The anticancer thiosemicarbazones Dp44mT and triapine lack inhibitory effects as catalytic inhibitors or poisons of DNA topoisomerase IIα.

The thiosemicarbazones Dp44mT (di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone) and triapine have potent antiproliferative activity and have been evaluated as anticancer agents. While these compounds strongly bind iron and copper, their mechanism(s) of action are incompletely understood. A recent report (Rao et al., Cancer Research 69:948-57, 2009) suggested that Dp44mT may, in part, exert its cytotoxicity through poisoning of DNA topoisomerase IIα. In the present report, a variety of assays were used to determine whether Dp44mT and triapine target topoisomerase IIα. Neither of these compounds inhibited topoisomerase IIα decatenation or induced cleavage of pBR322 DNA in the presence of enzyme. In cells, Dp44mT did not stabilize topoisomerase IIα covalent binding to DNA using an immunoblot band depletion assay, an ICE (immunodetection of complexes of enzyme-to-DNA) assay, and a protein-DNA covalent complex forming assay. Dp44mT did not display cross resistance to etoposide resistant K562 cells containing reduced topoisomerase IIα levels. Synchronized Dp44mT-treated CHO cells did not display a G2/M cell cycle block expected of a topoisomerase II inhibitor. A COMPARE analysis of Dp44mT using the NCI 60-cell line data indicated that inhibition of cell growth was poorly correlated with DNA topoisomerase IIα mRNA levels. In summary, we found no support for the conclusion that Dp44mT inhibits cell growth through the targeting of topoisomerase IIα. Since clinical trials of triapine are underway, it will be important to better understand the intracellular targeting and mechanisms of action of the thiosemicarbazones to support forward development of these agents and newer analogs.