Methylnaltrexone bromide is a selective, peripherally acting antagonist of opioid binding at the mu-opioid receptor.
As a quaternary amine, the ability of methylnaltrexone to cross the blood-brain barrier is restricted, allowing methylnaltrexone to function in tissues such as the gastrointestinal tract, without impacting opioid-mediated analgesic effects in the central nervous system.
In one study1, 154 patients were randomised to receive a single dose of methylnaltrexone or placebo. Rescue-free laxation* within 4 hours occurred in 62% and 58% in the methylnaltrexone 0.15mg/kg and 0.30mg/kg groups, respectively. In the placebo group, 14% of patients had rescue-free laxation within 4 hours (p<0.0001).
In another study2 133 patients were randomised to receive methylnaltrexone 0.15mg/kg or placebo every other day for 2 weeks. In the methylnaltrexone group, 48% of patients had laxation within 4 hours after the first study dose, as compared with 15% in the placebo group.
* A bowel movement without the use of additional laxatives
1. Poster presentation: Wellman C, Wilson, G, Israel, R. Methylnaltrexone in the treatment of opioid-induced constipation in patients with advanced illness: Results of two phase 3 trials.
2. Thomas J, Karver S, Cooney G et al. Methylnaltrexone for opioid-induced constipation in advanced illness. NEJM 2008: 358; 2332-43.
Further information: Wyeth