Topoisomerase I-related Parameters and Camptothecin Activity in the Colon Carcinoma Cell Lines from the National Cancer Institute Anticancer Screen¹

Camptothecin (CPT) derivatives are a new family of anticancer agents which are selective inhibitors of DNA topoisomerase I (top1) and have entered clinical trials with promising results. The cellular determinants for CPT activity were studied in the seven cell lines of the National Cancer Institute anticancer screen. These cell lines exhibit natural differences in sensitivity to CPT and can be divided into three groups, according to their increasing resistance: colo205, SW620, HCT116<HT29, HCC2998<HCT15, and KM12. The differential sensitivity range was approximately 17-fold between KM12 and colo205 cells. CPT uptake varied only by less than a factor of three among the cell lines. Top1 mRNA, measured by Northern blotting analysis, and top1 protein levels, measured by Western blotting, varied by 2-fold or less among the cell lines and were correlated neither with the CPT cytotoxicity nor the levels of cleavable complexes measured by alkaline elution in the various cell lines. An overall log-linear correlation was observed between CPT-induced top1-cleavable complexes and growth inhibition, indicating the importance of cleavable complex formation rather than top1 levels for cell killing in this panel of cell lines. Also, some cell lines displayed marked growth inhibition differences with minimal differences in cleavable complexes and S-phase fraction, suggesting that parameters downstream from the cleavable complexes are also critical for CPT cytotoxicity.