Novel 7-alkyl methylenedioxy-camptothecin derivatives exhibit increased cytotoxicity and induce persistent cleavable complexes both with purified mammalian topoisomerase I and in human colon carcinoma SW620 cells.

An alkylating camptothecin (CPT) derivative, 7-chloromethyl-10,11-methylenedioxy-camptothecin (7-CM-MDO-CPT) was recently shown to produce irreversible topoisomerase I (top1) cleavage complexes by binding to the +1 base of the scissile strand of a top1 cleavage site. We demonstrate that 7-CM-EDO-CPT (7-chloromethyl-10,11-ethylenedioxy-camptothecin) also induces irreversible top1-DNA complexes. 7-CM-MDO-CPT, 7-CM-EDO-CPT, and the nonalkylating derivative 7-ethyl-10,11-methylenedioxy-camptothecin (7-E-MDO-CPT) also induced reversible top1 cleavable complexes, which were markedly more stable to salt-induced reversal than those induced by 7-ethyl-10-hyroxy-CPT, the active metabolite of CPT-11. This greater stability of the top1 cleavable complexes was contributed by the 7-alkyl and the 10,11-methylene- (or ethylene-) dioxy substitutions. Studies in SW620 cells showed that 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT are more potent inducers of cleavable complexes and more cytotoxic than CPT. The reversal of the cleavable complexes induced by 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT was markedly slower after drug removal than that for CPT, which is consistent with the data with purified top1. By contrast to CPT, 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT were cytotoxic irrespective of the presence of 10 microM aphidicolin. These results suggest that 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT are more potent top1 poisons than CPT and produce long lasting top1 cleavable complexes and greater cytotoxicity than CPT in cells.