XK489 is a synthetic quinoxaline phenoxypropionic acid derivative with proapoptotic and antiproliferative activities. R(+)XK469 selectively inhibits topoisomerase II-beta, blocks activation of MEK/MAPK signaling kinases, stimulates caspases, and upregulates p53-dependent proteins, including cyclins A and B1, thereby arresting cancer cells in the G2/M phase of the cell cycle. Both R(+) and S(-) isomers of this agent are cytotoxic, although the R-isomer is more potent.