Radioimmunodetection of gliomas by administration of radiolabelled monoclonal antibodies. Experimental data.

Radiolabelled monoclonal antibodies (McAbs) raised against membrane components of an experimental rat glioma (79FR-G-41) were administered parenterally to immunodeficient mice bearing glioma grafts for tumor radioimmunodetection by external imaging. Purified McAbs (14AC1) of IgG2a isotype were labelled with Na131I (2mCi/50ml) using the Chloramin-T method. As control, for non-specific uptake of proteins in the tumor, normal mouse IgG were also iodinated. For radioimaging, nude mice bearing gliomas in the thigh muscle were injected intravenously with 15 micrograms of the 131I-McAb with an activity of approximately 150 mu Ci. Control tumor-bearing animals received the same amount of mouse 131I-IgG. Scans obtained immediately after injecting the intact 131I-14AC1 antibody and at 24, 48, 72, and 96 hours demonstrated accumulation in the tumor. The tumor was clearly visible 48 hours following injection of 131I-labelled antibody. At 96 hours after injection, the McAb showed a clearly higher uptake into the tumor as the control IgG. The biodistribution of the injected antibody was studied at 96 hours after injection following the last gamma-imaging. At this time the blood activity was still high, but the maximum activity was found in the tumor for the specific McAb. Using the 131I-14AC1 to image glioma transplants, it could be shown that grafts are permeable for the McAb. The time-course experiments administering 131I-14AC1 antibody and normal mouse 131I-IgG, demonstrated that the localization of 131I-I4AC1 antibody in glioma grafts is the result of specific antigen binding. The scintigrams using intact antibody without background subtraction provided adequate tumor visualization, but the activity in the blood was high even 96 hours after injection. More rapid clearance of blood - pool radioactivity would possibly be achieved with F(ab')2 fragments. These in vivo glioma imaging studies, together with related in vitro binding tests, indicate the potential value of monoclonal antiglioma antibodies not only for clinical tumor radioimmunodetection, but also for the evaluation of immunotherapeutic approaches to the glioma disease of man.