USE AND LIMITATIONS OF RADIOLABELED ANTI-CEA ANTIBODIES AND THEIR FRAGMENTS FOR PHOTOSCANNING DETECTION OF HUMAN COLORECTAL CARCINOMAS

Abstract

Patients (53) with carcinoma were injected with highly purified [131I]-labeled goat antibodies or fragments of antibodies against carcinoembryonic antigen (CEA). Each patient was tested by external photoscanning 4, 24, 36 and 48 h after injection. In 22 patients (16 of 38 injected with intact antibodies, 5 of 13 with F(ab'')2 fragments and 1 of 2 with Fab'' fragments), an increased concentration of 131I radioactivity corresponding the previously known tumor location was detected by photoscanning 36-48 h after injection. Blood pool and secreted radioactivity was determined by injecting, 15 min before scanning, [99mTc]-labeled normal serum albumin and free 99mTcO4-]. The computerized subtraction of 99mTc from 131I radioactivity enhanced the definition of tumor localization in the 22 positive patients. In spite of the computerized subtraction, interpretation of the scans remained doubtful for 12 patients and was entirely negative for 19 additional patients. To provide a more objective evaluation for the specificity of the tumor localization of antibodies, 14 patients scheduled for tumor resection were injected simultaneously with [131I]-labeled antibodies or fragments and with [125I]-labeled normal goat IgG or fragments. After surgery, the radioactivity of the 2 isotopes present in tumor or adjacent mormal tissues was measured in a dual channel scintillation counter. The antibodies or their fragments were 2-4 times more concentrated in the tumor than in the normal tissues. The injected antibodies formed immune complexes with circulating CEA, and the amount of immune complexes detectable in serum was roughly proportional to the level of circulating CEA.