Expression Behavior of 85-kDa Membrane Protein in Adriamycin-resistant Tumor Cells and the Inhibition of Human Tumor Growth in Athymic Mice by MRK-20 Monoclonal Antibody against the Protein

Treatment of tumors with monoclonal antibodies against tumor antigen is one of the selective modalities for cancer therapy. We examined the therapeutic effect of MRK-20 (IgG₁), a murine monoclonal antibody against resistance-associated 85-kDa membrane protein. The 85-kDa protein is expressed on the surface of multidrug-resistant cells induced by adriamycin. This protein is also expressed in some multiple-drug-resistant cells, including atypical multidrug-resistant cells. The protein, once lost during long-term culture without adriamycin, was rapidly induced by treatment with adriamycin but not with vinblastine or etoposide, suggesting a close relationship of the protein with adriamycin resistance but not with multidrug resistance. The antibody MRK-20 suppressed the growth of subcutaneously implanted tumors expressing the 85-kDa protein. Adriamycin-resistant human ovarian tumor 2780AD and innately resistant human erythroleukemia HEL cells in athymic mice were completely cured by treatment with MRK-20 antibody when the antibody was administered i.v. 2 days after s.c. tumor implantation. On the other hand, MRK-20 did not show any effect on the growth of the 85-kDa protein-negative A2780 human ovarian tumor. These results indicate that the effect of MRK-20 is highly specific to cells expressing 85-kDa protein.