8α,9α-Epoxycoleon-U-quinone (compound 3) is a p-glycoprotein (P-gp) regulator that is selective for cancer cells ( SI=2.0 ). 8α,9α-Epoxycoleon-U-quinone effectively inhibits P-gp activity in NCI-H460/R cells. 8α,9α-Epoxycoleon-U-quinone also reverses the resistance of cancer cells to Doxorubicin (DOX) and enhances the anticancer effect of DOX.