3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX): toxicological properties and risk assessment in drinking water

Cell-line and culture model specific responses to organic contaminants in house dust: Cell bioenergetics, oxidative stress, and inflammation endpoints

Exposure to organic contaminants in house dust is linked to the development or exacerbation of many allergic and immune disorders. In this work, we evaluate the effects of organic contaminants on different cell bioenergetics endpoints using five different cell lines (16HBE14o-, NuLi-1, A549, THP-1 and HepG2), and examine its effects on lung epithelial cells using conventional 2D and 3D (air-liquid interface/ALI) models. Proposed rapid bioenergetic assays relies on a quick, 40 min, exposure protocol that provides equivalent dose-response curves for ATP production, spare respiratory capacity, and cell respiration. Although cell-line differences play an important role in assay performance, established EC₅₀ concentrations for immortalized lung epithelial cells ranged from 0.11 to 0.15 mg/mL (∼2 µg of dust in a 96-well microplate format). Bioenergetic response of distinct cell types (i.e., monocytes and hepatocytes) was significantly different from epithelial cells; with HepG2 showing metabolic activity that might adversely affect results in 24 h exposure experiments. Like in cell bioenergetics, cell barrier function assay in ALI showed a dose dependent response. Although this is a physiologically relevant model, measurements are not as sensitivity as cytokine profiling and reactive oxygen species (ROS) assays. Observed effects are not solely explained by exposure to individual contaminants, this suggests that many causal agents responsible for adverse effects are still unknown. While 16HBE14o- cells show batter barrier formation characteristics, NuLi-1 cells are more sensitivity to oxidative stress induction even at low house dust extract concentrations, (NuLi-1 2.11-fold-change vs. 16HBE14o- 1.36-fold change) at 0.06 µg/mL. Results show that immortalized cell lines can be a suitable alternative to primary cells or other testing models, especially in the development of high-throughput assays. Observed cell line specific responses with different biomarker also highlights the importance of careful in-vitro model selection and potential drawbacks in risk assessment studies.

Carcinogenicity of the chlorination disinfection by-product MX

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone, better known by its historical name 'mutagen X' or MX, is a chlorination disinfection byproduct that forms from the reaction of chlorine and humic acids in raw water. MX has been measured in drinking water samples in several countries at levels that ranged from non-detectable to 310 ng/L. Although the concentration of MX in drinking water is typically 100- to 1000-fold lower than other common chlorinated by-products of concern (e.g., trihalomethanes), some have hypothesized that MX might play a role in the increased cancer risks that have been associated with the consumption of chlorinated water. This hypothesis is based on observations that MX, in some test systems, is extremely potent relative to trihalomethanes in inducing DNA damage and altering pathways involved in cell growth, and that in some epidemiological studies increased cancer rates are associated with the bacterial mutagenicity of disinfected water of which MX contributes a significant portion. MX also appears to be more potent than other chlorination by-products in causing cancer in animals. This article reviews the available evidence on the carcinogenicity of MX. MX induced cancer at multiple sites in male and female rats, acted as a tumor initiator and promoter, enhanced tumor yields in genetically modified rodents, induced a myriad of genotoxic effects in numerous in vitro and in vivo test systems, and was a potent inhibitor of gap junction intercellular communication. Although the precise mechanism of MX-induced DNA damage is not known, MX is able to cause DNA damage through an unusual mechanism of ionizing DNA bases due to its extremely high reductive potential. MX may also cause mutations through DNA adduction. This article develops a mean cancer potency estimate for MX of 2.3 (mg/kg-d)(-1) and an upper 95% percentile estimate of 4.5 (mg/kg-d)(-1), and examines the potential health risks posed by this chlorination contaminant in drinking water. A discussion of additional data that would be desirable to better characterize the risks posed by MX and other halogenated hydroxyfuranones follows.

Principles of risk assessment for determining the safety of chemicals: recent assessment of residual solvents in drugs and di(2-ethylhexyl) phthalate

Risk assessment of chemicals is essential for the estimation of chemical safety, and animal toxicity data are typically used in the evaluation process, which consists of hazard identification, dose-response assessment, exposure assessment, and risk characterization. Hazard identification entails the collection of all available toxicity data and assessment of toxicity endpoints based on findings for repeated dose toxicity, carcinogenicity or genotoxicity and species-specificity. Once a review is compiled, the allowable lifetime exposure level of a chemical is estimated from a dose-response assessment based on several measures. For non-carcinogens and non-genotoxic carcinogens, the no-observed-adverse-effect-level (NOAEL) is divided by uncertainty factors (e.g. with environmental pollutants) or safety factors (e.g. with food additives) to derive a tolerable daily intake (TDI) or acceptable daily intake (ADI), respectively. These factors include interspecies and individual differences, duration of exposure, quality of data, and nature of toxicity such as carcinogenicity or neurotoxicity. For genotoxic carcinogens, low dose extrapolation is accomplished with mathematical modeling (e.g. linearized multistage model) from the point of departure to obtain exposure levels that will be associated with an excess lifetime cancer risk of a certain level. Data for levels of chemicals in food, water and air, are routinely used for exposure assessment. Finally, risk characterization is performed to ensure that the established 'safe' level of exposure exceeds the estimated level of actual exposure. These principles have led to the evaluation of several existing chemicals. To establish a guideline for residual solvents in medicine, the permitted daily exposure (PDE), equivalent to TDI, of N,N-dimethylformamide was derived on the basis of developmental toxicity (malformation) and of N-methylpyrrolidone on the basis of the developmental neurotoxicity. A TDI for di(2-ethylhexyl)phthalate was derived from assessment of testicular toxicity.

Experimental cancer studies of chlorinated by-products

Chlorinated drinking water contains a number of different by-products formed during the chlorination process from organic matter. The carcinogenicity of only a fraction of them have been evaluated in experimental animals. The focus has been on compounds and groups of compounds that are most abundant in chlorinated drinking water or the in vitro toxicity data have suggested genotoxic potential. From trihalomethanes, chloroform causes liver tumors in mice and female rats and renal tumors in male mice and rats. Tumor formation by chloroform is strongly associated with cytotoxicity and regenerative cell proliferation in tissues and that has been considered to be one determinant of its carcinogenicity. From halogenic acetic acids, dichloroacetic acid (DCA) and trichlotoacetic acid (TCA) are hepatocarcinogenic in mice and DCA in male rats. Their genotoxicity is equivocal and nongenotoxic mechanisms, such as peroxisome proliferation and hypomethylation of DNA in the liver, likely contribute to tumor development. From chlorinated furanones (CHFs), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a multisite carcinogen in rats (e.g. in thyroid glands and liver) and it has caused DNA damage in vivo. MX may be a complete carcinogen because it also has promoter properties in vitro. Chlorinated drinking water may also contain brominated by-products providing the raw water contains bromide. At least some of them (bromodichloromethane, bromoform) have been shown to be carcinogenic in laboratory animals. Altogether, although several by-products have been shown to have carcinogenic potential in laboratory animals, it not yet possible to state which compounds or groups of by-products cause the cancer risk in chlorinated drinking water. The cellular mechanisms of their effects and these effects at low concentrations are still poorly understood. The few studies with mixtures of these by-products suggest that the mixture effects may be complex and unpredictable (inhibitory, additive, synergistic).