In vitro activity: Loperamide exhibits potent affinity and selectivity for the cloned micro (Ki = 3 nM) compared with the delta (Ki = 48 nM) and kappa (Ki = 1156 nM) human opioid receptors. Loperamide potently stimulates [35S]guanosine-5'-O-(3-thio)triphosphate binding with EC50 of 56 nM, and inhibits forskolin-stimulated cAMP accumulation (IC50 = 25 nM) in Chinese hamster ovary cells transfected with the human mu opioid receptor. Loperamide potently inhibits late-phase formalin-induced flinching after intrapaw injection (A50 = 6 mg). Loperamide is a strong inhibitor of CES2, with a K(i) of 1.5 muM, but it only weakly inhibits CES1A1 (IC50 = 0.44 mM). Loperamide reversibly blocks rises in [Ca2+]i evoked by high [K+] in a concentration-dependent manner, with an IC50 of 0.9 mM. Loperamide (0.1-50 mM) produces a concentration-dependent reduction of the peak IBa with an IC50 value of 2.5 mM and, at the highest concentration tested, could fully block IBa in the absence of any other pharmacological agent. Loperamide also attenuates NMDA-evoked currents recorded at a membrane potential of -60 mV, with an IC50 of 73 mM.

J Pharmacol Exp Ther. 1999 Apr;289(1):494-502; J Pharmacol Exp Ther. 2005 Jun;313(3):1011-6; Pharmacol Biochem Behav. 2005 May;81(1):114-21.