Quantitative analysis of receptor allosterism and its implication for drug discovery

Published on December 31, 2015   57 min
0:00
Hi, this is Rumin Zhang, a Research Scientist with Merck. Today, I'll be giving a lecture on the quantitative analysis of receptor allosterism and its implication for drug discovery. This lecture is based on the review paper we recently wrote and published in the expert opinion on drug discovery.
0:22
In this lecture, we will cover six aspects. First, I'll go over, with you, the operational model for receptor allosterism. And then I'll provide a high level classification of receptor modulators. Thirdly, I'll go over, with you, the optimal screening conditions for modulators. And fourth, we'll talk about a phenomena we termed as beta supremacy in allosterism. And fifth, we will cover the optimal assay design and data analysis by global curve fitting analysis. Lastly, address the impact of maximum system response, Emax, on the parameterization of receptor allosterism and this lack of impact on the structure-activity relationship, or SAR ranking.
1:12
Shown is a schematic for a general operational model for receptor allosterism. Let's focus on the lower left side. The R stands for receptor. Recepted self may have some basal level or constitutive level of receptor activity, because a small fraction of the receptor population may adopt a preexisting bioactive conformation that can be ready to talk to the signaling molecule proteins Arrestins and so forth, and they initiate some biological responses. So it has a basal level we termed KAI, as a level of basal or constitutive receptor activity. And then to the right, where you have agonist A binds to the receptor for mean a binary agonist receptor complex, your AR. That binary complex also possesses a level of activity which is described as τA. And of course, that activity derives from the fact that AR complex will also isomerize into a bioactive confirmation AR star. And then on the left, go up from receptor to RB, that B stands for the modulator molecule, separate from the agonist molecule. The modulator B can form a binary complex with the same receptor. In this case, it can occupy a site other than the agonist binding site, we call this an allosteric modulator. But B also can bind to the same site, an overlapping site with agonist, and behave as an allosterically competitive molecule. So that RB binary complex also may possess some level of biologic activity termed as τB. In a fourth thermodynamic circle, if A and B are mutually inclusive, then they should be able to form a ternary complex, together termed ARB. And that is the agonist receptor and modulator together forming a ternary complex. And that ternary complex can have its specific biologic activity described as τAB. In the field, τAB is often described as a product of two parameters. Beta times τA, meaning, how many folds of change relative to the agonist activity, τA. Well, that's the description of the biologic activity for the four receptor species, R, AR, RB, and ARB. Notice also when the agonist or the modulator molecule bind to the receptor, they have certain affinities. And those affinity constants are described as KA or KB, meaning the dissociation constant of AR or the dissociation constant of RB molecule. But in the presence of each other, that affinity constant may be modified by a factor termed alpha. Thus in the presence of agonists on the right side AR forming ARB, that binary affinity can be different from R forming RB in the absence of A. So the KB affinity constant can be modified by a fraction factor called alpha or affinity modifier. And likewise, by the principle of thermodynamic linkage, if the affinity of B binding to a receptor that's preoccupied by A, AR to ARB, if that affinity has been modified by alpha, then likewise, mutually, the affinity of A binding to the receptor in the presence of B, which is RB to ARB on that top, that affinity will also be modified by the same fold of alpha. So alpha is called the affinity modifier.
4:52
And then, right in the middle of the inner box, I designated an 'n', which is a slope factor or system response stiffness factor. And that one appears in the formula. As you see, the E over Emax, which is the particular efficacy at a given concentration of agonist and the given concentration of modulator compound B, and that particular response in relation to the maximum system response Emax, E over Emax, which is a fractional efficacy response is a function of basically three things. Here I simplify the equation to indicate only three parameters. One is what I call the affinity parameter K apparent, Kapp. And the other one is efficacy apparent, efficacy parameter called E apparent, Eapp. Notice, the affinity parameter Kapp is basically a summation of four different terms that are either affinity constant KA, KB, or the concentration of agonist A indicated here, A indicates the concentration of agonist, or B, the concentration of the modulator, or also the alpha which is the affinity modifier. So that's the affinity, aggregate affinity term, KAB, which consists of all this affinity and those terms. And then the EF, which is the aggregate apparent efficacy term. That has the same affinity parameters, as well as the concentration of A and B, but it also additionally has the four efficacy parameters. Notice the prefix to each of the four terms. KAI, that's the receptor basal activity. And then τA, that's the agonist bond receptor activity. And τB, that's the modulator B bond receptor activity. And then the beta times τA, which is really τAB, which is the ternary complex activity. So those are the two complex terms. And then the third term is the n, which is the system response factor. That fully describes the system response, all these eight parameters, and we can broadly classify these eight parameters into sort of two ways. One way of classifying is, they are either efficacy parameters or they are affinity parameters. Efficacy parameters include KAI τA, τB, τAB, or beta times τA. And then the affinity parameters are KA, KB, alpha and of course, the slope factor impacts the whole thing. It's neither classified as efficacy or affinity, it impacts all of them globally. And the second way of classifying these parameters is based on whether the parameter is specific for the agonist or specific for the modulator. Thus we have agonist specific or modulator-specific parameters. So the agonist specific parameters is on the bottom of the diagram, R to AR. Notice the KAI, the KA, the τA, and of course n slope, as it impacts the agonist response also. Those four can be called agonist specific parameters. And then all the rest KB, τB, τAB, or beta, and often all the modulator-specific parameters. So that's all about the operational model and the formula that describes the model.
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Quantitative analysis of receptor allosterism and its implication for drug discovery

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