Methylnaltrexone Bromide (Synonyms: MRZ 2663BR, Naltrexone methylbromide)

Methylnaltrexone Bromide is an antagonist of mu-opioid receptors (MOR) with IC50 value of 75nM.
MOR are a class of opioid receptors with high affinity for enkephalins and beta-endorphin but low affinity for dynorphins. Activation of MOR in the intestinal tract inhibits peristaltic action which causes constipation [1].
In patients with constipation, Methylnaltrexone Bromide plus standard care relieved the constipation symptoms with less time (38 days) and improved quality life of 0.02 years or 8.8 days [1]. Methylnaltrexone is a novel MOR antagonist, which does not pass the blood brain barrier. In the first two weeks of usage, Methylnaltrexone Bromide caused laxation in less than 24 hours for at least half of those patients without impairing pain control or causing serious adverse effects. In phase II studies, all 22 patients had fast laxation [2].
References:
[1]. Earnshaw SR, Klok RM, Iyer S, et al. Methylnaltrexone bromide for the treatment of opioid-induced constipation in patients with advanced illness--a cost-effectiveness analysis. Aliment Pharmacol Ther, 2010, 31(8): 911-921.
[2]. Bader S, Jaroslawski K, Blum HE, et al. Opioid-induced constipation in advanced illness: safety and efficacy of methylnaltrexone bromide. Clin Med Insights Oncol, 2011, 5: 201-211.