7‑hydroxymitragynine is an active metabolite of mitragynine and a key mediator of its analgesic effects

Mitragynina speciosa, more commonly known as kratom, is a plant native to Southeast Asia, the leaves of which have been used traditionally as a stimulant, analgesic, and treatment for opioid addiction. Recently, growing use of the plant in the United States and concerns that kratom represents an uncontrolled drug with potential abuse liability, have highlighted the need for more careful study of its pharmacological activity. The major active alkaloid found in kratom, mitragynine, has been reported to have opioid agonist and analgesic activity in vitro and in animal models, consistent with the purported effects of kratom leaf in humans. However, preliminary research has provided some evidence that mitragynine and related compounds may act as atypical opioid agonists, inducing therapeutic effects such as analgesia, while limiting the negative side effects typical of classical opioids. Here we report evidence that an active metabolite plays an important role in mediating the analgesic effects of mitragynine. We find that mitragynine is converted in vitro in both mouse and human liver preparations to the much more potent mu-opioid receptor agonist 7-hydroxymitragynine, and that this conversion is mediated by cytochrome P450 3A isoforms. Further, we show that 7-hydroxymitragynine is formed from mitragynine in mice and that brain concentrations of this metabolite are sufficient to explain most or all of the opioid-receptor-mediated analgesic activity of mitragynine. At the same time, mitragynine is found in the brains of mice at very high concentrations relative to its opioid receptor binding affinity, suggesting that it does not directly activate opioid receptors. The results presented here provide a metabolism-dependent mechanism for the analgesic effects of mitragynine and clarify the importance of route of administration for determining the activity of this compound. Further, they raise important questions about the interpretation of existing data on mitragynine and highlight critical areas for further research in animals and humans.</p

Morphine Stimulates Wound Healing Via Mu Opioid Receptor and Promotes Wound Closure in Sickle Mice

Abstract Abstract 2118 Painful non-healing leg ulcers in sickle cell disease (SCD) pose major treatment challenges for which there is no satisfactory therapy. We observed that the vascular and nerve architecture are abnormal, and mu opioid receptor (MOR) expression is decreased in the skin of BERK mice expressing sickle hemoglobin, as compared to HbA-BERK mice expressing normal human hemoglobin (Kohli et al., Blood, 2010). MOR mediates morphine-induced angiogenesis and analgesia, which are critical to the healing process. We hypothesized that MOR mediates the healing process and that MOR agonists such as morphine will promote healing by stimulating angiogenesis. Ischemic wounds (6 mm diameter) with impaired blood supply were created on the dorsal skin of sickle (BERK, and S+SAntilles) and control (HbA-BERK and C57BL6, respectively) mice; and mu-, delta-, and kappa-opioid receptor knockout (MOR-, DOR- and KOR-KO, respectively) mice and their 129S6 wild type controls. BERK and S+SAntilles did not survive ischemic wound surgery, but S+SAntilles survived non-ischemic, 4 mm punch biopsies on the leg. Wounds were treated topically twice a day with morphine, 3 mg/g Eucerin cream or with PBS-Eucerin cream. Periodic wound tracings were used to quantitate wound area with Adobe Photoshop and % area healed was calculated. Ischemic wounds (100% closure on d17 in control 129S6) took significantly longer to heal as compared to punch biopsies (100% closure on d8 in control C57BL6). Morphine significantly accelerated healing as compared to PBS-cream in control, S+SAntilles, DOR- and KOR-KO, but not in MOR-KO mice, suggesting that morphine-induced healing is MOR-dependent. Of note, PBS-treated ischemic wounds in MOR-KO healed significantly slower than 129S6 control wounds, indicating that MOR is involved in wound healing. Blood flow was significantly higher in the intact unwounded dorsal skin of MOR-KO as compared to 129S6 mice (p&lt;0.001). Significant decrease occurred after wounding in both 129S6 and MOR-KO, which was restored to baseline in morphine-treated 129S6 but not in MOR-KO wounds on d20. Thus, morphine promotes vascular function, which appears to be mediated by MOR in the healing wounds. In PBS-cream treated punch biopsy wounds, 100% wound closure occurred on day 8 as compared to day 6 with morphine treatment in both C57BL6 control and S+SAntilles sickle mice. Laser scanning confocal microscopy (LSCM) of healed wound scars 30d after wounding revealed significantly increased blood vessels (∼2-fold) in morphine-treated as compared to PBS treated in both C57BL6 and S+SAntilles mice. A significant increase was also noted in PGP 9.5-immunoreactive (ir) nerve fibers and lymphatics in morphine-treated scars. Pro-inflammatory and vaso-active neuropeptides, substance P (SP) and calcitonin gene related peptide (CGRP) were increased ∼2-fold in PBS-treated wound scars 30d post-wounding as compared to baseline in S+SAntilles mice, suggestive of increased neurogenic inflammation. However both neuropeptides returned to baseline levels in morphine-treated wounds. Similarly, both SP- and CGRP-ir in 129S6 and MOR-KO were increased significantly in PBS-treated mice but not in morphine-treated as compared to baseline before wounding. It is noteworthy, that SP- and CGRP-ir were significantly higher in the unwounded skin of MOR-, DOR- and KOR-KO as compared to 129S6 control mice, suggesting that all three ORs are involved in the regulation of neuropeptides. Therefore, even though wound healing and blood flow were not influenced by morphine in MOR-KO, neuropeptides were responsive to morphine treatment in MOR-KO. Together, these data suggest that MOR downregulation may contribute to impaired wound healing and that other ORs are critical to regulate neuropeptides to control neurogenic inflammation in the skin, which may in turn influence the healing process. Thus, morphine orchestrates wound healing by stimulating vascular function via MOR and modulating neurogenic inflammation via ORs in the skin. Since increased inflammation and vasculopathy underlie wound pathobiology in SCD, morphine may have a therapeutic effect on healing leg ulcers. Disclosures: No relevant conflicts of interest to declare. </jats:sec