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• Partial agonists: These ligands also increase the activity of the receptors but do not
  produce the maximal response shown by full agonists even when present in large
  excess such that all the receptors are occupied (Fig. 17.1).


• Inverse agonists: These agonists decrease the activity of constitutively active receptors
  to their inactive state.

Full agonist

Partial agonist

EC 50

100

50

0

Response (% maximum)

10 –9 10 –8 10 –7 10 –6 10 –5

Concentration of agonist (log scale)

(a)

Concentration of agonist (log scale)

10 –9 10 –8 10 –7 10 –6 10 –5

100 Increasing drug potency

50

0

Response (% maximum)

(b)

Fig. 17.1Dose–response curves for receptor agonists. (a) The biological effect (% maximum response) and the

concentration of a full agonist are plotted on a logarithmic scale. An equipotent partial agonist has a lower

efficacy than a full agonist – it cannot achieve the maximum response even when all the receptors are occupied.

EC 50 is the concentration of agonist that produces 50% maximum effect. (b) Dose–response curves for four full

agonist drugs of different potencies but equal efficacy. (Reproduced from Maxwell, S. R. J. and Webb, D. J.

(2008). Receptor functions.Medicine, 36 , 344–349, by permission of Elsevier Science.)

664 Cell membrane receptors and cell signalling