Development and characterization of chimeric anti-carcinoembryonic antigen monoclonal antibodies and their Fab fragments

Generation, immunologic characterization and antitumor effects of human monoclonal antibodies for carcinoembryonic antigen

We generated fully human mAbs (HmAbs) to carcinoembryonic antigen (CEA) using the KM mouse, which carries a human chromosome 14 fragment containing the entire Ig H chain loci and human kappa L chain segments in the mouse genome. Forty-six hybridoma clones producing HmAbs to CEA were thus obtained by fusing the P3-U1 mouse myeloma cells with splenocytes of the KM mice immunized with CEA. Among them, 22 clones produced HmAbs that reacted with CEA but not with 3 other CEA-related cell adhesion molecule (CEACAM) family members, CEACAM1, CEACAM6 and CEACAM8. In 12 HmAbs examined, 8 were IgG4, 2 were IgG3, 1 was IgG2, and the other was IgG1. The affinity constants for CEA of these HmAbs were comparable to those of the previously prepared mouse anti-CEA mAbs (MmAbs). BIAcore analyses revealed that 1 and 2 of the 22 HmAbs react with 2 epitopes defined by MmAbs on the domain N and the domain A1 or B1 of CEA, respectively. In the presence of human complement in vitro, 2 HmAbs tested showed substantial cytotoxicity, namely, 50-65%, against CEA-expressing tumor cells. With human lymphokine-activated killer cells in vitro, 3 HmAbs tested exhibited 40-65% Ab-dependent cell-mediated cytotoxicity against the tumor cells. Moreover, one of the HmAbs induced a significant inhibition of tumor growth when administered to mice xenografted with the CEA-expressing cells. Considering their lack of immunogenicity to humans, these CEA-specific HmAbs may be useful for immunotherapeutic approaches as well as for immunodiagnosis.

Antibody constructs for radioimmunodiagnosis and treatment of human pancreatic cancer

Pancreatic cancer (PC) is a common disease that is seldom cured. Current approaches to the treatment of PC are not effective because the non-specific nature of both chemotherapy and external beam radiation results in toxicity to normal tissue. Monoclonal antibodies (MAbs) can be used as selective carriers for delivering radionuclides, toxins, or cytotoxic drugs to malignant cell populations. Therefore, MAb-technology has led to a significant amount of research in targeted therapy. Targeted therapy would generally allow the concentration of cytotoxic agents in tumors and would markedly lessen the toxicity to normal tissues, which limits the dosage and effectiveness of systemically administered drugs. A variety of MAbs are being pre-clinically evaluated for the diagnosis and treatment of PC. Novel recombinant antibody constructs hold a promising future in both the diagnosis and treatment of cancer. By genetic-engineering methods, several high affinity antibody fragments with optimum tumor targeting properties, such as higher functional affinity (divalent and multivalent scFvs) and blood residence time (good tumor localization with high radiolocalization index), have been generated. Animal models have permitted the in vivo assessment of these antibody-based reagents, therapeutic/diagnostic radionuclide, radiolabeling conditions, and efficacy of administration regimes. For PC, immunoscintigraphy using MAbs has taken new strides. The use of MAbs and their fragments for radioimmunoguided surgery and therapy of PC has shown encouraging results at preclinical levels and warrants further attention.

Improved tumor detection by anti-CEA chimeric Fab oligomers with disulfide linkages in a pancreatic-carcinoma-xenograft model

We have investigated the effect of Fab oligomerization on imaging efficacy in a pancreatic-carcinoma xenograft model in mice. Recombinant mouse/human chimeric Fab of the anticarcinoembryonic antigen (CEA) monoclonal antibody A10, which has been shown to react specifically with gastrointestinal cancers, was used in this study. Fab homo-oligomers (dimers and trimers) were prepared by linkage of chimeric Fab with N-succinimidyl-3-(2-pyridyldithio)-propionate. Oligomers with S-S bonds showed 10-fold higher binding activity against human CEA than Fab, while the binding activity of oligomers was similar to that of F(ab')2. In mice bearing pancreatic-carcinoma xenografts, tumor uptake of S-S oligomers was significantly greater than that of monomeric Fab, while there was no difference in tumor uptake between S-S Fab trimers and F(ab')2. S-S oligomers showed more rapid clearance rates and uniform percolation in the tumor nodules than F(ab')2. At 18 hr after injection, clear scintigraphic detection of the pancreatic-carcinoma tumors was obtained with 123I-labeled S-S Fab dimers. At 24hr, improved tumor imaging was shown for 123I-labeled S-S Fab oligomers with slightly visible uptake in normal tissues, similar to that of F(ab')2. S-S oligomers of chimeric A10 Fab may be useful as rapid diagnostic tools of pancreatic carcinomas.

Therapy and imaging of pancreatic carcinoma xenografts with radioiodine-labeled chimeric monoclonal antibody A10 and its Fab fragment

Recombinant mouse/human chimeric monoclonal antibody A10 (ch-A10) and its Fab fragment (ch-Fab) react with carcinoembryonic antigen on various gastrointestinal carcinomas. We performed biodistribution studies with 125I-labeled ch-A10 and ch-Fab in an antigen-positive human pancreatic carcinoma (BxPC-3) xenograft model. We also evaluated the anti-tumor effect of 131I-labeled ch-A10, and studied the detection of BxPC-3 xenografts with 123I-labeled ch-Fab in whole body scintigraphy. In comparative biodistribution studies, the tumor uptake of 125I-labeled ch-A10 was significantly greater than that of 125I-labeled ch-Fab 24 h post-injection. However, the tumor-to-blood ratio was 46.8 for ch-Fab at 24 h post-injection, while it was only 1.4 for ch-A10. Microautoradiography studies showed that ch-Fab penetrated more uniformly into the tumor nodules than did ch-A10. In mice given a therapeutic dose of 131I-labeled ch-A10, a significant inhibition of tumor growth was seen, while control 131-I-labeled human IgG did not affect tumor growth. Leukocyte toxicity was observed within 3 weeks after injection of 131I-labeled ch-A10, but leukocyte counts recovered to normal levels at 8 weeks post-injection. In whole-body scintigraphy, clear and rapid tumor imaging was obtained with 200 microCi of 123I-labeled ch-Fab 24 h post-injection. These results suggest that radioiodine-labeled chimeric A10 antibodies could potentially be useful candidates for radioimmunotherapy and radioimmunodetection of pancreatic carcinomas.