Kenakin Terry, Jenkinson Stephen, Watson Christian
Department of Assay Development, GlaxoSmithKline Research and Development, 5 Moore Drive, Research Triangle Park, NC 27709, USA.
J Pharmacol Exp Ther. 2006 Nov;319(2):710-23. doi: 10.1124/jpet.106.107375. Epub 2006 Jul 20.
Insurmountable antagonism (maximal response to the agonist depressed) can result from a temporal inequilibrium involving a slow offset orthosteric antagonist or be the result of an allosteric modulation of the receptor. The former mechanism is operative when the antagonist, agonist, and receptors cannot come to proper equilibrium during the time allotted for collection of agonist response (hemi-equilibrium conditions). Allosteric effects (changes in the conformation of the receptor through binding of the allosteric modulator to a separate site) can preclude the agonist-induced production of response, leading to depression of maximal responses. In these cases, the effects on receptor affinity can be observed as well. The first premise of this article is that system-independent estimates of insurmountable antagonist potency can be made with no prior knowledge of molecular mechanism through the use of pA(2) (-log molar concentration of antagonist producing a 2-fold shift of the concentration response curve) measurements The relationship between the pA(2) and antagonist pK(B) (-log equilibrium dissociation constant of the antagonist-receptor complex) is described; the former is an extremely close approximation of the latter in most cases. The second premise is that specially designed experiments are required to differentiate orthosteric versus allosteric mechanisms; simply fitting of data to orthosteric or allosteric theoretical models can lead to ambiguous results. A strategy to determine whether the observed antagonism is orthosteric (agonist and antagonist competing for the same binding site on the receptor) or allosteric in nature is described that involves the detection of the hallmarks of allosteric response, namely saturation and probe dependence of effect.
不可克服的拮抗作用（激动剂的最大反应降低）可能源于涉及缓慢解离的正构拮抗剂的时间不平衡，或者是受体变构调节的结果。当前者机制起作用时，拮抗剂、激动剂和受体在收集激动剂反应的规定时间内（半平衡条件）无法达到适当的平衡。变构效应（变构调节剂与受体的另一位点结合导致受体构象改变）可阻止激动剂诱导的反应产生，导致最大反应降低。在这些情况下，对受体亲和力的影响也可观察到。本文的第一个前提是，通过使用pA₂（使浓度-反应曲线产生2倍位移的拮抗剂的-log摩尔浓度）测量，可以在不了解分子机制的情况下对不可克服的拮抗剂效力进行独立于系统的估计。描述了pA₂与拮抗剂pK₇（拮抗剂-受体复合物的-log平衡解离常数）之间的关系；在大多数情况下，前者是后者的极其近似值。第二个前提是，需要进行专门设计的实验来区分正构与变构机制；简单地将数据拟合到正构或变构理论模型可能会导致模糊的结果。描述了一种确定观察到的拮抗作用是正构（激动剂和拮抗剂竞争受体上的同一结合位点）还是变构性质的策略，该策略涉及检测变构反应的特征，即效应的饱和度和探针依赖性。
