Are the (New) Synthetic Opioids U-47700, Tramadol and Their Main Metabolites Prone to Time-Dependent Postmortem Redistribution?—A Systematic Study Using an In Vivo Pig Model.

This article focuses on the postmortem (PM) distribution and potential redistribution (PMR) of the synthetic opioids U-47700 and tramadol, along with their main metabolites N-desmethyl-U-47700 and O-desmethyltramadol (ODT), using a controlled toxicokinetic study in pigs. Following intravenous administration and euthanasia, tissue and fluid samples were collected up to 72 hours postmortem and analyzed via liquid chromatography–tandem mass spectrometry. The study found only low-to-moderate concentration changes over time in most tissues, with a notable increase of tramadol in liver tissue and decreases in bile and duodenum content, suggesting these latter matrices should be avoided for quantitative PM analysis. Central blood showed stable concentrations for both opioids and their metabolites, indicating it as the preferred matrix for PM quantification, while peripheral blood exhibited variable results. Overall, U-47700, tramadol, and their metabolites demonstrated a low tendency for PM redistribution under the study conditions.