Phenoxodiol (2H-1-benzopyran-7-0,1,3-(4-hydroxyphenyl)), a novel isoflavone derivative, inhibits DNA topoisomerase II by stabilizing the cleavable complex.

Cancer therapeutic drugs that inhibit DNA topoisomerase (topo) II by stabilizing the cleavable complex are collectively known as topo II poisons. Phenoxodiol is a synthetic derivative of the plant isoflavone daidzein and is currently undergoing clinical testing as a cancer therapeutic drug. The development of this agent as an antitumor drug was based to a large extent on its low toxicity in normal tissues but potent topo II inhibitory effects in rapidly dividing tumor cells. To evaluate phenoxodiol as a potential inhibitor of topoisomerases, we used the relaxation and nicking assays that can identify topo I inhibitors, and the unknotting and DNA cleavage assays that can identify topo II inhibitors. Phenoxodiol inhibited the catalytic activity of topo II in a dose-dependent manner and it stabilized the topo II-mediated cleavable complex, demonstrating that this agent is a topo II poison. Phenoxodiol's topo II inhibitory effects were comparable to those of other antitumor agents such as VP-16 and were stronger than those of genistein. Phenoxodiol did not inhibit topo I catalytic activity nor did it stabilize the topo I-mediated cleavable complex. These results demonstrate that phenoxodiol is a topo II-specific poison and suggest that this novel agent may find applications in cancer chemotherapy.