Abstract

Preparations from the leaves of the kratom plant (Mitragyna speciosa) are consumed for their opioid-like effects. Several deaths have been associated with kratom used concomitantly with some drugs. Pharmacokinetic interactions are potential underlying mechanisms of these fatalities. Accumulating in vitro evidence has demonstrated select kratom alkaloids, including the abundant indole alkaloid mitragynine, as reversible inhibitors of several cytochromes P450 (CYPs). The objective of this work was to refine the mechanistic understanding of potential kratom-drug interactions by considering both reversible and time-dependent CYP inhibition (TDI) in the liver and intestine. Mitragynine was tested against CYP2C9 (diclofenac 4′-hydroxylation), CYP2D6 (dextromethorphan O-demethylation), and CYP3A (midazolam 1′-hydroxylation) activities in human liver microsomes (HLMs) and CYP3A activity in human intestinal microsomes (HIMs). Comparing the absence to presence of NADPH during pre-incubation of mitragynine with HLMs or HIMs, an ~7-fold leftward shift in IC₅₀ (~20 to 3 μM) towards CYP3A resulted, prompting determination of TDI parameters (HLMs: K_I, 4.1 {plus minus} 0.9 μM; k_inact, 0.068 {plus minus} 0.01 min^-1; HIMs: K_I, 4.2 {plus minus} 2.5 μM; k_inact, 0.079 {plus minus} 0.02 min^-1). Mitragynine caused no leftward shift in IC₅₀ towards CYP2C9 (~40 μM) and CYP2D6 (~1 μM) but was a strong competitive inhibitor of CYP2D6 (K_i, 1.17 {plus minus} 0.07 μM). Using a recommended mechanistic static model, mitragynine (2-g kratom dose) was predicted to increase dextromethorphan and midazolam area under the plasma concentration-time curve (AUC) by 1.06 and 5.69-fold, respectively. The predicted midazolam AUC ratio exceeded the recommended cut-off (1.25), which would have been missed if TDI was not considered.

Significance Statement Kratom, a botanical natural product increasingly consumed for its opioid-like effects, may precipitate potentially serious pharmacokinetic interactions with drugs. The abundant kratom indole alkaloid, mitragynine, was shown to be a time-dependent inhibitor of hepatic and intestinal cytochrome P450 3A activity. A mechanistic static model predicted mitragynine to increase systemic exposure to the probe drug substrate midazolam by 5.7-fold, necessitating further evaluation via dynamic models and clinical assessment to advance understanding of consumer safety associated with kratom use.