The
DNA-intercalating
antitumor drug NB-506 is a potent topoisomerase
poison currently undergoing phase I/II clinical trials. It contains a planar indolocarbazole chromophore substituted with a
glucose residue. Up until now, it was thought that intercalation of the
drug into
DNA was essential for the stabilization of
topoisomerase I-
DNA covalent complexes. But, in the present study, we show that a regio-isomeric form of
NB-506 has lost its capacity to intercalate into
DNA, but remains an extremely potent
topoisomerase I poison. The new analogue contains two
hydroxyl groups at positions 2,10 instead of positions 1,11 in
NB-506. The relocation of the two
OH groups reduces considerably the strength of binding to
DNA and prevents the
drug from intercalating into the
DNA double helix. However, the
topoisomerase I inhibition capacity of the new analogue remains very high. The two
drug isomers are equally potent at maintaining the integrity of the
topoisomerase I-
DNA covalent complexes, but stimulate cleavage at different sites on
DNA.
NB-506 stabilizes
topoisomerase I preferentially at sites having a
pyrimidine (T or C) and a G on the 5' and 3' sides of the cleaved bond, respectively. The 2,10-isomer induces
topoisomerase I-mediated cleavage only at TG sites and, thus, behaves exactly as the reference
topoisomerase I poison camptothecin. Finally, cytotoxicity measurements performed with a panel of murine and human
cancer cell lines reveal that the newly designed
drug is considerably (up to 100-fold) more toxic to
tumor cells than the parent
drug NB-506. We conclude that the
DNA-binding and
topoisomerase I poisoning activities of
NB-506 can be viewed as two separate mechanisms.