This chemotherapy regimen is a two-pronged attack at the way cancer cells replicate. R115777 is a compound that may inhibit cancer cell growth by interfering with the p21 ras oncogene, while topotecan, a topoisomerase-1 inhibitor, works on cancer cells not subject to control by the ras oncogene. Animal studies suggest that the combination may be synergistic. Another advantage is that R115777 can be taken by mouth.