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Research Articles: Therapeutics, Targets, and Development

Inhibition of ABCG2-mediated transport by protein kinase inhibitors with a bisindolylmaleimide or indolocarbazole structure

¹ Medical Oncology Branch and ² Laboratory of Cell Biology, Center for Cancer Research, NIH, Bethesda, Maryland

Requests for reprints: Robert W. Robey, Building 10, Room 12C217, 9000 Rockville Pike, Bethesda, MD 20892. Phone: 301-496-0796; Fax: 301-402-1608. E-mail: robeyr{at}mail.nih.gov

Abstract

ABCG2 is a transporter with potential importance in cancer drug resistance, drug oral absorption, and stem cell biology. In an effort to identify novel inhibitors of ABCG2, we examined the ability of commercially available bisindolylmaleimides (BIM) and indolocarbazole protein kinase inhibitors (PKI) to inhibit ABCG2, given the previous demonstration that the indolocarbazole PKI UCN-01 interacted with the transporter. At a concentration of 10 µmol/L, all of the compounds tested increased intracellular fluorescence of the ABCG2-specific substrate pheophorbide a in ABCG2-transfected HEK-293 cells by 1.3- to 6-fold as measured by flow cytometry; the ABCG2-specific inhibitor fumitremorgin C increased intracellular fluorescence by 6.6-fold. In 4-day cytotoxicity assays, wild-type ABCG2-transfected cells were not more than 2-fold resistant to any of the compounds, suggesting that the PKIs are not significantly transported by ABCG2. BIMs I, II, III, IV, and V, K252c, and arcyriaflavin A were also able to inhibit [¹²⁵I]iodoarylazidoprazosin labeling of ABCG2 by 65% to 80% at 20 µmol/L, compared with a 50% to 70% reduction by 20 µmol/L fumitremorgin C. K252c and arcyriaflavin A were the most potent compounds, with IC₅₀ values for inhibition of [¹²⁵I]iodoarylazidoprazosin labeling of 0.37 and 0.23 µmol/L, respectively. K252c and arcyriaflavin A did not have any effect on the ATPase activity of ABCG2. Four minimally toxic compounds—BIM IV, BIM V, arcyriaflavin A, and K252c—reduced the relative resistance of ABCG2-transfected cells to SN-38 in cytotoxicity assays. We find that indolocarbazole and BIM PKIs directly interact with the ABCG2 protein and may thus increase oral bioavailability of ABCG2 substrates. [Mol Cancer Ther 2007;6(6):1877–85]

Grant support: Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 12/31/06; revised 3/28/07; accepted 4/26/07.