Methylnaltrexone bromide is the bromide salt form of methylnaltrexone, a quaternary methyl derivative of noroxymorphone. The methyl group and the quaternary salt formation increase the polarity and reduce the lipid solubility thereby restricts the blood–brain-barrier penetration. Methylnaltrexone has eight times higher affinity for MOR than for κ-opioid receptor (KOR) and δ-opioid receptor (DOR). Naltrexone forms interaction with Asp147 and Tyr148 along with a hydrogen bond with Lys233.