TOPOISOMERASE I-RELATED PARAMETERS AND CAMPTOTHECIN ACTIVITY IN THE COLON-CARCINOMA CELL-LINES FROM THE NATIONAL-CANCER-INSTITUTE ANTICANCER SCREEN

Camptothecin (CPT) derivatives are a new family of anticancer agents w hich are selective inhibitors of DNA topoisomerase I (top1) and have e ntered clinical trials with promising results. The cellular determinan ts for CPT activity were studied in the seven cell lines of the Nation al Cancer Institute anticancer screen. These cell lines exhibit natura l differences in sensitivity to CPT and can be divided into three grou ps, according to their increasing resistance: colo205, SW620, HCT116<H T29, HCC2998<HCT15, and KM12. The differential sensitivity range was a pproximately 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, measu red 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 cel l lines. An overall log-linear correlation was observed between CPT-in duced top1-cleavable complexes and growth inhibition, indicating the i mportance of cleavable complex formation rather than top1 levels for c ell killing in this panel of cell lines. Also, some cell lines display ed marked growth inhibition differences with minimal differences in cl eavable complexes and S-phase fraction, suggesting that parameters dow nstream from the cleavable complexes are also critical for CPT cytotox icity.