Assessment of Contribution of 7‐Hydroxymitragynine and Mitragynine Pseudoindoxyl to the MU‐Opioid Activity of Mitragynine

Our group has demonstrated that the primary kratom alkaloid mitragynine (MG) is metabolized in vivo into 7‐hydroxymitragynine (7‐OH‐MG), which is in turn converted into MG pseudoindoxyl (MG‐P). Here we compared their µ‐opioid receptor (MOR) pharmacology and contribution of the two metabolites to the MOR activity of MG. I n vitro displacement of 3 HDAMGO at human MOR yielded the rank order affinity (Ki in nM) of MG‐P (1.5) > morphine (4.0) > 7‐OH‐MG (78) > MG (709). Stimulation of 35 SGTPγS binding yielded the rank order efficacy (% stimulation normalized to DAMGO) of morphine (92) > 7‐OH‐MG (46) > MG‐P (32) > MG (4.0). In rats discriminating morphine (3.2 mg/kg, i.p.) from vehicle, morphine, 7‐OH‐MG and MG‐P produced 100% drug‐lever responding; MG produced a maximum of 80% drug‐lever responding. The rank order potency (ED 50 s in µmol/kg, i.p.) was MG‐P (0.47) > 7‐OH‐MG (0.66) > morphine (2.4) > MG (35). Rank order potency to decrease rates of responding was 7‐OH‐MG (11) > morphine (18) > MG‐P (24) > MG (87). Using hotplate (52°C) response latency to assess antinociception, the % maximum possible effects (MPEs; corresponding ED 50 s in µmol/kg, i.p.) were 93% (46) for morphine, 7% for MG (tested up to 129 µmol/kg), 78% (26) for 7‐OH‐MG, and 23% for MG‐P (tested up to 135 µmol/kg). The opioid antagonist naltrexone (0.085 µmol/kg, i.p.) antagonized the morphine‐like effects of MG, 7‐OH‐MG and MG‐P. For a time‐course study, oral MG (409 µmol/kg) produced up to 27% MPE over 360 min, while 736 µmol/kg MG produced up to 87% MPE at 30 min after administration. However, the antinociceptive ED 50 value of MG (567 µmol/kg) was comparable to MG's LD 50 value (572 µmol/kg), which indicates non‐specific behavioral disruption rather than robust antinociception. At 30 min after administration, oral 129 µmol/kg MG produced 100% drug‐lever responding in the rats discriminating morphine from vehicle (ED 50 =97 µmol/kg). At 5 min after administration, both 7‐OH‐MG and MG‐P (each 2.4 µmol/kg, i.p.) produced >95% drug‐lever responding. Given the low in vitro intrinsic activity of MG, the low antinociceptive effects of MG might have resulted from the possibility that MG antagonized the antinociceptive effects of 7‐OH‐MG converted from MG. However, the antinociceptive dose‐effect function of 7‐OH‐MG (2.4‐43 µmol/kg, i.p.) was potentiated rather than antagonized by MG (23‐129 µmol/kg, i.p.). These results suggest that all three kratom alkaloids are MOR agonists in vivo , with the intrinsic activity of 7‐OH‐MG being greater than both MG and MG‐P. Given the fast onset of action for both 7‐OH‐MG and MG‐P, the high potency of MG‐P to produce the morphine‐like discrimination, and the minimal antinociceptive effects of MG‐P, these results also may suggest that 7‐OH‐MG as well as MG‐P converted from MG contribute to the MOR activity of MG in vivo .