Methylnaltrexone, a novel peripheral opioid receptor antagonist for the treatment of opioid side effects

The Dynorphin/-Opioid Receptor System at the Interface of Hyperalgesia/Hyperkatifeia and Addiction

Purpose of Review

Drug addiction is characterized by compulsive drug seeking and use, accompanied by negative emotional states (hyperkatifeia) and heightened pain sensitivity (hyperalgesia) during withdrawal. Both hyperalgesia and hyperkatifeia are integral components of substance use disorders, negatively impacting treatment and recovery. The underlying neurobiological mechanisms of hyperalgesia and hyperkatifeia involve alterations of brain reward and stress circuits, including the dynorphin/κ-opioid receptor (KOR) system. The dynorphin/KOR system modulates pain perception, negative affect, and addictive behaviors. Here, we review the preclinical evidence of dynorphin/KOR signaling in opioid withdrawal-induced hyperalgesia and hyperkatifeia.

Recent Findings

In opioid dependence models, pharmacological and genetic interventions of the dynorphin/KOR system attenuate somatic and motivational signs of withdrawal and addictive-like behaviors, highlighting its therapeutic potential. Understanding the intricate interplay between dynorphin/KOR signaling, hyperalgesia, hyperkatifeia, and addiction offers novel insights into treatment strategies for opioid use disorder and other substance use disorders.

Summary

Further research is needed to elucidate precise mechanisms of the sexual dimorphism of dynorphin/KOR signaling and identify targeted interventions to mitigate hyperalgesia and hyperkatifeia and facilitate recovery from addiction.

Activation of neurons and satellite glial cells in the DRG produces morphine-induced hyperalgesia

Activation of neurons and glial cells in the dorsal root ganglion is one of the key mechanisms for the development of hyperalgesia. The aim of the present study was to examine the role of neuroglial activity in the development of opioid-induced hyperalgesia. Male rats were treated with morphine daily for 3 days. The resultant phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 in the dorsal root ganglion was analyzed by immunohistochemistry and Western blotting. Pain hypersensitivity was analyzed using behavioral studies. The amount of cytokine expression in the dorsal root ganglion was also analyzed. Repeated morphine treatment induced hyperalgesia and marked induction of phosphorylated ERK1/2 in the neurons and satellite glial cells on day 3. An opioid receptor antagonist, toll like receptor-4 inhibitor, MAP/ERK kinase (MEK) inhibitor and gap junction inhibitor inhibited morphine-induced hyperalgesia and ERK1/2 phosphorylation. Morphine treatment induced alteration of cytokine expression, which was inhibited by the opioid receptor antagonist, toll like receptor-4 inhibitor, MEK inhibitor and gap junction inhibitor. Dexamethasone inhibited morphine-induced hyperalgesia and ERK1/2 phosphorylation after morphine treatment. The peripherally restricted opioid receptor antagonist, methylnaltrexone, inhibited hyperalgesia and ERK1/2 phosphorylation. Morphine activates ERK1/2 in neurons and satellite glial cells in the dorsal root ganglion via the opioid receptor and toll like receptor-4. ERK1/2 phosphorylation is gap junction-dependent and is associated with the alteration of cytokine expression. Inhibition of neuroinflammation by activation of neurons and glia might be a promising target to prevent opioid-induced hyperalgesia.

Postoperative Ileus: a Pharmacological Perspective

Post-operative ileus (POI) is a frequent complication after abdominal surgery. The consequences of POI can be potentially serious such as bronchial inhalation or acute functional renal failure. Numerous advances in peri-operative management, particularly early rehabilitation, have made it possible to decrease POI. Despite this, the rate of prolonged POI ileus remains high and can be as high as 25% of patients in colorectal surgery. From a pathophysiological point of view, POI has two phases, an early neurological phase and a later inflammatory phase, to which we could add a "pharmacological" phase during which analgesic drugs, particularly opiates, play a central role. The aim of this review article is to describe the phases of the pathophysiology of POI, to analyse the pharmacological treatments currently available through published clinical trials and finally to discuss the different research areas for potential pharmacological targets.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

Role of central versus peripheral opioid system in antinociceptive and anti-inflammatory effect of botulinum toxin type A in trigeminal region

BACKGROUND

Although botulinum toxin type A (BT-A) is approved for chronic migraine treatment, its site and mechanism of action are still elusive. Recently our group discovered that suppression of CGRP release from dural nerve endings might account for antimigraine action of pericranially injected BT-A. We demonstrated that central antinociceptive effect of BT-A in sciatic region involves endogenous opioid system as well. Here we investigated possible interaction of BT-A with endogenous opioid system within the trigeminal region.

METHODS

In orofacial formalin test we investigated the influence of centrally acting opioid antagonist naltrexone (2 mg/kg, s.c.) versus peripherally acting methylnaltrexone (2 mg/kg, s.c.) on BT-A's (5 U/kg, s.c. into whisker pad) or morphine's (6 mg/kg, s.c.) antinociceptive effect and the effect on dural neurogenic inflammation (DNI). DNI was assessed by Evans blue-plasma protein extravasation.

RESULTS

Naltrexone abolished the effect of BT-A on pain and dural plasma protein extravasation, whereas peripherally acting methylnaltrexone did not change either BT-A's effect on pain or its effect on dural extravasation. Naltrexone abolished the antinociceptive and anti-inflammatory effects of morphine, as well. However, methylnaltrexone decreased the antinociceptive effect of morphine only partially in the second phase of the test and had no significant effect on morphine-mediated reduction in DNI.

CONCLUSIONS

Morphine acts on pain in trigeminal region both peripherally and centrally, whereas the effect on dural plasma protein extravasation seems to be only centrally mediated. However, the interaction of BT-A with endogenous opioid system, with consequent inhibition of nociceptive transmission as well as the DNI, occurs primarily centrally.

SIGNIFICANCE

Botulinum toxin type A (BT-A)'s axonal transport and potential transcytosis suggest that its antinociceptive effect might involve diverse neurotransmitters at different sites of trigeminal system. Here we discovered that the reduction in pain and accompanying DNI involves the interaction of BT-A with central endogenous opioid system (probably at the level of trigeminal nucleus caudalis).

Emerging therapies for patients with symptoms of opioid-induced bowel dysfunction

Opioid-induced bowel dysfunction (OIBD) comprises gastrointestinal (GI) symptoms, including dry mouth, nausea, vomiting, gastric stasis, bloating, abdominal pain, and opioid-induced constipation, which significantly impair patients’ quality of life and may lead to undertreatment of pain. Traditional laxatives are often prescribed for OIBD symptoms, although they display limited efficacy and exert adverse effects. Other strategies include prokinetics and change of opioids or their administration route. However, these approaches do not address underlying causes of OIBD associated with opioid effects on mostly peripheral opioid receptors located in the GI tract. Targeted management of OIBD comprises purely peripherally acting opioid receptor antagonists and a combination of opioid receptor agonist and antagonist. Methylnaltrexone induces laxation in 50%–60% of patients with advanced diseases and OIBD who do not respond to traditional oral laxatives without inducing opioid withdrawal symptoms with similar response (45%–50%) after an oral administration of naloxegol. A combination of prolonged-release oxycodone with prolonged-release naloxone (OXN) in one tablet (a ratio of 2:1) provides analgesia with limited negative effect on the bowel function, as oxycodone displays high oral bioavailability and naloxone demonstrates local antagonist effect on opioid receptors in the GI tract and is totally inactivated in the liver. OXN in daily doses of up to 80 mg/40 mg provides equally effective analgesia with improved bowel function compared to oxycodone administered alone in patients with chronic non-malignant and cancer-related pain. OIBD is a common complication of long-term opioid therapy and may lead to quality of life deterioration and undertreatment of pain. Thus, a complex assessment and management that addresses underlying causes and patomechanisms of OIBD is recommended. Newer strategies comprise methylnaltrexone or OXN administration in the management of OIBD, and OXN may be also considered as a preventive measure of OIBD development in patients who require opioid administration.

Natural product and natural product derived drugs in clinical trials

The 25 Natural Product (NP)-derived drugs launched since 2008 and the 100 NP-derived compounds and 33 Antibody Drug Conjugates (ADCs) in clinical trials or in registration at the end of 2013 are reviewed.

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.

Methylnaltrexone for the treatment of opioid-induced constipation

Opioids are the drugs of choice for treating moderate-to-severe pain, especially for patients in the end stage of cancer or other advanced illnesses, and also in critical care or for the treatment of chronic pain. Side effects such as nausea, pruritus, dizziness and constipation have to be controlled in order to use these drugs to their full potential. Opioid-induced bowel syndrome and constipation caused by activation of μ-receptors in the gut can have such distressing effects that some patients prefer to forego adequate pain control. Methylnaltrexone is a μ-opioid receptor antagonist that, unlike naltrexone or naloxone, does not pass the blood-brain barrier, and therefore does not impair the centrally mediated analgesic effect of opioids. It is licensed for the treatment of opioid-induced constipation in palliative care in more than 50 countries. This article presents practically relevant pharmacological data, basic research results and evidence from clinical research about methylnaltrexone, and outlines potential future therapeutic options for this promising drug.

Management of opioid-induced constipation in cancer patients: focus on methylnaltrexone

Opioids are one of the most widely used therapies for the palliative treatment of cancer pain; however, despite their proven analgesic efficacy, they are associated with several adverse effects. Associated with psychological distress and multiple concomitant clinical concerns, constipation is the most commonly occurring adverse effect of chronic opioid therapy in cancer patients. Whilst prophylaxis remains the first-line management option, methylnaltrexone is a recommended treatment option for opioid-related constipation if administration of laxatives is ineffective. Due to its inability to cross the blood-brain barrier, methylnaltrexone exerts a peripheral inhibition of opioid-related effects without influencing the opioid-induced central effects; as a result, the analgesic effect of opioids is unaffected. Moreover, multiple clinical trials, albeit not always conducted specifically in cancer patients, have demonstrated that up to 4 months' treatment with either intravenous or subcutaneous methylnaltrexone provides effective relief from opioid-related constipation and is well tolerated. Preliminary evidence indicates that the addition of methylnaltrexone to standard care for opioid-related constipation may also be advantageous from a pharmacoeconomic perspective. In addition, preliminary data suggest that methylnaltrexone could be associated with some further clinical benefits other than the treatment of opioid-related constipation, such as the improvement of gastric emptying, the relief of nausea/vomiting, and the reduction of the risk of regurgitation and pulmonary aspiration. This narrative review examines the most recent evidence and evaluates the current role of methylnaltrexone in the management of opioid-related constipation, and its potential efficacy in cancer patients. The pharmacokinetics, pharmacodynamics, efficacy and tolerability of methylnaltrexone are discussed.

Opioid-induced bowel dysfunction

OPINION STATEMENT

Opioid analgesics are commonly prescribed for moderate to severe pain. Opioids exert effects via receptors in the central and enteric nervous systems. Thus, central opioid analgesia can be limited by side effects involving the gastrointestinal tract, particularly by gastrointestinal motility delay. Opioid-induced bowel dysfunction is commonly treated with bulking agents, stimulant laxatives, lubiprostone, and tegaserod (removed from the market in March 2007). However, these treatments' efficacy in opioid bowel dysfunction has not been proven. Recent research has focused on developing peripheral μ opioid antagonists such as methylnatrexone and alvimopan. These drugs selectively block μ opioid receptors in the enteric nervous system without penetrating the blood-brain barrier and can avert adverse gastrointestinal symptoms of opioids without reducing central analgesia. Methylnaltrexone and alvimopan also reduce hospitalization duration in surgical patients with postoperative ileus. A second line of research has focused on peripheral κ opioid agonists that modulate nociception in the enteric nervous system without producing central nervous system side effects. Asimadoline and fedotozine reduce nociceptive reflexes caused by gut distention and improve pain symptoms in patients with irritable bowel syndrome. ADL 10-0101 (Adolor Corp., Exton, PA) is another peripheral κ opioid agonist that lowers pain scores in patients with chronic pancreatitis. Although peripheral κ opioid agonists are promising, clinical studies are needed to assess their efficacy in treating opioid-induced bowel dysfunction.

The Bowel Function Index for evaluating constipation in pain patients: definition of a reference range for a non-constipated population of pain patients

Opioid-induced constipation (OIC) is a severe, persisting side-effect of opioid therapy. The Bowel Function Index (BFI(a), numerical analogue scale 0 - 100), calculated as the mean of three variables (ease of defaecation, feeling of incomplete bowel evacuation, and personal judgement of constipation) was developed to evaluate bowel function in opioid-treated patients with pain. This clinician-administered tool allows easy measurement of OIC from the patient's perspective. The purpose of this investigation was to define a reference range reflecting BFI values in non-constipated chronic pain patients who were recruited into a cross-sectional survey and asked for their perceptions of constipation. The BFI scores were assessed and compared with those of patients with confirmed OIC obtained from two previously published trials. Results were analysed and a reference range of BFI values of 0 - 28.8, into which 95% of non-constipated chronic pain patients fell, was defined. This permits discrimination between chronic pain patients with, or without, constipation.

Can the intestinal dysmotility of critical illness be differentiated from postoperative ileus?

Gastrointestinal dysmotility is commonly noted in the intensive care unit and postoperative settings. Characterized by delayed passage of stool and flatus, nausea, vomiting, and abdominal distention, the condition is associated with nutritional deficiencies, risk of aspiration, and considerable allocation of health care resources. Knowledge of gastrointestinal function in health and illness continues to expand. While the factors that precipitate ileus differ between postoperative and critically ill patients, the two clinical scenarios seem to have similar mechanisms and share many of the same pathophysiologic patterns. By reviewing and comparing the literature on the respective mechanisms and contributing factors generated in these separate clinical settings, a common more comprehensive management strategy may be derived with the potential for newer innovative therapeutic options.

Neural correlates of an injury-free model of central sensitization induced by opioid withdrawal in humans

Preclinical evidence suggests that opioid withdrawal induces central sensitization (CS) that is maintained by supraspinal contributions from the descending pain modulatory system (DPMS). Here, in healthy human subjects we use functional magnetic resonance imaging to study the supraspinal activity during the withdrawal period of the opioid remifentanil. We used a crossover design and thermal stimuli on uninjured skin to demonstrate opioid withdrawal-induced hyperalgesia (OIH) without a CS-inducing peripheral stimulus. Saline was used in the control arm to account for effects of time. OIH in this injury-free model was observed in a subset of the healthy subjects (responders). Only in these subjects did opioid infusion and withdrawal induce a rise in activity in the mesencephalic-pontine reticular formation (MPRF), an area of the DPMS that has been previously shown to be involved in states of CS in humans, which became significant during the withdrawal phase compared with nonresponders. Paradoxically, this opioid withdrawal-induced rise in MPRF activity shows a significant negative correlation with the behavioral OIH score indicating a predominant inhibitory role of the MPRF in the responders. These data illustrate that in susceptible individuals central mechanisms appear to regulate the expression of OIH in humans in the absence of tissue injury, which might have relevance for functional pain syndromes where a peripheral origin for the pain is difficult to identify.

Review article: palliative care in gynecologic oncology

Patients with advanced gynecologic malignancies have a multitude of symptoms; pain, nausea, and vomiting, constipation, anorexia, diarrhea, dyspnea, as well as symptoms resulting from intestinal obstruction, hypercalcemia, ascites, and/or ureteral obstruction. Pain is best addressed through a multimodal approach. The optimum palliative management of end-stage malignant intestinal obstruction remains controversial, with no clear guidelines governing the choice of surgical versus medical management. Patient selection for palliative surgery, therefore, should be highly individualized because only carefully selected candidates may derive real benefit from such surgeries. There remains a real need for more emphasis on palliative care education in training programs.

Opioid antagonists for prevention and treatment of opioid-induced gastrointestinal effects

PURPOSE OF REVIEW

The therapeutic action of opioid analgesics is compromised by peripheral adverse effects, among which constipation is the most disabling as laxatives often fail to provide satisfactory relief. This review highlights recent advances in the specific control of opioid-induced constipation by opioid receptor antagonists with limited systemic bioavailability or a peripherally restricted site of action.

RECENT FINDINGS

The specific management of opioid-induced bowel dysfunction is currently based on three drug entities: oral alvimopan for the shortening of postoperative ileus associated with opioid-induced pain control after bowel resection, subcutaneous methylnaltrexone for the reduction of opioid-induced constipation in patients with advanced illness, and a fixed combination of oral prolonged-release naloxone with prolonged-release oxycodone for the treatment of noncancer and cancer pain. All three drug entities have been shown to attenuate opioid-induced motor stasis in the gut with a favorable adverse effect profile, while the analgesic effect of opioids remains unabated.

SUMMARY

The availability of opioid receptor antagonists with restricted access to the central nervous system provides a novel opportunity to specifically control opioid-induced constipation and other peripheral adverse effects of opioid analgesics. Further studies are needed to evaluate the long-term efficacy, safety and cost-effectiveness of this approach.

Pharmacologic management of chronic constipation

Chronic constipation is a common digestive problem in North America, with significant psychosocioeconomic implications. Dietary and lifestyle measures and low-cost traditional over-the-counter laxatives are usually the first line of therapy but help only half of the patients. Several newer agents that act by increasing colonic peristalsis, altering colonic secretion, and/or antagonizing enteric opioid receptors have been developed that are effective in treating constipation and its related symptoms as well as improving quality of life. This article focuses on the pharmacology of traditional and newer agents for the treatment of constipation.

Methylnaltrexone: the evidence for its use in the management of opioid-induced constipation

Introduction:

Constipation is a distressing side effect of opioid treatment, being so irksome in some cases that patients would rather suffer the pain than the side effect of opioid analgesics. Stool softeners or stimulating laxatives are often ineffective or even aggravate the situation. A new efficacious and safe drug is needed to limit the frequently observed side effects induced by effective opioid-based analgesic therapy and to improve the quality of life for patients, most of whom are impaired by a severe disease.

Aims:

The purpose of this article is to assess current evidence supporting the use of the peripherally acting μ-opioid receptor antagonist, methylnaltrexone, to restrict passage across the blood–brain barrier in patients with opioid-induced bowel dysfunction.

Evidence review:

There are now convincing data from phase II and multicenter phase III randomized, double-blind, placebo-controlled trials that methylnaltrexone induces laxation in patients with long-term opioid use without affecting central analgesia or precipitation of opioid withdrawal. Onset of the effect is rapid and improvement is maintained for at least 3 months during the drug treatment. The action of methylnaltrexone is dose dependent. Weight-related dosing appeared to be effective. There were no severe side effects or signs of opioid withdrawal. Adverse events, most frequently abdominal cramping or nausea, were usually mild to moderate. Methylnaltrexone is contraindicated in patients with known or suspected mechanical intestinal stenosis. Patients receiving methylnaltrexone must be monitored.

Place in therapy:

Methylnaltrexone applied subcutaneously every other day may be given to patients suffering from chronic constipation due to opioid therapy for whom laxatives do not provide adequate relief of their symptoms.

Managing opioid-induced constipation in advanced illness: focus on methylnaltrexone bromide

Constipation is a common symptom in palliative care patients which can generate considerable suffering. There is uncertainty about the choice of treatment options from varying recommendations for management of constipation and a varying clinical practice in palliative care settings. The purpose of the review was to evaluate the current recommendations of therapy guidelines for the management of opioid-induced constipation in palliative care patients with a focus on methylnaltrexone bromide. Recent findings in the literature and related information on the opioid-induced gastrointestinal disorders in patients with advanced illness, as well as information on the opioid-antagonist methylnaltrexone, are discussed. Knowledge of the role of definitions, the causes of constipation and the pathophysiology of opioid-induced constipation must be given high priority in the treatment of patients receiving opioids. Diagnosis and therapy of constipation, therefore, should relate to findings in clinical investigation. Opioid-induced constipation and its adequate treatment is an important issue for patients with advanced illness and also poses therapeutic challenge for clinicians in daily routine. Methylnaltrexone bromide may represent an important therapeutic option for palliative care patients who are suffering from opioid-induced constipation with failure of conventional prophylactic oral laxative treatment.

[Methylnaltrexone. A new approach for therapy of opioid-induced obstipation]

Chronic pain patients using opioids frequently suffer from constipation which compromises well-being. Such an opioid-induced gastro-intestinal complication can occur regularly in patients in palliative care as well as in analgesic sedated intensive care patients or during prolonged perioperative pain therapy. Discomfort and distress in the affected patients can be so severely pronounced that they would rather suffer from the pain than from the side effect of constipation. Conventional therapy can be insufficient in providing satisfactory relief of constipation, mostly because this opioid-induced bowel hypomotility can be laxative-resistant. Moreover, constipation does not decrease during the course of therapy as do other side effects. It is well known that opioid-induced constipation is mediated via activation of micro-opioid receptors in the gastrointestinal tract. Selective peripheral micro-receptor antagonists (such as methylnaltrexone, Relistor) can effectively treat opioid-induced constipation. An interference with central analgesia does not occur as the molecules cannot pass the blood-brain barrier due to their charged states. A reduction of opioid therapy or the development of withdrawal symptoms can be avoided. Studies have shown that methylnaltrexone is not only safe and efficient for chronically constipated palliative care patients but offers promising therapeutic options for further patient collectives.

Protease inhibitor-induced nausea and vomiting is attenuated by a peripherally acting, opioid-receptor antagonist in a rat model

BACKGROUND

Protease inhibitors such as ritonavir can cause nausea and vomiting which is the most common reason for discontinuation. Rats react to nauseous and emetic stimuli by increasing their oral intake of non-nutritive substances like kaolin, known as pica behavior. In this study, we evaluated the effects of methylnaltrexone, a peripherally acting mu-opioid receptor antagonist that does not affect analgesia, on ritonavir-induced nausea and vomiting in a rat pica model.

RESULTS

We observed that 24 to 48 hr after administration of oral ritonavir 20 mg/kg, kaolin consumption increased significantly in rats (P < 0.01). This increase was attenuated by pretreatment with an intraperitoneal injection of methylnaltrexone (0.3-3.0 mg/kg) in a dose dependent manner (P < 0.01) and also with naloxone (0.1-0.3 mg/kg) (P < 0.01). The areas under the curve for kaolin intake from time 0 to 120 hr were significantly reduced after administration of the opioid antagonists. Food intake was not significantly affected. Plasma naltrexone levels were measured after methylnaltrexone injection, and no detectable levels were found, indicating that methylnaltrexone was not demethylated in our experimental paradigm.

CONCLUSION

These results suggest that methylnaltrexone may have potential clinical utility in reducing nausea and vomiting in HIV patients who take ritonavir.

Peripherally acting mu-opioid receptor antagonists and postoperative ileus: mechanisms of action and clinical applicability

Postoperative ileus (POI), a transient cessation of coordinated bowel function after surgery, is an important health care problem. The etiology of POI is multifactorial and related to both the surgical and anesthetic pathways chosen. Opioids used to manage surgical pain can exacerbate POI, delaying gastrointestinal (GI) recovery. Peripherally acting mu-opioid receptor (PAM-OR) antagonists are designed to mitigate the deleterious effects of opioids on GI motility. This new class is investigational for POI management with the goal of accelerating the recovery of upper and lower GI tract function after bowel resection. In this review, we summarize the mechanisms by which POI occurs and the role of opioids and opioid receptors in the enteric nervous system, discuss the mechanism of action of PAM-OR antagonists, and review clinical pharmacology and Phase II/III POI trial results of methylnaltrexone and alvimopan. Finally, the role of anesthesiologists in managing POI in the context of a multimodal approach is discussed.

Novel opioid antagonists for opioid-induced bowel dysfunction and postoperative ileus

Peripherally acting mu-opioid receptor antagonists methylnaltrexone and alvimopan are a new class of drugs designed to reverse opioid-induced side-effects on the gastrointestinal system without compromising pain relief. This article gives an overview of the pharmacology, the efficacy, and adverse effects of these drugs. Both compounds seem to be generally well tolerated and effective for the treatment of opioid-related bowel dysfunction and postoperative ileus. Methylnaltrexone recently received approval by the US Food and Drug Administration (FDA) and the European Medicines Agency for treatment of opioid-related bowel dysfunction in patients with advanced illness. Alvimopan was recently approved by the FDA for treatment of postoperative ileus, but the use of the drug is restricted to inpatients because it has been associated with an increased rate of myocardial infarction. Further research should assess the effectiveness and safety of these drugs in clinical practice.

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.

Syntheses of novel high affinity ligands for opioid receptors

A series of novel high affinity opioid receptor ligands have been made whereby the phenolic-OH group of nalbuphine, naltrexone methiodide, 6-desoxonaltrexone, hydromorphone and naltrindole was replaced by a carboxamido group and the furan ring was opened to the corresponding 4-OH derivatives. These furan ring 'open' derivatives display very high affinity for mu and kappa receptors and much less affinity for delta. The observation that these target compounds have much higher receptor affinity than the corresponding ring 'closed' carboxamides significantly strengthens our underlying pharmacophore hypothesis concerning the bioactive conformation of the carboxamide group.

The effect of naloxone-3-glucuronide on colonic transit time in healthy men after acute morphine administration: a placebo-controlled double-blinded crossover preclinical volunteer study

BACKGROUND

Constipation is a significant side effect of opioid therapy. We have previously demonstrated that naloxone-3-glucuronide (NX3G) antagonizes the motility-lowering-effect of morphine in the rat colon.

AIM

To find out whether oral NX3G is able to reduce the morphine-induced delay in colonic transit time (CTT) without being absorbed and influencing the analgesic effect.

METHODS

Fifteen male volunteers were included. Pharmacokinetics: after oral administration of 0.16 mg/kg NX3G, blood samples were collected over a 6-h period. Pharmacodynamics: NX3G or placebo was then given at the start time and every 4 h thereafter. Morphine (0.05 mg/kg) or placebo was injected s.c. 2 h after starting and thereafter every 6 h for 24 h. CTT was measured over a 48-h period by scintigraphy. Pressure pain threshold tests were performed.

RESULTS

Neither NX3G nor naloxone was detected in the venous blood. The slowest transit time was observed during the morphine phase, which was significantly different from morphine with NX3G and placebo. The pain perception was not significantly influenced by NX3G.

CONCLUSIONS

Orally administered NX3G is able to reverse the morphine-induced delay of CTT in humans without being detected in peripheral blood samples. Therefore, NX3G may improve symptoms of constipation in-patients using opioid medication without affecting opioid-analgesic effects.

Peripherally acting mu-opioid receptor agonist attenuates neuropathic pain in rats after L5 spinal nerve injury

Studies in experimental models and controlled patient trials indicate that opioids are effective in managing neuropathic pain. However, side effects secondary to their central nervous system actions present barriers to their clinical use. Therefore, we examined whether activation of the peripheral mu-opioid receptors (MORs) could effectively alleviate neuropathic pain in rats after L5 spinal nerve ligation (SNL). Systemic loperamide hydrochloride (0.3-10 mg/kg, s.c.), a peripherally acting MOR-preferring agonist, dose-dependently reversed the mechanical allodynia at day 7 post-SNL. This anti-allodynic effect produced by systemic loperamide (1.5mg/kg, s.c.) was blocked by systemic pretreatment with either naloxone hydrochloride (10 mg/kg, i.p.) or methyl-naltrexone (5 mg/kg, i.p.), a peripherally acting MOR-preferring antagonist. It was also blocked by ipsilateral intraplantar pretreatment with methyl-naltrexone (43.5 microg/50 microl) and the highly selective MOR antagonist CTAP (5.5 microg/50 microl). However, this anti-allodynic effect of systemic loperamide was not blocked by intraplantar pretreatment with the delta-opioid receptor antagonist naltrindole hydrochloride (45.1 microg/50 microl). The anti-allodynic potency of systemic loperamide varied with time after nerve injury, with similar potency at days 7, 28, and 42 post-SNL, but reduced potency at day 14 post-SNL. Ipsilateral intraplantar injection of loperamide also dose-dependently (10-100 microg/50 microl) reversed mechanical allodynia on day 7 post-SNL. We suggest that loperamide can effectively attenuate neuropathic pain, primarily through activation of peripheral MORs in local tissue. Therefore, peripherally acting MOR agonists may represent a promising therapeutic approach for alleviating neuropathic pain.

Standardized concept for the treatment of gastrointestinal dysmotility in critically ill patients--current status and future options

Inhibition of gastrointestinal motility is a major problem in critically ill patients. Motor stasis gives rise to subsequent complications including intolerance to enteral feeding, enhanced permeability of the atrophic intestinal mucosa and conditions as severe as systemic inflammatory response syndrome, sepsis and multiple organ failure. Although the diagnosis of motility disturbances in critically ill patients is difficult, the type and site of the disturbance are important to consider in the analysis of the condition and in the choice of therapeutic approach. The pharmacological treatment of impaired gastrointestinal motility is difficult to handle for the clinician, because the underlying mechanisms are complex and not fully understood and the availability of pharmacological treatment options is limited. In addition, there is a lack of controlled studies on which to build an evidence-based treatment concept for critically ill patients. Notwithstanding this situation, there has been remarkable progress in the understanding of the integrated regulation of gastrointestinal motility in health and disease. These advances, which largely relate to the organization of the enteric nervous system and its signaling mechanisms, enable the intensivist to develop a standardized concept for the use of prokinetic agents in the treatment of impaired gastrointestinal motility in critically ill patients.

Methylnaltrexone mechanisms of action and effects on opioid bowel dysfunction and other opioid adverse effects

OBJECTIVE

To review the mechanisms of action of methylnaltrexone and its effects on opioid bowel dysfunction, as well as its effects on other opioid-induced adverse effects (ADEs), and its potential roles in clinical practice.

DATA SOURCES

A literature search using the MEDLINE and Cochrane Collaboration databases for articles published between 1966 and March 2007 was performed. Additional data sources were obtained from manual searches of recent journal articles, book chapters, and monographs. An updated literature search showed no additional publications.

STUDY SELECTION AND DATA EXTRACTION

Abstracts and original preclinical and clinical research reports published in the English language were identified for review. Review articles, commentaries, and news reports of this compound were excluded. Literature related to opioids, opioid receptors, opioid antagonists, methylnaltrexone, opioid-induced bowel dysfunction, constipation, nausea, and vomiting was evaluated and selected based on consideration of the support shown for the proof of concept, mechanistic findings, and timeliness. Fifty-eight original articles from preclinical studies and clinical trials using methylnaltrexone were identified. Pharmacologic action, benefits, and ADEs of methylnaltrexone were reviewed, with a focus on its effects on bowel dysfunction after opioids. Emphases were placed on its receptor binding activities and therapeutically relevant sites of action (peripheral vs central), in which peripheral opioid receptors in the body contribute to physiological and drug-induced effects.

DATA SYNTHESIS

Morphine and related opioids are associated with a number of limiting ADEs, including opioid-induced bowel dysfunction. Methylnaltrexone, a quaternary derivative of naltrexone, blocks peripheral effects of opioids while sparing central analgesic effects. It is currently under late-stage clinical investigation for the treatment of opioid-induced constipation in patients with advanced illness. Reported results showed the drug to be generally well-tolerated. The rapid reversal of constipation is very encouraging. Hastening postoperative discharge may also be possible.

CONCLUSIONS

Methylnaltrexone has the potential to prevent or treat opioid-induced peripherally mediated ADEs on bowel dysfunction without interfering with central analgesia. The study of methylnaltrexone leads to a greater understanding of the mechanisms of action of opioid pharmacology.

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.

Attenuation of vascular permeability by methylnaltrexone: role of mOP-R and S1P3 transactivation

Endothelial cell (EC) barrier dysfunction (i.e., increased vascular permeability) is observed in inflammatory states, tumor angiogenesis, atherosclerosis, and both sepsis and acute lung injury. Therefore, agents that preserve vascular integrity have important clinical therapeutic implications. We examined the effects of methylnaltrexone (MNTX), a mu opioid receptor (mOP-R) antagonist, on human pulmonary EC barrier disruption produced by edemagenic agents including morphine, the endogenous mOP-R agonist DAMGO, thrombin, and LPS. Pretreatment of EC with MNTX (0.1 muM, 1 h) or the uncharged mOP-R antagonist naloxone attenuated morphine- and DAMGO-induced barrier disruption in vitro. However, MNTX, but not naloxone, pretreatment of EC inhibited thrombin- and LPS-induced barrier disruption, indicating potential mOP-R-independent effects of MNTX. In addition, intravenously delivered MNTX attenuated LPS-induced vascular hyperpermeability in the murine lung. We next examined the mechanistic basis for this MNTX barrier protection and observed that silencing of mOP-R attenuated the morphine- and DAMGO-induced EC barrier disruption, but not the permeability response to either thrombin or LPS. Because activation of the sphingosine 1-phosphate receptor, S1P(3), is key to a number of barrier-disruptive responses, we examined the role of this receptor in the permeability response to mOP-R ligation. Morphine, DAMGO, thrombin, and LPS induced RhoA/ROCK-mediated threonine phosphorylation of S1P(3), which was blocked by MNTX, suggesting S1P(3) transactivation. In addition, silencing of S1P(3) receptor expression (siRNA) abolished the permeability response to each edemagenic agonist. These results indicate that MNTX provides barrier protection against edemagenic agonists via inhibition of S1P(3) receptor activation and represents a potentially useful therapeutic agent for syndromes of increased vascular permeability.