RECEPTOR IMAGING - COMPETITIVE OR COMPLEMENTARY TO ANTIBODY IMAGING

Both radiolabeled ligands to specific receptors on cell surfaces and r adiolabeled antibodies to specific cell surface epitopes provide new o pportunities to scintigraphically identify tumors. Both radiolabeled l igands and antibodies are characterized by high orders of affinity for their respective binding sites and offer greater specificity over the agents previously used for tumor imaging including gallium 67, thalli um 201, technetium 99m MIBI, and flourine-18-labeled deoxy; glucose. T he two classes of tumor-binding tracers differ primarily based on mole cular weight although the nonspecific portion of the immunoglobulins a re also antigenic. Increased molecular weight results in prolonged pla sma survival, which increases the interval available for tumor permeat ion but also produces increased nonspecific background activity, which impairs image contrast. At the present time, encouraging clinical res ults have been obtained with both agent types, but further development is necessary. Receptor-ligand tracers provide better contrast than an tibodies or antibody fragments. Receptor-ligand imaging technology awa its further developments in an understanding of the biology of recepto r expression in normal tissue and tumors and improved radiochemical te chniques and pharmacology to define the radioligands of choice. Radiol abeled antibodies will probably evolve in the direction of increased u se of antibody fragments and possibly the identification and polymeriz ation of epitope-recognition units in order to provide high-affinity, nonantigenic, small molecular weight tracers that will be more permeab le in tumors and clear more rapidly from background tissue. Rather tha n compete or complement each other, the techniques will likely produce a hybrid technology, radiolabeled molecular recognition units, with t he better features of with technologies including high binding affinit y (low dissociation constant) for surface membrane epitopes, including receptor sites. Copyright (C) 1997 by W.B. Saunders Company.