ANTI-BA46 MONOCLONAL-ANTIBODY MC3 - HUMANIZATION USING A NOVEL POSITIONAL CONSENSUS AND IN-VIVO AND IN-VITRO CHARACTERIZATION

Mc3 is a murine mAb that is highly effective in treating breast tumors in experimental radioimmunotherapy. Mc3 binds to BA46, a 46-kDa glyco protein of the human milk fat globule membrane that is also expressed in breast carcinoma cells. We cloned and sequenced cDNAs encoding the variable regions of Mc3 and constructed an IgG1,kappa human/mouse chim eric antibody. We then humanized the variable regions of Mc3 using a p ositional consensus method and retaining residues that might either co ntact the complementarity-determining regions or the opposite chain. T his positional consensus is novel in that it does not include residues with buried side chains. Humanized Mc3 retained full binding affinity , and fully competes with murine Mc3 for antigen binding. Humanized an d murine I-131-labeled Mc3 behaved identically in athymic nu/nu mice b iodistribution studies. The tumor uptake levels for both antibodies in creased over a period of 4 days within a range of 13-20% of the inject ed dose per g with extremely favorable tumor:normal ratios. Also, a si ngle therapeutic dose of I-131-labeled humanized Mc3 in the same anima l model reduced the average tumor size and produced one of five cures while in the uninjected control tumor growth continued unabated. We be lieve that these results justify the implementation of Phase I human c linical trials for imaging and radioimmunotherapy of breast cancer.