The cytotoxic actions of several classes of antitumor DNA intercalators are 
thought to result from some disturbance to DNA metabolism following trapping of 
the nuclear enzyme DNA topoisomerase II as a covalent complex on DNA. Here we 
have studied topoisomerase II trapping and DNA synthesis patterns in relation to 
the acute cytotoxic actions of 4'-(9-acridinylamino)methanesulfon-m-anisidide 
(mAMSA) or mitoxantrone on SV40 transformed human fibroblasts. These two DNA 
intercalators differed significantly in their cytotoxic potential, mitoxantrone 
being 24-fold more toxic than mAMSA when assayed by the inhibition of 
clonogenicity. Although both drugs induced G2 delay at cytotoxic concentrations, 
mAMSA-treated cells recovered normal cell cycle phase distributions within 24 h 
of removal of drug, while mitoxantrone-treated cells continued to accumulate in 
G2 up to 48 h following drug treatment with evidence of complete inhibition of 
entry into mitosis. Compared with mAMSA, mitoxantrone showed a similar capacity 
to induce cleavable complexes in cellular DNA, and only a 2-fold greater ability 
to inhibit DNA synthesis. Within a 4-h posttreatment period, mAMSA-treated cells 
recovered normal rates of DNA synthesis, whereas a continued depression of DNA 
synthesis was observed in mitoxantrone-treated cells. The recovery patterns of 
DNA synthesis correlated with the rapid disappearance of mAMSA-induced complexes 
(less than 27% lesions remaining 2 h after drug removal) and the persistence of 
mitoxantrone-induced complexes during a 4-h posttreatment period. This 
difference in complex longevity was observed in other human transformed 
fibroblast cell lines irrespective of differences in the absolute levels of 
complexes induced by either agent. We suggest that the results provide evidence 
that DNA intercalators may differ in the forms of complexes induced and that the 
comparatively high cytotoxicity of mitoxantrone relates to the ability of the 
drug to trap topoisomerase II complexes in a form which effects a long-term 
inhibition of DNA replication and G2 traverse.