Prolonged calcitonin receptor signaling by salmon, but not human calcitonin, reveals ligand bias

Ligand-Dependent Modulation of G Protein Conformation Alters Drug Efficacy

G protein-coupled receptor (GPCR) signaling, mediated by hetero-trimeric G proteins, can be differentially controlled by agonists. At a molecular level, this is thought to occur principally via stabilization of distinct receptor conformations by individual ligands. These distinct conformations control subsequent recruitment of transducer and effector proteins. Here, we report that ligand efficacy at the calcitonin GPCR (CTR) is also correlated with ligand-dependent alterations to G protein conformation. We observe ligand-dependent differences in the sensitivity of the G protein ternary complex to disruption by GTP, due to conformational differences in the receptor-bound G protein hetero-trimer. This results in divergent agonist-dependent receptor-residency times for the hetero-trimeric G protein and different accumulation rates for downstream second messengers. This study demonstrates that factors influencing efficacy extend beyond receptor conformation(s) and expands understanding of the molecular basis for how G proteins control/influence efficacy. This has important implications for the mechanisms that underlie ligand-mediated biased agonism. VIDEO ABSTRACT.

KBP-088, a novel DACRA with prolonged receptor activation, is superior to davalintide in terms of efficacy on body weight

This study aims to elucidate the mechanism behind the potent weight loss induced by dual amylin and calcitonin receptor agonists (DACRA) through comparison of the novel DACRA KBP-088 with the amylinomimetic davalintide with regard to in vitro receptor pharmacology and in vivo efficacy on food intake and body weight. KBP-088 and davalintide were tested for their ability to activate the amylin and calcitonin receptors as function of dose and time. Two doses of KBP-088 (1.67 and 5.0 μg/kg) were compared with similar davalintide doses in high-fat diet (HFD)-fed rats receiving subcutaneous dosing once daily for 62 days. Glucose tolerance was assessed after 3 and 7 wk of treatment. KBP-088 demonstrated activation of amylin and calcitonin receptors and prolonged receptor activation compared with davalintide as well as a potent reduction of acute food intake. KBP-088 transiently reduced food intake and induced and notably sustained a significant ∼16% vehicle-corrected weight loss without significant weight loss in the calorie-restricted control groups. Additionally, KBP-088 reduced white adipose tissues and adipocyte hypertrophy. Finally, KBP-088 alleviated hyperinsulinemia and improved oral glucose tolerance even with significantly lower insulin levels after 3 and 7 wk of treatment. KBP-088 is a potent amylin and calcitonin receptor agonist with prolonged receptor activation compared with davalintide. Moreover, KBP-088 induced and sustained significant weight loss and reduced overall adiposity and adipocyte hypertrophy in HFD rats. Finally, KBP-088 improved oral glucose tolerance and alleviated hyperinsulinemia, underscoring the potential of KBP-088 as an antiobesity agent with benefits on glucose control.

Allostery and Biased Agonism at Class B G Protein-Coupled Receptors

Class B G protein-coupled receptors (GPCRs) respond to paracrine or endocrine peptide hormones involved in control of bone homeostasis, glucose regulation, satiety, and gastro-intestinal function, as well as pain transmission. These receptors are targets for existing drugs that treat osteoporosis, hypercalcaemia, Paget's disease, type II diabetes, and obesity and are being actively pursued as targets for numerous other diseases. Exploitation of class B receptors has been limited by difficulties with small molecule drug discovery and development and an under appreciation of factors governing optimal therapeutic efficacy. Recently, there has been increasing awareness of novel attributes of GPCR function that offer new opportunity for drug development. These include the presence of allosteric binding sites on the receptor that can be exploited as drug binding pockets and the ability of individual drugs to enrich subpopulations of receptor conformations to selectively control signaling, a phenomenon termed biased agonism. In this review, current knowledge of biased signaling and small molecule allostery within class B GPCRs is discussed, highlighting areas that have progressed significantly over the past decade, in addition to those that remain largely unexplored with respect to these phenomena.

The role of kinetic context in apparent biased agonism at GPCRs

Biased agonism describes the ability of ligands to stabilize different conformations of a GPCR linked to distinct functional outcomes and offers the prospect of designing pathway-specific drugs that avoid on-target side effects. This mechanism is usually inferred from pharmacological data with the assumption that the confounding influences of observational (that is, assay dependent) and system (that is, cell background dependent) bias are excluded by experimental design and analysis. Here we reveal that ‘kinetic context', as determined by ligand-binding kinetics and the temporal pattern of receptor-signalling processes, can have a profound influence on the apparent bias of a series of agonists for the dopamine D₂ receptor and can even lead to reversals in the direction of bias. We propose that kinetic context must be acknowledged in the design and interpretation of studies of biased agonism.

Biased agonists act at a receptor to preferentially induce distinct intracellular signalling responses over others. Here the authors show how kinetics of ligand binding and signaling responses greatly influence observed bias profiles, and hence must be considered when studying biased agonists.

Amylin: Pharmacology, Physiology, and Clinical Potential

Amylin is a pancreatic β-cell hormone that produces effects in several different organ systems. Here, we review the literature in rodents and in humans on amylin research since its discovery as a hormone about 25 years ago. Amylin is a 37-amino-acid peptide that activates its specific receptors, which are multisubunit G protein-coupled receptors resulting from the coexpression of a core receptor protein with receptor activity-modifying proteins, resulting in multiple receptor subtypes. Amylin's major role is as a glucoregulatory hormone, and it is an important regulator of energy metabolism in health and disease. Other amylin actions have also been reported, such as on the cardiovascular system or on bone. Amylin acts principally in the circumventricular organs of the central nervous system and functionally interacts with other metabolically active hormones such as cholecystokinin, leptin, and estradiol. The amylin-based peptide, pramlintide, is used clinically to treat type 1 and type 2 diabetes. Clinical studies in obesity have shown that amylin agonists could also be useful for weight loss, especially in combination with other agents.

PTH receptor-1 signalling-mechanistic insights and therapeutic prospects

Parathyroid hormone/parathyroid hormone-related protein receptor (PTH/PTHrP type 1 receptor; commonly known as PTHR1) is a family B G-protein-coupled receptor (GPCR) that regulates skeletal development, bone turnover and mineral ion homeostasis. PTHR1 transduces stimuli from PTH and PTHrP into the interior of target cells to promote diverse biochemical responses. Evaluation of the signalling properties of structurally modified PTHR1 ligands has helped to elucidate determinants of receptor function and mechanisms of downstream cellular and physiological responses. Analysis of PTHR1 responses induced by structurally modified ligands suggests that PTHR1 can continue to signal through a G-protein-mediated pathway within endosomes. Such findings challenge the longstanding paradigm in GPCR biology that the receptor is transiently activated at the cell membrane, followed by rapid deactivation and receptor internalization. Evaluation of structurally modified PTHR1 ligands has further led to the identification of ligand analogues that differ from PTH or PTHrP in the type, strength and duration of responses induced at the receptor, cellular and organism levels. These modified ligands, and the biochemical principles revealed through their use, might facilitate an improved understanding of PTHR1 function in vivo and enable the treatment of disorders resulting from defects in PTHR1 signalling. This Review discusses current understanding of PTHR1 modes of action and how these findings might be applied in future therapeutic agents.

Can residence time offer a useful strategy to target agonist drugs for sustained GPCR responses?

Residence time describes the how long a ligand is bound to its target, and is attracting interest in drug discovery as a potential means of improving clinical efficacy by increasing target coverage. This concept, as originally applied to antagonists, is more complicated for G-protein-coupled receptor (GPCR) agonists because of the transiency of receptor responses (via desensitization and internalization). However, in some cases sustained GPCR agonist responses have been observed, with evidence consistent with a role for slow binding kinetics. We propose a model to explain our understanding of how residence time and rebinding might influence sustained signaling by internalized receptors. We also highlight the anticipated benefit for drug discovery of fully understanding and exploiting these phenomena to target desirable receptor response profiles selectively.

Sustained wash-resistant receptor activation responses of GPR119 agonists

G protein-coupled receptor 119 (GPR119) is involved in regulating metabolic homoeostasis, with GPR119 agonists targeted for the treatment of type-2 diabetes and obesity. Using the endogenous agonist oleoylethanolamide and a number of small molecule synthetic agonists we have investigated the temporal dynamics of receptor signalling. Using both a dynamic luminescence biosensor-based assay and an endpoint cAMP accumulation assay we show that agonist-driven desensitization is not a major regulatory mechanism for GPR119 despite robust activation responses, regardless of the agonist used. Temporal analysis of the cAMP responses demonstrated sustained signalling resistant to washout for some, but not all of the agonists tested. Further analysis indicated that the sustained effects of one synthetic agonist AR-231,453 were consistent with a role for slow dissociation kinetics. In contrast, the sustained responses to MBX-2982 and AZ1 appeared to involve membrane deposition. We also detect wash-resistant responses to AR-231,453 at the level of physiologically relevant responses in an endogenous expression system (GLP-1 secretion in GLUTag cells). In conclusion, our findings indicate that in a recombinant expression system GPR119 activation is sustained, with little evidence of pronounced receptor desensitization, and for some ligands persistent agonist responses continue despite removal of excess agonist. This provides novel understanding of the temporal responses profiles of potential drug candidates targetting GPR119, and highlights the importance of carefully examining the the mechanisms through which GPCRs generate sustained responses.

Lessons learned from the clinical development of oral peptides

The oral delivery of peptides and proteins has been hampered by an array of obstacles. However, several promising novel oral delivery systems have been developed. This paper reviews the most advanced oral formulation technologies, and highlights key lessons and implications from studies undertaken to date with these oral formulations. Special interest is given to oral salmon calcitonin (CT), glucagon-like peptide-1 (GLP-1), insulin, PYY-(3-36), recombinant human parathyroid hormone (rhPTH(1-31)-NH2) and PTH(1-34), by different technologies. The issues addressed include (i) interaction with water, (ii) interaction with food, (iii) diurnal variation, (iv) inter- and intra-subject variability, (v) correlation between efficacy and exposure and (vi) key deliverables of different technologies. These key lessons may aid research in the development of other oral formulations.