Expression of Human Tumor Mucin-Associated Carbohydrate Epitopes, Including Sialylated Tn, and Localization of Murine Monoclonal Antibodies CC49 and B72.3 in a Syngeneic Rat Colon Carcinoma Model

Using immunohistochemical techniques and whole-cell enzymelinked immunosorbent assay, we have determined that monoclonal antibodies (mAbs) B72.3 and CC49, which are widely used in the diagnosis and treatment of several human epithelial cancers, are expressed in a transplantable rat colon carcinoma cell line, K12-TRb. MAbs B72.3 and CC49 react with tumor-associated glycoprotein-72 (TAG-72) which is a carcinoma mucin molecule expressed in colon, breast, pancreatic, ovarian, lung, and gastric cancers. The carbohydrate epitope for mAb B72.3 is sialylated Tn (sTn), whereas CC49 reacts with an unknown carbohydrate epitope. K12-TRb is a transplantable rat colon carcinoma cell line derived from a dimethylhydrazine tumor which grows as progressive tumors in syngeneic BD IX rats. We found that the carbohydrate epitopes for mAbs B72.3 and CC49, including sTn, were more tumorrestricted in the rat than in humans. The only binding these had mAbs to normal rat tissue was to small-intestinal mucosa. MAbs B72.3 and CC49 were radiolabeled with iodine-125 (¹²⁵I) and injected intravenously into BD IX rats containing subcutaneously grown syngeneic K12-TRb tumors. Biodistribution experiments were conducted by dissecting groups of three rats on days 2, 4, 7, and 14 after injection of radiolabeled mAbs. These experiments confirmed that mAbs B72.3 and CC49 localize to K12-TRb tumors in vivo, and that the higher affinity mAb CC49 localized better than mAb B72.3. Gamma-camera imaging of subcutaneous K12-TRb tumors was successfully performed using¹²⁵I-labeled mAb CC49. The importance of this model is that mAbs B72.3 and CC49, immunoconjugates of these mAbs, and vaccines containing their corresponding carbohydrate epitopes, including sTn, can be studied in a relevant immunocompetent syngeneic rat colon carcinoma model.