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(Página creada con «right|thumb|320px|Espectro de eficacia de ligandos de receptores. La '''actividad intrínseca''' (AI) o '''eficacia''' se refiere a la cap...»)
 
[[Imagen:Efficacy spectrum.png|right|thumb|320px|Espectro de eficacia de ligandos de receptores.]]
 
La '''actividad intrínseca''' (AI) o '''eficacia''' se refiere a la capacidad relativa de un complejo fármaco-receptor para producir una respuesta funcional máxima. Esto debe distinguirse de la [[Constante de disociación|afinidad]], que es una medida de la capacidad del fármaco para unirse a su diana molecular, y el [[EC50|EC<sub>50</sub>]], que es una medida de la potencia de la droga y que es proporcional a la eficacia y afinidad. Este uso de la palabra "eficacia" se introdujo por Stephenson (1956)<ref name="pmid13383117">{{cite journal | author = Stephenson RP | title = A modification of receptor theory | journal = Br J Pharmacol Chemother | volume = 11 | issue = 4 | pages = 379–93 |date=December 1956 | pmid = 13383117 | pmc = 1510558 | doi = 10.1111/j.1476-5381.1956.tb00006.x| url = | issn = }}</ref> para describir la forma en que los [[agonista]]s varían en la respuesta que producen, incluso cuando ocupan el mismo número de receptores. Agonistas de alta eficacia pueden producir la respuesta máxima del sistema receptor ocupando una proporción relativamente baja de los receptores en ese sistema.<!--
 
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'''Intrinsic activity''' ('''IA''') or '''efficacy''' refers to the relative ability of a [[drug]]-[[receptor (biochemistry)|receptor]] complex to produce a maximum functional response. This must be distinguished from the [[dissociation constant#Protein-ligand binding|affinity]], which is a measure of the ability of the drug to bind to its molecular target, and the [[half maximal effective concentration|EC<sub>50</sub>]], which is a measure of the [[Potency (pharmacology)|potency]] of the drug and which is proportional to both efficacy and affinity. This use of the word "efficacy" was introduced by Stephenson (1956)<ref name="pmid13383117">{{cite journal | author = Stephenson RP | title = A modification of receptor theory | journal = Br J Pharmacol Chemother | volume = 11 | issue = 4 | pages = 379–93 |date=December 1956 | pmid = 13383117 | pmc = 1510558 | doi = 10.1111/j.1476-5381.1956.tb00006.x| url = | issn = }}</ref> to describe the way in which [[agonist]]s vary in the response they produce, even when they occupy the same number of receptors. High efficacy agonists can produce the maximal response of the receptor system while occupying a relatively low proportion of the receptors in that system.
 
Agonists of lower efficacy are not as efficient at producing a response from the drug-bound receptor, by stabilizing the active form of the drug-bound receptor. Therefore, they may not be able to produce the same maximal response, even when they occupy the entire receptor population, as the efficiency of transformation of the inactive form of the drug-receptor complex to the active drug-receptor complex may not be high enough to evoke a maximal response. Since the observed response may be less than maximal in systems with no spare receptor reserve, some low efficacy agonists are referred to as partial agonists.<ref name="urlGlaxo Wellcome pharmacology guide">{{cite web | url = http://www.pdg.cnb.uam.es/cursos/Barcelona2002/pages/Farmac/Comput_Lab/Guia_Glaxo/chap2c.html | title = In vitro pharmacology: concentration-response curves | author = | authorlink = | coauthors = | date = | work = Glaxo Wellcome pharmacology guide | publisher = | pages = | language = | archiveurl = | archivedate = | quote = | accessdate = 2009-07-11}}</ref>
 
However, it is worth bearing in mind that these terms are relative - even partial agonists may appear as full agonists in a different system/experimental setup, as when the number of receptors increases, there may be enough drug-receptor complexes for a maximum response to be produced, even with individually low efficacy of transducing the response. There are actually relatively few ''true'' full agonists or silent antagonists; many compounds usually considered to be full agonists (such as [[2,5-Dimethoxy-4-iodoamphetamine|DOI]]) are more accurately described as high efficacy partial agonists, as a partial agonist with efficacy over ~80-90% is indistinguishable from a full agonist in most assays. Similarly many antagonists (such as [[naloxone]]) are in fact partial agonists or inverse agonists, but with very low efficacy (less than 10%). Compounds considered partial agonists tend to have efficacy in between this range.
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