factor-like domain into which a cell adhesion sequence (RGD) is inserted and a COOH-terminaldomain with homology to the phospholipid binding

Evidence is presented for two different breast epitheial antigens that some epitopes have greater tumor specificity and are more effective targets for radioimmunotherapy than others. The two antigens, which are major components of the human milk fat globule membrane, are breast mucin and a Mr 46'000 glycoprotein (BA46). Of five monoclonal antibod ies (Mc5, Mc!, BrE-1, BrE-2, and BrE-3) against breast mucin, all rec ognize overlapping amino acid epitopes on the tandem repeat domain. However, each have unique and different tissue and tumor specificities and unique epitope structures on the fully glycosylated breast mucin. In preclinical studies, radioimmunoconjugates of all five monoclonal anti bodies inhibit growth of transplantable breast tumors in immunodeficient mice. In human clinical trials, radioiodinated Mc5 was very poor in localizing breast tumor metastases. On the other hand, WIn-labeled BrE-3 imaged almost 90% of breast tumors and showed promise in radioimmunotherapywhen labeled with °°Y. The failure ofMc5 in clinical trials may be partly attributed to the high levels of its epitope on circu lating mucin compared to the epitope of BrE-3. The Mc5 binding affinity increased significantly with glycosylation, while the BrE-3 epitope was masked by glycosylation. The BA46 glycoprotein is a breast tumor-asso ciated membrane antigen containing an NH2-terminal, epidermal growth factor-like domain into which a cell adhesion sequence (RGD) is inserted and a COOH-terminaldomain with homology to the phospholipid binding C1/C2 domain of coagulation factors V and VIII. It promotes cell attach ment in an RGD-dependent manner. Monoclonal antibody Mc8, which binds to the C2-like domain, is only moderately effective in experimental radioimmunotherapy, while Mc3, which binds an epitope in the EGF-like RGD domain, was highly effective in destroying breast tumors in nude mice. With °°Y-labeled Mc3, 6 of 7 mice are cured of the tumors. These results indicate that by selecting appropriate monoclonal antibodies, a normal antigen can he used as a target for radioimmunotherapy.