Key Points
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In vitro lead optimization cellular assays are used to refine initial lead molecules obtained from screening. This allows characterization of molecular activity on receptor genotypes.
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However, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds opening the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans.
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Specifically, the differences in in vitro assays and the therapeutic environment can come about if the receptor interacts with more than one G-protein (pleiotropic G-protein coupling) and especially if ligands induce ligand-specific receptor conformations that differentially interact with these G-proteins; both of these conditions have been shown to occur with numerous GPCRs.
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In addition, GPCR activities beyond G-protein coupling and the production of physiological response have been observed (some of these are therapeutically relevant — for example, receptor internalization to inhibit HIV infection), and these also have been seen to behave phenotypically in various cell systems.
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These data support the notion that lead molecules should be tested in the therapeutic environment as quickly as possible to reduce attrition of possibly useful drug candidates.
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These ideas also indicate some areas of possible therapeutic intervention emanating from such phenotypic conditions (that is, phantom genes where receptors assume a phenotype through combination of gene products in cells)
Abstract
Recombinant and natural cellular assays for human G-protein-coupled receptors are used to optimize initial lead molecules obtained from screening. Although the activity of these molecules can be assessed on human genotype receptors, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds. This opens the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans. This review discusses the mechanisms by which cells can impose phenotypic selectivity on molecules and approaches to reduce this practical problem for drug discovery.
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Glossary
- RECOMBINANT SYSTEMS
-
Cells transfected with DNA containing new genetic material in an order different from the original. Genetic engineering can be used to do this deliberately to produce new proteins in cells.
- EFFICACY
-
The property of a molecule that causes the receptor to change its behavior toward the host cell.
- AFFINITY
-
Ligands reside at a point of minimal energy within a binding locus of a protein according to a ratio of the rate that the ligand leaves the surface of the protein (koff) and the rate it approaches the protein surface (kon). This ratio is the equilibrium dissociation constant of the ligand–protein complex (denoted Keq = koff/kon) and defines the molar concentration of the ligand in the compartment containing the protein at which 50% of the protein has ligand bound to it at any one instant. The 'affinity' or attraction of the ligand for the protein is the reciprocal of Keq.
- PARTIAL AGONIST
-
Whereas a full agonist produces the system maximal response, a partial agonist produces a maximal response that is below that of the system maximum (and that of a full agonist). As well as producing a sub-maximal response, partial agonists produce antagonism of more efficacious full agonists.
- FULL AGONIST
-
This is the name given to an agonist that produces the full system maximal response (Emax). It is a system-dependent phenomenon and should not necessarily be associated with a particular agonist, as an agonist can be a full agonist in some systems and a partial agonist in others.
- 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 visible physiological response. An inverse agonist reverses this constitutive activity and so reduces, in a dose-dependent manner, the spontaneously elevated basal response of a constitutively active receptor system.
- INVERSE AGONISTS
-
These ligands reverse constitutive receptor activity. At present it is thought that this occurs because inverse agonists have a selectively higher affinity for the inactive versus the active conformation of the receptor. It is important to note that although inverse agonist activity requires constitutive activity to be observed, the property of the molecule that is responsible for this activity does not disappear when there is no constitutive activity. In these cases, inverse agonists function as simple competitive antagonists.
- HETERODIMER
-
In this case, a physical biochemical complex of two receptors formed in the membrane and interacting with ligands as one species. The pharmacological properties of heterodimers can differ from the properties of either reactant.
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Kenakin, T. Predicting therapeutic value in the lead optimization phase of drug discovery. Nat Rev Drug Discov 2, 429–438 (2003). https://doi.org/10.1038/nrd1110
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DOI: https://doi.org/10.1038/nrd1110
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