Pharmacokinetics: Mitragynine as alternative treatment in opioid withdrawal
A systematic review has recently described the rudimentary pharmacokinetics of mitragynine, but studies lack information on the role of metabolism and redistribution into tissues or excretion rate.
The investigators systematically searched the databases of PubMed, Scopus and Web of Science for all types of pharmacokinetic studies of mitragynine from inception to June 2018. A total of 17 articles met the inclusion criteria.
A lipophilic weak base, mitragynine is passively transported across the intestinal wall and blood-brain barrier. Around 85–95 percent is bound to plasma protein, while phase I and especially phase II enzymes metabolize mitragynine extensively.
“Actions on CYP enzymes are unlikely to impact drug metabolism at concentrations likely to exist in kratom-consuming humans,” the investigators said.
Mitragynine is rapidly absorbed in rats and humans following oral administration (Tmax, ~1.5 h; Cmax, ~0.3−1.8 μM). Its volume distribution is 37–90 L/kg, with a half-life of 3–9 h. Most of it is excreted in urine as metabolites. Bioavailability is poor, which is estimated at 21 percent, but a salt form of the alkaloid offers significantly higher oral bioavailability value.
In addition, mitragynine quickly penetrates and redistributes in the brain due to its high lipid solubility and permeability.
The investigators developed a quality assessment tool tailored for pharmacokinetic studies to rate some studies of lower value.
Mitragynine is a major psychoactive and analgesic alkaloid found in kratom leaves common in southern Thailand and northern states of the Malay Peninsula. Kratom is commercially available and used as an alternative for the treatment of opioid withdrawal.