Molecular Determinants of the Intrinsic Efficacy of the Antipsychotic Aripiprazole

Partial agonists of the dopamine D2 receptor (D2R) have been developed to treat the symptoms of schizophrenia without causing the side effects elicited by antagonists. The receptor-ligand interactions that determine the intrinsic efficacy of such drugs, however, are poorly understood. Aripiprazole has an extended structure comprising a phenylpiperazine primary pharmacophore and a 1,2,3,4-tetrahydroquinolin-2-one secondary pharmacophore. We combined site-directed mutagenesis, analytical pharmacology, ligand fragments and molecular dynamics simulations to identify the D2R-aripiprazole interactions that contribute to affinity and efficacy. We reveal that an interaction between the secondary pharmacophore of aripiprazole and a secondary binding pocket defined by residues at the extracellular portions of transmembrane segments 1, 2 and 7 determine the intrinsic efficacy of aripiprazole. Our findings reveal a hitherto unappreciated mechanism through which to fine-tune the intrinsic efficacy of D2R agonists

Discovery of a novel class of negative allosteric modulator of the dopamine D2 receptor through fragmentation of a bitopic ligand

We recently demonstrated that SB269652 (1) engages one protomer of a dopamine D2 receptor (D2R) dimer in a bitopic mode to allosterically inhibit the binding of dopamine at the other protomer. Herein, we investigate structural deter- minants for allostery, focusing on modifications to three moieties within 1. We find that orthosteric “head” groups with small 7-substituents were important to maintain the limited negative cooperativity of analogues of 1, and replacement of the tetrahydroisoquinoline head group with other D2R “privileged structures” generated orthosteric antagonists. Additionally, replacement of the cyclohexylene linker with polymethylene chains conferred linker length dependency in allosteric pharmacology. We validated the importance of the indolic NH as a hydrogen bond donor moiety for maintaining allostery. Replacement of the indole ring with azaindole conferred a 30-fold increase in affinity while maintaining negative cooperativity. Combined, these results provide novel SAR insight for bitopic ligands that act as negative allosteric modulators of the D2R

Potent prearranged positive allosteric modulators of the glucagon-like peptide-1 receptor

Drugs that allosterically modulate G protein-coupled receptor (GPCR) activity display higher specificity and may improve disease treatment. However, the rational design of compounds that target the allosteric site is difficult, as conformations required for receptor activation are poorly understood. Guided by photopharmacology, a set of prearranged positive allosteric modulators (PAMs) with restricted degrees of freedom was designed and tested against the glucagon-like peptide-1 receptor (GLP-1R), a GPCR involved in glucose homeostasis. Compounds incorporating a trans-stilbene comprehensively outperformed those with a cis-stilbene, as well as the benchmark BETP, as GLP-1R PAMs. We also identified major effects of ligand conformation on GLP-1R binding kinetics and signal bias. Thus, we describe a photopharmacology-directed approach for rational drug design, and introduce a new class of stilbene-containing PAM for the specific regulation of GPCR activity

Follicle-Stimulating Hormone Receptor: Advances and Remaining Challenges

International audienc

Understanding biased agonism at the dopamine D₂ receptor

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

Structure–Activity Relationships of Privileged Structures Lead to the Discovery of Novel Biased Ligands at the Dopamine D 2Receptor

Biased agonism at GPCRs highlights the potential for the discovery and design of pathway-selective ligands and may confer therapeutic advantages to ligands targeting the dopamine D2 receptor (D2R). We investigated the determinants of efficacy, affinity, and bias for three privileged structures for the D2R, exploring changes to linker length and incorporation of a heterocyclic unit. Profiling the compounds in two signaling assays (cAMP and pERK1/2) allowed us to identify and quantify determinants of biased agonism at the D2R. Substitution on the phenylpiperazine privileged structures (2-methoxy vs 2,3-dichloro) influenced bias when the thienopyridine heterocycle was absent. Upon inclusion of the thienopyridine unit, the substitution pattern (4,6-dimethyl vs 5-chloro-6-methoxy-4-methyl) had a significant effect on bias that overruled the effect of the phenylpiperazine substitution pattern. This latter observation could be reconciled with an extended binding mode for these compounds, whereby the interaction of the heterocycle with a secondary binding pocket may engender bias

Proof of concept study for designed multiple ligands targeting the dopamine D2, serotonin 5-HT2A, and muscarinic M1 acetylcholine receptors

Herein we describe the hybridization of a benzoxazinone M1 scaffold with D2 privileged structures derived from putative and clinically relevant antipsychotics to develop designed multiple ligands. The M1 mAChR is an attractive target for the cognitive deficits in key CNS disorders. Moreover, activity at D2 and 5-HT2A receptors has proven useful for antipsychotic efficacy. We identified 9 which retained functional activity at the target M1 mAChR and D2R and demonstrated high affinity for the 5-HT2AR

Structure–Activity Relationships of Privileged Structures Lead to the Discovery of Novel Biased Ligands at the Dopamine D₂ Receptor

Biased agonism at GPCRs highlights the potential for the discovery and design of pathway-selective ligands and may confer therapeutic advantages to ligands targeting the dopamine D₂ receptor (D₂R). We investigated the determinants of efficacy, affinity, and bias for three privileged structures for the D₂R, exploring changes to linker length and incorporation of a heterocyclic unit. Profiling the compounds in two signaling assays (cAMP and pERK1/2) allowed us to identify and quantify determinants of biased agonism at the D₂R. Substitution on the phenylpiperazine privileged structures (2-methoxy vs 2,3-dichloro) influenced bias when the thienopyridine heterocycle was absent. Upon inclusion of the thienopyridine unit, the substitution pattern (4,6-dimethyl vs 5-chloro-6-methoxy-4-methyl) had a significant effect on bias that overruled the effect of the phenylpiperazine substitution pattern. This latter observation could be reconciled with an extended binding mode for these compounds, whereby the interaction of the heterocycle with a secondary binding pocket may engender bias

A structure-activity analysis of biased agonism at the dopamine D2 receptor.

Biased agonism offers an opportunity for the medicinal chemist to discover pathway-selective ligands for GPCRs. A number of studies have suggested that biased agonism at the dopamine D2 receptor (D2R) may be advantageous for the treatment of neuropsychiatric disorders, including schizophrenia. As such, it is of great importance to gain insight into the SAR of biased agonism at this receptor. We have generated SAR based on a novel D2R partial agonist, tert-butyl (trans-4-(2-(3,4-dihydroisoquinolin-2(1H)-yl)ethyl)cyclohexyl)carbamate (4). This ligand shares structural similarity to cariprazine (2), a drug awaiting FDA approval for the treatment of schizophrenia, yet displays a distinct bias toward two different signaling end points. We synthesized a number of derivatives of 4 with subtle structural modifications, including incorporation of cariprazine fragments. By combining pharmacological profiling with analytical methodology to identify and to quantify bias, we have demonstrated that efficacy and biased agonism can be finely tuned by minor structural modifications to the head group containing the tertiary amine, a tail group that extends away from this moiety, and the orientation and length of a spacer region between these two moieties