Nonequilibrium drug antagonism.

Numerous studies during the past decade have demonstrated that certain pharmacological antagonists act through a relatively stable combination with specific receptors. This action cannot be considered to be truly irreversible. However, it is qualitatively different from classical competitive antagonism in that the agonist and antagonist are not in mass-action equilibrium with the receptors. The blockade produced may therefore be referred to as nonequilibrium. It is clearly different from ninecompetitive antagonism in which the antagonist acts at some point other than the site of action of the agonist in question. Nonequilibrium blockade cannot be adequately distinguished from competitive or noncompetitive blockade on the basis of effects on the agonist dose-response curves, or on the basis of duration of action. However, satisfactory differentiation can be made by combination of tests, several of which depend upon the fact that the persistence of a competitive blockade appears to ve limited by diffusion of antagonist into and out of the immediate environment of the receptors (bio-phase), whereas that of a nonequilibrium blockade is dependent upon the stability of the inhibitor-receptor complex. The action of the two established groups of ninequilibrium inhibitors, the β-haloalkylamine antihistaminies and adrenergic blocking agents, and the organophosphorus anticholinesterases, appears to develope in two stages. 1) The inhibitor is attached to the receptor by the same relatively weak forces (hydrogen bond, ionic, etc.) Which are involved in binding most agonists and classical competitive antagonists. During this stage the antagonist is in mass-action equlibrium with the agonist and may be remobed relatively casily by washing the tissue. 2) The blicking agent then reacts with the receptor or some adjacent grouping to from a much more stable bond and is no longer in competitive equilibrium with the agonist. Because of the competitive phase early in the development of blockade, prior occupancy of the receptors by agonist inhibits the development of blockade and provides direct evidence that the reaction is within the specific receptors. In the case of β-haloalkylamines, the development of a stable drug-receptor bond can be related directly to the chemical reactivity of the ethyleneiminium intermediates formed at physiological pH. The nature of the chemical structures with which these drugs react in tissues has not been established, but in vitro studies suggest that sulfhydryl groups may be involved. The availability of agents which form stable bonds with a variety of specific receptors provides an important tool for the analysis of drug effect. Use of these agents has made it possible to demonstrate that the receptors for adrenaline, histamine, acetylcholine and 5-hydroxytryptamine are distinct (48), that only a fraction of the total adrenergic or histamine receptors is necessary for the production of a maximal tissue response (48,111), and that activation of receptors is not an all-or-none process (109), Many other applications of these agents to the analysis of mechanisms of drug action will undoubtedly be developed.