title: CB2 cannabinoid receptor agonist enantiomers HU-433 and HU-308: An inverse relationship between binding affinity and biological potency.
abstract: Activation of the CB2 receptor is apparently an endogenous protective mechanism. Thus, it restrains inflammation and protects the skeleton against age-related bone loss. However, the endogenous cannabinoids, as well as Δ(9)-tetrahydrocannabinol, the main plant psychoactive constituent, activate both cannabinoid receptors, CB1 and CB2. HU-308 was among the first synthetic, selective CB2 agonists. HU-308 is antiosteoporotic and antiinflammatory. Here we show that the HU-308 enantiomer, designated HU-433, is 3-4 orders of magnitude more potent in osteoblast proliferation and osteoclast differentiation culture systems, as well as in mouse models, for the rescue of ovariectomy-induced bone loss and ear inflammation. HU-433 retains the HU-308 specificity for CB2, as shown by its failure to bind to the CB1 cannabinoid receptor, and has no activity in CB2-deficient cells and animals. Surprisingly, the CB2 binding affinity of HU-433 in terms of [(3)H]CP55,940 displacement and its effect on [(35)S]GTPγS accumulation is substantially lower compared with HU-308. A molecular-modeling analysis suggests that HU-433 and -308 have two different binding conformations within CB2, with one of them possibly responsible for the affinity difference, involving [(35)S]GTPγS and cAMP synthesis. Hence, different ligands may have different orientations relative to the same binding site. This situation questions the usefulness of universal radioligands for comparative binding studies. Moreover, orientation-targeted ligands have promising potential for the pharmacological activation of distinct processes.
authors.affiliation: Institute for Drug Research, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Reem
authors.initials: R
authors.lastname: Smoum
authors.affiliation: Bone Laboratory, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Saja
authors.initials: S
authors.lastname: Baraghithy
authors.affiliation: Institute for Drug Research, Hebrew University of Jerusalem, Jerusalem 91120, Israel; The Lise-Meitner Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Mukesh
authors.initials: M
authors.lastname: Chourasia
authors.affiliation: Institute for Drug Research, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Aviva
authors.initials: A
authors.lastname: Breuer
authors.affiliation: Bone Laboratory, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Naama
authors.initials: N
authors.lastname: Mussai
authors.affiliation: Bone Laboratory, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Malka
authors.initials: M
authors.lastname: Attar-Namdar
authors.affiliation: Bone Laboratory, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Natalya M
authors.initials: NM
authors.lastname: Kogan
authors.affiliation: Bone Laboratory, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Bitya
authors.initials: B
authors.lastname: Raphael
authors.affiliation: Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD, Scotland, United Kingdom; Molecular Pharmacology Group, Institute of Molecular, Cell and System Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom.
authors.forename: Daniele
authors.initials: D
authors.lastname: Bolognini
authors.affiliation: Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD, Scotland, United Kingdom;
authors.forename: Maria G
authors.initials: MG
authors.lastname: Cascio
authors.affiliation: Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD, Scotland, United Kingdom;
authors.forename: Pietro
authors.initials: P
authors.lastname: Marini
authors.affiliation: Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD, Scotland, United Kingdom;
authors.forename: Roger G
authors.initials: RG
authors.lastname: Pertwee
authors.affiliation: Institute for Drug Research, Hebrew University of Jerusalem, Jerusalem 91120, Israel; The Lise-Meitner Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Avital
authors.initials: A
authors.lastname: Shurki
authors.affiliation: Institute for Drug Research, Hebrew University of Jerusalem, Jerusalem 91120, Israel; raphaelm@ekmd.huji.ac.il.
authors.forename: Raphael
authors.initials: R
authors.lastname: Mechoulam
authors.affiliation: Bone Laboratory, Hebrew University of Jerusalem, Jerusalem 91120, Israel;
authors.forename: Itai
authors.initials: I
authors.lastname: Bab
mesh_terms: D000818:Animals; D016466:CHO Cells; D063386:Cannabinoid Receptor Agonists / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D002186:Cannabinoids / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D006224:Cricetinae; D003412:Cricetulus; D051379:Mice; D008810:Mice, Inbred C57BL; D043885:Receptor, Cannabinoid, CB2 / Q000819:agonists*; D013237:Stereoisomerism
chemical_list: D063386:Cannabinoid Receptor Agonists; D002186:Cannabinoids; D043885:Receptor, Cannabinoid, CB2; C402416:HU 308
keywords: enantiomers; endocannabinoids; osteoporosis; pose occupancy