Characterization of lipophilicity and blood partitioning of pyrrolizidine alkaloids and their N-oxides in vitro and in silico for toxicokinetic modeling.

Lipophilicity and blood partitioning are important determinants for predicting toxicokinetics using physiologically-based toxicokinetic (PBTK) modeling. In this study, the logarithm of the -octanol:water partition coefficient (log) and the blood-to-plasma concentration ratio ( ) were for the first time experimentally determined for the pyrrolizidine alkaloids (PAs) intermedine, lasiocarpine, monocrotaline, retrorsine and their -oxides (PANOs). Validated assays for log (miniaturized shake-flask method) and (LC-MS/MS-based depletion assay) determination were compared to an ensemble of models. Experimentally determined log indicate a higher affinity of PAs and PANOs to the aqueous compared to the organic phase. Depending on the method, determined log overpredicted the experimental values by ≥ 1 log unit for 3 out of 4 PAs (SPARC), 4 out of 6 PAs and PANOs (CLOGP), 5 out of 8 PAs and PANOs (KowWIN) and 3 out of 8 PAs and PANOs (S+logP). obtained suggested a low binding affinity of PAs and PANOs towards red blood cells. For all 8 PAs and PANOs, predicted deviated from experimental values by less than 50%. In conclusion, for PBTK modeling of PAs and PANOs, experimental log should be preferred, while predicted by the acid/base classification model is a suitable surrogate for experimental data.