Evaluating kratom alkaloids using PHASE

Exploring the Therapeutic Potential of Mitragynine and Corynoxeine: Kratom-Derived Indole and Oxindole Alkaloids for Pain Management

The search for effective pain management solutions remains a critical challenge, especially amidst growing concerns over the use of conventional opioids. In the US, opioid-related mortality rates have surged to as many as 80 deaths per 100,000 people in some states, with an estimated economic burden of USD 1.5 trillion annually-exceeding the gross domestic product (GDP) of most US industrial sectors. A remarkable breakthrough lies in the discovery that indole and oxindole alkaloids, produced by several genera within the plant Tribe Naucleeae, act on opioid receptors without activating the beta-arrestin-2 pathway, the primary driver of respiratory depression and overdose deaths. This systematic review explores the pharmacological properties, mechanisms of action, dosing considerations, interactions, and long-term effects of mitragynine and corynoxeine, alkaloids from the Southeast Asian plant Mitragyna speciosa (kratom) and others in the Tribe Naucleeae. Mitragynine, a partial opioid receptor agonist, and corynoxeine, known for its anti-inflammatory and neuroprotective effects, demonstrate significant therapeutic potential for managing diverse pain types-including neuropathic, inflammatory, nociceptive, visceral, and central pain syndromes-with a focus on cancer pain. Unlike traditional opioids, these compounds do not recruit beta-arrestin-2, avoiding key adverse effects such as respiratory depression, severe constipation, and rapid tolerance development. Their distinct pharmacological profiles make them innovative candidates for safer, non-lethal pain relief. However, challenges persist, including the unregulated nature of kratom products, inconsistencies in potency due to crude extract variability, potential for misuse, and adverse drug interactions. Addressing these issues requires establishing standardized quality control protocols, such as Good Manufacturing Practices (GMP), to ensure consistent potency and purity. Clear labeling requirements with dosage guidelines and warnings should be mandated to ensure safe use and prevent misuse. Furthermore, the implementation of regulatory oversight to monitor product quality and enforce compliance is essential. This review emphasizes the urgency of focused research to optimize dosing regimens, characterize the pharmacodynamic profiles of these alkaloids, and evaluate long-term safety. By addressing these gaps, the mitragynine- and corynoxeine-related drug classes can transition from promising plant-derived molecules to validated pharmacotherapeutic agents, potentially revolutionizing the field of pain management.

The neuropharmacology of kratom, a novel psychoactive natural product

Kratom (Mitragyna speciosa, Korth.) is a tropical tree that is indigenous to Southeast Asia. When ingested, kratom leaves or decoctions from the leaves have been reported to produce complex stimulant and opioid-like effects. For generations native populations in Southeast Asia have used kratom products to stave off fatigue, improve mood, alleviate pain and manage symptoms of opioid withdrawal. Over the past 15-20 years, kratom use has spread to Western nations including the United States, where many individuals are using kratom products for the self-management of pain, opioid use disorder, anxiety and depression. The increased use of kratom has triggered a surge in research into the biochemistry, pharmacology and behavioral effects of kratom and its active constituents, especially mitragynine and 7-hydroxymitragynine. In this review, we highlight some of the recent animal studies showing that kratom and its constituent compounds have potential beneficial effects in animal models of pain, anxiety, depression and opioid dependence. We also highlight studies showing that kratom can modulate the functioning of opioid, noradrenergic, serotonergic and dopaminergic systems. The highlighted studies strongly suggest that kratom and its constituents may form the basis for the development of novel therapeutic agents.

The Mitragyna speciosa (kratom) alkaloid mitragynine: Analysis of adrenergic α2 receptor activity in vitro and in vivo

Mitragynine, an alkaloid present in the leaves of Mitragyna speciosa (kratom), has a complex pharmacology that includes low efficacy agonism at μ-opioid receptors (MORs). This study examined the activity of mitragynine at adrenergic α2 receptors (Aα2Rs) in vitro and in vivo. Mitragynine displaced a radiolabeled Aα2R antagonist ([3H]RX821002) from human Aα2ARs in vitro with lower affinity (Ki = 1260 nM) than the agonists (-)-epinephrine (Ki = 263 nM) or lofexidine (Ki = 7.42 nM). Mitragynine did not significantly stimulate [35S]GTPγS binding at Aα2ARs in vitro, but in rats trained to discriminate 32 mg/kg mitragynine from vehicle (intraperitoneally administered; i.p.), mitragynine exerted an Aα2R agonist-like effect. Both α2R antagonists (atipamezole and yohimbine) and MOR antagonists (naloxone and naltrexone) produced rightward shifts in mitragynine discrimination dose-effect function and Aα2R agonists lofexidine and clonidine produced leftward shifts. In the mitragynine trained rats, Aα2R agonists also produced leftward shifts in discrimination dose-effect functions for morphine and fentanyl. In a separate rat cohort trained to discriminate 3.2 mg/kg i.p. morphine from vehicle, naltrexone produced a rightward shift, but neither an Aα2R agonist or antagonist affected morphine discrimination. In a hypothermia assay, both lofexidine and clonidine produced marked effects antagonized by yohimbine. Mitragynine did not produce hypothermia. Together, these data demonstrate that mitragynine acts in vivo like an Aα2R agonist, although its failure to induce hypothermia or stimulate [35S]GTPγS binding in vitro, suggests that mitragynine maybe a low efficacy Aα2R agonist.

Beneficial and adverse health effects of kratom (Mitragyna speciosa): A critical review of the literature

Used in Southeast Asia for generations, kratom gained popularity in the United States and elsewhere over the past several decades. Derived from Mitragyna speciosa, kratom preparations including leaves, teas, powders, capsules, and extracts may yield stimulant, analgesic, and opioid-like effects that occur dose-dependently based on concentrations of kratom's key alkaloids, mitragynine and 7-hydroxymitragynine. Such effects are responsible for kratom's potential as a reduced-harm alternative to opiates and as a withdrawal treatment. But these properties are also associated with tolerance development and addictive potential. Given mitragynine and 7-hydroxymitragynine activity on cytochrome P450 isoforms and opioid receptors, adverse effects among polysubstance users are a concern. Current literature on the toxicology of kratom is reviewed, including product alkaloid concentrations, in vitro and in vivo data, epidemiological evidence, and human case data. The potential harms and benefits of kratom products are discussed within an exposure assessment framework, and recommendations for industry are presented. Current evidence indicates that kratom may have therapeutic potential in some persons and that products present few risks with typical, non-polysubstance use. However, few studies identified alkaloid doses at which adverse effects were expected in humans or animals. Such research is needed to inform future assessments of kratom's risks and benefits.

Controversies in Assessment, Diagnosis, and Treatment of Kratom Use Disorder

PURPOSE OF REVIEW

We apply the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) criteria for substance use disorders (SUDs) to the herbal product kratom. Similarities and differences between kratom use disorder (KUD) and other SUDs are explored, along with assessment, diagnostic, and therapeutic recommendations for KUD.

RECENT FINDINGS

Literature reports of "kratom addiction" or KUD rarely specify the criteria by which patients were diagnosed. Individuals meeting DSM-5 KUD criteria typically do so via tolerance and withdrawal, using more than intended, and craving, not functional or ​psychosocial disruption, which occur rarely. Most clinicians who use medication to treat patients with isolated KUD select buprenorphine formulations, although there are no controlled studies showing that buprenorphine is safe or efficacious in this patient population. Diagnosis and treatment decisions for KUD should be systematic. We propose an algorithm that takes into consideration whether KUD occurs with comorbid opioid use disorder.

An insight review on the neuropharmacological effects, mechanisms of action, pharmacokinetics and toxicity of mitragynine

Mitragynine is one of the main psychoactive alkaloids in Mitragyna speciosa Korth. (kratom). It has opium-like effects by acting on μ-, δ-, and κ-opioid receptors in the brain. The compound also interacts with other receptors, such as adrenergic and serotonergic receptors and neuronal Ca2+ channels in the central nervous system to have its neuropharmacological effects. Mitragynine has the potential to treat diseases related to neurodegeneration such as Alzheimer's disease and Parkinson's disease, as its modulation on the opioid receptors has been reported extensively. This review aimed to provide an up-to-date and critical overview on the neuropharmacological effects, mechanisms of action, pharmacokinetics and safety of mitragynine as a prospective psychotropic agent. Its multiple neuropharmacological effects on the brain include antinociceptive, anti-inflammatory, antidepressant, sedative, stimulant, cognitive, and anxiolytic activities. The potential of mitragynine to manage opioid withdrawal symptoms related to opioid dependence, its pharmacokinetics and toxic effects were also discussed. The interaction of mitragynine with various receptors in the brain produce diverse neuropharmacological effects, which have beneficial properties in neurological disorders. However, further studies need to be carried out on mitragynine to uncover its complex mechanisms of action, pharmacokinetics, pharmacodynamic profiles, addictive potential, and safe dosage to prevent harmful side effects.

An update on the clinical pharmacology of kratom: uses, abuse potential, and future considerations

INTRODUCTION

Kratom (Mitragyna speciosa) has generated substantial clinical and scientific interest as a complex natural product. Its predominant alkaloid mitragynine and several stereoisomers have been studied for activity in opioid, adrenergic, and serotonin receptors. While awaiting clinical trial results, the pre-clinical evidence suggests a range of potential therapeutic applications for kratom with careful consideration of potential adverse effects.

AREAS COVERED

The focus of this review is on the pharmacology, pharmacokinetics, and potential drug-drug interactions of kratom and its individual alkaloids. A discussion on the clinical pharmacology and toxicology of kratom is followed by a summary of user surveys and the evolving concepts of tolerance, dependence, and withdrawal associated with kratom use disorder.

EXPERT OPINION

With the increasing use of kratom in clinical practice, clinicians should be aware of the potential benefits and adverse effects associated with kratom. While many patients may benefit from kratom use with few or no reported adverse effects, escalating dose and increased use frequency raise the risk for toxic events in the setting of polysubstance use or development of a use disorder.

A Critical Review of the Neuropharmacological Effects of Kratom: An Insight from the Functional Array of Identified Natural Compounds

Kratom (Mitragyna speciosa Korth. Havil) has been considered a narcotic drug for years, barred by the law in many parts of the world, while extensive research over the past few decades proves its several beneficial effects, some of which are still in ambiguity. In many countries, including Thailand, the indiscriminate use and abuse of kratom have led to the loss of life. Nonetheless, researchers have isolated almost fifty pure compounds from kratom, most of which are alkaloids. The most prevalent compounds, mitragynine and 7-hydroxy mitragynine, are reported to display agonist morphine-like effects on human μ-opioid receptors and antagonists at κ- and δ-opioid receptors with multimodal effects at other central receptors. Mitragynine is also credited to be one of the modulatory molecules for the Keap1-Nrf2 pathway and SOD, CAT, GST, and associated genes' upregulatory cascades, leading it to play a pivotal role in neuroprotective actions while evidently causing neuronal disorders at high doses. Additionally, its anti-inflammatory, antioxidative, antibacterial, and gastroprotective effects are well-cited. In this context, this review focuses on the research gap to resolve ambiguities about the neuronal effects of kratom and demonstrate its prospects as a therapeutic target for neurological disorders associated with other pharmacological effects.

Predicting binding between 55 cannabinoids and 4,799 biological targets by in silico methods

Recently, there has been an increase in cannabis-derived products being marketed as foods, dietary supplements, and other consumer products. Cannabis contains over a hundred cannabinoids, many of which have unknown physiological effects. Since there are large numbers of cannabinoids, and many are not commercially available for in vitro testing, an in silico tool (Chemotargets Clarity software) was used to predict binding between 55 cannabinoids and 4,799 biological targets (enzymes, ion channels, receptors, and transporters). This tool relied on quantitative structure activity relationships (QSAR), structural similarity, and other approaches to predict binding. From this screening, 827 cannabinoid-target binding pairs were predicted, which included 143 unique targets. Many cannabinoids sharing core structures (cannabinoid "types") had similar binding profiles, whereas most cannabinoids containing carboxylic acid groups were similar without regards to their core structure. For some of the binding predictions (43), in vitro binding data were available, and they agreed well with in silico binding data (median fourfold difference in binding concentrations). Finally, clinical adverse effects associated with 22 predicted targets were identified from an online database (Clarivate Off-X), providing important insights on potential human health hazards. Overall, in silico biological target predictions are a rapid means to identify potential hazards due to cannabinoid-target interactions, and the data can be used to prioritize subsequent in vitro and in vivo testing.

Modified Akuamma Alkaloids with Increased Potency at the Mu-opioid Receptor

Akuammine (1) and pseudoakuammigine (2) are indole alkaloids found in the seeds of the akuamma tree (Picralima nitida). Both alkaloids are weak agonists of the mu opioid receptor (μOR); however, they produce minimal effects in animal models of antinociception. To probe the interactions of 1 and 2 at the opioid receptors, we have prepared a collection of 22 semisynthetic derivatives. Evaluation of this collection at the μOR and kappa opioid receptor (κOR) revealed structural-activity relationship trends and derivatives with improved potency at the μOR. Most notably, the introduction of a phenethyl moiety to the N1 of 2 produces a 70-fold increase in potency and a 7-fold increase in selectivity for the μOR. The in vitro potency of this compound resulted in increased efficacy in the tail-flick and hot-plate assays of antinociception. The improved potency of these derivatives highlights the promise of exploring natural product scaffolds to probe the opioid receptors.

New science, drug regulation, and emergent public health issues: The work of FDA's division of applied regulatory science

The U.S. Food and Drug Administration (FDA) Division of Applied Regulatory Science (DARS) moves new science into the drug review process and addresses emergent regulatory and public health questions for the Agency. By forming interdisciplinary teams, DARS conducts mission-critical research to provide answers to scientific questions and solutions to regulatory challenges. Staffed by experts across the translational research spectrum, DARS forms synergies by pulling together scientists and experts from diverse backgrounds to collaborate in tackling some of the most complex challenges facing FDA. This includes (but is not limited to) assessing the systemic absorption of sunscreens, evaluating whether certain drugs can convert to carcinogens in people, studying drug interactions with opioids, optimizing opioid antagonist dosing in community settings, removing barriers to biosimilar and generic drug development, and advancing therapeutic development for rare diseases. FDA tasks DARS with wide ranging issues that encompass regulatory science; DARS, in turn, helps the Agency solve these challenges. The impact of DARS research is felt by patients, the pharmaceutical industry, and fellow regulators. This article reviews applied research projects and initiatives led by DARS and conducts a deeper dive into select examples illustrating the impactful work of the Division.

Interactive Effects of µ-Opioid and Adrenergic-α 2 Receptor Agonists in Rats: Pharmacological Investigation of the Primary Kratom Alkaloid Mitragynine and Its Metabolite 7-Hydroxymitragynine

The primary kratom alkaloid mitragynine is proposed to act through multiple mechanisms, including actions at µ-opioid receptors (MORs) and adrenergic-α 2 receptors (Aα 2Rs), as well as conversion in vivo to a MOR agonist metabolite (i.e., 7-hydroxymitragynine). Aα 2R and MOR agonists can produce antinociceptive synergism. Here, contributions of both receptors to produce mitragynine-related effects were assessed by measuring receptor binding in cell membranes and, in rats, pharmacological behavioral effect antagonism studies. Mitragynine displayed binding affinity at both receptors, whereas 7-hydroxymitragynine only displayed MOR binding affinity. Compounds were tested for their capacity to decrease food-maintained responding and rectal temperature and to produce antinociception in a hotplate test. Prototypical MOR agonists and 7-hydroxymitragynine, but not mitragynine, produced antinociception. MOR agonist and 7-hydroxymitragynine rate-deceasing and antinociceptive effects were antagonized by the opioid antagonist naltrexone but not by the Aα 2R antagonist yohimbine. Hypothermia only resulted from reference Aα 2R agonists. The rate-deceasing and hypothermic effects of reference Aα 2R agonists were antagonized by yohimbine but not naltrexone. Neither naltrexone nor yohimbine antagonized the rate-decreasing effects of mitragynine. Mitragynine and 7-hydroxymitragynine increased the potency of the antinociceptive effects of Aα 2R but not MOR reference agonists. Only mitragynine produced hypothermic effects. Isobolographic analyses for the rate-decreasing effects of the reference Aα 2R and MOR agonists were also conducted. These results suggest mitragynine and 7-hydroxymitragynine may produce antinociceptive synergism with Aα 2R and MOR agonists. When combined with Aα 2R agonists, mitragynine could also produce hypothermic synergism. SIGNIFICANCE STATEMENT: Mitragynine is proposed to target the µ-opioid receptor (MOR) and adrenergic-α2 receptor (Aα2R) and to produce behavioral effects through conversion to its MOR agonist metabolite 7-hydroxymitragynine. Isobolographic analyses indicated supra-additivity in some dose ratio combinations. This study suggests mitragynine and 7-hydroxymitragynine may produce antinociceptive synergism with Aα2R and MOR agonists. When combined with Aα2R agonists, mitragynine could also produce hypothermic synergism.

Current and emerging pharmacotherapies for opioid dependence treatments in adults: a comprehensive update

INTRODUCTION

Opioid use disorder (OUD) is characterized by compulsive opioid seeking and taking, intense drug craving, and intake of opioids despite negative consequences. The prevalence of OUDs has now reached an all-time high, in parallel with peak rates of fatal opioid-related overdoses, where 15 million individuals worldwide meet the criteria for OUD. Further, in 2020, 120,000 opioid-related deaths were reported worldwide with over 75,000 of those deaths occurring within the United States.

AREAS COVERED

In this review, we highlight pharmacotherapies utilized in patients with OUDs, including opioid replacement therapies, and opioid antagonists utilized for opioid overdoses and deterrent of opioid use. We also highlight newer treatments, such as those targeting the neuroimmune system, which are potential new directions for research given the recently established role of opioids in activating neuroinflammatory pathways, as well as over the counter remedies, including kratom, that may mitigate withdrawal.

EXPERT OPINION

To effectively treat OUDs, a deeper understanding of the current therapeutics being utilized, their additive effects, and the added involvement of the neuroimmune system are essential. Additionally, a complete understanding of opioid-induced neuronal alterations and therapeutics that target these abnormalities - including the neuroimmune system - is required to develop effective treatments for OUDs.

Exploring of cardiac autonomic activity with heart rate variability in long-term kratom (Mitragyna speciosa Korth.) users: a preliminary study

Background

Kratom is a psychoactive plant used to enhance productivity among laborers in Southeast Asian countries. Previous findings from in vitro research of mitragynine, a major component of kratom, suggested a possible risk of heart function abnormality. However, the cardiac autonomic function in long-term kratom users with chewing forms has never been studied. This study aimed to investigate heart rate variability (HRV) indices of cardiac autonomic function in long-term kratom chewers (LKC), compared to the control levels, and also to examine the correlation between HRV indices and relevant kratom use factors.

Method

A total number of 50 participants consisted of LKC (n = 31) who regularly chewed fresh kratom leaves for at least 2 years and demographically matched control subjects (n = 19). Resting electrocardiogram (ECG) signals were recorded from subjects for 3 min to analyze the ultrashort HRV in the frequency domain. The normalized low frequency (LFn) and high frequency (HFn) were chosen to be the HRV indices to evaluate cardiac autonomic function. The comparison of HRV indices between groups and the correlation between HRV indices and duration and quantity of kratom use was further conducted in statistical analysis.

Results

The LKC significantly increased LFn together with enhanced HFn compared to the control group tested, indicating that LKC changed cardiac autonomic function with parasympathetic dominance. Furthermore, no significant correlation between the HRV indices and the duration and quantity of kratom use was found, suggesting that the HRV indices were not relevant to these factors. The present study provided scientific-based evidence of cardiac autonomic modulation in long-term kratom chewers. LFn and HFn may be promising cardiac autonomic indicators for monitoring health outcomes in LKC.

Kratom as an opioid alternative: harm, or harm reduction? A systematic review of literature

Background: Kratom (Mitragyna speciosa Korth.) products are increasingly endorsed for self-management of multiple ailments, including as opioid substitution. The FDA has expressed that there is no evidence to indicate that this botanical is safe or effective for any medical use.Objective: We systematically review the current state of the literature concerning the impact of kratom and its alkaloids in all paradigms that involve opioids.Methods: A keyword search of online literature databases identified 16 preclinical studies, 25 case reports, and 10 observational studies meeting our pre-selected criteria.Results: All rodent models support alkaloids' action on opioid receptors, translating in their ability to mitigate opioid withdrawal. Some studies found mitragynine (MG) to have less reinforcing properties than morphine, and possessing tolerance-sparing properties when coadministered with morphine. Two studies that assessed 7-hydroxymitragynine (7OHMG) found it to substitute for morphine with potential for tolerance and dependence. Aside from addiction development, case reports outline a variety of confounding toxicities. Ten surveys of users, some conducted with assistance from pro-kratom lobbying organizations, find a high self-reported efficacy as an opioid substitute, with minimal reported adverse effects.Conclusion: With no reported controlled human clinical trials, in the light of rising concerns surrounding kratom's liabilities, there is insufficient evidence to allow any conclusions to be drawn. Case reports and observational studies carry significant bias toward harm and efficacy, respectively. Existing animal studies are heterogeneous in methodology and ultimately findings, with concern for interspecies variability and human translatability. Further research should investigate the safety and efficacy of using kratom alkaloids as opioid substitutes.

Kratom (Mitragyna speciosa) Validation: Quantitative Analysis of Indole and Oxindole Alkaloids Reveals Chemotypes of Plants and Products

Many consumers are turning to kratom (Mitragyna speciosa) to self-manage pain and opioid addiction. In the United States, an array of capsules, powders, and loose-leaf kratom products are readily available. Additionally, several online sites supply live kratom plants. A prerequisite to establishing quality control and quality assurance standards for the kratom industry, or understanding how alkaloid levels effect clinical outcomes, is the identification and quantitation of major and minor alkaloid constituents within available products and preparations. To this end, an ultra-high performance liquid chromatography-high resolution mass spectrometry method was developed for the analysis of 8 indole alkaloids (7-hydroxymitragynine, ajmalicine, paynantheine, mitragynine, speciogynine, isopaynantheine, speciociliatine, and mitraciliatine) and 6 oxindole alkaloids (isomitraphylline, isospeciofoleine, speciofoline, corynoxine A, corynoxeine, and rhynchophylline) in US-grown kratom plants and commercial products. These commercial products shared a qualitatively similar alkaloid profile, with 12 - 13 detected alkaloids and high levels of the indole alkaloid mitragynine (13.9 ± 1.1 - 270 ± 24 mg/g). The levels of the other major alkaloids (paynantheine, speciociliatine, speciogynine, mitraciliatine, and isopaynantheine) and the minor alkaloids varied in concentration from product to product. The alkaloid profile of US-grown M. speciosa "Rifat" showed high levels of the indole alkaloid speciogynine (7.94 ± 0.83 - 11.55 ± 0.18 mg/g) and quantifiable levels of isomitraphylline (0.943 ± 0.033 - 1.47 ± 0.18 mg/g). Notably, the alkaloid profile of a US-grown M. speciosa seedling was comparable to the commercial products with a high level of mitragynine (15.01 ± 0.20 mg/g). This work suggests that there are several M. speciosa chemotypes.

Metabolism of Speciociliatine, an Overlooked Kratom Alkaloid for its Potential Pharmacological Effects

Speciociliatine, a diastereomer of mitragynine, is an indole-based alkaloid found in kratom (Mitragyna speciosa). Kratom has been widely used for the mitigation of pain and opioid dependence, as a mood enhancer, and/or as an energy booster. Speciociliatine is a partial µ-opioid agonist with a 3-fold higher binding affinity than mitragynine. Speciociliatine has been found to be a major circulating alkaloid in humans following oral administration of a kratom product. In this report, we have characterized the metabolism of speciociliatine in human and preclinical species (mouse, rat, dog, and cynomolgus monkey) liver microsomes and hepatocytes. Speciociliatine metabolized rapidly in monkey, rat, and mouse hepatocytes (in vitro half-life was 6.6 ± 0.2, 8.3 ± 1.1, 11.2 ± 0.7 min, respectively), while a slower metabolism was observed in human and dog hepatocytes (91.7 ± 12.8 and > 120 min, respectively). Speciociliatine underwent extensive metabolism, primarily through monooxidation and O-demethylation metabolic pathways in liver microsomes and hepatocytes across species. No human-specific or disproportionate metabolites of speciociliatine were found in human liver microsomes. The metabolism of speciociliatine was predominantly mediated by CYP3A4 with minor contributions by CYP2D6.

The Chemical and Pharmacological Properties of Mitragynine and Its Diastereomers: An Insight Review

Mitragynine, is a naturally occurring indole alkaloid that can be isolated from the leaves of a psychoactive medicinal plant. Mitragyna speciosa, also known as kratom, is found to possess promising analgesic effects on mediating the opioid receptors such as µ (MOR), δ (DOR), and κ (KOR). This alkaloid has therapeutic potential for pain management as it has limited adverse effect compared to a classical opioid, morphine. Mitragynine is frequently regarded to behave like an opioid but possesses milder withdrawal symptoms. The use of this alkaloid as the source of an analgesic candidate has been proven through comprehensive preclinical and clinical studies. The present data have shown that mitragynine is able to bind to opioid receptors, particularly MOR, to exhibit the analgesic effect. Moreover, the chemical and pharmacological aspects of mitragynine and its diastereomers, speciogynine, speciociliatine, and mitraciliatine, are discussed. It is interesting to know how the difference in stereochemical configuration could lead to the difference in the bioactivity of the respective compounds. Hence, in this review, the updated pharmacological and toxicological properties of mitragynine and its diastereomers are discussed to render a comprehensive understanding of the pharmacological properties of mitragynine and its diastereomers based on their structure-activity relationship study.

The Lack of Contribution of 7-Hydroxymitragynine to the Antinociceptive Effects of Mitragynine in Mice: A Pharmacokinetic and Pharmacodynamic Study

Kratom (Mitragyna speciosa), a Southeast Asian tree, has been used for centuries in pain relief and mitigation of opium withdrawal symptoms. Mitragynine (MTG), the major kratom alkaloid, is being investigated for its potential to provide analgesia without the deleterious effects associated with typical opioids. Concerns have been raised regarding the active metabolite of MTG, 7-hydroxymitragynine (7HMG), which has higher affinity and efficacy at µ-opioid receptors than MTG. Here we investigated the hotplate antinociception, pharmacokinetics, and tissue distribution of MTG and 7HMG at equianalgesic oral doses in male and female C57BL/6 mice to determine the extent to which 7HMG metabolized from MTG accounts for the antinociceptive effects of MTG and investigate any sex differences. The mechanism of action was examined by performing studies with the opioid receptor antagonist naltrexone. A population pharmacokinetic/pharmacodynamic model was developed to predict the behavioral effects after administration of various doses of MTG and 7HMG. When administered alone, 7HMG was 2.8-fold more potent than MTG to produce antinociception. At equivalent effective doses of MTG and 7HMG, there was a marked difference in the maximum brain concentration of 7HMG achieved, i.e., 11-fold lower as a metabolite of MTG. The brain concentration of 7HMG observed 4 hours post administration, producing an analgesic effect <10%, was still 1.5-fold higher than the maximum concentration of 7HMG as a metabolite of MTG. These results provide strong evidence that 7HMG has a negligible role in the antinociceptive effects of MTG in mice. SIGNIFICANCE STATEMENT: Mitragynine (MTG) is being investigated for its potential to aid in pain relief, opioid withdrawal syndrome, and opioid use disorder. The active metabolite of MTG, 7-hydroxymitragynine (7HMG), has been shown to have abuse potential and has been implicated in the opioid-like analgesic effect after MTG administration. The results of this study suggest a lack of involvement of 7HMG in the antinociceptive effects of MTG in mice.

In vitro and in vivo pharmacology of kratom

Kratom products have been historically and anecdotally used in south Asian countries for centuries to manage pain and opioid withdrawal. The use of kratom products has dramatically increased in the United States. More than 45 kratom alkaloids have been isolated, yet the overall pharmacology of the individual alkaloids is still not well characterized. The purpose of this chapter is to summarize in vitro and in vivo opioid activities of the primary kratom alkaloid mitragynine and its more potent metabolite 7-hydroxymitragynine. Following are experimental procedures described to characterize opioid receptor activity; receptor binding and functional assays, antinociceptive assays, operant conditioning assays, and respiratory plethysmography. The capacity of kratom alkaloids to confer tolerance and physical dependence as well as their pharmacokinetic properties are also summarized. The data reviewed here suggest that kratom products and mitragynine possess low efficacy agonist activity at the mu-opioid receptor in vivo. In addition, kratom products and mitragynine have been demonstrated to antagonize the effects of high efficacy mu-opioid agonists. The data further suggest that 7-hydroxymitragynine formed in vivo by metabolism of mitragynine may be minimally involved in the overall behavioral profile of mitragynine and kratom, whereas 7-hydroxymitragynine itself, at sufficiently high doses administered exogenously, shares many of the same abuse- and dependence-related behavioral effects associated with traditional opioid agonists. The apparent low efficacy of kratom products and mitragynine at mu-opioid receptors supports the development of these ligands as effective and potentially safe medications for opioid use disorder.

Kratom Alkaloids: Interactions With Enzymes, Receptors, and Cellular Barriers

Parallel to the growing use of kratom, there is a wealth of evidence from self-report, preclinical, and early clinical studies on therapeutic benefits of its alkaloids in particular for treating pain, managing substance use disorder, and coping with emotional or mental health conditions. On the other hand, there are also reports on potential health risks concerning kratom use. These two aspects are often discussed in reviews on kratom. Here, we aim to highlight specific areas that are of importance to give insights into the mechanistic of kratom alkaloids pharmacological actions. This includes their interactions with drug-metabolizing enzymes and predictions of clinical drug-drug interactions, receptor-binding properties, interactions with cellular barriers in regards to barrier permeability, involvement of membrane transporters, and alteration of barrier function when exposed to the alkaloids.

Oxidative Metabolism as a Modulator of Kratom's Biological Actions

The leaves of Mitragyna speciosa (kratom), a plant native to Southeast Asia, are increasingly used as a pain reliever and for attenuation of opioid withdrawal symptoms. Using the tools of natural products chemistry, chemical synthesis, and pharmacology, we provide a detailed in vitro and in vivo pharmacological characterization of the alkaloids in kratom. We report that metabolism of kratom's major alkaloid, mitragynine, in mice leads to formation of (a) a potent mu opioid receptor agonist antinociceptive agent, 7-hydroxymitragynine, through a CYP3A-mediated pathway, which exhibits reinforcing properties, inhibition of gastrointestinal (GI) transit and reduced hyperlocomotion, (b) a multifunctional mu agonist/delta-kappa antagonist, mitragynine pseudoindoxyl, through a CYP3A-mediated skeletal rearrangement, displaying reduced hyperlocomotion, inhibition of GI transit and reinforcing properties, and (c) a potentially toxic metabolite, 3-dehydromitragynine, through a non-CYP oxidation pathway. Our results indicate that the oxidative metabolism of the mitragynine template beyond 7-hydroxymitragynine may have implications in its overall pharmacology in vivo.

Evaluation of Kratom Opioid Derivatives as Potential Treatment Option for Alcohol Use Disorder

Background and Purpose:Mitragyna speciosa extract and kratom alkaloids decrease alcohol consumption in mice at least in part through actions at the δ-opioid receptor (δOR). However, the most potent opioidergic kratom alkaloid, 7-hydroxymitragynine, exhibits rewarding properties and hyperlocomotion presumably due to preferred affinity for the mu opioid receptor (µOR). We hypothesized that opioidergic kratom alkaloids like paynantheine and speciogynine with reduced µOR potency could provide a starting point for developing opioids with an improved therapeutic window to treat alcohol use disorder.

Experimental Approach: We characterized paynantheine, speciociliatine, and four novel kratom-derived analogs for their ability to bind and activate δOR, µOR, and κOR. Select opioids were assessed in behavioral assays in male C57BL/6N WT and δOR knockout mice.

Key Results: Paynantheine (10 mg∙kg⁻¹, i.p.) produced aversion in a limited conditioned place preference (CPP) paradigm but did not produce CPP with additional conditioning sessions. Paynantheine did not produce robust antinociception but did block morphine-induced antinociception and hyperlocomotion. Yet, at 10 and 30 mg∙kg⁻¹ doses (i.p.), paynantheine did not counteract morphine CPP. 7-hydroxypaynantheine and 7-hydroxyspeciogynine displayed potency at δOR but limited µOR potency relative to 7-hydroxymitragynine in vitro, and dose-dependently decreased voluntary alcohol consumption in WT but not δOR in KO mice. 7-hydroxyspeciogynine has a maximally tolerated dose of at least 10 mg∙kg⁻¹ (s.c.) at which it did not produce significant CPP neither alter general locomotion nor induce noticeable seizures.

Conclusion and Implications: Derivatizing kratom alkaloids with the goal of enhancing δOR potency and reducing off-target effects could provide a pathway to develop novel lead compounds to treat alcohol use disorder with an improved therapeutic window.

A Novel Mitragynine Analog with Low-Efficacy Mu Opioid Receptor Agonism Displays Antinociception with Attenuated Adverse Effects

Mitragynine and 7-hydroxymitragynine (7OH) are the major alkaloids mediating the biological actions of the psychoactive plant kratom. To investigate the structure-activity relationships of mitragynine/7OH templates, we diversified the aromatic ring of the indole at the C9, C10, and C12 positions and investigated their G-protein and arrestin signaling mediated by mu opioid receptors (MOR). Three synthesized lead C9 analogs replacing the 9-OCH3 group with phenyl (4), methyl (5), or 3'-furanyl [6 (SC13)] substituents demonstrated partial agonism with a lower efficacy than DAMGO or morphine in heterologous G-protein assays and synaptic physiology. In assays limiting MOR reserve, the G-protein efficacy of all three was comparable to buprenorphine. 6 (SC13) showed MOR-dependent analgesia with potency similar to morphine without respiratory depression, hyperlocomotion, constipation, or place conditioning in mice. These results suggest the possibility of activating MOR minimally (G-protein Emax ≈ 10%) in cell lines while yet attaining maximal antinociception in vivo with reduced opioid liabilities.

Activity of Mitragyna speciosa ("Kratom") Alkaloids at Serotonin Receptors

Kratom alkaloids have mostly been evaluated for their opioid activity but less at other targets that could contribute to their physiological effects. Here, we investigated the in vitro and in vivo activity of kratom alkaloids at serotonin receptors (5-HTRs). Paynantheine and speciogynine exhibited high affinity for 5-HT1ARs and 5-HT2BRs, unlike the principal kratom alkaloid mitragynine. Both alkaloids produced antinociceptive properties in rats via an opioid receptor-independent mechanism, and neither activated 5-HT2BRs in vitro. Paynantheine, speciogynine, and mitragynine induced lower lip retraction and antinociception in rats, effects blocked by a selective 5-HT1AR antagonist. In vitro functional assays revealed that the in vivo 5-HT1AR agonistic effects may be due to the metabolites 9-O-desmethylspeciogynine and 9-O-desmethylpaynantheine and not the parent compounds. Both metabolites did not activate 5-HT2BR, suggesting low inherent risk of causing valvulopathy. The 5-HT1AR agonism by kratom alkaloids may contribute to the mood-enhancing effects associated with kratom use.

Kratom Alkaloids as Probes for Opioid Receptor Function: Pharmacological Characterization of Minor Indole and Oxindole Alkaloids from Kratom

Dry leaves of kratom (mitragyna speciosa) are anecdotally consumed as pain relievers and antidotes against opioid withdrawal and alcohol use disorders. There are at least 54 alkaloids in kratom; however, investigations to date have focused around mitragynine, 7-hydroxy mitragynine (7OH), and mitragynine pseudoindoxyl (MP). Herein, we probe a few minor indole and oxindole based alkaloids, reporting the receptor affinity, G-protein activity, and βarrestin-2 signaling of corynantheidine, corynoxine, corynoxine B, mitraciliatine, and isopaynantheine at mouse and human opioid receptors. We identify corynantheidine as a mu opioid receptor (MOR) partial agonist, whereas its oxindole derivative corynoxine was an MOR full agonist. Similarly, another alkaloid mitraciliatine was found to be an MOR partial agonist, while isopaynantheine was a KOR agonist which showed reduced βarrestin-2 recruitment. Corynantheidine, corynoxine, and mitraciliatine showed MOR dependent antinociception in mice, but mitraciliatine and corynoxine displayed attenuated respiratory depression and hyperlocomotion compared to the prototypic MOR agonist morphine in vivo when administered supraspinally. Isopaynantheine on the other hand was identified as the first kratom derived KOR agonist in vivo. While these minor alkaloids are unlikely to play the majority role in the biological actions of kratom, they represent excellent starting points for further diversification as well as distinct efficacy and signaling profiles with which to probe opioid actions in vivo.

Kratom use disorder: a primer for primary care physicians

Kratom is a substance similar to opioids that is often used for its euphoric effects, however it can be obtained legally in most of the United States. The substance is often not assessed on routine urine drug screen, however it is estimated that millions of people engage in kratom use each year and level of use is rising. Given the increasing prevalence of kratom use, and its potentially lethal consequences, it is imperative that primary care physicians be familiar with this substance and have a framework to approach identification and treatment of individuals with kratom use disorder. This manuscript offers a review of the epidemiology and pharmacology of kratom, along with guidance for care of individuals with kratom use disorder in the primary care setting.

Androgenic activation, impairment of the monoaminergic system and altered behavior in zebrafish larvae exposed to environmental concentrations of fenitrothion

Fenitrothion is an organophosphorus insecticide usually found in aquatic ecosystems at concentrations in the range of low ng/L. In this manuscript we show that 24 h exposure to environmental concentrations of fenitrothion, from ng/L to low μg/L, altered basal locomotor activity, visual-motor response and acoustic/vibrational escape response of zebrafish larvae. Furthermore, fenitrothion and expression of gap43a, gfap, atp2b1a, and mbp exhibited a significant non-monotonic concentration-response relationship. Once determined that environmental concentrations of fenitrothion were neurotoxic for zebrafish larvae, a computational analysis identified potential protein targets of this compound. Some of the predictions, including interactions with acetylcholinesterase, monoamine-oxidases and androgen receptor (AR), were experimentally validated. Binding to AR was the most suitable candidate for molecular initiating event, as indicated by both the up-regulation of cyp19a1b and sult2st3 and the non-monotonic relationship found between fenitrothion and the observed responses. Finally, when the integrity of the monoaminergic system was evaluated, altered levels of L-DOPA, DOPAC, HVA and 5-HIAA were found, as well as a significant up-regulation of slc18a2 expression at the lowest concentrations of fenitrothion. These data strongly suggest that concentrations of fenitrothion commonly found in aquatic ecosystems present a significant environmental risk for fish communities.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

High serum high-density lipoprotein and low serum triglycerides in Kratom users: A study of Kratom users in Thailand

Objective

The present study aims to examine the association between Kratom use and serum lipid level.

Method

This study compared the serum lipid profile of Kratom users and non-users living in Nam Phu Subdistrict, a special area that allows the traditional use of Kratom. The study subjects consisted of 581 individuals aged 18 and above. Binary logistic regression was used to determine an association between Kratom use and serum lipid level.

Results

The findings revealed an association between Kratom use and an elevated HDL level (≥60 mg/dL) with an adjusted OR of 1.82 (95% CI, 1.17-2.8), and an association between Kratom use and a triglyceride level <90 mg/dL with an adjusted OR of 1.75 (95% CI; 1.17-2.63). There were no associations between Kratom use and LDL as well as total cholesterol level.

Discussion and conclusions

This study provided additional evidence of Kratom use and a favorable lipid profile. Prevention of coronary heart disease or cerebrovascular disease via an improvement in the lipid profile may be a future pharmaceutical application of Kratom.

Exploring the Chemistry of Alkaloids from Malaysian Mitragyna speciosa (Kratom) and the Role of Oxindoles on Human Opioid Receptors

Ten indole and oxindole alkaloids (1-10) were isolated from the freshly collected leaves of Malaysian Mitragyna speciosa (Kratom). The chemical structures of these compounds were established on the basis of extensive 1D and 2D NMR and HRMS data analysis. The spectroscopic data of mitragynine oxindole B (4) are reported herein for the first time. The spatial configuration of mitragynine oxindole B (4) was confirmed by single-crystal X-ray diffraction. Simultaneous quantification of the isolated alkaloids in the M. speciosa leaf specimens collected from different locations in the northern region of Peninsular Malaysia was also performed using UPLC-MS/MS. The oxindole alkaloids (1-4) and the indole alkaloid (10) were assessed for binding affinity at opioid receptors. Corynoxine (1) showed high binding affinity to μ-opioid receptors with a K_i value of 16.4 nM. Further, corynoxine (1) was 1.8-fold more potent than morphine in rats subjected to a nociceptive hot plate assay. These findings have important implications for evaluating the combined effects of the minor oxindole alkaloids in the overall therapeutic activity of M. speciosa.

Pharmacological Comparison of Mitragynine and 7-Hydroxymitragynine: In Vitro Affinity and Efficacy for μ-Opioid Receptor and Opioid-Like Behavioral Effects in Rats

Relationships between µ-opioid receptor (MOR) efficacy and effects of mitragynine and 7-hydroxymitragynine are not fully established. We assessed in vitro binding affinity and efficacy and discriminative stimulus effects together with antinociception in rats. The binding affinities of mitragynine and 7-hydroxymitragynine at MOR (Ki values 77.9 and 709 nM, respectively) were higher than their binding affinities at κ-opioid receptor (KOR) or δ-opioid receptor (DOR). [35S]guanosine 5'-O-[γ-thio]triphosphate stimulation at MOR demonstrated that mitragynine was an antagonist, whereas 7-hydroxymitragynine was a partial agonist (Emax = 41.3%). In separate groups of rats discriminating either morphine (3.2 mg/kg) or mitragynine (32 mg/kg), mitragynine produced a maximum of 72.3% morphine-lever responding, and morphine produced a maximum of 65.4% mitragynine-lever responding. Other MOR agonists produced high percentages of drug-lever responding in the morphine and mitragynine discrimination assays: 7-hydroxymitragynine (99.7% and 98.1%, respectively), fentanyl (99.7% and 80.1%, respectively), buprenorphine (99.8% and 79.4%, respectively), and nalbuphine (99.4% and 98.3%, respectively). In the morphine and mitragynine discrimination assays, the KOR agonist U69,593 produced maximums of 72.3% and 22.3%, respectively, and the DOR agonist SNC 80 produced maximums of 34.3% and 23.0%, respectively. 7-Hydroxymitragynine produced antinociception; mitragynine did not. Naltrexone antagonized all of the effects of morphine and 7-hydroxymitragynine; naltrexone antagonized the discriminative stimulus effects of mitragynine but not its rate-decreasing effects. Mitragynine increased the potency of the morphine discrimination yet decreased morphine antinociception. Here we illustrate striking differences in MOR efficacy, with mitragynine having less than 7-hydroxymitragynine. SIGNIFICANCE STATEMENT: At human µ-opioid receptor (MOR) in vitro, mitragynine has low affinity and is an antagonist, whereas 7-hydroxymitragynine has 9-fold higher affinity than mitragynine and is an MOR partial agonist. In rats, intraperitoneal mitragynine exhibits a complex pharmacology including MOR agonism; 7-hydroxymitragynine has higher MOR potency and efficacy than mitragynine. These results are consistent with 7-hydroxymitragynine being a highly selective MOR agonist and with mitragynine having a complex pharmacology that combines low efficacy MOR agonism with activity at nonopioid receptors.

Liver injury associated with kratom, a popular opioid-like product: Experience from the U.S. drug induced liver injury network and a review of the literature

BACKGROUND

Kratom is a botanical product used as an opium substitute with abuse potential.

METHODS

Assessment of suspected cases of kratom-induced liver injury in a prospective US cohort.

RESULTS

Eleven cases of liver injury attributed to kratom were identified with a recent increase. The majority were male with median age 40 years. All were symptomatic and developed jaundice with a median latency of 14 days. The liver injury pattern was variable, most required hospitalization and all eventually recovered. Biochemical analysis revealed active kratom ingredients.

CONCLUSION

Kratom can cause severe liver injury with jaundice.

Preclinical pharmacokinetic study of speciociliatine, a kratom alkaloid, in rats using an UPLC-MS/MS method

Speciociliatine is a minor indole alkaloid found in kratom, a southeast Asian medicinal plant, used for centuries to increase energy, enhance mood, and mitigate pain and opioid dependence. An ultra-performance liquid chromatography tandem mass spectrometry method was developed and validated to quantify speciociliatine in rat plasma. The quantitation range was 3-600 ng/mL. The validated method was applied to a preclinical pharmacokinetic study in male Sprague-Dawley rats after 2.5 mg/kg intravenous (I.V.) and 20 mg/kg oral (P.O.) dosing. The plasma was analyzed to obtain concentration-time profiles and results were subjected to non-compartmental analysis to determine pharmacokinetic parameters including volume of distribution (6.2 ± 2.3 L/kg I.V.), clearance (0.7 ± 0.2 L/hr/kg), and absolute oral bioavailability (20.7 %). Speciociliatine had higher systemic exposure and lower clearance compared to the other kratom alkaloids mitragynine and corynantheidine. The speciociliatine pharmacokinetic parameters described here will help to better understand the overall effects reported with kratom product use.

Identification and characterization of plant-derived alkaloids, corydine and corydaline, as novel mu opioid receptor agonists

Pain remains a key therapeutic area with intensive efforts directed toward finding effective and safer analgesics in light of the ongoing opioid crisis. Amongst the neurotransmitter systems involved in pain perception and modulation, the mu-opioid receptor (MOR), a G protein-coupled receptor, represents one of the most important targets for achieving effective pain relief. Most clinically used opioid analgesics are agonists to the MOR, but they can also cause severe side effects. Medicinal plants represent important sources of new drug candidates, with morphine and its semisynthetic analogues as well-known examples as analgesic drugs. In this study, combining in silico (pharmacophore-based virtual screening and docking) and pharmacological (in vitro binding and functional assays, and behavioral tests) approaches, we report on the discovery of two naturally occurring plant alkaloids, corydine and corydaline, as new MOR agonists that produce antinociceptive effects in mice after subcutaneous administration via a MOR-dependent mechanism. Furthermore, corydine and corydaline were identified as G protein-biased agonists to the MOR without inducing β-arrestin2 recruitment upon receptor activation. Thus, these new scaffolds represent valuable starting points for future chemical optimization towards the development of novel opioid analgesics, which may exhibit improved therapeutic profiles.