Use of Physiologically Based Biokinetic (PBBK) Modeling to Study Estragole Bioactivation and Detoxification in Humans as Compared with Male Rats

Abstract

The extent of bioactivation of the herbal constituent estragole to its ultimate carcinogenic metabolite 1′-sulfooxyestragole depends on the relative levels of bioactivation and detoxification pathways. The present study investigated the kinetics of the metabolic reactions of both estragole and its proximate carcinogenic metabolite 1′-hydroxyestragole in humans in incubations with relevant tissue fractions. Based on the kinetic data obtained a physiologically based biokinetic (PBBK) model for estragole in human was defined to predict the relative extent of bioactivation and detoxification at different dose levels of estragole. The outcomes of the model were subsequently compared with those previously predicted by a PBBK model for estragole in male rat to evaluate the occurrence of species differences in metabolic activation. The results obtained reveal that formation of 1′-oxoestragole, which represents a minor metabolic route for 1′-hydroxyestragole in rat, is the main detoxification pathway of 1′-hydroxyestragole in humans. Due to a high level of this 1′-hydroxyestragole oxidation pathway in human liver, the predicted species differences in formation of 1′-sulfooxyestragole remain relatively low, with the predicted formation of 1′-sulfooxyestragole being twofold higher in human compared with male rat, even though the formation of its precursor 1′-hydroxyestragole was predicted to be fourfold higher in human. Overall, it is concluded that in spite of significant differences in the relative extent of different metabolic pathways between human and male rat there is a minor influence of species differences on the ultimate overall bioactivation of estragole to 1′-sulfooxyestragole.

A roadmap for the development of alternative (non-animal) methods for systemic toxicity testing

“…Those reactions were described well with Michaelis-Menten kinetics and the resulting Vmax and Km parameters were scaled to in vivo based on the microsomal or S9 protein content. The interplay of these multiple reactions was integrated in the PBBK model and the simulated concentrations of two estragole metabolites in the rat and human urine were reasonably consistent with the observed in vivo data considering the purpose of the modeling was to evaluate the dose-dependent changes in bioactivation, not to predict the absolute dose metrics (Anthony et al, 1987;Punt et al, 2008Punt et al, , 2009 (Paine et al, 1997). From the risk assessment point of view, other phase I and II enzymes in the GI tract should also be carefully considered in IVIVE.…”

“…A couple of publications by Punt and colleagues (Punt et al, 2008(Punt et al, , 2009) present an example of a more sophisticated QI-VIVE approach using metabolism data collected in a number of subcellular fractions. Although the intent of the study was to evaluate the relevance of carcinogenicity of estragole reported in high-dose animal studies to human exposure situations, a similar QIVIVE approach also could be applied for the interpretation of in vitro toxicity assays.…”

“…The key metabolism parameters to be estimated were rates of multiple biotransformation reactions that determine the level of carcinogenic species (1-sulfooxyestragole) in the liver. Due to the complexity of metabolism steps involved in formation of the ultimate carcinogenic metabolite of estragole as well as detoxication of parent compound and other intermediate metabolites, the approach using a combination of subcellular fractions along with different cofactors was more valuable for the purpose of their modeling than using a more integrated system such as hepatocytes (Punt et al, 2008(Punt et al, , 2009. By manipulating cofactors such as NADPH, UDPGA, NAD + , and PAPS in the selected in vitro system of microsomes or S9, multiple steps of estragole metabolism mediated by CYPs, UGTs, dehydrogenases, and SULTs, respectively, could be characterized.…”

A physiologically based in silico kinetic model predicting plasma cholesterol concentrations in humans

“…The average relative deviations between model predictions and experimental data were 36%, 49%, and 43% for TC, HDL-C, and non-HDL-C, respectively. This is considered successful within the present state-of-the-art of PBK modeling, where quantitative predictions may generally be correct within one order of magnitude (25)(26)(27)(28). These model predictions are generally within the experimental error margin given the small patient groups sizes (generally n < 20).…”

Bioactivation of herbal constituents: simple alerts in the complex system

“…For estragole that needs bioactivation to exert toxicity, detoxification of estragole 2¢, 3¢-oxides by epoxide hydrolases and glucuronidation of 1¢-hydroxyestragole would influence the toxicological effects [109,110]. Additionally, the formation of 1¢-oxoestragole is another important detoxification pathway of estragole in human [111]. Multiple herbal components might influence bioactivation of specific herbal ingredient through producing a significant impact on these competitive metabolism pathways.…”