Previously we reported [20] that there is no correlation between the cytotoxic activity of four new structural analogs of the antitumor
DNA intercalator 3-nitrobenzothiazolo[3,2-a]quinolinium
chloride (NBQ-2) and their interaction with
DNA. In the present study, we present evidence suggesting that the molecular basis for the anti-proliferative activity of these drugs is the inhibition of
topoisomerase II. The NBQ-2 derivatives inhibited the relaxation of
supercoiled DNA plasmid pRYG mediated by purified human
topoisomerase II. Inhibition of the decatenation of
kinetoplast DNA mediated by partially purified
topoisomerase II extracted from the human
histiocytic lymphoma U937 (a cell line previously shown to be sensitive to the drugs) was also caused by these drugs. The potency of the benzazolo[3,2-a]quinolinium drugs against
topoisomerase II in vitro was the following: 7-(1-propenyl)-3-nitrobenzimidazolo[3,2-a]quinolinium
chloride (NBQ-59) > 4-chlorobenzothiazolo[3,2-a]quinolinium
chloride (NBQ-76) > 7-ethyl-3-nitrobenzimidazolo[3,2-a]quinolinium
chloride (NBQ-48) > 7-benzyl-3-nitrobenzimidazolol[3,2-a]quinolinium
chloride (NBQ-38). This rank of potency for
topoisomerase II inhibition correlated very well with the cytotoxicity elicited by these drugs. Furthermore, significant levels of
topoisomerase II/DNA cleavage complex induced by these drugs in vivo were detected when U937 cells were treated with NBQ-59 and NBQ-76 whereas NBQ-38 and NBQ-48 produced negligible amounts of the cleavage complex. Our results strongly suggest that
topoisomerase II is the major cellular target of this family of compounds.