High-affinity monoclonal antibodies against P-glycoprotein

Cancer patients treated with one anticancer agent often develop resistance to a broad spectrum of chemotherapeutic agents. This type of multiple drug resistance (MDR) is often accompanied by a decrease in drug accumulation and an increase in expression of a 170,000-Da plasma membrane glycoprotein (P-170) that can effectively pump various anticancer agents out of cytoplasm. A panel of 12 IgG1, IgG2a, or IgG2b monoclonal antibodies was generated against the extracellular portion of P-glycoprotein by immunizing mice with a human MDR1 gene-transfected BA3T3 fibroblast line. We have characterized two of the anti-P-glycoprotein monoclonal antibodies, 15D3 and 17F9, in some detail. Both antibodies immunoprecipitate a 170- to 180-kDa protein from MDR cells, but do not block binding of the known anti-P-glycoprotein antibody MRK16, suggesting that 15D3 and 17F9 bind to a different epitope on the extracellular domain of P-glycoprotein than MRK16. Scatchard analysis revealed that 15D3 and 17F9 had association constants of 1.3 and 1.1 x 10(8) M-1, respectively. 15D3 and 17F9 had little effect on MDR cell growth except for a minor inhibition of KB-V1 cells when the cells were incubated in the presence of vinblastine. Neither antibody inhibited the efflux of P-glycoprotein substrates from MDR cells. Because of their strong binding activity, these antibodies may be useful for diagnostic detection of MDR in patients undergoing chemotherapy or as targeting components of immunotherapeutic agents.