In an attempt to characterize and overcome tumor cell resistance to amsacrine (m-AMSA), we studied the structure-activity relationships for amsacrine and seven of its analogs. Using the human leukemic cell line, CCRF-CEM, and its derivatives that express either P-glycoprotein (Pgp)-associated multidrug resistance (MDR) (CEM/VLB100) or altered topoisomerase II-associated MDR (at-MDR) (CEM/VM-1), we assessed antitumor effects of these drugs in a 48-hr growth inhibition assay. We also measured drug-topoisomerase II interactions in an intact cell assay that permits quantitation of drug-stabilized DNA-topoisomerase II complexes. We found that among the tested compounds, amsacrine has an intermediate effect on cell growth in all three cell lines. The CEM/VM-1 cells were 8.6-fold cross-resistant to m-AMSA, and the cross-resistance to the analogs was from 3.0- to 10.5-fold. In the CEM/VLBR100 cells, the resistance pattern was different: several analogs, including amsacrine, showed little or no cross-resistance (0.5- to 2.8- fold), whereas for those compounds with substituants at position 3 on the acridine ring, resistance was relatively higher (9.9- or 16.2-fold). Substituents at this position substantially decrease the lipophilicity of the two compounds examined, probably because they both contain amino groups that would be charged at physiologic pH. Compound 12489, having a 1′-NHSO2C6H4NH2 substituent, was very potent in the three cell lines, showing only a slightly higher IC50 value in the CEM/VM-1 line and a lower IC50 value in the CEM/VLB100 and in the CEM cells. In accordance with their growth-inhibitory properties, the compounds produced a range of effects on the stimulation (i.e., stabilization) of DNA-topoisomerase II complexes in intact CEM cells, whereas they had minimal effects in CEM/VM-1 cells. Dose-response experiments revealed that the stimulatory effect was concentration-dependent and biphasic for all the compounds. The results of the DNA-protein complex formation assays confirm that for the compounds tested, there is a relationship between their ability to stimulate DNA-protein complex formation at low drug concentrations and their growth-inhibitory action. Molecular modeling studies using energy-minimized structures of m-AMSA and selected amsacrine analogs revealed subtle changes that could have marked effects on their DNA binding geometries, and may account for their different biological activities. We conclude that certain substitutions on the anilinoacridine ring have a nearly 30-fold effect on the biological potency of these agents, and make some of these compounds susceptible to Pgp-mediated resistance. © 1993.