title: IWGT report on quantitative approaches to genotoxicity risk assessment I. Methods and metrics for defining exposure-response relationships and points of departure (PoDs).
abstract: This report summarizes the discussion, conclusions, and points of consensus of the IWGT Working Group on Quantitative Approaches to Genetic Toxicology Risk Assessment (QWG) based on a meeting in Foz do Iguaçu, Brazil October 31-November 2, 2013. Topics addressed included (1) the need for quantitative dose-response analysis, (2) methods to analyze exposure-response relationships & derive point of departure (PoD) metrics, (3) points of departure (PoD) and mechanistic threshold considerations, (4) approaches to define exposure-related risks, (5) empirical relationships between genetic damage (mutation) and cancer, and (6) extrapolations across test systems and species. This report discusses the first three of these topics and a companion report discusses the latter three. The working group critically examined methods for determining point of departure metrics (PoDs) that could be used to estimate low-dose risk of genetic damage and from which extrapolation to acceptable exposure levels could be made using appropriate mode of action information and uncertainty factors. These included benchmark doses (BMDs) derived from fitting families of exponential models, the No Observed Genotoxic Effect Level (NOGEL), and "threshold" or breakpoint dose (BPD) levels derived from bilinear models when mechanistic data supported this approach. The QWG recognizes that scientific evidence suggests that thresholds below which genotoxic effects do not occur likely exist for both DNA-reactive and DNA-nonreactive substances, but notes that small increments of the spontaneous level cannot be unequivocally excluded either by experimental measurement or by mathematical modeling. Therefore, rather than debating the theoretical possibility of such low-dose effects, emphasis should be placed on determination of PoDs from which acceptable exposure levels can be determined by extrapolation using available mechanistic information and appropriate uncertainty factors. This approach places the focus on minimization of the genotoxic risk, which protects against the risk of the development of diseases resulting from the genetic damage. Based on analysis of the strengths and weaknesses of each method, the QWG concluded that the order of preference of PoD metrics is the statistical lower bound on the BMD > the NOGEL > a statistical lower bound on the BPD. A companion report discusses the use of these metrics in genotoxicity risk assessment, including scaling and uncertainty factors to be considered when extrapolating below the PoD and/or across test systems and to the human.
authors.affiliation: Toxicology Consulting Services, Bonita Springs, FL, USA. Electronic address: jtmacgregor@earthlink.net.
authors.forename: James T
authors.initials: JT
authors.lastname: MacGregor
authors.affiliation: Bundesinstitut für Arzneimittel und Medizinprodukte, Bonn, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Frötschl
authors.affiliation: Health Canada, Ottawa, ON, Canada. Electronic address: paul.white@hc-sc.gc.ca.
authors.forename: Paul A
authors.initials: PA
authors.lastname: White
authors.affiliation: Ruston, LA, USA.
authors.forename: Kenny S
authors.initials: KS
authors.lastname: Crump
authors.affiliation: University of California, Riverside, CA, USA.
authors.forename: David A
authors.initials: DA
authors.lastname: Eastmond
authors.affiliation: Japan Bioassay Research Center, Hadano, Kanagawa, Japan.
authors.forename: Shoji
authors.initials: S
authors.lastname: Fukushima
authors.affiliation: F. Hoffmann-La Roche Ltd., Basel, Switzerland.
authors.forename: Melanie
authors.initials: M
authors.lastname: Guérard
authors.affiliation: Biosafety Research Center, Foods, Drugs and Pesticides, Iwata, Shizuoka, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Hayashi
authors.affiliation: RIVM-National Institute for Public Health and the Environment, Bilthoven, Netherlands.
authors.forename: Lya G
authors.initials: LG
authors.lastname: Soeteman-Hernández
authors.affiliation: Kao Corporation, Ichikai-Machi, Haga-Gun, Tochigi, Japan.
authors.forename: Toshio
authors.initials: T
authors.lastname: Kasamatsu
authors.affiliation: U.S. Food and Drug Administration, College Park, MD, USA.
authors.forename: Dan D
authors.initials: DD
authors.lastname: Levy
authors.affiliation: National Institute of Health Sciences, Tokyo, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Morita
authors.affiliation: Hoffmann-La Roche Inc., Basel, Switzerland.
authors.forename: Lutz
authors.initials: L
authors.lastname: Müller
authors.affiliation: U.S. Environmental Protection Agency, Washington, DC, USA.
authors.forename: Rita
authors.initials: R
authors.lastname: Schoeny
authors.affiliation: Pfizer, Inc., Groton, CT, USA.
authors.forename: Maik J
authors.initials: MJ
authors.lastname: Schuler
authors.affiliation: Sanofi, Vitry sur Seine, France.
authors.forename: Véronique
authors.initials: V
authors.lastname: Thybaud
authors.affiliation: Swansea University, Swansea, United Kingdom.
authors.forename: George E
authors.initials: GE
authors.lastname: Johnson
mesh_terms: D004247:DNA / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008957:Models, Genetic; D009152:Mutagenicity Tests / Q000379:methods / Q000592:standards; D009153:Mutagens / Q000032:analysis* / Q000633:toxicity*; D009154:Mutation; D009369:Neoplasms / Q000139:chemically induced / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D018570:Risk Assessment
chemical_list: D009153:Mutagens; D004247:DNA
keywords: Benchmark dose; Genotoxicity; Points of departure; Quantitative risk assessment