Development of a Web-Based Toolbox to Support Quantitative In-Vitro-to-In-Vivo Extrapolations (QIVIVE) within Nonanimal Testing Strategies

Abstract

The goal of the present study was to develop an online web-based toolbox that contains generic physiologically based kinetic (PBK) models for rats and humans, including underlying calculation tools to predict plasma protein binding and tissue:plasma distribution, to be used for quantitative in-vitro-to-in-vivo extrapolations (QIVIVE). The PBK models within the toolbox allow first estimations of internal plasma and tissue concentrations of chemicals to be made, based on the logP and pK_a of the chemicals and values for intestinal uptake and intrinsic hepatic clearance. As a case study, the toolbox was used to predict oral equivalent doses of in vitro ToxCast bioactivity data for the food additives methylparaben, propyl gallate, octyl gallate, and dodecyl gallate. These oral equivalent doses were subsequently compared with human exposure estimates, as a low tier assessment allowing prioritization for further assessment. The results revealed that daily intake levels of especially propyl gallate can lead to internal plasma concentrations that are close to in vitro biological effect concentrations, particularly with respect to the inhibition of human thyroid peroxidase (TPO). Estrogenic effects were not considered likely to be induced by the food additives, as daily exposure levels of the different compounds remained 2 orders of magnitude below the oral equivalent doses for in vitro estrogen receptor activation. Overall, the results of the study show how the toolbox, which is freely accessible through www.qivivetools.wur.nl, can be used to obtain initial internal dose estimates of chemicals and to prioritize chemicals for further assessment, based on the comparison of oral equivalent doses of in vitro biological activity data with human exposure levels.

Figure 1

Molecular structures of methylparaben and the different gallates included in the present study.

Figure 2

Structure of the PBK model that is integrated in the toolbox. CLint corresponds to the intrinsic hepatic clearance, GFR corresponds to the glomerular filtration rate, and fup corresponds to the fraction unbound in plasma.

Figure 3

Substrate depletion of methylparaben, propyl gallate, octyl gallate, and dodecyl gallate in incubations with rat (gray lines and dots) or human (black lines and dots) liver S9. The dots correspond to the observed time-dependent fraction of the concentration measured at t = 0 that remains in the incubation. For each experimental condition, the optimal S9 concentration (0.1, 0.5, or 1 mg/mL) was determined in pilot studies (data not shown).

Figure 4

Screenshots of the simulated total (bound and unbound) plasma concentrations of methylparaben, propyl gallate, octyl gallate, and dodecyl gallate at 0.1 mg/kg bw in humans and rats using the web-based toolbox.

Figure 5

PBK-model-predicted (smooth line) and in vivo observed (symbols) plasma concentrations of propyl gallate and octyl gallate in rats at different oral doses. In vivo rat data were taken from Tullberg et al. and represent reported average values for each sex of eight rats for 135 mg propyl gallate/kg bw, six rats for 14 mg propyl gallate/kg bw, and five rats for 17.5 mg octyl gallate/kg bw. More details on the in vivo and PBK-model-predicted TK parameters (Cmax and AUC) are presented in Table 2.

Figure 6

PBK-model-predicted (smooth line) and in vivo observed (symbols) plasma concentrations of propyl gallate in humans at different oral doses. In vivo human data were taken from Tullberg et al. and represent reported average values for each gender of five persons for 14 mg of propyl gallate/kg bw and four females and seven males for 1.4 mg of propyl gallate/kg bw. More details on the in vivo and PBK-model-predicted TK parameters (Cmax and AUC) are presented in Table 2.

Figure 7

Box (25–75th percentile) and whisker (1.5 × IQR) plots, depicting the range of AC50 values of the chemicals within the ToxCast data set. The AC50 values for ER-related assays (blue dots), the ToxCast-reported cytotoxicity limits (red dots), and the NCCT_TPO_AUR_dn assay (green dots) are highlighted within the graph. Methylparaben has not been tested in the NCCT_TPO_AUR_dn assay. Black dots represent assays with AC50 values lower than the 1.5 × IQR whisker.

Figure 8

Box (25–75th percentile) and whisker (1.5 × IQR) plots depicting the range of oral equivalent doses of the AC50 values (Figure 7) of the chemicals within the ToxCast data set and comparison of these oral equivalent doses to oral daily intake estimates (open dots).^,,, The oral equivalent doses related to ER-related assays (blue dots), the ToxCast-reported cytotoxicity limit (red dots), and the TPO inhibition assay (green dots) are highlighted within the graph.