Protein-linked DNA strand breaks induced by NSC 314622, a novel noncamptothecin topoisomerase I poison

NSC 314622 was found to have a cytotoxicity profile comparable to the topoisomerase I (top1) inhibitors camptothecin (CPT) and saintopin in the National Cancer Institute In Vitro Anticancer Drug Discovery Screen using the COMPARE analysis. In vitro data showed that NSC 314622 induced DNA cleavage in the presence of top1 at micromolar concentrations. Cleavage specificity was different from CPT in that NSC 314622 did not cleave all sites induced by CPT whereas some sites were unique to the NSC 314622 treatment. Top1-induced DNA cleavage was also more stable than cleavage induced by CPT. NSC 314622 did not induce DNA cleavage in the presence of human topoisomerase II. High concentrations of NSC 314622 did not produce detectable DNA unwinding, which suggests that NSC 314622 is not a DNA intercalator. DNA damage analyzed in human breast carcinoma MCF7 cells by alkaline elution showed that NSC 314622 induced protein-linked DNA single-strand breaks that reversed more slowly than CPT-induced strand breaks. CEM/C2, a CPT-resistant cell line because of a top1 point mutation [Cancer Res 55:1339-1346 (1995)], was cross-resistant to NSC 314622. These results demonstrate that NSC 314622 is a novel top1-targeted drug with a unique chemical structure.