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 mortality in the world and the second leading cause of neoplastic death in the United States. A major obstacle in the chemotherapy of this neoplasm is the emergence of multidrug resistance that is frequently associated with the expression of P-glycoprotein (p170) encoded by MDR1 (also known as PGY1) genes. Previously, we demonstrated that liposome-encapsulated doxorubicin is more cytotoxic than free doxorubicin in human promyelocytic leukemia and human breast cancer cells with the multidrug-resistant phenotype. Purpose: Our purpose was to investigate modulation of multidrug resistance by liposome-encapsulated vincristine (VCR) in a drug-resistant human colon cancer cell line HT-29^mdr1 and the potentiation of this modulation in combination with monoclonal antibody MRK-16 or verapamil. Methods: HT-29 parental cells and HT-29^mdr1 cells were exposed to free VCR or liposome-encapsulated VCR alone or in combination with MRK-16 or verapamil. Cytotoxicity of cells after various treatments was determined by neutral red staining, and cellular content of VCR was measured by using radiolabeled VCR; p170 expression of cells was assessed by azidopine. Results: HT-29^mdr1 cells express a high amount of p170, thus conferring sixfold to sevenfold resistance to VCR compared with the parent cell line. Liposome-encapsulated VCR lowers drug resistance in HT-29^mdr1 cells fourfold; IC₅₀ values (concentration that causes 50% reduction in cell number) were 12.5 ± 2.5 ng/mL compared with 42.5 ± 5.0 ng/mL with free VCR. IC₅₀ values for free VCR with empty liposomes were 25 ± 1.25 ng/mL. The combination of MRK-16 and free VCR produced a twofold increase in cytotoxicity over free VCR in p170-expressing cells; the combination of MRK-16 and liposome-encapsulated VCR produced a 10-fold potentiation of cytotoxicity. Nonspecific monoclonal antibody NR-LU-10 had no effect on cytotoxicity of HT-29^mdr1 cells with free VCR or liposome-encapsulated VCR. The combination of 1.5 μM verapamil potentiated the cytotoxicity of free VCR ninefold to 10-fold, 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, demonstrating a 15- to 17-fold potentiation of cytotoxicity. There were no significant differences in drug accumulation in HT-29^mdr1 cells when treated with liposome-encapsulated VCR or free VCR. Liposomes inhibited the photoaffinity labeling of azidopine to p170 HT-29^mdr1 cells. Conclusions: Liposome encapsulation of VCR effectively modulates multidrug resistance in human colon cancer cells and may become an important modality in treatment for colon cancers. [J Natl Cancer Inst 87:123-128, 1995]