Determination of ADME properties of mitragynine, 7-hydroxymitragynine, and mitraphylline

Traditionally, Mitragyna speciosa (Kratom) which is a popular herb in Southeast Asia, is used in the treatment of withdrawal symptoms of opiate addiction. Mitragynine, 7-hydroxymitragynine and mitraphylline are the main pharmacologically active alkaloids with wide spectrum of therapeutic properties. However, limited studies are reported in literature regarding their ADME properties. Hence, the current study focuses on evaluating the ADME properties of these three alkaloids and their effects on major efflux transporter P-gp. Mitragynine and 7-hydroxymitragynine were both stable in simulated intestinal fluid (SIF). While, in simulated gastric fluid (SGF), mitragynine (26% degradation) and 7-hydroxymitragynine (27% degradation) were highly unstable. In contrast, mitraphylline was stable in SGF but unstable in SIF (13.6% degradation). Mitragynine and 7-hydroxymitragynine showed moderate permeability across Caco-2 and MDR-MDCKII monolayers with no significant efflux. However, mitraphylline was subjected to efflux mediated by P-glycoprotein (P-gp) in both Caco-2 and MDR-MDCKII monolayers. In human liver microsomes (HLM), phase I metabolism was evident for mitraphylline and 7-hydroxymitragynine with half-life (T_1/2) of 24 and 50 min, respectively. Mitragynine was metabolically stable in both human liver microsomes (HLM) and S9 fractions. All the three compounds exhibited high plasma protein binding (> 90%) determined by equilibrium dialysis. In Calcein-AM uptake assay, both mitragynine and 7-hydroxymitragynine showed inhibition of P-gp (EC₅₀ 18 ± 3.6µM and 32 ± 1.9µM, respectively) similar to verapamil (EC₅₀ value of 34 ± 1.8µM) suggesting a possibility of drug interaction with conventional drugs that are P-gp substrates. Mitraphylline, however, showed no inhibition of P-gp.