A peripherally acting antagonist of the μ-opioid receptor (Ki = 7.42 nM; pA2 = 7.95); selective for the μ-opioid receptor over the δ-opioid receptor (Ki = 866 nM); acts as a partial agonist of κ-opioid receptors in vitro (EC50 = 47 nM for [35S]GTPγS binding); reverses morphine-induced decreases in gastrointestinal motility and analgesia in a hot-plate assay in rats (ED50s = 23.1 and 55.4 mg/kg, respectively); exhibits minimal brain uptake in a rat brain perfusion model