PENETRATION AND BINDING OF RADIOLABELED ANTICARCINOEMBRYONIC ANTIGEN MONOCLONAL-ANTIBODIES AND THEIR ANTIGEN-BINDING FRAGMENTS IN HUMAN-COLON MULTICELLULAR TUMOR SPHEROIDS

Abstract

The binding and penetration of two 125I-labeled anti-carcinoembryonic antigen (CEA) monoclonal antibodies (MAb) and their F(ab'')2 and Fab fragments were measured in multicellular spheroids of poorly (HT29) and moderately well differentiated (Co112) human colon adenocarcinomas which express different amounts of CEA. Spheroids cultured in vitro model tumor microenvironments where poor vascular supply may modulate antigen expression and accessibility. The two MAb studied, 202 and 35, were shown previously to react with different CEA epitopes and to have high affinities of 1.2 and 5.8 .times. 109 M-1, respectively. MAb 202 has also been shown to cross-react with antigens present on human granulocytes and normal epithelial cells from human lung and pancreas. Specific binding of intact MAb and fragments of both antibodies was demonstrated for both types of human colon carcinoma spheroids compared to mouse colon carcinoma (CL26) and mammary tumor (EMT6/Ro) spheroids. Total binding of MAb and fragments was greater (1.5- to 2.5-fold) after 4 h compared to 1 h of exposure; the amount of binding compared to control IgGI was 5- to 30-fold greater after 1-h incubation and 15 to 200 times greater after 4 h. This binding was stable as demonstrated by short and long wash experiments at 37.degree.C and 4.degree.C. The binding of F(ab'')2 and Fab fragments of the anti-CEA MAb 35 to spheroids of human colon Co112 was almost 2-fold greater than that of the intact MAb. However, for MAb 202, the binding of intact MAb and F(ab'')2 was greater than that of Fab fragments. In addition the binding of both intact and F(ab'')2 fragments of MAb 202 was greater than that obtained with MAb 35. Specific binding of both antibodies to HT29 spheroids, which express less CEA, was decreased for MAb and fragments of both 202 and 35. Autoradiography and immunoperoxidase experiments were performed to determine the penetration of MAb and fragments after incubation with intact spheroids. Comparisons were made with labeled MAb directly applied to frozen sections of spheroids. F(ab'')2 and Fab fragments of both antibodies were bound at the surface of intact spheroids and penetrated to eight to ten cells, but the intact MAb were localized mainly at the spheroid surface and the outer one to three cell layers. There was much less binding at the surfaces of HT29 compared to Co112 spheroids. An enzyme immunoassay using MAb 35 and 202 demonstrated that Co112 spheroids produced about 8-fold more CEA/mg of cell protein than did monolayer cultures. CEA production in HT20 spheroids was less than Co112 by a factor of about 48. CEA expression as determined on frozen sections of spheroids was very heterogeneous and often associated with differentiated acinar structures. Despite the presence of abundant CEA, even the F(ab'')2 and Fab fragments could not penetrate and bind significantly to most of these acini under these exposure conditions. These results demonstrate striking differences in binding of monoclonal antibodies and fragments to distinct CEA epitopes which might exhibit different biological expression.