The safety and efficacy of oral methylnaltrexone in preventing morphine-induced delay in oral-cecal transit time

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.

Insights into the Current and Possible Future Use of Opioid Antagonists in Relation to Opioid-Induced Constipation and Dysbiosis

Opioid receptor agonists, particularly those that activate µ-opioid receptors (MORs), are essential analgesic agents for acute or chronic mild to severe pain treatment. However, their use has raised concerns including, among others, intestinal dysbiosis. In addition, growing data on constipation-evoked intestinal dysbiosis have been reported. Opioid-induced constipation (OIC) creates an obstacle to continuing treatment with opioid analgesics. When non-opioid therapies fail to overcome the OIC, opioid antagonists with peripheral, fast first-pass metabolism, and gastrointestinal localized effects remain the drug of choice for OIC, which are discussed here. At first glance, their use seems to only be restricted to constipation, however, recent data on OIC-related dysbiosis and its contribution to the appearance of several opioid side effects has garnered a great of attention from researchers. Peripheral MORs have also been considered as a future target for opioid analgesics with limited central side effects. The properties of MOR antagonists counteracting OIC, and with limited influence on central and possibly peripheral MOR-mediated antinociception, will be highlighted. A new concept is also proposed for developing gut-selective MOR antagonists to treat or restore OIC while keeping peripheral antinociception unaffected. The impact of opioid antagonists on OIC in relation to changes in the gut microbiome is included.

Error traps in acute pain management in children

Providing effective acute pain management to hospitalized children can help improve outcomes, decrease length of stay, and increase patient and parental satisfaction. Error traps (circumstances that lead to erroneous actions or undesirable consequences) can result in inadequately controlled pain, unnecessary side effects, and adverse events. This article highlights five error traps encountered when managing acute pain in children. They include failure to appropriately assess pain, optimally utilize regional anesthesia, select suitable systemic analgesics, identify and treat medication-related side effects, and consider patient characteristics when choosing medication or dosing route. These issues are easily addressed when the clinician is cognizant of ways to anticipate, identify, and mitigate or avoid these errors.

European society of neurogastroenterology and motility guidelines on functional constipation in adults

INTRODUCTION

Chronic constipation is a common disorder with a reported prevalence ranging from 3% to 27% in the general population. Several management strategies, including diagnostic tests, empiric treatments, and specific treatments, have been developed. Our aim was to develop European guidelines for the clinical management of constipation.

DESIGN

After a thorough review of the literature by experts in relevant fields, including gastroenterologists, surgeons, general practitioners, radiologists, and experts in gastrointestinal motility testing from various European countries, a Delphi consensus process was used to produce statements and practical algorithms for the management of chronic constipation.

KEY RESULTS

Seventy-three final statements were agreed upon after the Delphi process. The level of evidence for most statements was low or very low. A high level of evidence was agreed only for anorectal manometry as a comprehensive evaluation of anorectal function and for treatment with osmotic laxatives, especially polyethylene glycol, the prokinetic drug prucalopride, secretagogues, such as linaclotide and lubiprostone and PAMORAs for the treatment of opioid-induced constipation. However, the level of agreement between the authors was good for most statements (80% or more of the authors). The greatest disagreement was related to the surgical management of constipation.

CONCLUSIONS AND INFERENCES

European guidelines on chronic constipation, with recommendations and algorithms, were developed by experts. Despite the high level of agreement between the different experts, the level of scientific evidence for most recommendations was low, highlighting the need for future research to increase the evidence and improve treatment outcomes in these patients.

Targeting mu opioid receptors to modulate gastrointestinal function: what have we learnt so far from the studies in functional bowel disorders?

Opioids have recently received much attention because of the epidemic in their use in some countries such as the USA and the UK. Concerns have been raised about the possibility that they can increase mortality in patients when used on a long-term basis. Moreover, they are known to induce paradoxical hyperalgesia as well as alterations of gut function. The analgesic properties of opioids are mediated by receptors located in the brain, but as opioid receptors are also expressed in the gastrointestinal tract, new drugs acting on these receptors have recently been developed to treat two functional disorders, namely irritable bowel syndrome with diarrhoea and opioid-induced constipation. The aim of this article is to highlight some interesting observations resulting from the development of these drugs in the field of functional gastrointestinal disorders.

Opioid receptors in the GI tract: targets for treatment of both diarrhea and constipation in functional bowel disorders?

Opioids have been used for centuries, mostly as a sedative and to treat pain. Currently, they are used on a global scale for the treatment of acute and chronic pain in diseases as osteoarthritis, fibromyalgia, and low back pain. Binding of opioids on opioid receptors can cause a range of different effects such as changes in stress response, analgesia, motor activity and autonomic functions. This review provide a synthetic summary of the most recent literature on the use of drugs acting on mu-receptors to treat two prevalent functional bowel disorders, presenting with opposite bowel habit. Eluxadoline and naloxegol, methylnaltrexone and naldemedine are recently FDA and/or EMA approved drugs demonstrated to be effective and safe for treatment respectively of irritable bowel syndrome subtype diarrhea and opioid induced constipation.

Insights on efficacious doses of PAMORAs for patients on chronic opioid therapy or opioid-naïve patients

BACKGROUND

Opioid-induced constipation (OIC) is a major side effect of opioid use. Centrally acting antagonists result in opioid withdrawal or worsening of pain and lead to use of peripherally acting mu-opioid receptor antagonists (PAMORAs). The required doses of the PAMORAs, methylnaltrexone and naloxegol, in the treatment of OIC are well established in chronic opioid users. OIC may occur after short duration of opioid treatment; the required doses of naloxone, naltrexone, and PAMORAs in opioid-naïve subjects (with no opioid use for at least 3 months) are unclear. The aim of this review was to evaluate the PAMORA dose required for opioid-naïve subjects to achieve similar beneficial effects on symptoms or valid surrogates to those observed in chronic opioid users.

METHODS

A PubMed search of μ-opioid antagonists to counter μ-opioid effects included terms: naloxone, naltrexone, methylnaltrexone, alvimopan, and naloxegol, as well as OIC and colonic transit.

KEY RESULTS

The approved dose of methylnaltrexone in chronic opioid users, 0.3 mg/kg subcutaneous (SQ), did not affect motility in opioid-naïve subjects. Trials investigating the required dose of alvimopan showed 0.5-1 mg dose was efficacious in treating OIC; a 10-fold higher dose (12 mg) of alvimopan is needed to block effects of codeine on small bowel and colonic transit in opioid-naïve subjects compared to chronic opioid users. Opioid-naïve users need 125 mg of naloxegol to reverse the effects of opioids on transit; this is in contrast to the 12.5 to 25 mg needed to treat OIC in chronic opioid users.

CONCLUSIONS & INFERENCES

Opioid-naïve subjects require a higher dose of PAMORA than chronic opioid users to achieve μ-opioid antagonist effect.

Phase 1, Randomized, Double-Blind, Placebo-Controlled Studies on the Safety, Tolerability, and Pharmacokinetics of Naldemedine in Healthy Volunteers

Naldemedine (S-297995) is a peripherally acting μ-opioid receptor antagonist for the treatment of opioid-induced constipation, a common side effect of opioid therapy. We determined the safety, tolerability, and pharmacokinetic profiles of oral naldemedine in healthy volunteers in 2 randomized, double-blind, placebo-controlled, phase 1 studies. In the single ascending dose study, subjects received a single dose of naldemedine (0.1-100 mg; n = 42) or placebo (n = 14). In the multiple ascending dose study, subjects received once-daily naldemedine (3-30 mg; n = 27) or placebo (n = 9) for 10 days. On day 1 of the single ascending dose studies and day 10 of the multiple ascending dose studies, respectively, the maximum plasma concentration levels of naldemedine were 1.98 to 2510 ng/mL and 73.8 to 700 ng/mL, peaked at 0.5 hours and 0.5 to 0.75 hours, and the fraction excreted in urine was 15.9% to 20.5% and 19.7% to 19.1%. There were no major safety or tolerability concerns even at naldemedine doses 150 to 500 times the therapeutic dose of 0.2 mg. The incidence of adverse events was not dose dependent. Gastrointestinal adverse events occurred more frequently with naldemedine vs placebo, and all of these were considered treatment related. Overall, naldemedine was rapidly absorbed, and no safety or tolerability issues were noted at the doses evaluated.

Formulary Drug Review: Naldemedine

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 in-services. A comprehensive target drug utilization evaluation/medication use evaluation (DUE/MUE) is also provided each month. With a subscription, the monographs are available online to subscribers. Monographs can be customized to meet the needs of a facility. Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. For more information about The Formulary Monograph Service, contact Wolters Kluwer customer service at 866-397-3433. The September 2017 monograph topics are brigatinib, durvalumab, edaravone, midostaurin, and sarilumab. The MUE is on sarilumab.

Established and emerging methods for assessment of small and large intestinal motility

BACKGROUND

Gastrointestinal symptoms are common in the general population and may originate from disturbances in gut motility. However, fundamental mechanistic understanding of motility remains inadequate, especially of the less accessible regions of the small bowel and colon. Hence, refinement and validation of objective methods to evaluate motility of the whole gut is important. Such techniques may be applied in clinical settings as diagnostic tools, in research to elucidate underlying mechanisms of diseases, and to evaluate how the gut responds to various drugs. A wide array of such methods exists; however, a limited number are used universally due to drawbacks like radiation exposure, lack of standardization, and difficulties interpreting data. In recent years, several new methods such as the 3D-Transit system and magnetic resonance imaging assessments on small bowel and colonic motility have emerged, with the advantages that they are less invasive, use no radiation, and provide much more detailed information.

PURPOSE

This review outlines well-established and emerging methods to evaluate small bowel and colonic motility in clinical settings and in research. The latter include the 3D-Transit system, magnetic resonance imaging assessments, and high-resolution manometry. Procedures, indications, and the relative strengths and weaknesses of each method are summarized.

Measurement of Gastrointestinal and Colonic Motor Functions in Humans and Animals

Accurately measuring the complex motor behaviors of the gastrointestinal tract has tremendous value for the understanding, diagnosis and treatment of digestive diseases. This review synthesizes the literature regarding current tests that are used in both humans and animals. There remains further opportunity to enhance such tests, especially when such tests are able to provide value in both the preclinical and the clinical settings.

The Impact of Opioid Treatment on Regional Gastrointestinal Transit

Background/Aims

To employ an experimental model of opioid-induced bowel dysfunction in healthy human volunteers, and evaluate the impact of opioid treatment compared to placebo on gastrointestinal (GI) symptoms and motility assessed by questionnaires and regional GI transit times using the 3-dimensional (3D)-Transit system.

Methods

Twenty-five healthy males were randomly assigned to oxycodone or placebo for 5 days in a double blind, crossover design. Adverse GI effects were measured with the bowel function index, gastrointestinal symptom rating scale, patient assessment of constipation symptom questionnaire, and Bristol stool form scale. Regional GI transit times were determined using the 3D-Transit system, and segmental transit times in the colon were determined using a custom Matlab^® graphical user interface.

Results

GI symptom scores increased significantly across all applied GI questionnaires during opioid treatment. Oxycodone increased median total GI transit time from 22.2 to 43.9 hours (P < 0.001), segmental transit times in the cecum and ascending colon from 5.7 to 9.9 hours (P = 0.012), rectosigmoid colon transit from 2.7 to 9.0 hours (P = 0.044), and colorectal transit time from 18.6 to 38.6 hours (P = 0.001). No associations between questionnaire scores and segmental transit times were detected.

Conclusions

Self-assessed GI adverse effects and increased GI transit times in different segments were induced during oxycodone treatment. This detailed information about segmental changes in motility has great potential for future interventional head-to-head trials of different laxative regimes for prevention and treatment of constipation.

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

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

Physiology, signaling, and pharmacology of opioid receptors and their ligands in the gastrointestinal tract: current concepts and future perspectives

Opioid receptors are widely distributed in the human body and are crucially involved in numerous physiological processes. These include pain signaling in the central and the peripheral nervous system, reproduction, growth, respiration, and immunological response. Opioid receptors additionally play a major role in the gastrointestinal (GI) tract in physiological and pathophysiological conditions. This review discusses the physiology and pharmacology of the opioid system in the GI tract. We additionally focus on GI disorders and malfunctions, where pathophysiology involves the endogenous opioid system, such as opioid-induced bowel dysfunction, opioid-induced constipation or abdominal pain. Based on recent reports in the field of pharmacology and medicinal chemistry, we will also discuss the opportunities of targeting the opioid system, suggesting future treatment options for functional disorders and inflammatory states of the GI tract.

[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").

Pharmacologic treatment of opioid-induced constipation

Opioids are potent analgesics for treating moderate to severe pain, but their use is associated with a number of adverse effects, especially opioid-induced constipation (OIC). If the centrally mediated analgesia of opioids could be separated from their peripherally mediated gastrointestinal effects, by a peripherally acting opioid receptor antagonist, opioid-induced bowel dysfunction could be prevented or reversed. There has been considerable interest in peripherally acting opioid antagonists or other compounds to treat OIC. Subcutaneous methylnaltrexone is the first approved therapeutic agent for treatment of OIC, and studies have been conducted using the oral formulation. This editorial contains a brief overview of other selected compounds to treat OIC. Other potential uses of peripherally acting opioid antagonist in clinical practice are also discussed.

Methylnaltrexone use in a seventeen-month-old female with progressive cancer and rectal prolapse

BACKGROUND

Opioid-induced constipation is commonly seen in pediatrics, especially at the end of life. As patients clinically decline, constipation often leads to increased pain and distress, while its enteral treatment becomes more challenging.

METHOD

There is little information about the safety and use of methylnaltrexone in children. We present the case of a 17-month-old girl with progressive leukemia who was at the end of her life and whose severe opioid-induced constipation and rectal prolapse was successfully treated with the μ-opioid-receptor antagonist methylnaltrexone. We selected a lower dose based on our lack of experience with methylnaltrexone in this age group and concern for potential complications given her rectal prolapse.

DISCUSSION

Opioid-induced constipation, abdominal distention, and rectal prolapse caused our patient's most distressing symptoms, even in the context of advanced cancer. A single dose of subcutaneous methylnaltrexone (0.12 mg/kg) resolved her constipation and rectal prolapse within one hour. Although evidence is limited, the drug has successfully been used in pediatric patients with no reported side effects to date. We recommend its use earlier in the course of severe opioid-induced constipation in children unable to tolerate an oral laxation regimen. Prospective research is needed to establish the parameters for use of this effective agent in children who cannot tolerate other regimens.

Colonic motility-promoting agents for treatment of chronic constipation

Chronic constipation (CC) is one of the most common functional gastroin testinal disorders. Possible etiologies for CC include alterations in gastrointestinal motility and secretion. Research efforts in CC have led to the identification of multifactorial and often overlapping etiologies including abnormalities in myenteric neurons, alterations in neurotransmitters and their receptors, and incoordination of the muscles of the pelvic floor or anorectum. In this article, we review the safety and efficacy of colonic motility-promoting agents, such as 5-HT modulators, motilin agonists, chloride-channel activators, opioid antagonists, uroguanylin and GC-2C receptor agonists, in the management of CC.

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.

Methods for the assessment of small-bowel and colonic transit

Transit assessment of the small intestine and colon is relevant in the study of physiology, pathophysiology, and pharmacodynamics, and there is increasing use of small-bowel and colonic transit measurements in clinical practice as well. The main methods that are applied in clinical practice are substrate-hydrogen breath tests for small-bowel transit and radiopaque markers for colonic transit. Over the past 2-3 decades, scintigraphy has become the preferred standard in research studies, particularly for studies of pathophysiology and pharmacodynamics. New approaches include experimental stable isotope measurement of orocecal transit and the recently approved method using a wireless motility capsule that is validated as an accurate measurement of small-bowel and colonic transit.

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

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

Methylnaltrexone in treatment of opioid-induced constipation in a pediatric patient

OBJECTIVES

Opioid-induced constipation persists as a challenge in the management of chronic pain treated with opioid therapy. Multiple opioid antagonists have been applied in attempt to combat the gastrointestinal side effects of opioid analgesia, however their lipid-soluble nature allows passage into the central nervous system and consequent antagonism of centrally mediated analgesia. In contrast, methylnaltrexone offers the advantage of peripheral receptor-specific opioid antagonism due to chemical alterations conferring greater polarity and less lipid solubility. We present use of enteral methylnatrexone to treat severe opioid-induced constipation in a young boy who had failed treatment with the non-specific opioid antagonist, naloxone. This case reports describes the safe transition from enteral naloxone to enteral methylnaltrexone and discusses the potential risk of relative opioid toxicity during the transition.

METHODS

Though methylnaltrexone has approved for subcutaneous use, the characteristics of the patient s disease required enteral administration which had not been described in pediatric dosing. Based on conservative extrapolation of data from adult dosing, a methylnaltrexone dosing regimen was selected and the naloxone was weaned over two days in an effort to avoid a relative opioid overdose.

RESULTS

The patient was successfully transitioned to methylnaltrexone from naloxone over two days. He did experience increased sedation during this time however no severe respiratory depression occurred due to the cessation of chronic central opioid antagonism causing a relative opioid toxicity. Following the institution of methylnaltrexone, his opioid requirement significantly decreased and his gastrointestinal symptoms improved.

DISCUSSION

Our case report demonstrates safe transition from enteral naloxone to enteral methylnaltrexone in a pediatric patient, avoiding the serious consequences of relative opioid toxicity. This patient experienced significant improvement of opioid-induced constipation and reduction in opioid requirements and it is possible that other patients would benefit as well. The role of enteral methylnaltrexone deserves further investigation.

The impact of opioid analgesics on the gastrointestinal tract function and the current management possibilities

Opioid-induced bowel dysfunction (OIBD) comprises gastrointestinal symptoms such as constipation, anorexia, nausea, vomiting, gastro-oesophageal reflux, delayed digestion, abdominal pain, bloating, hard stool and incomplete evacuation that significantly deteriorate patients' quality of life and compliance. Approximately one third of patients treated with opioids do not adhere to the opioid regimen or simply quit the treatment due to OIBD. Several strategies are undertaken to prevent or treat OIBD. Traditional oral laxatives are used but their effectiveness is limited and they display adverse effects. Other possibilities comprise opioid switch or changing the administration route. New therapies target opioid receptors in the gut that seem to be the main source of OIBD. One is a combination of an opioid and opioid antagonist (oxycodone/naloxone) in prolonged-release tablets, and another is a purely peripherally acting opioid receptor antagonist (methylnaltrexone) available in subcutaneous injections. The aim of this article is to review the pathomechanism and possible treatment strategies of OIBD.

Opioid-induced bowel dysfunction

OPINION STATEMENT

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

Quantitative determination of methylnaltrexone in human serum using liquid chromatography-tandem mass spectrometry

Methylnaltrexone (MNTX) is a novel peripherally acting μ-opioid antagonist that prevents peripheral side effects of opioid drugs such as constipation without affecting the analgesia. We developed a selective and sensitive assay to measure MTNX concentrations in human serum. The drug was measured after protein precipitation with perchloric acid using naltrexone as internal standard and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for detection. The chromatography was performed isocratically on a RP18 column using 25 mM ammonium acetate buffer (pH 4)/acetonitrile (90%/10%; flow rate 200 μl/min) as mobile phase. The MS/MS analysis was performed in positive ionization mode monitoring the m/z transitions 356.4/284.2 for MNTX and 342.4/324.2 for naltrexone. The method was validated according to selectivity, linearity, accuracy, precision, recovery, matrix effects and stability. The validation range for MNTX in serum was 0.5-250 ng/ml. The developed LC-MS/MS was shown to be valid and successfully applied to measure serum-concentration-time curves of MNTX in a pilot study in healthy volunteers.

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.

Opioid-induced constipation: challenges and therapeutic opportunities

There has been an alarming increase in the prescription of opiates and opioids for chronic non-cancer pain in the past 15 years. It is estimated that opiate-induced constipation (OIC) is experienced by ~40% of these patients, and that constipation and other gastrointestinal symptoms may dissuade patients from using the required analgesic dose to achieve effective pain relief. Opiates have several effects on gastrointestinal functions, and the inhibition of colonic transit and intestinal and colonic secretion results in constipation. Several different pharmacological approaches are being developed to prevent or treat OIC: prolonged release formulations that contain naloxone (a less specific opiate antagonist that is widely distributed) and a new class of peripherally restricted μ-opiate receptor antagonists, including methylnaltrexone, alvimopan, tapentadol, NKTR-118, and TD-1211. Novel patient response outcomes have been developed to facilitate demonstration of efficacy and safety of drugs in development for OIC.

Dexmedetomidine inhibits gastric emptying and oro-caecal transit in healthy volunteers

BACKGROUND

Dexmedetomidine is a potent and selective α2-adrenoceptor agonist used for perioperative and intensive care sedation with certain beneficial qualities. However, based on preclinical observations, it might inhibit gastric emptying and gastrointestinal transit, which could result in unwanted effects in intensive care patients. This study evaluated the effects of dexmedetomidine on gastric emptying and oro-caecal transit time in healthy volunteers.

METHODS

Twelve healthy male subjects were given 1 µg kg(-1) of dexmedetomidine i.v. over 20 min followed by a continuous i.v. infusion of 0.7 µg kg(-1) h(-1) for 190 min. For comparison, subjects were also given 0.10 mg kg(-1) of morphine hydrochloride i.v. over 20 min and a placebo infusion in a randomized order. Gastric emptying was assessed with the paracetamol absorption test and oro-caecal transit time with the hydrogen breath test.

RESULTS

The time to maximum paracetamol concentration in plasma was significantly longer, maximum paracetamol concentration was significantly lower, the area under the plasma paracetamol concentration-time curve was significantly smaller, and oro-caecal transit time was significantly longer during dexmedetomidine infusion compared with morphine or placebo infusion.

CONCLUSIONS

Dexmedetomidine markedly inhibits gastric emptying and gastrointestinal transit in healthy volunteers.

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.

Methylnaltrexone

Methylnaltrexone is a selective mu-opioid receptor antagonist that has restricted ability to cross the blood-brain barrier, thus enabling reversal of opioid-induced peripheral effects, such as constipation, without affecting the central effects, such as pain relief. Treatment with subcutaneous methylnaltrexone 0.15-0.30 mg/kg, relative to placebo, significantly increased the rescue-free laxation response rate within 4 hours of the first dose (primary endpoint) in adult patients with opioid-induced constipation and advanced illness in two randomized, double-blind, placebo-controlled, multicentre, phase III studies; one was a single-dose study (n = 154), the other a multiple-dose study (n = 133). In the multiple-dose study, rescue-free laxation response rates within 4 hours after at least two of the first four doses (coprimary endpoint) were also significantly higher in methylnaltrexone recipients than in placebo recipients. Moreover, median time to laxation after the first dose was significantly shorter in methylnaltrexone recipients than in placebo recipients in both studies. Methylnaltrexone was not associated with any significant changes in pain scores or central opioid withdrawal in these studies. Methylnaltrexone was generally well tolerated in clinical trials; most adverse events were of mild to moderate severity.

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.

Pharmacological management of constipation

The approach of this review is to give a pragmatic approach to using laxatives, based on a combination of what is known about mechanism of action and the available literature on evidence.

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: 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.

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 Pharmacotherapy

Many unique and clinically important medications were approved by the Food and Drug Administration from December 2007 through May 2008 for various conditions encountered in an internal medicine setting. These new treatments dramatically vary in their targeted body system and include agents for the cardiovascular system (nebivolol), central nervous system (desvenlafaxine), gastrointestinal tract (certolizumab, methylnaltrexone, and alvimopan), immunological function (etravirine), and metabolic function (sapropterin). This article discusses medications by their respective body system. Each review is comprised of an overview of the Food and Drug Administration–approved indication and the drug’s role in treatment of that disease state. Current dosing guidance, clinical efficacy and clinically relevant adverse drug reactions, drug interactions, contraindications, and precautions are also presented. This review is designed to focus on the new molecular entities and biological approvals clinicians may potentially encounter in an internal medicine practice.

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.