Abstract

Based on binding, functional, and pharmacological data, this study introduces SR147778 [5-(4-bromophenyl)-1-(2,4-dichloro-phenyl)-4-ethyl-N-(1-piperidinyl)-1H-pyrazole-3-carboxamide] as a highly potent, selective, and orally active antagonist for the CB1 receptor. This compound displays nanomolar affinity (K_i = 0.56 and 3.5 nM) for both the rat brain and human CB1 recombinant receptors, respectively. It has low affinity (K_i = 400 nM) for both the rat spleen and human CB2 receptors. Furthermore, it shows no affinity for any of the over 100 targets investigated (IC₅₀ > 1 μM). In vitro, SR147778 antagonizes the inhibitory effects of CP 55,940 [(1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol] on both the mouse vas deferens contractions (pA₂ value = 8.1) and on forskolin-stimulated adenylyl cyclase activity in the U373 MG cell lines (pA₂ value = 8.2) but not in Chinese hamster ovary (CHO) cells permanently expressing the human peripheral cannabinoid receptor (hCB2). SR147778 is able to block the mitogen-activated protein kinase activity induced by CP 55,940 in the CHO cell line expressing human brain cannabinoid receptor (IC₅₀ = 9.6 nM) but was inactive in cells expressing hCB2. After oral administration, SR147778 displaced the ex vivo [³H]-CP 55,940 binding to mouse brain membranes (ED₅₀ = 3.8 mg/kg) with a long duration of action, whereas it did not interact with the CB2 receptor expressed in the mouse spleen. Using different routes of administration, SR147778 (0.3–3 mg/kg) is shown to antagonize pharmacological effects (hypothermia, analgesia, and gastrointestinal transit) induced by R-(+)-(2,3-dihydro-5-methyl-3-[{4-morpholinyl}methyl] pyrol [1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl) methanone in mice. Finally, per se, SR147778 (0.3–10 mg/kg) is able to reduce ethanol or sucrose consumption in mice and rats and food intake in fasted and nondeprived rats.