Analysis of topoisomerase I/DNA complexes in patients administered topotecan

As part of a Phase II clinical trial of topotecan, DNA breakage in vivo was measured by detecting covalent topoisomerase/DNA intermediates in peripheral blood. The ICE (in vivo complex of enzyme) bioassay was used to assess topotecan activity in the peripheral blood of patients before, during, and after infusion therapy. The results can be summarized as: (a) ICE bioassay is a specific, antibody-based assay for topoisomerase I-mediated DNA damage. Topoisomerase I/DNA complex formation can be monitored unambiguously in the absence of topotecan to establish a basal level of endogenous enzyme action on DNA; (b) infusion of topotecan significantly stimulated formation of covalent enzyme/DNA complexes. Complexes were detected within 5 min postinfusion and increased over the course of a 30-min treatment; (c) after termination of infusion, complex formation decreased by 3-4-fold within 30 min, showing that cleavage complexes quickly reseal after drug withdrawal; and (d) formation of complexes varied widely between patients. The ICE bioassay can evaluate the effects of topoisomerase I inhibitors on target tissues; thus, it may valuable in predicting response to these drugs.