Functional selectivity in GPCR modulator screening

In high throughput screening systems, a single concentration of a new compound is tested in a biological system to detect direct effects (agonists) or effects on other ligands (antagonists). In this latter case, the chemical context of the assay is defined by a balance of maximal sensitivity (limited agonist concentration) and maximal window to observe effect (sizable agonist concentration to induce measurable effect). For allosteric modulators, there are other factors that should be considered in high throughput screening environments. Specifically, the saturable aspect of allosteric effect can dissociate the observed ordinate change in response (% inhibition) and potency of effect (concentration at which a given ordinate % effect is obtained). Also, the specter of probe dependence can be important in systems where the physiologically relevant agonist cannot be used for screening (i.e. HIV-1 entry). Finally, the interactive nature of allosteric systems can cause complex relationships between the chemical context of an assay and potency of allosteric modulator. For example, in cases where the efficacy of an agonist is reduced but the affinity is increased by a modulator, it may be more beneficial to have higher concentrations of agonist in the screening assay to optimize sensitivity to modulators. This must be balanced for allosteric potentiators with the need to retain a window to observe increased agonist effect.