Pyrazoloacridine (PA), an acridine congener with an unknown mechanism of action, has shown selective activity against solid tumor cells, cytotoxicity in noncycling and hypoxic cells, and promising antitumor activity in Phase I clinical trials. In the present study, the effect of PA on topoisomerase (topo) activity was evaluated using yeast strains lacking functional topo I or II, mammalian cell nuclear extracts, purified samples of mammalian topo I and topo II, and intact mammalian tissue culture cells. Clonogenic assays revealed that PA cytotoxicity in yeast strains was unaffected by selective loss of topo I or topo II activity. On the other hand, enzyme assays revealed that 2-4 microM PA abolished the catalytic activity of both topo I and topo II in vitro. In contrast to topotecan and etoposide, PA did not stabilize covalent topo-DNA complexes. Instead, PA inhibited topotecan-induced stabilization of covalent topo I-DNA complexes and etoposide-induced stabilization of topo II-DNA complexes in vitro and in intact cells. Consistent with these results, colony-forming assays indicated that short-term PA exposure inhibited the cytotoxicity of topotecan and etoposide, whereas prolonged PA exposure was itself toxic to these cells. Accumulation studies revealed that PA was concentrated as much as 250-fold in drug-treated cells, resulting in intranuclear concentrations that far exceeded those required to inhibit topo I and topo II. Collectively, these results not only suggest that PA can target both topo I and topo II at clinically achievable concentrations but also indicate that its mechanism is distinct from topo I and topo II poisons presently licensed for clinical use.

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