THE ORTHO-QUINONE METABOLITE OF THE ANTICANCER DRUG ETOPOSIDE (VP-16)IS A POTENT INHIBITOR OF THE TOPOISOMERASE-II DNA CLEAVABLE COMPLEX

Epipodophyllotoxin derivatives, such as etoposide (VP-16), constitute an important class of anticancer agents, the major cytotoxic effects o f which are associated with trapping of the topoisomerase II/DNA cleav able complex and formation of protein-DNA cross-links and nicked DNA. VP-16, however, can be metabolized to several highly reactive products , including an ortho-quinone (VPQ). The inhibitory activity of VPQ aga inst purified human topoisomerase II processing of supercoiled DNA was studied and compared with that of the parent compound, VP-16. Our res ults show that VPQ is a powerful inhibitor of topoisomerase II, which prevents DNA strand passage in the presence of ATP. As with VP-16, tra pping of the cleavable complex is highly reversible upon removal of di valent ions, which indicating that VPQ alters the cleavage-reunion equ ilibrium of topoisomerase II and DNA mainly by noncovalent interaction s, as does the parent compound. However, we observed several differenc es between the effects induced by VP-16 and VPQ, including a strong in hibition of the second DNA strand religation, which implies the involv ement of additional (asymmetric) mode(s) of interactions of the VPQ, p ossibly by interference with ATP binding by the homodimeric enzyme, an d/or involving covalent interactions. Reduced or oxidized glutathione prevented trapping of the topoisomerase/DNA cleavable complex by VPQ, but not by VP-16, probably by forming covalent adducts with the former .