Abstract: Three pairs of C-2 epimeric xanthones isolated from Garcinia hanburyi and method for efficiently separating the xanthone compounds into individual epimers, each of which possesses varying biological effects. The compounds are useful for their anticancer effects, particularly because they are shown to be non-substrates of the multidrug-resistance transporter. Some of the epimers have significant inhibitory effects on cytochrome P450 systems. The xanthone compounds of the present invention are gambogic acid, epigambogic acid, isogambogic acid, isoepigambogic acid, 30-hydroxygambogic acid and 30-hydroxyepigambogic acid.