Short Communication Presystemic Elimination of Trichloroethylene in Rats Following Environmentally Relevant Oral Exposures

1,1,2-Trichloroethylene (TCE), a volatile organic contaminant (VOC) of drinking water in the Unites States, is frequently present in trace amounts. TCE is currently classified by the International Agency for Research on Cancer and the U.S. Environmental Protection Agency as a probable human carcinogen, because it produces tumors in some organs of certain strains of mice or rats in chronic, high-dose bioassays. Previous studies (Toxicol Appl Pharmacol 60:509–526, 1981; Regul Toxicol Pharmacol 8:447–466, 1988) used physiological modeling principles to reason that the liver should remove virtually all of a well metabolized VOC, such as TCE, as long as concentrations in the portal blood were not high enough to saturate metabolism. To test this hypothesis, groups of unanesthetized male Sprague-Dawley rats received intravenous injections of 0.1, 1.0, or 2.5 mg TCE/kg as an aqueous emulsion. Other rats were gavaged with 0.0001, 0.001, 0.01, 0.1, 1, 2.5, 5, or 10 mg TCE/kg b.wt. Serial microblood samples were taken via an indwelling carotid artery cannula, to generate blood TCE versus time profiles. Headspace solid-phase microextraction gas chromatography with negative chemical ionization mass spectrometry (limit of quantitation 25 pg/ml) was used to quantify TCE. TCE was undetectable in rats given 0.0001 mg/kg, but it exhibited linear kinetics from 0.1 to 5.0 mg/kg. Bioavailability was consistent over this dosage range, ranging from 12.5 to 16.4%. The presence of these limited amounts of TCE in the arterial blood disprove the aforementioned hypothesis, yet demonstrate that first-pass hepatic and pulmonary elimination in the rat afford its extrahepatic organs protection from potential adverse effects by the majority of the low levels of TCE absorbed from drinking water. Extensive use of volatile organic chemicals (VOCs), including 1,1,2-trichloroethylene (TCE), has resulted in their common occurrence in drinking water supplies. TCE is the most frequently found chemical contaminant in groundwater in the proximity of hazardous waste sites in the United States. TCE was often detected in the blood of 982 nonoccupationally exposed adults evaluated in the National Health and Nutrition Examination Survey (Churchill et al., 2001) and more recently in a subset of 951 persons (Blount et al., 2006). Concentrations typically found in finished drinking water in the United States range from parts per trillion (ppt) to parts per billion (ppb) (Moran et al., 2007). Trace levels of TCE are primarily a health concern because the solvent is a potential human carcinogen. TCE-induced tumors seen in chronic, very high-dose rodent bioassays are organ-, species-, and strain-specific (National Research Council, 2006). Hepatocellular carcinoma, for example, is known to occur in only one strain of one species, the B6C3F1 mouse. Some strains of mice inhaling the chemical have developed lung tumors. A low incidence of kidney tumors has been reported in three of seven strains of rats tested. Leydig cell tumors have only been seen in male Sprague-Dawley (S-D) rats. There is still controversy about the relevance of these high-dose rodent tumors to humans and about human risks posed by very low exposures (Clewell and Andersen, 2004; Cohen et al., 2004; Caldwell and Keshava, 2006; Lock and Reed, 2006). Some researchers and administrators have taken the position that scientific evidence is insufficient to rule out that certain TCE metabolites may be mutagenic and therefore have no dosage threshold. TCE, in sufficient amounts, can produce noncancer effects in organs including the brain, liver, kidneys, testes, and immune system. Biotransformation plays a key role in modulating the toxicokinetics (TK) and the ensuing toxicity and carcinogenicity of TCE. The VOC is metabolized primarily via a CYP450-catalyzed oxidative pathway involving sequential formation of a series of products (Lash et al., 2000a). The initial step in oxidation of low TCE doses is catalyzed primarily in rodents and humans by CYP2E1, a constitutive CYP450 isoform (Lipscomb et al., 1997; Ramdhan et al., 2008). The second, relatively minor pathway involves conjugation of TCE with glutathione, followed by a series of subsequent metabolic activation and detoxification reactions (Lash et al., 2000a). This second pathway becomes important quantitatively only at quite high TCE doses. The majority of TCE biotransformation occurs in the liver, although metabolic activation of relatively small quantities of TCE reaching extrahepatic tissues, such as kidney (Lash et al., 2000b), testes (Forkert This work was supported in part by the U.S. Department of Energy [Cooperative Agreement No. DE-FC02-02CH11109]. Most of the data were previously presented as follows: Liu Y, Bartlett MG, White CA, Muralidhara S, Bruckner JV, and Fisher JW (2008) Characterization of presystemic elimination of trichloroethylene (TCE) in rats following environmentally relevant exposures. 47th Annual Meeting of the Society of Toxicology; 2008 Mar 15; Seattle, WA. Society of Technology, Reston, VA. Article, publication date, and citation information can be found at http://dmd.aspetjournals.org. doi:10.1124/dmd.109.028100. ABBREVIATIONS: VOC, volatile organic chemical; TCE, 1,1,2-trichloroethylene; ppt, parts per trillion; ppb, parts per billion; S-D, Sprague-Dawley; TK, toxicokinetics; MS, mass spectrometry; AUC0 , area under the blood TCE concentration versus time curve; Vd, volume of distribution; GI, gastrointestinal; t1⁄2, terminal elimination half-life; PBPK, physiologically based pharmacokinetic. 0090-9556/09/3710-1994–1998$20.00 DRUG METABOLISM AND DISPOSITION Vol. 37, No. 10 Copyright © 2009 by The American Society for Pharmacology and Experimental Therapeutics 28100/3510676 DMD 37:1994–1998, 2009 Printed in U.S.A. 1994 at A PE T Jornals on O cber 4, 2017 dm d.aspurnals.org D ow nladed from et al., 2002), and lungs (Forkert et al., 2006), can have a toxicologically significant impact in situ. There are a number of protection and repair systems that guard against cytotoxic, mutagenic, and carcinogenic actions of TCE and other chemicals. One of these processes is first-pass or presystemic elimination. Ingested chemicals that are absorbed into venous mesenteric blood vessels are conveyed via the portal vein through the liver before reaching the arterial circulation and extrahepatic organs. Lee et al. (1996) report that a substantial proportion of oral TCE, a well metabolized VOC, is eliminated its first pass through the liver and lungs of male rats. Weisel and Jo (1996) were essentially unable to detect TCE in the exhaled breath of persons who consumed 0.5 liter of water containing 20 or 40 g TCE/liter. Andersen (1981) proposed that the liver was capable of removing essentially all of the orally administered VOCs with high extraction ratios from portal blood, if their concentrations were not high enough to saturate metabolism. If true, this effect could have profound implications for theoretical cancer risks in extrahepatic tissues. Our research group has recently developed analytical techniques (Liu et al., 2008a,b) that are sensitive enough to evaluate the efficiency of first-pass elimination of trace levels of TCE in drinking water. The overall objective of the current project was to test the hypothesis of Andersen (1981), by directly characterizing the linearity of the kinetics and delineating the bioavailability of a series of very low oral doses of TCE in rats. Materials and Methods Chemicals. Analytical-grade TCE was purchased from Sigma-Aldrich (St. Louis, MO). Sulfuric acid was purchased from Mallinckrodt Baker, Inc. (Phillipsburg, NJ). High-performance liquid chromatography-grade acetonitrile was obtained from Thermo Fisher Scientific (Waltham, MA). Deionized water was generated from a Siemens deionized water system (Warrendale, PA). Ultrahigh purity helium and methane were purchased from National Welders (Charlotte, NC). Alkamuls, formerly Emulphor, a polyethoxylated vegetable oil, was obtained from sanofi-aventis (Bridgewater, NJ) and used to prepare stable aqueous TCE emulsions the day of dosing. Animals. Male S-D rats (270–380 g) were obtained from Charles River Laboratories (Raleigh, NC). All protocols for this study were approved by the institution’s Animal Care and Use Committee. The animals were housed in pairs in polycarbonate cages in an Association for Assessment and Accreditation of Laboratory Animals-approved animal care facility with a 12-h light cycle (light: 7:00 AM–7:00 PM) at 22 2 C and 55 5% relative humidity for at least 7 days before use. Food (5001 Rodent Diet; PMI Nutrition International, Brentwood, MO) and boiled tap water were provided ad libitum