Bioavailability of hydrophobic organic chemicals on an in vitro metabolic transformation using rat liver S9 fraction

In vitro approach to refine bioconcentration and biotransformation predictions of organic persistent pollutants using cell lines

The application of the 3Rs (Replacement, Reduction, and Refinement) in animal experimentation has recently concentrated its efforts on utilizing cellular systems to predict toxicity in organisms. In this context, while refining the data obtained from cell lines, this study assesses their bioaccumulation potential and various methods for extrapolating the in vitro metabolization rate constant to support modelled bioaccumulation assessments for fish and their limitations. For this purpose, the concentrations of the parent compound, phenanthrene, and its major metabolites within the cells and in the medium at various exposure times were quantified. A chemical distribution model (mass balance) was applied to calculate the concentrations of the cell-bioaccessible compounds (Cfree) based on the experimentally determined concentrations. An elevated matching was observed between the in vitro bioconcentration factor (BCF) and the in vivo BCFs reported in the literature for zebrafish liver cells (ZFL). This study demonstrates the importance of further investigating in vitro biotransformation kinetics. The results obtained with the approach developed here provide valuable information to enhance current models. Additionally, it underscores the potential of cell lines as a strategy for rapid, simple, and cost-effective predictions without the need for animal experimentation.

In Vitro Metabolism and p53 Activation of Genotoxic Chemicals: Abiotic CYP Enzyme vs Liver Microsomes

Chemicals often require metabolic activation to become genotoxic. Established test guidelines recommend the use of the rat liver S9 fraction or microsomes to introduce metabolic competence to in vitro cell-based bioassays, but the use of animal-derived components in cell culture raises ethical concerns and may lead to quality issues and reproducibility problems. The aim of the present study was to compare the metabolic activation of cyclophosphamide (CPA) and benzo[a]pyrene (BaP) by induced rat liver microsomes and an abiotic cytochrome P450 (CYP) enzyme based on a biomimetic porphyrine catalyst. For the detection of genotoxic effects, the chemicals were tested in a reporter gene assay targeting the activation of the cellular tumor protein p53. Both chemicals were metabolized by the abiotic CYP enzyme and the microsomes. CPA showed no activation of p53 and low cytotoxicity without metabolic activation, but strong activation of p53 and increased cytotoxicity upon incubation with liver microsomes or abiotic CYP enzyme. The effect concentration causing a 1.5-fold induction of p53 activation was very similar with both metabolization systems (within a factor of 1.5), indicating that genotoxic metabolites were formed at comparable concentrations. BaP also showed low cytotoxicity and no p53 activation without metabolic activation. The activation of p53 was detected for BaP upon incubation with active and inactive microsomes at similar concentrations, indicating experimental artifacts caused by the microsomes or NADPH. The activation of BaP with the abiotic CYP enzyme increased the cytotoxicity of BaP by a factor of 8, but no activation of p53 was detected. The results indicate that abiotic CYP enzymes may present an alternative to rat liver S9 fraction or microsomes for the metabolic activation of test chemicals, which are completely free of animal-derived components. However, an amendment of existing test guidelines would require testing of more chemicals and genotoxicity end points.

A novel method to derive a human safety limit for PFOA by gene expression profiling and modelling

Perfluorooctanoic acid (PFOA) is a persistent environmental contaminant that can accumulate in the human body due to its long half-life. This substance has been associated with liver, pancreatic, testicular and breast cancers, liver steatosis and endocrine disruption. PFOA is a member of a large group of substances also known as "forever chemicals" and the vast majority of substances of this group lack toxicological data that would enable their effective risk assessment in terms of human health hazards. This study aimed to derive a health-based guidance value for PFOA intake (ng/kg BW/day) from in vitro transcriptomics data. To this end, we developed an in silico workflow comprising five components: (i) sourcing in vitro hepatic transcriptomics concentration-response data; (ii) deriving molecular points of departure using BMDExpress3 and performing pathway analysis using gene set enrichment analysis (GSEA) to identify the most sensitive molecular pathways to PFOA exposure; (iii) estimating freely-dissolved PFOA concentrations in vitro using a mass balance model; (iv) estimating in vivo doses by reverse dosimetry using a PBK model for PFOA as part of a quantitative in vitro to in vivo extrapolation (QIVIVE) algorithm; and (v) calculating a tolerable daily intake (TDI) for PFOA. Fourteen percent of interrogated genes exhibited in vitro concentration-response relationships. GSEA pathway enrichment analysis revealed that "fatty acid metabolism" was the most sensitive pathway to PFOA exposure. In vitro free PFOA concentrations were calculated to be 2.9% of the nominal applied concentrations, and these free concentrations were input into the QIVIVE workflow. Exposure doses for a virtual population of 3,000 individuals were estimated, from which a TDI of 0.15 ng/kg BW/day for PFOA was calculated using the benchmark dose modelling software, PROAST. This TDI is comparable to previously published values of 1.16, 0.69, and 0.86 ng/kg BW/day by the European Food Safety Authority. In conclusion, this study demonstrates the combined utility of an "omics"-derived molecular point of departure and in silico QIVIVE workflow for setting health-based guidance values in anticipation of the acceptance of in vitro concentration-response molecular measurements in chemical risk assessment.

Binding of Per- and Polyfluoroalkyl Substances (PFAS) to Serum Proteins: Implications for Toxicokinetics in Humans

Per- and polyfluoroalkyl substances (PFAS) are a diverse class of highly persistent anthropogenic chemicals that are detectable in the serum of most humans. PFAS exposure has been associated with many adverse effects on human health including immunotoxicity, increased risk of certain cancers, and metabolic disruption. PFAS binding to the most abundant blood serum proteins (human serum albumin [HSA] and globulins) is thought to affect transport to active sites, toxicity, and elimination half-lives. However, few studies have investigated the competitive binding of PFAS to these proteins in human serum. Here, we use C18 solid-phase microextraction fibers to measure HSA-water and globulin-water distribution coefficients (DHSA/w, Dglob/w) for PFAS with carbon chains containing 4 to 13 perfluorinated carbons (ηpfc = 4-13) and several functional head-groups. PFAS with ηpfc < 7 were highly bound to HSA relative to globulins, whereas PFAS with ηpfc ≥ 7 showed a greater propensity for binding to globulins. Experimentally measured DHSA/w and Dglob/w and concentrations of serum proteins successfully predicted the variability in PFAS binding in human serum. We estimated that the unbound fraction of serum PFAS varied by up to a factor of 2.5 among individuals participating in the 2017-2018 U.S. National Health and Nutrition Examination Survey. These results suggest that serum HSA and globulins are important covariates for epidemiological studies aimed at understanding the effects of PFAS exposure.

Bioconcentration Assessment in Fish Based on In Vitro Intrinsic Clearance: Predictivity of an Empirical Model Compared to In Vitro-In Vivo Extrapolation Models

To estimate the bioconcentration factor (BCF), the in vitro intrinsic clearance (CLIN VITRO,INT) from rainbow trout liver S9 fractions (RT-S9) can be applied to in vitro-in vivo extrapolation (IVIVE) models, yet uncertainties remain in model parameterization. An alternative model approach is evaluated: a regression model was built in the form log BCF = a × log Kow + b × log CLIN VITRO,INT. The coefficients a and b were fitted based on a training set of 40 chemicals. A high robustness of the coefficients and good accuracy of BCF prediction were found on independent datasets of neutral organic chemicals (measured log Kow 3.3-6.2). BCF predictions were similar to or in better agreement with in vivo BCFs compared to IVIVE models (2.4- to 2.9- vs 2.8- to 3.6-fold misprediction) for training and test sets. Species-matched models (trout, carp) did not result in improvements. This study presents the largest dataset on CLIN VITRO,INT and BCFs to assess predictivity of the RT-S9 assay. The robustness of the regression statistics on different datasets and the high statistical weight of the CLIN VITRO,INT term illustrate the predictive power of the RT-S9 assay as an important step toward regulatory acceptance to replace animal experiments.

Miniaturized method for the quantification of persistent organic pollutants and their metabolites in HepG2 cells: assessment of their biotransformation

Biotransformation can greatly influence the accumulation and, subsequently, toxicity of substances in living beings. Although traditionally these studies to quantify metabolization of a compound have been carried out with in vivo species, currently, in vitro test methods with very different cell lines are being developed for their evaluation. However, this is still a very limited field due to multiple variables of a very diverse nature. So, an increasing number of analytical chemists are working with cells or other similar biological samples of very small size. This makes it necessary to address the development of analytical methods that allow determining their concentration both inside the cells and in their exposure medium. The aim of this study is to develop a set of analytical methodologies for the quantification of polycyclic aromatic hydrocarbons, PAHs (phenanthrene, PHE), and polybrominated diphenyl ethers, PBDEs (2,2',4,4'-tetrabromodiphenyl ether, BDE-47), and their major metabolites in cells and their exposure medium. Analytical methodologies, based on miniaturized ultrasound probe-assisted extraction, gas chromatography-mass spectrometry-microelectron capture detector (GC-MS-µECD), and liquid chromatography-fluorescence detector (LC-FL) determination techniques, have been optimized and then applied to a biotransformation study in HepG2 at 48 h of exposure. Significant concentrations of the major metabolites of PHE (1-OH, 2-OH, 3-OH, 4-OH-, and 9-OH-PHE) and BDE-47 (5-MeO-, 5-OH-, and 3-OH-BDE-47) were detected and quantified inside the cells and in the exposure medium. These results provide a new method for determination and improve information on the metabolization ratios for a better knowledge of the metabolic pathways and their toxicity.

Development of a bioprinter-based method for incorporating metabolic competence into high-throughput in vitro assays

The acceptance and use of in vitro data for hazard identification, prioritization, and risk evaluation is partly limited by uncertainties associated with xenobiotic metabolism. The lack of biotransformation capabilities of many in vitro systems may under- or overestimate the hazard of compounds that are metabolized to more or less active metabolites in vivo. One approach to retrofitting existing bioassays with metabolic competence is the lid-based Alginate Immobilization of Metabolic Enzymes (AIME) method, which adds hepatic metabolism to conventional high-throughput screening platforms. Here, limitations of the lid-based AIME method were addressed by incorporating bioprinting, which involved depositing S9-encapsulated microspheres into standard 384-well plates with requisite cofactors for phase I and II hepatic metabolism. Objectives of this study included: 1) compare the lid-based and AIME bioprinting methods by assessing the enzymatic activity of a common cytochrome P450 (CYP) enzyme, 2) use biochemical assays with the bioprinting method to characterize additional measures of phase I and II metabolic activity, and 3) evaluate the bioprinting method by screening 25 chemicals of known metabolism-dependent bioactivity in the VM7Luc estrogen receptor transactivation (ERTA) assay. A comparison of the two methods revealed comparable precision and dynamic range. Activity of additional CYP enzymes and glucuronidation was observed using the AIME bioprinting method. The ERTA experiment identified 19/21 ER-active test chemicals, 14 of which were concordant with expected biotransformation effects (73.7%). Additional refinement of the AIME bioprinting method has the potential to expand high-throughput screening capabilities in a robust, accessible manner to incorporate in vitro metabolic competence.

Replacing animal-derived components in in vitro test guidelines OECD 455 and 487

The evaluation of single substances or environmental samples for their genotoxic or estrogenic potential is highly relevant for human- and environment-related risk assessment. To examine the effects on a mechanism-specific level, standardized cell-based in vitro methods are widely applied. However, these methods include animal-derived components like fetal bovine serum (FBS) or rat-derived liver homogenate fractions (S9-mixes), which are a source of variability, reduced assay reproducibility and ethical concerns. In our study, we evaluated the adaptation of the cell-based in vitro OECD test guidelines TG 487 (assessment of genotoxicity) and TG 455 (detection of estrogenic activity) to an animal-component-free methodology. Firstly, the human cell lines A549 (for OECD TG 487), ERα-CALUX® and GeneBLAzer™ ERα-UAS-bla GripTite™ (for OECD TG 455) were investigated for growth in a chemically defined medium without the addition of FBS. Secondly, the biotechnological S9-mix ewoS9R was implemented in comparison to the induced rat liver S9 to simulate in vivo metabolism capacities in both OECD test guidelines. As a model compound, Benzo[a]pyrene was used due to its increased genotoxicity and endocrine activity after metabolization. The metabolization of Benzo[a]Pyrene by S9-mixes was examined via chemical analysis. All cell lines (A549, ERα-CALUX® and GeneBLAzer™ Erα-UAS-bla GripTite™) were successfully cultivated in chemically defined media without FBS. The micronucleus assay could not be conducted in chemically defined medium due to formation of cell clusters. The methods for endocrine activity assessment could be conducted in chemically defined media or reduced FBS content, but with decreased assay sensitivity. The biotechnological ewoS9R showed potential to replace rat liver S9 in the micronucleus in FBS-medium with A549 cells and in the ERα-CALUX® assay in FBS- and chemically defined medium. Our study showed promising steps towards an animal-component free toxicity testing. After further improvements, the new methodology could lead to more reproducible and reliable results for risk assessment.

Bioaccumulation Screening of Neutral Hydrophobic Organic Chemicals in Air-Breathing Organisms Using In Vitro Rat Liver S9 Biotransformation Assays

To advance methods for bioaccumulation assessment of organic substances in air-breathing organisms, the present study developed an in vitro approach for screening neutral hydrophobic organic substances for their bioaccumulation potential in air-breathing organisms consisting of (1) depletion assays for chemicals in rat liver S9 subcellular fractions, (2) in vitro-in vivo extrapolation, and (3) whole-organism bioaccumulation modeling to assess the biomagnification potential of neutral organic substances in the rat. Testing of the in vitro method on 14 test chemicals of potentially biomagnifying substances showed that the bioassays could be conducted with a high level of reproducibility and that in vitro-derived elimination rate constants were in good agreement with in vivo-determined elimination rate constants in the rat. Exploring the potential of the in vitro approach for screening organic chemicals for bioaccumulation in air-breathing organisms indicated that chemical substances that exhibit a depletion rate constant in the S9 in vitro bioassay ≥0.3 h^-1 are not expected to biomagnify in rats independent of their octanol-water partitioning coefficient (K_OW ) or octanol-air partitioning coefficient (K_OA ). The high level of reproducibility achieved in the test, combined with the good agreement between in vitro-derived and in vivo-determined depuration rates, suggests that the in vitro approach in combination with a K_OA - and K_OW -based screening approach has good potential for screening chemicals in commerce for their bioaccumulation potential in air-breathing organisms in a cost-effective and expedient manner, especially if the bioassay can be automated. Environ Toxicol Chem 2022;41:2565-2579. © 2022 SETAC.

Could chemical exposure and bioconcentration in fish be affected by slow binding kinetics in blood?

The possible implications of slow binding kinetics on respiratory uptake, bioconcentration and exposure of chemicals were evaluated in the present study. Most physiological and chemical information needed for such an evaluation is already known from the literature or can be estimated. However, data for binding kinetics of chemicals in fish plasma have not been reported in the literature yet. In the first part of this study, we therefore experimentally investigated the plasma binding kinetics for ten chemicals, including pollutants like polycyclic aromatic hydrocarbons and a pesticide. The determined desorption rate constants were in the range of 0.4 s-1 to 0.1 s-1. In the second part of this study, we present a comparative modeling analysis of generic predictions with binding kinetics of different velocities. For doing so, a model that explicitly represents binding kinetics in blood was developed and applied for different hypothetical scenarios. The evaluation showed that slow sorption kinetics only limits respiratory uptake and thus influences the levels of bioaccumulation for extreme and, by that, rather unlikely parameter combinations (i.e. for strongly sorbing chemicals with very slow binding kinetics). It can therefore be assumed that limitations on respiratory uptake due to slow binding kinetics in blood are rather unlikely for most chemicals.

Optimization of a pre-metabolization procedure using rat liver S9 and cell-extracted S9 in the Ames fluctuation test

Many environmental pollutants pose a toxicological hazard only after metabolic activation. In vitro bioassays using cell lines or bacteria have often no or reduced metabolic activity, which impedes their use in the risk assessment. To improve the predictive capability of in vitro assays, external metabolization systems like the liver S9 fraction are frequently combined with in vitro toxicity assays. While it is typical for S9 fractions that samples and testing systems are combined in the same exposure system, we propose to separate the metabolism step and toxicity measurement. This allows for a modular combination of metabolic activation by enzymes isolated from rat liver (S9) or a biotechnological alternative (ewoS9^R) with in vitro bioassays that lack metabolic capacity. Benzo(a)pyrene and 2-aminoanthracene were used as model compounds to optimize the conditions for the S9 metabolic degradation/activation step. The Ames assay with Salmonella typhimurium strains TA98 and TA100 was applied to validate the set-up of decoupling the S9 activation/metabolism from the bioassay system. S9 protein concentration of 0.25 mg_protein/mL, a supplement of 0.13 mM NADPH and a pre-incubation time of 100 min are recommended for activation of samples prior to dosing them to in vitro bioassays using the regular dosing protocols of the respective bioassay. EwoS9^R performed equally well as Moltox S9, which is a step forward in developing true animal-free in vitro bioassays. After pre-incubation with S9 fraction, chemicals induced bacteria revertants in both the TA98 and the TA100 assay as efficiently as the standard Ames assay. The pre-incubation of chemicals with S9 fraction could serve for a wide range of cellular in vitro assays to efficiently combine activation and toxicity measurement, which may greatly facilitate the application of these assays for chemical hazard assessment and monitoring of environmental samples.

Examining Uncertainty in In Vitro-In Vivo Extrapolation Applied in Fish Bioconcentration Models

In vitro biotransformation rates were determined for 30 chemicals, mostly fragrance ingredients, using trout liver S9 fractions (RT-S9) and incorporated into in vitro-in vivo extrapolation (IVIVE) models to predict bioconcentration factors (BCFs). Predicted BCFs were compared against empirical BCFs to explore potential major uncertainties involved in the in vitro methods and IVIVE models: (i) in vitro chemical test concentrations; (ii) different gill uptake rate constant calculations (k₁); (iii) protein binding (different calculations and measurement of the fraction of unbound chemical, f_U); (iv) species differences; and (v) extrahepatic biotransformation. Predicted BCFs were within 0.5 log units for 44% of the chemicals compared to empirical BCFs, whereas 56% were overpredicted by >0.5 log units. This trend of overprediction was reduced by alternative k₁ calculations to 32% of chemicals being overpredicted. Moreover, hepatic in vitro rates scaled to whole body biotransformation rates (k_B) were compared against in vivo k_B estimates. In vivo k_B was underestimated for 79% of the chemicals. Neither lowering the test concentration, nor incorporation of new measured f_U values, nor species matching avoided the tendency to overpredict BCFs indicating that further improvements to the IVIVE models are needed or extrahepatic biotransformation plays an underestimated role.