This chapter explains the measurement of affinity and efficacy in functional assays. One major approach to the testing of drug activity is with functional assays. Functional assays give flexibility in terms of what biochemical functional response can be monitored for drug activity. These are composed of any biological system that yields a biochemical product or physiological response to drug stimulation. Such assays detect molecules that produce biological response or those that block the production of physiological response. These can be whole tissues, cells in culture, or membrane preparations. Like biochemical binding studies, the pharmacological output can be tailored by using selective stimulation. Whereas the output can be selected by the choice of radioligand or other traceable probe with binding studies, in functional studies the output can be selected by choice of agonist. When necessary, selective antagonists can be used to obviate unwanted functional responses and isolate the receptor of interest. This practice was more prevalent in isolated tissue studies where the tissue was chosen for the presence of the target receptor, and in some cases this came with concomitant presence of other related and obfuscating receptor responses. In recombinant systems, a surrogate host cell line with a blank cellular background can often be chosen. This results in much more selective systems and less need for selective agonist probes.
