By screening 1990 compounds from the NCI diversity set library against human topoisomerase II α, we have identified NSC 35866 - a C6-substituated analog of guanine as being an inhibitor human topoisomerase II α. In subsequent pharmacological characterization this compound was found to inhibit both the DNA strand passage and ATPase reactions of human topoisomerase II α. In ATPase assays NSC 35866 primarily inhibited the DNA-stimulated fraction of topoisomerase II ATPase activity. NSC 35866 was also found to stabilize a salt-stable non-covalent complex of human topoisomerase II α on DNA in an ATP dependent manner, but did not form detectable levels of covalent cleavage complexes with DNA. In accordance with these biochemical data NSC 35866 could antagonize etoposide-induced cytotoxicity and DNA breaks in human small cell lung cancer (SCLC) OC-NYH cells. NSC 35866 was also found to induce a salt stable complex of topoisomerase II α on DNA in SCLC OC-NYH cells as determined in band depletion assay. Together these data suggest that NSC 35866 functions as a catalytic topoisomerase II inhibitor. Structure-activity studies are ongoing in order to determine in more detail the mechanism of topoisomerase II inhibition by substituted purine analogs.

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