The camptothecin-resistant topoisomerase I mutant F361S is cross-resistantto antitumor rebeccamycin derivatives. A model for topoisomerase I inhibition by indolocarbazoles

DNA topoisomerase I is a major cellular target for antitumor indolocarbazol e derivatives (IND) such as the antibiotic rebeccamycin and the synthetic a nalogue NB-506 which is undergoing phase I clinical trials. We have investi gated the mechanism of topoisomerase I inhibition by a rebeccamycin analogu e, R-3, using the wild-type human topoisomerase I and a well-characterized recombinant enzyme, F361S. The catalytic activity of this mutant remains fu lly intact, but the enzyme is resistant to inhibition by camptothecin (CPT) . Here we show that the mutated enzyme is cross-resistant to the rebeccamyc in analogue. Despite their profound structural differences, CPT and R-3 int erfere similarly with the activity of the wild-type and mutant topoisomeras e I enzymes, and the drug-induced cleavable complexes are equally sensitive to the NaCl concentration. CPT and IND likely recognize identical structur al elements of the topoisomerase I-DNA covalent complex; however, differenc es do exist in terms of sequence-specificity of topoisomerase I-mediated DN A cleavage. For the first time, a molecular model showing that CPT and IND share common steric and electronic features is proposed. The model helps to identify a specific pharmacophore for topoisomerase I inhibitors.