03.02.2.2 Full and Partial Agonism

There are several types of agonism including full, and partial agonism, which are best understood from graphs. In a graph of effect against long agonist concentration, a full agonist gives the same maximum response as the endogenous ligand, and has a nominally efficacy of 1 (Figure 3.18).

Figure 3.18 Full and partial agonist (Copyright QUT, Sheila Doggrell)

A partial agonist has a smaller maximum effect than the endogenous ligand or full agonist, and has an efficacy less than 1.

An example of a partial agonist is buprenorphine. Morphine stimulates opioid m-receptors to induce analgesia, but another effect mediated by opioid m-receptors is respiratory depression, which can occur with the higher concentrations of morphine. Indeed, respiratory depression is a common side effect associated with full agonists at opioid m-receptors. Buprenorphine is a partial agonist at opioid m-receptors. Buprenorphine causes analgesia but, as it has a lesser maximum effect; it is unlikely to cause any excessive respiratory depression. This means that buprenorphine can be a safer drug than morphine to use.