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Selection of Tumor-specific Epitopes on Target Antigens for Radioimmunotherapy of Breast Cancer¹

Cancer Research Fund of Contra Costa, Walnut Creek, California 94596

Evidence is presented for two different breast epithelial 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 M_r 46,000 glycoprotein (BA46). Of five monoclonal antibodies (Mc5, Mc1, BrE-1, BrE-2, and BrE-3) against breast mucin, all recognize 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 antibodies 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, ¹¹¹In-labeled BrE-3 imaged almost 90% of breast tumors and showed promise in radioimmunotherapy when labeled with ⁹⁰Y. The failure of Mc5 in clinical trials may be partly attributed to the high levels of its epitope on circulating 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-associated membrane antigen containing an NH₂-terminal, epidermal growth factor-like domain into which a cell adhesion sequence (RGD) is inserted and a COOH-terminal domain with homology to the phospholipid binding C1/C2 domain of coagulation factors V and VIII. It promotes cell attachment 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 ⁹⁰Y-labeled Mc3, 6 of 7 mice are cured of the tumors. These results indicate that by selecting appropriate monoclonal antibodies, a normal antigen can be used as a target for radioimmunotherapy.