Toxicity-distribution relationships among 3-alkylfurans in mouse liver and kidney

Mechanism-based cytotoxicity trend prediction of furan-containing pollutants present in a mixture

Human exposure to furan-containing pollutants (FCPs) has raised concerns due to their high risk of toxicity. A substantial number of approximately 8500 recorded compounds containing a furan ring exist which have been analytically or in biologically studied. A significant portion of these compounds is found in the everyday environments of individuals, particularly when ingested through food. Consequently, there is a need for a universal approach to rapidly predict the potential toxicity trends of FCPs. In this study, we developed a bromine labeling-based platform that combines LC-ICP-MS and LC-ESI-MS techniques to absolutely quantify FCP-induced protein adduction. The LC-ESI-MS approach facilitated the identification of FCP-derived protein adducts and optimized liquid chromatographic conditions for analyte separation. By employing a well-designed bromine-containing compound as a general internal standard, LC-ICP-MS-based technique enabled to absolutely assess bromine-labeled protein adduction. The protein adduction efficiencies of furan, 2-methylfuran, and 2,5-dimethylfuran were found to be 2.68, 2.90, and 0.37 molecules per 10,000 FCP molecules that primary hepatocytes received, respectively. Furthermore, we observed that 2-methylfuran exhibited the highest cytotoxicity, followed by furan and 2,5-dimethylfuran, which aligned with the order of their protein adduction. Thus, the protein adduction efficiency of FCPs could serve as a potential index for predicting their toxicity trends.

A Metabolic Activation-Based Chemoproteomic Platform to Profile Adducted Proteins Derived from Furan-Containing Compounds

Human exposure to widespread furan-containing compounds (FCCs) has drawn much attention due to the high risk of their toxicities. Identifying adducted proteins resulting from the metabolic activation of FCCs is the core to learning the mechanism of FCCs' toxic action. We succeeded in establishing a metabolic activation-based chemoproteomic platform to map FCC-derived protein adducts in cultured primary hepatocytes treated with FCCs and to pinpoint the modification sites, using click chemistry but without alkynylation or azidation of FCCs to be investigated. The proposed platform was systematically verified by biomimetic synthesis, liver microsomal incubation, and primary hepatocyte culture. A mixture of furan, 2-methylfuran, and 2,5-dimethylfuran as model was tested by use of the established platform. A total of hepatic 171 lysine-based adducted proteins and 145 cysteine-based adducted proteins by the reactive metabolites of the three FCCs were enriched and identified (Byonic score ≥ 100). The target proteins were found to mainly participate in ATP synthesis. The technique was also successfully applied to furan-containing natural products. The established platform made it possible to profile covalently adducted proteins, because of potential exposure to a vast inventory of over two million of FCCs documented.

Mechanistic study of bergamottin-induced inactivation of CYP2C9

Bergamottin (BGM) is a major furanocoumarin constituent of grapefruit and is reported to have inhibitory effects on cytochrome P450 enzymes. This study investigated the chemical interactions between BGM and the enzyme CYP2C9. BGM exhibited time-, concentration-, and NADPH-dependent inhibition of CYP2C9. Co-incubation with diclofenac, a reversible inhibitor of CYP2C9, attenuated the time-dependent enzyme inhibition. Exhaustive dialysis did not restore enzyme activity post-inhibition. Glutathione (GSH) and catalase/superoxide dismutase failed to reverse BGM-induced CYP2C9 inactivation. A GSH trapping study suggested that BGM was metabolized to an epoxide and/or γ-ketoenal that may have been responsible for the enzyme inactivation. In conclusion, BGM can be characterized as a mechanism-based inactivator of CYP2C9 acting via the formation of an epoxide and/or γ-ketoenal.

Scientific Opinion on Flavouring Group Evaluation 13 Revision 3 (FGE.13Rev3): furfuryl and furan derivatives with and without additional side-chain substituents and heteroatoms from chemical group 14

The Panel on Food additives and Flavourings of the EFSA was requested to update Flavouring Group Evaluation 13 using the Procedure as outlined in Commission Regulation (EC) No 1565/2000, to include an evaluation of the flavouring substances 2-ethyl-5-methylfuran [FL-no: 13.125] and 2-octylfuran [FL-no: 13.162]. FGE.13 revision 3 (FGE.13Rev3) deals with 26 flavourings substances of which 24 have been already evaluated to be of no safety concern. For [FL-no: 13.125] and [FL-no: 13.162], a concern for genotoxicity was raised in FGE.13Rev1. This concern could be ruled out based on new genotoxicity data on supporting substances in FGE.67Rev3. Subsequently, [FL-no: 13.125 and 13.162] were evaluated, through a stepwise approach that integrates intake from current uses, toxicological threshold of concern (TTC), and available data on metabolism and toxicity, along the B-side of the Procedure, making use of a BMDL of 8.51 mg/kg body weight (bw) per day. The Panel derived this BMDL from an oral subchronic toxicity study with the supporting substance 2-pentylfuran [FL-no: 13.059]. Using this BMDL, for [FL-no: 13.125 and 13.162], adequate margins of safety were calculated based on the MSDI approach. The Panel concluded that the 26 candidate substances in FGE.13Rev3 do not give rise to safety concerns at their levels of dietary intake, when estimated on the basis of the MSDI approach. Adequate specifications for the materials of commerce have been provided for all 26 substances. Data on uses and use levels are needed for [FL-no: 13.130]. For 21 flavouring substances [FL-no: 13.011, 13.102, 13.108, 13.113, 13.114, 13.122, 13.125, 13.127, 13.129, 13.132, 13.133, 13.135, 13.136, 13.139, 13.141, 13.143, 13.146, 13.149, 13.162, 13.178 and 13.185], the mTAMDI intake estimates are above the TTC for their structural class and more reliable data on uses and use levels are required to finalise their evaluation.

Toward of Safer Phenylbutazone Derivatives by Exploration of Toxicity Mechanism

A drug design for safer phenylbutazone was been explored by reactivity and docking studies involving single electron transfer mechanism, as well as toxicological predictions. Several approaches about its structural properties were performed through quantum chemistry calculations at the B3LYP level of theory, together with the 6-31+G(d,p) basis sets. Molecular orbital and ionization potential were associated to electron donation capacity. The spin densities contribution showed a preferential hydroxylation at the para-positions of phenyl ring when compared to other positions. In addition, on electron abstractions the aromatic hydroxylation has more impact than alkyl hydroxylation. Docking studies indicate that six structures 1, 7, 8 and 13–15 have potential for inhibiting human as well as murine COX-2, due to regions showing similar intermolecular interactions to the observed for the control compounds (indomethacin and refecoxib). Toxicity can be related to aromatic hydroxylation. In accordance to our calculations, the derivatives here proposed are potentially more active as well safer than phenylbutazone and only structures 8 and 13–15 were the most promising. Such results can explain the biological properties of phenylbutazone and support the design of potentially safer candidates.

Risks for public health related to the presence of furan and methylfurans in food

The European Commission asked EFSA for a scientific evaluation on the risk to human health of the presence of furan and methylfurans (2-methylfuran, 3-methylfuran and 2,5-dimethylfuran) in food. They are formed in foods during thermal processing and can co-occur. Furans are produced from several precursors such as ascorbic acid, amino acids, carbohydrates, unsaturated fatty acids and carotenoids, and are found in a variety of foods including coffee and canned and jarred foods. Regarding furan occurrence, 17,056 analytical results were used in the evaluation. No occurrence data were received on methylfurans. The highest exposures to furan were estimated for infants, mainly from ready-to-eat meals. Grains and grain-based products contribute most for toddlers, other children and adolescents. In adults, elderly and very elderly, coffee is the main contributor to dietary exposure. Furan is absorbed from the gastrointestinal tract and is found in highest amounts in the liver. It has a short half-life and is metabolised by cytochrome P450 2E1 (CYP2E1) to the reactive metabolite, cis-but-2-ene-1,4-dialdehyde (BDA). BDA can bind covalently to amino acids, proteins and DNA. Furan is hepatotoxic in rats and mice with cholangiofibrosis in rats and hepatocellular adenomas/carcinomas in mice being the most prominent effects. There is limited evidence of chromosomal damage in vivo and a lack of understanding of the underlying mechanism. Clear evidence for indirect mechanisms involved in carcinogenesis include oxidative stress, gene expression alterations, epigenetic changes, inflammation and increased cell proliferation. The CONTAM Panel used a margin of exposure (MOE) approach for the risk characterisation using as a reference point a benchmark dose lower confidence limit for a benchmark response of 10% of 0.064 mg/kg body weight (bw) per day for the incidence of cholangiofibrosis in the rat. The calculated MOEs indicate a health concern. This conclusion was supported by the calculated MOEs for the neoplastic effects.

A novel bis-furan scaffold for transthyretin stabilization and amyloid inhibition

The design and synthesis of a novel bis-furan scaffold tailored for high efficiency at inhibiting transthyretin amyloid formation is reported. In vitro results show that the discovered compounds are more efficient inhibitors of amyloid formation than tafamidis, a drug currently used in the treatment of familial amyloid polyneuropathy (FAP), despite their lower molecular weight and lipophilicity. Moreover, ex vivo experiments with the strongest inhibitor in the series, conducted in human blood plasma from normal and FAP Val30Met-transthyretin carriers, disclose remarkable affinity and selectivity profiles. The promises and challenges facing further development of this compound are discussed under the light of increasing evidence implicating transthyretin stability as a key factor not only in transthyretin amyloidoses and several associated co-morbidities, but also in Alzheimer's disease.

A 28-day Gavage Toxicity Study in Fischer 344 Rats with 3-methylfuran

3-Methylfuran is produced in foods during food processing and preservation techniques that involve heat treatment such as cooking, jarring, canning, and pasteurization. Currently, there are no studies available on the toxicity of 3-methylfuran. We conducted a 28-day gavage toxicity study (7 days per week) using doses of 0.0, 0.1, 0.3, 1.5, 3.0, 6.0, 12.0, and 25.0 mg/kg bw/day in order to determine the dose range needed to establish a no observed adverse effect level and to better characterize nonneoplastic effects including those affecting hematology, clinical biochemistry, gross morphology, and histopathology. Histological changes of the liver were noted in all treated animals and gross changes were noted beginning at 3.0 mg/kg bw/kg. Alterations in the activity of serum enzymes indicative of effects on the liver were observed, including increases in levels of alanine transaminase and alkaline phosphatase at the highest dose. There was a significant increase in serum thyroxine (T4) and triiodothyronine (T3), which was not accompanied by histological changes in the thyroid. For the most part, statistically significant changes were seen only at the highest dose for hematology and at the 2 highest doses for clinical chemistry parameters. In contrast, mild histological lesions in the liver were observed even at the lowest dose of 0.1 mg/kg bw/day.

A 28-day Gavage Toxicity Study in Male Fischer 344 Rats with 2-methylfuran

In most thermally treated products, a series of alkylated furan derivatives have been found, in particular 2-substituted alkylfurans such as 2-methylfuran. These methyl analogs are metabolically activated in a similar fashion as the parent furan, yielding highly reactive unsaturated dialdehydes. There is currently limited toxicological data available for 2-methyl furan exposure by any route that makes conducting a risk assessment difficult. In this pilot study, we report the general toxicology findings affecting tissue morphology, histopathology, clinical biochemistry, and hematology in a 28-day gavage study. The liver was the primary target organ that developed dose-dependent toxicity. Relative liver weights were increased by 42% at 25.0 mg/kg/body weight (bw)/day. Histological changes in the liver were observed at 0.4, 1.5, 3.0, 6.0, 12.0, and 25.0 mg/kg bw/day. These changes were not accompanied by clinical changes in the serum enzyme markers such as alanine transaminase, alkaline phosphatase, and aspartate transaminase. Clinical biochemistry markers for kidney were altered, but these were not accompanied by histological changes. The prostate was significantly decreased in size at the 25.0 mg/kg bw/day dose of 2-methyfuran. Some hematological parameters were also altered.

Reactive metabolites in the biotransformation of molecules containing a furan ring

Many xenobiotics containing a furan ring are toxic and/or carcinogenic. The harmful effects of these compounds require furan ring oxidation. This reaction generates an electrophilic intermediate. Depending on the furan ring substituents, the intermediate is either an epoxide or a cis-enedione with more ring substitution favoring epoxide formation. Either intermediate reacts with cellular nucleophiles such as protein or DNA to trigger toxicities. The reactivity of the metabolite determines which cellular nucleophiles are targeted. The toxicity of a particular furan is also influenced by the presence of competing metabolic pathways or efficient detoxification routes. GSH plays an important role in modulating the harmful effects of this class of compound by reacting with the reactive metabolite. However, this may not represent a detoxification step in all cases.

Developing structure-activity relationships for the prediction of hepatotoxicity

Drug-induced liver injury is a major issue of concern and has led to the withdrawal of a significant number of marketed drugs. An understanding of structure-activity relationships (SARs) of chemicals can make a significant contribution to the identification of potential toxic effects early in the drug development process and aid in avoiding such problems. This process can be supported by the use of existing toxicity data and mechanistic understanding of the biological processes for related compounds. In the published literature, this information is often spread across diverse sources and can be varied and unstructured in quality and content. The current work has explored whether it is feasible to collect and use such data for the development of new SARs for the hepatotoxicity endpoint and expand upon the limited information currently available in this area. Reviews of hepatotoxicity data were used to build a structure-searchable database, which was analyzed to identify chemical classes associated with an adverse effect on the liver. Searches of the published literature were then undertaken to identify additional supporting evidence, and the resulting information was incorporated into the database. This collated information was evaluated and used to determine the scope of the SARs for each class identified. Data for over 1266 chemicals were collected, and SARs for 38 classes were developed. The SARs have been implemented as structural alerts using Derek for Windows (DfW), a knowledge-based expert system, to allow clearly supported and transparent predictions. An evaluation exercise performed using a customized DfW version 10 knowledge base demonstrated an overall concordance of 56% and specificity and sensitivity values of 73% and 46%, respectively. The approach taken demonstrates that SARs for complex endpoints can be derived from the published data for use in the in silico toxicity assessment of new compounds.

Electrophilic Intermediates Produced by Bioactivation of Furan

The industrial and environmental chemical, furan, is a liver toxicant and carcinogen in laboratory animals. It has been classified as a possible human carcinogen. The mechanism of tumor induction is unknown. However, toxicity is initiated by cytochrome P450 catalyzed oxidation of furan to an alpha,beta-unsaturated dialdehyde, cis-2-butene-1,4-dial. This metabolite reacts readily with protein and DNA nucleophiles and is a bacterial mutagen in Ames assay strain TA104. Metabolism studies indicate that this reactive metabolite is formed in vivo. It is also an intermediate leading to other metabolites whose role in furan-derived toxicities has yet to be explored.

Furan-mediated uncoupling of hepatic oxidative phosphorylation in Fischer-344 rats: an early event in cell death

Furan is a potent rodent hepatotoxicant and carcinogen. The present study was done to examine the effects of furan on hepatic energy metabolism both in vivo and in vitro in male F-344 rats. Furan produced concentration- and incubation time-dependent irreversible reductions in ATP in freshly isolated F-344 rat hepatocytes. Furan-mediated depletion of ATP occurred prior to cell death and was prevented by including 1-phenylimidazole, a cytochrome P450 inhibitor, in the suspensions. Male F-344 rats were treated with furan (0-30 mg/kg, po) and killed 24 hr later to prepare hepatic mitochondria. Furan produced dose-dependent increases in state 4 respiration and ATPase activity. Both of these changes were prevented by 1-phenylimidazole cotreatment. In a separate series of experiments, mitochondria were prepared from isolated rat hepatocytes following incubation with furan (2-100 microM) for 1-4 hr. Furan produced incubation time- and concentration-dependent increases in state 4 respiration and ATPase activity. Furan-mediated mitochondrial changes were prevented by adding 1-phenylimidazole to the hepatocyte suspensions. These results indicate that the ene-dialdehyde metabolite of furan uncouples hepatic oxidative phosphorylation in vivo and in vitro. In vitro studies using an isolated hepatocyte suspension/culture system demonstrated that the concentration response for furan-mediated mitochondrial changes in suspension corresponded with the concentration responses for cell death after 24 hr. Including 1-phenylimidazole or oligomycin plus fructose in hepatocyte suspensions prevented furan-induced cell death after 24 hr in culture. The results of this study indicate that furan-induced uncoupling of oxidative phosphorylation is an early, critical event in cytolethality both in vivo and in vitro.

Expression of myc, fos, and Ha-ras in the livers of furan-treated F344 rats and B6C3F1 mice

Furan administered by gavage for 2 yr has been reported to induce hepatocellular carcinomas in male and female B6C3F1 mice and in male but not female F344 rats. Chronic exposure studies in our laboratory using bioassay conditions showed extensive hepatocellular toxicity and sustained increases in regenerative cell proliferation after 1, 3, and 6 wk of treatment in male and female rats and male mice. Altered expression of growth-control genes associated with this hyperproliferative state may enhance the susceptibility of these genes to mutation or may provide a selective growth advantage to preneoplastic cells. Quantitative northern blot analysis of mRNA was used to examine the expression of the oncogenes myc, fos, and Ha-ras in the livers of animals treated with furan. In male rats, a single administration of 30 mg/kg furan produced necrosis and a subsequent wave of cell proliferation 48 h after treatment and induced transient peaks in the expression of myc, fos, and Ha-ras 6-24 h after treatment. In male rat liver from our cell proliferation studies, only a slight increase in myc expression was seen at the end of week 1 of treatment. However, beginning at week 3 and increasing at week 6, up to a 15-fold increase over control values was observed in the expression of myc in the treated animals. The only other notable increase in expression observed in any animals from the cell proliferation study was a threefold increase in myc at week 6 in treated female rats. The absence of an increase in Ha-ras expression in the male mouse liver suggests that the unique pattern of Ha-ras mutations previously reported in furan-induced mouse liver tumors is not due to increased mutational susceptibility related to overexpression of this gene. The lack of sustained expression of myc, fos, and Ha-ras in rapidly proliferating liver suggests that continuous expression of these genes is not necessary to maintain increased rates of cell replication. The large increase in myc expression in male but not female rats suggests an adaptive change that may be related to the sex-specific incidence of furan-induced hepatocellular carcinomas in rats.

Pulmonary absorption and disposition of [14C]thiophene in rats following nose-only inhalation exposure

The absorption, disposition, and metabolism of [14C]thiophene was investigated in rats following nose-only inhalation exposure at 8000 ppm for 1 h. Under these exposure conditions, it was estimated that approximately 16.3% (493 mumol) of the inhaled thiophene was absorbed from the respiratory system. Within 72 h following exposure, a total of 488 mumol of thiophene equivalents (99% of that retained) was excreted, of which 360.4 mumol (73.9% of the total excreted radioactivity) was in expired air, 120.7 mumol (24.8%) was in urine, 3 mumol (0.6%) was in feces, and 3.7 mumol (0.8%) was in the cage wash. Excretion took place primarily within the first 8 h, during which 91% of the total radioactivity excreted was collected. The thiophene equivalents remaining in tissues at 72 h were estimated to total 5.1 mumol (1.0% of the retained radioactivity). Exhaled radioactivity was identified as thiophene. No 14CO2 was detected in the expired air. After 1 h following exposure, the elimination of thiophene equivalents from plasma was monophasic, with a half-time of 3.6 h. The elimination of thiophene equivalents from blood cells was biphasic, with half-times of 2.9 h and 9.1 d. The blood cells/plasma concentration ratios of thiophene equivalents ranged from 3 to 13, with the higher ratio observed at the 12-h time interval. At 72 h after exposure, blood cells contained the highest concentration of thiophene equivalents, approximately fourfold higher than that of the liver, which contained the second highest concentration. Kidney, heart, and lung contained similar but lower concentrations than liver, while brain, fat, and skeletal muscles contained the lowest concentrations. In summary, this study demonstrates that thiophene was absorbed from the respiratory system, and the majority of the absorbed thiophene was eliminated unchanged in the exhaled air, while a smaller fraction was metabolized and eliminated in urine.

Toxicity of alkyldihydrofurans to metabolically active organs in the mouse

Alkylfurans inflict toxicity in several mammalian species to lung, liver and kidney. Organ specificity of the alkylfurans is a sensitive function of the nature of the alkyl group. To determine if this toxicity requires an aromatic ring in the compound, we synthesized 4-methyl-2,3-dihydrofuran, 4-ethyl-2,3-dihydrofuran and 4-pentyl-2,3-dihydrofuran and determined their toxicity to lung, liver and kidney in mice. Lung damage was evaluated by light microscopy and the incorporation of [14C]thymidine into lung DNA. The results indicated that 4-methyl-2,3-dihydrofuran and 4-ethyl-2,3-dihydrofuran were toxic to the lung whereas 4-pentyl-2,3-dihydrofuran did not produce lung toxicity. Histological examination of liver sections revealed that 4-ethyl-2,3-dihydrofuran induced vacuolar degeneration of hepatocytes. Kidney toxicity was evaluated by light microscopy and determining plasma urea levels. Both 4-ethyl-2,3-dihydrofuran and 4-pentyl-2,3-dihydrofuran exhibited kidney toxicity, while equimolar doses of 4-methyl-2,3-dihydrofuran did not damage the kidney. A quantitative comparison of the nephrotoxicity of 4-pentyl-2,3-dihydrofuran with the corresponding aromatic compound 3-pentylfuran was made. We also sought to determine if renal injury resulting from these 2 agents is related to their oxidative metabolism. Uptake of organic ions by kidney slices and plasma urea nitrogen levels were used to assess renal function. 3-Pentylfuran caused greater renal injury than an equimolar dose of 4-pentyl-2,3-dihydrofuran. Phenobarbital pretreatment protected mice against 3-pentylfuran-induced nephrotoxicity. Cotreatment with piperonyl butoxide did not affect renal injury resulting from 3-pentylfuran. N-octylimidazole significantly reduced 3-pentylfuran-induced nephrotoxicity as well as that caused by 4-pentyl-2,3-dihydrofuran. These results point to metabolic activation as a basis for the nephrotoxicity induced by both compounds.