Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.6.3.44 (P-glycoprotein)
 13,344 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have previously shown that phenothiazines sensitize multidrug resistant (MDR) cells to chemotherapeutic drugs in a manner related to specific structural features, and have identified structurally related thioxanthenes with increased anti-MDR activity. We have now studied the structure-activity relationships of 16 thioxanthenes in the human breast cancer line MCF-7 AdrR. trans-Thioxanthene stereoisomers were 2- to 7-fold more potent than cis-thioxanthenes for antagonizing MDR. The most potent thioxanthenes possessed a halogenated tricyclic ring connected by a 3-carbon alkyl bridge to a piperazinyl or piperadinyl side group. The chemosensitizing effects of the lead compound, trans-flupenthixol, its stereoisomer cis-flupenthixol, its phenothiazine homologue fluphenazine, and the calcium channel blocker verapamil, were further studied in a series of sensitive and MDR cell lines. trans-Flupenthixol caused a greater reversal of cellular resistance to doxorubicin, vinblastine, vincristine, and colchicine in MCF-7 AdrR, KB-V1, and P388/DOX MDR cells than the other chemosensitizers. In particular, trans-flupenthixol was 2- to 3-fold more potent for reversing MDR than equimolar concentrations of verapamil. Furthermore, trans-flupenthixol fully reversed resistance to doxorubicin, vincristine, and colchicine in MDR MCF-7 and NIH 3T3 cells transfected with the mdr1 gene. None of these agents altered MDR in a non-P-glycoprotein expressing MCF-7 cell line selected with mitoxantrone, nor in any of the parental cell lines. The stereoselective antagonism of the flupenthixol isomers on several putative cellular targets was studied to explore the mechanism of their chemosensitizing activity. cis- and trans-flupenthixol were equally active inhibitors of protein kinase C and calmodulin. Both cis- and trans-flupenthixol were also potent inhibitors of [3H]azidopine binding to P-glycoprotein. The apparent lack of clinical toxicity of trans-flupenthixol makes it an attractive drug for possible use in the modulation of tumor resistance in vivo if appropriate tissue concentrations can be achieved.
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PMID:Cellular and biochemical characterization of thioxanthenes for reversal of multidrug resistance in human and murine cell lines. 196 58

P-glycoprotein (P-gp) is an energy-dependent drug extrusion pump with broad specificity for diverse hydrophobic anticancer agents and compounds known to reverse multidrug resistance (MDR). Among MDR reversing agents, phenothiazines (PTZs) and related compounds may sensitize MDR by interacting with a specific binding site(s) on P-gp and by other mechanisms. In order (1) to identify a binding site for PTZs and related compounds on P-gp, (2) to examine whether these compounds and other MDR modulators bind to the same domains of P-gp, and (3) to identify proteins with high specificity for these neuroleptic agents and other MDR modulators, we used a butyrophenone D2-dopamine receptor photoaffinity probe, N-(p-azido-3-[125I]iodophenethyl)spiperone ([125I]NAPS). [125I]NAPS was actively effluxed from vincristine (VCR)-resistant SH-SY5Y/VCR human neuroblastoma cells, and nonradioactive I-NAPS was a potent chemosensitizing agent. After photolabeling, the probe bound specifically and with high efficiency to P-gp and to another multidrug binding 17-kDa membrane-bound protein, spiperophilin, in these cells. The efficiency of [125I]NAPS binding to P-gp was 5-6-fold more than [3H]azidopine and [125I]arylazidoprazosin ([125I]AAP), known photoaffinity analogs for P-gp. [125I]NAPS photolabeling of P-gp was preferentially competed by MDR-related drugs, with vinblastine > VCR > colchicine > doxorubicin > actinomycin D. Many drugs that are known to reverse MDR were potent inhibitors of [125I]NAPS binding to P-gp. While PTZs and related compounds were potent inhibitors of [125I]NAPS binding to P-gp, most of them enhanced the binding of [125I]AAP significantly. cis-Flupentixol increased the binding of [125I]AAP to P-gp 9-fold more than did trans-flupentixol, but both were potent inhibitors of [125I]NAPS binding, suggesting their stereoselective effect on the [125I]AAP binding site. Proteolysis of [125I]NAPS-bound P-gp with Staphylococcus aureus V8 protease revealed that this probe binds to two major peptides, 6 and 8 kDa, and a number of minor ones, while [125I]AAP binds to only an 8-kDa peptide. These results suggest that modulators of MDR may interact with separate or overlapping domains. Furthermore, most MDR modulators, dopaminergic drugs, and beta-adrenergic antagonists used also inhibited binding of [125I]-NAPS to spiperophilin, suggesting that this protein may be a target for these drugs.
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PMID:N-(p-azido-3-[125I]iodophenethyl)spiperone binds to specific regions of P-glycoprotein and another multidrug binding protein, spiperophilin, in human neuroblastoma cells. 790 76

Human P-glycoprotein (Pgp) confers multidrug resistance to cancer cells by ATP-dependent extrusion of a great many structurally dissimilar hydrophobic compounds. The manner in which Pgp recognizes these different substrates is unknown. The protein shows internal homology between its N- and C-terminal halves, each comprised of six putative transmembrane helices and a consensus ATP binding/utilization site. Photoactive derivatives of certain Pgp substrates specifically label two regions, one on each half of the protein. In this study, using [125I]iodoarylazidoprazosin ([125I]IAAP), a photoactive analog of prazosin, we have demonstrated the presence of two nonidentical drug-interaction sites within Pgp. Taking advantage of a highly susceptible trypsin cleavage site in the linker region of Pgp, we characterized the [125I]IAAP binding to the N- and C-terminal halves. cis(Z)-Flupentixol, a modulator of Pgp function, preferentially increased the affinity of [125I]IAAP for the C-terminal half of the protein (C-site) by reducing the Kd from 20 to 6 nM without changing the labeling or affinity (Kd = 42-46 nM) of the N-terminal half (N-site). Also, the concentration of vinblastine (Pgp substrate) and cyclosporin A (Pgp modulator) required for 50% inhibition of [125I]IAAP binding to the C-site was increased 5- to 6-fold by cis(Z)-flupentixol without any effect on the N-site. In addition, [125I]IAAP binding to the N-site was less susceptible than to C-site to inhibition by vanadate which blocks ATP hydrolysis and drug transport. These data demonstrate the presence of at least two nonidentical substrate interaction sites in Pgp.
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PMID:Evidence for two nonidentical drug-interaction sites in the human P-glycoprotein. 938 Jun 80

Cancers are frequently chemoresistant because of overexpression of P-glycoprotein. Two different approaches to improve cancer treatment are currently being investigated in clinical trials: inhibition of P-glycoprotein function by reversing agents, and alleviation of leukocytopenia by MDR1 gene transfer to normal bone marrow of patients. We report here that retroviral vectors encoding a mutant P-glycoprotein (MDR1-F983A) protect hematopoietic cells from anticancer drugs even in the presence of trans-(E)-flupentixol, an inhibitor of P-glycoprotein. Transfer of either mutant or wild-type MDR1 to K562 erythroleukemia cells or primary murine bone marrow resulted in reduced accumulation of daunomycin and vinblastine because of increased drug efflux.trans-(E)-Flupentixol at concentrations up to 10 microM failed to reverse drug efflux mediated by the product of the mutant MDR1 while wild-type P-glycoprotein was inhibited. In the presence of 2 microM trans-(E)-flupentixol chemoresistance to daunomycin was circumvented only in K562 cells transduced with wild-type, but not with mutant, MDR1. Moreover, drug resistance of KB-8-5 epidermoid cancer cells, which express the wild-type MDR1 gene at levels comparable to clinical specimens from multidrug-resistant cancers, was fully overcome in the presence of trans-(E)-flupentixol. Vectors expressing mutant P-glycoprotein may help improve chemotherapy by allowing safe dose intensification under conditions in which multidrug-resistant cancers are rendered drug sensitive by reversing agents.
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PMID:Chemoprotection of hematopoietic cells by a mutant P-glycoprotein resistant to a potent chemosensitizer of multidrug-resistant cancers. 1072 34

The human multidrug transporter P-glycoprotein (Pgp, ABCB1) contributes to the poor bioavailability of many anticancer and antimicrobial agents as well as to drug resistance at the cellular level. For rational design of effective Pgp inhibitors, a clear understanding of its mechanism of action and functional regulation is essential. In this study, we demonstrate that inhibition of Pgp-mediated drug transport by cis-(Z)-flupentixol, a thioxanthene derivative, occurs through an allosteric mechanism. Unlike competitive inhibitors, such as cyclosporin A and verapamil, cis-(Z)-flupentixol does not interfere with substrate ([(125)I]iodoarylazidoprazosin) recognition by Pgp, instead it prevents substrate translocation and dissociation, resulting in a stable but reversible Pgp-substrate complex. cis-(Z)-Flupentixol-induced complex formation requires involvement of the Pgp substrate site, because agents that either physically compete (cyclosporin A) for or indirectly occlude (vanadate) the substrate-binding site prevent formation of the complex. Allosteric modulation by cis-(Z)-flupentixol involves a conformational change in Pgp detectable by monoclonal antibody UIC2 binding to a conformation-sensitive external epitope of Pgp. The conformational change observed is distinct from that induced by Pgp substrates or competitive inhibitors. A single amino acid substitution (F983A) in TM12 of Pgp that impairs inhibition by cis-(Z)-flupentixol of Pgp-mediated drug transport also affects stabilization of the Pgp-substrate complex as well as the characteristic conformational change. Taken together, our results describe the molecular mechanism by which the Pgp modulator cis-(Z)-flupentixol allosterically inhibits drug transport.
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PMID:Allosteric modulation of human P-glycoprotein. Inhibition of transport by preventing substrate translocation and dissociation. 1264 84