NB-506 is a
topoisomerase I (top1) inhibitor in clinical trials. In this study, we used a series of
camptothecin (
CPT)-resistant cell lines with known top1 alterations. We show that three mutations in different domains of the top1
enzyme that confer
CPT resistance also confer cross-resistance to
NB-506. The
CPT-resistant cell lines and corresponding mutations were: human prostate
carcinoma cells DU-145/RC1 (mutation R364H), Chinese hamster fibroblasts DC3F/C10 (mutation G503S), and human
leukemia CEM/C2 cells (N722S). This result suggests that
NB-506 and
CPT share a common binding site in the top1-DNA complex. We next used these three cell lines and their parental cells to study the relationship between top1
poisoning by
NB-506 and antiproliferative activity. We found that the
CPT-resistant cells were only 2-10-fold resistant to
NB-506, which suggests that
NB-506 targets other cellular processes/pathways besides top1. This conclusion was further supported by the limited cross-resistance of top1-deficient murine
leukemia P388/CPT45 cells (2-fold). Cross-resistance was also limited for J-109,382, an isomer of
NB-506 that does not intercalate into
DNA, indicating that the non-top1-mediated antiproliferative activity of
NB-506 is not attributable to
DNA intercalation. Together, these data indicate that
NB-506 and indolocarbazoles are promising agents to overcome
CPT resistance.