XENOGRAFT LOCALIZATION USING PRETARGETED STREPTAVIDIN-CONJUGATED MONOCLONAL-ANTIBODY AND IN(111)-LABELED BIOTIN

In order to increase the tumor/background ratio of radiolabeled monoclonal antibody, we have investigated pretargeted streptavidin-conjugated antibody followed by radiolabeled biotin in a xenograft model. Nude mice bearing subcutaneous H.Ep-2 tumors were injected intravenously with 9 mug streptavidin-conjugated HMFG1 antibody. Time was allowed for tumor localization and blood clearance (1-7 days) before intraperitoneal injection of In-111-labeled biotin (0.1 mug-100 mug). Animals were dissected at times from 2 to 48 h after biotin injection, and radioactivity in tumor, blood and normal organs measured. Tumor/tissue ratios were compared with those obtained with HMFG1 directly labeled with In-111, and with In-111-labeled biotin alone, or following unconjugated streptavidin. Improved tumor/blood ratios were observed with the pretargeted antibody, due to the rapid clearance of biotin from the circulation. These ratios increased with biotin dose, and with time after biotin administration. Injection of 100 mug In-111-labeled biotin 1 day after streptavidin-conjugated antibody resulted in a 4-fold increase in tumor/blood and tumor/liver ratios compared with In-111-labeled antibody, as early as 2 h after radiolabel administration. Although the absolute amount of radiolabel in the tumor was much lower with the two-step strategy than with directly-labeled antibody, the increase in tumor to background ratio suggests that pretargeted antibodies may result in superior immunoscintigraphy.