Receptor binding assays saturability

There should be specific, saturable binding to the receptor, accompanied by pharmacological characteristics appropriate to the functional effects, demonstrable using a radioactive, eg, tritium or iodine-125, ligand to label the receptor. Radioligand binding assays (1,6) have become a significant means by which to identify and characterize receptors and enzymes (see Immunoassays Radioactive tracers). Isolation of the receptor or expression of the receptor in another cell, eg, an oocyte can be used to confirm the existence of a discrete entity. 

Towards this end, studies were carried out which included the radio-labelling of the somatostatin receptor ligand DOTA-Tyr -TATE (1,4,7,10-tetraazacyclotetradecane-N,N, N",N" -tetraacetic acid tyrosineS-octreotate, or DOTATATE) with both Lu and I, and the quality control of the resulting complexes. In vitro studies of the biological affinity of the Lu-DOTATATE and I-DOTATATE for the somatostatin membrane receptor were carried out with the help of competition and saturation binding assays. In vivo studies of the biodistribution of Lu-DOTATATE and I-DOTATA TL in animals, either alone or in competition, were also carried out. The radiotherapeutic effect of "Lu-DOTATATE was evaluated by cytometry measurements. Estimation of absorbed doses of Lu-DOTATATE was carried out by mathematical modelling. 

Universally, competitive binding assays are characterized by two important features—saturability and specificity—each of which must be demonstrated to secure the validity of the technique and to document that the ligand/receptor interaction examined in vitro IS related to the physiologically important events that occur in VIVO. These aspects of ligand binding are discussed respectively in the following two sections of this chapter. 

An immediate problem that must be overcome when conducting an RRA is the relatively low density of receptors specific for the ligand vis-a-vis other tissue constituents that are available for nonspecific interactions. Indeed, in many competitive binding assays, the amount of ligand specifically bound when all receptors are occupied (saturated) is on the order of fmoles/mg protein Because m most applications, and particularly for RRAs, non-... 

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