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DOI: 10.1055/a-2523-3987
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 modeling. In this study, the logarithm of the n-octanol : water partition coefficient and the blood-to-plasma concentration ratio were, for the first time, experimentally determined for the pyrrolizidine alkaloids intermedine, lasiocarpine, monocrotaline, retrorsine, and their N-oxides. Validated in vitro assays for determination of the n-octanol : water partition coefficient (miniaturized shake-flask method) and the blood-to-plasma conentration ratio (LC-MS/MS-based depletion assay) were compared to an ensemble of in silico models. The experimentally determined octanol : water partition coefficient indicates a higher affinity of pyrrolizidine alkaloids and their N-oxides to the aqueous compared to the organic phase. Depending on the method, in silico determined n-octanol : water partition coefficients overpredicted the experimental values by ≥ 1 log unit for three out of four pyrrolizidine alkaloids (SPARC), four out of six (CLOGP), five out of eight (KowWIN), and three out of eight (S+logP) pyrrolizidine alkaloids and their N-oxides. The blood-to-plasma concentration ratio obtained in vitro suggested a low binding affinity of pyrrolizidine alkaloids and their N-oxides towards red blood cells. For all eight pyrrolizidine alkaloids and their N-oxides, in silico predicted blood-to-plasma ratios deviated from experimental values by less than 50%. In conclusion, for physiologically-based toxicokinetic modeling of pyrrolizidine alkaloids and their N-oxides, the experimental octanol : water partition coefficient should be preferred, while the blood-to-plasma concentration ratio predicted by the acid/base classification model is a suitable surrogate for experimental data.
Keywords
logP - octanol : water partition coefficient - blood-to-plasma ratio - drug distribution - pharmacokinetics - PBTK modelingSupporting Information
- Supporting Information
Linear regression of logP values (Fig. 1 aS), and Rb values (Fig 1 bS) of reference substances experimentally determined in this work versus literature, the effect of the concentration on the Rb value of reference substances (Fig. 2S), experimentally determined and predicted logP values of reference substances (Fig. 3S), representative ddMS2 data of reference substances (Figs. 4S–10S), predicted pKa values and predominant charge state at experimental pH (logP determination) of PAs, PANOs and reference substances (Table 1S), logP values and Rb values of reference substances (Table 2S), MS/MS transitions and parameters (Table 3S), and mass spectrometric peak areas and calculated recovery of reference substances for logP determination (Table 4S) are available as Supporting Information.
Publication History
Received: 29 July 2024
Accepted after revision: 05 January 2025
Accepted Manuscript online:
23 January 2025
Article published online:
21 February 2025
© 2025. The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution License, permitting unrestricted use, distribution, and reproduction so long as the original work is properly cited. (https://creativecommons.org/licenses/by/4.0/)
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