• Gordon Munro, Camilla A E Baek, Helle K Erichsen, Alexander N Nielsen, Elsebet Ø Nielsen, Jørgen Scheel-Kruger, Pia Weikop, and Dan Peters.
• NeuroSearch A/S, 93 Pederstrupvej, DK-2750 Ballerup, Denmark. gmu@neurosearch.dk
• Neuropharmacology. 2008 Feb 1;54(2):331-43.

AbstractTramadol is an atypical analgesic with a unique dual mechanism of action. It acts on monoamine transporters to inhibit reuptake of noradrenaline (NA) and serotonin (5-HT), and consequent upon metabolism, displays potent agonist activity at micro-opioid receptors. Here, we present data for the novel compound NS7051, which was shown to have sub-micromolar affinity (Ki=0.034microM) for micro-opioid receptors and inhibited reuptake of 5-HT, NA and DA (IC(50)=4.2, 3.3 and 3.5microM in cortex, hippocampus and striatum respectively). NS7051 (1-30mg/kg, s.c.) produced a dose-dependent naloxone-reversible increase in the hot plate withdrawal latency, and was also analgesic in the tail flick test. In models of persistent and chronic inflammatory nociception, NS7051 reversed flinching behaviours during interphase and second phase of the formalin test (ED(50)=1.7 and 1.8mg/kg, s.c.), and hindpaw weight-bearing deficits induced by complete Freund's adjuvant injection (ED50=1.2mg/kg, s.c.). In the chronic constriction injury model of neuropathic pain, mechanical allodynia and hyperalgesia were both reversed by NS7051 (ED50=6.7 and 4.9mg/kg, s.c.). Tramadol was also active in all pain models although at considerably higher doses (20-160mg/kg, s.c.). No ataxia was observed at antiallodynic doses giving therapeutic indices of 19 and 3 for NS7051 and tramadol. The combined opioid receptor agonism and monoamine reuptake inhibitory properties of NS7051 inferred from behavioural studies appear to contribute to its well tolerated antinociceptive profile in rats. However, unlike tramadol this did not correlate with the ability to increase hippocampal monoamine levels measured by microdialysis in anesthetised rats.

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