• Yakov Rotshteyn, Thomas A Boyd, and Chun-Su Yuan.
• Progenics Pharmaceuticals, 777 Old Saw Mill River Road, Tarrytown, New York, NY 10591, USA.
• Expert Opin Drug Metab Toxicol. 2011 Feb 1;7(2):227-35.

IntroductionOpioid-induced constipation is a major side effect of the use of opioid pain medications in a palliative care population. At present, the only approved treatment for opioid-induced constipation is methylnaltrexone bromide subcutaneous injection. Methylnaltrexone is a peripherally restricted opioid antagonist with μ-opioid receptor selectivity that can reduce opioid activity in peripheral organs such as the gastrointestinal tract while sparing the pain relief afforded by the pain medications.Areas CoveredThis article addresses the pharmacokinetics of parenterally administered methylnaltrexone, including the studies in humans that form the basis for our understanding, and information on the disposition, metabolism and elimination of the drug. From this review, the reader will gain an understanding of the body's handling of methylnaltrexone following intravenous or subcutaneous administration.Expert OpinionStudies conducted to date indicate that methylnaltrexone has high bioavailability after subcutaneous administration at therapeutic dose levels, a terminal half-life of ∼ 8 - 9 h, minimal metabolism, elimination involving renal and non-renal routes, and a limited potential for drug-drug interactions. Combined with high efficacy and good tolerability, the predictable pharmacokinetic behavior of methylnaltrexone facilitates its successful utilization in clinical practice for the treatment of opioid-induced constipation in patients with advanced medical illness.

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