Stimulation of topoisomerase II-mediated DNA cleavage by an indazole analogue of lucanthone

Lucanthone is an antitumour drug used as an adjuvant in radiation therapy. The drug intercalates into DNA and inhibits topoisomerase II. An indazole a nalogue of lucanthone (IA-5) was examined for its ability to modulate topoi somerase II-DNA cleavable complex formation in vitro. The drug contains a m ethylbenzothiopyranoindazole chromophore instead of the methyl-thioxantheno ne nucleus of lucanthone. Using a radiolabelled linear plasmid DNA as a sub strate, both lucanthone and the indazole analogue were shown to promote the cleavage of DNA by human topoisomerase II. Sequencing experiments with dif ferent restriction fragments indicated that the indazole drug promoted DNA cleavage primarily at sites having a C on the 3' side of the cleaved bond ( -1 position). By contrast, in the same sequencing methodology lucanthone ex erted a much weaker effect on topoisomerase II. The sequence selectivity of IA-5 is reminiscent of that of the anticancer drug mitoxantrone and its an thrapyrazole analogue losoxantrone, which is structurally close to IA-5. Bi nding to DNA anal topoisomerase II inhibition are two distinct processes co ntributing separately to the cytotoxic activity of the indazole drug. (C) 1 999 Elsevier Science Inc.