Resistance of tumor cells to chemotherapy drugs is an important factor in the failure of tumor therapy.Among them,drug efflux caused by overexpression of P-glycoprotein (P-gp),a member of ATP-binding cassette transporter,is one of the main mechanisms of MDR.In this study,six known neo-clerodane diterpenoids were isolated from Scutellaria barbata.Using modern separation and purification methods,their structures were elucidated as scutebarbatine (1),scutebarbatine B (2),suctebartine F (3),clerdinin B (4),scutellin A (5) and scutehennanine D (6).Among them,Compound 4 was isolated from Scutellaria barbata for the first time.In addition,the reversal effect of compounds on the resistance to adriamycin (Adr) in HepG2/ADR cells was evaluated.The results showed that compound 1,2,3,6(20 μM) had reversal activity when it was combined with adriamycin(Adr),with reversal index (RI) values from 14.04 to 39.42.Western blot analysis indicated that P-glycoprotein expression in HepG2/Adr cells was significantly higher than that in sensitive strains,which might be the main factor causing drug resistance.Fluorescence results showed that the compounds could significantly promote the accumulation of Adr in HepG2/Adr cells.However,the compounds did not affect the P-glycoprotein expression.These results indicated that compound 1,2,3 and 6 might reverse tumor multidrug resistance by inhibiting the efflux function of P-glycoprotein.