Alvimopan: an oral, peripherally acting, mu-opioid receptor antagonist for the treatment of opioid-induced bowel dysfunction--a 21-day treatment-randomized clinical trial

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.

The effects of methylnaltrexone alone and in combination with acutely administered codeine on gastrointestinal and colonic transit in health

BACKGROUND

The short-term effects of methylnaltrexone (MNTX), a peripherally acting mu-opioid receptor antagonist, on gastrointestinal and colonic transit remain unclear.

AIM

To compare the effects of placebo, codeine, subcutaneous (s.c.) MNTX and codeine with s.c. MNTX on gastrointestinal and colonic transit of solids in healthy humans.

METHODS

In a randomized, parallel-group, double-blind, placebo-controlled trial of 48 healthy volunteers, effects of 6 consecutive days of placebo [s.c. and p.o. (orally), n = 8], codeine (p.o. 30 mg q.d.s., n = 8), MNTX (s.c. 0.30 mg/kg, n = 16) and combined MNTX and codeine (same doses and routes, n = 16) on gastrointestinal and colonic transit were assessed. A validated scintigraphic method was used to measure transit during the last 48 h of treatment. Bowel function was estimated during treatment as well as 1 week preceding treatment using standard diaries. Analysis of covariance was used to assess treatment effects.

RESULTS

Codeine delayed colonic transit [geometric centre at 24 h (P = 0.04) and ascending colon t(1/2) (P = 0.02)] and reduced stool frequency (P = 0.002), but had no effect on stool form. MNTX did not affect transit, stool frequency or stool form, either alone or with codeine (P > 0.3). No drug interaction effects were detected (P > 0.15).

CONCLUSION

Methylnaltrexone does not alter gastrointestinal or colonic transit and does not reverse acute codeine-associated delayed gut transit in health.

The role of opioid receptor antagonists in the treatment of opioid-induced constipation: a review

Opioid-induced constipation (OIC) is associated with negative impact of opioid analgesics on opioid receptors located in the gut wall. Until recently, OIC was treated symptomatically only, with different laxatives which did not target the pathophysiology of OIC. Recently, several opioid receptor antagonists have been introduced in the treatment of OIC. Methylnaltrexone (MNTX) is a peripheral mu-opioid receptor antagonist for subcutaneous administration, which does not evoke symptoms of opioid abstinence. MNTX is indicated for patients with OIC who are not amenable to therapy with oral laxatives. In clinical trials, the effectiveness of MNTX assessed as its ability to induce spontaneous bowel movement, is 50%-60% of treated patients; MNTX demonstrates significant superiority over placebo. Another product is combination of oral formulation of prolonged release oxycodone and prolonged release naloxone (PR oxycodone/PR naloxone), indicated for patients who require opioid administration for chronic pain and have already developed OIC, and for those who need opioid therapy and take the drug to prevent OIC. Naloxone administered orally displays local, antagonist effects on opioid receptors in the gut wall, negligible systemic bioavailability, and significantly reduces the oxycodone constipating effect. PR oxycodone/PR naloxone has similar analgesic efficacy, but causes less constipation and less laxative consumption in comparison with patients treated with oxycodone alone. Both products are expensive, therefore their administration should be carefully considered. On the other hand, uncontrolled OIC and the necessity to perform rectal invasive procedures (enema, manual evacuation) lead not only to increased health care costs, but most importantly, cause severe patient suffering.

Update on the management of constipation in the elderly: new treatment options

Constipation disproportionately affects older adults, with a prevalences of 50% in community-dwelling elderly and 74% in nursing-home residents. Loss of mobility, medications, underlying diseases, impaired anorectal sensation, and ignoring calls to defecate are as important as dyssynergic defecation or irritable bowel syndrome in causing constipation. Detailed medical history on medications and co-morbid problems, and meticulous digital rectal examination may help identify causes of constipation. Likewise, blood tests and colonoscopy may identify organic causes such as colon cancer. Physiological tests such as colonic transit study with radio-opaque markers or wireless motility capsule, anorectal manometry, and balloon expulsion tests can identify disorders of colonic and anorectal function. However, in the elderly, there is usually more than one mechanism, requiring an individualized but multifactorial treatment approach. The management of constipation continues to evolve. Although osmotic laxatives such as polyethylene glycol remain mainstay, several new agents that target different mechanisms appear promising such as chloride-channel activator (lubiprostone), guanylate cyclase agonist (linaclotide), 5HT₄ agonist (prucalopride), and peripherally acting μ-opioid receptor antagonists (alvimopan and methylnaltrexone) for opioid-induced constipation. Biofeedback therapy is efficacious for treating dyssynergic defecation and fecal impaction with soiling. However, data on efficacy and safety of drugs in elderly are limited and urgently needed.

Critical illness, gastrointestinal complications, and medication therapy during enteral feeding in critically ill adult patients

Critically ill patients who are subjected to high stress or with severe injury can rapidly break down their body protein and energy stores. Unless adequate nutrition is provided, malnutrition and protein wasting may occur, which can negatively affect patient outcome. Enteral nutrition (EN) is the mainstay of nutrition support therapy in patients with a functional gastrointestinal (GI) tract who cannot take adequate oral nutrition. EN in critically ill patients provides the benefits of maintaining gut functionality, integrity, and immunity as well as decreasing infectious complications. However, the ability to provide timely and adequate EN to critically ill patients is often hindered by GI motility disorders and complications associated with EN. This paper reviews the GI complications and intolerances associated with EN in critically ill patients and provides recommendations for their prevention and treatment. It also addresses the role of commonly used medications in the intensive care unit and their impact on GI motility and EN delivery.

Pharmacological treatment of chronic pain - the need for CHANGE

BACKGROUND

Although chronic pain affects around 20% of adults in Europe and the USA, there is substantial evidence that it is inadequately treated. In June 2009, an international group of pain specialists met in Brussels to identify the reasons for this and to achieve consensus on strategies for improving pain management.

SCOPE

Literature on chronic pain management was reviewed, and information presented to and discussed by a panel of experts.

FINDINGS

It was agreed that guidelines are not universally accepted by those involved in pain management, and pain treatment seems to be driven mainly by tradition and personal experience. Other factors include poor communication between patients and physicians, the side effects of analgesic drugs, and limited individualisation of therapy. Difficulty in maintaining the balance between adequate pain relief and acceptable tolerability, particularly with strong opioids, can lead to the establishment of a 'vicious circle' that alternates between lack of efficacy and unpleasant side effects, prompting discontinuation of treatment. The medical community's understanding of the physiological differences between nociceptive pain and neuropathic pain, which is often more severe and difficult to treat, could be improved. Increasing physicians' knowledge of the pharmacological options available to manage these different pain mechanisms offers the promise of better treatment decisions and more widespread adoption of a multi-mechanistic approach; this could involve loosely combining two substances from different drug classes, or administering an analgesic with two different mechanisms of action. In some circumstances, a single compound capable of addressing both nociceptive and neuropathic pain is desirable.

CONCLUSIONS

To improve patient outcomes, a thorough understanding of pain mechanisms, sensitisation and multi-mechanistic management is required. Universal, user-friendly educational tools are therefore required to familiarise physicians with these topics, and also to improve communication between physicians and their pain patients, so that realistic expectations of treatment can be established.

Methylnaltrexone: a novel approach for the management of opioid-induced constipation in patients with advanced illness

In April 2008, the US FDA granted approval to methylnaltrexone (Relistor), the first peripheral micro-opioid-receptor antagonist for the treatment of opioid-induced constipation in advanced-illness patients receiving palliative care and for whom other laxative therapies failed to achieve adequate results. Methylnaltrexone, a quaternary derivative of naltrexone, introduces a novel mechanism of action that selectively antagonizes the peripheral micro-receptors in the GI tract without effects on the CNS. In clinical trials, subcutaneous methylnaltrexone reversed opioid-induced constipation after the first dose in approximately 50-60% of the patients. In most of the cases, effective laxation occurred within 1 h. The therapeutic benefit was sustained in multiple-dose studies. Owing to the nature of the population studied, safety data are available for approximately 4 months of use. Although it is not the focus of this article, methylnaltrexone's mechanism of action suggests it could be beneficial for other peripheral, opioid-induced adverse effects, such as opioid-related nausea, vomiting, urinary retention, pruritus or postoperative ileus.

Recent advances in the use of opioids for cancer pain

Opioids are the mainstay of treatment for moderate to severe cancer pain. In recent years there have been many advances in the use of opioids for cancer pain. Availability and consumption of opioids have increased and opioids other than morphine (including methadone, fentanyl, oxycodone) have become more widely used. Inter-individual variation in response to opioids has been identified as a significant challenge in the management of cancer pain. Many studies have been published demonstrating the benefits of opioid switching as a clinical maneuver to improve tolerability. Constipation has been recognized as a significant burden in cancer patients on opioids. Peripherally restricted opioid antagonists have been developed for the prevention and management of opioid induced constipation. The phenomenon of breakthrough pain has been characterized and novel modes of opioid administration (transmucosal, intranasal, sublingual) have been explored to facilitate improved management of breakthrough cancer pain. Advances have also been made in the realm of molecular biology. Pharmacogenetic studies have explored associations between clinical response to opioids and genetic variation at a DNA level. To date these studies have been small but future research may facilitate prospective prediction of response to individual drugs.

[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.

Efficacy and safety of mu-opioid antagonists in the treatment of opioid-induced bowel dysfunction: systematic review and meta-analysis of randomized controlled trials

CONTEXT

Opioid-induced bowel dysfunction (OBD) is characterized by constipation, incomplete evacuation, bloating, and increased gastric reflux. OBD occurs both acutely and chronically, in multiple disease states, resulting in increased morbidity and reduced quality of life.

OBJECTIVE

To compare the efficacy and safety of traditional and peripherally active opioid antagonists vs conventional interventions for OBD.

DESIGN

We searched MEDLINE, the Cochrane Central Register of Controlled Trials and EMBASE. Additional reports were identified from the reference lists of retrieved articles.

STUDY SELECTION

Studies were included if they were randomized controlled trials that investigated the efficacy of mu-opioid antagonists for OBD.

DATA EXTRACTION

Data were extracted by two independent investigators and included demographic variables, diagnoses, interventions, efficacy, and adverse events.

RESULTS OF DATA SYNTHESIS

Twenty-two articles met inclusion criteria and provided data on 2,352 opioid antagonist-treated patients. The opioid antagonist investigated was alvimopan (eight studies), methylnaltrexone (six), naloxone (seven), and nalbuphine (one). Meta-analysis demonstrated that methylnaltrexone and alvimopan are efficacious in reversing opioid-induced increased gastrointestinal transit time and constipation, and that alvimopan is safe and efficacious in treating postoperative ileus. The incidence of adverse events with opioid antagonists was similar to placebo and generally reported as mild-to-moderate.

CONCLUSIONS

Insufficient evidence exists for the safety or efficacy of naloxone or nalbuphine in the treatment of OBD. Long-term efficacy and safety of any of the opioid antagonists is unknown, as is the incidence or nature of rare adverse events. Alvimopan and methylnaltrexone both show promise in treating OBD, but further data will be required to fully assess their place in therapy.

Clinical guidelines for the use of chronic opioid therapy in chronic noncancer pain

Use of chronic opioid therapy for chronic noncancer pain has increased substantially. The American Pain Society and the American Academy of Pain Medicine commissioned a systematic review of the evidence on chronic opioid therapy for chronic noncancer pain and convened a multidisciplinary expert panel to review the evidence and formulate recommendations. Although evidence is limited, the expert panel concluded that chronic opioid therapy can be an effective therapy for carefully selected and monitored patients with chronic noncancer pain. However, opioids are also associated with potentially serious harms, including opioid-related adverse effects and outcomes related to the abuse potential of opioids. The recommendations presented in this document provide guidance on patient selection and risk stratification; informed consent and opioid management plans; initiation and titration of chronic opioid therapy; use of methadone; monitoring of patients on chronic opioid therapy; dose escalations, high-dose opioid therapy, opioid rotation, and indications for discontinuation of therapy; prevention and management of opioid-related adverse effects; driving and work safety; identifying a medical home and when to obtain consultation; management of breakthrough pain; chronic opioid therapy in pregnancy; and opioid-related policies.

PERSPECTIVE

Safe and effective chronic opioid therapy for chronic noncancer pain requires clinical skills and knowledge in both the principles of opioid prescribing and on the assessment and management of risks associated with opioid abuse, addiction, and diversion. Although evidence is limited in many areas related to use of opioids for chronic noncancer pain, this guideline provides recommendations developed by a multidisciplinary expert panel after a systematic review of the evidence.

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.

Validation of the Bowel Function Index to detect clinically meaningful changes in opioid-induced constipation

OBJECTIVE

The Bowel Function Index (BFI) is a clinician-administered, patient-reported, 3-item questionnaire to evaluate opioid-induced constipation in cancer and non-cancer chronic pain patients. The objective of the present analysis was to evaluate the psychometric characteristics of the BFI using data from clinical studies of oral prolonged release (PR) oxycodone/naloxone.

METHODS

OXN2401 was a multicenter, controlled, randomized, double-blind, parallel-group study including oral PR oxycodone combined with oral PR naloxone as well as oral PR oxycodone combined with corresponding naloxone placebo. OXN3401 and OXN3001 were 12-week multicenter, controlled, randomized, double-blind, parallel-group studies of a fixed combination of oral PR oxycodone/naloxone versus PR oxycodone. In addition, a placebo group was included in study OXN3401. BFI psychometric characteristics (reliability, reproducibility, convergent/known groups validity, and responsiveness) were evaluated.

RESULTS

Demographic data (n=985) were comparable and analyses indicated a high degree of internal consistency (Cronbach's alpha >0.7). Change of less than 5 points in BFI was indicative of high reproducibility. Correlations between BFI item and total scores to stool frequency were statistically significant and in the low-to-moderate range (OXN2401 -0.23 to -0.29, p < 0.001; OXN3401 range -0.26 to -0.40, p < 0.001; OXN3001 -0.14 to -0.15, p < 0.05). Data indicate that a BFI score change of ≥12 points represents a clinically meaningful change in constipation.

LIMITATIONS

This publication for validation of BFI only includes data from three clinical trials. However, another publication of an additional specifically designed cross-sectional validation study is in preparation.

CONCLUSION

The BFI is a valid and reliable instrument for the assessment of opioid-induced constipation in chronic pain patients. Psychometric analyses from clinical trials support the BFI's psychometric properties.

Prokinetic Drugs in the Intensive Care Unit: Reviewing the Evidence

Delay in gastric emptying is common in the critically ill, and can lead to abdominal distension, diarrhoea or constipation, and vomiting, and may contribute to increased incidence of reflux and nosocomial infection. Prokinetic agents increase the rate of luminal transit as well as the force of contraction, and are commonly used in the intensive care unit. This article summarises the current state of knowledge about the use of prokinetics and explores potential agents which might be used in the future in this group of patients.

Constipation: evaluation and treatment of colonic and anorectal motility disorders

This article focuses on the colonic and anorectal motility disturbances that are associated with chronic constipation and their management. Functional chronic constipation consists of three overlapping subtypes: slow transit constipation, dyssynergic defecation, and irritable bowel syndrome with constipation. The Rome criteria may serve as a useful guide for making a clinical diagnosis of functional constipation. Today, an evidence-based approach can be used to treat patients with chronic constipation. The availability of specific drugs for the treatment of chronic constipation, such as tegaserod and lubiprostone, has enhanced the therapeutic armamentarium for managing these patients. Randomized controlled trials have also established the efficacy of biofeedback therapy in the treatment of dyssynergic defecation.

A systematic review of treatments for constipation in children and young adults undergoing cancer treatment

Constipation is a common problem in children and young people with cancer. Treatment of this complication is subject to wide variation in practice. We undertook a systematic review of randomized controlled trials to build a rational approach to prophylaxis and treatment of this complication. Randomized controlled trials of any intervention (pharmacologic, physical, complementary, or alternative) to prevent or treat constipation were to be included if they included children and young people 16 years old and younger who were undergoing treatment for malignancy. Of the 1336 abstracts retrieved from the searches, only 3 papers were identified for further assessment, and no studies were suitable for inclusion. There are no good data on which to base the management of constipation in children and young people with cancer. This is not to say that we do not know if laxatives work-they are clearly effective. Our ignorance is of the comparative value of different agents. The practical problems with undertaking specific trials of supportive care measures are large, and integration of such questions into treatment trials is essential.

Alvimopan

Each month, subscribers to The Formulary Monograph Service receive 5 to 6 well-documented monographs on drugs that are newly released or are in late phase 3 trials. The monographs are targeted to Pharmacy & Therapeutics Committees. Subscribers also receive monthly 1-page summary monographs on agents that are useful for agendas and pharmacy/nursing inservices. A comprehensive target drug utilization evaluation (DUE) is also provided each month. With a subscription, the monographs are sent to you in print and are also available online. Monographs can be customized to meet the needs of your facility. Subscribers to the The Formulary Monograph Service also receive access to a pharmacy bulletin board, The Formulary Information Exchange (The F.I.X.). All topics pertinent to clinical and hospital pharmacy are discussed on The F.I.X.

Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. If you would like information about The Formulary Monograph Service or The FIX., call The Formulary at 800-322-4349. The October 2008 monograph topics are on clevidipine butyrate injectable emulsion, tetrabenazine, tenofovir disoproxil fumarate tablets, ferumoxytol, and saxagliptin. The DUE is on clevidipine butyrate injectable emulsion.

Development of peripheral opioid antagonists' new insights into opioid effects

The recent approval by the US Food and Drug Administration of 2 medications--methylnaltrexone and alvimopan--introduces a new class of therapeutic entities to clinicians. These peripherally acting mu-opioid receptor antagonists selectively reverse opioid actions mediated by receptors outside the central nervous system, while preserving centrally mediated analgesia. Methylnaltrexone, administered subcutaneously, has been approved in the United States, Europe, and Canada. In the United States, it is indicated for the treatment of opioid-induced constipation in patients with advanced illness (eg, cancer, AIDS) who are receiving palliative care, when response to laxative therapy has not been sufficient. Alvimopan, an orally administered medication, has been approved in the United States to facilitate recovery of gastrointestinal function after bowel resection and primary anastomosis. Clinical and laboratory studies performed during the development of these drugs have indicated that peripheral receptors mediate other opioid effects, including decreased gastric emptying, nausea and vomiting, pruritus, and urinary retention. Laboratory investigations with these compounds suggest that opioids affect fundamental cellular processes through mechanisms that were previously unknown. These mechanisms include modifications of human immunodeficiency virus penetration, tumor angiogenesis, vascular permeability, and bacterial virulence.

Subcutaneous methylnaltrexone for the treatment of opioid-induced constipation in patients with advanced illness: a double-blind, randomized, parallel group, dose-ranging study

Methylnaltrexone, a peripherally-acting quaternary opioid antagonist, is an investigational treatment for opioid-induced constipation in patients with advanced illness. This randomized, parallel-group, repeated dose, dose-ranging trial included a double-blind phase for one week followed by an open-label phase for a maximum of three weeks. Opioid-treated patients with advanced illness who met criteria for opioid-induced constipation despite laxative therapy were potentially eligible. Double-blind treatment occurred on Days 1, 3, and 5; open-label therapy could be administered as often as every other day. The initial dose range of 1mg, 5mg, or 12.5mg was extended by adding a 20mg group during the study while still maintaining the double blind; the initial open-label dose of 5mg could be titrated. The primary outcome was a laxation response within four hours after the first dose. Thirty-three patients received at least one dose of methylnaltrexone. Only one of 10 patients (10%) who received the 1mg dose experienced laxation within four hours of dosing. The median time to laxation was >48 hours for the 1mg dose group, compared to 1.26 hours for all patients receiving >or=5mg (P=0.0003). There was no apparent dose-response above 5mg. Most adverse events were related to the gastrointestinal system, were mild, and did not lead to discontinuation. In conclusion, methylnaltrexone relieved opioid-induced constipation at doses >or=5mg in patients with advanced illness, and did not reduce analgesia or cause opioid withdrawal symptoms.

Mu-opioid antagonists for opioid-induced bowel dysfunction

BACKGROUND

Opioid-induced bowel dysfunction (OBD) is characterized by constipation, incomplete evacuation, bloating, and increased gastric reflux. OBD occurs both acutely and chronically, in multiple disease states, resulting in increased morbidity and reduced quality of life.

OBJECTIVES

To compare the efficacy and safety of traditional and peripherally active opioid antagonists versus conventional interventions for OBD.

SEARCH STRATEGY

We searched MEDLINE, the Cochrane Central Register of Controlled Trials and EMBASE in January 2007. Additional reports were identified from the reference lists of retrieved papers.

SELECTION CRITERIA

Studies were included if they were randomized controlled trials that investigated the efficacy of mu-opioid antagonists for OBD.

DATA COLLECTION AND ANALYSIS

Data were extracted by two independent review authors and included demographic variables, diagnoses, interventions, efficacy, and adverse events.

MAIN RESULTS

Twenty-three studies met inclusion criteria and provided data on 2871 opioid antagonist-treated patients. The opioid antagonists investigated were alvimopan (nine studies), methylnaltrexone (six), naloxone (seven), and nalbuphine (one). Meta-analysis demonstrated that methylnaltrexone and alvimopan were better than placebo in reversing opioid-induced increased gastrointestinal transit time and constipation, and that alvimopan appears to be safe and efficacious in treating postoperative ileus. The incidence of adverse events with opioid antagonists was similar to placebo and generally reported as mild-to-moderate.

AUTHORS' CONCLUSIONS

Insufficient evidence exists for the safety or efficacy of naloxone or nalbuphine in the treatment of OBD. Long-term efficacy and safety of any of the opioid antagonists is unknown, as is the incidence or nature of rare adverse events. Alvimopan and methylnaltrexone both show promise in treating OBD, but further data will be required to fully assess their place in therapy.

Opioid-induced bowel dysfunction

Opioid-induced bowel dysfunction is a distressing condition that may persist indefinitely in the clinical setting. As we understand more about normal gastrointestinal (GI) physiology, we are also beginning to understand more fully how opioids cause bowel dysfunction. Current therapeutic interventions for opioid-induced bowel dysfunction can be burdensome and sometimes lack efficacy. Systemic opioid antagonists administered orally can induce laxation, but can unpredictably induce systemic or local GI tract withdrawal symptoms. Two new investigational agents, alvimopan and methylnaltrexone, are peripherally acting opioid antagonists that do not cross the blood-brain barrier. Studies to date show promise that these agents may relieve opioid-induced bowel dysfunction in a well-tolerated manner without reversing central analgesia.

Alvimopan, a peripherally acting mu-opioid receptor (PAM-OR) antagonist for the treatment of opioid-induced bowel dysfunction: results from a randomized, double-blind, placebo-controlled, dose-finding study in subjects taking opioids for chronic non-cancer pain

Our objective was to investigate the efficacy and safety of alvimopan, a peripherally acting mu-opioid receptor (PAM-OR) antagonist, in subjects with non-cancer pain and opioid-induced bowel dysfunction (OBD), and to identify at least one treatment regimen that improves OBD. Following a 2-week baseline period, 522 subjects reporting <3 spontaneous bowel movements (SBMs)/week (with >or=25% accompanied by a sensation of incomplete evacuation, straining, or lumpy hard stools), requiring analgesia equivalent to >or=30 mg oral morphine/day were randomized to alvimopan 0.5mg twice daily (BID), 1mg once daily (QD), 1mg BID, or placebo for 6 weeks. Compared with placebo, there was a statistically and clinically significant increase in mean weekly SBM frequency over the initial 3 weeks of treatment (primary endpoint) with alvimopan 0.5mg BID (+1.71 mean SBMs/week), alvimopan 1mg QD (+1.64) and alvimopan 1mg BID (+2.52); P<0.001 for all comparisons. Increased SBM frequency and additional treatment effects, including improvements in symptoms such as straining, stool consistency, incomplete evacuation, abdominal bloating/discomfort, and decreased appetite, were sustained over 6 weeks. The most frequently reported adverse events were abdominal pain, nausea, and diarrhea, occurring more frequently in the higher dosage groups. The alvimopan 0.5mg BID regimen demonstrated the best benefit-to-risk profile for managing OBD with alvimopan in this study population, with a side effect profile similar to that of placebo. There was no evidence of opioid analgesia antagonism. Competitive peripheral antagonism of opioids with alvimopan can restore GI function and relieve OBD without compromising analgesia.

Peripherally acting opioid antagonists in the treatment of opiate-related constipation: a systematic review

Many patients treated with opioids suffer from constipation. Opiate- or opioid-related constipation is not only a frequent but also a distressing symptom and difficult to treat. There is emerging evidence regarding a novel approach to the management of opiate-related constipation. The aim of this paper is to collect, critically appraise, and summarize the evidence on the effectiveness of recently developed peripherally acting micro-receptor antagonists in the treatment of opiate-related constipation. A comprehensive search of 11 computerized databases was conducted and efforts were made to identify unpublished and ongoing research. Twenty studies were identified; 13 were randomized controlled trials (RCTs) and 7 were Phase II studies assessing toxicity. Studies were mainly executed in healthy volunteers or members of methadone programs with opioid-induced constipation as a model to mimic the condition of patients on opioids. Two RCTs were conducted in hospice patients. Quality of study design and validity of the findings was assessed in all studies. Data show proof of concept but do not allow a definitive answer concerning the effectiveness of the peripherally acting micro-opioid antagonists methylnaltrexone and alvimopan in managing opiate-related constipation. Further research is needed. If future Phase III trials provide supportive data, opioid antagonists may become a standard therapeutic option for the treatment of opiate-related constipation in patients with advanced cancer.

Emerging pharmacologic options for treating postoperative ileus

PURPOSE

Characteristics of the ideal drug therapy for postoperative ileus (POI); the pharmacology, efficacy, and safety of currently available nonselective opioid antagonists and the new peripherally selective opioid antagonists methylnaltrexone and alvimopan for the treatment of POI; and formulary considerations associated with the introduction of these new POI drug therapies are discussed.

SUMMARY

The ideal drug therapy for treating POI would selectively antagonize the inhibitory effects on the gastrointestinal (GI) tract of all of the potential factors implicated in the pathophysiology of POI (neurogenic, inflammatory, hormonal, and pharmacologic mediators). The most promising target to date is inhibition of the adverse GI effects of endogenous and exogenous opioids. Selective inhibition of the mu-opioid receptors in the GI tract, without reversing centrally mediated opioid-induced analgesia, may be beneficial in reducing POI. The nonselective opioid antagonists naloxone and nalmefene have not been studied for POI, and they cross the blood-brain barrier. Therefore, they are not appropriate for preventing or treating POI. The peripherally selective opioid antagonist methylnaltrexone shortens the duration of POI and the hospital length of stay (LOS). Alvimopan, a more extensively studied peripherally selective opioid antagonist, has been shown to reduce the duration of POI, frequency of postoperative nausea and vomiting, and hospital LOS. Both methylnaltrexone and alvimopan also appear effective for treating opioid-induced constipation. Preliminary results of a long-term study of alvimopan safety have revealed some potential concerns, and the significance of the adverse effects must be understood before the most appropriate role of alvimopan in patient care can be determined. Restricting the prescribing of new POI drug therapies to certain types of patients, surgeries, and prescribers; incorporating these therapies into preoperative and postoperative policies, procedures, and protocols; and the potential cost savings from reducing hospital LOS are among the considerations in adding these agents to health-system formularies.

CONCLUSION

Peripherally selective opioid receptor antagonists are promising new drug therapies that can reduce the clinical and economic burden of POI.

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.

Constipation: evaluation and treatment of colonic and anorectal motility disorders

This article focuses on the colonic and anorectal motility disturbances that are associated with chronic constipation and their management. Functional chronic constipation consists of three overlapping subtypes: slow transit constipation, dyssynergic defecation, and irritable bowel syndrome with constipation. The Rome criteria may serve as a useful guide for making a clinical diagnosis of functional constipation. Today, an evidence-based approach can be used to treat patients with chronic constipation. The availability of specific drugs for the treatment of chronic constipation, such as tegaserod and lubiprostone, has enhanced the therapeutic armamentarium for managing these patients. Randomized controlled trials have also established the efficacy of biofeedback therapy in the treatment of dyssynergic defecation.

Mechanisms underlying feed intolerance in the critically ill: Implications for treatment

Malnutrition is associated with poor outcomes in critically ill patients. Although nutritional support is yet to be proven to improve mortality in non-malnourished critically ill patients, early enteral feeding is considered best practice. However, enteral feeding is often limited by delayed gastric emptying. The best method to clinically identify delayed gastric emptying and feed intolerance is unclear. Gastric residual volume (GRV) measured at the bedside is widely used as a surrogate marker for gastric emptying, but the value of GRV measurement has recently been disputed. While the mechanisms underlying delayed gastric emptying require further investigation, recent research has given a better appreciation of the pathophysiology. A number of pharmacological strategies are available to improve the success of feeding. Recent data suggest a combination of intravenous metoclopramide and erythromycin to be the most successful treatment, but novel drug therapies should be explored. Simpler methods to access the duodenum and more distal small bowel for feed delivery are also under investigation. This review summarises current understanding of the factors responsible for, and mechanisms underlying feed intolerance in critical illness, together with the evidence for current practices. Areas requiring further research are also highlighted.

Cancer-related constipation

Cancer-related constipation is common and a significant detractor from patient quality of life. It has many possible causes and is still not well understood. Information is lacking on therapies for cancer-related constipation among current medications approved by the US Food and Drug Administration (FDA). Most agents have only been formally tested in comparison with placebo in chronic idiopathic constipation if at all. Few comparative studies of laxatives have been performed to establish superiority or synergy. As we understand more about the physiology of the gastrointestinal tract, new targeted therapies have become available. These include a selective chloride channel activator, lubiprostone, and a selective 5HT4 serotonin receptor agonist, tegaserod, both of which have been FDA approved for chronic idiopathic constipation. The role of these agents in cancer-related constipation remains to be seen. On the horizon are two investigational peripherally acting opioid receptor antagonists, alvimopan and methylnaltrexone. Preliminary results in cancer-related constipation suggest that these agents may be important additions to our treatment repertoire.

[Novel therapeutic approaches in the treatment of irritable bowel syndrome]

The treatment of irritable bowel syndrome due to the heterogeneous clinical symptoms and coexisting psychiatric disorders is still controversial. Although several agents with different mechanisms of action are widely used in clinical practice, there are only few drugs available with strong evidence of their efficacy, safety and tolerability at present. The etiology of irritable bowel syndrome is considered to be multifactorial: experimental and clinical research on visceral hypersensitivity, motility and brain-gut axis involving its neurotransmitters and receptors created the foundation of novel therapeutic approaches. Albeit nowadays several drugs (alosetron, tegaserod) have been registered in a few countries for the treatment of irritable bowel syndrome, further large clinical trials are required related to the new chemical entities.

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.

Constipation and palliative care - where are we now?

Constipation is an unpleasant and distressing symptom that many palliative care patients may experience, often having a profound effect on their quality of life. The many management options available reflect the multifactorial nature of constipation. The article explores the complexity of constipation in palliative care and highlights the challenge of managing opioid-induced constipation. Advances in the pharmacological and non-pharmacological management of constipation are reviewed and discussed in the light of relevant research. Further discussion includes definitions, incidence and causes of constipation in palliative care.

The in vitro pharmacology of the peripherally restricted opioid receptor antagonists, alvimopan, ADL 08-0011 and methylnaltrexone

This study characterized the pharmacology of the peripherally restricted opioid receptor antagonists, alvimopan, its metabolite, ADL 08-0011, and methylnaltrexone. The activities of the compounds were investigated with respect to human or guinea pig opioid receptor binding and function in recombinant cell lines and mechanical responsiveness of the guinea pig ileum. Alvimopan and ADL 08-0011 had higher binding affinity than methylnaltrexone at human mu opioid receptors (pK (i) values of 9.6, 9.6, and 8.0, respectively). The compounds had different selectivities for the mu receptor over human delta and guinea pig kappa opioid receptors. ADL 08-0011 had the highest mu receptor selectivity. With respect to their mu opioid receptor functional activity ([(35)S]GTPgammaS incorporation), methylnaltrexone had a positive intrinsic activity, consistent with partial agonism, unlike alvimopan and ADL 08-0011, which had negative intrinsic activities. Alvimopan, ADL 08-0011, and methylnaltrexone antagonized inhibitory responses mediated by the mu opioid agonist, endomorphin-1 (pA (2) values of 9.6, 9.4, and 7.6, respectively) and by U69593, a kappa opioid agonist (pA (2) values of 8.4, 7.2, and 6.7, respectively). In morphine-naive guinea pig ileum, methylnaltrexone reduced, while alvimopan and ADL 08-0011 increased, the amplitude of electrically evoked contractions and spontaneous mechanical activity. In tissue from morphine-dependent animals, alvimopan and ADL 08-0011 increased spontaneous activity to a greater degree than methylnaltrexone. The data suggested that alvimopan-induced contractions resulted predominantly from an interaction with kappa opioid receptors. It is concluded that alvimopan, ADL 08-0011, and methylnaltrexone differ in their in vitro pharmacological properties, particularly with respect to opioid receptor subtype selectivity and intrinsic activity. The clinical significance of the data from this study remains to be determined.