title: Incorporating population variability and susceptible subpopulations into dosimetry for high-throughput toxicity testing.
abstract: Momentum is growing worldwide to use in vitro high-throughput screening (HTS) to evaluate human health effects of chemicals. However, the integration of dosimetry into HTS assays and incorporation of population variability will be essential before its application in a risk assessment context. Previously, we employed in vitro hepatic metabolic clearance and plasma protein binding data with in vitro in vivo extrapolation (IVIVE) modeling to estimate oral equivalent doses, or daily oral chemical doses required to achieve steady-state blood concentrations (Css) equivalent to media concentrations having a defined effect in an in vitro HTS assay. In this study, hepatic clearance rates of selected ToxCast chemicals were measured in vitro for 13 cytochrome P450 and five uridine 5'-diphospho-glucuronysyltransferase isozymes using recombinantly expressed enzymes. The isozyme-specific clearance rates were then incorporated into an IVIVE model that captures known differences in isozyme expression across several life stages and ethnic populations. Comparison of the median Css for a healthy population against the median or the upper 95th percentile for more sensitive populations revealed differences of 1.3- to 4.3-fold or 3.1- to 13.1-fold, respectively. Such values may be used to derive chemical-specific human toxicokinetic adjustment factors. The IVIVE model was also used to estimate subpopulation-specific oral equivalent doses that were directly compared with subpopulation-specific exposure estimates. This study successfully combines isozyme and physiologic differences to quantitate subpopulation pharmacokinetic variability. Incorporation of these values with dosimetry and in vitro bioactivities provides a viable approach that could be employed within a high-throughput risk assessment framework.
authors.affiliation: The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709-2137 bwetmore@thehamner.org.
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Wetmore
authors.affiliation: The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709-2137.
authors.forename: Brittany
authors.initials: B
authors.lastname: Allen
authors.affiliation: The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709-2137.
authors.forename: Harvey J
authors.initials: HJ
authors.lastname: Clewell
authors.affiliation: The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709-2137.
authors.forename: Timothy
authors.initials: T
authors.lastname: Parker
authors.affiliation: United States Environmental Protection Agency, Office of Research and Development, National Center for Computational Toxicology, Research Triangle Park, North Carolina 27711.
authors.forename: John F
authors.initials: JF
authors.lastname: Wambaugh
authors.affiliation: Simcyp Limited (a Certara company), Blades Enterprise Centre, John Street, Sheffield S2 4SU, UK.
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Almond
authors.affiliation: The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709-2137.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Sochaski
authors.affiliation: The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709-2137 bwetmore@thehamner.org.
authors.forename: Russell S
authors.initials: RS
authors.lastname: Thomas
mesh_terms: D000284:Administration, Oral; D000367:Age Factors; D023401:Animal Use Alternatives; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D014453:Glucuronosyltransferase / Q000235:genetics / Q000378:metabolism*; D057166:High-Throughput Screening Assays / Q000379:methods* / Q000706:statistics & numerical data; D006801:Humans; D007527:Isoenzymes; D008657:Metabolic Clearance Rate; D008954:Models, Biological; D011994:Recombinant Proteins; D018570:Risk Assessment; D061987:Sf9 Cells; D018411:Spodoptera; D018675:Toxicity Tests / Q000379:methods* / Q000706:statistics & numerical data; D014162:Transfection; D015262:Xenobiotics / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000633:toxicity
chemical_list: D007527:Isoenzymes; D011994:Recombinant Proteins; D015262:Xenobiotics; D003577:Cytochrome P-450 Enzyme System; D014453:Glucuronosyltransferase
keywords: In vitro in vivo extrapolation; dosimetry; population variability; reaction phenotyping; risk assessment; toxicokinetics