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

Abstract

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