Alvimopan dihydrate is a selective antagonist of opioid receptor with IC50 value of 1.7 nM.Opioid receptor is a G protein-coupled receptor with opioids as ligands.The dissociation rate of alvimopan from the micro opioid receptor (t(1/2)=30-44 min) was slower than those of the antagonists N-methylnaltrexone (t(1/2)=0.46 min) and naloxone (t(1/2)=0.82 min), and was similar to that of the long acting partial agonist buprenorphine (t(1/2)=44 min). When preincubation with the micro opioid receptor, the affinity and potency of alvimopan increased apparently, which is consistent with its long duration of action [1].Patients with opioid induced bowel dysfunction (OBD) orally received alvimopan 0.5 mg twice daily (BID), 1 mg once daily (QD), 1 mg BID, or placebo for 6 weeks. Alvimopan significantly increased spontaneous bowel movements (SBMs) frequency each week in the last three weeks with alvimopan 0.5 mg BID (+1.71 mean SBMs/week), alvimopan 1 mg QD (+1.64) and alvimopan 1 mg BID (+2.52), respectively. While, Alvimopan caused side effects such as abdominal pain, nausea, and diarrhea, which was the same as the placebo. Alvimopan restored GI function and relieved OBD without affecting analgesia [2].References:[1]. Cassel JA, Daubert JD, DeHaven RN. [(3)H]Alvimopan binding to the micro opioid receptor: comparative binding kinetics of opioid antagonists. Eur J Pharmacol, 2005, 520(1-3): 29-36.[2]. Webster L, Jansen JP, Peppin J, et al. Alvimopan, a peripherally acting mu-opioid receptor (PAM-OR) antagonist for the treatment of opioid-induced bowel dysfunction: results from a randomized, double-blind, placebo-controlled, dose-finding study in subjects taking opioids for chronic non-cancer pain. Pain, 2008, 137(2): 428-440.