High-specific-activity 111In-labeled Anticarcinoembryonic Antigen Monoclonal Antibody: Biodistribution and Imaging in Nude Mice Bearing Human Colon Cancer Xenografts

Tumor imaging and biodistribution of an indium-labeled monoclonal antibody (MAB) to carcinoembryonic antigen (CEA) [anti-CEA MAB-diethylenetriaminepentaacetic acid (DTPA)-111In] have been investigated using LS174T human colon cancer xenografts in nude mice. Antibody specificity, dose, and specific activity were examined with respect to tumor uptake and quality of scintiscans at different times following injection. The CEA-bearing LS174T tumors were imaged specifically with anti-CEA MAB-DTPA-111In. Using 62.5 ng of indium-labeled MAB (50 microCi/micrograms) the ratio of activity in tissue expressed as a percentage of the total radioactive dose injected into the animal per gram tissue for tumor:blood increased from 0.66 +/- 0.02 (SE) at 1 h to 14.8 +/- 1.1 at 72 h. Scintiscan quality improved with the rise in tumor:blood ratio until 48 h. At longer intervals insufficient counts remained for imaging. The tumor:blood ratio and the scintiscan quality were not improved by increasing the MAB dose to 625 or 6250 ng but good images were obtained at longer times postinjection. By decreasing the 111In from 50 to 10 microCi/micrograms of MAB, the unbound 111In was decreased from 7 microCi/micrograms (14%) to 0.2 microCi/micrograms (2%). Even with the lower specific activity (9.8 microCi/micrograms) of the 10-microCi/micrograms preparation, scintiscan quality at the 62.5-ng dose was maintained. This anti-CEA MAB-DTPA-111In preparation was stable, retained immunological activity, did not require column chromatography to remove unbound 111In, was specific for a CEA-bearing tumor, and was effective for tumor imaging over a wide range of antibody doses (3 to 300 micrograms MAB/kg body weight). This anti-CEA MAB-DTPA-111In preparation is feasible and practical for imaging CEA-bearing tumors in humans.