A method for the simultaneous determination of mercapturic acids as biomarkers of exposure to 2-chloroprene and epichlorohydrin in human urine

Comparison of urinary mercapturic acid excretions in users of various tobacco/nicotine products

Urinary mercapturic acids (MAs) are detoxification products for electrophiles occurring in the human body. They are suitable biomarkers of exposure to directly acting electrophilic chemicals or to chemicals which generate the electrophile during its metabolism. We determined the urinary excretion of 19 MAs in habitual users of combustible cigarettes (CCs), electronic cigarettes (ECs), heated tobacco products (HTPs), oral tobacco (OT), and nicotine replacement therapy (NRT) products, and nonusers (NUs) of any tobacco/nicotine products. The 19 MAs are assumed to be physiologically formed primarily from 15 toxicants with three of them belonging to IARC Group 1 (human carcinogen), seven to Group 2A (probable human carcinogen), four to Group 2B (possible human carcinogen), and one to Group 3 (not classifiable as carcinogen). Smoking (CC) was found to be associated with significantly elevated exposure to ethylene oxide (or ethylene), 1,3-butadiene, benzene, dimethylformamide, acrolein, acrylamide, styrene, propylene oxide, acrylonitrile, crotonaldehyde, and isoprene compared with the other user groups and NU. Users of HTPs revealed slight elevation in the MAs related to acrolein, acrylamide, and crotonaldehyde compared with the other non-CC groups. Vaping (EC) was not found to be associated with any of the MAs studied. In conclusion, the determination of urinary MAs is a useful tool for assessing the exposure to toxicants (mainly potential carcinogens) in users of various tobacco/nicotine products. Our data also give cause to clarify the role of vaping (EC) in urinary excretion of DHPMA (precursor: glycidol).

Extended Analysis and Evidence Integration of Chloroprene as a Human Carcinogen

β-Chloroprene is used in the production of polychloroprene, a synthetic rubber. In 2010, Environmental Protection Agency (EPA) published the Integrated Risk Information System "Toxicological Review of Chloroprene," concluding that chloroprene was "likely to be carcinogenic to humans." This was based on findings from a 1998 National Toxicology Program (NTP) study showing multiple tumors within and across animal species; results from occupational epidemiological studies; a proposed mutagenic mode of action; and structural similarities with 1,3-butadiene and vinyl chloride. Using mouse data from the NTP study and assuming a mutagenic mode of action, EPA calculated an inhalation unit risk (IUR) for chloroprene of 5 × 10^-4 per µg/m³ . This is among the highest IURs for chemicals classified by IARC or EPA as known or probable human carcinogens and orders of magnitude higher than the IURs for carcinogens such as vinyl chloride, benzene, and 1,3-butadiene. Due to differences in pharmacokinetics, mice appear to be uniquely responsive to chloroprene exposure compared to other animals, including humans, which is consistent with the lack of evidence of carcinogenicity in robust occupational epidemiological studies. We evaluated and integrated all lines of evidence for chloroprene carcinogenicity to assess whether the 2010 EPA IUR could be scientifically substantiated. Due to clear interspecies differences in carcinogenic response to chloroprene, we applied a physiologically based pharmacokinetic model for chloroprene to calculate a species-specific internal dose (amount metabolized/gram of lung tissue) and derived an IUR that is over 100-fold lower than the 2010 EPA IUR. Therefore, we recommend that EPA's IUR be updated.

Isotope dilution LC/ESI--MS-MS quantitation of urinary 1,4-bis(N-acetyl-S-cysteinyl)-2-butanone in mice and rats as the biomarker of 1-chloro-2-hydroxy-3-butene, an in vitro metabolite of 1,3-butadiene

1-Chloro-2-hydroxy-3-butene (CHB) is a possible metabolite of 1,3-butadiene, a carcinogenic air pollutant. To demonstrate its formation in vivo, it is desirable to develop a practical biomarker and the corresponding analysis method. CHB can undergo alcohol dehydrogenase- and cytochromes P450 enzymes (P450)-mediated oxidation to yield 1-chloro-3-buten-2-one (CBO), which readily forms glutathione conjugates. We hypothesized that CBO-derived mercapturic acids, which are the expected biotransformed products of CBO-glutathione conjugates, could be used as CHB biomarkers. Thus, in the present study, we investigated the in vivo biotransformation of CHB into CBO-derived mercapturic acids. Because the reaction of CBO with N-acetyl-l-cysteine yields two products, 1,4-bis(N-acetyl-S-cysteinyl)-2-butanone (NC1) and 1-chloro-4-(N-acetyl-S-cysteinyl)-2-butanone (NC2), we first developed an isotope dilution LC/ESI^--MS-MS method to quantitate urinary NC1 and NC2, and then determined their concentrations in urine of C57BL/6 mice and Sprague-Dawley rats administered CHB. Since no NC2 was detected in samples, the LC/ESI^--MS-MS method was optimized specifically for NC1. NC1 was enriched through solid phase extraction with the recovery being 75-82%. The limits of detection and quantitation were 6.8 and 34 fmol/0.1 mL for mouse urine, and 4.5 and 7.1 fmol/0.1 mL for rat urine, respectively. In urine of animals before CHB administration, no NC1 was detected; in mice administered CHB at 10 and 30 mg/kg, and rats at 5 and 15 mg/kg, NC1 was detected and its concentrations in urine from animals given higher doses were 3-6 fold higher than those given lower doses. Moreover, the NC1 concentrations in urine during 0-8 h were 4-6 fold and 10-11 fold higher than those during 8-24 h for mice and rats, respectively. The results demonstrated that CHB could be in vivo biotransformed into NC1, which could be used as a practical CHB biomarker.

Weighted gene co-expression network analysis of pneumocytes under exposure to a carcinogenic dose of chloroprene

AIMS

Occupational exposure to chloroprene via inhalation may lead to acute toxicity and chronic pulmonary diseases, including lung cancer. Currently, most research is focused on epidemiological studies of chloroprene production workers. The specific molecular mechanism of carcinogenesis by chloroprene in lung tissues still remains obscure, and specific candidate therapeutic targets for lung cancer are lacking. The present study identifies specific gene modules and valuable hubs associated with lung cancer.

MAIN METHODS

We downloaded the dataset GSE40795 from the Gene Expression Omnibus (GEO) and divided the dataset into the non-carcinogenic dose chloroprene exposed mice group and the carcinogenic dose chloroprene exposed mice group. With a systemic biological view, we discovered significantly altered gene modules between the two groups and identified hub genes in the carcinogenic dose exposed group using weighted co-expression network analysis (WGCNA).

KEY FINDINGS

A total of 2434 differentially expressed genes were identified. Twelve gene modules with multiple biological activities were related to the carcinogenesis of chloroprene in lung tissue. Seven hub genes that were critical for the carcinogenesis of chloroprene in lung tissue were ultimately identified, including Cftr, Hip1, Tbl1x, Ephx1, Cbr3, Antxr2 and Ccnd2. They were implicated in inflammatory response, cell transformation, gene transcription regulation, phase II detoxification, angiogenesis, cell adhesion, motility and the cell cycle.

SIGNIFICANCE

The seven hub genes may become valuable candidates for risk assessment biomarkers and therapeutic targets in lung cancer.

Rapid determination of four short-chain alkyl mercapturic acids in human urine by column-switching liquid chromatography-tandem mass spectrometry

We developed and validated an analytical method for the simultaneous determination of methyl mercapturic acid (MeMA), ethyl mercapturic acid (EtMA), n-propyl mercapturic acid (PrMA) and iso-propyl mercapturic acid (iPrMA) in human urine. These alkyl mercapturic acids are known or presumed biomarkers of exposure to several alkylating agents including methyl bromide, dimethyl sulfate, ethyl bromide, 1-bromopropane and 2-bromopropane. The method involves a column switching arrangement for online solid phase extraction of the analytes with subsequent analytical separation and detection using liquid chromatography and tandem mass spectrometry. Within day and day-to-day imprecision was determined to range from 4.5 to 12.2%. The analytical method is distinguished by its wide linear working range of up to 2,500 μg/L with detection limits ranging from 2.0 μg/L (for PrMA) to 5.1 μg/L (for MeMA) that render possible the application in various biomonitoring studies regarding exposure to alkylating agents. The results of a pilot study on urine samples of 30 individuals occupationally non-exposed to alkylating agents using the new procedure confirmed the background excretion of MeMA (<5.1-35.6 μg/L) and PrMA (<2.0-95.7 μg/L).