Alvimopan dihydrate (ADL 8-2698 dihydrate) is a potent, selective, orally active and reversible u-opioid receptor antagonist, with an IC 50 of 1.7 nM. Alvimopan dihydrate is highly selective for u-opioid receptor ( Ki =0.47 nM) over kappa- and delta-opioid receptors ( Kis =100, 12 nM, respectively). Alvimopan dihydrate can be used for the research of postoperative ileus (1) (2) (3).