The role of endogenous opioids in the control of gastrointestinal motility: predictions from in vitro modelling

Targeting TRPV1 and TRPA1: A feasible strategy for natural herbal medicines to combat postoperative ileus

Under physiological or pathological conditions, transient receptor potential (TRP) channel vanilloid type 1 (TRPV1) and TRP ankyrin 1 (TRPA1) possess the ability to detect a vast array of stimuli and execute diverse functions. Interestingly, increasing works have reported that activation of TRPV1 and TRPA1 could also be beneficial for ameliorating postoperative ileus (POI). Increasing research has revealed that the gastrointestinal (GI) tract is rich in TRPV1/TRPA1, which can be stimulated by capsaicin, allicin and other compounds. This activation stimulates a variety of neurotransmitters, leading to increased intestinal motility and providing protective effects against GI injury. POI is the most common emergent complication following abdominal and pelvic surgery, and is characterized by postoperative bowel dysfunction, pain, and inflammatory responses. It is noteworthy that natural herbs are gradually gaining recognition as a potential therapeutic option for POI due to the lack of effective pharmacological interventions. Therefore, the focus of this paper is on the TRPV1/TRPA1 channel, and an analysis and summary of the processes and mechanism by which natural herbs activate TRPV1/TRPA1 to enhance GI motility and relieve pain are provided, which will lay the foundation for the development of natural herb treatments for this disease.

Impact of opioids on esophageal motility

Opioids are well known to cause adverse effects on the gastrointestinal tract including nausea, vomiting, and constipation. Data regarding how opioids affect the esophagus are more limited. Opioid-induced esophageal dysfunction (OIED) is a clinical syndrome defined by chronic opioid use (≥3 months), esophageal symptoms (mainly dysphagia), and esophageal motility abnormalities diagnosed by manometry including achalasia type III, hypercontractile esophagus, distal esophageal spasm, and esophagogastric junction outflow obstruction. Up until now, the effect of opioids on esophageal motility assessed by the functional lumen imaging probe (FLIP) had not been described. In this issue of NGM, Patel et al. report that FLIP assessment in patients with esophageal symptoms showed that chronic opioid users have a significant increase in repetitive retrograde contractions, but no significant reduction in distensibility at the esophagogastric junction compared to non-users. Additionally, perceptive symptoms were higher, and quality of life metrics were lower in the chronic opioid users. This review article will discuss our current understanding of OIED and provide context for this latest study in chronic opioid users. Further investigation with larger prospective studies is needed to understand the pathophysiology, diagnosis, and management of OIED.

The Guts of the Opioid Crisis

Bidirectional interactions of the gut epithelium with commensal bacteria are critical for maintaining homeostasis within the gut. Chronic opioid exposure perturbs gut homeostasis through a multitude of neuro-immune-epithelial mechanisms, resulting in the development of analgesic tolerance, a major underpinning of the current opioid crisis. Differences in molecular mechanisms of opioid tolerance between the enteric and central pain pathways pose a significant challenge for managing chronic pain without untoward gastrointestinal effects.

Risk factors for upper and lower type prolonged postoperative ileus following surgery for Crohn's disease

PURPOSE

Prolonged postoperative ileus (PPOI) is common after bowel resections, especially in Crohn's disease (CD). The pathophysiology of PPOI is not fully understood. PPOI could affect only the upper or lower gastrointestinal (GI) tract. The aim of this study was to assess risk factors for diverse types of PPOI, particularly to differentiate PPOI of upper and lower GI tract.

METHODS

A retrospective analysis of 163 patients with CD undergoing ileocecal resection from 2015 to 2020 in a single center was performed. PPOI of the upper GI tract was predefined as the presence of vomiting or use of nasogastric tube longer than the third postoperative day. Lower PPOI was predefined as the absence of defecation for more than three days. Independent risk factors were identified by multivariable logistic regression analysis.

RESULTS

Overall incidence of PPOI was 42.7%. PPOI of the upper GI tract was observed in 30.7% and lower PPOI in 20.9% of patients. Independent risk factors for upper PPOI included older age, surgery by a resident surgeon, hand-sewn anastomosis, prolonged opioid analgesia, and reoperation, while for lower PPOI included BMI ≤ 25 kg/m², preoperative anemia, and absence of ileostomy.

CONCLUSION

This study identified different risk factors for upper and lower PPOI after ileocecal resection in patients with CD. A differentiated upper/lower type approach should be considered in future research and clinical practice. High-risk patients for each type of PPOI should be closely monitored, and modifiable risk factors, such as preoperative anemia and opioids, should be avoided if possible.

Dexamethasone on postoperative gastrointestinal motility: A placebo-controlled, double-blinded, randomized controlled trial

BACKGROUND AND AIM

Following abdominal surgery, patients usually experience a transient episode of impaired gastrointestinal motility. This study aimed to determine whether a single preoperative dose of dexamethasone can promote the recovery of gastrointestinal function in patients following elective gastrointestinal surgery.

METHODS

In this single-center, two-arm, parallel, randomized controlled trial, we studied 126 patients (aged 18-80 years) who underwent elective open or laparoscopic bowel surgery for malignant or benign pathology. At the induction of anesthesia, a treatment group (n = 64) received a single dose of 8-mg intravenous dexamethasone, and a control group (n = 62) received normal saline.

RESULTS

Intravenous administration of 8-mg dexamethasone significantly decreased the time to return of flatus by an average of approximately 8 h (P < 0.05). Abdominal distension was significantly reduced on the third day after surgery in the dexamethasone group (P < 0.05), and the time to tolerance of a liquid diet was shorter in the dexamethasone group (P < 0.01). There were no significant differences in other secondary outcomes, including postoperative pain, complication rates, length of hospital stay, or time to first defecation, between the two groups.

CONCLUSIONS

A single intravenous dose of 8-mg dexamethasone at induction of anesthesia significantly decreases the time to return of flatus, improves abdominal distension at 72 h, and promotes tolerance of a liquid diet. Although further studies are required to confirm our results, we recommend that dexamethasone should be used more widely in gastrointestinal surgery.

Opioid-induced esophageal dysfunction

PURPOSE OF REVIEW

Chronic opioid use is common and can cause opioid-induced esophageal dysfunction (OIED). We will discuss the pathophysiology, diagnosis, and management of OIED.

RECENT FINDINGS

OIED is diagnosed based on symptoms, opioid use, and manometric evidence of distal esophageal spasm, esophagogastric junction outflow obstruction, achalasia type III, or jackhammer esophagus. Chronic opioid use appears to interfere with inhibitory signals involved in control of esophageal motility, allowing for unchecked excitatory stimuli, and leading to spastic contractility and impaired esophagogastric junction relaxation. Patients may present with dysphagia and chest pain. OIED is significantly more prevalent in patients taking the stronger opioids oxycodone and hydrocodone compared with the weaker opioid tramadol. Based on 24-h morphine equivalent doses, patients with OIED take higher opioid doses than those without OIED. Impaired inhibitory signaling was recently demonstrated in a study showing reduced deglutitive inhibition during multiple rapid swallows in patients taking opioids.

SUMMARY

OIED is frequent in chronic opioid users undergoing manometry for esophageal symptoms, especially at higher doses or with stronger opioids. OIED appears to be due to impaired inhibitory signals in the esophagus. Opioid cessation or dose reduction is recommended, but studies examining management of OIED are lacking.

Opioid-Induced Foregut Dysfunction

The impact of opioid use on the lower gastrointestinal tract is well described, but recent opioid crisis has caused increased awareness of the detrimental effects of these drugs on esophageal and gastroduodenal motility. Opioid use has been associated with increased incidence of spastic esophageal motility disorders and gastroduodenal dysfunction. Opioid receptors are present with high abundance in the myenteric and submucosal plexus of the enteric nervous system. Activation of these receptors leads to suppressed excitability of the inhibitory musculomotor neurons and unchecked tonic contraction of the autogenic musculature (such as the lower esophageal sphincter and the pylorus).

Human native kappa opioid receptor functions not predicted by recombinant receptors: Implications for drug design

If activation of recombinant G protein-coupled receptors in host cells (by drugs or other ligands) has predictive value, similar data must be obtained with native receptors naturally expressed in tissues. Using mouse and human recombinant κ opioid receptors transfected into a host cell, two selectively-acting compounds (ICI204448, asimadoline) equi-effectively activated both receptors, assessed by measuring two different cell signalling pathways which were equally affected without evidence of bias. In mouse intestine, naturally expressing κ receptors within its nervous system, both compounds also equi-effectively activated the receptor, inhibiting nerve-mediated muscle contraction. However, whereas ICI204448 acted similarly in human intestine, where κ receptors are again expressed within its nervous system, asimadoline was inhibitory only at very high concentrations; instead, low concentrations of asimadoline reduced the activity of ICI204448. This demonstration of species-dependence in activation of native, not recombinant κ receptors may be explained by different mouse/human receptor structures affecting receptor expression and/or interactions with intracellular signalling pathways in native environments, to reveal differences in intrinsic efficacy between receptor agonists. These results have profound implications in drug design for κ and perhaps other receptors, in terms of recombinant-to-native receptor translation, species-dependency and possibly, a need to use human, therapeutically-relevant, not surrogate tissues.

The effect of opioid agonists and antagonists on gastrointestinal motility in mice selected for high and low swim stress-induced analgesia

BACKGROUND

The opioid system in the gastrointestinal (GI) tract plays an important physiological role, but is also responsible for the side effect of opioid drugs, including troublesome constipation in chronic pain treatment. The aim of this study was to characterize and validate a new mouse model to study the effects of opioid agonists and antagonists in the GI tract.

METHODS

Six-week-old male Swiss-Webster mice, divergently bred for high (HA) and low (LA) swim stress-induced analgesia (SSIA), were used in the study. To assess the influence of opioid agonists (morphine and loperamide) and antagonists (naloxone hydrochloride, NLX and naloxone methiodide, NLXM) on GI motility, whole GI transit (whole GIT) and upper GIT assays were conducted. To evaluate the expression of opioid receptors in the ileum and colon of HA and LA mice, immune staining was performed.

KEY RESULTS

The effect of morphine was more pronounced in HA line, whereas loperamide exerted a stronger effect in LA mice. Furthermore, NLX and NLXM differentially abolished the inhibitory action of the central and peripheral opioid system on whole and upper GIT in HA and LA mice. The differences in GI motility between HA and LA mice coexisted with parallel changes in the expression of opioid receptors in the ileum and colon.

CONCLUSIONS & INFERENCES

Differences in the activity of the endogenous opioid system are responsible for the vulnerability to changes in GI motility during treatment with opioids. Our findings validate the HA/LA model for further studies on opioids in the GI tract.

Postoperative ileus: Recent developments in pathophysiology and management

BACKGROUND & AIMS

Postoperative ileus (POI) is a frequent occurrence after abdominal and other types of surgery, and is associated with significant morbidity and costs to health care providers. The aims of this narrative review were to provide an update of classification systems, preventive techniques, pathophysiological mechanisms, and treatment options for established POI.

METHODS

The Web of Science, MEDLINE, PubMed and Google Scholar databases were searched using the key phrases 'ileus', 'postoperative ileus' and 'definition', for relevant studies published in English from January 1997 to August 2014.

RESULTS

POI is still a problematic and frequent complication of surgery. Fluid overload, exogenous opioids, neurohormonal dysfunction, and gastrointestinal stretch and inflammation are key mechanisms in the pathophysiology of POI. Evidence is supportive of thoracic epidural analgesia, avoidance of salt and water overload, alvimopan and gum chewing as measures for the prevention of POI, and should be incorporated into perioperative care protocols. Minimal access surgery and avoidance of nasogastric tubes may also help. Novel strategies are emerging, but further studies are required for the treatment of prolonged POI, where evidence is still lacking.

CONCLUSIONS

Although POI is often inevitable, methods to reduce its duration and facilitate recovery of postoperative gastrointestinal function are evolving rapidly. Utilisation of standardised diagnostic classification systems will help improve applicability of future studies.

Opioid receptors and associated regulator of G protein signaling are involved in the cathartic colon of rats

A cathartic colon is characteristic of slow transit constipation (STC), which can result following the long-term use of irritant laxatives. In the present study, the involvement of three opioid receptor subtypes (μ, MOR; δ, DOR; and κ, KOR), regulator of G protein signaling 4 (RGS-4) and β-arrestin-2 were investigated in the cathartic colon of rats. A rat model of a cathartic colon was established by feeding the animals with phenolphthalein, while normal rats were used as a control. The mRNA and protein expression levels of the opioid receptors, RGS-4 and β-arrestin-2 were detected in the rat colon using semi-quantitative reverse transcription polymerase chain reaction and western blot analysis, respectively. The rat model of a cathartic colon was successfully established using the phenolphthalein stimulus, and was shown to result in shrunken myenteric neurons and loose muscle fibers in the intestinal wall. The mRNA and protein expression levels of the three opioid receptor subtypes, RGS-4 and β-arrestin-2 were significantly higher in the cathartic colon group when compared with the levels in the normal control group (all P<0.01). With regard to the protein expression levels, MOR protein increased 2.4 fold, DOR expression increased 1.5 fold, KOR levels increased 1.5 fold, RGS-4 protein increased 3.5 fold and β-arrestin-2 expression increased 2.0 fold. Therefore, the expression levels of opioid receptors were found to increase in the cathartic colons of the rats, indicating that opioid receptors and downstream RGS-4 and β-arrestin-2 signaling may play an important role in the pathogenesis of STC.

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

Acetylcholine serves as a derepressor in Loperamide-induced Opioid-Induced Bowel Dysfunction (OIBD) in zebrafish

The mechanisms underlying gut development, especially peristalsis, are widely studied topics. However, the causes of gut peristalsis-related diseases, especially Opioid-Induced Bowel Dysfunction (OIBD) disorder, have not been well defined. Therefore, our study used zebrafish, a popular model for studying both gut development and peristalsis, and DCFH-DA, a dye that clearly labels the live fish gut lumen, to characterize the formation process of gut lumen as well as the gut movement style in vivo. By applying Loperamide Hydrochloride (LH), the μ-opioid receptor-specific agonist, we established an OIBD-like zebrafish model. Our study found that acetylcholine (ACh) was a key transmitter that derepressed the phenotype induced by LH. Overall, the study showed that the antagonistic role of ACh in the LH-mediated opioid pathway was evolutionarily conserved; moreover, the OIBD-like zebrafish model will be helpful in the future dissection of the molecular pathways involved in gut lumen development and pathology.

Randomized placebo-controlled study of intravenous methylnaltrexone in postoperative ileus

OBJECTIVE

This phase 2 study evaluated the safety and activity of intravenous methylnaltrexone on the duration of postoperative ileus in patients undergoing segmental colectomy.

METHODS

Adults (aged 18 years or older) with American Society of Anesthesiologists physical status of I, II, or III who underwent segmental colectomy, including partial colectomy, sigmoidectomy, cecectomy, or anterior proctosigmoidectomy, via laparotomy with general anesthesia, received intravenous methylnaltrexone 0.30 mg/kg or placebo every 6 h beginning within 90 min after end of surgery. Treatment continued until 24 h after the patient tolerated solid foods, was discharged, or for 7 d maximum. Efficacy endpoints included measures of gastrointestinal recovery and time to discharge eligibility.

RESULTS

A total of 65 patients (methylnaltrexone, n = 33; placebo, n = 32) were randomized. Mean time to first bowel movement was accelerated by 20 h (p = 0.038) and time to discharge eligibility was accelerated by 33 h (p = 0.049) with methylnaltrexone vs placebo. Opioid use was similar between groups until postoperative day 4, then fluctuated in the placebo group. Methylnaltrexone was generally well tolerated.

CONCLUSIONS

In this study, intravenous methylnaltrexone significantly decreased time to postoperative bowel recovery and eligibility for hospital discharge by ∼1 d, with an adverse event profile similar to placebo. These were two of several exploratory endpoints; not all efficacy endpoints showed a significant difference between methylnaltrexone and placebo. The efficacy results in this trial were not seen in two subsequent large-scale studies.

Effects of methylnaltrexone on guinea pig gastrointestinal motility

The purpose of the present study was to compare the effects of methylnaltrexone (MNTX), a peripherally acting μ opioid receptor (μOR) antagonist, on gastrointestinal (GI) motility in naïve vs. opiate chronically treated guinea pigs in vitro and in vivo. We have used the electrically stimulated muscle twitch contractions of longitudinal muscle-myenteric plexus (LMMP) preparations and total GI transit as measure of GI motility. In LMMP preparations of naïve guinea pigs, MNTX (1-30 μM) induced a significant, dose-response reduction of morphine-induced inhibition of electrically stimulated muscle twitch contractions, with an IC50 of 9.4 10(-8)M. By contrast, MNTX abolished the inhibitory effect of acute morphine at any concentrations tested (1-30 μM) in the guinea pigs chronically treated with opiates. In vivo, MNTX (10-50 mg s.c.) did not affect GI transit in naïve guinea pigs when administered acutely or for five consecutive days, but reversed the GI transit delay induced by chronic morphine treatment. These findings show that MNTX is effective in reversing opiate-induced inhibition of GI motility acting at peripheral μ opioid receptors, but does not exert a pharmacologic effect on GI transit in the absence of opiate stimulation.

Localization and regulation of fluorescently labeled delta opioid receptor, expressed in enteric neurons of mice

BACKGROUND & AIMS

Opioids and opiates inhibit gastrointestinal functions via μ, δ, and κ receptors. Although agonists of the δ opioid receptor (DOR) suppress motility and secretion, little is known about the localization and regulation of DOR in the gastrointestinal tract.

METHODS

We studied mice in which the gene that encodes the enhanced green fluorescent protein (eGFP) was inserted into Oprd1, which encodes DOR, to express an approximately 80-kilodalton product (DOReGFP). We used these mice to localize DOR and to determine how agonists regulate the subcellular distribution of DOR.

RESULTS

DOReGFP was expressed in all regions but was confined to enteric neurons and fibers within the muscularis externa. In the submucosal plexus, DOReGFP was detected in neuropeptide Y-positive secretomotor and vasodilator neurons of the small intestine, but rarely was observed in the large bowel. In the myenteric plexus of the small intestine, DOReGFP was present in similar proportions of excitatory motoneurons and interneurons that expressed choline acetyltransferase and substance P, and in inhibitory motoneurons and interneurons that contained nitric oxide synthase. DOReGFP was present mostly in nitrergic myenteric neurons of colon. DOReGFP and μ opioid receptors often were co-expressed. DOReGFP-expressing neurons were associated with enkephalin-containing varicosities, and enkephalin-induced clathrin- and dynamin-mediated endocytosis and lysosomal trafficking of DOReGFP. DOReGFP replenishment at the plasma membrane was slow, requiring de novo synthesis, rather than recycling.

CONCLUSIONS

DOR localizes specifically to submucosal and myenteric neurons, which might account for the ability of DOR agonists to inhibit gastrointestinal secretion and motility. Sustained down-regulation of DOReGFP at the plasma membrane of activated neurons could induce long-lasting tolerance to DOR agonists.

Opioid receptors in the gastrointestinal tract

Opium is arguably one of the oldest herbal medicines, being used as analgesic, sedative and antidiarrheal drug for thousands of years. These effects mirror the actions of the endogenous opioid system and are mediated by the principal mu-, kappa- and delta-opioid receptors. In the gut, met-enkephalin, leu-enkephalin, beta-endorphin and dynorphin occur in both neurons and endocrine cells. When released, opioid peptides activate opioid receptors on the enteric circuitry controlling motility and secretion. As a result, inhibition of gastric emptying, increase in sphincter tone, induction of stationary motor patterns and blockade of peristalsis ensue. Together with inhibition of ion and fluid secretion, these effects cause constipation, one of the most frequent and troublesome adverse reactions of opioid analgesic therapy. Although laxatives are most frequently used to ameliorate opioid-induced bowel dysfunction, their efficacy is unsatisfactory. Specific antagonism of peripheral opioid receptors is a more rational approach. This goal is addressed by the use of opioid receptor antagonists with limited absorption such as oral prolonged-release naloxone and opioid receptor antagonists that do not penetrate the blood-brain barrier such as methylnaltrexone and alvimopan. Preliminary evidence indicates that peripherally restricted opioid receptor antagonists may act as prokinetic drugs in their own right.

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.

Methylnaltrexone in the treatment of opioid-induced constipation

Constipation is a significant problem related to opioid medications used to manage pain. This review attempts to outline the latest findings related to the therapeutic usefulness of a μ opioid receptor antagonist, methylnaltrexone in the treatment of opioid-induced constipation. The review highlights methylnaltrexone bromide (Relistor™; Progenics/Wyeth) a quaternary derivative of naltrexone, which was recently approved in the United States, Europe and Canada. The Food and Drug Administration in the United States approved a subcutaneous injection for the treatment of opioid bowel dysfunction in patients with advanced illness who are receiving palliative care and when laxative therapy has been insufficient. Methylnaltrexone is a peripherally restricted, μ opioid receptor antagonist that accelerates oral-cecal transit in patients with opioid-induced constipation without reversing the analgesic effects of morphine or inducing symptoms of opioid withdrawal. An analysis of the mechanism of action and the potential benefits of using methylnaltrexone is based on data from published basic research and recent clinical studies.

Opioid-induced bowel dysfunction: prevalence, pathophysiology and burden

As a result of the undesired action of opioids on the gastrointestinal (GI) tract, patients receiving opioid medication for chronic pain often experience opioid-induced bowel dysfunction (OBD), the most common and debilitating symptom of which is constipation. Based on clinical experience and a comprehensive MEDLINE literature review, this paper provides the primary care physician with an overview of the prevalence, pathophysiology and burden of OBD. Patients with OBD suffer from a wide range of symptoms including constipation, decreased gastric emptying, abdominal cramping, spasm, bloating, delayed GI transit and the formation of hard dry stools. OBD can have a serious negative impact on quality of life (QoL) and the daily activities that patients feel able to perform. To relieve constipation associated with OBD, patients often use laxatives chronically (associated with risks) or alter/abandon their opioid medication, potentially sacrificing analgesia. Physicians should have greater appreciation of the prevalence, symptoms and burden of OBD. In light of the serious negative impact OBD can have on QoL, physicians should encourage dialogue with patients to facilitate optimal symptomatic management of the condition. There is a pressing need for new therapies that act upon the underlying mechanisms of OBD.

Treatment of opioid-induced gut dysfunction

Opioid analgesics are the mainstay in the treatment of moderate-to-severe pain, yet their use is frequently associated with adverse effects, the most common and debilitating being constipation. Opioid-induced motor stasis results from blockade of gastrointestinal peristalsis and fluid secretion, and reflects the action of the endogenous opioid system in the gut. Methylnaltrexone and alvimopan are new investigational drugs that selectively target peripheral mu-opioid receptors because they are poorly absorbed in the intestine and do not enter the brain. Clinical studies have proved the concept that these drugs prevent opioid-induced bowel dysfunction without interfering with analgesia. As reviewed in this article, opioid receptor antagonists with a peripherally restricted site of action also hold therapeutic promise in postoperative ileus and chronic constipation due to the fact that they have been found to stimulate intestinal transit.

Alvimopan for the Management of Postoperative Ileus

OBJECTIVE:

To review the pharmacology, pharmacokinetics, clinical efficacy, safety, dosage, and administration of alvimopan, a peripherally acting μ-opioid receptor antagonist, in the management of postoperative ileus (POI).

DATA SOURCES:

A literature search (1980–October 2004%) applying the terms alvimopan, ADL 8–2698, and LY246736 was conducted using MEDLINE. Information was also obtained from scientific congress abstracts and data on file with the manufacturer.

STUDY SELECTION AND DATA EXTRACTION:

Studies and abstracts investigating alvimopan and POI were considered for inclusion; however, they were restricted to English-language articles.

DATA SYNTHESIS:

Alvimopan is a novel, peripherally acting μ-opioid receptor antagonist that is currently under evaluation for the management of POI. POI presents significant clinical challenges that can delay patient recovery and contribute to increased morbidity and prolonged hospitalization after surgery. Clinical trials have demonstrated that alvimopan, at oral doses of 6 and 12 mg, can accelerate time to recovery of gastrointestinal (GI) function and time to hospital discharge following abdominal surgery. The incidence of adverse events with alvimopan therapy was shown to be similar to that of placebo.

CONCLUSIONS:

Alvimopan is well tolerated and effective at accelerating GI recovery and time to discharge in patients who have undergone bowel resection or hysterectomy when administered prior to surgery and twice daily thereafter until discharge or for up to 7 days. Alvimopan potentially offers significant benefits for patients with POI over currently available treatments.

Preclinical studies of opioids and opioid antagonists on gastrointestinal function

Opioid receptors in the gastrointestinal (GI) tract mediate the effects of endogenous opioid peptides and exogenously administered opioid analgesics, on a variety of physiological functions associated with motility, secretion and visceral pain. The studies reviewed or reported here describe a range of in vivo activities of opioid receptor antagonists upon GI function in rodents, focusing on mu receptors. Naloxone, and the peripherally acting mu-opioid receptor antagonists alvimopan and methylnaltrexone, reverse morphine-induced inhibition of GI transit in mice and rats, and morphine- or loperamide-induced inhibition of castor oil-induced diarrhoea in mice. At doses producing maximal reversal of morphine-induced effects upon GI transit, only the central nervous system (CNS) penetrant antagonist naloxone was able to reverse morphine-induced analgesia. Both central and peripheral opioid antagonists may affect GI function and/or visceromotor sensitivity in the absence of exogenous opioid analgesics, suggesting a constitutive role for endogenous opioid peptides in the control of GI physiology. Furthermore, in contrast to naloxone, alvimopan does not produce hypersensitivity to the visceromotor response induced by nociceptive levels of colorectal distension in a rodent model of post-inflammatory colonic hypersensitivity, suggesting that in the periphery endogenous mu-opioid receptor-mediated mechanisms do not regulate colonic sensitivity. The data support the hypothesis that peripherally acting opioid antagonists may be able to selectively block opioid receptors in the GI tract, thereby preserving normal GI physiology, while not blocking the effects of endogenous opioid peptides or exogenous opioid analgesics in the CNS. These findings suggest that the primary sites of action of mu-opioid agonists with respect to inhibition of GI function are in the periphery, whereas analgesic activity resides primarily in the CNS.