Introduction and Early Concepts

The development of new drugs usually requires the synthesis of large numbers of structurally related compounds. If a set of agonists of this kind is tested on a particular tissue, the compounds are often found to fall into two categories. Some can elicit a maximal tissue response and are described as full agonists in that experimental situation. Others cannot elicit this maximal response, no matter how high their concentration, and are termed partial agonists. Examples include  

FIGURE 1.5 Comparison of the log concentration-response relationships for P-adrenoceptor-mediated actions on six tissues of a full and a partial agonist (isoprenaline [closed circles] and prenalterol [open circles], respectively). The ordinate shows the response as a fraction of the maximal response to isoprenaline. (From Kenakin, T. P. and Beek, D., J. Pharmacol. Exp. Ther., 213, 406-413, 1980.) 

FIGURE 1.6 Interaction between the full agonist histamine and the H2-receptor partial agonist impromidine on isolated ventricular strips from human myocardium. The concentration-response curve on the left is for histamine alone, and those on the right show the response to impromidine acting either on its own (open squares) or in the presence of a constant concentration (100 pM) of histamine (open diamonds). (From English, T. A. H. et al Br. J. Pharmacol., 89, 335-340, 1986.) 

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