Abstract 2529: Structure-activity relationship studies of the type II topoisomerase poison amsacrine
Authors: Ketron Adam , Graves David E. , Osheroff Neil 
Published: June 2012

Abstract
Type II topoisomerases are essential, ubiquitously expressed enzymes that are required for the modulation of chromosomal topology. A critical intermediate in the catalytic cycle of topoisomerase II is the generation of the cleavage complex, a transient enzyme-linked DNA double-stranded break. Several classes of widely prescribed anticancer drugs kill cells specifically by stabilizing the topoisomerase II-DNA cleavage complex, resulting in an accumulation of genetic damage and eventually apoptosis. The anilinoacridine, amsacrine, is a topoisomerase II-targeted drug effective in the treatment of relapsed acute myeloid leukemia. The structure of the drug includes a DNA-intercalative acridine moiety and a substituent-bearing pendant ring. Amsacrine enhances in vitro topoisomerase II-mediated DNA cleavage ∼8-fold. It has long been known that movement of the methoxy group from the meta- to the ortho- position (relative to the methanesulfonamide group) abrogates drug action, but the basis for this effect is unknown. Therefore, the effects of eliminating the methoxy group on drug activity were determined. The resulting compound was intermediate to the meta- and ortho-methoxy compounds, suggesting that the meta-methoxy group favors drug-enzyme interaction while the ortho-methoxy group inhibits it. Substitution of the methanesulfonamide group with a hydroxy group or a proton attenuated enzyme-mediated DNA cleavage ∼5-fold or ∼2-fold, respectively. This implies that the methanesulfonamide promotes, but is not essential for, drug-enzyme interaction. To investigate the significance of DNA intercalation in the interaction between amsacrine and the topoisomerase II-DNA cleavage complex, the relative potencies of amsacrine and derivatives as DNA intercalators were determined. Correlations between structure and DNA intercalation were observed, but not between DNA intercalation and drug activity. This suggests that DNA intercalation alone is not sufficient for drug action; specific interaction with the enzyme through the pendant ring groups is required for effective drug activity. To further investigate the significance of DNA intercalation in amsacrine activity, the effects of the isolated pendant ring on enzyme-mediated DNA cleavage were determined. This compound enhanced DNA cleavage in the absence of DNA intercalation with efficacy and site-specificity similar to that of amsacrine, but with a 100-fold lower potency. Unlike amsacrine, the activity of the pendant ring was partially redox-dependent. We conclude from these results that the interaction between amsacrine and topoisomerase II is mediated through the pendant ring substituents, while the acridine moiety protects against redox activity and significantly increases the local concentration of amsacrine at the active site of the enzyme. This work was supported by NIH grants GM033944 and T32 GM065086.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2529. doi:10.1158/1538-7445.AM2011-2529