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.