Development and translational application of a rat-to-human PBPK model for isopropoxate to support forensic exposure...

Isopropoxate is a new synthetic etomidate analog and emerging new psychoactive substance, posing severe public health risks due to its high abuse liability and overdose potential. Ethical constraints preclude clinical pharmacokinetic investigations, necessitating alternative mechanistic approaches. In this study, we developed the first physiologically based pharmacokinetic (PBPK) model for isopropoxate by integrating in vitro data of human liver microsomes and in vivo data of rats. The rat PBPK model accurately described dose-dependent plasma concentration-time profiles, with predicted-to-observed ratios remaining within the accepted two-fold range, as reflected by fold errors (FE) of 0.944-1.12 for C_max and 0.744-1.68 for AUC_0-∞ across all dose levels. In addition, dedicated PBPK submodels were established and validated for the primary metabolite of etomidate acid with FE ranging from 0.800 to 1.01 for C_max. Human pharmacokinetics were extrapolated using in vitro-in vivo extrapolation (IVIVE) and allometric scaling, predicting a high oral bioavailability of 94.9% and a systemic clearance of 0.31 mL/min/kg. The model facilitates pharmacokinetic interpretation, postmortem back-calculation of exposure timing, and assessment of metabolite production, thereby supporting forensic and overdose risk evaluation. Beyond predicting pharmacokinetics, the developed PBPK framework provides a practical tool for forensic toxicology by supporting interpretation of parent-metabolite relationships in biological specimens, reconstruction of exposure timelines under different scenarios, and mechanistic comparison of exposure profiles following different routes of administration.