At the beginning of twentieth century, J. N. Langley and P. Erlich independently recognized the fundamental features of drug-receptor interaction, i.e., specificity, the basis of cellular recognition and activation and the cellular response . The concept of a receptor includes the key attributes of ligand recognition and signal transduction. The signal transduction process may be mediated through an integral part of the receptor structure or involve receptor interactions with additional nonreceptor proteins . The final proof of the existence of the pharmacological receptors came from the recent advances in biochemistry and molecular biology to purify, sequence, clone, and express receptor proteins. The discovery and development of receptors as drug targets stems from the drug interactions with receptor molecules located in the plasma membranes or in the cytosol of target cells. Several receptors have now been very well characterized. Gene sequences of hundreds of orphan receptors have been identified by homology analysis of genome databases. The ligands of these orphan receptors have to be characterized to determine their functions in order to convert these receptors into new drug targets.
Analysis of the drug targets in current drug therapy showed that there are about 500 molecular targets . Receptors, including cell membrane receptors, nuclear receptors, ion channels, and orphan receptors, represent more than 60%
of the drug discovery targets (Fig. 1). Ligand binding provides a direct approach of in vitro receptor-binding assays. The principle of receptor-binding assays is straightforward. In the conventional ligand-receptor-binding assay, a suitable (high-affinity) radiolabeled ligand is incubated with the chosen receptor preparation. The free radioligand is separated from the receptor-bound ligand, and the total ligand bound to receptor (nonspecific binding plus specific binding) is determined by counting in a scintillation counter. The nonspecific binding is determined in the presence of a large excess (100-1000X) of unlabeled ligand to block the receptor-binding sites of interest and represents the radioligand bound to other receptor sites and to the separation medium such as glass fiber, filter, or assay tubes (Fig. 2A). Specific binding (the ligand bound to the specific receptor-binding sites) is calculated by subtracting the nonspecific binding from the total binding.
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