Enhancement of cellular accumulation of cyclosporine by anti-P-glycoprotein monoclonal antibody MRK-16 and synergistic modulation of multidrug resistance.

Abstract

Drug resistance is a major obstacle to successful cancer chemotherapy. P-glycoprotein, which transports certain antitumor agents out of resistant tumor cells, is known to be a major factor in some types of multidrug resistance. Studies have shown that verapamil and the immunosuppressors cyclosporine and FK-506 can reverse multidrug resistance in vitro and in vivo and that the P-glycoprotein monoclonal antibody MRK-16 increases drug toxicity in multidrug-resistant tumors. The purpose of this in vitro study was to establish effective treatment modalities for overcoming multidrug resistance. We assessed the synergistic effects of verapamil, cyclosporine, or FK-506 in combination with MRK-16 and antitumor agents. Human myelogenous leukemia K562 cells and multidrug-resistant K562/ADM cells were treated with vincristine or doxorubicin combined with MRK-16 and cyclosporine alone or together; MRK-16 and verapamil alone or together; or MRK-16 and FK-506. The effects of MRK-16 and cyclosporine or verapamil on the accumulation of vincristine and doxorubicin were examined in K562/ADM cells, and the mechanisms of action were analyzed. MRK-16 and cyclosporine synergistically enhanced the antitumor effects of vincristine and of doxorubicin in K562/ADM cells. However, the combined use of MRK-16 with verapamil or FK-506 did not show such synergistic effects in these cells. Studies of the effect of MRK-16 on cellular accumulation of cyclosporine and verapamil revealed that MRK-16 substantially increased accumulation of cyclosporine in K562/ADM cells, but did not increase accumulation of verapamil. MRK-16 and cyclosporine synergistically enhanced the antitumor effects of vincristine and doxorubicin because MRK-16 increased cellular accumulation of cyclosporine. These results, together with our previous finding that intravenous administration of MRK-16 induced regression of multidrug-resistant subcutaneous tumors in athymic mice, support the hypothesis that the combined use of MRK-16 and cyclosporine might increase the efficacy of antitumor agents against multidrug-resistant tumors expressing P-glycoprotein. Clinical phase I trials of MRK-16 in the treatment of multidrug-resistant tumors are under consideration.