A method of inhibiting topoisomerase I mediated DNA cleavage including administering a pharmaceutically effective amount of a compound having the following formula or a pharmaceutically acceptable salt thereof: wherein R1 and R2 are independently the same or different and are hydrogen, an alkyl group, an aminoalkyl group, an alkylaminoalkyl group, a haloalkyl group, a hydroxyalkyl group, an alkenyl group, an alkynyl group, an alkoxyl group, an aryloxy group, a carbamoyloxy group, a halogen, a hydroxyl group, a nitro group, a cyano group, an azido group, a formyl group, a hydrazino group, —C(O)Rf, an amino group, an alkylamino group, a dialkylamino group, —SRc, or R1 and R2 together form a group of the formula —O(CH2)nO— wherein n represents the integer 1 or 2; R3 is H, F, a halogen atom, a nitro group, an amino group, a hydroxyl group, or a cyano group; or R2 and R3 together form —O(CH2)nO— (n=1 or 2); R4 is H, F, a C1-3 alkyl group, a C2-3 alkenyl group, a C2-3 alkynyl group, or a C1-3 alkoxyl group; R5 is a C1-10 alkyl group, or a propargyl group.