The tryptophan hydroxylase inhibitor LX1031 shows clinical benefit in patients with nonconstipating irritable bowel syndrome

Early-Life Stress Induced by Neonatal Maternal Separation Leads to Intestinal 5-HT Accumulation and Causes Intestinal Dysfunction

Background

The early childhood period is a critical development stage, and experiencing stress during this time may increase the risk of gastrointestinal disorders, including irritable bowel syndrome (IBS). Neonatal maternal separation (NMS) in rodent models has been shown to cause bowel dysfunctions similar to IBS, and 5-HT is considered to be a key regulator regulating intestinal function, but the precise underlying mechanisms remain unclear.

Results

We established a maternal separation stress mouse model to simulate early-life stress, exploring the expression patterns of 5-HT under chronic stress and its mechanisms affecting gut function. We observed a significant increase in 5-HT expression due to NMS, leading to disruptions in intestinal structure and function. However, inhibiting 5-HT reversed these effects, suggesting its potential as a therapeutic target. Furthermore, our research revealed that excess 5-HT in mice with early life stress increased intestinal neural network density and promoted excitatory motor neuron expression. Mechanistically, 5-HT activated the Wnt signaling pathway through the 5-HT4 receptor, promoting neurogenesis within the intestinal nervous system.

Conclusion

These findings shed light on the intricate changes induced by early life stress in the intestines, confirming the regulatory role of 5-HT in the enteric nervous system and providing potential insights for the development of novel therapies for gastrointestinal disorders.

Multiomics Analysis Revealed Colorectal Cancer Pathogenesis

Gut microbiota-derived microbial compounds may link to the pathogenesis of colorectal cancer (CRC). However, the role of the host-microbiome in the incidence and progression of CRC remains elusive. We performed 16S rRNA sequencing, metabolomics, and proteomic studies on samples from 85 CRC patients who underwent colonoscopy examination and found two distinct changed patterns of microbiome in CRC patients. The relative abundances of Catabacter and Mogibacterium continuously increased from intramucosal carcinoma to advanced stages, whereas Clostridium, Anaerostipes, Vibrio, Flavonifractor, Holdemanella, and Hungatella were significantly altered only in intermediate lesions. Fecal metabolomics analysis exhibited consistent increases in bile acids, indoles, and urobilin as well as a decrease in heme. Serum metabolomics uncovered the highest levels of bilin, glycerides, and nucleosides together with the lowest levels of bile acids and amino acids in the stage of intermediate lesions. Three fecal and one serum dipeptides were elevated in the intermediate lesions. Proteomics analysis of colorectal tissues showed that oxidation and autophagy through the PI3K/Akt-mTOR signaling pathway contribute to the development of CRC. Diagnostic analysis showed multiomics features have good predictive capability, with AUC greater than 0.85. Our overall findings revealed new candidate biomarkers for CRC, with potentially significant diagnostic and prognostic capabilities.

Efficient Synthesis of Chiral Aryl Alcohol with a Novel Kosakonia radicincitans Isolate in Tween 20/L-carnitine: Lysine-Containing Synergistic Reaction System

Chiral trifluoromethyl alcohols as vital intermediates are of great interest in fine chemicals and especially in pharmaceutical synthesis. In this work, a novel isolate Kosakonia radicincitans ZJPH202011 was firstly employed as biocatalyst for the synthesis of (R)-1-(4-bromophenyl)-2,2,2-trifluoroethanol ((R)-BPFL) with good enantioselectivity. By optimizing fermentation conditions and bioreduction parameters in aqueous buffer system, the substrate concentration of 1-(4-bromophenyl)-2,2,2-trifluoroethanone (BPFO) was doubled from 10 to 20 mM, and the enantiomeric excess (ee) value for (R)-BPFL increased from 88.8 to 96.4%. To improve biocatalytic efficiency by strengthening the mass-transfer rate, natural deep-eutectic solvents, surfactants and cyclodextrins (CDs) were introduced separately in the reaction system as cosolvent. Among them, L-carnitine: lysine (C: Lys, molar ratio 1:2), Tween 20 and γ-CD manifested higher (R)-BPFL yield compared with other same kind of cosolvents. Furthermore, based on the excellent performance of both Tween 20 and C: Lys (1:2) in enhancing BPFO solubility and ameliorating cell permeability, a Tween 20/C: Lys (1:2)-containing integrated reaction system was then established for efficient bioproduction of (R)-BPFL. After optimizing the critical factors involved in BPFO bioreduction in this synergistic reaction system, BPFO loading increased up to 45 mM and the yield reached 90.0% within 9 h, comparatively only 37.6% yield was acquired in neat aqueous buffer. This is the first report on K. radicincitans cells as new biocatalyst applied in (R)-BPFL preparation, and the developed Tween 20/C: Lys-containing synergistic reaction system has great potential for the synthesis of various chiral alcohols.

Structure-Based Design of Xanthine-Imidazopyridines and -Imidazothiazoles as Highly Potent and In Vivo Efficacious Tryptophan Hydroxylase Inhibitors

Tryptophan hydroxylases catalyze the first and rate-limiting step in the biosynthesis of serotonin, a well-known neurotransmitter that plays an important role in multiple physiological functions. A reduction of serotonin levels, especially in the brain, can cause dysregulation leading to depression or insomnia. In contrast, overproduction of peripheral serotonin is associated with symptoms like carcinoid syndrome and pulmonary arterial hypertension. Recently, we developed a class of TPH inhibitors based on xanthine-benzimidazoles, characterized by a tripartite-binding mode spanning the binding sites of the cosubstrate pterin and the substrate tryptophan and by chelation of the catalytic iron ion. Herein, we describe the structure-based development of a second generation of xanthine-imidiazopyridines and -imidazothiazoles designed to inhibit TPH1 in the periphery while preventing the interaction with TPH2 in the brain. Lead compound 32 (TPT-004) shows superior pharmacokinetic and pharmacodynamic properties as well as efficacy in preclinical models of peripheral serotonin attenuation and colorectal tumor growth.

5-HT7 receptor-dependent intestinal neurite outgrowth contributes to visceral hypersensitivity in irritable bowel syndrome

Irritable bowel syndrome (IBS) is characterized by visceral hypersensitivity (VH) associated with abnormal serotonin/5-hydroxytryptamine (5-HT) metabolism and neurotrophin-dependent mucosal neurite outgrowth. The underlying mechanisms of VH remain poorly understood. We investigated the role of 5-HT₇ receptor in mucosal innervation and intestinal hyperalgesia. A high density of mucosal nerve fibres stained for 5-HT₇ was observed in colonoscopic biopsy specimens from IBS patients compared with those from healthy controls. Staining of 5-HT₃ and 5-HT₄ receptors was observed mainly in colonic epithelia with comparable levels between IBS and controls. Visceromotor responses to colorectal distension were evaluated in two mouse models, one postinfectious with Giardia and subjected to water avoidance stress (GW) and the other postinflammatory with trinitrobenzene sulfonic acid-induced colitis (PT). Increased VH was associated with higher mucosal density of 5-HT₇-expressing nerve fibres and elevated neurotrophin and neurotrophin receptor levels in the GW and PT mice. The increased VH was inhibited by intraperitoneal injection of SB-269970 (a selective 5-HT₇ antagonist). Peroral multiple doses of CYY1005 (a novel 5-HT₇ ligand) decreased VH and reduced mucosal density of 5-HT₇-expressing nerve fibres in mouse colon. Human neuroblastoma SH-SY5Y cells incubated with bacteria-free mouse colonic supernatant, 5-HT, nerve growth factor, or brain-derived neurotrophic factor exhibited nerve fibre elongation, which was inhibited by 5-HT₇ antagonists. Gene silencing of HTR7 also reduced the nerve fibre length. Activation of 5-HT₇ upregulated NGF and BDNF gene expression, while stimulation with neurotrophins increased the levels of tryptophan hydroxylase 2 and 5-HT₇ in neurons. A positive-feedback loop was observed between serotonin and neurotrophin pathways via 5-HT₇ activation to aggravate fibre elongation, whereby 5-HT₃ and 5-HT₄ had no roles. In conclusion, 5-HT₇-dependent mucosal neurite outgrowth contributed to VH. A novel 5-HT₇ antagonist could be used as peroral analgesics for IBS-related pain.

Role of Ion Channels in the Chemotransduction and Mechanotransduction in Digestive Function and Feeding Behavior

The gastrointestinal tract constantly communicates with the environment, receiving and processing a wide range of information. The contents of the gastrointestinal tract and the gastrointestinal tract generate mechanical and chemical signals, which are essential for regulating digestive function and feeding behavior. There are many receptors here that sense intestinal contents, including nutrients, microbes, hormones, and small molecule compounds. In signal transduction, ion channels are indispensable as an essential component that can generate intracellular ionic changes or electrical signals. Ion channels generate electrical activity in numerous neurons and, more importantly, alter the action of non-neurons simply and effectively, and also affect satiety, molecular secretion, intestinal secretion, and motility through mechanisms of peripheral sensation, signaling, and altered cellular function. In this review, we focus on the identity of ion channels in chemosensing and mechanosensing in the gastrointestinal tract.

Structure-Based Design of Xanthine-Benzimidazole Derivatives as Novel and Potent Tryptophan Hydroxylase Inhibitors

Tryptophan hydroxylases catalyze the first and rate-limiting step in the synthesis of serotonin. Serotonin is a key neurotransmitter in the central nervous system and, in the periphery, functions as a local hormone with multiple physiological functions. Studies in genetically altered mouse models have shown that dysregulation of peripheral serotonin levels leads to metabolic, inflammatory, and fibrotic diseases. Overproduction of serotonin by tumor cells causes severe symptoms typical for the carcinoid syndrome, and tryptophan hydroxylase inhibitors are already in clinical use for patients suffering from this disease. Here, we describe a novel class of potent tryptophan hydroxylase inhibitors, characterized by spanning all active binding sites important for catalysis, specifically those of the cosubstrate pterin, the substrate tryptophan as well as directly chelating the catalytic iron ion. The inhibitors were designed to efficiently reduce serotonin in the periphery while not passing the blood-brain barrier, thus preserving serotonin levels in the brain.

Effect of Shugan Jianpi Recipe Combined with Cross Moxibustion on Biochemical Examination Indexes and Total Score of TCM Symptoms in Patients with Spleen-Stomach Damp-Heat Diarrhea Irritable Bowel Syndrome

Objective

To explore the effect of Shugan Jianpi recipe combined with cross moxibustion on biochemical examination indexes and total score of TCM symptoms in patients with spleen-stomach damp-heat diarrhea irritable bowel syndrome (IBS).

Methods

Sixty patients with spleen-stomach damp-heat diarrhea (IBS) treated in our hospital from January 2019 to September 2021 were enrolled. The patients were randomly assigned into the control group and study group. The control group was treated with Chang Shugan Jianpi recipe, and the study group was treated with Shugan Jianpi recipe combined with cross moxibustion. The curative effect, single symptom score, total score of TCM clinical symptoms, plasma gastrointestinal hormone level, IBS-QOL score, and recurrence were compared.

Results

First of all, we compared the curative effects. The study group was significantly effective in 24 cases, effective in 5 cases, and ineffective in 1 case, and the effective rate was 96.67%. In the control group, 13 cases were markedly effective, 10 cases were effective, and 7 cases were ineffective, and the effective rate was 76.67%. The effective rate of the study group was higher compared to the control group (P < 0.05). Secondly, the individual symptom scores were compared. Compared between the two groups, the scores of diarrhea times, stool characteristics, abdominal pain, and abdominal distension in the study group were lower compared to the control group (P < 0.05). After treatment, the total score of TCM clinical symptoms decreased. Compared between the two groups, the total score of TCM clinical symptoms in the study group was lower compared to the control group at 1 week, 2 weeks, and 4 weeks after treatment (P < 0.05). After treatment, the levels of 5-HT and VIP decreased. The levels of 5-HT and VIP in the study group were lower compared to the control group (P < 0.05). The scores of anxieties, health worry, behavioral disorder, social reaction, somatic intention, interpersonal relationship, dietary concern, and sexual behavior in the study group were lower compared to the control group. The IBS-QOL scores were significantly lower compared to the control group (P < 0.05). Finally, we compared the recurrence. The recurrence rate in the study group was lower compared to the control group (P < 0.05).

Conclusion

The prescription combined with cross moxibustion has the effect of soothing the liver and invigorating the spleen, resolving dampness and stomach, and can reduce the main clinical symptoms of patients with diarrhea IBS of spleen-stomach damp-heat type, and the overall effect is significant. It can also enhance the emotional state of anxiety and depression and achieve the overall psychological and physical balance and health state, and the recurrence rate is low, which can be further applied in clinic.

Herpes Simplex Virus-1 Induced Serotonin-Associated Metabolic Pathways Correlate With Severity of Virus- and Inflammation-Associated Ocular Disease

Herpes simplex virus-associated diseases are a complex interaction between cytolytic viral replication and inflammation. Within the normally avascular and immunoprivileged cornea, HSV ocular infection can result in vision-threatening immune-mediated herpetic keratitis, the leading infectious cause of corneal blindness in the industrialized world. Viral replicative processes are entirely dependent upon numerous cellular biosynthetic and metabolic pathways. Consistent with this premise, HSV infection was shown to profoundly alter gene expression associated with cellular amino acid biosynthetic pathways, including key tryptophan metabolism genes. The essential amino acid tryptophan is crucial for pathogen replication, the generation of host immune responses, and the synthesis of neurotransmitters, such as serotonin. Intriguingly, Tryptophan hydroxylase 2 (TPH2), the neuronal specific rate-limiting enzyme for serotonin synthesis, was the most significantly upregulated gene by HSV in an amino acid metabolism PCR array. Despite the well-defined effects of serotonin in the nervous system, the association of peripheral serotonin in disease-promoting inflammation has only recently begun to be elucidated. Likewise, the impact of serotonin on viral replication and ocular disease is also largely unknown. We therefore examined the effect of HSV-induced serotonin-associated synthesis and transport pathways on HSV-1 replication, as well as the correlation between HSV-induced ocular serotonin levels and disease severity. HSV infection induced expression of the critical serotonin synthesis enzymes TPH-1, TPH-2, and DOPA decarboxylase (DDC), as well as the serotonin transporter, SERT. Concordantly, HSV-infected cells upregulated serotonin synthesis and its intracellular uptake. Increased serotonin synthesis and uptake was shown to influence HSV replication. Exogenous addition of serotonin increased HSV-1 yield, while both TPH-1/2 and SERT pharmacological inhibition reduced viral yield. Congruent with these in vitro findings, rabbits intraocularly infected with HSV-1 exhibited significantly higher aqueous humor serotonin concentrations that positively and strongly correlated with viral load and ocular disease severity. Collectively, our findings indicate that HSV-1 promotes serotonin synthesis and cellular uptake to facilitate viral replication and consequently, serotonin's proinflammatory effects may enhance the development of ocular disease.

Unraveling Autocrine Signaling Pathways through Metabolic Fingerprinting in Serous Ovarian Cancer Cells

Focusing on defining metabolite-based inter-tumoral heterogeneity in ovarian cancer, we investigated the metabolic diversity of a panel of high-grade serous ovarian carcinoma (HGSOC) cell-lines using a metabolomics platform that interrogate 731 compounds. Metabolic fingerprinting followed by 2-dimensional and 3-dimensional principal component analysis established the heterogeneity of the HGSOC cells by clustering them into five distinct metabolic groups compared to the fallopian tube epithelial cell line control. An overall increase in the metabolites associated with aerobic glycolysis and phospholipid metabolism were observed in the majority of the cancer cells. A preponderant increase in the levels of metabolites involved in trans-sulphuration and glutathione synthesis was also observed. More significantly, subsets of HGSOC cells showed an increase in the levels of 5-Hydroxytryptamine, γ-aminobutyrate, or glutamate. Additionally, 5-hydroxytryptamin synthesis inhibitor as well as antagonists of γ-aminobutyrate and glutamate receptors prohibited the proliferation of HGSOC cells, pointing to their potential roles as oncometabolites and ligands for receptor-mediated autocrine signaling in cancer cells. Consistent with this role, 5-Hydroxytryptamine synthesis inhibitor as well as receptor antagonists of γ-aminobutyrate and Glutamate-receptors inhibited the proliferation of HGSOC cells. These antagonists also inhibited the three-dimensional spheroid growth of TYKNU cells, a representative HGSOC cell-line. These results identify 5-HT, GABA, and Glutamate as putative oncometabolites in ovarian cancer metabolic sub-type and point to them as therapeutic targets in a metabolomic fingerprinting-based therapeutic strategy.

Tryptophan-derived serotonin-kynurenine balance in immune activation and intestinal inflammation

Endogenous tryptophan metabolism pathways lead to the production of serotonin (5‐hydroxytryptamine; 5‐HT), kynurenine, and several downstream metabolites which are involved in a multitude of immunological functions in both health and disease states. Ingested tryptophan is largely shunted to the kynurenine pathway (95%) while only minor portions (1%–2%) are sequestered for 5‐HT production. Though often associated with the functioning of the central nervous system, significant production of 5‐HT, kynurenine and their downstream metabolites takes place within the gut. Accumulating evidence suggests that these metabolites have essential roles in regulating immune cell function, intestinal inflammation, as well as in altering the production and suppression of inflammatory cytokines. In addition, both 5‐HT and kynurenine have a considerable influence on gut microbiota suggesting that these metabolites impact host physiology both directly and indirectly via compositional changes. It is also now evident that complex interactions exist between the two pathways to maintain gut homeostasis. Alterations in 5‐HT and kynurenine are implicated in the pathogenesis of many gastrointestinal dysfunctions, including inflammatory bowel disease. Thus, these pathways present numerous potential therapeutic targets, manipulation of which may aid those suffering from gastrointestinal disorders. This review aims to update both the role of 5‐HT and kynurenine in immune regulation and intestinal inflammation, and analyze the current knowledge of the relationship and interactions between 5‐HT and kynurenine pathways.

Amino Acid Trp: The Far Out Impacts of Host and Commensal Tryptophan Metabolism

Tryptophan (Trp) is an essential amino acid primarily derived from the diet for use by the host for protein synthesis. The intestinal tract is lined with cells, both host and microbial, that uptake and metabolize Trp to also generate important signaling molecules. Serotonin (5-HT), kynurenine and its downstream metabolites, and to a lesser extent other neurotransmitters are generated by the host to signal onto host receptors and elicit physiological effects. 5-HT production by neurons in the CNS regulates sleep, mood, and appetite; 5-HT production in the intestinal tract by enterochromaffin cells regulates gastric motility and inflammation in the periphery. Kynurenine can signal onto the aryl hydrocarbon receptor (AHR) to elicit pleiotropic responses from several cell types including epithelial and immune cells, or can be further metabolized into bioactive molecules to influence neurodegenerative disease. There is a remarkable amount of cross-talk with the microbiome with regard to tryptophan metabolites as well. The gut microbiome can regulate the production of host tryptophan metabolites and can use dietary or recycled trp to generate bioactive metabolites themselves. Trp derivatives like indole are able to signal onto xenobiotic receptors, including AHR, to elicit tolerogenic effects. Here, we review studies that demonstrate that tryptophan represents a key intra-kingdom signaling molecule.

The placebo response rate in pharmacological trials in patients with irritable bowel syndrome: a systematic review and meta-analysis

BACKGROUND

Clinical trials in irritable bowel syndrome are associated with high placebo response rates. We aimed to identify the magnitude of the placebo response and the contributing factors to this occurrence.

METHODS

We did a systematic review and meta-analysis with a search of MEDLINE, EMBASE, and the Cochrane Central Register of Controlled Trials between April 1, 1959, and April 30, 2020. We included all randomised controlled trials that compared an active pharmacotherapeutic agent with placebo and had a dichotomous outcome of response to therapy (in terms of global improvement or improvement in abdominal pain) in adults (aged ≥18 years) with irritable bowel syndrome. Exclusion criteria were trials reporting on treatment satisfaction as a dichotomous outcome of response to therapy or clinician-reported outcomes and a treatment duration of less than 4 weeks. Our main outcome was identification of the magnitude of the pooled placebo response rate for the following endpoints: global improvement, abdominal pain, and US Food and Drug Administration (FDA) endpoints. We extracted information from published reports and pooled proportions through meta-analysis with random effects. The study was registered with PROSPERO, CRD42020170908.

FINDINGS

Of the 6863 publications identified, 70 articles describing 73 randomised controlled trials were included in our analysis. The pooled placebo response rate was 27·3% (95% CI 24·3-30·9) using the global improvement endpoint, 34·4% (31·2-37·8) using the abdominal pain endpoint, and 17·9% (15·2-21·0) using the composite FDA endpoint responder definition, all with substantial heterogeneity between the trials. Studies published before 2006, and those done in Europe, with a parallel design, a run-in period of 2 weeks or less, a dose schedule of three times a day or more, or a smaller sample size of the control group were significantly associated with an increased pooled placebo response rate.

INTERPRETATION

More than a quarter of patients with irritable bowel syndrome had a placebo response in terms of global improvement, with multiple associated moderators. We recommend future trials apply a run-in period of at least 2 weeks and dose once or twice a day to minimise the placebo response rate.

FUNDING

None.

Inflammation and Overlap of Irritable Bowel Syndrome and Functional Dyspepsia

Irritable bowel syndrome (IBS) and functional dyspepsia (FD) are common functional gastrointestinal disorders (FGIDs) and account for a large proportion of consulting patients. These 2 disorders overlap with each other frequently. The pathogenesis of IBS or FD is complicated and multi-factors related, in which infectious or non-infectious inflammation and local or systemic immune response play significant roles. There are few studies focusing on the mechanism of inflammation in patients with overlap syndrome of irritable bowel syndrome and functional dyspepsia (IBS-FD). This review focuses on current advances about the role of inflammation in the pathogenesis of IBS and FD and the possible mechanism of inflammation in IBS-FD.

Serotonin in the Pathogenesis of Lymphocytic Colitis

Lymphocytic colitis (LC) is a chronic inflammatory disease associated with watery diarrhea, abdominal pain, and colonic intraepithelial lymphocytosis. Serotonin (5-hydroxytryptamine, 5-HT) is reported to increase in certain colon diseases; however, little is known regarding its metabolism in LC. In the present work, the level of 5-HT in serum and the number of enteroendocrine cells (EECs) as well as the expression of the 5-HT rate-limiting enzyme tryptophan hydroxylase 1 (TPH1) in colonic biopsies and urine 5-hydroxyindoeoacetic acid (5-HIAA) were determined in 36 LC patients that were treated with budesonide and 32 healthy controls. The 5-HT serum and 5-HIAA urine levels were measured using ELISA, the EEC number was determined immunohistochemically, and the colonic TPH1 mRNA expression was determined using RT-PCR. The levels of 5-HT and 5-HIAA and the number of EECs were higher in LC patients than in the controls, and positive correlations were observed between the 5-HT and 5-HIAA levels, 5-HT and EEC number, TPH1 mRNA and EEC number, as well as the severity of disease symptoms and 5-HIAA. Budesonide decreased the levels of 5-HT, 5-HIAA, and TPH1 expression and the number of EECs to values that did not differ from those for controls. In conclusion, the serotonin metabolism may be important for LC pathogenesis, and the urinary level of 5-HIAA may be considered as a non-invasive marker of this disease activity.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

An update on efficacy and safety considerations for the latest drugs used to treat irritable bowel syndrome

INTRODUCTION

Irritable bowel syndrome (IBS), globally affecting 11.2% of the population and imposing a direct annual cost of $1.7bn-$10bn in the US, is one of the today's major therapeutic challenges. Therefore, there is urgent need to address this issue through reviewing the tolerability and efficacy of available medications.

AREAS COVERED

Over the past decade, related experiments were cited through Clinicaltrials.gov, PubMed, WHO ICTRP, and Cochrane library. Pharmacological parameters of approved medications available in the USFDA, EMA, TGA and PMDA were also stated.

EXPERT OPINION

Anti-spasmodics are used as the first-line treatment in pain-predominant IBS and IBS-D, among which calcium channel blockers and neurokinin-type 2 receptor antagonists seem to replace anti-cholinergic drugs. As second-line treatments, rifaximin is considered to be the best for IBS-D though it has lower efficacy than alosetron and eluxadoline. For IBS-C, linaclotide is the most effective and the safest second-line therapy, following laxatives/fibers, which may be replaced by tenapanor, in the future. When moderate to severe IBS is associated with severe pain or comorbid psychological disorders, gut-brain neuromodulators could also be prescribed. Regarding all this, there is still a paramount need to conduct careful clinical studies on efficacy, safety and cost-effectiveness of current approved and non-approved treatments.

Inhibition of serotonin synthesis: A novel therapeutic paradigm

The rate-limiting enzyme in serotonin synthesis is tryptophan hydroxylase (TPH). There are two independent serotonin systems in the body characterized by two isoforms of TPH, TPH1 and TPH2. While TPH2 synthesizes serotonin in the brain, TPH1 is expressed in the gut and in other peripheral tissues and supplies platelets in the circulation with serotonin. This duality of the serotonin system is enforced by the blood-brain barrier which is impermeable for serotonin. In the brain serotonin acts as neurotransmitter and is a main target for the treatment of psychiatric disorders. In the periphery it is released by platelets at the site of activation and elicits numerous physiological effects. TPH1 deficient mice were shown to be protected from diverse diseases including hemostatic, inflammatory, fibrotic, gastrointestinal, and metabolic disorders and therefore serotonin synthesis inhibition emerged as a reasonable therapeutic paradigm. Recently the first TPH inhibitor, telotristat ethyl, came on the market for the treatment of carcinoid syndrome. This review summarizes the state of development and the therapeutic opportunities of such compounds.

Abnormalities of mucosal serotonin metabolism and 5-HT3 receptor subunit 3C polymorphism in irritable bowel syndrome with diarrhoea predict responsiveness to ondansetron

BACKGROUND

Irritable bowel syndrome with diarrhoea (IBS-D) is a common condition, greatly reducing the quality of life with few effective treatment options available.

AIM

To report the beneficial response shown in our trial with the 5-hydroyxtryptamine (5-HT) receptor 3 antagonist, ondansetron in IBS-D METHODS: A randomised, placebo-controlled, cross-over trial of 5 weeks of ondansetron versus placebo in 125 patients meeting modified Rome III criteria for IBS-D as previously described. Patients were compared to 21 healthy controls. 5-HT and 5-HIAA were measured in rectal biopsies. Whole gut transit time was assessed using a radio-opaque marker technique. Whole blood DNA was genotyped for an insertion polymorphism in the promoter region of the serotonin transporter gene SLC6A4, as well as single nucleotide polymorphisms (SNPs) of the tryptophan hydroxylase gene TPH1 and 5-HT₃ receptor genes HTR3A, C and E.

RESULTS

Patients' biopsies showed significantly higher 5-HIAA levels (2.1 (1.2-4.2) pmol/mg protein vs 1.1 (0.4-1.5) in controls, P < .0001). 39 patients used < 4 mg/d ("super-responders") while 55 required ≥ 4 mg/d. 5-HT concentrations in rectal biopsies were significantly lower in super-responders (21.3 (17.0-31.8) vs 37.7 (21.4-61.4), P = .0357) and the increase in transit time on ondansetron was significantly greater (15.6 (1.8-31) hours vs 3.9 (-5.1-17.9) hours). Stool consistency responders were more likely to carry the CC genotype of the SNP p.N163K rs6766410 of the HTR3C gene (33% vs 14%, P = .0066).

CONCLUSION

IBS-D patients have significant abnormalities in mucosal 5-HT metabolism. Those with the lowest concentration of 5-HT in rectal biopsies showed the greatest responsiveness to ondansetron.

Asymmetric Hydrogenation of Aryl Perfluoroalkyl Ketones Catalyzed by Rhodium(III) Monohydride Complexes Bearing Josiphos Ligands

The asymmetric hydrogenation of 2,2,2-trifluoroacetophenones and aryl perfluoroalkyl ketones was developed using a unique, well-defined chloride-bridged dinuclear rhodium(III) complex bearing Josiphos-type diphosphine ligands. These complexes were prepared from [RhCl(cod)]₂ , Josiphos ligands, and hydrochloric acid. As catalyst precursors, they allow for the efficient and enantioselective synthesis (up to 99 % ee) of chiral secondary alcohols with perfluoroalkyl groups. This system does not require an activating base for the hydrogenation of 2,2,2-trifluoroacetophenones. Additionally, the enantioselective C=O hydrogenations of 2-phenyl-3-(haloacetyl)-indoles, a class of privileged structures in medicinal chemistry, is reported for the first time.

Predicting human disease mutations and identifying drug targets from mouse gene knockout phenotyping campaigns

Two large-scale mouse gene knockout phenotyping campaigns have provided extensive data on the functions of thousands of mammalian genes. The ongoing International Mouse Phenotyping Consortium (IMPC), with the goal of examining all ∼20,000 mouse genes, has examined 5115 genes since 2011, and phenotypic data from several analyses are available on the IMPC website (www.mousephenotype.org). Mutant mice having at least one human genetic disease-associated phenotype are available for 185 IMPC genes. Lexicon Pharmaceuticals' Genome5000™ campaign performed similar analyses between 2000 and the end of 2008 focusing on the druggable genome, including enzymes, receptors, transporters, channels and secreted proteins. Mutants (4654 genes, with 3762 viable adult homozygous lines) with therapeutically interesting phenotypes were studied extensively. Importantly, phenotypes for 29 Lexicon mouse gene knockouts were published prior to observations of similar phenotypes resulting from homologous mutations in human genetic disorders. Knockout mouse phenotypes for an additional 30 genes mimicked previously published human genetic disorders. Several of these models have helped develop effective treatments for human diseases. For example, studying Tph1 knockout mice (lacking peripheral serotonin) aided the development of telotristat ethyl, an approved treatment for carcinoid syndrome. Sglt1 (also known as Slc5a1) and Sglt2 (also known as Slc5a2) knockout mice were employed to develop sotagliflozin, a dual SGLT1/SGLT2 inhibitor having success in clinical trials for diabetes. Clinical trials evaluating inhibitors of AAK1 (neuropathic pain) and SGLT1 (diabetes) are underway. The research community can take advantage of these unbiased analyses of gene function in mice, including the minimally studied 'ignorome' genes.

Impacts of Psychological Stress on Osteoporosis: Clinical Implications and Treatment Interactions

The significant biochemical and physiological effects of psychological stress are beginning to be recognized as exacerbating common diseases, including osteoporosis. This review discusses the current evidence for psychological stress-associated mental health disorders as risk factors for osteoporosis, the mechanisms that may link these conditions, and potential implications for treatment. Traditional, alternative, and adjunctive therapies are discussed. This review is not intended to provide therapeutic recommendations, but, rather, the goal of this review is to delineate potential interactions of psychological stress and osteoporosis and to highlight potential multi-system implications of pharmacological interventions. Review of the current literature identifies several potentially overlapping mechanistic pathways that may be of interest (e.g., glucocorticoid signaling, insulin-like growth factor signaling, serotonin signaling) for further basic and clinical research. Current literature also supports the potential for cross-effects of therapeutics for osteoporosis and mental health disorders. While studies examining a direct link between osteoporosis and chronic psychological stress are limited, the studies reviewed herein suggest that a multi-factorial, personalized approach should be considered for improved patient outcomes in populations experiencing psychological stress, particularly those at high-risk for development of osteoporosis.

New treatments and therapeutic targets for IBS and other functional bowel disorders

Functional bowel disorders (FBDs) are a spectrum of disorders characterized by combinations of symptoms attributable to the lower gastrointestinal tract. Most current first-line therapies for IBS and other FBDs target the predominant symptom and mainly affect one symptom in the symptom complex. Additional broadly effective treatment alternatives targeting the entire symptom complex are needed. New drugs for FBDs (such as lubiprostone, linaclotide, plecanatide, prucalopride, eluxadoline and rifaximin) target key mechanisms in the pathophysiology of these disorders and improve both the abnormal bowel habit and other key symptoms, such as abdominal pain and bloating. The current development of new treatment alternatives is focusing on different aspects of the complex pathophysiology of IBS and other FBDs: gut microenvironment (via diet and modulation of gut microbiota), enterohepatic circulation of bile acids, gastrointestinal secretion, motility and sensation, gut-brain interactions, gut barrier function and the immune system within the gastrointestinal tract. Studies also suggest that personalized treatment of IBS and other FBDs is possible using various diagnostic markers.

Peripheral Serotonin Synthesis as a New Drug Target

The first step in serotonin (5-HT) biosynthesis is catalyzed by tryptophan hydroxylase (TPH). There are two independent sources of the monoamine that have distinct functions: first, the TPH1-expressing enterochromaffin cells (ECs) of the gut; second, TPH2-expressing serotonergic neurons. TPH1-deficient mice revealed that peripheral 5-HT plays important roles in platelet function and in inflammatory and fibrotic diseases of gut, pancreas, lung, and liver. Therefore, TPH inhibitors were developed which cannot pass the blood-brain barrier to specifically block peripheral 5-HT synthesis. They showed therapeutic efficacy in several rodent disease models, and telotristat ethyl is the first TPH inhibitor to be approved for the treatment of carcinoid syndrome. We review this development and discuss further therapeutic options for these compounds.

Advancements in drug development for diarrhea-predominant irritable bowel syndrome

INTRODUCTION

Diarrhea-predominant irritable bowel syndrome (IBS-D) is a common disorder characterized by a complex pathophysiology hampering optimal targeted drug development. Recent advances in our understanding of key underlying mechanisms prompted novel therapeutics including novel pharmacological approaches.

AREAS COVERED

This review summarizes the latest advancements in the pipeline of IBS-D drugs focusing on new pharmacological targets, efficacy and safety of medicinal products considering the recent harmonization of regulatory requirements by the FDA and the EMA.

EXPERT OPINION

The new 5-HT₃ receptor antagonist ramosetron appears a promising therapeutic approach devoid of significant adverse events, although it is presently unavailable in Western countries, most likely because of the precautionary approach taken by regulatory agencies with this drug class. New pharmacological concepts on full agonists/antagonists, mixed-receptor activity and novel drug targets may streamline the present drug pipeline along with the adherence on new regulatory guidelines on outcome measures. Eluxadoline can be taken as an example of this paradigm shift. It has now been granted marketing authorization for IBS-D on both sides of the Atlantic, but it is still considered as a second-line agent by the NICE. There is still much work to be done to fully cover clinical needs of patients with IBS-D.

The TPH1 rs211105 gene polymorphism affects abdominal symptoms and quality of life of diarrhea-predominant irritable bowel syndrome

The gastrointestinal symptoms of irritable bowel syndrome are strongly related to impaired quality of life (QOL), especially in diarrhea-predominant. The gene polymorphisms associated with serotonin, or 5-hydroxytryptamine, alter gastrointestinal symptoms and mental status. We aimed to evaluate the effects of gene polymorphisms on gastrointestinal symptoms, psychological conditions, and QOL, and compare these between patients with diarrhea-predominant irritable bowel syndrome (n = 62) and healthy controls (n = 64). The gene polymorphisms of 5-HTTLPR, 5-HTTVNTR, TPH1 rs453773, and TPH1 rs211105 were evaluated. Gastrointestinal symptoms, depressive state, and QOL were assessed using the Gastrointestinal Symptom Rating Scale, Self-rating Depression Scale, and Short-Form-36. Gene polymorphisms did not significantly differ in frequency between the two groups. The scores for diarrhea, abdominal pain, and indigestion significantly correlated with the physical component summary score. Only the group of patients with diarrhea-predominant irritable bowel syndrome showed a significant correlation between the TPH1 rs211105 T/T genotype and lower scores for role physical and mental health, and higher scores for indigestion and diarrhea. 5-HTTLPR l/s was associated with lower score of role emotional in the diarrhea-predominant irritable bowel syndrome and higher scores in the controls. The gene polymorphisms of 5-hydroxytryptamine signaling effected gastrointestinal symptoms and QOL, especially of the patients with diarrhea-predominant irritable bowel syndrome.

Enteric serotonin and oxytocin: endogenous regulation of severity in a murine model of necrotizing enterocolitis

Necrotizing enterocolitis (NEC), a gastrointestinal inflammatory disease of unknown etiology that may also affect the liver, causes a great deal of morbidity and mortality in premature infants. We tested the hypothesis that signaling molecules, which are endogenous to the bowel, regulate the severity of intestinal and hepatic damage in an established murine NEC model. Specifically, we postulated that mucosal serotonin (5-HT), which is proinflammatory, would exacerbate experimental NEC and that oxytocin (OT), which is present in enteric neurons and is anti-inflammatory, would oppose it. Genetic deletion of the 5-HT transporter (SERT), which increases and prolongs effects of 5-HT, was found to increase the severity of systemic manifestations, intestinal inflammation, and associated hepatotoxicity of experimental NEC. In contrast, genetic deletion of tryptophan hydroxylase 1 (TPH1), which is responsible for 5-HT biosynthesis in enterochromaffin (EC) cells of the intestinal mucosa, and TPH inhibition with LP-920540 both decrease the severity of experimental NEC in the small intestine and liver. These observations suggest that 5-HT from EC cells helps to drive the inflammatory damage to the gut and liver that occurs in the murine NEC model. Administration of OT decreased, while the OT receptor antagonist atosiban exacerbated, the intestinal inflammation of experimental NEC. Data from the current investigation are consistent with the tested hypotheses-that the enteric signaling molecules, 5-HT (positively) and OT (negatively) regulate severity of inflammation in a mouse model of NEC. Moreover, we suggest that mucosally restricted inhibition of 5-HT biosynthesis and/or administration of OT may be useful in the treatment of NEC.NEW & NOTEWORTHY Serotonin (5-HT) and oxytocin reciprocally regulate the severity of intestinal inflammation and hepatotoxicity in a murine model of necrotizing enterocolitis (NEC). Selective depletion of mucosal 5-HT through genetic deletion or inhibition of tryptophan hydroxylase-1 ameliorates, while deletion of the 5-HT uptake transporter, which increases 5-HT availability, exacerbates the severity of NEC. In contrast, oxytocin reduces, while the oxytocin receptor antagonist atosiban enhances, NEC severity. Peripheral tryptophan hydroxylase inhibition may be useful in treatment of NEC.

New therapeutic perspectives in irritable bowel syndrome: Targeting low-grade inflammation, immuno-neuroendocrine axis, motility, secretion and beyond

Irritable bowel syndrome (IBS) is a chronic, recurring, and remitting functional disorder of the gastrointestinal tract characterized by abdominal pain, distention, and changes in bowel habits. Although there are several drugs for IBS, effective and approved treatments for one or more of the symptoms for various IBS subtypes are needed. Improved understanding of pathophysiological mechanisms such as the role of impaired bile acid metabolism, neurohormonal regulation, immune dysfunction, the epithelial barrier and the secretory properties of the gut has led to advancements in the treatment of IBS. With regards to therapies for restoring intestinal permeability, multiple studies with prebiotics and probiotics are ongoing, even if to date their efficacy has been limited. In parallel, much progress has been made in targeting low-grade inflammation, especially through the introduction of drugs such as mesalazine and rifaximin, even if a better knowledge of the mechanisms underlying the low-grade inflammation in IBS may allow the design of clinical trials that test the efficacy and safety of such drugs. This literature review aims to summarize the findings related to new and investigational therapeutic agents for IBS, most recently developed in preclinical as well as Phase 1 and Phase 2 clinical studies.

Eluxadoline in the treatment of diarrhea-predominant irritable bowel syndrome. The SEPD perspective

Functional gut disorders, including diarrhea-predominant irritable bowel syndrome, are highly prevalent conditions worldwide that significantly impact health economy and patient quality of life, yet lacking fully satisfactory therapeutic options. These circumstances fostered research on various molecules with more specific therapeutic targets, including opioid receptors. Eluxadoline (Allergan's Vibercy® in the USA, Truberzi® in Europe) is a locally-acting mixed mu- and kappa-opioid receptor agonist, and delta-opioid receptor antagonist, that was licensed in 2015 by the Food and Drug Administration (FDA) and in 2016 by the European Medicines Agency (EMA) for use in diarrhea-predominant irritable bowel syndrome. Eluxadoline provides, with advantage over the current standard of care, control of both stool consistency and abdominal pain, good tolerability in most cases, and improved quality of life, hence it deserves consideration when approaching a patient with this disorder. As with any recently approved therapy, adequate pharmacovigilance is to be expected, as well as studies to inform on different scenarios such as on-demand therapy, loss of response assessment, use as rescue therapy for other molecules, and cost-effectiveness, to further characterize and more accurately position eluxadoline within the therapeutic spectrum.

Heterogeneity of enterochromaffin cells within the gastrointestinal tract

Enterochromaffin cells were the first endocrine cells of the gastrointestinal tract to be chemically distinguished, almost 150 years ago. It is now known that the chromaffin reaction of these cells was due to their content of the reactive aromatic amine, 5-hydroxytryptamine (5-HT, also known as serotonin). They have commonly been thought to be a special class of gut endocrine cells (enteroendocrine cells) that are distinct from the enteroendocrine cells that contain peptide hormones. The study by Martin et al. in the current issue of this journal reveals that the patterns of expression of nutrient receptors and transporters differ considerably between chromaffin cells of the mouse duodenum and colon. However, even within regions, chromaffin cells differ; in the duodenum there are chromaffin cells that contain both secretin and 5-HT, cholecystokinin and 5-HT, and all three of secretin, cholecystokinin, and 5-HT. Moreover, the ratios of these different cell types differ substantially between species. And, in terms of function, 5-HT has many roles, including in appetite, motility, fluid secretion, release of digestive enzymes and bone metabolism. The paper thus emphasizes the need to define the many different classes of enterochromaffin cells and relate this to their roles.

Identification of a Novel Allosteric Inhibitory Site on Tryptophan Hydroxylase 1 Enabling Unprecedented Selectivity Over all Related Hydroxylases

Pulmonary arterial hypertension (PAH) has demonstrated multi-serotonin receptor dependent pathologies, characterized by increased tone (5-HT_1B receptor) and complex lesions (SERT, 5-HT_1B, 5-HT_2B receptors) of the pulmonary vasculature together with right ventricular hypertrophy, ischemia and fibrosis (5-HT_2B receptor). Selective inhibitors of individual signaling elements – SERT, 5-HT_2A, 5HT_2B, and combined 5-HT2_A/B receptors, have all been tested clinically and failed. Thus, inhibition of tryptophan hydroxylase 1 (TPH1), the rate limiting step in 5-HT synthesis, has been suggested as a more broad, and thereby more effective, mode of 5-HT inhibition. However, selectivity over non-pathogenic enzyme family members, TPH2, phenylalanine hydroxylase, and tyrosine hydroxylase has hampered therapeutic development. Here we describe the site/sequence, biochemical, and biophysical characterization of a novel allosteric site on TPH1 through which selectivity over TPH2 and related aromatic amino acid hydroxylases is achieved. We demonstrate the mechanism of action by which novel compounds selectively inhibit TPH1 using surface plasma resonance and enzyme competition assays with both tryptophan ligand and BH4 co-factor. We demonstrate 15-fold greater potency within a human carcinoid cell line versus the most potent known TPH1/2 non-specific inhibitor. Lastly, we detail a novel canine in vivo system utilized to determine effective biologic inhibition of newly synthesized 5-HT. These findings are the first to demonstrate TPH1-selective inhibition and may pave the way to a truly effective means to reduce pathologic 5-HT and thereby treat complex remodeling diseases such as PAH.

Tyrosine and tryptophan hydroxylases as therapeutic targets in human disease

INTRODUCTION

The ancient and ubiquitous monoamine signalling molecules serotonin, dopamine, norepinephrine, and epinephrine are involved in multiple physiological functions. The aromatic amino acid hydroxylases tyrosine hydroxylase (TH), tryptophan hydroxylase 1 (TPH1), and tryptophan hydroxylase 2 (TPH2) catalyse the rate-limiting steps in the biosynthesis of these monoamines. Genetic variants of TH, TPH1, and TPH2 genes are associated with neuropsychiatric disorders. The interest in these enzymes as therapeutic targets is increasing as new roles of these monoamines have been discovered, not only in brain function and disease, but also in development, cardiovascular function, energy and bone homeostasis, gastrointestinal motility, hemostasis, and liver function. Areas covered: Physiological roles of TH, TPH1, and TPH2. Enzyme structures, catalytic and regulatory mechanisms, animal models, and associated diseases. Interactions with inhibitors, pharmacological chaperones, and regulatory proteins relevant for drug development. Expert opinion: Established inhibitors of these enzymes mainly target their amino acid substrate binding site, while tetrahydrobiopterin analogues, iron chelators, and allosteric ligands are less studied. New insights into monoamine biology and 3D-structural information and new computational/experimental tools have triggered the development of a new generation of more selective inhibitors and pharmacological chaperones. The enzyme complexes with their regulatory 14-3-3 proteins are also emerging as therapeutic targets.

Mechanosensory Signaling in Enterochromaffin Cells and 5-HT Release: Potential Implications for Gut Inflammation

Enterochromaffin (EC) cells synthesize 95% of the body 5-HT and release 5-HT in response to mechanical or chemical stimulation. EC cell 5-HT has physiological effects on gut motility, secretion and visceral sensation. Abnormal regulation of 5-HT occurs in gastrointestinal disorders and Inflammatory Bowel Diseases (IBD) where 5-HT may represent a key player in the pathogenesis of intestinal inflammation. The focus of this review is on mechanism(s) involved in EC cell "mechanosensation" and critical gaps in our knowledge for future research. Much of our knowledge and concepts are from a human BON cell model of EC, although more recent work has included other cell lines, native EC cells from mouse and human and intact mucosa. EC cells are "mechanosensors" that respond to physical forces generated during peristaltic activity by translating the mechanical stimulus (MS) into an intracellular biochemical response leading to 5-HT and ATP release. The emerging picture of mechanosensation includes Piezo 2 channels, caveolin-rich microdomains, and tight regulation of 5-HT release by purines. The "purinergic hypothesis" is that MS releases purines to act in an autocrine/paracrine manner to activate excitatory (P2Y₁, P2Y₄, P2Y₆, and A_2A/A_2B) or inhibitory (P2Y₁₂, A₁, and A₃) receptors to regulate 5-HT release. MS activates a P2Y₁/G_αq/PLC/IP₃-IP₃R/SERCA Ca²⁺signaling pathway, an A_2A/A_2B-Gs/AC/cAMP-PKA signaling pathway, an ATP-gated P2X₃ channel, and an inhibitory P2Y₁₂-G_i/o/AC-cAMP pathway. In human IBD, P2X₃ is down regulated and A_2B is up regulated in EC cells, but the pathophysiological consequences of abnormal mechanosensory or purinergic 5-HT signaling remain unknown. EC cell mechanosensation remains poorly understood.

IBS and IBD - separate entities or on a spectrum?

The acute phase of IBD with inflamed gut and often ulcerated mucosa is clearly different from the apparently normal mucosa characteristic of IBS. However, more detailed assessment has detected immune activation, increased gut permeability, increased mucosal serotonin availability, abnormalities of enteric nerve structure and function, and dysbiosis in gut microbiota in IBS - all features seen in IBD. Furthermore, as treatments for inflammation in IBD have become more effective it is now apparent that ∼1 in 3 patients with IBD in remission from inflammation still have persistent abnormalities of sensation, motility and gut microbiota, which might cause IBS-like symptoms. This Perspective explores the overlap between IBS and IBD and their treatments, proposing future directions for research in this stimulating area.

Serotonin biosynthesis as a predictive marker of serotonin pharmacodynamics and disease-induced dysregulation

The biogenic amine serotonin (5-HT) is a multi-faceted hormone that is synthesized from dietary tryptophan with the rate limiting step being catalyzed by the enzyme tryptophan hydroxylase (TPH). The therapeutic potential of peripheral 5-HT synthesis inhibitors has been demonstrated in a number of clinical and pre-clinical studies in diseases including carcinoid syndrome, lung fibrosis, ulcerative colitis and obesity. Due to the long half-life of 5-HT in blood and lung, changes in steady-state levels are slow to manifest themselves. Here, the administration of stable isotope labeled tryptophan (heavy “h-Trp”) and resultant in vivo conversion to h-5-HT is used to monitor 5-HT synthesis in rats. Dose responses for the blockade of h-5-HT appearance in blood with the TPH inhibitors L-para-chlorophenylalanine (30 and 100 mg/kg) and telotristat etiprate (6, 20 and 60 mg/kg), demonstrated that the method enables robust quantification of pharmacodynamic effects on a short time-scale, opening the possibility for rapid screening of TPH1 inhibitors in vivo. In the bleomycin-induced lung fibrosis rat model, the mechanism of lung 5-HT increase was investigated using a combination of synthesis and steady state 5-HT measurement. Elevated 5-HT synthesis measured in the injured lungs was an early predictor of disease induced increases in total 5-HT.

Eluxadoline for the treatment of diarrhoea-predominant irritable bowel syndrome

INTRODUCTION

Irritable bowel syndrome (IBS) treatment is challenging physicians because of its multifactorial physiopathology. In particular, abdominal pain and diarrhea management lack one unique effective pharmacological remedy. Opioid receptors, present in the central nervous system (CNS) and the enteric nervous system (ENS), are involved in visceral sensitivity and gastrointestinal motility control. To date only a few opioid receptor modulators are currently in use for the treatment of IBS but with dosage limitations due to the early development of severe constipation.

AREAS COVERED

In this drug evaluation manuscript we review the irritable bowel syndrome therapeutic needs and chemistry, pharmacokinetics and -dynamics, clinical study results with the new opioid receptor ligand eluxadoline for the treatment of diarrhea-predominant irritable bowel syndrome (IBS-D).

EXPERT OPINION

Eluxadoline shows a peculiar pharmacological profile with μ-opioid agonism and δ-opioid antagonism actions. Its efficacy over placebo for the treatment of abdominal pain and diarrhea in IBS-D has been demonstrated in short- and long-term clinical studies in humans. Its safety has been evaluated in the same studies. Interestingly, eluxadoline showed a low rate of constipation development in IBS patients in comparison with known effects of other opioid receptor modulators. Patients with a history of acute pancreatitis should not be treated with eluxadoline.

High risk of drug-induced microscopic colitis with concomitant use of NSAIDs and proton pump inhibitors

BACKGROUND

Microscopic colitis (MC) is a chronic bowel disorder characterised by watery diarrhoea. Nonsteroidal anti-inflammatory drugs (NSAIDs), proton pump inhibitors (PPIs), selective serotonin reuptake inhibitors (SSRIs) and statins have been associated with MC. However, underlying mechanisms remain unclear.

AIM

To study the association between exposure to these drugs and MC, with attention to time of exposure, duration, dosage and combined exposure, and to test hypotheses on underlying pharmacological mechanisms.

METHODS

A case-control study was conducted using the British Clinical Practice Research Datalink. MC cases (1992-2013) were matched to MC-naive controls on age, sex and GP practice. Drug exposure was stratified according to time of exposure, duration of exposure or dosage. Conditional logistic regression analysis was applied to calculate adjusted odds ratios (AORs).

RESULTS

In total, 1211 cases with MC were matched to 6041 controls. Mean age was 63.4 years, with 73.2% being female. Current use of NSAIDs (AOR 1.86, 95% CI 1.39-2.49), PPIs (AOR 3.37, 95% CI 2.77-4.09) or SSRIs (AOR 2.03, 95% CI 1.58-2.61) was associated with MC compared to never or past use. Continuous use for 4-12 months further increased the risk of MC. Strongest associations (fivefold increased risk) were observed for concomitant use of PPIs and NSAIDs. Statins were not associated with MC.

CONCLUSIONS

Current exposure to NSAIDs, PPIs or SSRIs and prolonged use for 4-12 months increased the risk of MC. Concomitant use of NSAIDs and PPIs showed the highest risk of MC. Acid suppression related dysbiosis may contribute to the PPI effect, which may be exacerbated by NSAID-related side-effects.

Oridonin Alleviates Visceral Hyperalgesia in a Rat Model of Postinflammatory Irritable Bowel Syndrome: Role of Colonic Enterochromaffin Cell and Serotonin Availability

The aim of this present study was to investigate the effect of oridonin on visceral hyperalgesia and colonic serotonin availability in a rat model of trinitrobenzenesulfonic acid-induced postinflammatory irritable bowel syndrome (PI-IBS). Rats were randomly divided into five groups: normal control, PI-IBS model, PI-IBS+low-dose oridonin (5 mg/kg), PI-IBS+median-dose oridonin (10 mg/kg), and PI-IBS+high-dose oridonin (20 mg/kg). Rats in control and model groups were orally administered with water by gavage, whereas rats in oridonin-treated groups were orally administered with different dosages of oridonin, and drugs were given for 14 consecutive days. Compared with the control group, the pain threshold pressure was significantly reduced in PI-IBS rats. The colonic enterochromaffin (EC) cell number, serotonin content, and the protein expression of tryptophan hydroxylase (TPH) were markedly increased and the protein expression of serotonin reuptake transporter was significantly decreased in PI-IBS rats. The spleen index in PI-IBS rats was decreased, and the levels of tumor necrosis factor (TNF)-α, interferon (IFN)-γ, interleukin (IL)-4, and IL-13 in the colon of PI-IBS rats were also markedly decreased. Oridonin treatment dose dependently increased pain threshold pressure, and markedly decreased colon EC cell numbers, TPH expression, and serotonin content in PI-IBS rats. Oridonin treatment also significantly increased the spleen index as well as the levels of TNF-α, IFN-γ, IL-4, and IL-13 in the colon of PI-IBS rats. Results of this study demonstrate that the analgesic effect of oridonin in PI-IBS rats is associated with reduced colonic EC cell hyperplasia and 5-HT availability, the regulatory effect of oridonin on colonic cytokine production may be correlated with its effect on colonic EC cell number.

Inhibition of Peripheral Synthesis of Serotonin as a New Target in Neuroendocrine Tumors

: The carcinoid syndrome represents a set of signs and symptoms associated with neuroendocrine tumors (NETs) that occur primarily when metastases are developed in the liver, resulting in the worsening of quality of life. Serotonin plays a central role in the physiology of carcinoid syndrome by promoting intestinal motility. Somatostatin analogs (SSAs) have widely demonstrated their efficacy as symptomatic relievers of carcinoid syndrome, but this control is ephemeral, being reduced by approximately 50% within the first year. The exact mechanisms of resistance to SSAs are not fully understood, but it is believed that serotonin might be involved. Patients with carcinoid syndrome present with a significant increase in serotonin plasma levels and, consequently, in the soluble urinary metabolite 5-hydroxyindole acetic acid. Telotristat etiprate is a potent inhibitor of tryptophan hydroxylase, a rate-limiting enzyme in the synthesis of serotonin, that has demonstrated in the phase III TELESTAR clinical trial a significant improvement in the control of bowel movements in patients with NETs who have carcinoid syndrome and who have progressed to an SSA. Based on these results, telotristat etiprate has emerged as a potential new option in the treatment algorithm of symptomatic control of functioning NETs. However, some issues need to be clarified, such as the safety profile of the drug outside clinical trials, the benefit in quality of life, and the possible impact on tumor growth, as well as its role within sequencing or combination treatment strategies with pre-existing drugs effective in NET treatment.

IMPLICATIONS FOR PRACTICE

This article reviews the literature about carcinoid syndrome, which affects patients diagnosed with neuroendocrine tumors. Few articles have been published about this syndrome and its pathophysiology. Somatostatin analogs provide symptomatic relief; however, patients may become refractory to this strategy, usually within the first year of treatment. In this context, as an agent with an innovative mechanism of action, telotristat etiprate has demonstrated activity in a phase III trial, and findings may offer a path to an improve quality of life and prolonged survival for certain patients.

Expression of serotonin, chromogranin-A, serotonin receptor-2B, tryptophan hydroxylase-1, and serotonin reuptake transporter in the intestine of dogs with chronic enteropathy

Serotonin regulates many intestinal motor and sensory functions. Altered serotonergic metabolism has been described in human gastrointestinal diseases. The objective of our study was to compare expression of several components of the serotonergic system [serotonin (5-HT), serotonin reuptake transporter protein (SERT), tryptophan hydroxylase-1 (TPH-1), 5-HT receptor2B (5-HT2B)] and the enterochromaffin cell marker chromogranin-A (CgA) in the intestinal mucosa between dogs with chronic enteropathy and healthy controls. Serotonin and CgA expression were determined by immunohistochemistry using banked and prospectively obtained, paraffin-embedded canine gastrointestinal biopsies (n = 11), and compared to a control group of canine small intestinal sections (n = 10). Expression of SERT, TPH-1, and 5-HT2B were determined via real-time reverse transcription (qRT)-PCR using prospectively collected endoscopic duodenal biopsies (n = 10) and compared to an additional control group of control duodenal biopsies (n = 8, control group 2) showing no evidence of intestinal inflammation. Dogs with chronic enteropathies showed strong staining for both 5-HT and CgA. Mean positive cells per high power field (HPF) were significantly increased for both compounds in dogs with chronic enteropathies (p < 0.001 for 5-HT; p < 0.05 for CgA). The number of 5-HT-positive and CgA-positive cells/HPF showed significant correlation in the entire group of dogs, including both diseased and healthy individuals (Pearson r(2) = 0.2433, p = 0.016). No significant differences were observed for SERT, TPH-1, or 5-HT2B expression; however, dogs with chronic enteropathy showed greater variability in expression of TPH-1 and 5-HT2B We conclude that components of the neuroendocrine system show altered expression in the intestinal mucosa of dogs with chronic enteropathy. These changes may contribute to nociception and clinical signs in these patients.

Inflammation in irritable bowel syndrome: Myth or new treatment target?

Low-grade intestinal inflammation plays a key role in the pathophysiology of irritable bowel syndrome (IBS), and this role is likely to be multifactorial. The aim of this review was to summarize the evidence on the spectrum of mucosal inflammation in IBS, highlighting the relationship of this inflammation to the pathophysiology of IBS and its connection to clinical practice. We carried out a bibliographic search in Medline and the Cochrane Library for the period of January 1966 to December 2014, focusing on publications describing an interaction between inflammation and IBS. Several evidences demonstrate microscopic and molecular abnormalities in IBS patients. Understanding the mechanisms underlying low-grade inflammation in IBS may help to design clinical trials to test the efficacy and safety of drugs that target this pathophysiologic mechanism.

Pharmacological, Pharmacokinetic, and Pharmacogenomic Aspects of Functional Gastrointestinal Disorders

This article reviews medications commonly used for the treatment of patients with functional gastrointestinal disorders. Specifically, we review the animal models that have been validated for the study of drug effects on sensation and motility; the preclinical pharmacology, pharmacokinetics, and toxicology usually required for introduction of new drugs; the biomarkers that are validated for studies of sensation and motility endpoints with experimental medications in humans; the pharmacogenomics applied to these medications and their relevance to the FGIDs; and the pharmacology of agents that are applied or have potential for the treatment of FGIDs, including psychopharmacologic drugs.

Serotonergic Mechanisms Regulating the GI Tract: Experimental Evidence and Therapeutic Relevance

Serotonin (5-hydroxytryptamine; 5-HT) is best known as a neurotransmitter critical for central nervous system (CNS) development and function. 95% of the body's serotonin, however, is produced in the intestine where it has been increasingly recognized for its hormonal, autocrine, paracrine, and endocrine actions. This chapter provides the most current knowledge of the critical autocrine and paracrine roles of 5-HT in intestinal motility and inflammation as well as its function as a hormone in osteocyte homeostasis. Therapeutic applications in each of these areas are also discussed.

New and Investigational Agents for Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) affects about 15 % of the US population and results in significant morbidity and health care costs. There remains a significant unmet need for effective treatments particularly for the pain component of IBS and other functional gastrointestinal disorders (FGIDs). Progress made in our understanding of pathophysiological mechanisms such as the role of altered bile acid metabolism, neurohormonal regulation, immune dysfunction, the epithelial barrier and secretory properties of the gut has led to advancements in therapeutic armamentarium for IBS. This review discusses the new drugs for constipation and diarrhea-predominant IBS subtypes that have been tested or have been under investigation over the last 3-4 years. Overall, there is a promising pipeline of investigational drugs for the future treatment of IBS and related FGIDs.

Systematic review: instruments to assess abdominal pain in irritable bowel syndrome

BACKGROUND

Consensus on standard methods to assess chronic abdominal pain in patients with irritable bowel syndrome (IBS) is currently lacking.

AIM

To systematically review the literature with respect to instruments of measurement of chronic abdominal pain in IBS patients.

METHODS

Systematic literature search was performed in PubMed/Medline databases for studies using pain measurement instruments in patients with IBS.

RESULTS

One hundred and ten publications were reviewed. A multitude of different instruments is currently used to assess chronic abdominal pain in IBS patients. The single-item methods, e.g. the validated 10-point numeric rating scale (NRS), and questionnaires assessing gastrointestinal symptoms severity, focus mostly on the assessment of only the intensity of abdominal pain. Of these questionnaires, the validated IBS-Symptom Severity Scale includes the broadest measurement of pain-related aspects. General pain questionnaires and electronic momentary symptom assessment tools have been used to study abdominal pain in IBS patients, but have not yet been validated for this purpose. The evidence for the use of provocation tests, e.g. the rectal barostat with balloon distention, for measurement of abdominal pain in IBS is weak, due to the poor correlation between visceral pain thresholds assessed by provocation tests and abdominal pain as assessed by retrospective questionnaires.

CONCLUSIONS

The multitude of different instruments to measure chronic abdominal pain in IBS makes it difficult to compare endpoints of published studies. There is need for validated instruments to assess chronic abdominal pain in IBS patients, that overcome the limitations of the currently available methods.

Visceral hypersensitivity in irritable bowel syndrome: evidence for involvement of serotonin metabolism--a preliminary study

BACKGROUND

Altered serotonergic (5-HT) metabolism and visceral perception have been associated with the pathogenesis of irritable bowel syndrome (IBS). Aim of this preliminary study was to assess the effect of the direct precursor of 5-HT, 5-hydroxytryptophan (5-HTP), on systemic 5-HT metabolites and visceral perception and to assess potential differential responses between IBS and controls.

METHODS

15 IBS patients and 15 healthy volunteers participated in this randomized double-blind placebo controlled study. Visceroperception was measured by rectal barostat. The 100 mg 5-HTP or placebo was ingested orally. Serotonergic metabolites were assessed in platelet poor plasma.

KEY RESULTS

5-HTP induces rectal allodynia in a significant number of healthy controls; IBS patients exhibit lowered pain thresholds in both placebo and 5-HTP conditions. 5-HTP induces rectal hyperalgesia in hypersensitive but not in non-hypersensitive IBS patients. Administration of 5-HTP significantly increased plasma 5-HTP levels (p < 0.001), did not affect 5-HT levels (p > 0.05), while levels of the main metabolite of 5-HT, 5-hydroxyindoleacetic acid, increased significantly (p < 0.05) in both groups. The magnitude of these changes observed in 5-HT metabolites was significantly greater in IBS patients.

CONCLUSIONS & INFERENCES

Oral administration of 5-HTP induced significant alterations in systemic 5-HT metabolites that were accompanied by increased visceroperception of pain in controls and hypersensitive IBS patients. Changes in 5-HT metabolism appear to be important factors involved in visceral hypersensitivity as the 5-HTP-induced pro-nociceptive response was observed in all hypersensitive IBS patients and to a lesser magnitude in a significant number of healthy controls but in none of the non-hypersensitive IBS patients.

Secretory diarrhoea: mechanisms and emerging therapies

Diarrhoeal disease remains a major health burden worldwide. Secretory diarrhoeas are caused by certain bacterial and viral infections, inflammatory processes, drugs and genetic disorders. Fluid secretion across the intestinal epithelium in secretory diarrhoeas involves multiple ion and solute transporters, as well as activation of cyclic nucleotide and Ca(2+) signalling pathways. In many secretory diarrhoeas, activation of Cl(-) channels in the apical membrane of enterocytes, including the cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, increases fluid secretion, while inhibition of Na(+) transport reduces fluid absorption. Current treatment of diarrhoea includes replacement of fluid and electrolyte losses using oral rehydration solutions, and drugs targeting intestinal motility or fluid secretion. Therapeutics in the development pipeline target intestinal ion channels and transporters, regulatory proteins and cell surface receptors. This Review describes pathogenic mechanisms of secretory diarrhoea, current and emerging therapeutics, and the challenges in developing antidiarrhoeal therapeutics.

Adult Tph2 knockout mice without brain serotonin have moderately elevated spine trabecular bone but moderately low cortical bone thickness

Disruption of serotonin synthesis in neurons and the periphery by knockout (KO) of mouse genes for tryptophan hydroxylases (peripheral Tph1 and neuronal Tph2) has been claimed to decrease (Tph2 KO) and increase (Tph1 KO) bone mass. In this report, adult male and female Tph2 KO mice were observed to have elevated spine trabecular bone. Female Tph2 KO mice have reduced midshaft femur cortical bone thickness. Bone mass was normal in male and female Tph1 KO mice examined as part of a Tph1/Tph2 double knockout (DKO) mouse cohort.

Emerging drug for diarrhea predominant irritable bowel syndrome

INTRODUCTION

Irritable bowel syndrome (IBS) is one of the most common gastrointestinal disorders with a 9 - 23% prevalence estimated in the general population. Patients can be subdivided into those who tend to have predominant diarrhea (IBS-D) or predominant constipation (IBS-C). Total annual productivity loss related to IBS in US is estimated at $205 million, with a significant impairment of health-related quality of life. A gold standard for the treatment of IBS is not established. Symptoms might improve with the use of few drugs and behavioral therapy, however, data concerning efficacy, safety and tolerability are limited. Therefore, development and validation of new therapies targeting at the molecular level are widely awaited.

AREAS COVERED

We will specifically describe in this review Phase II and Phase III trials, with specific focus on treatment of IBS-D patients. Unfortunately, it is difficult to draw definite conclusions from Phase II and Phase III trials, because of the known high placebo effect.

EXPERT OPINION

Drugs active on opioid receptor subtypes and neurokinin (NK) receptors seem to be the most promising, but substantial progress of information in this field is still needed. The achievement of more insights on the pathogenesis of IBS could surely better drive and target the therapy, but still strong efforts are awaited.