Avidin-biotin system for delivery of diagnostic agents

Radiolabeled monoclonal antibodies specific for tumor-associated antigens are used for the diagnosis and therapy of malignant tumors. However, the blood clearance of antibodies is slow and tumor-to-nontumor ratios of radioactivity in the current system are not high. To increase tumor-to-nontumor ratios, the concept of pretargeting has been proposed. The strategy is based on the separate administration of the monoclonal antibody and the radiolabel. Because avidin and streptavidin bind to biotin selectively with extremely high affinity, the avidin-biotin system has been applied for pretargeting. With this method, high tumor-to-nontumor ratios are reported, not only in animal experiments, but also in clinical trials. The avidin-biotin system is also used to reduce the background radioactivity of the directly labeled antibody as well as radioimmunoguided surgery.