CI-958, a new
DNA-intercalating
drug derived from a series of substituted 2H-[1] benzothiopyrano[4,3,2-cd]
indazoles, is being tested in clinical trails because of its curative properties against murine solid
tumor models and because it has demonstrated activity in a pilot phase II study of patients with
hormone-refractory
prostate cancer. However, the mechanism of anticancer action of
CI-958 has not been established. Because
CI-958 binds to
DNA and
DNA helicases are profoundly affected by
DNA-binding drugs, we examined the effects of
CI-958 on human
DNA helicase action.
DNA helicase activity was measured by strand dissociation of double-stranded (
ds) DNA with a gel electrophoresis assay, and
ATPase activities were determined on thin-layer chromatography by measurement of the conversion of
ATP to
ADP. For human helicase blockade,
CI-958 is slightly more potent than
doxorubicin (EC50 values 0.17 and 0.26 microM, respectively). We observed no difference in helicase-blockade EC50 values recorded for three helicase substrates containing A-T rich, G-C rich, and both types of
oligonucleotide sequences. The effects of
CI-958 helicase blockade and
DNA-dependent ATPase activities were similar for the two reactions. The kinetics of the blockade by
CI-958 of the human
DNA helicase indicates that it involves a reversible ternary complex of helicase-
drug-dsDNA.
CI-958 produces potent blockade of human
DNA helicases with no apparent strong DNA sequence-binding preference. Similar potency against helicase strand dissociation and
DNA-dependent ATPase suggests that the mechanism against these reactions is the same. The blockade of
DNA helicases by
CI-958 may be central in its mechanism of action as an anticancer
drug.