[Chemical study of citrus plants in the search for cancer chemopreventive agents].

The constituents of Citrus plants were investigated to develop useful agents that are effective in cancer chemoprevention. We examined the roots and bark of the roots of various Citrus plants, resulting in the isolation of many novel compounds. Their structures were determined using spectroscopic methods, especially 2D-NMR spectra. The following new compounds of constituents were found: dimeric coumarins, dimeric acridone alkaloids, and acridone-coumarin dimers. Auraptene and nobiletin are known as useful constituents of the peel of Citrus plants for cancer chemoprevention. However, we found that 3,5,6,7,8,3',4'-heptamethoxyflavone (HPT) has both antitumor promotion and initiation activities and is more effective than auraptene and nobiletin. We synthesized pentaallyl quercetin (QPA), a useful antitumor compound that has the additional effects of a P-glycoprotein (P-gp) inhibitor. The inhibitory effects of QPA on P-gp were more effective than those of the typical P-gp inhibitors cyclosporin A and verapamil. Both HPT and QPA have antitumor promotion activity and are potential candidates for effective multidrug resistance agents in cancer chemotherapy.