DIFFERENTIAL STABILIZATION OF EUKARYOTIC DNA TOPOISOMERASE-I CLEAVABLE COMPLEXES BY CAMPTOTHECIN DERIVATIVES

Camptothecins belong to a group of anticancer agents with a specific m echanism of action: stabilization and trapping of eukaryotic DNA topoi somerase I (top1) cleavable complexes. Two water-soluble camptothecin derivatives are in clinical trial, and their anticancer activity appea rs promising: topotecan and CPT-11. The latter is hydrolyzed to its ac tive metabolite, SN-38. We have previously reported that SN-38 is amon g the most cytotoxic camptothecin derivatives and that the cleavable c omplexes induced by SN-38 are more stable than those induced by CPT in human colon carcinoma cells [Tanizawa et al. (1994) J. Natl. Cancer I nst. 86, 836-842]. Top1 inhibition was further investigated by determi ning the salt-induced religation rates of top1-cleavable complexes in fragments from the top1 cDNA. Religation depended on both the local DN A base sequence and the drug structure. Cleavable complexes induced by SN-38 and 10,11-methylenedioxycamptothecin were markedly more stable (less rapidly reversible) than those induced by CPT, topotecan, and 9- aminocamptothecin. The stability of 10-hydroxycamptothecin-induced cle avable complexes was intermediate to those of CPT and SN-38, indicatin g that both the 10-hydroxy and the 7-ethyl group of SN-38 probably int eract with the drug binding site of top1-cleavable complexes. A DNA ol igonucleotide containing a single top1 cleavage site was also used to compare the camptothecin derivatives. The salt stability of drug-induc ed cleavable complexes in the top1 oligonucleotide was correlated with the drug potencies to induce top1 cleavage. Cell killing requires tha t trapped cleavable complexes be converted to DNA damage as a result o f replication fork collision. Therefore, the persistence of cleavable complexes may have important implications for anticancer drug efficacy .