Four
monoclonal antibodies against
carcinoembryonic antigen (CEA) have been selected from 32 hybrids that produce
antibodies against this
antigen, by the criteria of high affinity for CEA and low cross-reactivity with granulocyte
glycoprotein(s). The specificity of
tumor localization in vivo of the four MAb, and their F(ab')2 and
Fab fragments was compared in nude mice bearing grafts of a serially transplanted, CEA-producing, human colon
carcinoma. The distribution of radiolabeled MAb and their fragments after
intravenous injection was analyzed by direct measurement of radioactivity in
tumor and normal organs, as well as by whole-body scanning and by autoradiography of
tumor sections. Paired labeling experiments, in which 131I-labeled antibody or fragments and 125I-labeled control
IgG are injected simultaneously, were undertaken to determine the relative
tumor uptakes of each labeled
protein. The
tumor antibody uptake divided by that of control
IgG defines the specificity index of localization.
Tumor antibody uptakes (as compared with the whole mouse), ranging between 7 and 15, and specificity indices ranging between 3.4 and 6.8, were obtained with the four intact MAb at day 4-5 after injection. With F(ab')2 fragments of the four MAb, at day 3, the
tumor antibody uptakes ranged between 12 and 24 and the specificity indices between 5.3 and 8.2. With the
Fab fragments prepared from the two most promising MAb, the antibody uptakes reached values of 34 and 82 at day 2-3 and the specificity indices were as high as 12 and 19. The scanning results paralleled those obtained by direct measurement of radioactivity. With intact MAb,
tumor grafts of 0.5-1 g gave very contrasted positive scans 3 d after injection. Using MAb fragments,
tumors of smaller size were detectable earlier. The best results were obtained with
Fab fragments of MAb 35, which gave clear detections of
tumors weighing only 0.1 g as early as 48 h after injection. Autoradiographs of
tumor sections from mice injected with 125I-labeled MAb demonstrated that the radioactivity was localized in the
tumor tissues and not in the stromal connective tissue of mouse origin. The highest radioactivity concentration was localized in areas known to contain CEA such as the pseudolumen of glands and the apical side of
carcinoma cells. The penetration of radioactivity in the central part of
tumor nodules and the pseudolumen appeared to be increased with the use of MAb fragments.