Structure-activity study of the interaction of bioreductive benzoquinone alkylating agents with DNA topoisomerase II

Abstract

Purpose: Quantitative structure-activity studies were performed on a series of benzoquinone mustard (BM) bifunctional alkylating agents to determine whether DNA topoisomerase II (topo II) inhibition was responsible for cell growth inhibition. Methods: Topo II inhibition was evaluated by decatenation and agarose gel electrophoresis assays. Results: The BM compounds were shown to potently inhibit the decatenation activity of topo II. Though BM compounds promoted the formation of protein-DNA complexes in isolated nuclei and cells, this effect was undiminished when levels of topo II varied. The BM compounds had little activity in a topo II-mediated DNA cleavage assay, suggesting that they do not function as topo II poisons. Rather, BM-induced protein-DNA complex formation was likely due to the bifunctional alkylating reactivity of these compounds. Finally, the growth inhibitory properties of these compounds did not correlate with their ability to inhibit topo II, indicating that these compounds did not exert their cellular activity through inhibition of topo II. Some BM compounds reacted very quickly with glutathione and cysteine, likely initially through an electrophilic Michael addition. In the absence of cysteine, the growth inhibitory effects of BM were increased tenfold, indicating the modulatory effect of cysteine sulfhydryl adducts. EPR studies showed that a semiquinone-free radical was produced by some BM compounds. Conclusions: BM compounds likely exert their action through DNA cross-linking and/or by inducing oxidative stress. Although topo II is not a direct target of these agents, this enzyme may play a role in processing the consequences of direct DNA adduction and/or oxidative DNA damage.