Kappa Opioid Receptor Agonists: New Targets In The Treatment Of Pain

Background and Purpose: Pain, although necessary for survival, can become pathological affecting an estimated 1 in 5 adults globally. It is also the most common reason people seek medical attention. Mu opioid receptor (MOPr) agonists, such as morphine, are the gold standard treatment for pain. Although these drugs have excellent analgesic properties, adverse effects such as addiction, tolerance and respiratory depression make their use problematic. An estimated 10,000 New Zealanders are addicted to prescription opiates, highlighting the need for better drugs to treat pain. Kappa opioid receptor (KOPr) agonists have analgesic properties and, unlike MOPr agonists, are also anti-addictive. Unfortunately, adverse effects such as sedation and dysphoria, have limited their therapeutic potential. The discovery of KOPr agonists that have analgesic properties without inducing adverse effects can allow for better, more efficient treatments of pain. We are the first to report the analgesic potential of novel Salvinorin A (Sal A) analogues: Tetrahydropyran Salvinorin A (THP Sal A) and Mesyl Salvinorin B (Mesyl Sal B).  Experimental Approach and Compounds Tested: This study uses animal behavioural models to characterise the analgesic, anti-oedematous, sedative and hypothermic effects of a structurally new class of KOPr agonists including Sal A, THP Sal A and Mesyl Sal B. The known peripherally restricted KOPr agonist, ICI 204,448, was used to evaluate the peripherally mediated analgesic mechanisms of KOPr agonists. The tail-flick and intradermal formalin test were used to assess acute central and peripheral pain processes respectively. Sedative effects were monitored via rotarod performance test; thermoregulatory effects were also recorded.   Key Results: ICI 204,448 attenuated inflammatory pain at a dose of 1 mg/kg (P<0.05, 30 min) and 2 mg/kg (P<0.001, 30-35 min). Although it showed no centrally mediated analgesic effects, it was found to be sedative at a dose of 2 mg/kg (P<0.01, 15-60 min). Sal A (2 mg/kg) attenuated inflammatory pain (P<0.01, 25-35 min) at the same dose it was sedative (P<0.01, 2-15 min). Although it treated acute thermal pain at a non-sedative dose (1 mg/kg, P<0.001, 10-15 min), it has a short duration of action (˜15 min). THP Sal A attenuated both thermal and inflammatory pain. Unfortunately, it was also sedative at both 1 mg/kg (P<0.01, 15-45 min) and 2 mg/kg (P<0.001, 15-90 min). Mesyl Sal B significantly attenuated both central (1mg/kg, P<0.01, 30-60 min) and peripheral (2 mg/kg, P<0.01, 30 min) pain processes. Although Mesyl Sal B was found to have a weak analgesic effect in all pain assays, it was not sedative.  Conclusions and Implications: KOPr agonists attenuate acute nociceptive and inflammatory pain. Structural modification of Sal A at the C-2 position alters its analgesic effects in vivo. Substitution with a tetrahydropyran group greatly improves central analgesic effects; however, sedative effects were also observed. Although substitution with a mesylate group produced no sedative effects, it had reduced effects on central and peripheral pain processes. The lack of sedation by Mesyl Sal B makes it a good target for future research in pain. Its longer duration of action compared to Sal A suggests it has a better metabolic profile. The creation of more soluble KOPr compounds would allow for better dose-testing to evaluate therapeutic potential of KOPr analgesics.