Discovery and Pharmacological Evaluation of a Diphenethylamine Derivative (HS665), a Highly Potent and Selective κ Opioid Receptor Agonist Mariana Spetea, † Ilona P. Berzetei-Gurske, ‡ Elena Guerrieri, † and Helmut Schmidhammer* ,† † Department of Pharmaceutical Chemistry, Institute of Pharmacy and Center for Molecular Biosciences Innsbruck, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria ‡ Biosciences Division, SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025, United States * S Supporting Information ABSTRACT: Here we report on the design, synthesis, and biological characterization of novel κ opioid (KOP) receptor ligands of diphenethylamines. In opioid receptor binding and functional assays, the N-cyclobutylmethyl substituted derivative 4 (HS665) showed the highest affinity and selectivity for the KOP receptor and KOP agonist potency. Compound 4 inhibited acetic acid induced writhing after subcutaneous administration in mice via KOP receptor-mediated mechanisms, being equipotent as an analgesic to the KOP agonist U50,488. ■ INTRODUCTION The κ opioid (KOP) receptor belongs to the family of seven- transmembrane G-protein-coupled receptors (GPCRs), and it plays a significant role in a broad range of physiological functions. 1,2 Stimulation of the KOP receptor results in significant analgesia, while it is not involved in the unwanted side effects of respiratory depression, inhibition of gastro- intestinal motility, dependence, or abuse liability, as in the case of the μ opioid (MOP) receptor. 2 KOP agonists appear to have some advantages over the widely used MOP analgesics. On the other hand, the therapeutic utility of KOP agonists is associated with dose-limiting effects including dysphoria, sedation, and psychotomimetic effects. 1,2 Besides the analgesic activity, 3 KOP agonists have also shown other beneficial actions such as antipruritic, 4 antiarthritic, 5,6 anti-inflammatory, 5,6 and neuro- protective effects. 7 At present, the main classes of available chemically distinct KOP receptor agonists include peptides (e.g., dynorphin analogues), 8 benzomorphans (e.g., bremazo- cine, pentazocine), 9 morphinans (e.g., TRK-820), 9 arylaceta- mides (e.g., U50,488, U69,593), 9 diazabicyclononanones (e.g., HZ2), 9 neoclerodane diterpenes (e.g., salvinorin A), 9,10 benzodiazepines (e.g., tifluadom). 9 Several of such compounds are employed as research tools or are in clinical use. Recent reviews on small molecule and peptide ligands as agonists at the KOP receptor and their potential for drug development have been published. 8-10 Inhibiting KOP receptors has been proposed to be useful for the treatment of stress-related conditions (e.g., depression and anxiety), drug addiction, and eating disorders. 1,8,11,12 The first competitive KOP receptor antagonist was norbinaltorphimine (nor-BNI), 13 followed later by 5′ -guanidinonaltrindole (GNTI), 14 both derived from naltrexone. The structurally distinct JDTic, a trans-3,4-dimethyl-4-(3-hydroxyphenyl)- piperidine derivative, was discovered as a highly potent and selective KOP receptor antagonist. 15 Recently, the crystal structure of the human KOP receptor in complex with JDTic was reported revealing important features of the ligand-binding pocket that contribute to the high affinity and selectivity of JDTic for the human KOP receptor. 16 The pharmacology of currently available selective KOP antagonists shows a delay in onset of action and extremely long-lasting effects in vivo, which might limit their therapeutic utility. 17,18 The development of KOP receptor ligands with improved pharmacokinetic and pharmacodynamic properties and safety profile is an important direction in pharmaceutical research toward the discovery of useful clinical agents. Earlier observations were made on the dopamine D 2 receptor agonist 1 (RU 24213, Figure 1), 19 a diphenethylamine derivative, reported to display moderate affinity to KOP receptors and to act as KOP antagonist. 20 Moreover, its n-pentyl analogue 2 (Figure 1) also exhibited moderate affinity to the KOP receptor and showed KOP antagonist activity in vivo. Compound 2 antagonized diuresis and antinociceptive effects in rats after subcutaneous (sc) administration produced by the KOP receptor agonist U50,488. 21 Therefore, such simple molecules Received: August 31, 2012 Figure 1. Structures of investigated compounds. CPM, cyclo- propylmethyl; CBM, cyclobutylmethyl. Brief Article pubs.acs.org/jmc © XXXX American Chemical Society A dx.doi.org/10.1021/jm301258w | J. Med. Chem. XXXX, XXX, XXX-XXX