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Cross Species Comparisons of Toxicokinetic Parameters for In Vitro-In Vivo Correlation Analyses
Citation:
Jarnagin, A., E. Korol-Bexell, A. Brennan, J. Ford, J. Wambaugh, D. Macmillan, M. Hughes, AND B. Wetmore. Cross Species Comparisons of Toxicokinetic Parameters for In Vitro-In Vivo Correlation Analyses. SOT, Nashville, TN, March 19 - 23, 2023. https://doi.org/10.23645/epacomptox.22294393
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Description:
In toxicology, chemical hazard evaluations have historically relied on rodent data. Increasing use of New Approach Methods (NAMs) has enabled a shift to a combination of human in vitro toxicity testing and toxicokinetic (TK) data in a process known as in vitro-in vivo extrapolation (IVIVE) to predict in vivo points of departure. Concentrations at which bioactivity is observed in vitro are used in conjunction with chemical TK parameters to estimate the administered equivalent dosage (AED) required to achieve the bioactive blood concentrations. At the US EPA, a high-throughput TK (HTTK) approach using hepatic clearance and plasma protein binding (PPB) data to estimate human internal concentrations has demonstrated good predictivity during in vitro-in vivo correlation (IVIVC) analyses, although it has exhibited a consistent overestimation of steady state concentrations. Moreover, these IVIVC analyses rely primarily on clinical drug data where in vivo human TK data exists rather than commercial chemicals of specific interest to the EPA. Given the dependence on in vivo rodent TK data to perform IVIVC analyses for non-drug chemicals, a more thorough evaluation of rat-specific IVIVE is warranted to better understand HTTK model performance in rat while also revealing potential interspecies differences in absorption, distribution, metabolism and excretion (ADME). While earlier work on a subset of parameters showed cross-species agreement, certain sets of chemicals are known to exhibit species differences. Moreover, differences in metabolic enzymes, permeability/transport mechanisms and/or physiologies may limit the utility of a rodent species to model a specific human ADME process. In this case study, chemicals covering a range of uses, non-drug functional group presence and metabolic capabilities were selected for in vitro and in vivo TK analyses. In in vivo studies, male Sprague Dawley (SD) rats were dosed orally and by intravenous injection one time with 1 or 5 mg/kg, respectively, the blood serially collected at 0-72 hr after dosing to obtain ADME inputs including clearance (Cl), bioavailability (F), and volume of distribution (Vd). In parallel, in vitro evaluations of PPB and hepatic and intestinal clearance were performed using mixed pools of human and SD rat tissues. In vivo TK parameters spanned a wide range: Cl ranged from less than 0.01 (acifluorfen) to over 500 mL/hr (acephate); F ranged from ~0.005 (diazinon) to 1 (acephate); and Vd ranged from ~0.1 (acifluorfen) and over 10,000 mL (acephate, oral route). Bioavailability was inversely related to likelihood of human intestinal clearance as defined by CYP3A4 substrate specificity: chemicals not predicted in silico to be CYP3A4 substrates showed the highest bioavailability (acephate, acifluorfen) while predicted substrates exhibited F = 0.4 or lower (e.g., diazinon, difenoconazole). Comparisons between human and rat in vitro PPB showed good agreement (r2>0.9) except for n-butyl paraben, which was rapidly degraded in rodent plasma, likely due to higher activity of rodent plasma esterases. Although there was agreement in rank order of hepatic clearance across the two species, rodent microsomal Cl was consistently higher than that in humans, typically by 2-3 fold. Similar to hepatic clearance comparisons, qualitative but not quantitative agreement was noted in intestinal clearance evaluations. Further analyses are planned to evaluate performance of HT-IVIVE in predicting rat TK and to pinpoint species differences that may hinder application of rodent TK data in refining human HT-IVIVE. This abstract does not necessarily reflect the views of the U.S. EPA.
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DOI: Cross Species Comparisons of Toxicokinetic Parameters for In Vitro-In Vivo Correlation Analyses
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