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.