The development and prevalidation of an in vitro mutagenicity assay based on MutaMouse primary hepatocytes, Part II: Assay performance for the identification of mutagenic chemicals

The use of canonical dose-response models for benchmark dose analysis of continuous toxicological data

The benchmark dose (BMD) approach employs dose-response modeling to determine the dose associated with a small change in response relative to the background response. Here, we introduce a conceptual framework for modeling continuous data that is based on key risk assessment principles and requirements. Based on this framework, we define a class of dose-response models sharing the same four biologically interpretable model parameters, while exhibiting five common properties that are essential from a risk assessment perspective: such models are denoted as "canonical" models. The first two canonical properties are straightforward: property 1. The models should predict positive values only (as measurements of continuous endpoints are typically positive) and property 2. the outcomes should not depend on the measurement unit. Canonical property 3 reflects the observation that toxicological dose-response data related to different subgroups (e.g. species, sexes, and exposure durations) are typically (at least approximately) parallel on a log-dose scale, which is at the same time an implicit assumption in defining fundamental toxicological concepts, such as extrapolation factors, relative potency factors (RPFs), and relative sensitivity factors (RSFs). Property 4 is needed to enable comparisons of the sensitivity of endpoints differing in maximum response. A fifth canonical property reflects our view that choices regarding the dose-response model expression, the assumed distribution for the within-group variation, and the benchmark response (BMR) that is being used should be internally consistent. The canonical models that we discuss are suitable to fit parallel dose-response curves to combined datasets related to different subgroups (e.g. species, sexes, and exposure durations). Doing so provides a tool to check canonical property 3 of the particular data analyzed. We provide a review of empirical evidence indicating that this property has general validity, which is highly fortunate, as this legitimizes the use of extrapolation factors and RPFs in risk assessment. We then evaluate to what extent the approaches in current BMD guidance by European Food Safety Authority (EFSA) or U.S. Environmental Protection Agency (US-EPA) comply with the principles of canonical dose-response modeling, concluding that this is only partly the case. The latter can have unfavorable and sometimes far-reaching consequences. For instance, some of the recommended non-canonical models result in different BMDs when changing the measurement unit (e.g. µg to mg). As another example, the BMD tool recently developed by EFSA implements covariate analysis in such a way that canonical property 3 cannot possibly be represented by any of the models. As another disadvantage, non-canonical models preclude the effective development and use of prior distributions in a Bayesian approach. Finally, we argue that a concomitant but important advantage of only using canonical models is that BMD methodology will be more transparent, so that risk assessors will be better able to understand it, and BMDs with high societal impact can be more easily defended. The present paper may be a helpful tool for toxicologists and risk assessors to critically follow the developments in BMD methodology at the conceptual level.

The transgenic MutaMouse hepatocyte mutation assay in vitro: Mutagenicity and mutation spectra of six substances with different mutagenic mechanisms

Mutagenicity testing is a component of the hazard assessment of industrial chemicals, biocides, and pesticides. Mutations induced by test substances can be detected by in vitro and in vivo methods that have been adopted as OECD Test Guidelines. One of these in vivo methods is the Transgenic Rodent Assay (TGRA), OECD test guideline no. 488. An analogous in vitro TGRA has been described, but experience with this test method is limited. In this study, six in vivo TGRA positive mutagens were tested in the in vitro TGRA based on primary MutaMouse hepatocytes. In addition to the functional read-out of the lacZ reporter gene, induced mutations were analysed by next-generation sequencing (NGS). Five of the six in vivo TGRA positive mutagens (N-ethyl-N-nitrosourea (ENU), ethyl methanesulfonate (EMS), mitomycin C (MMC), benzo[a]pyrene (B[a]P), and azathioprine (AZA), but not cyproterone acetate) mutated the lacZ gene in vitro. NGS identified mutations which matched the mutagenic mechanisms described in the literature. The alkylating agent ENU induced a greater proportion of A:T to T:A transversions than did the other alkylating agent, EMS, whereas EMS increased smaller deletions (1-4 bp). G:C to T:A transversions accounted for the majority of mutations identified after treatments with MMC and B[a]P, both of which form monoadducts at the guanine N2 position. AZA induced mainly G:C to A:T transitions, explained by the structural similarity of one of its metabolites to guanine. An increased proportion of mid-size changes (0.3-2.5 kb) was detected only for the crosslinking mutagen MMC. The in vitro TGRA based on primary MutaMouse hepatocytes is a promising in vitro assay for the assessment of mutation induction, reflecting many aspects of the corresponding in vivo TGRA and allowing for mutation spectra analysis to evaluate the induced mutations.

In vitro mutagenicity of selected environmental carcinogens and their metabolites in MutaMouse FE1 lung epithelial cells

Chemicals in commerce or under development must be assessed for genotoxicity; assessment is generally conducted using validated assays (e.g. Tk mouse lymphoma assay) as part of a regulatory process. Currently, the MutaMouse FE1 cell mutagenicity assay is undergoing validation for eventual use as a standard in vitro mammalian mutagenicity assay. FE1 cells have been shown to be metabolically competent with respect to some cytochrome P450 (CYP) isozymes; for instance, they can convert the human carcinogen benzo[a]pyrene into its proximate mutagenic metabolite. However, some contradictory results have been noted for other genotoxic carcinogens that require two-step metabolic activation (e.g. 2-acetylaminofluorene and 2-amino-3-methylimidazo[4,5-f]quinoxaline). Here, we examined three known or suspected human carcinogens, namely acrylamide, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 4-aminobiphenyl (4-ABP), together with their proximate metabolites (i.e. glycidamide, N-OH-PhIP and N-OH-4-ABP), to aid in the validation of the FE1 cell mutagenicity assay. Assessments of the parent compounds were conducted both in the presence and absence of an exogenous metabolic activation mixture S9; assessments of the metabolites were in the absence of S9. The most potent compound was N-OH-PhIP -S9, which elicited a mutant frequency (MF) level 5.3-fold over background at 5 µM. There was a 4.3-fold increase for PhIP +S9 at 5 µM, a 1.7-fold increase for glycidamide -S9 at 3.5 mM and a 1.5-fold increase for acrylamide +S9 at 4 mM. Acrylamide -S9 elicited a marginal 1.4-fold MF increase at 8 mM. Treatment with PhIP -S9, 4-ABP ±S9 and N-OH-4-ABP -S9 failed to elicit significant increases in lacZ MF with any of the treatment conditions tested. Gene expression of key CYP isozymes was quantified by RT-qPCR. Cyp1a1, 1a2 and 1b1 are required to metabolise PhIP and 4-ABP. Results showed that treatment with both compounds induced expression of Cyp1a1 and Cyp1b1 but not Cyp1a2. Cyp2e1, which catalyses the bioactivation of acrylamide to glycidamide, was not induced after acrylamide treatment. Overall, our results confirm that the FE1 cell mutagenicity assay has the potential for use alongside other, more traditional in vitro mutagenicity assays.