Geldanamycin (GM) is a representative Hsp90 inhibitor and has shown promising anti-cancer activity. Because of its adverse effects on liver, its less toxic derivatives; 17-(allylamino)-17-demethoxygeldanamycin (17AAG) and 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (DMAG) are currently being evaluated for the treatment. GM contains a quinone group, which activated by cytochrome P-450 reductase to form semiquinone radicals that can in-turn reduce oxygen to form superoxide. We hypothesized that the differential toxicity against liver among these drugs reflects the production of superoxide. To determine the reactivity with cytochrome P-450 reductase, each of these drugs mixed with NADPH and the enzyme and the oxidation of NADPH was monitored. Secondly, after incubation of the rat primary hepatocytes with each of these drugs and DCFH-DA, the intracellular reactive oxygen species (ROS) was monitored by DCF fluorescence. Thirdly, the cell viability was estimated by mitochondrial respiration (MTT assay) after overnight incubation of the cells with each of these drugs. Finally, by electron spin resonance (ESR) methods, the ROS was identified after each of these drugs mixed with NADPH and the enzyme. GM, DMAG, and 17-AAG were found to support oxidation of NADPH in enzyme incubation indicating the generation of semiquinone and superoxide, which was identified by ESR spectra. Cyclic voltammetry studies show that the redox pair of quione/semiquinone had the lowest midpoint potential for GM suggesting the ease of reduction. Intracellular ROS generation estimated by DCF fluorescence studies followed the order GM >> 17AAG \#8776; DMAG. Cell toxicity measured using cell viability studies followed the order GM >> 17AAG \#8776; DMAG. These studies suggest that hepatotoxicity exhibited by the Hsp90 inhibitors belonging to the ansamycin analogs is related to ROS production.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1854.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

American Association for Cancer Research
2009