Inhibition of DNA topoisomerases II and/or I by pyrazolo[1,5-a]indole derivatives and their growth inhibitory activities

DNA topoisomerases (topos) I and II are molecular targets of several potent anticancer agents. Thus inhibitors of these enzymes are potential candidates or model compounds for anticancer drugs. We found some of the totally synthetic pyrazolo[ 1,5-a] indole derivatives, GS-2, -3, and -4, to be strong inhibitors of topo II, and GS-5 was found to be a dual inhibitor of topos I and II (IC50 values were in the range of 10-30 muM). Because of the DNA-intercalating activity of these compounds affecting supercoil structure of closed circular DNA, the method of evaluation of topo I inhibition designed for such compounds by Pommier et al. (Nucleic Acids Res 15: 6713-6731, 1987) was employed. Results showed that only GS-5 with a hydroxyl group at position C-6 was found to be a strong inhibitor of topo I with an IC50 of similar to10 muM. Inhibition of topo I and/or topo II by these compounds does not involve significant accumulation of DNA-topo I/II cleavable complexes, demonstrating that they are not topo poisons but catalytic inhibitors. In the "band depletion" analysis for in vivo targeting of topo I and II, these compounds were shown to suppress depletion of intracellular free enzymes by the topo poisons etoposide and/or camptothecin, indicating that they do target topo I and/or II in living cells. These compounds also exhibit moderate to strong growth-inhibitory activity in panels of human cancer cell lines. This study shows pyrazolo[ 1,5-a] indole derivatives to be a novel group of anticancer chemotherapeutic agents with single or dual catalytic inhibitory activities against topo I and topo II.