In vitro cytotoxicity of Stealth liposomes co-encapsulating doxorubicin and verapamil on doxorubicin-resistant tumor cells

Multidrug resistance (MDR) is a major obstacle to successful clinical cancer chemotherapy. A novel doxorubicin anti-resistant Stealth liposomes (DARSLs), prepared by co-encapsulating doxorubicin (DOX) and verapamil (VER) into stealth liposomes, has been developed. The average particle size of DARSLs was 118.1 +/- 22.3 nm. Encapsulation efficiencies of DOX and VER in DARSLs were greater than 95% and 70%, respectively. The IC50 of DARSLs as measured by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay in multidrug resistant rat prostate cancer Mat-LyLu-B2 (MLLB2) cells was 0.079 +/- 0.017 mu M, 13 fold less than that for liposomal DOX with free VER (LDFV 0.96 +/- 0.46 mu M) but only about 2 times less than FDFV. The IC50 cytotoxicity on MLLB2 cells of the various formulations was as follows: DARSLs similar to LDLV < FDFV < FDLV < LDFV < LD < FD, (LD: liposomal DOX; LV: liposomal VER; FD: free DOX; FV: free VER). Similar cytotoxicities were shown between DARSLs and FDFV in DOX-resistant human uterus sarcoma MES-SA/DX5 cells, reversing DOX-resistance to that shown by FD on DOX-sensitive MES-SA cells. For MLLB2 cells, DARSLs was the most cytotoxic, but its intracellular concentration of DOX, measured as mean cellular fluorescence with flow cytometry was lower (p < 0.01) than that observed with the FDFV formulation. In conclusion, DARSLs was an effective DOX formulation which could overcome drug resistance in DOX-resistant tumor cells, but its mechanisms of action may be complex.