Cytotoxic Effects of Tanshinones from Salvia miltiorrhiza on Doxorubicin-Resistant Human Liver Cancer Cells

P-Glycoprotein (Pgp) overexpression and alterations in p53 oncogene expression are known to affect chemotherapeutic efficacy in the treatment of human hepatocellular carcinoma (HCC). The present study has demonstrated the anti-HCC potential of cryptotanshinone (1), dihydrotanshinone (2), tanshinone I (3), and tanshinone IIA (4), the active lipophilic constituents of Salvia miltiorrhiza, using MTT and caspase-3 activity assays and poly(ADP-ribose) polymerase cleavage in HepG2, Hep3B, and PLC/PRF/5 cells. THLE-3, a normal human immortalized liver cell line, was used to demonstrate the selective growth inhibitory effect of 3 for a HCC cell line. Compound 1 suppressed doxorubicin efflux, a process mediated by P-glycoprotein, in a Pgp-overexpressed HepG2 subclone (R-HepG2 cells). Despite its moderate cytostatic and pro-apoptotic effects and minimal influence on doxorubicin efflux, 4 provided the best synergism with doxorubicin as determined by the Combination Index, the Loewe additivity model, and the Bliss independence criterion.