Pharmacology can be divided into two branches: pharmacodynamics and pharmacokinetics. Simply put pharmacodynamics is how a compound affects the body whereas pharmacokinetics is how the body affects a compound. In preclinical in vitro investigation, pharmacodynamics looks at the mechanism of action of a compound upon entrance into the body, as well as the measurable physiological effect. Pharmacokinetics examines the absorption (how the compound gets into the system), distribution (where the compound goes once it enters the systemic circulation), metabolism (how the body breaks the compound down), and excretion (how the body eliminates the compound), referred to as ADME.

In Vitro Pharmacodynamics

In vitro pharmacodynamics focuses on the interaction of a compound with body systems. Compounds that are ingested into the body may interact with receptors to produce an effect. Receptors are proteins that are found inside or on the surface of cells within the human body. Humans produce chemical messengers (i.e., hormones or neurotransmitters) that interact with these receptors and these are called endogenous compounds. Compounds that are supplied externally through food, supplements, or medication are considered exogenous compounds.

The interactions that can occur between an exogenous compound and a receptor can be defined in several ways. If the compound binds like an endogenous chemical messenger, it is considered a receptor agonist. To describe this type of interaction the theory of “lock and key” is often used where the exogenous compound perfectly fits into the receptor and unlocks and opens that receptor causing the same downstream effects that an endogenous chemical messenger would. Other exogenous compounds are defined as partial agonists indicating that they fit into a receptor but not perfectly, causing some expected physiological effects from the partial activation of the receptor, but not all. There are also exogenous compounds that act as antagonists which bind to a receptor and block it – not allowing others compounds to come in and produce the effects of that receptor.

VIDEO: Receptor Pharmacology

The receptor pharmacology of select kratom alkaloids have been investigated in different in vitro models. In vitro models are laboratory models that mimic human cell-based systems but take place entirely outside of the body. The body contains many types of receptors that regulate the functions of the human body. Kratom alkaloids have been found to have activity at several central nervous system (CNS) receptor types including adrenergic, dopaminergic, opioid, and serotonergic. Within each receptor type – there are many subtypes (1).

Kratom contains over forty indole alkaloids. The most abundant, mitragynine, shows partial agonism at select opioid receptors as well as moderate and moderate and nonselective agonism at dopaminergic and adrenergic receptors, respectively (2-8). Speciociliatine is a partial agonist at select opioid receptors (5). Speciogynine and paynantheine display partial agonism at select serotonin receptors (2, 7). Speciogynine has moderate and nonselective activity at select adrenergic receptor subtypes while corynantheidine is an opioid agonist at select subtypes and shows agonism at select adrenergic receptors (3, 9-11). A metabolite of mitragynine, 7-hydroxymitragynine, is a full opioid agonist at the μ-opioid receptor (4, 5, 8, 10).

The CNS pharmacology of kratom alkaloids most likely contributes to the varying effects described after ingestion of kratom products. This unique polypharmacology makes kratom one of the most interesting complex natural products to study with incredible potential.

VIDEO: Alkaloid Binding

In vitro Pharmacokinetics

Any exogenous compound must be absorbed, distributed to the site of action, metabolized, and excreted. Supplements and drugs are primarily metabolized and prepared for excretion by the liver. Therefore, in vitro systems of metabolism use human liver hepatocytes, recombinant cytochrome P450 (CYP450) metabolizing enzymes, or liver S9 fractions to understand the fate of a compound.

Kratom alkaloids are lipophilic (“fat-loving”) compounds which allows them to passively diffuse through the intestinal wall, so are readily absorbed into the systemic circulation when delivered orally in solution and distribute to the brain across the blood brain barrier (12, 13). Once in the brain, alkaloids interact with receptors to produce a physiological response. To facilitate excretion, alkaloids are first metabolized by liver enzymes to produce more water-soluble compounds that can be filtered out of the body through urine (1).

There are two phases of drug metabolism characterized as Phase I (oxidation, reduction, hydroxylation catalyzed by enzymes) and Phase II (glucuronidation and sulfonation). Human urine samples were analyzed for metabolites of kratom alkaloids (mitragynine, speciogynine, speciociliatine, and paynantheine) and found both Phase I and Phase II metabolites (14-18).

VIDEO: In Vitro Absorption, Distribution, Metabolism, and Excretion (ADME)

To date, metabolite extensive metabolic studies have only been performed for mitragynine and speciociliatine in human liver fractions (S9 and hepatocytes) focusing on Phase I metabolism by drug metabolizing enzymes (19, 20). It was found that both mitragynine and speciociliatine were metabolized primarily by CYP450 isoform 3A4 to O-demethylated and mono-oxidative metabolites with minor contributions from other CYP450 isoforms. Speciociliatine is more metabolically stable in human liver microsomes than mitragynine with a half-life of 41.8 minutes compared to 20.0 minutes for mitragynine (5).

One of the active metabolites of mitragynine, 7-hydroxymitragynine is metabolized by CYP3A4. This metabolite is a full agonist at μ-opioid receptors meaning it must be examined. Further study into other potential active metabolites of minor kratom alkaloids, as well as the contribution of minor alkaloids themselves to the overall effect of kratom use is warranted.

IN VITRO

In Vitro Pharmacodynamics

In vitro Pharmacokinetics

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