Fundamental aspects of radioreceptor assays

Significant advances in understanding the actions of hormones, neurotransmitters and drugs resulted from the development of new analytical procedures. Among these advances the discovery that, for a given drug (or hormone or neurotransmitter), there exists a receptor site on membranes to which the substance must attach in order to evoke a biological response, led to the setting up of the radioreceptor assays. Thus ligand-binding techniques can be used as assays for measuring drug levels in biological tissues and fluids. Radioreceptor assays are based on the principle of competitive protein-binding methods; when a radioligand and an unlabelled ligand are present together with a specific receptor preparation, the amount of radioactive ligand bound to the receptor is a quantitative function of the amount of unlabelled ligand present in the incubation medium. The technology of the radioreceptor assays is simple and rapid to perform when optimal conditions for binding are determined. Radioreceptor assays are highly sensitive, reliable, precise and accurate. Their chief limitation relates to specificity as any substance having an appreciable affinity for the receptor displaces the specifically bound radioligand. Paradoxically this lack of specificity may be in some cases advantageous in that it allows for the detection of the parent compound and active metabolites in proportion to their affinity for the specific receptor. During recent years, radioreceptor assays have been applied to quantitative determination of hormones, neurotransmitters and drugs. Among these, the radioreceptor assay of benzodiazepines is a typical example. In addition to drug analysis and drug monitoring, radioreceptor assays have been found to be versatile tools for basic studies on the receptor itself, on the endogenous ligand(s) for the receptor and on the discovery and the identification of biologically active chemical entities (pharmacological screening). The future of radioreceptor assays in the pharmacological sciences appears to be promising.