Pharmacological demarcation of the growth hormone, gut motility and feeding effects of ghrelin using a novel ghrelin receptor agonist

Efficacy and Safety of Ghrelin Agonists in Patients with Diabetic Gastroparesis: A Systematic Review and Meta-Analysis

Background/Aims

Ghrelin agonists are emerging prokinetic agents for treating gastroparesis. Although recent clinical trials have demonstrated their efficacy in patients with diabetic gastroparesis (DG), the impact of such agents on symptoms and gastric dysmotility remains unclear. We performed a systematic review and meta-analysis to evaluate the efficacy and safety of ghrelin agonists in patients with DG.

Methods

A search of common electronic databases (MEDLINE, Embase, and Cochrane Central Register of Controlled Trials) was preformed, using keyword combinations that referenced ghrelin and DG and retrieving all eligible randomized controlled trials (RCTs) of ghrelin agonists versus placebo in patients with DG. The primary outcome measure was the change in patient-reported overall gastroparesis symptom scores. Secondary outcomes included the change in gastric emptying time, specific symptoms related to gastroparesis, and adverse events. A random-effects model was applied to all study outcomes. Heterogeneity among studies was determined by the chi-square test and I² statistics.

Results

We selected six RCTs of patients with DG (n=557) for meta-analysis. Ghrelin agonist administration (vs placebo) significantly improved overall gastroparesis symptoms (standardized mean difference, –0.34; 95% confidence interval, –0.56 to –0.13) and significantly improved symptoms related to gastroparesis, including nausea, vomiting, early satiety, and abdominal pain. Adverse events recorded for ghrelin agonists and placebo did not differ significantly. There was no significant heterogeneity among eligible studies.

Conclusions

Compared with placebo, ghrelin agonists are effective and well-tolerated for the treatment of DG.

Fluorinated phenylalanines: synthesis and pharmaceutical applications

Recent advances in the chemistry of peptides containing fluorinated phenylalanines (Phe) represents a hot topic in drug research over the last few decades. ᴅ- or ʟ-fluorinated phenylalanines have had considerable industrial and pharmaceutical applications and they have been expanded also to play an important role as potential enzyme inhibitors as well as therapeutic agents and topography imaging of tumor ecosystems using PET. Incorporation of fluorinated aromatic amino acids into proteins increases their catabolic stability especially in therapeutic proteins and peptide-based vaccines. This review seeks to summarize the different synthetic approaches in the literature to prepare ᴅ- or ʟ-fluorinated phenylalanines and their pharmaceutical applications with a focus on published synthetic methods that introduce fluorine into the phenyl, the β-carbon or the α-carbon of ᴅ-or ʟ-phenylalanines.

Advances in therapeutic peptides targeting G protein-coupled receptors

Dysregulation of peptide-activated pathways causes a range of diseases, fostering the discovery and clinical development of peptide drugs. Many endogenous peptides activate G protein-coupled receptors (GPCRs) - nearly 50 GPCR peptide drugs have been approved to date, most of them for metabolic disease or oncology, and more than 10 potentially first-in-class peptide therapeutics are in the pipeline. The majority of existing peptide therapeutics are agonists, which reflects the currently dominant strategy of modifying the endogenous peptide sequence of ligands for peptide-binding GPCRs. Increasingly, novel strategies are being employed to develop both agonists and antagonists, to both introduce chemical novelty and improve drug-like properties. Pharmacodynamic improvements are evolving to allow biasing ligands to activate specific downstream signalling pathways, in order to optimize efficacy and reduce side effects. In pharmacokinetics, modifications that increase plasma half-life have been revolutionary. Here, we discuss the current status of the peptide drugs targeting GPCRs, with a focus on evolving strategies to improve pharmacokinetic and pharmacodynamic properties.

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.

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.

Therapeutic applications of ghrelin agonists in the treatment of gastroparesis

There remains an unmet need for effective pharmacologic treatments for gastroparesis. Ghrelin is the endogenous ligand for the growth hormone secretagogue receptor and has been shown to regulate energy homeostasis and exert prokinetic effects on gastrointestinal motility. In recent years, several ghrelin receptor agonists have been studied in clinical trials of patients with diabetic gastroparesis. The intravenous macrocyclic peptidomimetic, TZP-101, initially suggested improvement in gastroparesis symptoms with intravenous administration when compared to placebo. However, in subsequent studies of oral preparations, TZP-102 failed to confirm these results. Another ghrelin receptor agonist, RM-131, was recently shown to significantly accelerate gastric emptying (GE) in patients with type 1 and type 2 diabetes and delayed GE. RM-131 reduced total Gastroparesis Cardinal Symptom Index-Daily Diary (GCSI-DD) and composite scores among type 1 diabetics. Continued development of ghrelin agonists should be explored in attempts to expand therapeutic options for the treatment of gastroparesis.

Patterns of Brain Activation and Meal Reduction Induced by Abdominal Surgery in Mice and Modulation by Rikkunshito

Abdominal surgery inhibits food intake and induces c-Fos expression in the hypothalamic and medullary nuclei in rats. Rikkunshito (RKT), a Kampo medicine improves anorexia. We assessed the alterations in meal microstructure and c-Fos expression in brain nuclei induced by abdominal surgery and the modulation by RKT in mice. RKT or vehicle was gavaged daily for 1 week. On day 8 mice had no access to food for 6–7 h and were treated twice with RKT or vehicle. Abdominal surgery (laparotomy-cecum palpation) was performed 1–2 h before the dark phase. The food intake and meal structures were monitored using an automated monitoring system for mice. Brain sections were processed for c-Fos immunoreactivity (ir) 2-h after abdominal surgery. Abdominal surgery significantly reduced bouts, meal frequency, size and duration, and time spent on meals, and increased inter-meal interval and satiety ratio resulting in 92–86% suppression of food intake at 2–24 h post-surgery compared with control group (no surgery). RKT significantly increased bouts, meal duration and the cumulative 12-h food intake by 11%. Abdominal surgery increased c-Fos in the prelimbic, cingulate and insular cortexes, and autonomic nuclei, such as the bed nucleus of the stria terminalis, central amygdala, hypothalamic supraoptic (SON), paraventricular and arcuate nuclei, Edinger-Westphal nucleus (E-W), lateral periaqueduct gray (PAG), lateral parabrachial nucleus, locus coeruleus, ventrolateral medulla and nucleus tractus solitarius (NTS). RKT induced a small increase in c-Fos-ir neurons in the SON and E-W of control mice, and in mice with surgery there was an increase in the lateral PAG and a decrease in the NTS. These findings indicate that abdominal surgery inhibits food intake by increasing both satiation (meal duration) and satiety (meal interval) and activates brain circuits involved in pain, feeding behavior and stress that may underlie the alterations of meal pattern and food intake inhibition. RKT improves food consumption post-surgically that may involve modulation of pain pathway.

New ghrelin agonist, HM01 alleviates constipation and L-dopa-delayed gastric emptying in 6-hydroxydopamine rat model of Parkinson's disease

BACKGROUND

Constipation and L-dopa-induced gastric dysmotility are common gastrointestinal (GI) symptoms in Parkinson's disease (PD). We investigated the novel ghrelin agonist, HM01 influence on GI motor dysfunctions in 6-hydroxydopamine (6-OHDA) rats.

METHODS

HM01 pharmacological profiles were determined in vitro and in vivo in rats. We assessed changes in fecal output and water content, and gastric emptying (GE) in 6-OHDA rats treated with orogastric (og) HM01 and L-dopa/carbidopa (LD/CD, 20/2 mg/kg). Fos immunoreactivity (ir) cells in specific brain and lumbosacral spinal cord were quantified.

KEY RESULTS

HM01 displayed a high binding affinity to ghrelin receptor (Ki: 1.42 ± 0.36 nM), 4.3 ± 1.0 h half-life and high brain/plasma ratio. 6-OHDA rats had reduced daily fecal output (22%) and water intake (23%) compared to controls. HM01 (3 and 10 mg/kg) similarly reversed the decreased 4-h fecal weight and water content in 6-OHDA rats. Basal GE was not modified in 6-OHDA rats, however, LD/CD (once or daily for 8 days) delayed GE in 6-OHDA and control rats that was prevented by HM01 (3 mg/kg acute or daily before LD/CD). HM01 increased Fos-ir cell number in the area postrema, arcuate nucleus, nucleus tractus solitarius, and lumbosacral intermediolateral column of 6-OHDA rats where 6-OHDA had a lowering effect compared to controls.

CONCLUSIONS & INFERENCES

6-OHDA rats display constipation- and adipsia-like features of PD and L-dopa-inhibited GE. The new orally active ghrelin agonist, HM01 crosses the blood-brain barrier and alleviates these alterations suggesting a potential benefit for PD with GI disorders.

Ghrelin receptor modulators: a patent review (2011 - 2014)

INTRODUCTION

Over the past 3 years, several patents appeared dealing with the discovery of compounds able to modulate ghrelin actions: agonists for the treatment of cachexia, as diagnostic agents for GH deficiency or for the increase in gastrointestinal motility, antagonists and inverse agonists as anorexigenic agents for the treatment of obesity and type 2 diabetes. This research has been conducted by several pharmaceutical companies and some compounds have entered clinical trials, but, to date, compounds acting on the ghrelin receptor do not represent clinical options yet.

AREAS COVERED

A comprehensive description and categorization of patents related to each type of compounds is provided, together with data related to these compounds that appeared in the scientific literature.

EXPERT OPINION

Ghrelin appears to mediate a myriad of actions, and some of these appear to be due to unknown mechanisms (a second putative ghrelin receptor, putative receptors for unacylated ghrelin); several agonists, antagonists and inverse agonists at ghrelin receptor have been developed but their mechanism of action into CNS is poorly understood. The therapeutic potential of compounds acting on ghrelin receptor is still to be fully assessed, but the results obtained to date are encouraging for the successful clinical translation of compounds able to treat several pathologies.

Preclinical gastrointestinal prokinetic efficacy and endocrine effects of the ghrelin mimetic RM-131

AIMS

The 28 amino acid hormone ghrelin, the natural ligand for the growth hormone secretagogue, or ghrelin receptor (GHR), has diverse physiological functions, including a possible role as a gastrointestinal prokinetic. The synthetic ghrelin mimetic RM-131 is in Phase II clinical trials for treatment of diabetic gastroparesis and other gastrointestinal (GI) disorders. We aimed to determine the relative potency of RM-131, when compared to other GI ghrelin mimetics, to predict efficacy and determine the role of RM-131 in models of inflammatory bowel disease.

MAIN METHODS

We evaluated and compared ghrelin, RM-131 and other synthetic ghrelin mimetics for their prokinetic potency in models of gastrointestinal disorders in the rat and we evaluated the endocrine (rats and dogs) and anti-inflammatory effects (mice) of the ghrelin mimetic RM-131.

KEY FINDINGS

The pentapeptide RM-131 increased gastric emptying in rodent models of ileus. RM-131 is about 100-fold more potent than human ghrelin and is 600 to 1800-fold more potent, when compared to several investigational ghrelin mimetics tested in clinical trials. RM-131 has anti-inflammatory effects and significantly increases survival and reduces macroscopic markers of tissue damage in a TNBS model of inflammatory bowel disease. RM-131 treatment shows a transient increase in growth hormone levels in Beagle dogs and rats, returning to baseline upon chronic treatment. Significant effects on glucose and insulin are not observed in chronic studies.

SIGNIFICANCE

RM-131's potency, efficacy and endocrine profile, are promising attributes for the treatment of diverse functional gastrointestinal disorders in humans.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Treatments for severe gastroparesis

Severe gastroparesis is a kind of gastroparesis that is refractory to conventional drug therapy and requires nutritional support and frequently emergency hospitalization. The selection of treatment for severe gastroparesis has always been a dilemma for clinicians. Currently, there have been limited reports on the treatment of severe gastroparesis. This article sums up the primary treatments, drug treatments and other kinds of treatments for severe gastroparesis and discusses the prospects for the treatment of this refractory disease.

Diabetic gastroparesis: recent insights into pathophysiology and implications for management

Delayed gastric emptying affects a substantial proportion of patients with long-standing diabetes, and when associated with symptoms and/or disordered glycemic control, affects quality of life adversely. Important clinicopathological insights have recently been gained by the systematic analysis of gastric biopsies from patients with severe diabetic gastroparesis, which may stimulate the development of new therapies in the coming decade. Experience with prokinetic therapies and treatments, such as pyloric botulinum toxin injection and gastric electrical stimulation, has established that relief of symptoms does not correlate closely with acceleration of delayed gastric emptying, and that well-designed controlled trials are essential to determine the efficacy of emerging therapies.

Ghrelin as a target for gastrointestinal motility disorders

The therapeutic potential of ghrelin and synthetic ghrelin receptor (GRLN-R) agonists for the treatment of gastrointestinal (GI) motility disorders is based on their ability to stimulate coordinated patterns of propulsive GI motility. This review focuses on the latest findings that support the therapeutic potential of GRLN-R agonists for the treatment of GI motility disorders. The review highlights the preclinical and clinical prokinetic effects of ghrelin and a series of novel ghrelin mimetics to exert prokinetic effects on the GI tract. We build upon a series of excellent reviews to critically discuss the evidence that supports the potential of GRLN-R agonists to normalize GI motility in patients with GI hypomotility disorders such as gastroparesis, post-operative ileus (POI), idiopathic chronic constipation and functional bowel disorders.

Ghrelin in gastrointestinal disease

Enteroendocrine cells of the gastric fundus are the predominant source of ghrelin production, although ghrelin gene transcripts and ghrelin-producing cells have been identified throughout the gastrointestinal tract. Various infectious, inflammatory and malignant disorders of the gastrointestinal system have been shown to alter ghrelin production and secretion and consequently to affect endocrine ghrelin levels and activity. Animal studies have demonstrated that ghrelin and synthetic ghrelin mimetics can reduce the severity of gastric and colonic inflammation and human clinical trials are underway to determine the efficacy of ghrelin in improving motility disorders. This review summarises the impact of gastrointestinal disease on ghrelin synthesis and secretion and the potential use of ghrelin and its mimetics for the treatment of these diseases.

Randomised clinical trial: ghrelin agonist TZP-101 relieves gastroparesis associated with severe nausea and vomiting--randomised clinical study subset data

BACKGROUND

Limited therapeutic options exist for severe gastroparesis, where severe nausea and vomiting can lead to weight loss, dehydration and malnutrition due to inadequate caloric and fluid intake. TZP-101 (ulimorelin) is a ghrelin receptor agonist that accelerates gastric emptying and improves upper gastrointestinal symptoms in diabetic patients with gastroparesis.

AIM

To assess effects of TZP-101 in diabetic gastroparesis patients with severe nausea/vomiting and baseline severity scores of ≥3.5 (range: 0-5) on the Gastroparesis Cardinal Symptom Index (GCSI) Nausea/Vomiting subscale.

METHODS

Patients were hospitalised and received four single daily 30-min infusions of one of six TZP-101 doses (range 20-600 μg/kg) or placebo. Efficacy was assessed by symptom improvement.

RESULTS

At baseline, 23 patients had a mean severity score for GCSI Nausea/Vomiting of 4.45±0.44. Statistically significant improvements over placebo occurred in the 80 μg/kg group for end of treatment changes from baseline in GCSI Nausea/Vomiting subscale (reduction in score of -3.82±0.76, P=0.011) and the GCSI Total score (-3.14±0.78, P=0.016) and were maintained at the 30-day follow-up assessment (-2.02±1.63, P=0.073 and -1.99±1.33, P=0.032 respectively). The proportion of days with vomiting was reduced significantly (P=0.05) in the 80 μg/kg group (mean of 1.2 days of vomiting for four treatment days) compared with placebo (mean of 3.2 days of vomiting across 4 treatment days).

CONCLUSIONS

TZP-101 substantially reduced the frequency and severity of nausea and vomiting as well as overall gastroparesis symptoms. The results are consistent with gastrointestinal motility effects of TZP-101, supporting further investigation of TZP-101 in the management of severe gastroparesis.

Safety and efficacy of ghrelin agonist TZP-101 in relieving symptoms in patients with diabetic gastroparesis: a randomized, placebo-controlled study

BACKGROUND

Gastroparesis, a chronic disorder of abnormal gastric motility, is common in patients with diabetes mellitus. A synthetic, selective ghrelin receptor agonist, TZP-101, is in clinical development for treatment of gastroparesis. This double-blind, randomized, placebo-controlled study evaluated the safety and efficacy of multiple TZP-101 doses in patients with moderate to severe symptomatic diabetic gastroparesis.

METHODS

Patients were admitted to the hospital and adaptively randomized to receive a single 30-min intravenous infusion of 20, 40, 80, 160, 320, or 600 μg kg(-1) TZP-101, (n = 57) or placebo, (n = 19) for four consecutive days. Symptoms were evaluated daily with the patient-rated Gastroparesis Cardinal Symptom Index (GCSI) and Gastroparesis Symptom Assessment (GSA). Clinicians rated gastroparesis symptoms on treatment day 4.

KEY RESULTS

  The 80 μg kg(-1) dose was identified as the most effective dose. On day 4, there was statistically significant improvement compared with placebo in the severity of GCSI Loss of Appetite and Vomiting scores for that dose group (P = 0.034 and P = 0.006). In addition, at the 80 μg kg(-1) dose, the proportion of patients with at least 50% improvement in vomiting score was significantly different (P = 0.019) compared with placebo. Meal-related GSA scores for Postprandial fullness were significantly improved in the 80 μg kg(-1) TZP-101 group compared with placebo (P = 0.012). Clinicians rated the 80 μg kg(-1) group better improved than placebo for overall symptom assessment (P = 0.047). Safety profiles were similar in the placebo and TZP-101 groups and all doses were well-tolerated.

CONCLUSIONS & INFERENCES

TZP-101 appears to be safe, well-tolerated, and effective at acutely addressing several gastroparesis symptoms.

Integrating GHS into the Ghrelin System

Oligopeptide derivatives of metenkephalin were found to stimulate growth-hormone (GH) release directly by pituitary somatotrope cells in vitro in 1977. Members of this class of peptides and nonpeptidyl mimetics are referred to as GH secretagogues (GHSs). A specific guanosine triphosphatate-binding protein-associated heptahelical transmembrane receptor for GHS was cloned in 1996. An endogenous ligand for the GHS receptor, acylghrelin, was identified in 1999. Expression of ghrelin and homonymous receptor occurs in the brain, pituitary gland, stomach, endothelium/vascular smooth muscle, pancreas, placenta, intestine, heart, bone, and other tissues. Principal actions of this peptidergic system include stimulation of GH release via combined hypothalamopituitary mechanisms, orexigenesis (appetitive enhancement), insulinostasis (inhibition of insulin secretion), cardiovascular effects (decreased mean arterial pressure and vasodilation), stimulation of gastric motility and acid secretion, adipogenesis with repression of fat oxidation, and antiapoptosis (antagonism of endothelial, neuronal, and cardiomyocyte death). The array of known and proposed interactions of ghrelin with key metabolic signals makes ghrelin and its receptor prime targets for drug development.

The prokinetic face of ghrelin

This review evaluated published data regarding the effects of ghrelin on GI motility using the PubMed database for English articles from 1999 to September 2009. Our strategy was to combine all available information from previous literature, in order to provide a complete structured review on the prokinetic properties of exogenous ghrelin and its potential use for treatment of various GI dysmotility ailments. We classified the literature into two major groups, depending on whether studies were done in health or in disease. We sub-classified the studies into stomach, small intestinal and colon studies, and broke them down further into studies done in vitro, in vivo (animals) and in humans. Further more, the reviewed studies were presented in a chronological order to guide the readers across the scientific advances in the field. The review shows evidences that ghrelin and its (receptor) agonists possess a strong prokinetic potential to serve in the treatment of diabetic, neurogenic or idiopathic gastroparesis and possibly, chemotherapy-associated dyspepsia, postoperative, septic or post-burn ileus, opiate-induced bowel dysfunction and chronic idiopathic constipation. Further research is necessary to close the gap in knowledge about the effect of ghrelin on the human intestines in health and disease.

New frontiers in endocrinology of eating disorders

Alterations of both central and peripheral feeding regulatory substances occur in the acute phases of anorexia nervosa (AN) and bulimia nervosa (BN) and, generally, reverse after recovery. Some of these alterations are believed not only to sustain the altered eating behavior but also to contribute to certain psychopathological aspects and/or etiopathogenetic processes of eating disorders (EDs). It has been suggested that EDs are clinical conditions linked to reward-related mechanisms leading to a kind of addiction to self-starvation and/or overeating. Most of the feeding regulatory substances, which are dysregulated in EDs, are also implicated in the modulation of reward, emotional, and cognitive functions, thus representing possible links between altered nutritional regulation, motivated behaviors and reward processes. In this chapter, the ED literature dealing with ghrelin, brain-derived neurotrophic factor, opioid peptides, and endocannabinoids, which have prominent effects on eating behavior, body weight, reward, emotional, and cognitive functions, is reviewed in view of the above suggested links. Moreover, the potential therapeutics of new medications developed on the basis of neuroendocrine aberrations found in EDs is also presented.

Actions and therapeutic pathways of ghrelin for gastrointestinal disorders

Ghrelin is a peptide hormone that possesses unique orexigenic properties. By acting on the growth-hormone secretagogue receptor 1a, ghrelin induces a short-term increase in food consumption, which ultimately induces a positive energy balance and increases fat deposition. Reduced ghrelin levels have been observed in obese patients and after bariatric surgery. In particular, bariatric procedures that involve gastric resection or bypass lead to reduced ghrelin levels. Administration of physiological doses of exogenous ghrelin to humans does not significantly alter gastric motility; however, administration of high doses stimulates gastric motility, with increased gastric tone and emptying, and increased activity of migrating motor complexes in the small bowel. The potential of ghrelin agonists to be used as prokinetics is being tested in patients with gastroparesis and postoperative ileus. Ghrelin acts directly on pancreatic islet cells to reduce insulin production. Findings from studies in animals have revealed that small-molecule ghrelin antagonists favorably influence glucose tolerance, appetite suppression and weight loss. Other studies have demonstrated that ghrelin antagonists retard gastric emptying only at very high doses, which suggests that these agents will probably not induce upper gastrointestinal symptoms. The potential of this new class of therapeutic agents to influence appetite and glycemic control strongly indicates that they should be tested in clinical trials.

Effect of the ghrelin receptor agonist TZP-101 on colonic transit in a rat model of postoperative ileus

Ghrelin, the natural ligand of the growth hormone secretagogue receptor (ghrelin receptor), is an orexigenic gut hormone with prokinetic action in the upper gastrointestinal tract. Previously we have shown in a rodent model of postoperative ileus that the synthetic ghrelin receptor agonist TZP-101 prevents the delay in gastric emptying and improves small intestinal transit. The goal of the present study was to investigate whether TZP-101 affects colonic transit and food intake in rats with postoperative ileus. Fasted rats were treated with morphine and subjected to laparotomy under isoflurane anesthesia. Following surgery the animals were placed in clean home cages and fecal pellet output and food intake were monitored for 48 h. TZP-101 or vehicle were administered as 3 i.v. bolus infusions at 0 h, 2 h and 4 h post-surgery. TZP-101 (0.03-1 mg/kg) dose-dependently decreased the time to first bowel movement and increased fecal pellet output measured at 12 h and 24 h post-surgery compared to the vehicle. The administration of TZP-101 was not associated with a significant alteration in food intake. In conclusion, this study provides the first experimental evidence that a novel ghrelin receptor agonist improves large bowel function in rats with postoperative ileus, suggesting that TZP-101 may be useful in the clinic to accelerate upper gastrointestinal transit and to shorten the time to the first bowel movement following surgery.