title: Affinity, potency and efficacy of tramadol and its metabolites at the cloned human mu-opioid receptor.
abstract: The present study was conducted to characterise the centrally active analgesic drug tramadol hydrochloride [(1RS,2RS)-2-[(dimethyl-amino)-methyl]-1-(3-methoxyphenyl)-cyclohe xanol hydrochloride] and its metabolites M1, M2, M3, M4 and M5 at the cloned human mu-opioid receptor. Membranes from stably transfected Chinese hamster ovary (CHO) cells were used to determine the four parameters of the ligand-receptor interaction: the affinity of (+/-)-tramadol and its metabolites was determined by competitive inhibition of [3H]naloxone binding under high and low salt conditions. The agonist-induced stimulation of [35S]GTPgammaS binding permits the measurement of potency (EC50), efficacy (Emax = maximal stimulation) and relative intrinsic efficacy (effect as a function of receptor occupation). The metabolite (+)-M1 showed the highest affinity (Ki=3.4 nM) to the human mu-opioid receptor, followed by (+/-)-M5 (Ki=100 nM), (-)-M1 (Ki=240 nM) and (+/-)-tramadol (Ki=2.4 microM). The [35S]GTPgammaS binding assay revealed an agonistic activity for the metabolites (+)-M1, (-)-M1 and (+/-)-M5 with the following rank order of intrinsic efficacy: (+)-M1>(+/-)-M5>(-)-M1. The metabolites (+/-)-M2, (+/-)-M3 and (+/-)-M4 displayed only weak affinity (Ki> 10 microM) and had no stimulatory effect on GTPgammaS binding. These data indicate that the metabolite (+)-M1 is responsible for the mu-opioid-derived analgesic effect.
authors.affiliation: Department of Molecular Pharmacology, Grünenthal GmbH, Aachen, Germany. Clemens.Gillen@grunenthal.de
authors.forename: C
authors.initials: C
authors.lastname: Gillen
authors.forename: M
authors.initials: M
authors.lastname: Haurand
authors.forename: D J
authors.initials: DJ
authors.lastname: Kobelt
authors.forename: S
authors.initials: S
authors.lastname: Wnendt
mesh_terms: D000701:Analgesics, Opioid / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D016466:CHO Cells; D003001:Cloning, Molecular; D006224:Cricetinae; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006801:Humans; D009270:Naloxone / Q000378:metabolism; D009292:Narcotic Antagonists / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000187:drug effects*; D014147:Tramadol / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D017450:Receptors, Opioid, mu; D009270:Naloxone; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D014147:Tramadol