Understanding biased agonism at the dopamine D₂ receptor
2017-03-03T00:35:02Z (GMT) by
The phenomenon of “biased agonism” presents an attractive avenue for drug development as it allows the separation of therapeutic effects from side effects mediated by the same target. A prototypical G protein-coupled receptor at which biased agonism has been extensively studied is the dopamine D₂ receptor, an important therapeutic target for current treatments of Parkinson’s disease and schizophrenia. There is increasing evidence that biased agonism is important for the antipsychotic efficacy of dopamine D₂ receptor partial agonists, such as aripiprazole and cariprazine. However, a clear relationship between biased agonism at the dopamine D₂ receptor and antipsychotic efficacy remains elusive, not least due to discrepancies in literature describing aripiprazole as ‘biased’ or ‘unbiased’, despite the same signalling endpoints being studied using the same cell background. To clarify such conflicts, and to aid the drug discovery efforts aimed at identifying novel dopamine D₂ receptor biased agonists, the focus of this thesis is to gain greater insight into the mechanisms that mediate biased agonism at the dopamine D₂ receptor. Through the utilization of both mutagenesis-based and structure-activity-based approaches, a secondary binding pocket was identified for being crucial in the affinity, efficacy, and bias of different ligands at the dopamine D₂ receptor. A structure-activity relationship study indicated that both efficacy and biased agonism can be finely tuned by minor structural modifications to the head group, the tail group, and the orientation and length of a spacer region of cariprazine. In particular, it was demonstrated that modifications to the tail region, and thus the interaction with a potential secondary binding site, alter the orientation of the head group within the orthosteric binding site regulating both efficacy and biased agonism. These results were corroborated with a mutagenesis study, in which mutations within a putative secondary binding site significantly impacted the affinity and efficacy of a number of dopamine D₂ receptor agonists. Finally, it was demonstrated that “kinetic context”, as determined by both ligand-binding kinetics and the kinetics intrinsic to different cellular signalling processes, can dramatically impact observations of biased agonism. Such findings illustrate, for the first time, the importance of incorporating kinetic profiling in future studies focussed on biased agonism to allow a more informed selection of preclinical candidates and thus an improved foundation for drug discovery of biased agonists.