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

Systematic review with meta-analysis: efficacy and safety of treatments for opioid-induced constipation

BACKGROUND

When opioid-induced constipation is treated with centrally acting opioid antagonists, there may be opioid withdrawal or aggravation of pain due to inhibition of μ-opioid analgesia. This led to the development of peripherally acting μ-opioid receptor antagonists (PAMORAs).

AIM

To evaluate the efficacy of available PAMORAs and other approved or experimental treatments for relieving constipation in patients with opioid-induced constipation, based on a systematic review and meta-analysis of published studies.

METHODS

A search of MEDLINE, EMBASE and EBM Reviews Cochrane Central Register of Controlled Trials was completed in July 2019 for randomised trials compared to placebo. FDA approved doses or highest studied dose was evaluated. Efficacy was based on diverse endpoints, including continuous variables (the bowel function index, number of spontaneous bowel movements and stool consistency based on Bristol Stool Form Scale), or responder analysis (combination of >3 spontaneous bowel movements or complete spontaneous bowel movements plus 1 spontaneous bowel movement or complete spontaneous bowel movements, respectively, over baseline [so-called FDA endpoints]). Adverse effects evaluated included central opioid withdrawal, serious adverse events, abdominal pain and diarrhoea.

RESULTS

We included 35 trials at low risk of bias enrolling 13 566 patients. All PAMORAs demonstrated efficacy on diverse patient response endpoints. There was greater efficacy with approved doses of the PAMORAs (methylnaltrexone, naloxegol and naldemidine), with lower efficacy or lower efficacy and greater adverse effects with combination oxycodone with naloxone, lubiprostone and linaclotide.

CONCLUSIONS

Therapeutic response in opioid-induced constipation is best achieved with the PAMORAs, methylnaltrexone, naloxegol and naldemidine, which are associated with low risk of serious adverse events.

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.

Integrated Palliative Care and Oncologic Care in Non-Small-Cell Lung Cancer

OPINION STATEMENT

Palliative care integrated into standard medical oncologic care will transform the way we approach and practice oncologic care. Integration of appropriate components of palliative care into oncologic treatment using a pathway-based approach will be described in this review. Care pathways build on disease status (early, locally advanced, advanced) as well as patient and family needs. This allows for an individualized approach to care and is the best means for proactive screening, assessment, and intervention, to ensure that all palliative care needs are met throughout the continuum of care. Components of palliative care that will be discussed include assessment of physical symptoms, psychosocial distress, and spiritual distress. Specific components of these should be integrated based on disease trajectory, as well as clinical assessment. Palliative care should also include family and caregiver education, training, and support, from diagnosis through survivorship and end of life. Effective integration of palliative care interventions have the potential to impact quality of life and longevity for patients, as well as improve caregiver outcomes.

The effect of methylnaltrexone on the side effects of intrathecal morphine after orthopedic surgery under spinal anesthesia

Methylnaltrexone is a peripheral opioid receptor antagonist that does not cross the blood-brain barrier; so without interference with pain relief, it could reverse the peripheral opioid side effects such as constipation, pruritus, postoperative ileus, and urinary retention. This study has been designed to evaluate the effect of methylnaltrexone on postoperative side effects of intrathecal morphine. In seventy-two 18- to 55-year-old patients scheduled for elective orthopedic operations under spinal anesthesia, neuraxial blockade was achieved using 10 mg 0.5% hyperbaric bupivacaine and 0.1 mg preservative-free morphine sulfate. The first group (M) received 12 mg methylnaltrexone, while the second group (P) received normal saline, subcutaneously, immediately after spinal block in a randomized, double-blind fashion. There was a significant decrease in the rate of nausea and vomiting in group M, but there was no significant difference in the rate of pruritus or urinary retention between the two groups. Pain score was significantly lower in group M. Respiratory depression or decreased level of consciousness was not reported in any patient. Subcutaneous administration of methylnaltrexone was not effective in decreasing postoperative urinary retention and pruritus, but lowered the rate of nausea and vomiting and pain score after intrathecal bupivacaine and morphine.

Opioid-induced bowel dysfunction

Opioid-induced bowel dysfunction (OIBD) is a potentially debilitating side effect of chronic opioid use. It refers to a collection of primarily gastrointestinal motility disorders induced by opioids, of which opioid-induced constipation (OIC) is the most common. Management of OIBD is difficult, and affected patients will often limit their opioid intake at the expense of experiencing more pain, to reduce the negative impact of OIBD on their quality of life. Effective pharmacologic therapy for OIC is considered an unmet need and several agents have recently been given priority review and approval for OIC. Furthermore, multiple agents currently in development show promise in treating OIC without significant impact on analgesia or precipitation of withdrawal symptoms. The approval and availability of such medications would represent a significant improvement in the management of OIC and OIBD in patients with chronic pain.

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.

Clinical management of pain in advanced lung cancer

Lung cancer is the most common cancer in the world and pain is its most common symptom. Pain can be brought about by several different causes including local effects of the tumor, regional or distant spread of the tumor, or from anti-cancer treatment. Patients with lung cancer experience more symptom distress than patients with other types of cancer. Symptoms such as pain may be associated with worsening of other symptoms and may affect quality of life. Pain management adheres to the principles set out by the World Health Organization's analgesic ladder along with adjuvant analgesics. As pain can be caused by multiple factors, its treatment requires pharmacological and non-pharmacological measures from a multidisciplinary team linked in with specialist palliative pain management. This review article examines pain management in lung cancer.

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.

Subcutaneous methylnaltrexone for treatment of acute opioid-induced constipation: phase 2 study in rehabilitation after orthopedic surgery

BACKGROUND

Methylnaltrexone has been shown to be effective for treating opioid-induced constipation (OIC) in chronic settings, but its effects on acute OIC have not been studied.

OBJECTIVE

To assess safety and efficacy of subcutaneous methylnaltrexone in patients with acute OIC after orthopedic procedures.

DESIGN

Double-blind, randomized, parallel-group, placebo-controlled, hypothesis-generating phase 2 study.

SETTING

Sixteen US hospitals and rehabilitation facilities.

PATIENTS

Adult patients with acute OIC after orthopedic surgical procedure, expected to require opioids for at least 7 days postrandomization.

INTERVENTIONS

Patients received once-daily subcutaneous methylnaltrexone 12 mg or placebo for up to 4 or 7 days.

MEASUREMENTS

All endpoints were exploratory and included the percentage of patients achieving laxation within 2 and 4 hours of first dose and time to laxation.

RESULTS

Thirty-three patients received at least 1 dose of study drug (methylnaltrexone, n = 18; placebo, n = 15). Within 2 and 4 hours, significantly more patients receiving methylnaltrexone achieved laxation (2 hours: 33.3% vs 0%, P = 0.021; 4 hours: 38.9% vs 6.7%, P = 0.046) compared with placebo. Time to laxation was significantly shorter with methylnaltrexone (median = 15.8 hours) versus placebo (median = 50.9 hours), P = 0.0197. The most common adverse events related to the gastrointestinal tract. Pain scores remained stable and were similar to those of placebo, and signs and symptoms of opioid withdrawal did not emerge in patients receiving methylnaltrexone.

CONCLUSIONS

Methylnaltrexone was generally well tolerated and was active in inducing laxation in this study of patients experiencing acute OIC following orthopedic surgery.

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.

Novel opioid antagonists for opioid-induced bowel dysfunction

INTRODUCTION

Adverse effects frequently limit the therapeutic benefits of opioid analgesics. Gastrointestinal adverse effects are common, burdensome, and can compromise the quality of life. It is estimated that up to 81% of patients still report constipation despite regular use of laxatives. Thus, the development of opioid antagonists that selectively target receptors in the gut without affecting central analgesia has provided new perspectives on the treatment of opioid-induced gastrointestinal adverse effects.

AREAS COVERED

In this paper, we review the pathophysiology, prevalence, and burden of opioid-induced bowel dysfunction (OBD). In addition, this study aims to provide a better understanding of the mechanism of action and reviews the efficacy, safety and the latest research on novel opioid antagonists for OBD.

EXPERT OPINION

Two strategies effectively relieve OBD without interfering with centrally mediated analgesia: the administration of opioid antagonists with limited systemic absorption and peripherally acting mu-opioid receptor antagonists (PAMORA) that selectively target mu-receptors in the gastrointestinal tract. Methylnaltrexone and alvimopan are two recently marketed PAMORA and provide a new mechanism-based approach for the treatment of opioid-induced gastrointestinal dysfunction. However, its use in clinical practice is limited by various reasons such as its relatively low response rates and higher costs. Nevertheless, at least four new oral PAMORA (NKTR-118, TD-1211, ADL-7445, and ADL-5945) are under clinical development, further expanding the possibilities for a new paradigm for OBD management.

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.

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.

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. 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.

Methylnaltrexone treatment of opioid-induced constipation in patients with advanced illness

Methylnaltrexone, a peripherally acting mu-opioid receptor antagonist with restricted ability to cross the blood-brain barrier, reverses opioid-induced constipation (OIC) without affecting analgesia. A double-blind study in patients with advanced illness and OIC demonstrated that methylnaltrexone significantly induced laxation within four hours after the first dose compared with placebo. In this study, patients with advanced illness and OIC on stable doses of opioids and laxatives were randomized to methylnaltrexone 0.15mg/kg (n=62) or placebo (n=71) subcutaneously every other day for two weeks. Laxation was assessed daily. Constipation distress, bowel status change, pain, laxative use, and opioid withdrawal symptoms were assessed weekly using standardized scales. Additional analyses to further characterize response to methylnaltrexone revealed that among patients with a bowel movement within four hours following the first dose, the median time to response was 0.5 hours for methylnaltrexone. Response rates among methylnaltrexone-treated patients who had responded to all previous doses were 57%-100% for doses two to seven. Among methylnaltrexone-treated patients who did not respond to the first or to the first two consecutive doses, 35% and 26% responded to the second and third dose, respectively. Higher percentages of patients and clinicians rated bowel status as improved in the methylnaltrexone than the placebo group. Fewer methylnaltrexone than placebo patients reported use of common laxative types, particularly enemas, during the study. Subcutaneous methylnaltrexone promptly and predictably induced laxation, improved constipation distress, and was associated with less laxative use in patients with advanced illness and OIC.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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

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

Use of FDA approved methamphetamine to allow adjunctive use of methylnaltrexone to mediate core anti-growth factor signaling effects in glioblastoma

Methylnaltrexone (MNTX) was recently FDA approved to treat opiate induced constipation. It happens to also indirectly reduce Src activity. Src is a 54 kDa tyrosine kinase, crucial in signaling of, and link between, vascular endothelial growth factor (VEGF), and epidermal growth factor (EGF). Glioblastomas use both EGF and VEGF signaling to enhance growth and neo-angiogenesis. Stem cell sub-fractions of glioblastomas are enriched for high VEGF synthesizing cells so this is a particularly valuable adjunctive target during cytotoxic treatment with drugs like temozolomide. MNTX does not cross the blood-brain barrier (BBB). Methamphetamine (MA) temporarily opens the BBB and therefore may allow methylnaltrexone entry into glioblastoma tissue. MA is FDA approved, marketed to treat attention problems in children. MA-MNTX combination should be tested as glioblastoma treatment adjunct. Temozolomide CSF levels are 10-20% of blood levels. Thus MA may also allow greater brain tissue temozolomide levels yet with lower systemic exposure.

Motility disorders in the ICU: recent therapeutic options and clinical practice

PURPOSE OF REVIEW

Motility disturbances often occur in critically ill patients resulting in an increased rate of morbidity and mortality. Only limited options for treatment of gastrointestinal dysfunction have been introduced. Factors contributing to motility disorders in the ICU patient, and recent therapeutic approaches are reviewed in the following.

RECENT FINDINGS

Despite the growing use of early enteral nutrition in the ICU and improvements in patients' outcome, feed intolerance and motility disorders in critical illness remain unsolved. Evaluation of pathophysiological patterns such as antro-pyloric dysfunction has led to a better knowledge of gut function, whereas development of new prokinetic agents is scarce, and enthusiasm has been cut by the withdrawal of some propulsive agents from the market.

SUMMARY

The complexity of gastrointestinal motor function poses a challenge to the pharmacological modulation of gut motility. There has been progress in the understanding of pathophysiologic patterns, whereas therapeutic options are still rare. Metoclopramide and erythromycin are the best evaluated and still the most promising prokinetic agents. Only a few studies in critical illness are available, and the definite value of novel propulsive agents such as motilin agonists and mu-receptor antagonists is unclear due to small patient populations. The most reasonable approach of motility disorders in critical illness seems to be an individual assessment of all associated risk factors combined with early enteral nutrition and use of prokinetic agents.

Reversal of opioid-induced gastric dysfunction in a critically ill burn patient after methylnaltrexone

Peripheral-acting mu opiate receptor antagonists have been extensively studied for the treatment of opiate-induced constipation in advanced illness for the prophylaxis of postoperative ileus. We document the first intensive care patient to receive methylnaltrexone in an attempt to facilitate enteral nutrition. Gastric residuals markedly decreased and enteral feeding increased after administration of i.v. methylnaltrexone. The patient's ileus resolved coincident with the first injection.

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.

Natural products to drugs: natural product-derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or are in registration (as at 31st December 2007) have been reviewed, as well as natural product-derived compounds for which clinical trials have been halted or discontinued since 2005. Also discussed are natural product-derived drugs launched since 2005, new natural product templates and late-stage development candidates.