A compound selected from the group consisting of 1-(2-fluoro-5-((4-oxo-3,4,5,6,7,8-hexahydrophthalazin-1-yl)methyl)phenyl)pyrrolidine-2,5-dione and 4-(3-(1,4-diazepan-1-ylcarbonyl)-4-fluorobenzyl)-5,6,7,8-tetrahydrophthalazin-1(2H)-one, or a pharmaceutically acceptable salt thereof, for use in treating leukemia, colon cancer, glioblastomas, lymphomas, melanomas, carcinomas of the breast or cervical carcinomas in a mammal by administering thereto a therapeutically acceptable amount of said compound.