Peripheral opioids for functional GI disease: a reappraisal

Antinociceptive Effects of Kappa-Opioid Receptor Agonists

Preclinical models that assess "pain" in rodents typically measure increases in behaviors produced by a "pain stimulus." A large literature exists showing that kappa opioid receptor (KOR) agonists can decrease these "pain-stimulated behaviors" following many different pain stimuli. Despite showing apparent antinociceptive properties in these preclinical models, KOR agonists failed as analgesics in clinical trials. Recent studies that assessed decreases in behavior due to a pain stimulus show that KOR agonists are not effective in restoring these "pain-depressed behaviors" to normal levels, which agrees with the lack of effectiveness for KOR agonists in clinical trials. One current explanation for the failure of previous KOR agonists in clinical trials is that those agonists activated beta-arrestin signaling and that KOR agonists with a greater bias for G protein signaling will be more successful. However, neither G protein-biased agonists nor beta-arrestin-biased agonists are very effective in assays of pain-depressed behavior, which suggests that novel biased agonists may still not be effective analgesics. This review provides a concise account of the effectiveness of KOR agonists in preclinical models of pain-stimulated and pain-depressed behaviors following the administration of different pain stimuli. Based on the previous results, it may be appropriate to include both behaviors when testing the analgesic potential of KOR agonists.

Enkephalinase inhibitors, potential therapeutics for the future treatment of diarrhea predominant functional gastrointestinal disorders

The endogenous opioid system (EOS) is considered being a crucial element involved in the pathophysiology of irritable bowel syndrome (IBS) as it regulates gastrointestinal (GI) homeostasis through modulation of motility and water and ion secretion/absorption. Along with opioid receptors (ORs), the following components of EOS can be distinguished: 1. endogenous opioid peptides (EOPs), namely enkephalins, endorphins, endomorphins and dynorphins, and 2. peptidases, which regulate the metabolism (synthesis and degradation) of EOPs. Enkephalins, which are δ-opioid receptors agonists, induce significant effects in the GI tract as they act as potent pro-absorptive neurotransmitters. The action of enkephalins and other EOPs is limited, since EOPs are easily and rapidly inactivated by a natural metalloendopeptidase (enkephalinase/neprilysin) and aminopeptidase N. Studies show that the activity of EOPs can be enhanced by inhibition of these enzymes. In this review, we discuss the antidiarrheal and antinociceptive potential of enkephalinase inhibitors. Furthermore, our review is to answer the question whether enkephalinase inhibitors may be helpful in the future treatment of diarrhea predominant functional GI disorders.

Pharmacological Approach for Managing Pain in Irritable Bowel Syndrome: A Review Article

Context

Visceral pain is a leading symptom for patients with irritable bowel syndrome (IBS) that affects 10% - 20 % of the world population. Conventional pharmacological treatments to manage IBS-related visceral pain is unsatisfactory. Recently, medications have emerged to treat IBS patients by targeting the gastrointestinal (GI) tract and peripheral nerves to alleviate visceral pain while avoiding adverse effects on the central nervous system (CNS). Several investigational drugs for IBS also target the periphery with minimal CNS effects.

Evidence of Acquisition

In this paper, reputable internet databases from 1960 - 2016 were searched including Pubmed and ClinicalTrials.org, and 97 original articles analyzed. Search was performed based on the following keywords and combinations: irritable bowel syndrome, clinical trial, pain, visceral pain, narcotics, opioid, chloride channel, neuropathy, primary afferent, intestine, microbiota, gut barrier, inflammation, diarrhea, constipation, serotonin, visceral hypersensitivity, nociceptor, sensitization, hyperalgesia.

Results

Certain conventional pain managing drugs do not effectively improve IBS symptoms, including NSAIDs, acetaminophen, aspirin, and various narcotics. Anxiolytic and antidepressant drugs (Benzodiazepines, TCAs, SSRI and SNRI) can attenuate pain in IBS patients with relevant comorbidities. Clonidine, gabapentin and pregabalin can moderately improve IBS symptoms. Lubiprostone relieves constipation predominant IBS (IBS-C) while loperamide improves diarrhea predominant IBS (IBS-D). Alosetron, granisetron and ondansetron can generally treat pain in IBS-D patients, of which alosetron needs to be used with caution due to cardiovascular toxicity. The optimal drugs for managing pain in IBS-D and IBS-C appear to be eluxadoline and linaclotide, respectively, both of which target peripheral GI tract.

Conclusions

Conventional pain managing drugs are in general not suitable for treating IBS pain. Medications that target the GI tract and peripheral nerves have better therapeutic profiles by limiting adverse CNS effects.

Potential biomarkers

A "biomarker" (biological marker) is an indicator of a bodily function that can be objectively measured. A wide range of possible biomarkers for IBS have been considered but at present only gut transit measured using radio-isotope markers meet the criteria of reproducibility and availability. While barostat studies perform reasonably in expert centers, to do them reproducibly requires considerable effort and standardization. This makes them unsuitable for widespread use. However radio-isotope tests are expensive and of limited availability so the search for other more convenient markers including blood and stool tests is still an important goal for the future.

[Opioid-induced bowel dysfunction: a literature analysis on pathophysiology and treatment]

Bowel dysfunction is a frequent and serious side effect of opioid analgetics. In spite of its common occurrence, in the course of clinical routine, it is frequently ignored or underestimated. Authors of the analysed literature widely agree that a prophylactic and routine pharmacotherapy is necessary. For this purpose, laxatives, enemas and suppositories, prokinetic agents and opioid antagonists can be considered. Bulk-forming laxatives did not prove to be effective, since the quantity of fluid intake required for the action usually cannot be provided. Furthermore, the benefit of emollient agents is doubted. As a monotherapy they are not sufficient. By contrast, stimulant and osmotic laxatives proved to be active. Prokinetic drugs are not recommended because of their serious side effects. Effective abatement of opioid-induced obstipation by opioid antagonists has been proven in numerous studies. However, the loss of analgesia and opioid withdrawal symptoms were described as adverse effects. Development of quaternary opioid antagonists such as methylnaltrexone was allowed for mitigating these adverse effects.

New treatments for irritable bowel syndrome in women

The estimated prevalence of irritable bowel syndrome (IBS) in Western countries is 7-15%, with a female:male ratio of 2-2.5:1 in IBS patients who seek healthcare services; however, the female predominance is lower in the general population. IBS has a significant impact on health-related quality of life and is associated with a significant healthcare and economic burden. Management of IBS is comprised of general measures and pharmacologic and nonpharmacologic treatment. However, there are ongoing efforts to find more effective therapeutic approaches. As advancements in the understanding of the pathophysiology of IBS continue to grow, new and effective treatments with novel mechanisms of action that have the potential to improve relief of IBS symptoms over current treatments are likely to be developed. This article provides an overview of current and emerging therapies for IBS and also highlights sex and gender differences in clinical trials and treatment response.

Effect of oral naloxone hydrochloride on gastrointestinal transit in premature infants treated with morphine

BACKGROUND

Opioids are common drugs for pain treatment in preterm newborn infants, in spite of several adverse effects. Constipation is a frequent problem when opioids are used in both adults and neonates. Although several studies indicate that the oral administration of naloxone hydrochloride (NH) improves intestinal motility during opioid therapy, there is still a lack of evidence in newborns.

AIM

The aim of this study was to assess the efficacy of NH against reduced intestinal motility during opioid treatment.

METHODS

A retrospective cohort study was performed. We analysed the medical records of fifteen infants (Group 1) treated with continuous morphine (MO) infusion and fourteen infants (Group 2) treated with both oral NH (3 microg/kg 4 times daily) and MO.

RESULTS

There was no statistically significant difference in the total MO dose. Infants treated both with NH and MO had a tendency to improve their mean stool frequency/day. A statistically significant improvement was observed in the mean total food intake (mL/kg/day) of the infants treated with NH (p = 0.014). No difference in the mean food retention between the two groups was observed.

CONCLUSION

Orally administrated NH seems to improve intestinal motility resulting in increased food intake/day and improved stool frequency/day in premature newborn infants treated with MO. Further studies are needed to corroborate these findings.

Neuromodulatory processes of the brain-gut axis

Brain-gut axis represents a complex reflex circuit that integrates the communication between cortex and the digestive system. Disturbances of the neuromodulatory processes in the brain-gut axis generate functional digestive disorders mainly centered on the pain symptoms and motility disorders. This article reviews structural and patho-physiological aspects of the brain-gut axis and explains how the neuromodulatory interventions currently used in order to treat GI conditions related to the brain-gut axis disturbances. The neuromodulation can be realized by pharmacological targeting mainly receptors in the periphery or using electrical stimulation applied at different levels of the nervous system or directly in the muscular layers of the bowels resulting in modulation of the digestive system activity. The efficacy of the methods using electrostimulation is dependent on the parameters of the physical system used: amplitude, frequency, burst time of the electrical current and also the positioning of the electrodes. While pharmacological interventions are largely used at the moment, neuromodulatory interventions involving electrical stimulation showed clinical efficacy in research trials and have promise.

Asimadoline, a kappa-opioid agonist, and satiation in functional dyspepsia

BACKGROUND

Asimadoline, a kappa-opioid agonist, reduces visceral sensitivity in experimental animal models and may decrease satiation and postprandial fullness in healthy individuals. However, its effect on satiation in functional dyspepsia is unclear, and any symptom benefit has not been explored.

AIM

To evaluate the effects of asimadoline on satiation volume and postchallenge symptoms in functional dyspepsia.

METHODS

A randomized, double-blind trial evaluated gastric satiation and symptoms before and after 8 weeks of asimadoline 0.5 mg (n = 13) or 1.0 mg (n = 13) or placebo (n = 14) b.d. in patients with functional dyspepsia (Rome II). Gastrointestinal Symptom Rating Scale and Nepean Dyspepsia Index were used to assess symptoms during the 8-week treatment.

RESULTS

Over 8 weeks of treatment, asimadoline had no significant effect on maximum-tolerated volume or aggregate symptom score with nutrient drink challenge, and on the mean of the total daily symptom severity score compared to placebo. In a post hoc analysis, asimadoline 0.5 mg significantly increased the maximum-tolerated volume (1217 mL +/- 90.2) compared to placebo (807 mL +/- 111.8) in patients with higher postprandial fullness scores (P = 0.01).

CONCLUSION

Asimadoline overall did not significantly alter maximum-tolerated volume, symptoms postnutrient challenge or symptoms over 8 weeks in functional dyspepsia.

Adynamic ileus and acute colonic pseudo-obstruction

Ileus and colonic pseudo-obstruction cause functional obstruction of intestinal transit, without mechanical obstruction, because of uncoordinated or attenuated intestinal muscle contractions. Ileus usually arises from an exaggerated intestinal reaction to abdominal surgery that is often exacerbated by numerous other conditions. Colonic pseudo-obstruction is induced by numerous metabolic disorders, drugs that inhibit intestinal motility, severe illnesses, and extensive surgery. It presents with massive colonic dilatation with variable, moderate small bowel dilatation. Both conditions are initially treated with supportive measures that include intravenous rehydration, correction of electrolyte abnormalities, discontinuation of antikinetic drugs, and treatment of other contributing disorders. Specific therapies for colonic pseudo-obstruction include neostigmine (an anticholinesterase) for pharmacologic colonic decompression and colonoscopic decompression.

The narcotic bowel syndrome: clinical features, pathophysiology, and management

Narcotic bowel syndrome (NBS) is a subset of opioid bowel dysfunction that is characterized by chronic or frequently recurring abdominal pain that worsens with continued or escalating dosages of narcotics. This syndrome is underrecognized and may be becoming more prevalent. In the United States this may be the result of increases in using narcotics for chronic nonmalignant painful disorders, and the development of maladaptive therapeutic interactions around its use. NBS can occur in patients with no prior gastrointestinal disorder who receive high dosages of narcotics after surgery or acute painful problems, and among patients with functional gastrointestinal disorders or other chronic gastrointestinal diseases who are managed by physicians who are unaware of the hyperalgesic effects of chronic opioids. The evidence for the enhanced pain perception is based on the following: (1) activation of excitatory antianalgesic pathways within a bimodal opioid regulation system, (2) descending facilitation of pain at the rostral ventral medulla and pain facilitation via dynorphin and cholecystokinin activation, and (3) glial cell activation that produces morphine tolerance and enhances opioid-induced pain. Treatment involves early recognition of the syndrome, an effective physician-patient relationship, graded withdrawal of the narcotic according to a specified withdrawal program, and the institution of medications to reduce withdrawal effects.