Tannins as potent inhibitors of DNA topoisomerase II in vitro.
Fifty-two out of 60 tannins, including gallo-, ellagi-, condensed, and complex tannins, are inhibitors of human DNA topoisomerase II in vitro. Thirty-six compounds that completely inhibited enzyme activity at a concentration of 500 nM or less, as assessed by ATP-dependent unknotting of P4 phage DNA, were at least 100-fold more potent than the clinically useful antitumor agent etoposide (VP-16). Relative inhibitory activity was primarily related to the number of phenolic hydroxyl groups (galloyl and hexahydroxydiphenoyl moieties) found in the active structures, with more groups generally conferring increased potencies. Unlike VP-16 and some DNA intercalative agents that stabilize the topoisomerase II-DNA cleavage intermediate, none of the active compounds induced protein-linked DNA breaks in cultured cells. Some of the tannins reduced VP-16-induced protein-linked DNA breaks by 20% or more, but one of these compounds, (-)-epicatechin, was not an inhibitor in vitro. Our data suggest that some tannins, such as sangiin H-6, that are potent inhibitors of catalytic double DNA-strand passage in vitro may target intracellular enzyme activity in a similar fashion to known poisons that interfere with formation of the enzyme-DNA covalent intermediate.