Tolerability, gut effects, and pharmacokinetics of methylnaltrexone following repeated intravenous administration in humans

No Effect of Methylnaltrexone on Acute Pancreatitis Severity: A Multicenter Randomized Controlled Trial

INTRODUCTION

Opioids used to manage severe pain in acute pancreatitis (AP) might exacerbate the disease through effects on gastrointestinal and immune functions. Methylnaltrexone, a peripherally acting µ-opioid receptor antagonist, may counteract these effects without changing analgesia.

METHODS

This double-blind, randomized, placebo-controlled trial included adult patients with AP and systemic inflammatory response syndrome at 4 Danish centers. Patients were randomized to receive 5 days of continuous intravenous methylnaltrexone (0.15 mg/kg/d) or placebo added to the standard of care. The primary end point was the Pancreatitis Activity Scoring System score after 48 hours of treatment. Main secondary outcomes included pain scores, opioid use, disease severity, and mortality.

RESULTS

In total, 105 patients (54% men) were randomized to methylnaltrexone (n = 51) or placebo (n = 54). After 48 hours, the Pancreatitis Activity Scoring System score was 134.3 points in the methylnaltrexone group and 130.5 points in the placebo group (difference 3.8, 95% confidence interval [CI] -40.1 to 47.6; P = 0.87). At 48 hours, we found no differences between the groups in pain severity (0.0, 95% CI -0.8 to 0.9; P = 0.94), pain interference (-0.3, 95% CI -1.4 to 0.8; P = 0.55), and morphine equivalent doses (6.5 mg, 95% CI -2.1 to 15.2; P = 0.14). Methylnaltrexone also did not affect the risk of severe disease (8%, 95% CI -11 to 28; P = 0.38) and mortality (6%, 95% CI -1 to 12; P = 0.11). The medication was well tolerated.

DISCUSSION

Methylnaltrexone treatment did not achieve superiority over placebo for reducing the severity of AP.

Clinical Outcomes of Intravenous Methylnaltrexone in Children: A Single-Arm Retrospective Cohort Study

OBJECTIVES

Constipation is a common adverse event of opioid use that is often difficult to treat. Methylnaltrexone is a therapeutic option for opioid-induced constipation (OIC) approved for oral and subcutaneous use in adults. These administration routes are not always feasible in the pediatric population. The primary objective of this research was to quantify the response rate of methylnaltrexone in pediatric patients when it was administered via the intravenous (IV) route.

METHODS

This retrospective study evaluated patients ages <18 years who received IV methylnaltrexone between January 1, 2013, and June 30, 2020, for OIC. Efficacy was evaluated through documentation of bowel evacuation within 4 hours of methylnaltrexone administration. Adverse events observed within 24 hours of administration were attributed to methylnaltrexone.

RESULTS

Methylnaltrexone was administered to 134 unique patients during the study period. Of these, 46 met exclusion criteria, resulting in 88 patients being included in the study. Patients with an underlying hematology/oncology diagnosis consisted of 77% of the study population, and 23% of patients had an -underlying medical/surgical diagnosis. The response rate to IV methylnaltrexone was 25% (CI, 16-34).

CONCLUSIONS

The results of this retrospective chart review demonstrate the potential role of IV methylnaltrexone in the pediatric population. Despite the overall lower response rate relative to that reported in adults, IV methylnaltrexone possesses a unique mechanism of action that may serve as an alternative treatment option for patients unable to use the oral and subcutaneous administration routes. There were no significant adverse events seen in the study.

Perioperative subcutaneous methylnaltrexone does not enhance gastrointestinal recovery after posterior short-segment spinal arthrodesis surgery: a randomized controlled trial

BACKGROUND CONTEXT

Postoperative ileus is a major barrier to gastrointestinal recovery following surgery. Opioid analgesics likely play an important causative role, particularly in spinal or orthopedic surgeries not involving bowel manipulation. Methylnaltrexone, a peripherally-acting µ-opioid receptor antagonist, is a potential prophylactic treatment.

PURPOSE

To assess the influence of perioperative subcutaneous methylnaltrexone administration on gastrointestinal recovery following short-segment lumbar arthrodesis surgeries.

DESIGN

This is a randomized, double-blind, controlled trial.

PATIENT SAMPLE

Eligible patients undergoing posterior short-segment lumbar arthrodesis surgeries at a single institution between February 2019 and April 2021 were enrolled in this study.

OUTCOME MEASURES

The primary outcome measure was time-to-first bowel movement. Secondary outcome measures included time-to-discharge/discharge eligibility. Exploratory outcome measures included daily postoperative opioid consumption and pain scores.

METHODS

In this study, eligible patients were enrolled to receive either methylnaltrexone or placebo perioperatively. Time-to-bowel movement, time-to-discharge/discharge eligibility, intra and postoperative analgesic administration, and pain scores were recorded and compared.

RESULTS

Eighty two patients in total were enrolled; 41 to the methylnaltrexone and 41 to the placebo group. Both groups were similar in their baseline characteristics. There was no difference in median (range) time-to-bowel movement between the 2 groups [61.8 hours (35.7-93.6) versus 50.7 hours (17.8-110.8), p = .391]. There was also no difference in time-to-discharge/discharge eligibility [105.0 hours (81.0 - 201.3) versus 90.7 (77.5 - 184.5), p=.784]. Finally, there were no differences in either postoperative opioid consumption or numeric rating scores for back, leg, or abdominal pain on postoperative days 0 to 4 (p>.05).

CONCLUSIONS

Methylnaltrexone did not accelerate gastrointestinal recovery and did not affect opioid consumption or pain scores following short-segment spinal surgery as compared to placebo. Additional studies will be needed to identify effective opioid receptor antagonist dosing regimens for patients undergoing either short- or long-segment spinal arthrodesis procedures.

Effects of the peripherally acting μ-opioid receptor antagonist methylnaltrexone on acute pancreatitis severity: study protocol for a multicentre double-blind randomised placebo-controlled interventional trial, the PAMORA-AP trial

Background

Moderate to severe acute pancreatitis (AP) is associated with a high rate of complications and increased mortality, yet no targeted pharmacologic treatment currently exists. As pain is a dominant symptom in AP, patients are exposed to excess levels of both endo- and exogenous opioids, which may have harmful effects on the course of AP. This trial investigates the effects of the peripherally acting μ-opioid receptor antagonist (PAMORA) methylnaltrexone on disease severity and clinical outcomes in patients with moderate to severe AP.

Methods

PAMORA-AP is a multicentre, investigator-initiated, double-blind, randomised, placebo-controlled, interventional trial, which will be conducted at four referral centres for acute pancreatitis in Denmark. Ninety patients with early-onset AP (pain onset within 48 h) as well as predicted moderate to severe disease (two or more systemic inflammatory response syndrome criteria upon admission) will be prospectively included. Subsequently, participants will be randomised (1:1) to intravenous treatment with either methylnaltrexone or matching placebo (Ringer’s lactate) during 5 days of admission. The primary endpoint will be the group difference in disease severity as defined and measured by the Pancreatitis Activity Scoring System (PASS) score 48 h after randomisation. Secondary endpoints include daily PASS scores; disease severity according to the Atlanta classification; quantification of need for analgesics, nutritional support, intravenous fluid resuscitation and antibiotics; duration of hospital admissions, readmission rates and mortality. Pain intensity and gut function will be self-reported using validated questionnaires. Exploratory endpoints include circulating levels of pro-and anti-inflammatory markers, polyethylene glycol recovery from the urine, circulating levels of blood markers of intestinal permeability, the prevalence of pancreatic complications on computed tomography (CT) scans, and colon transit time assessed using a CT-based radiopaque marker method.

Discussion

This trial aims to evaluate the PAMORA methylnaltrexone as a novel targeted pharmacotherapy in patients with moderate to severe AP with the potential benefit of improved patient outcomes.

Trial registration

ClinicalTrials.govNCT04743570. Registered on 28 January 2021. EudraCT 2020-002313-18.

Design and feasibility of a double-blind, randomized trial of peri-operative methylnaltrexone for postoperative ileus prevention after adult spinal arthrodesis

BACKGROUND

Postoperative ileus (POI) is a common complication with no proven prophylactic measures in place. While perioperative opioid use has been implicated in POI development, current treatments fail to target this disease mechanism. Methylnaltrexone (MNTX) has been used to prevent the effects of opioids on the bowel and could reduce the incidence of POI when administered preoperatively.

METHODS

In this phase IIb randomized controlled trial, we assessed the effect of perioperative MNTX on time-to-first-bowel movement following spinal arthrodesis surgeries.

RESULTS

82 patients were randomly selected in a 1:1 ratio to be included in either the treatment or placebo groups. Comparison of relevant factors of included patients to patients who refused to participate (n = 21) and to a prior retrospective series (n = 241) revealed no differences in age, male sex, liver disease, and number of surgical levels. Overall treatment fidelity (98% adherence) and retention (100% at one-month follow-up) were high. The predicted POI incidence (9.3-11.1%) was also equivalent to a prior retrospective series. However, the overall observed POI incidence (3.7%) was lower than expected, which could reflect a superimposed 'trial effect' related to standardized care in a research setting.

CONCLUSIONS

Since exposure to significant opioid doses represents a barrier to enhanced recovery after surgery, the results of this innovative trial may provide further guidance for the peri-operative use of opioid-receptor blockers. Here, we show that MNTX can be effectively administered in the peri-operative period with appropriate follow-up achieved in a representative population of patients undergoing spinal surgery.

TRIAL REGISTRATION NUMBERS

Clinicaltrials.gov - NCT03852524 and Institutional Review Board - 2018H0260.

Development of a postoperative ileus risk assessment scale: identification of intraoperative opioid exposure as a significant predictor after spinal surgery

OBJECTIVE

Postoperative ileus (POI) is associated with abdominal pain, nausea, vomiting, and delayed mobilization that in turn lead to diminished patient satisfaction, increased hospital length of stay (LOS), and increased healthcare costs. In this study, the authors developed a risk assessment scale to predict the likelihood of developing POI following spinal surgery.

METHODS

The authors undertook a retrospective review of a prospectively maintained registry of consecutive patients who underwent arthrodesis/fusion surgeries between May 2013 and December 2017. They extracted clinical information, including cumulative intraoperative and postoperative opioid doses using standardized converted morphine milligram equivalent (MME) values. Univariate and multivariate analyses were performed and several categorical and continuous variables were evaluated in a binary logistic regression model built with backward elimination to assess for independent predictors. A points-based prediction model was developed and validated to determine the risk of POI.

RESULTS

A total of 334 patients who underwent spinal fusion surgeries were included. Fifty-six patients (16.8%) developed POI, more frequently in those who underwent long-segment surgeries compared to short-segment surgeries (33.3% vs 10.4%; p < 0.001). POI was associated with an increased LOS when compared with patients who did not develop POI (8.0 ± 4.5 days vs 4.4 ± 2.4 days; p < 0.01). The incidences of liver disease (16% vs 3.7%; p = 0.01) and substance abuse history (12.0% vs 3.2%; p = 0.04) were higher in POI patients than non-POI patients undergoing short-segment surgeries. While the incidences of preoperative opioid intake (p = 0.23) and cumulative 24-hour (87.7 MME vs 73.2 MME; p = 0.08) and 72-hour (225.6 MME vs 221.4 MME; p = 0.87) postoperative opioid administration were not different, remifentanil (3059.3 µg vs 1821.5 µg; p < 0.01) and overall intraoperative opioid (326.7 MME vs 201.7 MME; p < 0.01) dosing were increased in the POI group. The authors derived a multivariate model based on the 5 most significant factors predictive of POI (number of surgical levels, intraoperative MME, liver disease, age, and history of substance abuse) and calculated relative POI risks using a derived 32-point system.

CONCLUSIONS

Intraoperative opioid administration, incorporated in a comprehensive risk assessment scale, represents an early and potentially modifiable predictor of POI. These data indicate that potential preventive strategies, implemented as part of enhanced recovery after surgery protocols, could be instituted in the preoperative phase of care to reduce POI incidence.

Effects of Methylnaltrexone on Ticagrelor-Induced Antiplatelet Effects in Coronary Artery Disease Patients Treated With Morphine

OBJECTIVES

The aim of this study was to assess if intravenous methylnaltrexone can counteract the effects of morphine on the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of ticagrelor.

BACKGROUND

Morphine delays the onset of action of oral P2Y₁₂ receptor inhibitors, including ticagrelor, by inhibiting gastric emptying and leading to delayed drug absorption. Methylnaltrexone is a peripheral opioid receptor antagonist that has the potential to prevent opioid-induced peripherally mediated side effects (e.g., gastric emptying inhibition) without affecting analgesia.

METHODS

In this prospective, randomized, double-blind, placebo-controlled, crossover study, aspirin-treated patients with stable coronary artery disease (n = 30) were randomized to receive methylnaltrexone (0.3 mg/kg intravenous) or matching placebo. After methylnaltrexone or placebo administration, all patients received morphine (5 mg intravenous). This was followed 15 min later by a 180-mg loading dose of ticagrelor. Patients crossed over to the alternative study treatment after 7 ± 2 days of washout. PK and PD assessments were performed at 12 time points (6 pre- and 6 post-crossover). PK analysis included measurement of plasma levels of ticagrelor and its major active metabolite (AR-C124910XX). PD assessments included VerifyNow P2Y₁₂, light transmittance aggregometry, and vasodilator-stimulated phosphoprotein.

RESULTS

Only marginal changes in plasma levels of ticagrelor (and its major active metabolite) were observed with ticagrelor: maximum plasma concentration and area under the plasma concentration versus time curve from time 0 to the last measurable concentration were 38% and 30% higher, respectively, in patients receiving methylnaltrexone compared with those receiving placebo, but no differences in time to maximum plasma concentration were observed. There were no differences in P2Y₁₂ reaction units by VerifyNow P2Y₁₂ between groups at each time point, including 2 h (the primary endpoint; p = 0.261). Similarly, there were no differences in PD markers assessed by light transmittance aggregometry and vasodilator-stimulated phosphoprotein.

CONCLUSIONS

In patients with coronary artery disease receiving morphine, intravenous administration of the peripheral opioid receptor antagonist methylnaltrexone leads to only marginal changes in plasma levels of ticagrelor and its major metabolite, without affecting levels of platelet reactivity. (Effect of Methylnaltrexone on the PK/PD Profiles of Ticagrelor in Patients Treated With Morphine; NCT02403830).

Opioid-Induced Constipation and Bowel Dysfunction: A Clinical Guideline

OBJECTIVE

To formulate timely evidence-based guidelines for the management of opioid-induced bowel dysfunction.

SETTING

Constipation is a major untoward effect of opioids. Increasing prescription of opioids has correlated to increased incidence of opioid-induced constipation. However, the inhibitory effects of opioids are not confined to the colon, but also affect higher segments of the gastrointestinal tract, leading to the coining of the term "opioid-induced bowel dysfunction."

METHODS

A literature search was conducted using Medline, EMBASE, and EMBASE Classic, and the Cochrane Central Register of Controlled Trials. Predefined search terms and inclusion/exclusion criteria were used to identify and categorize relevant papers. A series of statements were formulated and justified by a comment, then labeled with the degree of agreement and their level of evidence as judged by the Strength of Recommendation Taxonomy (SORT) system.

RESULTS

From a list of 10,832 potentially relevant studies, 33 citations were identified for review. Screening the reference lists of the pertinent papers identified additional publications. Current definitions, prevalence, and mechanism of opioid-induced bowel dysfunction were reviewed, and a treatment algorithm and statements regarding patient management were developed to provide guidance on clinical best practice in the management of patients with opioid-induced constipation and opioid-induced bowel dysfunction.

CONCLUSIONS

In recent years, more insight has been gained in the pathophysiology of this "entity"; new treatment approaches have been developed, but guidelines on clinical best practice are still lacking. Current knowledge is insufficient regarding management of the opioid side effects on the upper gastrointestinal tract, but recommendations can be derived from what we know at present.

Protocol for a randomised control trial of methylnaltrexone for the treatment of opioid-induced constipation and gastrointestinal stasis in intensive care patients (MOTION)

INTRODUCTION

Gastrointestinal dysmotility and constipation are common problems in intensive care patients. The majority of critical care patients are sedated with opioids to facilitate tolerance of endotracheal tubes and mechanical ventilation, which inhibit gastrointestinal motility and lead to adverse outcomes. Methylnaltrexone is a peripheral opioid antagonist that does not cross the blood-brain barrier and can reverse the peripheral side effects of opioids without affecting the desired central properties. This trial will investigate whether methylnaltrexone can reverse opioid-induced constipation and gastrointestinal dysmotility.

METHODS

This is a single-centre, multisite, double-blind, randomised, placebo-controlled trial. 84 patients will be recruited from 4 intensive care units (ICUs) within Imperial College Healthcare NHS Trust. Patients will receive intravenous methylnaltrexone or placebo on a daily basis if they are receiving opioid infusion to facilitate mechanical ventilation and have not opened their bowels for 48 hours. All patients will receive standard laxatives as per the clinical ICU bowel protocol prior to randomisation. The primary outcome of the trial will be time to significant rescue-free laxation following randomisation. Secondary outcomes will include tolerance of enteral feed, gastric residual volumes, incidence of pneumonia, blood stream and Clostridium difficile infection, and any reversal of central opioid effects.

ETHICS AND DISSEMINATION

The trial protocol, the patient/legal representative information sheets and consent forms have been reviewed and approved by the Harrow Research Ethics Committee (REC Reference 14/LO/2004). An independent Trial Steering Committee and Data Monitoring Committee are in place, with patient representation. On completion, the trial results will be published in peer-reviewed journals and presented at national and international scientific meetings.

TRIAL REGISTRATION NUMBER

2014-004687-37; Pre-results.

Diagnosis and management of chronic constipation in adults

Constipation is a heterogeneous, polysymptomatic, multifactorial disease. Acute or transient constipation can be due to changes in diet, travel or stress, and secondary constipation can result from drug treatment, neurological or metabolic conditions or, rarely, colon cancer. A diagnosis of primary chronic constipation is made after exclusion of secondary causes of constipation and encompasses several overlapping subtypes. Slow-transit constipation is characterized by prolonged colonic transit in the absence of pelvic floor dysfunction. This subtype of constipation can be identified using either the radio-opaque marker test or wireless motility capsule test, and is best treated with laxatives such as polyethylene glycol or newer agents such as linaclotide or lubiprostone. If unsuccessful, subspecialist referral should be considered. Dyssynergic defecation results from impaired coordination of rectoanal and pelvic floor muscles, and causes difficulty with defecation. The condition can be identified using anorectal manometry and balloon expulsion tests and is best managed with biofeedback therapy. Opioid-induced constipation is an emerging entity, and several drugs including naloxegol, methylnaltrexone and lubiprostone are approved for its treatment. In this Review, we provide an overview of the burden and pathophysiology of chronic constipation, as well as a detailed discussion of the available diagnostic tools and treatment options.

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

BACKGROUND

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

METHODS

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

KEY RESULTS

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

CONCLUSIONS & INFERENCES

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

Methylnaltrexone: its pharmacological effects alone and effects on morphine in healthy volunteers

RATIONALE

Methylnaltrexone bromide (MTNX) is a peripherally acting mu-opioid receptor antagonist, prescribed for the treatment of opioid-induced constipation in patients with advanced illness who are receiving palliative care. Studies have used this drug to determine if other opioid-induced effects besides constipation are altered by MTNX in humans and have suggested, based on their results, that these other effects are altered by peripheral opioid actions.

OBJECTIVE

The primary objective of this report is to present results that provide indirect evidence that MTNX has centrally mediated effects, albeit slight, and secondarily to describe the effects of MTNX on psychopharmacological effects of morphine.

METHODS

In a crossover, randomized, placebo-controlled, double-blind study, 29 healthy volunteers received 0.45 mg/kg MTNX or saline subcutaneously, followed by saline intravenously. In three other conditions, 0.143 mg/kg of morphine sulfate administered intravenously was preceded by subcutaneous administration of 0, 0.225, or 0.45 mg/kg MTNX. Before and after drug administration, subjective and physiological measures, including pupil diameter, were assessed.

RESULTS

Two separate analyses confirmed that 0.45 mg/kg MTNX alone induced a slight degree of miosis, a centrally mediated opioid agonist effect. This dose had minimal subjective effects. MTNX at either or both the 0.225 and 0.45 mg/kg dose reduced some subjective effects of morphine without altering miosis.

CONCLUSIONS

We present indirect evidence that MTNX crosses the blood-brain barrier in humans. Therefore, whether the reductions in subjective effects of morphine by MTNX that were observed in past studies and in this study can be attributed to peripheral mechanisms is open to question.

[Is the pharmacological treatment of constipation in palliative care evidence based? : a systematic literature review]

MATERIALS AND METHODS

To evaluate the evidence for clinically established pharmacological therapies for constipation in palliative care, a systematic literature review was performed in different databases (Cochrane Library, Embase, PubMed, Ovid MEDLINE, CINAHL), textbooks, and publications.

RESULTS

Whereas 130 randomized controlled trials were found with patients outside of palliative care settings, only 10 controlled studies with patients in end-of-life situations were identified: three RCTs with methylnaltrexone and one with the combination of oxycodone and naloxone showed the effect and safety of opiate antagonists for patients who are not at risk of gastrointestinal perforation. There have been no studies which test methylnaltrexone against the optimization of therapy with conventional laxatives. Six other controlled studies of limited quality in design and execution and with only few participants tested naloxone, senna, lactulose, Co-danthramer, an Ayurvedic preparation (Misrakasneham), magnesium hydroxide, fluid paraffin, sodium picosulfate and docusate without finding statistically significant differences in efficacy or side effects. Most patients in these studies had cancer. Only case studies with few patients in palliative care were found for meglumine, neostigmine, and other substances mentioned above.

CONCLUSION

Evidence on medical treatment of constipation in palliative care is sparse and guidelines have to refer to evidence from outside the palliative care setting and to expert opinions. Results from studies with other patient groups can only be transferred with limitations to very ill patients at the end of life who might have a higher risk for potential side effects such as gastrointestinal perforation in case of abdominal tumor manifestation. Therefore further studies are required to evaluate the medical treatment of multiple reasons for constipation in these patients. These studies should focus on feasibility, clinical relevance and quality of life. The English full text version of this article will be available in SpringerLink as of November 2012 (under "Supplemental").

Sphingolipids in lung endothelial biology and regulation of vascular integrity

Of the multiple and diverse homeostatic events that involve the lung vascular endothelium, participation in preserving vascular integrity and therefore organ function is paramount. We were the first to show that the lipid growth factor and angiogenic factor, sphingosine-1-phosphate, is a critical agonist involved in regulation of human lung vascular barrier function (Garcia et al. J Clin Invest, 2011). Utilizing both in vitro models and preclinical murine, rat, and canine models of acute and chronic inflammatory lung injury, we have shown that S1Ps, as well as multiple S1P analogues such as FTY720 and ftysiponate, serve as protective agents limiting the disruption of the vascular EC monolayer in the pulmonary microcirculation and attenuate parenchymal accumulation of inflammatory cells and high protein containing extravasated fluid, thereby reducing interstitial and alveolar edema. The vasculo-protective mechanism of these therapeutic effects occurs via ligation of specific G-protein-coupled receptors and an intricate interplay of S1P with other factors (such as MAPKS, ROCKs, Rho, Rac1) with rearrangement of the endothelial cytoskeleton to form strong cortical actin rings in the cell periphery and enhanced cell-to-cell and cell-to-matrix tethering dynamics. This cascade leads to reinforcement of focal adhesions and paracellular junctional complexes via cadherin, paxillin, catenins, and zona occludens. S1P through its interaction with Rac and Rho influences the cytoskeletal rearrangement indicated in the later stages of angiogenesis as a stabilizing force, preventing excessive vascular permeability. These properties translate into a therapeutic potential for acute and chronic inflammatory lung injuries. S1P has potential for providing a paradigm shift in the approach to disruption of critical endothelial gatekeeper function, loss of lung vascular integrity, and increased vascular permeability, defining features of acute lung injury (ALI), and may prove to exhibit an intrinsically protective role in the pulmonary vasculature ameliorating agonist- or sepsis-induced pulmonary injury and vascular leakage.

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.

Deconstructing 14-phenylpropyloxymetopon: minimal requirements for binding to mu opioid receptors

A series of phenylpropyloxyethylamines and cinnamyloxyethylamines were synthesized as deconstructed analogs of 14-phenylpropyloxymetopon and analyzed for opioid receptor binding affinity. Using the Conformationally Sampled Pharmacophore modeling approach, we discovered a series of compounds lacking a tyrosine mimetic, historically considered essential for μ opioid binding. Based on the binding studies, we have identified the optimal analogs to be N-methyl-N-phenylpropyl-2-(3-phenylpropoxy)ethanamine, with 1520 nM, and 2-(cinnamyloxy)-N-methyl-N-phenethylethanamine with 1680 nM affinity for the μ opioid receptor. These partial opioid structure analogs will serve as the novel lead compounds for future optimization studies.

Most recent developments and modifications of 14-alkylamino and 14-alkoxy-4,5-epoxymorphinan derivatives

The 14-position of natural opiates (e.g. morphine) are unsubstituted, however synthetic approaches have uncovered that functionalizing position 14 gives rise to a wide range of diverse activities. This review focuses on SAR of the position, with the aim of aiding in the search for opioid analgesics with improved clinical profiles.

Characterization of abdominal pain during methylnaltrexone treatment of opioid-induced constipation in advanced illness: a post hoc analysis of two clinical trials

CONTEXT

Methylnaltrexone is a selective peripherally acting mu-opioid receptor antagonist that decreases the constipating effects of opioids without affecting centrally mediated analgesia. In two double-blind, placebo-controlled, Phase III studies of methylnaltrexone for opioid-induced constipation in patients with advanced illness, abdominal pain was the most common adverse event (AE) reported.

OBJECTIVES

This analysis sought to further characterize the Medical Dictionary for Regulatory Activities-defined abdominal pain AEs experienced in these studies.

METHODS

A post hoc analysis of verbatim descriptions was used to further assess AEs characterized as abdominal pain in both trials. Descriptive summary statistics were used to assess severity of abdominal pain, effect of abdominal pain on global pain scores, and other characteristics. Logistic regression analysis was used to determine the association of baseline characteristics with abdominal pain.

RESULTS

Most verbatim descriptions of abdominal pain referred to "abdominal cramps" or "cramping." Abdominal pain AEs were mostly mild to moderate in severity and did not affect patients' global evaluation of pain. The incidence of abdominal pain AEs in methylnaltrexone-treated patients was greatest after the first dose and decreased with subsequent doses. No association between abdominal pain AEs and most baseline patient characteristics was noted.

CONCLUSION

Abdominal pain AEs in methylnaltrexone-treated patients in clinical trials are usually described as "cramps" or "cramping," are mostly mild to moderate in severity, and decrease in incidence with subsequent dosing.

Opioid-induced constipation in advanced illness: safety and efficacy of methylnaltrexone bromide

Constipation, one of the major side effects of opiates used in palliative care, can impair patients’ quality of life to a point where it prevents sufficient pain control. Methylnaltrexone is a novel μ-receptor antagonist, which does not pass the blood brain barrier. It is licensed to treat opiate induced constipation for patients with advanced diseases. This review article presents an overview of pharmacology and safety of its application, evidence of its efficacy and economic aspects of its use in clinical practice. Available data are limited but strongly suggest that methylnaltrexone causes laxation in less than 24 hours for at least half of those patients over the first two weeks of usage without impairing pain control or causing serious adverse effects. To avoid danger of gastrointestinal perforation it is contraindicated for patients at risk for that complication. More research is needed to evaluate its long-term efficacy and economic impact.

Prophylactic use of laxative for constipation in critically ill patients

BACKGROUND

This study was designed to evaluate the use of laxative prophylaxis for constipation in intensive care unit (ICU) and the impact of early versus late bowel movement on patient's outcome.

METHODS

The study was a prospective, randomized controlled trial in critically ill ventilated adult patients, who were expected to stay on ventilator for >72 h. Control group did not receive any intervention for bowel movement for the first 72 h, whereas interventional group received prophylactic dose of lactulose 20 cc enterally every 12 h for the first 72 h. The parameters measured during the study were admission diagnosis, age, gender, comorbid conditions, admission Simplified Acute Physiologic Score (SAPS II), sedative and narcotic agents with doses and duration, timing and tolerance of nutrition, daily assessment of bowel movement, total use of prokinetic, doses of suppositories, and enema for first bowel movement, total number of days on ventilator, weaning failures, extubation or tracheostomy, ICU length of stay, and death or discharge.

RESULTS

A total of 100 patients were enrolled, 50 patients in each control and interventional group. Mean age was 38.8 years, and both groups had male predominance. Mean SAPS II score for both was 35. Mean dose of Fentanyl (323.8 ± 108.89 mcg/h in control and 345.83 ± 94.43 mcg/h in interventional group) and mean dose of Midazolam (11.1 ± 4.04 mg/h in control and 12.4 ± 3.19 mg/h in interventional group). There were only two (4%) patients in control, while nine (18%) patients in interventional group who had bowel movement in <72 h (P < 0.05). Mean ventilator days were 16.19, and 17.36 days in control and interventional groups, respectively. Subgroup analysis showed that the patients who moved bowel in <5 days in both groups had mean ventilator days of 18.5, whereas it was 15.88 days for the patients who moved bowel after 5 days in both groups (P< 0.05). Mean ICU days for control was 21.15 ± 10.44 and 20.77 ± 8.33 days for interventional group. Forty-eight (96%) patients in each group were discharged from the ICU. Two (4%) patients died in ICU in each group.

CONCLUSIONS

Laxative prophylaxis can be used successfully to prevent constipation in ICU patients. Late bowel movement >5 days is associated with less ventilator days, compared to early <5 days bowel movement.

Safety and efficacy of methylnaltrexone in shortening the duration of postoperative ileus following segmental colectomy: results of two randomized, placebo-controlled phase 3 trials

PURPOSE

Postoperative ileus contributes to surgical morbidity and is associated with prolonged hospitalization and increased health care costs. The efficacy and safety of the peripherally acting μ-opioid receptor antagonist methylnaltrexone in shortening the duration of postoperative ileus following segmental colectomy was evaluated.

METHODS

Two identically designed, multicenter, double-blind, parallel-group, placebo-controlled studies randomly assigned patients undergoing segmental colectomy (study 1, N = 515; study 2, N = 533) to receive 12 or 24 mg of methylnaltrexone intravenously or placebo every 6 hours starting within 90 minutes of surgery completion, continuing for up to 10 days or up to 24 hours after gastrointestinal recovery. The primary efficacy end point was the time from the end of surgery to the first bowel movement. Safety was evaluated via standard assessments (ie, adverse events and related withdrawals, physical examinations, laboratory tests, vital signs, electrocardiograms) and assessment of surgical complications.

RESULTS

The primary and secondary efficacy outcomes (time to discharge eligibility, time to hospital discharge, and clinically meaningful events of nausea and vomiting following segmental colectomy) did not differ significantly between patients treated with either a dose of methylnaltrexone or with placebo. Rates of adverse events and serious adverse events were comparable across all treatment groups in both studies. The most commonly observed adverse events were nausea, pyrexia, and vomiting.

CONCLUSIONS

Although the efficacy of methylnaltrexone in reducing the duration of postoperative ileus was not demonstrated in these studies, intravenous methylnaltrexone at doses of 12 mg and 24 mg was safe, in general, and well tolerated in postcolectomy patients. The utility of intravenous methylnaltrexone in treating postoperative ileus remains unproven.

Subcutaneous methylnaltrexone for treatment of opioid-induced constipation in patients with chronic, nonmalignant pain: a randomized controlled study

Methylnaltrexone is effective for opioid-induced constipation (OIC) in advanced illness patients. This 4-week, double-blind, randomized, placebo-controlled study investigated the effect of subcutaneous methylnaltrexone on OIC in patients receiving opioids for chronic, nonmalignant pain. Patients (N = 460) received subcutaneous methylnaltrexone 12 mg once daily (QD) or every other day (alternating with placebo) compared with placebo. Assessments included bowel movement count, time of bowel movement, straining, sense of complete evacuation, Bristol Stool Form Scales, and quality of life. Within 4 hours of first dose, 34.2% of patients in both methylnaltrexone groups had rescue-free bowel movements (RFBMs) versus 9.9% on placebo (P < .001). The estimated number needed to treat was about 4. On average, 28.9% of methylnaltrexone QD and 30.2% of methylnaltrexone alternate-day dosing resulted in RFBMs within 4 hours versus 9.4% QD and 9.3% alternate-day placebo injections (both P < .001). Both methylnaltrexone groups had significantly shorter time to first RFBM (P < .001) and greater increase in number of weekly RFBMs (P < .05) versus placebo. Adverse events included abdominal pain, diarrhea, nausea, and hyperhidrosis. Bristol Stool Form Scale scores (P = .002) and sensation of complete evacuation (P < .04) were significantly superior with methylnaltrexone QD; both methylnaltrexone groups reported no or mild straining during RFBMs in the first 2 weeks (P < .02). At 4 weeks, a significantly greater improvement in patient-reported, constipation-specific quality of life was seen in the alternate-day dosing (P < .05) and QD (P < .001) groups.

PERSPECTIVE

We present data demonstrating that subcutaneous methylnaltrexone 12 mg given once daily (QD) or every other day provides significant relief of OIC and was generally well tolerated in patients with chronic, nonmalignant pain. These results expand on prior effectiveness observed for the treatment of OIC in advanced illness patients to a broader population.

Creating the animated intensive care unit

Critical care medicine has matured greatly as a field in the past decade. Much has been learned concerning the institution of life support therapies to sustain patients with diverse and multiple organ failures, thus providing patients with a window of opportunity to recover from potentially life-ending insults. The management of critically ill patients has increasingly involved creation of a highly controlled environment by care providers, with patients immobilized, tethered to devices, and receiving multiple drugs to facilitate the entire process. Although it has been assumed that such control of the patient has been necessary to implement essential therapies and to tailor life support systems such as mechanical ventilation, this assumption may be unfounded or at least overplayed, as knowledge of the adverse effects of this approach have been identified and quantified. Extant information, based on observational studies and a few interventional trials, would suggest a radically different approach to care is warranted, even given the difficulties in reversing the current culture of critical care management. Specifically, methods to avoid entirely, or minimize, neuromuscular blockade and sedation are supported by recent literature. These methods include the use of noninvasive ventilation in appropriately selected patients, the development of mechanical ventilators more synchronous with patient efforts and needs, and the use of sedation strategies to avoid drug accumulations with protracted effects. These methods, in turn, afford opportunities to avoid extreme immobilization and institute physiotherapy earlier than previously had been thought possible. In addition to the neuropsychiatric and neuromuscular benefits that could derive from minimizing opiate administration in critically ill patients, gut hypomotility could be avoided. This, in turn, could facilitate earlier and more complete enteral nutrition. Even when opioids have to be administered in generous amounts for control of pain that may accompany critical illness, it is now possible to block the peripheral actions of these medications with the μ-receptor antagonist methylnaltrexone. Other new drugs being introduced into the critical care unit such as dexmedetomidine may also provide a greater ability to achieve analgesia and anxiolysis without some of the adverse concomitant effects seen with more traditional drug regimens. The ultimate goal of this multipronged program to facilitate the maintenance of patients who are more interactive with their care providers, and the life support provided in the intensive care unit would be to speed the pace of recovery and to diminish the need for the protracted rehabilitation that often follows survival from critical illness.

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.

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 potentiates the anti-angiogenic effects of mTOR inhibitors

Background

Recent cancer therapies include drugs that target both tumor growth and angiogenesis including mammalian target of rapamycin (mTOR) inhibitors. Since mTOR inhibitor therapy is associated with significant side effects, we examined potential agents that can reduce the therapeutic dose.

Methods

Methylnaltrexone (MNTX), a peripheral mu opioid receptor (MOR) antagonist, in combination with the mTOR inhibitors temsirolimus and/or rapamycin, was evaluated for inhibition of VEGF-induced human pulmonary microvascular endothelial cell (EC) proliferation and migration as well as in vivo angiogenesis (mouse Matrigel plug assay).

Results

MNTX inhibited VEGF-induced EC proliferation and migration with an IC50 of ~100 nM. Adding 10 nM MNTX to EC shifted the IC50 of temsirolimus inhibition of VEGF-induced proliferation and migration from ~10 nM to ~1 nM and from ~50 to ~10 nM respectively. We observed similar effects with rapamycin. On a mechanistic level, we observed that MNTX increased EC plasma membrane-associated tyrosine phosphate activity. Inhibition of tyrosine phosphatase activity (3,4-dephostatin) blocked the synergy between MNTX and temsirolimus and increased VEGF-induced tyrosine phosphorylation of Src with enhanced PI3 kinase and mTOR Complex 2-dependent phosphorylation of Akt and subsequent activation of mTOR Complex 1 (rapamycin and temsirolimus target), while silencing Src, Akt or mTOR complex 2 components blocked VEGF-induced angiogenic events.

Conclusions

Our data indicate that MNTX exerts a synergistic effect with rapamycin and temsirolimus on inhibition of VEGF-induced human EC proliferation and migration and in vivo angiogenesis. Therefore, addition of MNTX could potentially lower the dose of mTOR inhibitors which could improve therapeutic index.

[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.

Current and novel therapeutic options for irritable bowel syndrome management

Irritable bowel syndrome is a functional gastrointestinal disorder affecting up to 3-15% of the general population in western countries. It is characterised by unexplained abdominal pain, discomfort, and bloating in association with altered bowel habits. The pathophysiology of irritable bowel syndrome is multifactorial involving disturbances of the brain-gut axis. The pathophysiology provides the rationale for pharmacotherapy: abnormal gastrointestinal motor functions, visceral hypersensitivity, psychosocial factors, autonomic dysfunction, and mucosal immune activation. Understanding the mechanisms, and their mediators or modulators including neurotransmitters and receptors have led to several therapeutic approaches including agents acting on the serotonin receptor or serotonin transporter system, antidepressants, novel selective anticholinergics, alpha-adrenergic agonists, opioid agents, cholecystokinin-antagonists, neurokinin-antagonists, somatostatin receptor agonists, corticotropin releasing factor antagonists, chloride channel activators, guanylate cyclase-c agonists, melatonin, atypical benzodiazepines, antibiotics, immune modulators and probiotics. The mechanisms and current evidence regarding efficacy of these agents are reviewed.

Effectiveness of two opioid antagonists in treating opioid-induced constipation

AIM

This literature review reports the effectiveness of two peripheral opioid antagonists to relieve constipation caused by prolonged use of opioids, primarily in patients with advanced cancer.

BACKGROUND

Opioid-induced constipation can cause great discomfort to patients who use opioids for prolonged periods and on occasion decline pain-relief in an effort to help aid laxation. The use of peripheral opioid antagonists can help reverse the slowing of the gut caused by the opioids, but the correct one must be used in order not to reverse the important analgesic effects the opioids provide.

DATA SOURCES

Information was obtained by searching the databases British Nursing Index, CINAHL and Medline.

REVIEW METHODS

Selection criteria were as follows: original research in the form of randomized controlled trials; articles had to be written in English; articles had to focus on the effectiveness of naloxone and methylnaltrexone in the treatment of opioid-induced constipation; the search was limited to the period 1990-2008.

RESULTS

Nineteen articles were included in the study. The results from the studies indicate that due to fewer withdrawal effects and less reversal of analgesia experienced, methylnaltrexone is more effective at relieving opioid-induced constipation.

CONCLUSION

The recent development of methylnaltrexone into a commercially-viable drug indicates its effectiveness. It may help patients suffering from the intense discomfort that constipation can cause.

Methylnaltrexone: the answer to opioid-induced constipation?

Opioid-induced constipation is a significant problem particularly for end stage cancer patients, methadone users, patients suffering from chronic pain as well as surgical patients. Until recently, there were few efficacious treatment options that did not have significant side effects. Methylnaltrexone is a promising drug for the treatment of opioid-induced constipation. It is an opioid-receptor antagonist that blocks the peripheral gastrointestinal opioid receptors responsible for opioid-induced bowel dysfunction. Due to the drug's polarity, it does not cross the blood-brain barrier; therefore, it does not block the central opioid receptors, thus, retaining effective analgesia. Methylnaltrexone has been recently approved by the FDA in the subcutaneous form for the treatment of opioid-induced bowel dysfunction, whereas the intravenous and oral forms remain under investigation.

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.

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.

Methylnaltrexone, a new peripherally acting mu-opioid receptor antagonist being evaluated for the treatment of postoperative ileus

Postoperative ileus (POI), a transient impairment of bowel function, is considered an inevitable response after open abdominal surgery. It leads to significant patient morbidity and increased hospital costs and length of stay. The pathophysiology is multifactorial, involving neurogenic, hormonal, inflammatory and pharmacologic mediators. Several treatments have been shown to reduce the duration of POI, and a multimodal approach combining several of these interventions seems to be the most effective treatment option. Various drug therapies have been evaluated for the treatment of POI, although most have not shown any benefit. Peripherally active mu-opioid receptor antagonists are a new class of compounds that selectively block the peripheral (i.e., gastrointestinal [GI]) effects of opioids while preserving centrally mediated analgesia. Recently, alvimopan was approved in the US for the treatment of POI after abdominal surgery with bowel resection. Methylnaltrexone is a peripherally active mu-opioid receptor antagonist that has been shown to antagonize the inhibitory effects of opioids on GI transit without impairing analgesia. Phase II data indicated that methylnaltrexone was effective for improving GI recovery, reducing POI and shortening the time to discharge readiness in patients who underwent segmental colectomy. Two Phase III trials have been completed, and one is underway at present. Preliminary results from the two completed trials indicate that methylnaltrexone was not better than placebo for the primary or secondary outcomes. Further analyses of these data, clinical trial designs and the various dosage forms are necessary to determine the potential role of methylnaltrexone in the treatment of POI.

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.