Mitragynine and 7- hydroxymitragynine: Pharmacodynamic studies

Abstract

Mitragyna speciosa is reported to be beneficial for the management of chronic pain and opioid withdrawal in the evolving opioid epidemic. Data on the blood–brain barrier (BBB) transport of mitragynine and 7-hydroxymitragynine, the active compounds of the plant, are still lacking and inconclusive. Here, we present for the first time the rate and the extent of mitragynine and 7-hydroxymitragynine transport across the BBB, with an investigation of their post-BBB intra-brain distribution.

We utilized an in vitro BBB model to study the rate of BBB permeation of the compounds and their interaction with efflux transporter P-glycoprotein (P-gp). Mitragynine showed higher apical-to-basolateral (A-B, i.e. blood-to-brain side) permeability than 7-hydroxymitragynine.

7-Hydroxymitragynine showed a tendency to efflux, with efflux ratio (B-A/A-B) of 1.39. Both were found to inhibit the P-gp and are also subject to efflux by the P-gp. Assessment of the extent of BBB transport in vivo in rats from unbound brain to plasma concentration ratios (Kp,uu,brain) revealed extensive efflux of both compounds, with less than 10 percent of unbound mitragynine and 7-hydroxymitragynine in plasma crossing the BBB.

By contrast, the extent of intra-brain distribution was significantly different, with mitragynine having 18-fold higher brain tissue uptake in brain slice assay compared with 7-hydroxymitragynine. Mitragynine showed a moderate capacity to accumulate inside brain parenchymal cells, while 7-hydroxymitragynine showed restricted cellular barrier transport.

The presented findings from this systematic investigation of brain pharmacokinetics of mitragynine and 7-hydroxymitragynine are essential for design and interpretation of in vivo experiments aiming to establish exposure–response relationship.

Mitragynine (MG) and 7-hydroxymitragynine (7-OHMG) are alkaloids of the plant Mitragyna speciosa Korth; widely known as kratom from its Thai name, while in Malaysia, the plant is called ketum. Ketum is native to Southeast Asia and has been used traditionally to treat minor ailments and to increase physical endurance for manual labourers, and historically used as a substitute for opium (Burkill & Haniff 1930; Burkill 1935; Suwanlert 1975). Current trends in the use of ketum are focused on either treatment purposes, i.e. for pain and managing opioid withdrawal (Boyer et al. 2008; Vicknasingam et al. 2010), or consumed for non-medical purposes in the form of a cocktail with other illicit substances to induce euphoria (Singh, Narayanan, & Vicknasingam 2016; Singh et al. 2017). A recent CNN article titled “Can the kratom plant help fix the opioid crisis?” highlighted positive reports from users of ketum products (which are sold as supplements), to manage chronic pain and to wean users off opiates (Nadia Kounang, CNN Health+ 2017). However, regular consumption of ketum drink caused dependence and craving among users (Singh, Müller, & Vicknasingam 2014), and administration of the alkaloid MG caused addiction-related behaviours in rodent models, of which partly mediated by opioid receptors (Yusoff et al. 2016; Yusoff et al. 2017). Impaired cognitive functions in rodents given with MG were also reported (Yusoff et al. 2016; Ismail et al. 2017).

Further understandings on the detailed mechanisms of action of MG and 7-OHMG and their ability to cross the blood–brain barrier (BBB) are urgently needed. The BBB, formed by the brain microvascular endothelium, regulates molecular movement into and out of the brain. It is important to establish the rate and extent of BBB transport of these agents to evaluate their potential as central nervous system (CNS) therapeutics, mainly for pain and management of opioid withdrawal, especially at a time when use and abuse of opioids are epidemic (Schaefer, Tome, & Davis 2017).

Pharmacological activities, primarily central actions of MG and 7-OHMG, are widely reported yet often controversial [refer to reviews by Hassan et al. (2013) and Suhaimi et al. (2016)]. Most notably, both compounds showed antinociceptive activity, with 7-OHMG demonstrating higher activity than morphine, tested in tail-flick and hot-plate tests in mice after subcutaneous and oral administrations (Matsumoto et al. 2004). In a recent dose–response study in mice, MG showed 66-fold lower tail flick antinociceptive activity judged by ED50 than morphine, while 7-OHMG showed 5-fold and 350-fold higher activity than morphine and MG, respectively (Váradi et al. 2016). However, the authors did not provide any information on the plasma or brain concentration–effect relationship that makes direct comparison of substances based only on ED50 too vague, due to the vast differences in both systemic and CNS pharmacokinetics of studied compounds.

Mitragynine and 7-OHMG exert their effects through interaction with opioid receptors, both as partial agonists at human μ-opioid receptor and as competitive antagonists at human κ-opioid and δ-opioid receptors (Kruegel et al. 2016). Using bioluminescence resonance energy transfer functional assays for determination of in vitro human μ-opioid receptor functional activity, Kruegel et al. reported EC50 value of MG as 339 nM and almost 10-fold lower EC50 of 7-OHMG, 34.5 nM (Kruegel et al. 2016). However, there is currently no evidence that these concentrations could be attained at the target site in vivo neither in humans nor in rodents.

It is well established that only unbound (free) drug can interact with the target and initiate a pharmacological effect (Hammarlund-Udenaes et al. 2008; Loryan et al. 2014). Hence, for establishing a trustworthy exposure response relationship, the assessment of unbound brain interstitial fluid (ISF) concentration must be investigated. However, the presence of the BBB does not allow use of unbound plasma concentration as a surrogate for brain ISF (Hammarlund-Udenaes et al. 2008). The recently described combinatory mapping approach (CMA) validated against cerebral microdialysis has been proven to be a good tool for the evaluation of neuropharmacokinetics (neuroPK) of drugs, in particular the extent of BBB transport and post-BBB intra-brain distribution in whole brain and brain regions of interest (ROI) (Loryan et al. 2014; Loryan et al. 2016). In addition, primary brain endothelial cell culture models are crucial for evaluation of the rate of BBB transport and characterization of its underlying mechanisms (Abbott et al. 2014).

While there are many findings reflecting pharmacological activities of MG and 7-OHMG, data on the rate and the extent of BBB transport as well as mechanisms for BBB permeation of these compounds are relatively sparse and inconclusive. In the current study, we address several aspects of MG and 7-OHMG neuroPK and present comparative characteristics on the rate of BBB transport evaluated using an in vitro BBB model from primary porcine brain endothelial cells (PBECs) and interaction with the efflux transporter P-glycoprotein (P-gp); the extent of BBB transport assessed by means of unbound brain (or brain ROIs) to unbound plasma concentration ratio (Kp,uu) in rats using the CMA methodology; and the extent of post-BBB intra-brain distribution using the brain slice assay.