Agonist, inverse

Sanofi-Synthelabo researchers discovered pyrazole 53 and analogs to have potent Cannabinoid receptor-1 (CB-1) antagonist/inverse agonist activity and have progressed 53 into development for treatment of obesity and alcohol dependence. The synthesis of 53 was accomplished by heating the diketone sodium salt 51 with the aryl hydrazine hydrochloride in acetic acid to provide the intermediate 52, which was further derivatized... 

The most probable mechanism for inverse agonism is the same one operable for positive agonism namely, selective receptor state affinity. However, unlike agonists that have a selectively higher affinity for the receptor active state (to induce G-protein activation and subsequent physiological response) inverse agonists have a selectively higher affinity for the inactive receptor state and thus uncouple already spontaneously coupled [RaG] species in the system. 

FIGURE 3.10 Constitutive activity due to receptor overexpression visualization through binding and function, (a) Constitutive activity observed as receptor species ([RaG]/[RL0J) and cellular function ([RaG]/ ([RaG] + 3), where P = 0.03. Stimulus-response function ([RaG]/([RaG] + p)) shown in inset. The output of the [RaG] function becomes the input for the response function. Dotted line shows relative amounts of elevated receptor species and functional response at [R]/KG= 1. (b) Effects of an inverse agonist in a system with [R]/ Kq= 1 (see panel a) as observed through receptor binding and cellular function. 

While the extended ternary complex model accounts for the presence of constitutive receptor activity in the absence of ligands, it is thermodynamically incomplete from the standpoint of the interaction of receptor and G-protein species. Specifically, it must be possible from a thermodynamic point of view for the inactive state receptor (ligand bound and unbound) to interact with G-proteins. The cubic ternary complex model accommodates this possibility [23-25]. From a practical point of view, it allows for the potential of receptors (whether unbound or bound by inverse agonists) to sequester G-proteins into a nonsignaling state. 

There are some specific differences between the cubic and extended ternary complex models in terms of predictions of system and drug behavior. The first is that the receptor, either ligand bound or not bound, can form a complex with the G-protein and that this complex need not signal (i.e., [ARiG] and [RjG]). Under these circumstances an inverse agonist (one that stabilizes the inactive state of the receptor) theoretically can form inactive ternary complexes and thus sequester G-proteins away from signaling pathways. There is evidence that this can occur with cannabi-noid receptor [26]. The cubic ternary complex model also... 

Bouaboula, M., Perrachon, S., Milligan, L., Canatt, X., Rinaldi-Carmona, M., Portier, M., Barth, F., Calandra, B., Pecceu, F., Lupker, J., Maffrand, J.-P., Le Fur, G., and Casellas, P. (1997). A selective inverse agonist for central cannabinoid receptor inhibits mitogen-activated protein kinase activation stimulated by insulin or insulin-like growth factor. J. Biol. Ckem. 272 22330-22339. 

Analyses for Inverse Agonists in Constitntively Active Receptor Systems... 

FIGURE 6.12 Schild analysis for constitutively active receptor systems, (a) Competitive antagonism by the inverse agonist in a constitutively active receptor system with DR values calculated at the EC80. 

There are two ways to estimate the potency of an inverse agonist from the system described by Equation 6.17. The first is to observe the concentration of inverse agonist that... 

Equation 6.19 predicts an increasing IC50 with either increases in L or 1. In systems with low-efficacy inverse agonists or in systems with low levels of constitutive activity, the observed location parameter is still a close estimate of the KB (equilibrium dissociation constant of the ligand-receptor complex, a molecular quantity that transcends test system type). In general, the observed potency of inverse agonists only defines the lower limit of affinity. 

This expression predicts that the modifying term will always be <1 for an inverse agonist (p < 1). Therefore, the calculation of the affinity of an inverse agonist from dextral displacement data (pA2 measurement) will always overestimate the potency of the inverse agonist. However, since... 

The same principles (Schild analysis) can be applied to competitive antagonists that demonstrate either positive (partial agonists) or negative (inverse agonists). 

Functional effects of an inverse agonist with the operational model (6.8.4)... 

Functional Effects of an Inverse Agonist with the Operational Model... 

The pA2 calculation is derived by equating the response produced by the full agonist in the absence of the inverse agonist (Equation 6.64 with [B] = 0) to the response in the presence of a concentration of the inverse agonist that produces a dose ratio of 2 (by definition the pA2). For calculation of KB from 10-pAT... 

Used to estimate system independent potency of an inverse agonist in a constitutively active receptor system. 

Operational model for inverse agonist interaction with agonist variable slope... 

Operational Model for Inverse Agonist Interaction with Agonist Variable Slope ... 

FIGURE 11.2 Paired experimental data. Values of constitutive calcitonin receptor activity [1 -(Tr/Tj) units] in transiently transfected melanophores. Five separate experiments are shown. Points to the left indicate the basal level of constitutive activity before (filled circles) and after (open circles) addition of 100 nM AC512 (calcitonin receptor inverse agonist). Lines join values for each individual experiment. Points to the right are the mean values for constitutive activity in control (filled circles) and after AC512 (open circles) for all five experiments (bars represent standard errors of the mean). Data shown in Table 11.3. 

As can be seen from the analysis in Table 11.3, the paired t-test indicates that the effect of AC512 on the constitutive activity is significant at the 99% level of confidence (p<0.01 and AC512 is an inverse agonist and does decrease the constitutive receptor activity of calcitonin receptors). 

Constitutive receptor activity, receptors spontaneously produce conformations that activate G-proteins in the absence of agonists. This activity, referred to as constitutive activity, can be observed in systems in which the receptor expression levels are high and the resulting levels of spontaneously activating receptor species produce a visible physiological response. An inverse agonist reverses this constitutivie activity and thus reduces, in a dose-dependent... 

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