Affinity, potency and efficacy of tramadol and its metabolites at the cloned human mu-opioid receptor

Naunyn Schmiedebergs Arch Pharmacol. 2000 Aug;362(2):116-21. doi: 10.1007/s002100000266.


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.

MeSH terms

  • Analgesics, Opioid / metabolism
  • Analgesics, Opioid / pharmacology*
  • Animals
  • Binding, Competitive / drug effects
  • CHO Cells
  • Cloning, Molecular
  • Cricetinae
  • Guanosine 5'-O-(3-Thiotriphosphate) / metabolism
  • Humans
  • Naloxone / metabolism
  • Narcotic Antagonists / metabolism
  • Receptors, Opioid, mu / drug effects*
  • Tramadol / metabolism
  • Tramadol / pharmacology*


  • Analgesics, Opioid
  • Narcotic Antagonists
  • Receptors, Opioid, mu
  • Naloxone
  • Guanosine 5'-O-(3-Thiotriphosphate)
  • Tramadol