Biological Properties of Chimeric Domain-deleted Anticarcinoma Immunoglobulins

Abstract CC49 is a second-generation monoclonal antibody (MAb) that has high affinity for the tumor-associated pancarcinoma antigen tumorassociated glycoprotein-72. In clinical trials using gamma scanning, radiolabeled CC49 has facilitated the detection of more than 90% of carcinomas. We report here the development of a constant heavy-chain 2 (C H 2) domain-deleted chimeric (c) CC49 MAb by transfecting an expression construct consisting of the CC49 murine variable region and a C H 2 domain-deleted human IgG1 constant region into cCC49κ producing SP2/0 murine myeloma cells. As determined by SDS-PAGE, the intact cCC49ΔC H 2 has a molecular weight of 153,000 and, under reducing conditions, molecular weights of 43,000 and 27,000. The plasma clearance and tumor-targeting properties of cCC49ΔC H 2 were evaluated and compared with those of mouse/human chimeric forms cCC49ΔC H 1 and intact cCC49. Previous studies have shown that the in vitro antigen-binding properties of cCC49ΔC H 1 are similar to those of cCC49. Biodistribution studies reported here, using 131 I-labeled cCC49ΔC H 1 and 125 I-labeled cCC49 in athymic mice bearing human colon carcinoma xenografts, demonstrated that both cMAbs localized to the tumor and cleared from the normal tissues similarly. However, in comparison with 125 I-labeled cCC49, 131 I-labeled cCC49ΔC H 2 localized to tumors earlier and had a significantly lower percentage of the injected dose of cMAb/g (%ID/g) in normal tissues than cCC49. Immunoscintigraphy of 131 I-labeled cCC49ΔC H 2 and 125 I-labeled cCC49 in athymic mice bearing human tumor xenografts demonstrated a clear image of the tumor by 24 h after i.v. administration of the ΔC H 2 cMAb versus the 72 h required for cCC49. Biodistribution studies using 177 Lu-conjugated cCC49ΔC H 1 and cCC49 showed no significant difference between the radiolocalization indices (%ID/g in tumor divided by %ID/g in normal tissue). 177 Lu-conjugated cCC49ΔC H 2, however, had lower %ID/g values in tumor xenografts and lower radiolocalization indices than either 177 Lu-conjugated cCC49ΔC H 1 or 177 Lu-conjugated cCC49. Pharmacokinetic studies in non-tumor-bearing athymic mice using cCC49ΔC H 1 and cCC49 revealed no significant difference between these cMAbs. However, the plasma clearance of cCC49ΔC H 2 in non-tumor-bearing mice was significantly faster than that of cCC49. These results were similar when the cMAbs were labeled with either iodine or lutetium. In nonhuman primates, 131 I-labeled cCC49ΔC H 2 cleared significantly faster than 125 I-labeled cCC49. The similar plasma clearance and tumor localization of cCC49 and cCC49ΔC H 1 suggest that these two cMAbs may be used in similar clinical settings. However, because of the unique pharmacokinetics and tumor targeting of cCC49ΔC H 2 versus cCC49 or cCC49ΔC H 1, this chimeric immunoglobulin form may be useful in clinical settings that require efficient tumor targeting and rapid serum and whole-body clearance.