Scintigraphic studies in animals and in humans have demonstrated uptake of radiolabeled antibody by both normal and tumor tissue. Normal tissues most commonly visualized are blood, liver, spleen, kidneys, lymph nodes, bone, and thyroid. A number of factors have been demonstrated to affect the uptake by normal and tumor tissue, including radioisotope properties, immunoglobulin characteristics, antibody specificity, tumor size, vascular permeability, and antigen expression. Clarification of the mechanism of tumor and normal tissue uptake depends upon comparison of scintigraphic findings with analysis of tissue for such factors as radioactivity, antigen content, and tumor size.

One of the major limitations of 111In labeled monoclonal antibody imaging has been extensive 111In uptake by histologically normal liver, especially in a host bearing a large tumor mass. By high performance liquid chromatography and sodium dodecyl sulfate-polyacrylamide-gel electrophoresis analysis of liver and blood it can be demonstrated that much of the liver uptake is related to the formation of antigen:antibody complexes. The normal liver intensity can be decreased by inhibition of radiolabeled complex formation.

Understanding of the mechanisms of tissue uptake, both normal and tumor, and of radiolabeled antibody metabolism is crucial to the rational planning and use of radioimmunoconjugates for tumor imaging and treatment. Animal and human studies complement one another in examination of these mechanisms.

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Presented at the “Second Conference on Radioimmunodetection and Radioimmunotherapy of Cancer,” September 8–10, 1988, Princeton, NJ. Supported in part by NIH Cancer Center Core Grant CA 33572 and Program Project Grant CA 43904, and a grant from Medi+Physics Inc., Emeryville, CA.

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