Reversal of multidrug resistance by transferrin-conjugated liposomes co-encapsulating doxorubicin and verapamil

Purpose: Liposomes co-encapsulating doxorubicin (DOX) and verapamil (VER), and conjugated to transferrin (Tf-L-DOX/VER) were synthesized and evaluated in K562 leukemia cells. The design of this formulation was aimed at selective targeting of tumor cells, reducing cardiotoxicity of DOX and VER, as well as overcoming P-glycoprotein (Pgp)-mediated multidrug resistance (MDR) phenotype. Methods: The liposomes were prepared by polycarbonate membrane extrusion, followed by pH-gradient driven remote loading and Tf conjugation. Kinetics of in vitro release of DOX and VER from liposomes was determined by measuring changes in the concentration of encapsulated drugs. Uptake of Tf-conjugated liposomes by K562 cells was evaluated by fluorescence microscopy and by fluorometry. Cytotoxicities of various formulations of DOX were determined by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolum bromide (MTT) assay. Results: Efficiencies for liposomal loading of DOX and VER were 95% and 70%, respectively. The mean particle diameter for the liposomes was approximately 110nm. Rates of release for DOX and VER were similar in singly-loaded and co-loaded liposomes. Tf-L-DOX/VER showed efficient uptake by the TfR+ K562 cells. In DOX-resistant K562 cells (K562/DOX), Tf-L-DOX/VER showed 5.2 and 2.8 times greater cytotoxicity (IC₅₀ = 4.18 μM) than non-targeted liposomes (L-DOX/VER) (IC₅₀ = 21.7 μM) and Tf-L-targeted liposomes loaded with DOX alone (Tf-L-DOX) (IC₅₀ = 11.5 μM), respectively. Conclusions: The combination of TfR targeting and co-encapsulation of DOX and VER was highly effective in overcoming drug resistance in K562 leukemia cells.