Receptor imaging: Competitive or complementary to antibody imaging?
Both radiolabeled ligands to specific receptors on cell surfaces and radiolabeled antibodies to specific cell surface epitopes provide new opportunities to scintigraphically identify tumors. Both radiolabeled ligands 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, thallium 201, technetium 99m MIBI, and flourine-18-labeled deoxyglucose. The two classes of tumor-binding tracers differ primarily based on molecular weight although the nonspecific portion of the immunoglobulins are also antigenic. Increased molecular weight results in prolonged plasma survival, which increases the interval available for tumor permeation but also produces increased nonspecific background activity, which impairs image contrast. At the present time, encouraging clinical results have been obtained with both agent types, but further development is necessary. Receptor-ligand tracers provide better contrast than antibodies or antibody fragments. Receptor-ligand imaging technology awaits further developments in an understanding of the biology of receptor expression in normal tissue and tumors and improved radio-chemical techniques and pharmacology to define the radioligands of choice. Radiolabeled antibodies will probably evolve in the direction of increased use of antibody fragments and possibly the identification and polymerization of epitope-recognition units in order to provide high-affinity, nonantigenic, small molecular weight tracers that will be more permeable in tumors and clear more rapidly from background tissue. Rather than compete or complement each other, the techniques will likely produce a hybrid technology, radiolabeled molecular recognition units, with the better features of both technologies including high binding affinity (low dissociation constant) for surface membrane epitopes, including receptor sites.