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Indoloquinoxaline Compounds That Selectively Antagonize P-Glycoprotein

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Tumor cells often develop drug resistance through overexpression of membrane transport proteins that effectively efflux anticancer agents. The pharmacologies of the two best-studied transporters, P-glycoprotein (Pgp) and MRP1, are partially overlapping but distinct. To improve the therapeutic potential of drug resistance reversing agents, we have developed a program to identify compounds with selectivity for Pgp or MRP1. Screening of a commercial library of compounds identified indoloquinoxaline compounds with transporter selectivity, and certain examples were synthesized and further evaluated. 1,4-Dibutoxy-6H-indolo[2,3-b]quinoxaline and 4,7-dibutoxy-2,3-dihydrobenzimidazole-2-spiro-3-indolin-2-one were synthesized by condensation of 3,6-dibutoxy-1,4-diaminobenzene and isatin. Neither compound was cytotoxic to MCF-7 cells, nor did either one affect the sensitivity of MCF-7/VP or HL-60/ADR cells at doses up to at least 20 μM, indicating that they do not antagonize MRP1. In contrast, each compound, at doses as low as 0.25 μM, sensitized NCI/ADR cells to vinblastine, actinomycin D, Taxol, and doxorubicin, indicating that they effectively reverse Pgp-mediated multidrug resistance (MDR). Furthermore, the compounds sensitized two additional cell lines that overexpress Pgp to this panel of anticancer drugs. However, these compounds did not affect the sensitivities of MCF-7 or T24 cells to these cytotoxic drugs, and did not alter the sensitivities of any of the tested cell lines to cisplatin or 5-fluorouracil. Both compounds enhanced the intracellular accumulation of [3H]vinblastine by NCI/ADR cells, but did not inhibit photoaffinity labeling of Pgp by [3H]azidopine at concentrations up to at least 100 μM. Therefore, these novel nontoxic indoloquinoxalines selectively sensitize Pgp-overexpressing cells to drugs that are subject to transport by this protein, without modulating the sensitivities of MRP1-overexpressing or non-Pgp cells to cytotoxic drugs. Because of this transporter selectivity, we predict that these compounds will be effective MDR modulators in vivo.

Keywords: Key words: Drug resistance; MRP1;; P-glycoprotein

Document Type: Research Article


Affiliations: Department of Pharmacology, Pennsylvania State University, 500 University Drive, Hershey, PA 17033

Publication date: May 1, 2001

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  • Formerly: Oncology Research Incorporating Anti-Cancer Drug Design
    Oncology Research Featuring Preclinical and Clincal Cancer Therapeutics publishes research of the highest quality that contributes to an understanding of cancer in areas of molecular biology, cell biology, biochemistry, biophysics, genetics, biology, endocrinology, and immunology, as well as studies on the mechanism of action of carcinogens and therapeutic agents, reports dealing with cancer prevention and epidemiology, and clinical trials delineating effective new therapeutic regimens.

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