A series of 13 anthrapyrazole (AP) compounds that were analogs of piroxantrone and losoxantrone were synthesized. Their cell growth inhibitory effects, DNA binding and inhibition of DNA topoisomerase IIα (topo II) decatenation catalytic activities were determined. The IC50s for cell growth inhibition of the various analogs ranged from 0.1 to 9 μM with AP10 being the most active compound. The change in DNA melting temperature (ΔTm) at a drug concentration of 2 μM was used as an indication of strength of DNA binding. While some of the analogs bound DNA more strongly than the doxorubicin control, others bound DNA more weakly. QSAR analysis showed that there was a good correlation of cell growth inhibitory activity with ΔTm suggesting that these compounds may act by targeting DNA. However, cell growth inhibition was not well correlated with inhibition of catalytic topoisomerase IIα activity suggesting that these anthrapyrazoles do not act by targeting topoisomerase II. The analogs were also examined for their ability to stabilize a DNA-topo II covalent complex and act as a topoisomerase IIα poison by measuring their ability to cleave pBR322 DNA to produce linear DNA. The analogs were not able to induce DNA cleavage and, thus, it was concluded that they did not act as topo II poisons. Also the growth inhibitory effects of the analogs on K562 and K/VP.5 cells (containing one-fifth the topo II content of the parental K562 cells) were not significantly different, a result that is also consistent with these compounds exerting their activity by mechanisms distinct from topo II poisoning. In summary these results indicate that even though the anthrapyrazoles inhibited topo II catalytic activity this may not be the mechanism by which they inhibited cell growth. Support: CIHR and a Canada Research Chair in Drug Development to BH, ROI CA90787

[Proc Amer Assoc Cancer Res, Volume 46, 2005]