(T0930-10-64) Pharmacokinetic Studies of 7-hydroxymitragynine, the Active Metabolite of Mitragynine, in Rats

Purpose: Kratom (Mitragyna speciosa) is a tropical evergreen tree from Southeast Asia that has been widely used for the self-treatment of pain. In addition, it has demonstrated potential to treat opioid withdrawal symptoms. Mitragynine is the major alkaloid found in kratom and is hypothesized to be responsible for the pharmacological effects^1,2. However, the active metabolite of mitragynine, 7-hydroxymitragynine (7-HMG), has raised concerns due to its ability to substitute for morphine, in a dose-dependent manner, in the rat self-administration model³. 7-HMG is a μ-opioid receptor (MOR) agonist showing 22-fold greater binding affinity towards MOR compared with mitragynine. The metabolite of 7-HMG, mitragynine pseudoindoxyl, shows 119-fold tighter binding affinity at MOR than mitragynine^4,5. Although there are pharmacokinetic studies available in the literature, none of these have determined the oral bioavailability of 7-HMG^6-8. To address this void in understanding, we performed oral and intravenous pharmacokinetic studies of 7-HMG, and quantified it along with its active metabolite, mitragynine pseudoindoxyl.

Methods: Single dose oral and intravenous pharmacokinetic studies of 7-HMG were conducted in male rats. 7-HMG was dosed orally (5 mg/kg) or intravenously (2.5 mg/kg) to each rat. During the pharmacokinetic study, rats were connected to Basi Culex automated blood sampling system. Blood samples were collected at pre-dose, 0.08, 0.17, 0.25, 0.5, 0.75, 1, 2, 4, 6, 8, 12, 18 and 24 hours, post-dose. Plasma was separated from whole blood after centrifuging and plasma concentration of 7-HMG and its metabolite, mitragynine pseudoindoxyl were determined using a validated ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method. The pharmacokinetic parameters were calculated by non-compartmental analysis using Phoenix WinNonlin, version 6.4.

Results: 7-HMG showed a second-order pharmacokinetic profile in both oral and intravenous studies. The essential pharmacokinetic parameters were calculated, including the apparent volume of distribution (V_z: 5.6 ± 0.8 L/kg) which reflects the extracellular distribution of 7-HMG in the body, and the process of 7-HMG elimination from the body (clearance: 8.0 ± 0.7 L/hr/kg) after intravenous dosing. After oral dosing 7-HMG, C_max, T_max, and oral bioavailability were also determined. The C_max was 5.3 ± 1.3 ng/mL after normalizing the dose, T_max was 0.3 ± 0.1 h which indicates fast absorption of 7-HMG. The oral bioavailability of 7-HMG was 2.8% which shows the rate and extent to which 7-HMG is absorbed and becomes systemically available. Mitragynine pseudoindoxyl only can be quantified within an hour after intravenous administration and was below the lower limit of quantification in plasma samples collected after oral dosing.

Conclusion:
This research demonstrates the low oral bioavailability of 7-HMG when administered alone. In addition, the plasma concentration of mitragynine pesudoindoxyl, the active metabolite of 7-HMG, was only quantifiable after IV administration. This preclinical study can be applied to further PK/PD modeling of mitragynine and 7-HMG.

References: 1. Avery, Bonnie A., et al. "Comparative pharmacokinetics of mitragynine after oral administration of Mitragyna speciosa (Kratom) leaf extracts in rats." Planta medica 85.04 (2019): 340-346.
2. Maxwell, Elizabeth A., et al. "Pharmacokinetics and safety of mitragynine in beagle dogs." Planta medica 86.17 (2020): 1278-1285.
3. Hemby, Scott E., et al. "Abuse liability and therapeutic potential of the Mitragyna speciosa (kratom) alkaloids mitragynine and 7‐hydroxymitragynine." Addiction Biology24.5 (2019): 874-885.
4. Obeng, Samuel, et al. "Investigation of the adrenergic and opioid binding affinities, metabolic stability, plasma protein binding properties, and functional effects of selected indole-based kratom alkaloids." Journal of medicinal chemistry 63.1 (2019): 433-439.
5. Váradi, András, et al. "Mitragynine/corynantheidine pseudoindoxyls as opioid analgesics with mu agonism and delta antagonism, which do not recruit β-arrestin-2." Journal of medicinal chemistry 59.18 (2016): 8381-8397.
6. Vuppala, Pradeep K., et al. "Development and validation of a UPLC‐MS/MS method for the determination of 7‐hydroxymitragynine, a μ‐opioid agonist, in rat plasma and its application to a pharmacokinetic study." Biomedical chromatography 27.12 (2013): 1726-1732.
7. Maxwell, Elizabeth A., et al. "Oral pharmacokinetics in beagle dogs of the mitragynine metabolite, 7-hydroxymitragynine." European Journal of drug metabolism and pharmacokinetics46 (2021): 459-463.
8. Berthold, Erin C., et al. "The lack of contribution of 7-hydroxymitragynine to the antinociceptive effects of mitragynine in mice: a pharmacokinetic and pharmacodynamic study." Drug Metabolism and Disposition 50.2 (2022): 158-167.