Identification, Optimization, and Pharmacology of Acylurea GHS-R1a Inverse Agonists

Structure-Guided Design of Nurr1 Agonists Derived from the Natural Ligand Dihydroxyindole

The neuroprotective transcription factor Nurr1 was recently found to bind the dopamine metabolite 5,6-dihydroxyindole (DHI) providing access to Nurr1 ligand design from a natural template. We screened a custom set of 14 k extended DHI analogues in silico for optimized descendants to select 24 candidates for microscale synthesis and in vitro testing. Three out of six primary hits were validated as novel Nurr1 agonists with up to sub-micromolar binding affinity, highlighting the druggability of the Nurr1 surface region lining helix 12. In vitro profiling confirmed cellular target engagement of DHI descendants and demonstrated remarkable additive effects of combined Nurr1 agonist treatment, indicating diverse binding sites mediating Nurr1 activation, which may open new avenues in Nurr1 modulation.

Optimization of a Pyrimidinone Series for Selective Inhibition of Ca2+/Calmodulin-Stimulated Adenylyl Cyclase 1 Activity for the Treatment of Chronic Pain

Adenylyl cyclase type 1 (AC1) is involved in signaling for chronic pain sensitization in the central nervous system and is an emerging target for the treatment of chronic pain. AC1 and a closely related isoform AC8 are also implicated to have roles in learning and memory signaling processes. Our team has carried out cellular screening for inhibitors of AC1 yielding a pyrazolyl-pyrimidinone scaffold with low micromolar potency against AC1 and selectivity versus AC8. Structure-activity relationship (SAR) studies led to analogues with cellular IC50 values as low as 0.25 μM, selectivity versus AC8 and other AC isoforms as well as other common neurological targets. A representative analogue displayed modest antiallodynic effects in a mouse model of inflammatory pain. This series represents the most potent and selective inhibitors of Ca2+/calmodulin-stimulated AC1 activity to date with improved drug-like physicochemical properties making them potential lead compounds for the treatment of inflammatory pain.

Advances in the Development of Nonpeptide Small Molecules Targeting Ghrelin Receptor

Ghrelin is an octanoylated peptide acting by the activation of the growth hormone secretagogue receptor, namely, GHS-R1a. The involvement of ghrelin in several physiological processes, including stimulation of food intake, gastric emptying, body energy balance, glucose homeostasis, reduction of insulin secretion, and lipogenesis validates the considerable interest in GHS-R1a as a promising target for the treatment of numerous disorders. Over the years, several GHS-R1a ligands have been identified and some of them have been extensively studied in clinical trials. The recently resolved structures of GHS-R1a bound to ghrelin or potent ligands have provided useful information for the design of new GHS-R1a drugs. This perspective is focused on the development of recent nonpeptide small molecules acting as GHS-R1a agonists, antagonists, and inverse agonists, bearing classical or new molecular scaffolds, as well as on radiolabeled GHS-R1a ligands developed for imaging. Moreover, the pharmacological effects of the most studied ligands have been discussed.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

Ligand-independent activity of the ghrelin receptor modulates AMPA receptor trafficking and supports memory formation

The biological signals of hunger, satiety, and memory are interconnected. The role of the hormone ghrelin in regulating feeding and memory makes ghrelin receptors attractive targets for associated disorders. We investigated the effects of the high ligand-independent activity of the ghrelin receptor GHS-R1a on the physiology of excitatory synapses in the hippocampus. Blocking this activity produced a decrease in the synaptic content of AMPA receptors in hippocampal neurons and a reduction in GluA1 phosphorylation at Ser⁸⁴⁵ Reducing the ligand-independent activity of GHS-R1a increased the surface diffusion of AMPA receptors and impaired AMPA receptor-dependent synaptic delivery induced by chemical long-term potentiation. Accordingly, we found that blocking this GHS-R1a activity impaired spatial and recognition memory in mice. These observations support a role for the ligand-independent activity of GHS-R1a in regulating AMPA receptor trafficking under basal conditions and in the context of synaptic plasticity that underlies learning.

Ghrelin Based Therapy of Metabolic Diseases

BACKGROUND

Ghrelin, a unique 28 amino acid peptide hormone secreted by the gastric X/A like cells, is an endogenous ligand of the growth hormone secretagogue receptor (GHSR). Ghrelin-GHSR signaling has been found to exert various physiological functions, including stimulation of appetite, regulation of body weight, lipid and glucose metabolism, and increase of gut motility and secretion. This system is thus critical for energy homeostasis.

OBJECTIVE

The objective of this review is to highlight the strategies of ghrelin-GHSR based intervention for therapy of obesity and its related metabolic diseases.

RESULTS

Therapeutic strategies of metabolic disorders targeting the ghrelin-GHSR pathway involve neutralization of circulating ghrelin by antibodies and RNA spiegelmers, antagonism of ghrelin receptor by its antagonists and inverse agonists, inhibition of ghrelin O-acyltransferase (GOAT), as well as potential pharmacological approach to decrease ghrelin synthesis and secretion.

CONCLUSION

Various compounds targeting the ghrelin-GHSR system have shown promising efficacy for the intervention of obesity and relevant metabolic disorders in animals and in vitro. Further clinical trials to validate their efficacy in human beings are urgently needed.

Identification of novel GPR81 agonist lead series for target biology evaluation

GPR81 is a novel drug target that is implicated in the control of glucose and lipid metabolism. The lack of potent GPR81 modulators suitable for in vivo studies has limited the pharmacological characterization of this lactate sensing receptor. We performed a high throughput screen (HTS) and identified a GPR81 agonist chemical series containing a central acyl urea scaffold linker. During SAR exploration two additional new series were evolved, one containing cyclic acyl urea bioisosteres and another a central amide bond. These three series provide different selectivity and physicochemical properties suitable for in-vivo studies.

A one-pot route to N-acyl ureas: a formal four-component hydrolytic reaction involving aminonitrones and isocyanide dibromides

A one-pot route to N-acyl ureas proceeds via generation of electrophilically activated 2-substituted 1,2,4-oxadiazolium salts. The conformation of the N-acyl ureas is stabilized via moderate strength (6.2–7.8 kcal mol⁻¹) resonance-assisted hydrogen bonds.

Pharmacological Modulation of Ghrelin to Induce Weight Loss: Successes and Challenges

PURPOSE OF REVIEW

Obesity is affecting over 600 million adults worldwide and has numerous negative effects on health. Since ghrelin positively regulates food intake and body weight, targeting its signaling to induce weight loss under conditions of obesity seems promising. Thus, the present work reviews and discusses different possibilities to alter ghrelin signaling.

RECENT FINDINGS

Ghrelin signaling can be altered by RNA Spiegelmers, GHSR/Fc, ghrelin-O-acyltransferase inhibitors as well as antagonists, and inverse agonists of the ghrelin receptor. PF-05190457 is the first inverse agonist of the ghrelin receptor tested in humans shown to inhibit growth hormone secretion, gastric emptying, and reduce postprandial glucose levels. Effects on body weight were not examined. Although various highly promising agents targeting ghrelin signaling exist, so far, they were mostly only tested in vitro or in animal models. Further research in humans is thus needed to further assess the effects of ghrelin antagonism on body weight especially under conditions of obesity.

Pd-Catalyzed Carbonylation of Acyl Azides

Pd-catalyzed reactions of azides with CO to access an isocynate intermediate have been developed extensively in recent years. However, the catalytic carbonylation of sensitive acyl azides has not been reported. Herein, we report a simple Pd-catalyzed carbonylation reaction of acyl azides with broad substrate scope, high efficiency, and simple operation under mild conditions, which provides facile access to acyl ureas. In addition, a mechanistic study was carried out by both experiment and DFT calculation. Control experiments and kinetic study revealed that the real active palladium species were Pd(0). The result of kinetic study suggested that palladium catalyst, azide, and CO were all involved in the turnover-limiting step except for amine. Further DFT study suggested that an unprecedented five-membered palladacycle intermediate was the key intermediate in the carbonylation reaction.

The Role of Ghrelin in Regulating Synaptic Function and Plasticity of Feeding-Associated Circuits

Synaptic plasticity of the neuronal circuits associated with feeding behavior is regulated by peripheral signals as a response to changes in the energy status of the body. These signals include glucose, free fatty acids, leptin and ghrelin and are released into circulation, being able to reach the brain. Ghrelin, a small peptide released from the stomach, is an orexigenic hormone produced in peripheral organs, and its action regulates food intake, body weight and glucose homeostasis. Behavioral studies show that ghrelin is implicated in the regulation of both hedonic and homeostatic feeding and of cognition. Ghrelin-induced synaptic plasticity has been described in neuronal circuits associated with these behaviors. In this review, we discuss the neuromodulatory mechanisms induced by ghrelin in regulating synaptic plasticity in three main neuronal circuits previously associated with feeding behaviors, namely hypothalamic (homeostatic feeding), ventral tegmental (hedonic and motivational feeding) and hippocampal (cognitive) circuits. Given the central role of ghrelin in regulating feeding behaviors, and the altered ghrelin levels associated with metabolic disorders such as obesity and anorexia, it is of paramount relevance to understand the effects of ghrelin on synaptic plasticity of neuronal circuits associated with feeding behaviors.

Identification and Pharmacological Profile of an Indane Based Series of Ghrelin Receptor Full Agonists

Cachexia and muscle wasting are very common among patients suffering from cancer, chronic obstructive pulmonary disease, and other chronic diseases. Ghrelin stimulates growth hormone secretion via the ghrelin receptor, which subsequently leads to increase of IGF-1 plasma levels. The activation of the GH/IGF-1 axis leads to an increase of muscle mass and functional capacity. Ghrelin further acts on inflammation, appetite, and adipogenesis and for this reason was considered an important target to address catabolic conditions. We report the synthesis and properties of an indane based series of ghrelin receptor full agonists; they have been shown to generate a sustained increase of IGF-1 levels in dog and have been thoroughly investigated with respect to their functional activity.

Indazole-6-phenylcyclopropylcarboxylic Acids as Selective GPR120 Agonists with in Vivo Efficacy

GPR120 agonists have therapeutic potential for the treatment of diabetes, but few selective agonists have been reported. We identified an indazole-6-phenylcyclopropylcarboxylic acid series of GPR120 agonists and conducted SAR studies to optimize GPR120 potency. Furthermore, we identified a (S,S)-cyclopropylcarboxylic acid structural motif which gave selectivity against GPR40. Good oral exposure was obtained with some compounds displaying unexpected high CNS penetration. Increased MDCK efflux was utilized to identify compounds such as 33 with lower CNS penetration, and activity in oral glucose tolerance studies was demonstrated. Differential activity was observed in GPR120 null and wild-type mice indicating that this effect operates through a mechanism involving GPR120 agonism.

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Can We Selectively Reduce Appetite for Energy-Dense Foods? An Overview of Pharmacological Strategies for Modification of Food Preference Behavior

Excessive intake of food, especially palatable and energy-dense carbohydrates and fats, is largely responsible for the growing incidence of obesity worldwide. Although there are a number of candidate antiobesity drugs, only a few of them have been proven able to inhibit appetite for palatable foods without the concurrent reduction in regular food consumption. In this review, we discuss the interrelationships between homeostatic and hedonic food intake control mechanisms in promoting overeating with palatable foods and assess the potential usefulness of systemically administered pharmaceuticals that impinge on the endogenous cannabinoid, opioid, aminergic, cholinergic, and peptidergic systems in the modification of food preference behavior. Also, certain dietary supplements with the potency to reduce specifically palatable food intake are presented. Based on human and animal studies, we indicate the most promising therapies and agents that influence the effectiveness of appetite-modifying drugs. It should be stressed, however, that most of the data included in our review come from preclinical studies; therefore, further investigations aimed at confirming the effectiveness and safety of the aforementioned medications in the treatment of obese humans are necessary.

Intriguing possibilities and beneficial aspects of transporter-conscious drug design

It has been revealed that many types of drugs interact with transporter proteins within an organism. Transporter proteins absorb or excrete materials, including drugs and nutrients, across the cell membrane. Some hydrophobic drugs are excreted from the cell as xenobiotics by ATP-binding cassette (ABC) transporters. However, solute carrier (SLC) transporters are tissue-specifically expressed and have substrate specificities. Thus, transporter-conscious drug design is an excellent method of delivering drugs to pharmaceutical target organs and provides advantages in absorption, distribution, excretion, and toxicity of drugs (ADMET) due to transport systems. In fact, based on this strategy, the bioavailability of prodrugs designed as peptide transporter 1 (PEPT1) substrates was better than that of the corresponding parent compounds due to the transport system in the small intestine. Furthermore, in central nervous system (CNS) drug developing, drug delivery into brain across the blood-brain barrier (BBB) is a serious problem. However, this problem can be also solved by the use of the transport systems at the BBB. Therefore, transporter-consciously designed drugs not only may effectively elicit activity but also may control adverse side effects caused by off-targets and drug-drug interactions and, consequently, may show good performance in clinical trials. In this review, I introduce possibilities and advantages of transporter-conscious drug designs.

Rational design of dual peptides targeting ghrelin and Y2 receptors to regulate food intake and body weight

Ghrelin and Y2 receptors play a central role in appetite regulation inducing opposite effects. The Y2 receptor induces satiety, while the ghrelin receptor promotes hunger and weight gain. However, the food regulating system is tightly controlled by interconnected pathways where redundancies can lead to poor efficacy and drug tolerance when addressing a single molecule. We developed a multitarget strategy to synthesize dual peptides simultaneously inhibiting the ghrelin receptor and stimulating the Y2 receptor. Dual peptides showed dual activity in vitro, and one compound induced a slight diminution of food intake in a rodent model of obesity. In addition, stability studies in rats revealed different behaviors between the dual peptide and its corresponding monomers. The Y2 receptor agonist was unstable in blood, while the dual peptide showed an intermediate stability compared to that of the highly stable ghrelin receptor inverse agonist.