AM833 Is a Novel Agonist of Calcitonin Family G Protein-Coupled Receptors: Pharmacological Comparison with Six Selective and Nonselective Agonists

Modulation of amylin and calcitonin receptor activation by hybrid peptides

Calcitonin peptide hormone controls calcium homeostasis by activating the calcitonin receptor. When the calcitonin receptor forms a complex with an accessory protein, the complex functions as the receptors for another peptide hormone amylin. The amylin receptors are the drug target for diabetes and obesity treatment. Since human amylin can produce aggregates, rat amylin that does not form aggregates has been commonly used for research. Interestingly, calcitonin originated from salmons was reported to interact with human amylin receptors with higher affinity/potency than endogenous rat amylin. Here, the peptide hybrid was made of a rat amylin N-terminal fragment and a salmon calcitonin C-terminal fragment. This novel hybrid peptide showed higher potency for human amylin receptor 1/2 activation by 6- to 8-fold than endogenous rat amylin. To further examine the role of the peptide C-terminal fragment in receptor activation, another hybrid peptide was made where salmon calcitonin N-terminal 21 amino acids were fused with rat amylin C-terminal 11 amino acids. The rat amylin C-terminal fragment was previously reported to have relatively low affinity for calcitonin receptor extracellular domain. As expected, this calcitonin-amylin hybrid peptide decreased the potency for calcitonin receptor activation by 3-fold compared to salmon calcitonin. The hybrid strategy used in this study significantly changed the peptide potency for amylin and calcitonin receptor activation. These results provide insight into the role of peptide C-terminal fragments in modulating amylin and calcitonin receptor activation.

Central nervous system pathways targeted by amylin in the regulation of food intake

Amylin is a peptide hormone co-released with insulin from pancreatic β-cells during a meal and primarily serves to promote satiation. While the caudal hindbrain was originally implicated as a major site of action in this regard, it is becoming increasingly clear that amylin recruits numerous central nervous system pathways to exert multifaceted effects on food intake. In this Review, we discuss the evidence derived from preclinical studies showing that amylin and the related peptide salmon calcitonin (sCT) directly or indirectly target genetically distinct neurons in the caudal hindbrain (nucleus tractus solitarius and area postrema), rostral hindbrain (lateral parabrachial nucleus), midbrain (lateral dorsal tegmentum and ventral tegmental area) and hypothalamus (arcuate nucleus and parasubthalamic nucleus) via activation of amylin and/or calcitonin receptors. Given that the stable amylin analogue cagrilintide is under clinical development for the treatment of obesity, it is important to determine whether this drug recruits overlapping or distinct central nervous system pathways to that of amylin and sCT with implications for minimising any aversive effects it potentially causes. Such insight will also be important to understand how amylin and sCT analogues synergize with other molecules as part of dual or triple agonist therapies for obesity, especially the glucagon-like peptide 1 receptor (GLP-1R) agonist semaglutide, which has been shown to synergistically lower body weight with cagrilintide (CagriSema) in clinical trials.

Amylin receptor subunit interactions are modulated by agonists and determine signaling

Three amylin receptors (AMYRs) mediate the metabolic actions of the peptide hormone amylin and are drug targets for diabetes and obesity. AMY1R, AMY2R, and AMY3R are heterodimers consisting of the G protein-coupled calcitonin receptor (CTR) paired with a RAMP1, -2, or -3 accessory subunit, respectively, which increases amylin potency. Little is known about AMYR subunit interactions and their role in signaling. Here, we show that the AMYRs have distinct basal subunit equilibriums that are modulated by peptide agonists and determine the cAMP signaling phenotype. Using a novel biochemical assay that resolves the AMYR heterodimers and free subunits, we found that the AMY1/2R subunit equilibriums favored free CTR and RAMP1/2, and rat amylin and αCGRP agonists promoted subunit association. A stronger CTR-RAMP3 transmembrane domain interface yielded a more stable AMY3R, and human and salmon calcitonin agonists promoted AMY3R dissociation. Similar changes in subunit association-dissociation were observed in live cell membranes, and G protein coupling and cAMP signaling assays showed how these altered signaling. Our findings reveal regulation of heteromeric GPCR signaling through subunit interaction dynamics.

Converting the Amyloidogenic Islet Amyloid Polypeptide into a Potent Nonaggregating Peptide Ligand by Side Chain-to-Side Chain Macrocyclization

The islet amyloid polypeptide (IAPP), also known as amylin, is a hormone playing key physiological roles. However, its aggregation and deposition in the pancreatic islets are associated with type 2 diabetes. While this peptide adopts mainly a random coil structure in solution, its secondary conformational conversion into α-helix represents a critical step for receptor activation and contributes to amyloid formation and associated cytotoxicity. Considering the large conformational landscape and high amyloidogenicity of the peptide, as well as the complexity of the self-assembly process, it is challenging to delineate the delicate interplay between helical folding, peptide aggregation, and receptor activation. In the present study, we probed the roles of helical folding on the function-toxicity duality of IAPP by restricting its conformational ensemble through side chain-to-side chain stapling via azide-alkyne cycloaddition. Intramolecular macrocyclization (i; i + 4) constrained IAPP into α-helix and inhibited its aggregation into amyloid fibrils. These helical derivatives slowed down the self-assembly of unmodified IAPP. Site-specific macrocyclization modulated the capacity of IAPP to perturb lipid bilayers and cell plasma membrane and reduced, or even fully inhibited, the cytotoxicity associated with aggregation. Furthermore, the α-helical IAPP analogs showed moderate to high potency toward cognate G protein-coupled receptors. Overall, these results indicate that macrocyclization represents a promising strategy to protect an amyloidogenic peptide hormone from aggregation and associated toxicity, while maintaining high receptor activity.

Targeting the incretin system in obesity and type 2 diabetes mellitus

Obesity and type 2 diabetes mellitus (T2DM) are widespread, non-communicable diseases that are responsible for considerable levels of morbidity and mortality globally, primarily in the form of cardiovascular disease (CVD). Changes to lifestyle and behaviour have insufficient long-term efficacy in most patients with these diseases; metabolic surgery, although effective, is not practically deliverable on the scale that is required. Over the past two decades, therapies based on incretin hormones, spearheaded by glucagon-like peptide 1 (GLP1) receptor agonists (GLP1RAs), have become the treatment of choice for obesity and T2DM, and clinical evidence now suggests that these agents have benefits for CVD. We review the latest advances in incretin-based pharmacotherapy. These include 'GLP1 plus' agents, which combine the known advantages of GLP1RAs with the activity of additional hormones, such as glucose-dependent insulinotropic peptide, glucagon and amylin, to achieve desired therapeutic goals. Second-generation non-peptidic oral GLP1RAs promise to extend the benefits of GLP1 therapy to those who do not want, or cannot have, subcutaneous injection therapy. We conclude with a discussion of the knowledge gaps that must be addressed before incretin-based therapies can be properly deployed for maximum benefit in the treatment of obesity and T2DM.

Type-2 Diabetes, Pancreatic Amylin, and Neuronal Metabolic Remodeling in Alzheimer's Disease

Type-2 diabetes raises the risk for Alzheimer's disease (AD)-type dementia and the conversion from mild cognitive impairment to dementia, yet mechanisms connecting type-2 diabetes to AD remain largely unknown. Amylin, a pancreatic β-cell hormone co-secreted with insulin, participates in the central regulation of satiation, but also forms pancreatic amyloid in persons with type-2 diabetes and synergistically interacts with brain amyloid β (Aβ) pathology, in both sporadic and familial Alzheimer's disease (AD). Growing evidence from studies of tumor growth, together with early observations in skeletal muscle, indicates amylin as a potential trigger of cellular metabolic reprogramming. Because the blood, cerebrospinal fluid, and brain parenchyma in humans with AD have increased concentrations of amylin, amylin-mediated pathological processes in the brain may involve neuronal metabolic remodeling. This review summarizes recent progress in understanding the link between prediabetic hypersecretion of amylin and risk of neuronal metabolic remodeling and AD and suggests nutritional and medical effects of food constituents that might prevent and/or ameliorate amylin-mediated neuronal metabolic remodeling.

Variable CGRP family peptide signaling durations and the structural determinants thereof

Calcitonin gene-related peptides alpha and beta (αCGRP, βCGRP), adrenomedullin (AM), and adrenomedullin 2/intermedin (AM2/IMD) function in pain signaling, neuroimmune communication, and regulation of the cardiovascular and lymphatic systems by activating either of two class B GPCRs, CLR and CTR, in complex with a RAMP1, -2, or -3 modulatory subunit. Inspired by our recent discovery that AM2/IMD(1-47) activation of CLR-RAMP3 elicits long duration cAMP signaling, here we used a live-cell cAMP biosensor assay to characterize the signaling kinetics of the two CGRP peptides and several bioactive AM and AM2/IMD fragments with variable N-terminal extensions. Remarkably, AM2/IMD(8-47) and AM2/IMD-53 exhibited even longer duration signaling than the 1-47 fragment. AM2/IMD(8-47) was a striking 8-fold longer acting than AM(13-52) at CLR-RAMP3. In contrast, the N-terminal extension of AM had no effect on signaling duration. AM(1-52) and (13-52) were equally short-acting. Analysis of AM2/IMD-AM mid-region chimeras and AM2/IMD R23 and R33 point mutants showed the importance of these residues for long-duration signaling and identified AM2/IMD peptides that exhibited up to 17-fold diminished signaling duration at CLR-RAMP3, while retaining near wildtype signaling potencies. βCGRP was ∼ 3-fold longer acting than αCGRP at the CGRP (CLR-RAMP1) and the amylin1 (CTR-RAMP1) receptors. Chimeric CGRP peptides showed that the single residue difference near the N-terminus, and the two differences in the mid-region, equally contributed to the longer duration of βCGRP signaling. This work uncovers key temporal differences in cAMP signaling among the CGRP family peptides, elucidates the structural bases thereof, and provides pharmacological tools for studying long-duration AM2/IMD signaling.

The processing intermediate of human amylin, pro-amylin(1-48), has in vivo and in vitro bioactivity

Amylin is released by pancreatic beta-cells in response to a meal and its major soluble mature form (37 amino acid-peptide) produces its biological effects by activating amylin receptors. Amylin is derived from larger propeptides that are processed within the synthesizing beta-cell. There are suggestions that a partially processed form, pro-amylin(1-48) is also secreted. We tested the hypothesis that pro-amylin(1-48) has biological activity and that human pro-amylin(1-48) may also form toxic pre-amyloid species. Amyloid formation, the ability to cross-seed and in vitro toxicity were similar between human pro-amylin(1-48) and amylin. Human pro-amylin(1-48) was active at amylin-responsive receptors, though its potency was reduced at rat, but not human amylin receptors. Pro-amylin(1-48) was able to promote anorexia by activating neurons of the area postrema, amylin's primary site of action, indicating that amylin can tolerate significant additions at the N-terminus without losing bioactivity. Our studies help to shed light on the possible roles of pro-amylin(1-48) which may be relevant for the development of future amylin-based drugs.

Development of a Novel Assay for Direct Assessment of Selective Amylin Receptor Activation Reveals Novel Differences in Behavior of Selective and Nonselective Peptide Agonists

Dual amylin and calcitonin receptor agonists (DACRAs) show promise as efficacious therapeutics for treatment of metabolic disease, including obesity. However, differences in efficacy in vivo have been observed for individual DACRAs, indicating that detailed understanding of the pharmacology of these agents across target receptors is required for rational drug development. To date, such understanding has been hampered by lack of direct, subtype-selective, functional assays for the amylin receptors (AMYRs). Here, we describe the generation of receptor-specific assays for recruitment of Venus-tagged Gs protein through fusion of luciferase to either the human calcitonin receptor (CTR), human receptor activity-modifying protein (RAMP)-1, RAMP1 (AMY1R), human RAMP2 (AMY2R), or human RAMP3 (AMY3R). These assays revealed a complex pattern of receptor activation by calcitonin, amylin, or DACRA peptides that was distinct at each receptor subtype. Of particular note, although both of the CT-based DACRAs, sCT and AM1784, displayed relatively similar behaviors at CTR and AMY1R, they generated distinct responses at AMY2R and AMY3R. These data aid the rationalization of in vivo differences in response to DACRA peptides in rodent models of obesity. Direct assessment of the pharmacology of novel DACRAs at AMYR subtypes is likely to be important for development of optimized therapeutics for treatment of metabolic diseases. SIGNIFICANCE STATEMENT: Amylin receptors (AMYRs) are important obesity targets. Here we describe a novel assay that allows selective functional assessment of individual amylin receptor subtypes that provides unique insight into the pharmacology of potential therapeutic ligands. Direct assessment of the pharmacology of novel agonists at AMYR subtypes is likely to be important for development of optimized therapeutics for treatment of metabolic diseases.

Structural insight into selectivity of amylin and calcitonin receptor agonists

Amylin receptors (AMYRs), heterodimers of the calcitonin receptor (CTR) and one of three receptor activity-modifying proteins, are promising obesity targets. A hallmark of AMYR activation by Amy is the formation of a 'bypass' secondary structural motif (residues S19-P25). This study explored potential tuning of peptide selectivity through modification to residues 19-22, resulting in a selective AMYR agonist, San385, as well as nonselective dual amylin and calcitonin receptor agonists (DACRAs), with San45 being an exemplar. We determined the structure and dynamics of San385-bound AMY3R, and San45 bound to AMY3R or CTR. San45, via its conjugated lipid at position 21, was anchored at the edge of the receptor bundle, enabling a stable, alternative binding mode when bound to the CTR, in addition to the bypass mode of binding to AMY3R. Targeted lipid modification may provide a single intervention strategy for design of long-acting, nonselective, Amy-based DACRAs with potential anti-obesity effects.

Cagrilintide: A Long-Acting Amylin Analog for the Treatment of Obesity

Despite the worldwide epidemic of obesity, there remain few approved pharmacological treatment options to bridge the gap between lifestyle therapy and bariatric surgery. Cagrilintide is an amylin-analog, now being developed in combination with the GLP-1 agonist semaglutide to achieve sustained weight loss in persons with overweight and obesity. Amylin, released with insulin from beta cells in the pancreas, induces its satiating effect via both the homoeostatic and hedonic regions of the brain. Semaglutide, a GLP-1 receptor agonist, reduces appetite via GLP-1 receptors in the hypothalamus and increases the production of insulin, and reduces glucagon secretion, delaying gastric emptying. These separate, but related mechanisms of action of an amylin-analog and a GLP-1 receptor agonist appear to have an additive effect on appetite reduction. Given the heterogeneity and complex pathogenesis of obesity, combination therapy with multiple pathophysiological targets is a logical approach to increasing weight loss response with pharmacotherapy. Cagrilintide alone, as well as cagrilintide in combination with semaglutide have shown promising weight loss in clinical trials that supports the further development of this therapy for sustained weight management.

Calcitonin/PAC1 receptor splice variants: a blind spot in migraine research

The neuropeptides calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide (PACAP) and their receptors are linked to migraine neurobiology. Recent antimigraine therapeutics targeting the signaling of these neuropeptides are effective; however, some patients respond suboptimally, indicating an incomplete understanding of migraine pathophysiology. The CGRP- and PACAP-responsive receptors can be differentially spliced. It is known that receptor splice variants can have different pathophysiological effects in other receptor-mediated pain pathways. Despite considerable knowledge on the structural and pharmacological differences of the CGRP- and PACAP-responsive receptor splice variants and their expression in migraine-relevant tissues, their role in migraine is rarely considered. Here we shine a spotlight on the calcitonin and PACAP (PAC1) receptor splice variants and examine what implications they may have for drug activity and design.

Body weight lowering effect of glucose-dependent insulinotropic polypeptide and glucagon-like peptide receptor agonists is more efficient in RAMP1/3 KO than in WT mice

The glucose-dependent insulinotropic polypeptide (GIPR) and glucagon-like peptide (GLP-1R) receptor agonists are insulin secretagogues that have long been shown to improve glycemic control and dual agonists have demonstrated successful weight loss in the clinic. GIPR and GLP-1R populations are located in the dorsal vagal complex where receptor activity-modifying proteins (RAMPs) are also present. According to recent literature, RAMPs not only regulate the signaling of the calcitonin receptor, but also that of other class B G-protein coupled receptors, including members of the glucagon receptor family such as GLP-1R and GIPR. The aim of this study was to investigate whether the absence of RAMP1 and RAMP3 interferes with the action of GIPR and GLP-1R agonists on body weight maintenance and glucose control. To this end, WT and RAMP 1/3 KO mice were fed a 45% high fat diet for 22 weeks and were injected daily with GLP-1R agonist (2 nmol/kg/d; NN0113-2220), GIPR agonist (30 nmol/kg/d; NN0441-0329) or both for 3 weeks. While the mono-agonists exerted little to no body weight lowering and anorectic effects in WT or RAMP1/3 KO mice, but at the given doses, when both compounds were administered together, they synergistically reduced body weight, with a greater effect observed in KO mice. Finally, GLP-1R and GIP/GLP-1R agonist treatment led to improved glucose tolerance, but the absence of RAMPs resulted in an improvement of the HOMA-IR score. These data suggest that RAMPs may play a crucial role in modulating the pharmacological actions of GLP-1 and GIP receptors.

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Elucidating the Interactome of G Protein-Coupled Receptors and Receptor Activity-Modifying Proteins

G protein-coupled receptors (GPCRs) are known to interact with several other classes of integral membrane proteins that modulate their biology and pharmacology. However, the extent of these interactions and the mechanisms of their effects are not well understood. For example, one class of GPCR-interacting proteins, receptor activity-modifying proteins (RAMPs), comprise three related and ubiquitously expressed single-transmembrane span proteins. The RAMP family was discovered more than two decades ago, and since then GPCR-RAMP interactions and their functional consequences on receptor trafficking and ligand selectivity have been documented for several secretin (class B) GPCRs, most notably the calcitonin receptor-like receptor. Recent bioinformatics and multiplexed experimental studies suggest that GPCR-RAMP interactions might be much more widespread than previously anticipated. Recently, cryo-electron microscopy has provided high-resolution structures of GPCR-RAMP-ligand complexes, and drugs have been developed that target GPCR-RAMP complexes. In this review, we provide a summary of recent advances in techniques that allow the discovery of GPCR-RAMP interactions and their functional consequences and highlight prospects for future advances. We also provide an up-to-date list of reported GPCR-RAMP interactions based on a review of the current literature. SIGNIFICANCE STATEMENT: Receptor activity-modifying proteins (RAMPs) have emerged as modulators of many aspects of G protein-coupled receptor (GPCR)biology and pharmacology. The application of new methodologies to study membrane protein-protein interactions suggests that RAMPs interact with many more GPCRs than had been previously known. These findings, especially when combined with structural studies of membrane protein complexes, have significant implications for advancing GPCR-targeted drug discovery and the understanding of GPCR pharmacology, biology, and regulation.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

Novel Noninvasive Approaches to the Treatment of Obesity: From Pharmacotherapy to Gene Therapy

Recent insights into the pathophysiologic underlying mechanisms of obesity have led to the discovery of several promising drug targets and novel therapeutic strategies to address the global obesity epidemic and its comorbidities. Current pharmacologic options for obesity management are largely limited in number and of modest efficacy/safety profile. Therefore, the need for safe and more efficacious new agents is urgent. Drugs that are currently under investigation modulate targets across a broad range of systems and tissues, including the central nervous system, gastrointestinal hormones, adipose tissue, kidney, liver, and skeletal muscle. Beyond pharmacotherapeutics, other potential antiobesity strategies are being explored, including novel drug delivery systems, vaccines, modulation of the gut microbiome, and gene therapy. The present review summarizes the pathophysiology of energy homeostasis and highlights pathways being explored in the effort to develop novel antiobesity medications and interventions but does not cover devices and bariatric methods. Emerging pharmacologic agents and alternative approaches targeting these pathways and relevant research in both animals and humans are presented in detail. Special emphasis is given to treatment options at the end of the development pipeline and closer to the clinic (ie, compounds that have a higher chance to be added to our therapeutic armamentarium in the near future). Ultimately, advancements in our understanding of the pathophysiology and interindividual variation of obesity may lead to multimodal and personalized approaches to obesity treatment that will result in safe, effective, and sustainable weight loss until the root causes of the problem are identified and addressed.

THERAPY OF ENDOCRINE DISEASE: Amylin and calcitonin - physiology and pharmacology

Type 2 diabetes is a common manifestation of metabolic dysfunction due to obesity and constitutes a major burden for modern health care systems, in concert with the alarming rise in obesity worldwide. In recent years, several successful pharmacotherapies improving glucose metabolism have emerged and some of these also promote weight loss, thus, ameliorating insulin resistance. However, the progressive nature of type 2 diabetes is not halted by these new anti-diabetic pharmacotherapies. Therefore, novel therapies promoting weight loss further and delaying diabetes progression are needed. Amylin, a beta cell hormone, has satiating properties and also delays gastric emptying and inhibits postprandial glucagon secretion with the net result of reducing postprandial glucose excursions. Amylin acts through the six amylin receptors, which share the core component with the calcitonin receptor. Calcitonin, derived from thyroid C cells, is best known for its role in humane calcium metabolism, where it inhibits osteoclasts and reduces circulating calcium. However, calcitonin, particularly of salmon origin, has also been shown to affect insulin sensitivity, reduce the gastric emptying rate and promote satiation. Preclinical trials with agents targeting the calcitonin receptor and the amylin receptors, show improvements in several parameters of glucose metabolism including insulin sensitivity and some of these agents are currently undergoing clinical trials. Here, we review the physiological and pharmacological effects of amylin and calcitonin and discuss the future potential of amylin and calcitonin-based treatments for patients with type 2 diabetes and obesity.

Mediators of Amylin Action in Metabolic Control

Amylin (also called islet amyloid polypeptide (IAPP)) is a pancreatic beta-cell hormone that is co-secreted with insulin in response to nutrient stimuli. The last 35 years of intensive research have shown that amylin exerts important physiological effects on metabolic control. Most importantly, amylin is a physiological control of meal-ending satiation, and it limits the rate of gastric emptying and reduces the secretion of pancreatic glucagon, in particular in postprandial states. The physiological effects of amylin and its analogs are mediated by direct brain activation, with the caudal hindbrain playing the most prominent role. The clarification of the structure of amylin receptors, consisting of the calcitonin core receptor plus receptor-activity modifying proteins, aided in the development of amylin analogs with a broad pharmacological profile. The general interest in amylin physiology and pharmacology was boosted by the finding that amylin is a sensitizer to the catabolic actions of leptin. Today, amylin derived analogs are considered to be among the most promising approaches for the pharmacotherapy against obesity. At least in conjunction with insulin, amylin analogs are also considered important treatment options in diabetic patients, so that new drugs may soon be added to the only currently approved compound pramlintide (Symlin^®). This review provides a brief summary of the physiology of amylin’s mode of actions and its role in the control of the metabolism, in particular energy intake and glucose metabolism.

Long-acting amylin analogues for the management of obesity

PURPOSE OF REVIEW

To summarize recent developments of long-acting amylin analogues for the treatment of obesity and to outline their mode of action.

RECENT FINDINGS

Amylin is a pancreatic hormone acting to control energy homeostasis and body weight. Activity at the calcitonin and amylin receptors in the area postrema seems to - at least partly - be responsible for these effects of amylin. Both preclinical and early-stage clinical studies investigating long-acting amylin receptor analogues demonstrate beneficial effects on body weight in obesity. Cagrilintide, a novel amylin analogue suitable for once-weekly administration, is in phase II clinical development and has shown promising body weight reducing effects alone and in combination with the glucagon-like peptide 1 receptor agonist semaglutide.

SUMMARY

Long-acting amylin analogues have emerged as a possible pharmacotherapy against obesity, but more studies are needed to support the utility and long-term effects of this strategy in relevant populations.

Lipidated Calcitonin Gene-Related Peptide (CGRP) Peptide Antagonists Retain CGRP Receptor Activity and Attenuate CGRP Action In Vivo

Signaling through calcitonin gene-related peptide (CGRP) receptors is associated with pain, migraine, and energy expenditure. Small molecule and monoclonal antibody CGRP receptor antagonists that block endogenous CGRP action are in clinical use as anti-migraine therapies. By comparison, the potential utility of peptide antagonists has received less attention due to suboptimal pharmacokinetic properties. Lipidation is an established strategy to increase peptide half-life in vivo. This study aimed to explore the feasibility of developing lipidated CGRP peptide antagonists that retain receptor antagonist activity in vitro and attenuate endogenous CGRP action in vivo. CGRP peptide analogues based on the archetypal CGRP receptor antagonist, CGRP_8-37, were palmitoylated at the N-terminus, position 24, and near the C-terminus at position 35. The antagonist activities of the lipidated peptide analogues were tested in vitro using transfected Cos-7 cells expressing either the human or mouse CGRP receptor, amylin subtype 1 (AMY₁) receptor, adrenomedullin (AM) receptors, or calcitonin receptor. Antagonist activities were also evaluated in SK-N-MC cells that endogenously express the human CGRP receptor. Lipidated peptides were then tested for their ability to antagonize endogenous CGRP action in vivo using a capsaicin-induced dermal vasodilation (CIDV) model in C57/BL6J mice. All lipidated peptides except for the C-terminally modified analogue retained potent antagonist activity compared to CGRP_8-37 towards the CGRP receptor. The lipidated peptides also retained, and sometimes gained, antagonist activities at AMY₁, AM₁ and AM₂ receptors. Several lipidated peptides produced robust inhibition of CIDV in mice. This study demonstrates that selected lipidated peptide antagonists based on αCGRP_8-37 retain potent antagonist activity at the CGRP receptor and are capable of inhibition of endogenous CGRP action in vivo. These findings suggest that lipidation can be applied to peptide antagonists, such as αCGRP_8-37 and are a potential strategy for antagonizing CGRP action.

A structural basis for amylin receptor phenotype

Amylin receptors (AMYRs) are heterodimers of the calcitonin (CT) receptor (CTR) and one of three receptor activity-modifying proteins (RAMPs), AMY₁R, AMY₂R, and AMY₃R. Selective AMYR agonists and dual AMYR/CTR agonists are being developed as obesity treatments; however, the molecular basis for peptide binding and selectivity is unknown. We determined the structure and dynamics of active AMYRs with amylin, AMY₁R with salmon CT (sCT), AMY₂R with sCT or human CT (hCT), and CTR with amylin, sCT, or hCT. The conformation of amylin-bound complexes was similar for all AMYRs, constrained by the RAMP, and an ordered midpeptide motif that we call the bypass motif. The CT-bound AMYR complexes were distinct, overlapping the CT-bound CTR complexes. Our findings indicate that activation of AMYRs by CT-based peptides is distinct from their activation by amylin-based peptides. This has important implications for the development of AMYR therapeutics.

Hypophagia induced by salmon calcitonin, but not by amylin, is partially driven by malaise and is mediated by CGRP neurons

Objective

The behavioral mechanisms and the neuronal pathways mediated by amylin and its long-acting analog sCT (salmon calcitonin) are not fully understood and it is unclear to what extent sCT and amylin engage overlapping or distinct neuronal subpopulations to reduce food intake. We here hypothesize that amylin and sCT recruit different neuronal population to mediate their anorectic effects.

Methods

Viral approaches were used to inhibit calcitonin gene-related peptide (CGRP) lateral parabrachial nucleus (LPBN) neurons and assess their role in amylin’s and sCT’s ability to decrease food intake in mice. In addition, to test the involvement of LPBN CGRP neuropeptidergic signaling in the mediation of amylin and sCT’s effects, a LPBN site-specific knockdown was performed in rats. To deeper investigate whether the greater anorectic effect of sCT compared to amylin is due do the recruitment of additional neuronal pathways related to malaise multiple and distinct animal models tested whether amylin and sCT induce conditioned avoidance, nausea, emesis, and conditioned affective taste aversion.

Results

Our results indicate that permanent or transient inhibition of CGRP neurons in LPBN blunts sCT-, but not amylin-induced anorexia and neuronal activation. Importantly, sCT but not amylin induces behaviors indicative of malaise including conditioned affective aversion, nausea, emesis, and conditioned avoidance; the latter mediated by CGRP^LPBN neurons.

Conclusions

Together, the present study highlights that although amylin and sCT comparably decrease food intake, sCT is distinctive from amylin in the activation of anorectic neuronal pathways associated with malaise.

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CGRP neurons mediate the effect of the amylin agonist salmon calcitonin (sCT) on food intake.

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Amylin's hypophagic effect does not require CGRP neurons.

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sCT-induced anorexia but not amylin is associated with malaise.

Targeting the Enteroendocrine System for Treatment of Obesity

Mimetics of the anorexigenic gut hormone glucagon-like peptide 1 (GLP-1) were originally developed as insulinotropic anti-diabetic drugs but also evoke significant weight loss, leading to their recent approval as obesity therapeutics. Co-activation of receptors for GLP-1 and other gut hormones which reduce food intake - peptide YY (PYY_3-36), cholecystokinin (CCK) and glucose-dependent insulinotropic peptide (GIP) - is now being explored clinically to enhance efficacy. An alternative approach involves pharmacologically stimulating endogenous secretion of these hormones from enteroendocrine cells (EECs) to recapitulate the metabolic consequences of bariatric surgery, where highly elevated postprandial levels of GLP-1 and PYY_3-36 are thought to contribute to improved glycaemia and weight loss.

Amylin as a Future Obesity Treatment

Obesity is defined as abnormal or excessive fat accumulation that contributes to detrimental health impacts. One-third of the population suffers from obesity, and it is important to consider obesity as a chronic disease requiring chronic treatment. Amylin is co-secreted with insulin from β pancreatic cells upon nutrient delivery to the small intestine as a satiety signal, acts upon sub-cortical homeostatic and hedonic brain regions, slows gastric emptying, and suppresses post-prandial glucagon responses to meals. Therefore, new pharmacological amylin analogues can be used as potential anti-obesity medications in individuals who are overweight or obese. In this narrative review, we analyse the efficacy, potency, and safety of amylin analogues. The synthetic amylin analogue pramlintide is an approved treatment for diabetes mellitus which promotes better glycaemic control and small but significant weight loss. AM833 (cagrilintide), an investigational novel long-acting acylated amylin analogue, acts as a non-selective amylin receptor. This calcitonin G protein-coupled receptor agonist can serve as an attractive novel treatment for obesity, resulting in reduction of food intake and significant weight loss in a dose-dependent manner.

Drugs for Treating Obesity

Older medications approved for chronic weight management (orlistat, naltrexone/bupropion, liraglutide 3 mg and, in the USA, phentermine/topiramate) have not been widely adopted by health care providers. Those medications produce only modest additional weight loss when used to augment lifestyle intervention. However, semaglutide 2.4 mg weekly has recently emerged and produces much more weight loss - on average 15% weight loss at 1 year. Semaglutide's enhanced efficacy and that its class (GLP-1 receptor analogs) is well-known may result in more clinicians adopting pharmacotherapy. Furthermore, the first dedicated cardiovascular outcome trial powered for superiority testing an anti-obesity medication (SELECT) is underway with semaglutide 2.4 mg. A positive outcome will further promote the concept that weight management should be a primary target for cardiometabolic disease control. In phase 3, tirzepatide and cagrilintide/semaglutide combination are showing promise for even greater weight loss efficacy. Another recently approved medication takes a personalized medicine approach; setmelanotide is approved as a therapy for those with some of the ultra-rare genetic diseases characterized by severe, early onset obesity. This chapter reviews the currently available and anticipated medications for chronic weight management as well as those approved for the genetic and syndromic obesities.

Development of Cagrilintide, a Long-Acting Amylin Analogue

A hallmark of the pancreatic hormone amylin is its high propensity toward the formation of amyloid fibrils, which makes it a challenging drug design effort. The amylin analogue pramlintide is commercially available for diabetes treatment as an adjunct to insulin therapy but requires three daily injections due to its short half-life. We report here the development of the stable, lipidated long-acting amylin analogue cagrilintide (23) and some of the structure-activity efforts that led to the selection of this analogue for clinical development with obesity as an indication. Cagrilintide is currently in clinical trial and has induced significant weight loss when dosed alone or in combination with the GLP-1 analogue semaglutide.