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 mu-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.