Abstract
Symadex is the lead acridine compound of a novel class of imidazoacridinones (IAs) currently undergoing phase II clinical trials for the treatment of various cancers. Recently, we have shown that Symadex is extruded by ABCG2-overexpressing lung cancer A549/K1.5 cells, thereby resulting in a marked resistance to certain IAs. To identify the IA residues essential for substrate recognition by ABCG2, we here explored the ability of ABCG2 to extrude and confer resistance to a series of 23 IAs differing at defined residue(s) surrounding their common 10-azaanthracene structure. Taking advantage of the inherent fluorescent properties of IAs, ABCG2-dependent efflux and drug resistance were determined in A549/K1.5 cells using flow cytometry in the presence or absence of fumitremorgin C, a specific ABCG2 transport inhibitor. We find that a hydroxyl group at one of the R1, R2, or R3 positions in the proximal IA ring was essential for ABCG2-mediated efflux and consequent IA resistance. Moreover, elongation of the common distal aliphatic side chain attenuated ABCG2-dependent efflux, thereby resulting in the retention of parental cell sensitivity. Hence, the current study offers novel molecular insight into the structural determinants that facilitate ABCG2-mediated drug efflux and consequent drug resistance using a unique platform of fluorescent IAs. Moreover, these results establish that the IA determinants mediating cytotoxicity are precisely those that facilitate ABCG2-dependent drug efflux and IA resistance. The possible clinical implications for the future design of novel acridines that overcome ABCG2-dependent multidrug resistance are discussed.
Footnotes
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ABBREVIATIONS: AML, acute myeloid leukemia; IA, imidazoacridinone; TA, triazoloacridinone; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; FTC, fumitremorgin C; ABC, ATP-binding cassette; PhA, pheophorbide; PBS, phosphate-buffered saline; TMR, tetramethylrosamine; RF, resistance-fold; AF, accumulation-fold; ClogP, calculated log P; MDR, multidrug resistance.
- Received January 15, 2009.
- Accepted February 25, 2009.
- The American Society for Pharmacology and Experimental Therapeutics
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