neo-Clerodane diterpenoids from Scutellaria barbata mediated inhibition of P-glycoprotein in MCF-7/ADR cells.

Ten new (1-10) and seventeen known (11-27) neo-clerodane diterpenoids substituted with nicotinoyloxyl were isolated from the plant Scutellaria barbata and their structures were established by extensive spectroscopic analysis. Chemoreversal effects of these neo-clerodane diterpenoids on multidrug resistance were evaluated in breast cancer multidrug-resistant MCF-7/ADR cells that overexpress P-glycoprotein. Four compounds (11, 14, 16, and 18) exhibited better chemoreversal abilities than the classical P-gp inhibitor verapamil and the most potent compound 11 reduced IC50 value of adriamycin in MCF-7/ADR cells from 58.8 μM to 1.3 μM. Mechanistic investigations showed that compound 11 reversed multidrug resistance through suppressing the activity of P-gp and restraining the expression of P-glycoprotein. In the present study, the structure-activity relationships of neo-clerodane diterpenoids were also discussed.