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Partial Agonists Notes

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Lecture 7 - Drugs: From Molecules to Man (19/02/2018)

Partial Agonists
Affinity and Efficacy of a drug

Affinity: Describes how tight the binding is.

Efficacy: Describes what the drug does once bound.
 Agonists display full efficacy.
 Antagonists display zero efficacy.
 Partial agonists also exist and were discovered by Ariens and Stephenson in the 1950s by investigating the effects of homologous series.

Efficacy is basically 'what the drug does' once bound. Full agonists have full efficacy, and competitive antagonists have zero efficacy.
Just like affinity, efficacy is not an all or nothing concept so it can have values that lie in between.

Investigation into Alkyltrimethylammonium drugs

Alkyltrimethylammonium drugs have three methyl groups attached to a nitrogen, but differ in the identity of the fourth group (R).

 Butyl [4]
 Heptyl [7]
 Decyl [9]

Ariens and Stephenson used a rats ileum to measure the degree of contraction when certain drugs were applied.
This graph illustrated their results.

The results displayed how Butyl, Heptyl and Decyl each produced different effects due to the degree of Agonism and Antagonism.

A partial agonist is essentially halfway between an agonist and an antagonist. As a consequence its also possible to see antagonism of a full agonist by a partial agonist.

For example, if we added a near maximal concentration of a full agonist to a tissue together with an near maximal concentration of a partial agonist, the overall response would be smaller than we would see with just the full agonist.

This is even though the number of occupied receptors would be greater. The reason for this is simple: the two drugs will compete and some of the receptors will bind each type of drug. The response evoked by the partial agonist is smaller, so the overall response is reduced.

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