Inhibition of the action of the topoisomerase II poison amsacrine by simple aniline derivatives: evidence for drug-protein interactions.

The action of the anticancer drug amsacrine appears to involve molecular interactions with both DNA and topoisomerase II. It has been shown previously that DNA intercalators can inhibit the action of amsacrine and several other topoisomerase II poisons, presumably as a result of interference with the DNA binding sites for the enzyme. We show here that drug molecules such as N-phenylmethanesulfonamide, which mimic the anilino side chain of amsacrine, inhibit the cytotoxicity against cultured Lewis lung murine carcinoma of amsacrine, amsacrine analogues including asulacrine and DACA (N-[2-(dimethylamino)-ethyl]acridine-4-carboxamide dihydrochloride), and etoposide. In contrast, the cytotoxicity of doxorubicin was slightly increased by co-incubation with N-phenylmethanesulfonamide. The cytotoxicity of amsacrine was also modulated in human Jurkat leukemia, HCT-8 colon, and HT-29 colon cell lines. Because o-AMSA, an amsacrine analogue containing a methoxy group in the ortho rather than in the meta position, is known to be inactive as an antitumor drug, the abilities of the ortho and meta methoxy-substituted derivatives of methyl-N-phenylcarbamate to reverse the cytotoxicity of amsacrine, asulacrine, and DACA were compared. The ortho substitution decreased activity while meta substitution slightly increased it, suggesting that the side chains were binding to a similar site to that occupied by amsacrine. To determine whether the side chain variants actively inhibited the formation of DNA-topoisomerase II covalent complexes, cultured cells were treated with amsacrine or asulacrine, harvested, and lysed directly on acrylamide gels before electrophoresis and Western blotting to identify non-DNA-bound topoisomerase II. Extractable topoisomerase II was depleted in cells incubated with amsacrine but partially restored by coculture with methyl-N-phenylcarbamate. The findings are consistent with the hypothesis that low molecular weight molecules can modulate the effects of topoisomerase II poisons by directly interacting with the enzyme.