Partial Agonist

Abstract

Partial agonists bind to and activate a receptor, but are not able to elicit the maximum possible response that is pro- duced by full agonists. The maximum response produced by a partial agonist is called its intrinsic activity and may be expressed on a percentage scale where a full agonist produced a 100% response. A key property of partial agonists is that they display both agonistic and antagonis- tic effects. In the presence of a full ▶agonist,a partial agonistwill act as an ▶antagonist, competing with the full agonist for the same receptor and thereby reducing the ability of the full agonist to produce its maximum effect. The balance of activity between agonist and antagonist effects varies from one substance to another, according to their intrinsic activities, and is also influenced by the test system used to measure their effects. Weak partial agonists are those compounds, possessing low intrinsic activity, that are able to produce only a small percentage of the total response produced by an agonist and which act predominantly as antagonists. Strong partial agonists may come close to mimicking the maximum effects of a full agonist and may display only weak antagonistic abili- ty. When the test system under study has a large receptor reserve, weak partial agonists show greater agonist activity then when the system has a small receptor reserve.