in various leukocyte lineages with highest expression in CD56+ natural killer cells. Recently, a polymorphism in exon 26 (C3435T) of this gene was shown to correlate with intestinal PGP expression and function in humans. Carriers homozygous for this polymorphism (TT) showed more than two-fold lower PGP expression and higher digoxin plasma concentrations compared to the CC group. However, it is not known whether this mutation in the MDR1 gene is also associated with altered PGP function in peripheral blood cells. We therefore assessed efflux of the PGP-substrate rhodamine 123 from CD56+ natural killer cells. Leukocytes were isolated from whole blood of 10 CC, 10 CT and 11 TT healthy Caucasian individuals. Using flow cytometry, rhodamine fluorescence was determined in CD56+ cells. Moreover, MDRI mRNA was quantified in leukocytes by real-time polymerase chain reaction. Subjects with CC genotype revealed a significantly lower rhodamine fluorescence (i.e. higher PGP function) compared to individuals with TT genotype (51.1 +/- 11.4% versus 67.5 +/- 9.5%, p < 0.01). Heterozygous individuals had an intermediate rhodamine fluorescence (61.4 +/- 6.3%). MDR1 mRNA normalized for cyclophilin was lowest in the TT population (1.29 +/- 1.01), intermediate in heterozygous subjects (1.60 +/- 0.76) and highest in the CC group (1.91 +/- 0.94; not significant). In summary, subjects being homozygous for C in position 3435 of the MDR1 gene have a more pronounced efflux of rhodamine from CD56+ natural killer cells and a higher MDR1 mRNA expression in leukocytes than subjects with the TT genotype. Measurement of rhodamine efflux using flow-cytometry from peripheral blood cells allows assessment of genetically determined differences in P-glycoprotein function.

Oral delivery of taxanes

Oral treatment with cytotoxic agents is to be preferred as this administration route is convenient to patients, reduces administration costs and facilitates the use of more chronic treatment regimens. For the taxanes paclitaxel and docetaxel, however, low oral bioavailability has limited development of treatment by the oral route. Preclinical studies with mdr1a P-glycoprotein knock-out mice, which lack functional P-glycoprotein activity in the gut, have shown significant bioavailability of orally administered paclitaxel. Additional studies in wild-type mice revealed good bioavailability after oral administration when paclitaxel was combined with P-glycoprotein blockers such as cyclosporin A or the structurally related compound SDZ PSC 833. Based on the extensive preclinical research, the feasibility of oral administration of paclitaxel and docetaxel in cancer patients was recently demonstrated in our Institute. Co-administration of cyclosporin A strongly enhanced the oral bioavailability of both paclitaxel and docetaxel. For docetaxel in combination with cyclosporin A an oral bioavailability of 90% was achieved with an interpatient variability similar to that after intravenous drug administration; for paclitaxel the oral bioavailability is estimated at approximately 50%. The safety of the oral route for both taxanes is good. A phase II study of weekly oral docetaxel in combination with cyclosporin A is currently ongoing.

Transporters involved in the elimination of drugs in the kidney: organic anion transporters and organic cation transporters

Transporters in the kidney mediate the secretion or reabsorption of many compounds and thereby influence the plasma levels of their substrates. Organic anion transporters and organic cation transporters are two major classes of secretory transporters in the mammalian kidney. During the past decade, significant progress has been made in the cloning, functional expression, and initial characterization of these transporters. To date, five organic cation transporters and nine organic anion transporters have been cloned. In this review, we summarize the available data on organic anion and organic cation transporters, focusing in particular on their molecular characteristics, tissue distribution, and inhibitor and substrate selectivities. Currently we have a good understanding of the inhibitor selectivities for most of these transporters, and with the development of more robust assays, we will soon have a better understanding of their substrate selectivities. Based on the available data, summarized in this review, it appears that many compounds interact with multiple transporters. Furthermore, there appears to be substantial overlap in the selectivities of organic cation transporters, and the same appears true for organic anion transporters. At the present time, it is unclear what the roles of these multiple transporters are in renal drug elimination. With the development of new assays, reagents, and experimental methods, we will soon have a better understanding of the roles of each transporter isoform in the renal elimination of drugs.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

Influence of P-glycoprotein on brain uptake of [18F]MPPF in rats

The aim of this study was to determine if the brain uptake of 4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-[18F]fluorobenzamido ]ethylpiperazine ([18F]MPPF), a radioligand for the imaging of 5-HT1A receptors, is influenced by the action of P-glycoprotein. Anesthetized male Wistar rats were injected i.v. with the 5-HT1A receptor antagonist [18F]MPPF (2 MBq, S.A.>110 TBq/mmol) after treatment with saline (controls) or with the 5-HT1A receptor antagonist 1-(2'-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine (NAN-190) (2.5 mg/kg i.v.). After 60 min, the animals were sacrificed and 13 areas of the brain were dissected for ex vivo gamma counting. The regional distribution of radioactivity was also assessed in brain slices using a storage phosphor system. Modulation of P-glycoprotein was achieved by injection of cyclosporin A (50 mg/kg) 30 min prior to injection of [18F]MPPF.The distribution of 18F-derived radioactivity corresponded to regional 5-HT1A receptor density as known from autoradiography. Modulation of P-glycoprotein with cyclosporin A caused a 5- to 10-fold increase in the uptake of [18F]MPPF. Tissue/cerebellum ratios in the brain correlated with receptor densities determined by in vitro autoradiography. Measurements of plasma radioactivity showed that the increased brain uptake of [18F]MPPF is partially due to a rise in ligand delivery after treatment with cyclosporin A (area under the curve, AUC, increased by a factor of 1.8). Biodistribution experiments in wild type and mdr1a(-/-) knockout mice confirmed that [18F]MPPF is a substrate for P-glycoprotein.

P-Glycoprotein in cell cultures: a combined approach to study expression, localisation, and functionality in the confocal microscope

Madin Darby canine kidney (MDCK) cells transfected with the multidrug resistance mdr1 gene, MDR1-MDCK (Pastan et al., 1988, Proc. Natl. Acad. Sci. USA 85 4486-4470), were used in a combined approach to study expression, localisation and functionality of the P-glycoprotein (P-gp) membrane transporter in the same cell culture preparations. Cells were characterised with regard to their growth curve, transepithelial electrical resistance (TEER), and cytoarchitecture. Efflux of the P-gp substrate rhodamine123 (rho123) was monitored with confocal laser scanning microscopy (CLSM). The transfected cells grew in multilayers. After reaching confluence they exhibited a complete tight junction (TJ) network. P-gp was strongly expressed at the uppermost apical surface of the multilayer already after 4 days in culture. The lower cell layers were not clearly polarised. P-gp-mediated transport could be followed by efflux of the fluorescent rho123 from the cells into the apical extracellular space. Verapamil, a P-gp inhibitor, significantly decreased efflux. For MDCK parent cells the rho123 assay was negative up to about day 20, and only at later times (day 25) low P-gp activity was detected. These results clearly show that despite the fact that the transfected cells form irregular layers, they provide a good model for screening of P-gp substrates and inhibitors.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Genetics of alpha-amanitin resistance in a natural population of Drosophila melanogaster

The genetic basis of variation in resistance to natural toxins is of interest for both ecological and evolutionary genetics. The wide variety of larval resources used by Drosophila, both within and between species, makes flies an excellent system for studying causes and consequences of selection resulting from exposure to natural toxins associated with different resources. In this study we carry out a genetic analysis of alpha-amanitin resistance in a population sample of Drosophila melanogaster. Data from mapping crosses of chromosome III support a role for a naturally occurring polymorphism in a multidrug resistance gene (Mdr65A) in alpha-amanitin resistance. However, there are no amino acid differences between resistant and sensitive chromosomes at Mdr65A. Therefore, if Mdr65A mutants contribute to the difference between alpha-amanitin-resistant and alpha-amanitin-sensitive third chromosome lines, the underlying cause is a gene regulatory mutation.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.