Kruegel AC, Uprety R, Grinnell SG, et al. 7-Hydroxymitragynine Is an Active Metabolite of Mitragynine and a Key Mediator of Its Analgesic Effects. ACS Cent Sci. 2019;5(6):992-1001. doi:10.1021/acscentsci.9b00141
Summary by KratomScience
A study published in 2019 in the open access journal ACS Central Science found what may be a game changer in the pharmacology of mitragynine.
It was known previously that the effects of mitragynine (MG) come from binding to the mu opioid receptor (MOR). In previous studies naloxone, an opioid blocker used in Suboxone and Narcan, also blocked the effects of MG, proving its function on the MOR.
But researchers in this study found evidence MG administered orally is first metabolized into 7-hydroxomitragynine (7-OH), which then attaches to the MOR to produce more potent pain relieving effects.
Considered a more potent but less abundant alkaloid of kratom, the presence of a greater concentration of 7-OH in ground leaf kratom or extracts was thought to produce a more potent kratom product. With evidence found here that MG metabolizes into 7-OH, this may not be the case, as “the analgesia induced by mitragynine appears to depend largely on formation of 7-OH as a metabolite and not on the parent compound.” Extract is more potent because of the higher concentration of alkaloids, but the potency may not come from an imbalance of alkaloids as it varies from alkaloids found in fresh natural leaf. This also calls into question whether a higher level of 7-OH from the drying process contributes to potency.
Pure MG in studies in mice was found to produce greater analgesic effects if administered orally than from subcutaneous injections (under the skin). This led researchers to hypothesize that the metabolism of mitragynine is what drives its pain relieving properties. In vitro studies of both human and mouse liver microsomes showed clear evidence that MG was disappearing as 7-OH was emerging. Further study demonstrated that MG is metabolized into 7-OH by CYP3A4, a liver enzyme that oxidizes drugs and other toxins.
An in vivo experiment administered rats with MG alone, then collected blood and plasma samples which showed the presence of 7-OH. Another experiment took two groups of mice, and administered one group with MG and the other with 7-OH. Both groups of mice were found to have similar analgesic effects and similar concentrations of 7-OH in the brain, as MG metabolizes into 7-OH but 7-OH remains stable in the liver.
Tests on rodents don’t automatically suggest the same results in humans because of differing metabolisms, and metabolism varies from human to human. Humans who have abnormally higher or lower levels of certain liver enzymes may be more or less sensitive to kratom, as has been proven with codeine. This also suggests that kratom consumers pay attention to their metabolism and liver health in order to avoid negative or impotent effects of kratom.
Finally, despite news reports that list kratom as causing respiratory depression, this study points to evidence the opposite is true as…
…we have also found that both mitragynine and 7-OH are G protein-biased agonists of the mu-opioid receptor (MOR).21,25 We have thus classified the kratom alkaloids as “atypical opioids” and have postulated that these compounds may provide a greater therapeutic window between analgesia and classical opioid side effects. For example, other MOR agonists that activate G protein signaling without recruiting the beta-arrestin pathway exhibit attenuated respiratory depression and reduced inhibition of gastrointestinal (GI) transit compared to classical opioids.26−30 In fact, an early study on the pharmacology of mitragynine demonstrated its superiority compared to the classical opioid codeine in this regard, providing preliminary support for this hypothesis.24