The intrinsic activity (English: intrinsic activity ) is a measure of the potency to change the cell function, which results from the binding of a ligand to a receptor . This measure is an important parameter in pharmacodynamics .
When a ligand (e.g. a drug ) reaches its site of action, it binds to the receptor there and forms a ligand-receptor complex with it. While the affinity is a measure of the strength of the bond between the binding partners, the intrinsic activity is a measure of the strength of the effect that results from this bond.
The intrinsic activity is calculated according to the formula , where IA is the intrinsic activity, W max is the maximum action of the agonist and E max is the theoretical maximum effect. The value of the intrinsic activity is always between 0 and 1. If an active ingredient has an intrinsic activity of 0, it does not trigger any effect via the receptor and is therefore a pure antagonist ; if the intrinsic activity is 1, the receptor binding achieves the maximum effect, the substance is then accordingly a pure agonist . Active ingredients with an intrinsic activity between 0 and 1 are called partial agonists . Substances that achieve the opposite effect are called inverse agonists .
It should be noted that the classic model, according to which a ligand acts “monofunctionally” on the receptor, is no longer up-to-date and needs to be updated. Rather, a ligand can address different signaling pathways in a differentiated manner. So it can act as an agonist and an antagonist in parallel on one and the same receptor on different signaling pathways.
Since the intrinsic activity varies from tissue to tissue, the term efficacy has been replaced.
Intrinsic sympathomimetic activity
The term intrinsic sympathomimetic activity, ISA for short , which is a term for the stimulating effect of some β-receptor blockers , such as celiprolol or pindolol , on the receptors occupied by them, exists analogously to intrinsic activity .
- A clear example is the substance SB 242084. A PLA2 inverse agonist and a PLC agonist at the 5-HT 2C receptor .
- Jonathan D. Urban et al. a .: Functional selectivity and classical concepts of quantitative pharmacology. In: Journal of Pharmacology and Experimental Therapeutics , Vol. 320 (2007), Issue 1, pp. 1-13, , PMID 16803859 PDF .