7- and 10-Substituted Camptothecins: Dependence of Topoisomerase I-DNA Cleavable Complex Formation and Stability on the 7- and 10-Substituents

7-Alkyl, 7-alkyl-10-hydroxy, 7-alkyl-10-methoxy, and 7-alkyl-10,11-methylenedioxy analogs of camptothecin have been synthesized and evaluated for their ability to trap human DNA topoisomerase I in cleavable complexes. The 7-alkyl chain lengths varied linearly from methyl to butyl. The concentration required to produce cleavable complexes with purified topoisomerase I in 50% of the plasmid DNA (EC50) was reduced by 1 order of magnitude by the introduction of a 10-methoxy or 7-alkyl group compared with camptothecin. The EC50 values were reduced by 2 orders of magnitude with a 10-hydroxy or 10,11-methylenedioxy moiety compared with camptothecin. The steady-state EC50 concentrations for all of the analogs tested were slightly dependent on substitution at the 7-position, but this dependence was least with the 10-methoxy series. The kinetics of the reversibility of the complexes formed with all analogs was only slightly influenced by the length of the 7-substitution, with the trend that ethyl or greater lengths led to slightly reduced rate constants for cleavable complex reversal. These results were also observed for DNA-protein cross-link formation by the analogs in isolated CEM cell nuclei. Our data indicate that in vitro cleavable complex stability, as determined by the apparent rate constants for complex dissociation, does not reflect the in vitro biological activity of these camptothecin analogs. However, complex stability in vivo may be important for the antitumor activity of the compounds.