• Laura Diego, Rabia Atayee, Pieter Helmons, and Charles F von Gunten.
• Universitat Pompeu Fabra, Department of Experimental and Health Sciences, Edifici Parc de Recerca Biomèdica de Barcelona, Carrer Dr. Aiguader, 88. 08003 Barcelona, Spain. ldiegod@sefap.org
• Expert Rev Gastroenterol Hepatol. 2009 Oct 1;3(5):473-85.

AbstractIn 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.

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