Salvicine, a novel DNA topoisomerase II inhibitor, exerting its effects by trapping enzyme-DNA cleavage complexes.

Salvicine, a structurally modified diterpenoid quinone derived from Salvia prionitis, is a novel anticancer drug candidate. The compound has significant in vitro and in vivo activity against malignant tumor cells and xenografts, especially some human solid tumor models. This anticancer activity of salvicine is associated with its ability to induce tumor cell apoptosis. Salvicine was also found to have a profound cytotoxic effect on multidrug-resistant (MDR) cell lines by down-regulating the expression of MDR-1 mRNA of MDR cells. Salvicine acted as a topoisomerase II (Topo II) poison through its marked enhancement effect on Topo II-mediated DNA double-strand breaks as observed in the DNA cleavage assay. Strong inhibitory activity of salvicine against Topo II was observed in a kDNA decatenation assay, with an approximate IC(50) value of 3 microM. A similar result was obtained by a Topo II-mediated supercoiled DNA relaxation assay. In contrast, no inhibitory activity was observed against the catalytic activity of Topo I. When the effects of salvicine on individual steps of the catalytic cycle of Topo II were dissected, it was found that the mechanism by which salvicine inactivates Topo II is different from that by other anti-Topo II agents. Salvicine greatly promoted Topo II-DNA binding and inhibited pre- and post-strand Topo II-mediated DNA religation without interference with the forward cleavage steps. In addition, salvicine was not a DNA intercalative agent, as demonstrated by DNA unwinding assays. The results of this study indicate that the inhibitory activity of salvicine against Topo II was derived from its ability to stabilize DNA strand breaks through interactions with the enzyme alone or with the DNA-enzyme complex. It is therefore postulated that salvicine acts on Topo by trapping the DNA-Topo II complex, which in turn produces anticancer effects.