Intercalation into DNA is not required for inhibition of topoisomerase I by indolocarbazole antitumor agents.
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.
Bailly
C
C
Laboratoire de Pharmacologie Antitumorale du Centre Oscar Lambret and Institut National de la Santé et de la Recherche Médicale U-524, IRCL, Lille, France. bailly@lille.inserm.fr
Dassonneville
L
L
Colson
P
P
Houssier
C
C
Fukasawa
K
K
Nishimura
S
S
Yoshinari
T
T
D000818:Animals; D000970:Antineoplastic Agents / Q000737:chemistry / Q000494:pharmacology*; D002227:Carbazoles / Q000737:chemistry / Q000494:pharmacology*; D004247:DNA / Q000187:drug effects* / Q000378:metabolism; D018983:DNA Footprinting; D015195:Drug Design; D005960:Glucosides / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D020128:Inhibitory Concentration 50; D007364:Intercalating Agents / Q000737:chemistry / Q000494:pharmacology; D051379:Mice; D013237:Stereoisomerism; D059004:Topoisomerase I Inhibitors; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D002227:Carbazoles; D005960:Glucosides; D007364:Intercalating Agents; C092476:NB 506; D059004:Topoisomerase I Inhibitors; D004247:DNA