DIFFERENCES IN INHIBITION OF CHROMOSOME SEPARATION AND G(2) ARREST BYDNA TOPOISOMERASE-II INHIBITORS MERBARONE AND VM-26

Merbarone, a novel DNA topoisomerase II (topo II) inhibitor, differs f rom teniposide (VM-26) in that it inhibits topo II activities without stabilizing topo II-DNA covalent complexes. Thus, while the cellular e ffects of VM-26 are the consequences of inhibition of topo II catalyti c activities and generation of topo II-mediated DNA damage, those of m erbarone may be due to inactivation of topo II catalytic function. To address the issues of mechanisms of cell cycle effects and pharmacolog ical actions of these two topo II inhibitors in mammalian cells, we us ed synchronized cultures of HeLa cells to study the effects of these d rugs on cell cycle processes where topo II is essential (e.g., chromos ome separation) or possibly involved (e.g., G(2) arrest, DNA replicati on). We found that both drugs inhibited chromosome separation and cell division without preventing cells from exiting mitosis. Both drugs ca used S-phase retardation, G(2) arrest, and phase-specific cytotoxicity in that they are more toxic to S, M, and G(2) cells than G(0)/G(1) ce lls. However, merbarone produced the above effects in convergent dosag es that were within one to five times its 90% inhibitory cytotoxic con centration, whereas the concentrations of VM-26 to cause quantitativel y similar effects were quite divergent. VM-26 is 50-100-fold more effi cient in causing G(2) arrest than in inhibiting chromosome separation. Furthermore, at concentrations showing similar levels of S-phase supp ression, VM-26 caused significant DNA breaks, while merbarone had no s uch effect. Our data suggest that the effects of merbarone and VM-26 d uring mitosis are most likely due to inhibition of topo II function. W e conclude that while G(2) arrest by VM-26 is related to topo II-media ted DNA damage and its sequelae, G(2) arrest by merbarone likely resul ts from different mechanisms.