REVERSAL OF MULTIDRUG-RESISTANCE IN HUMAN COLON-CANCER CELLS EXPRESSING THE HUMAN MDR1 GENE BY LIPOSOMES IN COMBINATION WITH MONOCLONAL-ANTIBODY OR VERAPAMIL

Background: colorectal cancer is a major cause of cancer-related morta lity in the world and the second leading cause of neoplastic death in the United States. A major obstacle in the chemotherapy of this neopla sm is the emergence of multidrug resistance that is frequently associa ted with the expression of P-glycoprotein (p170) encoded by MDR1 (also known as PGY1) genes. Previously, we demonstrated that liposome-encap sulated doxorubicin is more cytotoxic than free doxorubicin in human p romyelocytic leukemia and human breast cancer cells with the multidrug -resistant phenotype. Purpose: Our purpose was to investigate modulati on of multidrug resistance by liposome-encapsulated vincristine (VCR) in a drug-resistant human colon cancer cell line HT-29mdr1 cells were exposed to free VCR or liposome-encapsulated VCR alone or in combinati on with MRK-16 or verapamil. Cytotoxicity of cells after various treat ments was determined by neutral red staining, and cellular content of VCR was measured by using radiolabeled VCR;p170 expression of cells wa s assessed by azidopine. Results: HT-29mdr1 cells express a high amoun t of p170, thus conferring sixfold to sevenfold resistance to VCR comp ared with the parent cell line. Liposome-encapsulated VCR lowers drug resistance in HT-29mdr1 cells fourfold; IC50 values (concentration tha t causes 50% reduction in cell number) were 12.5 +/- 2.5 ng/mL compare d with 42.5 +/- ng/mL with free VCR. IC50 values for free VCR with emp ty liposomes were 25 +/- 1.25 ng/mL. The combination of MRK-16 and fre e VCR produced a twofold increase in cytoxicity over free VCR in p170- expressing cells; the combination of MRK-16 and liposome-encapsulated VCR produced a 10-fold potentiation of cytoxicity. Nonspecific monoclo nal antibody NR-LU-10 had no effect on cytotoxicity of HT-29mdr1 cells with free VCR or liposome-encapsulated VCR. The combination of 1.5 mu M verapamil potentiated the cytotoxicity of free VCR ninefold to 10-fo ld, IC50 values reduced to 5.0 +/- 1.5 ng/mL, and in combination with liposome-encapsulated VCR, IC50 values reduced to 2.5 +/- 1.0 ng/mL, d emonstrating a 15- to 17- fold potentiation of cytotoxicity. There wer e no significant differences in drug accumulation in HT-29mdr1 cells w hen treated with liposome-encapsulated VCR or free VCR. Liposomes inhi bited the photo affinity labeling of azidopine to p170 HT-29mdr1 cells . Conclusions: Liposome encapsulation of VCR effectively modulates mul tidrug resistance in human colon cancer cells and may become an import ant modality in treatment for colon cancers.