EFFECT OF PYRAZOLOACRIDINE (NSC-366140) ON DNA TOPOISOMERASE-I AND TOPOISOMERASE-II

Pyrazoloacridine (PA), an acridine congener with an unknown mechanism of action, has shown selective activity against solid tumor cells, cyt otoxicity in noncycling and hypoxic cells, and promising antitumor act ivity 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, puri fied samples of mammalian topo I and topo II, and intact mammalian tis sue culture cells, Clonogenic assays revealed that PA cytotoxicity in yeast strains was unaffected by selective loss of topo I or topo II ac tivity, On the other hand, enzyme assays revealed that 2-4 mu M PA abo lished the catalytic activity of both topo I and topo IT in vitro, In contrast to topotecan and etoposide, PA did not stabilize covalent top o-DNA complexes, Instead, PA inhibited topotecan-induced stabilization of covalent topo I-DNA complexes and etoposide-induced stabilization of topo II-DNA complexes ill vitro and in intact cells, Consistent wit h these results, colony-forming assays indicated that shortterm PA exp osure inhibited the cytotoxicity of topotecan and etoposide, whereas p rolonged PA exposure was itself toxic to these cells, Accumulation stu dies revealed that PA was concentrated as much as 250-fold in drug-tre ated cells, resulting in intranuclear concentrations that far exceeded those required to inhibit topo I and topo II, Collectively, these res ults not only suggest that PA can target both topo I and topo II at cl inically achievable concentrations but also indicate that its mechanis m is distinct from topo I and topo II poisons presently licensed for c linical use.