Biological properties of chimeric domain-deleted anticarcinoma immunoglobulins

CC49 is a second-generation monoclonal antibody (MAb) that has high affinity for the tumor-associated pancarcinoma antigen tumor-associated 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 (CH2) domain-deleted chimeric (c) CC49 MAb by transfecting an expression construct consisting of the CC49 murine variable region and a CH2 domain-deleted human IgG1 constant region into cCC49 kappa producing SP2/0 murine myeloma cells. As determined by SDS-PAGE, the intact cCC49 delta CH2 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 cCC49 delta CH2 were evaluated and compared with those of mouse/human chimeric forms cCC49 delta CH1 and intact cCC49. Previous studies have shown that the in vitro antigen-binding properties of cCC49 delta CH1 are similar to those of cCC49. Biodistribution studies reported here, using 131I-labeled cCC49 delta CH1 and 125I-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 125I-labeled cCC49, 131I-labeled cCC49 delta CH2 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 131I-labeled cCC49 delta CH2 and 125I-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 delta CH2 cMAb versus the 72 h required for cCC49. Biodistribution studies using 177Lu-conjugated cCC49 delta CH1 and cCC49 showed no significant difference between the radiolocalization indices (% ID/g in tumor divided by % ID/g in normal tissue). 177Lu-conjugated cCC49 delta CH2, however, had lower % ID/g values in tumor xenografts and lower radiolocalization indices than either 177Lu-conjugated cCC49 delta CH1 or 177Lu-conjugated cCC49. Pharmacokinetic studies in non-tumor-bearing athymic mice using cCC49 delta CH1 and cCC49 revealed no significant difference between these cMAbs. However, the plasma clearance of cCC49 delta CH2 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, 131I-labeled cCC49 delta CH2 cleared significantly faster than 125I-labeled cCC49. The similar plasma clearance and tumor localization of cCC49 and cCC49 delta CH1 suggest that these two cMAbs may be used in similar clinical settings. However, because of the unique pharmacokinetics and tumor targeting of cCC49 delta CH2 versus cCC49 or cCC49 delta CH1, this chimeric immunoglobulin form may be useful in clinical settings that require efficient tumor targeting and rapid serum and whole-body clearance.