Postmortem distribution of mitragynine and 7-hydroxymitragynine in 51 cases

The prevalence of mitragynine (kratom) in forensic toxicology casework has steadily increased over time. Readily available and currently legal, mitragynine is widely used for its stimulant and, depending on concentration, sedative effects. Our laboratory analyzed various fluid and tissue specimens from 51 postmortem cases to investigate the distribution of mitragynine and its active metabolite 7-hydroxymitragynine. Central and peripheral blood concentrations were compared, with an average heart blood to femoral blood ratio being 1.37 for mitragynine and 1.08 for 7-hydroxymitragynine. This ratio >1.0 suggests that mitragynine has some propensity toward postmortem redistribution; however, the difference in concentrations of mitragynine and 7-hydroxymitragynine is not statistically significant. Large average mitragynine to 7-hydroxymitragynine ratios of 30.9 in femoral blood and 32.4 in heart blood were observed compared to average ratios of 14.8 in vitreous humor and 16.9 in urine. In addition, the stability of these two compounds was investigated in both matrix and organic solvent. When stored refrigerated (4°C), mitragynine was stable for up to 30 days and 7-hydroxymitragynine was stable for up to 7 days with an analyte loss of <20%. Following 60 days of refrigerated storage, 7-hydroxymitragynine concentrations dropped over 50% from initial concentrations. Methanolic preparations of mitragynine and 7-hydroxymitragynine were stable following 3 months of storage at -20°C.