Resistance to chemotherapy occurs in cancer cells because of intrinsic or acquired changes in expression of specific proteins. We have studied resistance to natural product chemotherapeutic agents such as doxorubicin, Vinca alkaloids, and taxol. In most cases, cells become simultaneously resistant to multiple drugs because of reductions in intracellular drug concentrations. For the natural product drugs, this cross-resistance is frequently due to expression of an energy-dependent drug efflux system (ABC transporter) known as P-glycoprotein (P-gp), the product of the MDR1 or ABCB1 gene, or to other members of the ABC transporter family, including ABCB5. To explore the possibility that other members of the ABC family of transporters may be involved in drug resistance in cancer, we have developed real-time PCR for detection of most of the 48 known ABC transporters;these techniques have been used to correlate expression of novel ABC transporters in cancer cell lines of known drug resistance. Expression of approximately 30 ABC transporters has been shown to correlate in the NCI-60 cell lines with resistance to specific cytotoxic drugs. Furthermore, this analysis has revealed that some drugs are more toxic to P-gp-expressing cells than to non-expressors, suggesting a novel approach to treatment of MDR cancers. Several different chemical classes with this property, including thiosemicarbazides, have been identified. One compound, NSC73306, has been studied in detail and shown to kill P-gp-expressing cells with high specificity by blocking them in S phase. Treatment with NSC73306 and related drugs also results in increased turnover of ABCB1 mRNA. Cells that survive NSC73306 treatment do not express P-gp and are sensitive to chemotherapy with natural product drugs such as anthracyclines, paclitaxel and Vinca alkaloids. A quantitative structure activity analysis of NSC73306 analogs and a further correlation analysis in the NCI-60 cell lines has yielded many additional compounds with improved ability to kill selectively P-gp-expressing cells, but also with improved solubility properties. In addition, the compound tiopronin, which is a sulfhydryl donor used clinically to treat cystinosis, and some of its derivatives have been shown to be powerful selective agents for killing MDR cells. In this case, expression of P-gp is not required for multidrug-resistant cells to be sensitive to tiopronin, which appears to kill through Reactive Oxygen Species. Technology enabling a high-throughput screen for new agents that are substrates, inhibitors or specifically kill P-gp-expressing cells has been developed and preliminary screens reveal new chemical entities that are P-gp substrates of kill P-gp-expressing cells. In cell lines derived from patients with Kaposis sarcoma, exposure to HIV protease inhibitors results in overexpression of ABCB1, for which the protease inhibitors are important substrates.Studies on the normal function of P-gp suggest that it is involved in normal uptake and distribution of many drugs. C11-desmethoxy-loperamide has been developed in collaboration with Robert Innis in NIMH to PET image distribution of this specific P-gp substrate in cancers and in the brain, with and without treatment with potent inhibitors of P-gp such as tariquidar. PET ligands that are weak bases are trapped in lysosomes, amplifying the uptake signal in the brain, in some normal tissues, and in cancers. We have shown that among three most prominent transporters at the blood-brain barrier (ABCB1, ABCC1, ABCG2), this compound is specific for ABCB1 (P-gp). A synonymous polymorphism of P-gp (C3435T, no amino acid change) in the setting of a specific P-gp haplotype can affect efficiency of P-gp pumping by altering the rhythm of protein folding and changing substrate and inhibitor interactions with P-gp. This haplotype appears to change mRNA folding, and cause a major translational delay which results in altered conformation of P-gp. Stable transfectants of porcine LLC-PK1 cells with the haplotype form of P-gp show altered drug resistance and inhibitor sensitivity compared to wild-type P-gp transfectants. We have created a highly sensitive, quantitative assay for ABC transporter mRNAs in human cancer samples using TaqMan Low Density Arrays. In initial studies of several cancers whose cell lines are represented in the NCI-60 cell lines we have found that patterns of expression of ABC transporter genes in clinical cancers differ substantially from those of the established cell lines, irrespective of the manner in which these cells are cultured (monolayer, 3D culture or xenografts). This result indicates that current in vitro cancer cell models may not be adequate to study drug resistance in cancer and efforts are underway to develop novel ex vivo cancer cell growth conditions that maintain in vivo patterns of expression of MDR genes. Studies on regulation of expression of ABCG2 during development indicate that it is regulated during development post-transcriptionally by a microRNA (miRNA) that targets the 3 prime non-coding region of ABCG2 mRNA. ABCG2 mRNA levels are high in pluripotent stem cells, but are not expressed as protein until the cells begin to differentiate.

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National Cancer Institute (NCI)
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Kannan, Pavitra; Schain, Martin; Kretzschmar, Warren W et al. (2017) An automated method measures variability in P-glycoprotein and ABCG2 densities across brain regions and brain matter. J Cereb Blood Flow Metab 37:2062-2075
Esser, Lothar; Zhou, Fei; Pluchino, Kristen M et al. (2017) Structures of the Multidrug Transporter P-glycoprotein Reveal Asymmetric ATP Binding and the Mechanism of Polyspecificity. J Biol Chem 292:446-461
Kumar, Jeyan S; Wei, Bih-Rong; Madigan, James P et al. (2016) Bioluminescent imaging of ABCG2 efflux activity at the blood-placenta barrier. Sci Rep 6:20418
Bakhsheshian, Joshua; Wei, Bih-Rong; Hall, Matthew D et al. (2016) In Vivo Bioluminescent Imaging of ATP-Binding Cassette Transporter-Mediated Efflux at the Blood-Brain Barrier. Methods Mol Biol 1461:227-39
Weidner, Lora D; Fung, King Leung; Kannan, Pavitra et al. (2016) Tariquidar Is an Inhibitor and Not a Substrate of Human and Mouse P-glycoprotein. Drug Metab Dispos 44:275-82
Fung, King Leung; Kapoor, Khyati; Pixley, Jessica N et al. (2016) Using the BacMam Baculovirus System to Study Expression and Function of Recombinant Efflux Drug Transporters in Polarized Epithelial Cell Monolayers. Drug Metab Dispos 44:180-8
Pluchino, Kristen M; Hall, Matthew D; Moen, Janna K et al. (2016) Human-Mouse Chimeras with Normal Expression and Function Reveal That Major Domain Swapping Is Tolerated by P-Glycoprotein (ABCB1). Biochemistry 55:1010-23
Liow, Jeih-San; Zoghbi, Sami S; Hu, Shuo et al. (2016) (18)F-FCWAY, a serotonin 1A receptor radioligand, is a substrate for efflux transport at the human blood-brain barrier. Neuroimage 138:134-140
Kumar, Jeyan S; Miller Jenkins, Lisa M; Gottesman, Michael M et al. (2016) The Drug Excipient Cyclodextrin Interacts With d-Luciferin and Interferes With Bioluminescence Imaging. Mol Imaging 15:
Greene, James M; Levy, Doron; Herrada, Sylvia P et al. (2016) Mathematical Modeling Reveals That Changes to Local Cell Density Dynamically Modulate Baseline Variations in Cell Growth and Drug Response. Cancer Res 76:2882-90

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