Methods of using a compound, its pharmaceutically acceptable salts, and / or its pro-
drug esters, in isolated form, to treat 
cancer, and methods for isolating, for formulating, and for administering the compound, salt, and / or pro-
drug ester as an antitumor agent, wherein the compound, salt, or pro-
drug ester has the following structure: wherein:R1, R2, R5, R7, and R8 are each separately selected from the group consisting of a 
hydrogen atom, a 
halogen atom, and saturated C1-C24 
alkyl, unsaturated C1-C24 alkenyl, cycloalkyl, cycloalkenyl, alkoxy, cycloalkoxy, 
aryl, substituted 
aryl, heteroaryl, substituted heteroaryl, amino, substituted amino, nitro, substituted nitro, phenyl, and substituted phenyl groups,R3, R4, and R6 are each separately selected from the group consisting of a 
hydrogen atom, a 
halogen atom, and saturated C1-C12 
alkyl, unsaturated C1-C12 alkenyl, cycloalkyl, alkoxy, cycloalkoxy, 
aryl, substituted aryl, heteroaryl, substituted heteroaryl, amino, substituted amino, nitro, and substituted nitro groups,X1 and X2 are separately selected from the group consisting of an 
oxygen atom, and a 
sulfur atom, andthe dashed bond represents a bond selected from the group consisting of a carbon-carbon 
single bond and a carbon-carbon 
double bond. Most preferably, R3 and R4 are 
hydrogen, and each are involved in hydrogen bonds, and / or the dashed bond is a 
double bond, such that the chemical backbone of the compound substantially retains a substantially planar conformation.