Efficacy
Efficacy is the ability to get a job done satisfactorily. The word comes from the same roots as, and for practical purposes is synonymous with, "effectiveness". It is used in pharmacology and medicine to refer to both the maximum response achievable from a pharmaceutical drug in research settings, and to the capacity for sufficient therapeutic effect or beneficial change in clinical settings.
In pharmacology, efficacy (Emax) is the maximum response achievable from an applied or dosed agent, for instance, a small molecule drug.Intrinsic activity is a relative term that describes a drug's efficacy relative to a drug with the highest observed efficacy. It is a purely descriptive term that has little or no mechanistic interpretation.
In order for a drug to have an effect, it needs to bind to its target, and then to affect the function of this target. The target of a drug is commonly referred to as a receptor, but can in general be any chemically sensitive site on any molecule found in the body. The nature of such binding can be quantified by characterising how tightly these molecules, the drug and its receptor, interact: this is known as the affinity. Efficacy, on the other hand, is a measure of the action of a drug once binding has occurred. The maximum response, Emax, will be reduced if efficacy is sufficiently low, but any efficacy greater than 20 or so gives essentially the same maximum response.
The definition of efficacy has been discussed by. The only way in which absolute measures of efficacy have been obtained is by single ion channel analysis of ligand gated ion channels. It is still not possible to do this for G protein-linked receptors.
In the case of the glycine receptor and the nicotinic acetylcholine receptor (muscle type), it has been proposed by Sivilotti et al. that opening of the ion channel involves two steps after agonist is bound. Firstly a conformation change to a higher affinity (but still shut) form, followed by the conformation change from shut to open. It was found that partial agonism results from deficiency in the first step, and that the opening and shutting steps are essentially the same for both full and partial agonists. This has been confirmed and extended by Sine and colleagues (2009). The implication of this work is that efficacy has to be defined by at least two equilibrium constants (or, more generally, by four rate constants).
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