The 1'-substituent on the anilino ring of the antitumor drug amsacrine is a critical element for topoisomerase II inhibition and cytotoxicity.

The mechanism of action of the antitumor drug amsacrine involves intercalation of the acridine chromophore into DNA and inhibition of topoisomerase II. The substituent at position 1' on the aniline is believed to be essential to the formation of the topoisomerase II/DNA cleavable complex and therefore to the cytotoxicity of the drug. To further delineate the role of the 1'-substituent, we investigated the effects on topoisomerase II activities of three anilinoacridine derivatives that differ only by the nature of the substituent at position 1'. The results of the cytotoxicity assays performed with cells sensitive (DC-3F) and resistant [DC-3F/9-hydroxy-ellipticine (9-OH-E)] to topoisomerase inhibitors are correlated with the effects of the drugs on topoisomerase II-mediated DNA cleavage in vitro. The influence of topoisomerase II alpha on the mechanism of action of the drugs was examined using resistant DC-3F/9-OH-E cells transfected with a plasmid carrying a wild-type human topoisomerase II alpha cDNA. Depending on the nature of the 1'-substituent of the drugs, the restoration of normal topoisomerase II alpha catalytic activity in human topoisomerase II alpha cDNA-transfected DC-3F/9-OH-E cells either does not modify the susceptibility of the cells to the drug or partially reverses the resistance phenotype. The molecular and cellular studies reveal that topoisomerase II alpha is implicated in the cytotoxicity of amsacrine and confirm that the substituent at position 1' on the anilino ring of amsacrine governs the interaction with topoisomerase II.