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 -met hylenedioxy-camptothecin (7-CM-MDO-CPT) was recently shown to produce irreversible topoisomerase 1 (top1) cleavage complexes by binding to t he +1 base of the scissile strand of a top1 cleavage site. We demonstr ate 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-c amptothecin (7-E-MDO-CPT) also induced reversible top1 cleavable compl exes, which were markedly more stable to salt-induced reversal than th ose induced by 7-ethyl-10-hyroxy-CPT, the active metabolite of CPT-11. This greater stability of the top1 cleavable complexes was contribute d by the 7-alkyl and the 10,11-methylene- (or ethylene-) dioxy substit utions. 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 m ore cytotoxic than CPT. The reversal of the cleavable complexes induce d by 7-E-MDO-CPT, 7-CM-MDO-CPT, and 7-CM-EDO-CPT was markedly slower a fter 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, an d 7-CM-EDO-CPT were cytotoxic irrespective of the presence of 10 mu M 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 l asting top1 cleavable complexes and greater cytotoxicity than CPT in c ells.