Biotransformation of pentachlorophenol, aniline and biphenyl in isolated rainbow trout (Oncorhynchus mykiss) hepatocytes: comparison with in vivo metabolism

A weight of evidence approach for bioaccumulation assessment

Bioaccumulation assessments conducted by regulatory agencies worldwide use a variety of methods, types of data, metrics, and categorization criteria. Lines of evidence (LoE) for bioaccumulation assessment can include bioaccumulation metrics such as in vivo bioconcentration factor (BCF) and biomagnification factor (BMF) data measured from standardized laboratory experiments, and field (monitoring) data such as BMFs, bioaccumulation factors (BAFs), and trophic magnification factors (TMFs). In silico predictions from mass-balance models and quantitative structure-activity relationships (QSARs) and a combination of in vitro biotransformation rates and in vitro-in vivo extrapolation (IVIVE) models can also be used. The myriad bioaccumulation metrics and categorization criteria and underlying uncertainty in measured or modeled data can make decision-making challenging. A weight of evidence (WoE) approach is recommended to address uncertainty. The Bioaccumulation Assessment Tool (BAT) guides a user through the process of collecting and generating various LoE required for assessing the bioaccumulation of neutral and ionizable organic chemicals in aquatic (water-respiring) and air-breathing organisms. The BAT includes data evaluation templates (DETs) to critically evaluate the reliability of the LoE used in the assessment. The DETs were developed from standardized testing guidance. The approach used in the BAT is consistent with OECD and SETAC WoE principles and facilitates the implementation of chemical policy objectives in chemical assessment and management. The recommended methods are also iterative and tiered, providing pragmatic methods to reduce unnecessary animal testing. General concepts of the BAT are presented and case study applications of the tool for hexachlorobenzene (HCB) and β-hexachlorocyclohexane (β-HCH) are demonstrated. The BAT provides a consistent and transparent WoE framework to address uncertainty in bioaccumulation assessment and is envisaged to evolve with scientific and regulatory developments. Integr Environ Assess Manag 2023;19:1235-1253. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Investigating the bioaccumulation potential of anionic organic compounds using a permanent rainbow trout liver cell line

Permanent rainbow trout (Oncorhynchus mykiss) cell lines represent potential in vitro alternatives to experiments with fish. We here developed a method to assess the bioaccumulation potential of anionic organic compounds in fish, using the rainbow trout liver-derived RTL-W1 cell line. Based on the availability of high quality in vivo bioconcentration (BCF) and biomagnification (BMF) data and the substances' charge state at physiological pH, four anionic compounds were selected: pentachlorophenol (PCP), diclofenac (DCF), tecloftalam (TT) and benzotriazol-tert-butyl-hydroxyl-phenyl propanoic acid (BHPP). The fish cell line acute toxicity assay (OECD TG249) was used to derive effective concentrations 50 % and non-toxic exposure concentrations to determine exposure concentrations for bioaccumulation experiments. Bioaccumulation experiments were performed over 48 h with a total of six time points, at which cell, medium and plastic fractions were sampled and measured using high resolution tandem mass spectrometry after online solid phase extraction. Observed cell internal concentrations were over-predicted by KOW-derived predictions while pH-dependent octanol-water partitioning (DOW) and membrane lipid-water partitioning (DMLW) gave better predictions of cell internal concentrations. Measured medium and cell internal concentrations at steady state were used to calculate RTL-W1-based BCF, which were compared to DOW- or DMLW-based model approaches and in vivo data. With the exception of PCP, the cell-derived BCF best compared to DOW-based model predictions, which were higher than predictions based on DMLW. All methods predicted the in vivo BCF for diclofenac well. For PCP, the cell-derived BCF was lowest although all BCF predictions underestimated the in vivo BCF by ≥ 1 order of magnitude. The RTL-W1 cells, and all other prediction methods, largely overestimated in vivo BMF, which were available for PCP, TT and BHPP. We conclude that the RTL-W1 cell line can supplement BCF predictions for anionic compounds. For BMF estimations, however, in vitro-in vivo extrapolations need adaptation or a multiple cell line approach.

In vitro metabolism of pesticides and industrial chemicals in fish

Metabolism is one of the most important factors in controlling the toxicity and bioaccumulation of pesticides in fish. In vitro systems using subcellular fractions, cell lines, hepatocytes and tissues of a specific organ, each of which is characterized by usability, enzyme activity and chemical transport via membrane, have been applied to investigate the metabolic profiles of pesticides. Not only species and organs but also the fishkeeping conditions are known to greatly affect the in vitro metabolism of pesticides. A comparison of the metabolic profiles of pesticides and industrial chemicals taken under similar conditions has shown that in vitro systems using a subcellular S9 fraction and hepatocytes qualitatively reproduce many in vivo metabolic reactions. More investigation of these in vitro systems for pesticides is necessary to verify their applicability to the estimation of pesticide metabolism in fish.

Which Molecular Features Affect the Intrinsic Hepatic Clearance Rate of Ionizable Organic Chemicals in Fish?

Greater knowledge of biotransformation rates for ionizable organic compounds (IOCs) in fish is required to properly assess the bioaccumulation potential of many environmentally relevant contaminants. In this study, we measured in vitro hepatic clearance rates for 50 IOCs using a pooled batch of liver S9 fractions isolated from rainbow trout (Oncorhynchus mykiss). The IOCs included four types of strongly ionized acids (carboxylates, phenolates, sulfonates, and sulfates), three types of strongly ionized bases (primary, secondary, tertiary amines), and a pair of quaternary ammonium compounds (QACs). Included in this test set were several surfactants and a series of beta-blockers. For linear alkyl chain IOC analogues, biotransformation enzymes appeared to act directly on the charged terminal group, with the highest clearance rates for tertiary amines and sulfates and no clearance of QACs. Clearance rates for C₁₂-IOCs were higher than those for C₈-IOC analogues. Several analogue series with multiple alkyl chains, branched alkyl chains, aromatic rings, and nonaromatic rings were evaluated. The likelihood of multiple reaction pathways made it difficult to relate all differences in clearance to specific molecular features the tested IOCs. Future analysis of primary metabolites in the S9 assay is recommended to further elucidate biotransformation pathways for IOCs in fish.

Cross-Species Extrapolation of Uptake and Disposition of Neutral Organic Chemicals in Fish Using a Multispecies Physiologically-Based Toxicokinetic Model Framework

The potential to bioconcentrate is generally considered to be an unwanted property of a substance. Consequently, chemical legislation, including the European REACH regulations, requires the chemical industry to provide bioconcentration data for chemicals that are produced or imported at volumes exceeding 100 tons per annum or if there is a concern that a substance is persistent, bioaccumulative, and toxic. For the filling of the existing data gap for chemicals produced or imported at levels that are below this stipulated volume, without the need for additional animal experiments, physiologically-based toxicokinetic (PBTK) models can be used to predict whole-body and tissue concentrations of neutral organic chemicals in fish. PBTK models have been developed for many different fish species with promising results. In this study, we developed PBTK models for zebrafish (Danio rerio) and roach (Rutilus rutilus) and combined them with existing models for rainbow trout (Onchorhynchus mykiss), lake trout (Salvelinus namaycush), and fathead minnow (Pimephales promelas). The resulting multispecies model framework allows for cross-species extrapolation of the bioaccumulative potential of neutral organic compounds. Predictions were compared with experimental data and were accurate for most substances. Our model can be used for probabilistic risk assessment of chemical bioaccumulation, with particular emphasis on cross-species evaluations.

Effective synthesis of sulfate metabolites of chlorinated phenols

Chlorophenols are an important class of persistent environmental contaminants and have been implicated in a range of adverse health effects, including cancer. They are readily conjugated and excreted as the corresponding glucuronides and sulfates in the urine of humans and other species. Here we report the synthesis and characterization of a series of ten chlorophenol sulfates by sulfation of the corresponding chlorophenols with 2,2,2-trichloroethyl (TCE) chlorosulfate using N,N-dimethylaminopyridine (DMAP) as base. Deprotection of the chlorophenol diesters with zinc powder/ammonium formate yielded the respective chlorophenol sulfate ammonium salts in good yield. The molecular structure of three TCE-protected chlorophenol sulfate diesters and one chlorophenol sulfate monoester were confirmed by X-ray crystal structure analysis. The chlorophenol sulfates were stable for several months if stored at -20 °C and, thus, are useful for future toxicological, environmental and human biomonitoring studies.

Biphenyl-4-yl 2,2,2-trichloro-ethyl sulfate

The mol-ecular structure of the title compound, C(14)H(11)Cl(3)O(4)S, displays a biphenyl dihedral angle of 4.9 (2)° between the benzene rings, which is significantly smaller than the calculated dihedral angle of 41.2° of biphenyl derivatives without ortho substituents. The C(Ar)-O bond length of 1.432 (4) Å is comparable with other sulfuric acid biphenyl-4-yl ester 2,2,2-trichloro-ether ester derivatives without electronegative substituents in the sulfated phenyl ring.

Establishment and validation of primary hepatocytes of the African sharptooth catfish (Clarias gariepinus)

In vitro systems such as primary cells and continuous cell lines are gaining momentum in ecotoxicological studies. Cytotoxicity tests with fish cells as well as tests using specific endpoints such as CYP1A induction are valuable in the toxicity assessment of environmental samples. The main objective of this study was to establish and validate the use of primary hepatocytes from the African sharptooth catfish (Clarias gariepinus) as an in vitro toxicity monitoring system. The successful isolation of primary hepatocytes from the sharptooth catfish was achieved using an in situ perfusion method. The primary hepatocytes responded to CYP1A induction, while a continuous Chinese hamster ovary (CHO-K1) cell line showed no activity when exposed to various concentrations of benzo[a]pyrene (B[a]P) (p<0.0001). Cytotoxicity, as measured by the methyl thiazol tetrazolium (MTT) assay, was not observed following a 72 h exposure of the primary hepatocytes and the CHO-K1 cell line to different B[a]P concentrations. However, the hepatocytes were damaged at higher B[a]P concentrations (>10(-6)M) as shown by transmission electron microscopy. This cytotoxicity effect was also confirmed by the trypan blue exclusion assay (TD(50) of 10(-6)M). Differences in the results between the MTT and trypan blue exclusion assays are probably due to mitochondria that are still metabolically active, causing the tetrazolium salt to be dehydrogenated. The internal architecture of normal primary hepatocytes included large quantities of rough endoplasmic reticulum (often in close proximity to the nucleus), mitochondria, aggregates and scattered glycogen, a few lipid droplets and spherical nuclei with distinct nucleoli. The primary catfish hepatocyte cell culture system, expressing CYP1A when exposed to B[a]P, could be used as a biomarker for aromatic hydrocarbon pollutants in aquatic ecosystems of southern and East Africa.

Accumulation and metabolism of the anticancer flavonoid 5,7-dimethoxyflavone compared to its unmethylated analog chrysin in the Atlantic killifish

The use of dietary flavonoids as potential chemopreventive agents is a concept of increasing interest. Recent findings indicate that methylated flavones have the advantage of increased metabolic stability. One such compound, the naturally-occurring 5,7-dimethoxyflavone (5,7-DMF), has been shown to be a potential chemopreventive agent in human cancer originating from the liver, mouth, esophagus and lung. As bioavailability is a key issue for potential in vivo effects, the tissue accumulation and biliary elimination of 5,7-DMF and its non-methylated analog chrysin were examined in a small fish model (Fundulus heteroclitus). The fish were exposed to 5,7-DMF, chrysin or vehicle control (DMSO<0.01%) in seawater for 8h. Toxicity was not observed at the 5microM exposure level. Tissues and bile were harvested and analyzed by HPLC and LC/MS for quantitation and identification of parent compound and metabolites. 5,7-DMF accumulated 20-fold to 100-fold in all tissues examined, with the highest accumulation in liver and brain, whereas chrysin was barely detectable in any tissues except the liver. The bile of chrysin-exposed fish contained very low concentrations of unchanged chrysin but high concentrations of two glucuronic acid conjugates. In the bile of 5,7-DMF-exposed fish, the parent compound was detectable in significant amounts along with glucuronic acid conjugates of O-demethylated 5,7-DMF. In conclusion, our study demonstrated high tissue accumulation and limited metabolism of 5,7-DMF compared to chrysin in vivo, making this flavone a promising chemopreventive molecule.

Bioconcentration factor of relatively low concentrations of chlorophenols in Japanese medaka

Bioconcentration factors (BCF) for pentachlorophenol (PCP) and 2,4-dichlorophenol (2,4-DCP) in Japanese medaka (Oryzias latipes) were determined at five different concentrations of the chemicals, between 0.1 and 10 microg/l (PCP), 0.3 and 30 microg/l (2,4-DCP), in the ambient water. Medaka were exposed to each chemicals in a continuous-flow system during the embryonic development period and 60 days after hatching from eggs collected in the laboratory. Both the exposure time and the aqueous concentrations are much more realistic and closer to natural aquatic environments than those used in conventional BCF studies. The BCF values of PCP were from (4.9+/-2.8)x10(3) at the aqueous concentration of 0.074+/-0.028 microg/l to (2.1+/-1.4)x10(3) at 9.70+/-0.56 microg/l. The BCF value of 2,4-DCP were from (3.4+/-3.0)x10(2) at 0.235+/-0.060 microg/l to 92+/-27 at 27.3+/-1.6 microg/l. Generally, BCF values increased as the aqueous concentrations of PCP or 2,4-DCP decreased. This finding suggests that a relatively low and realistic aqueous concentration of these compounds is necessary to more accurately determine their BCF values in natural aquatic environments. Conventional BCF experiments at higher aqueous concentrations may underestimate the BCF values.

Effects of prochloraz and nonylphenol diethoxylate on hepatic biotransformation enzymes in trout: a comparative in vitro/in vivo-assessment using cultured hepatocytes

The suitability of cultured rainbow trout hepatocytes as a model system for the assessment of xenobiotic effects on hepatic biotransformation enzymes in fish was examined. Two model water pollutants, the imidazole fungicide prochloraz and the alkylphenolic compound nonylphenol diethoxylate (NP2EO), were investigated in a comparative in vitro/in vivo approach. Biotransformation enzymes were measured in cultured rainbow trout hepatocytes following exposure to xenobiotics in vitro, or in the liver of juvenile rainbow trout (Oncorhynchus mykiss) exposed in vivo. The patterns of biochemical responses to the model pollutants were generally similar between in vitro and in vivo investigations. Levels of cytochrome P4501A (CYP1A) protein and the catalytic activity of the CYP1A-dependent enzyme 7-ethoxyresorufin-O-deethylase (EROD) were induced in vitro after 24 h of exposure to 1.0 microM prochloraz. In vitro, higher prochloraz concentrations induced only the levels of CYP1A above control levels, but not EROD activity. In vivo exposure of juvenile trout to 0.27 microM prochloraz resulted in an induction of CYP1A and EROD after 7 and 14 days, while 0.027 microM prochloraz had no effects. In vitro, the 6beta- and 16beta-hydroxylation of testosterone was significantly decreased by 1.0-3.0 microM prochloraz, while in vivo these variables were significantly inhibited after exposure to 0.27 microM prochloraz for 7 and 14 days. NP2EO did not affect EROD activity in vitro. In vivo, EROD activity and CYP1A remained unchanged following 7 days of exposure to 0.32 or 1.30 microM NP2EO. NP2EO (15-50 microM) inhibited the 16beta-hydroxylation and glucuronidation of testosterone in vitro. In vivo, 7 days of exposure to 0.32 or 1.30 microM NP2EO resulted in a significant inhibition of the 6beta- and 16beta-hydroxylation of testosterone. The good qualitative correspondence between in vitro and in vivo results indicates that studies using trout hepatocytes allow the identification of biochemical targets of xenobiotic effects in fish liver. However, more research is needed before quantitative predictions, e.g. of effective concentrations, can be made from in vitro investigations.

Metabolic fate of 2,4-dichloroaniline, prochloraz and nonylphenol diethoxylate in rainbow trout: a comparative in vivo/in vitro approach

The metabolism and distribution of 2,4-dichloroaniline (2,4-DCA), prochloraz and 4-n-nonylphenol diethoxylate (NP2EO) were investigated in vivo and in vitro in rainbow trout (Oncorhynchus mykiss). Each compound was administered p.o. (10 mg/kg wet weight) and urine was collected during 48 h (2,4-DCA, prochloraz) or 72 h (NP2EO). Fish were sacrificed, the gall bladder was excised and radioactivity was measured in tissues, viscera and carcasses. Metabolic profiles were performed by radio-HPLC and when possible metabolites were identified by LC/MS. For comparison, the biotransformation of these xenobiotics was also investigated in freshly isolated hepatocytes. The metabolic pathways of 2,4-DCA have been identified leading to the glucuronide conjugate (in vivo) and to the glucuronide conjugate and the hydroxylamine metabolite (in vitro). This difference highlights the usefulness of the hepatocyte system in metabolic studies, since the formation of the hydroxylamine reactive metabolite cannot be demonstrated in vivo. For prochloraz, we observed that residue levels are significantly higher in males than in females for gill, fat, brain and carcasses, however, the reasons for this difference remain unclear. Although, the presence of glucuronide conjugates was detected in vivo and in vitro, the chemical structure of isolated metabolites has to be determined. However, the comparison of the in vivo versus in vitro metabolic profiles indicates that several peaks, probably corresponding to intermediate metabolites, were present only in hepatocyte incubations. Biotransformation of NP2EO occurred in vivo and in vitro in rainbow trout, but did not result in the formation of 4-n-NP. The major metabolite present in bile corresponded to the NP2EO-glucuronide but this metabolite was not found in vitro. It is concluded that hepatocytes may produce a different metabolic pattern than in the whole fish, but may also give evidence of a metabolic pathway difficult to apprehend in vivo.

Metabolic activity in primary cultures of fish hepatocytes

In aquatic toxicology, isolated liver cells from fish can be used as a tool to generate initial information on the hepatic metabolism of xenobiotics, and on the mechanisms of xenobiotic activation or deactivation. This isolation of teleost liver cells is achieved by enzymic dissociation, and monolayer cultures of fish hepatocytes in serum-free medium maintain good viability for 3-8 days. During in vitro culture, fish liver cells express stable levels of phase I and phase II enzymes, such as cytochrome P4501A or glutathione S-transferase, and the cells show an induction of biotransformation enzymes after exposure to xenobiotics. The xenobiotic metabolite pattern produced by fish hepatocytes in vitro is generally similar to that observed in vivo. Limitations to more-intensive application of cultured fish hepatocytes as a screen in aquatic hazard assessment are partly due to the rather limited scope of existing studies, i.e. the focus on one particular species (rainbow trout), and on one particular biotransformation enzyme (cytochrome P4501A), as well as a lack of comparative in vitro/in vivo studies.

In vivo effects of phenolic compounds on blood parameters of a marine fish (Dicentrarchus labrax)

Sea bass (Dicentrarchus labrax) were injected intraperitoneally once (single dose) or three times (fractionated dose) with phenol or OH-phenols (hydroquinone, resorcinol, and pyrocatechol). On the basis of the lethal doses, OH-phenols were more toxic than phenol, and pyrocatechol was the most powerful compound. Hematological, metabolic and antioxidant blood parameters were measured 3 days after the end of the treatment. Metabolic variations as specific effects on erythrocytes were revealed and differences between single and fractionated doses were observed. OH-phenols-treated fish showed disorders in the metabolic toxicity indicators as hypoglycemia, low blood urea nitrogen level (BUN) and decrease of alkaline phosphatase activity (ALP). In addition, quantitative structure-activity relationships were developed using the n-octanol:water partition coefficient (log K(ow)). Positive correlations were found with ALP, plasma glucose and hemoglobin.