NTP technical report on the toxicity and metabolism studies of chloral hydrate (CAS No. 302-17-0). Administered by gavage to F344/N rats and B6C3F1 mice

Abstract

Chloral hydrate is widely used as a sedative and a hypnotic in pediatric medicine. It is also a byproduct of water chlorination. Chloral hydrate has been shown to be genotoxic in numerous prokaryotic and eukaryotic assay systems including human lymphocytes in vitro. One of its metabolites, trichloroacetic acid, has demonstrated hepatocarcinogenic activity in mice. Trichloroethylene and perchloroethylene, both of which are metabolized to chloral hydrate, have been shown to be carcinogenic in rats and/or mice. Because of this evidence of carcinogenicity and because of the wide-spread use of chloral hydrate, 16- or 17-day range-finding toxicity studies and separate 16- or 17-day metabolism studies were performed in F344/N rats and B6C3F1 mice in preparation for further long-term rodent studies. In addition, in vitro studies of the metabolism and DNA-binding capacity of chloral hydrate and its metabolites were performed. Genetic toxicity studies were conducted in Salmonella typhimurium, cultured Chinese hamster ovary cells, Drosophila melanogaster, and mouse bone marrow cells. For the range-finding studies, groups of eight male and eight female F344/N Nctr BR rats and B6C3F1/Nctr BR (C57BL/6N x C3H/HeN MTV-) mice were administered 0, 50, 100, 200, 400, or 800 mg chloral hydrate per kg body weight in water by gavage 5 days per week for 17 days (rats) or 16 days (mice) for a total of 12 doses. One male rat receiving 800 mg/kg died after five doses. Two 800 mg/kg female rats died after dosing ended but before study termination. One male mouse in each group except the 400 mg/kg group died before the end of the study. Two 800 mg/kg female mice also died before the end of the study. The final mean body weight of 800 mg/kg male rats and the mean body weight gains of 400 and 800 mg/kg males were significantly less than those of the vehicle controls. The mean body weight gains of all groups of dosed male mice were significantly greater than that of the vehicle control group. The only clinical finding in rats and mice attributed to chloral hydrate treatment was light sedation in the 400 mg/kg groups and heavy sedation in the 800 mg/kg groups; sedation subsided within 30 minutes or 3 hours, respectively. The liver weights of 400 mg/kg male mice and 800 mg/kg male and female mice were significantly greater than those of the vehicle control groups. No chemical-related lesions were observed in rats or mice. Male and female rats and mice were administered a single dose of 50 or 200 mg chloral hydrate per kg body weight in water by gavage, or 12 doses of 50 or 200 mg/kg over 17 days (rats) or 16 days (mice). Plasma concentrations of chloral hydrate and its metabolites were determined 15 minutes, 1, 3, 6, and 24 hours, and 2, 4, 8, and 16 days after receiving 1 or 12 doses. Maximum concentrations of chloral hydrate were observed at the initial sampling point of 15 minutes. By 1 hour, the concentrations had dropped substantially, and by 3 hours, chloral hydrate could not be detected in rats or mice. Trichloroacetic acid was the major metabolite detected in the plasma. In rats, the concentrations rose slowly, with the peaks occurring between 1 and 6 hours after treatment. In mice, the peak concentrations were found 1 hour after dosing. The concentrations then slowly decreased such that by 2 days the metabolite could no longer be detected in rats or mice. Trichloroethanol was assayed both as the free alcohol and its glucuronide. In rats, the maximum concentrations of free trichloroethanol occurred at 15 minutes, while the peak concentrations of trichloroethanol glucuronide were found at 1 hour; by 3 hours, concentrations of both metabolites approached background levels. In mice, the maximum concentrations of both metabolites occurred at 15 minutes, and by 1 to 3 hours concentrations approached background levels. The plasma concentrations of chloral hydrate and its metabolites were dose dependent in rats and mice. In mice, plasma concentrations of trichloroacetic acid were significantly higher after a single dose than after 12 doses. None of the metabolic parameters appears to account for species differences that may exist in hepatocarcinogenicity. The data from the study of metabolism and DNA adduct formation indicated that in vitro metabolism of 200 microM to 5 mM chloral hydrate by male B6C3F1 mouse liver microsomes (control microsomes) generated free radical intermediates that resulted in endogenous lipid peroxidation, forming malondialdehyde, formaldehyde, acetaldehyde, acetone, and propionaldehyde. Similar concentrations of trichloroacetic acid and trichloroethanol, the primary metabolites of chloral hydrate, also generated free radicals and induced lipid peroxidation. Lipid peroxidation induced by trichloroacetic acid nearly equaled that induced by chloral hydrate, while that from trichloroethanol was three- to fourfold less. Metabolism of 200 microM to 5 mM chloral hydrate, trichloroacetic acid, and trichloroethanol by liver microsomes of B6C3F1 mice pretreated with pyrazole (pyrazole-induced microsomes) yielded lipid peroxidation products at concentrations two- to threefold greater than those from liver microsomes of untreated mice. Additionally, chloral hydrate-induced lipid peroxidation catalyzed by control and pyrazole-induced microsomes was reduced significantly by 2,4-dichloro-6-phenylphenoxyethylamine, a general cytochrome P450 inhibitor. Human lymphoblastoid transgenic cells expressing cytochrome P(450)2E1 metabolized 200 to 5,000 micrograms/mL chloral hydrate to reactants inducing mutations, whereas the parental cell line was inactive. The malondialdehyde-modified DNA adduct, 3-(2-deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2 alpha]purin-10(3H)-one (MDA-MG-1), formed from the metabolism of 1 mM chloral hydrate, trichloroacetic acid, and trichloroethanol by control B6C3F1 mouse liver microsomes, mouse pyrazole-induced microsomes, male F344/N rat liver microsomes, and human liver microsomes in the presence and absence of calf thymus DNA was also determined. When incubated in the absence of calf thymus DNA, the amount of malondialdehyde formed from metabolism by pyrazole-induced mouse microsomes was twice that from rat or human liver microsomes. Amounts of chloral hydrate-induced and trichloroacetic acid-induced lipid peroxidation products formed from metabolism by rat and human liver microsomes were similar, and these quantities were about twice those formed from the metabolism of trichloroethanol. The quantity of MDA-MG-1 formed from the metabolism of chloral hydrate, trichloroacetic acid, and trichloroethanol by mouse, rat, and human liver microsomes exhibited a linear correlation with the quantity of malondialdehyde formed under incubation conditions in the absence of calf thymus DNA. Chloral hydrate was shown to be mutagenic in vitro and in vivo. At doses from 1,000 to 10,000 micrograms/plate, it induced mutations in S. typhimurium strain TA100, with and without S9 activation; an equivocal response was obtained in S. typhimurium strain TA98 in the absence of S9, and no mutagenicity was detected with strain TA1535 or TA1537. Chloral hydrate at doses from 1,700 to 5,000 micrograms/mL induced sister chromatid exchanges; at doses from 1,000 to 3,000 micrograms/mL, chromosomal aberrations were induced in cultured Chinese hamster ovary cells, with and without S9. Results of a sex-linked recessive lethal test in D. melanogaster were unclear; administration of chloral hydrate by feeding produced an inconclusive increase in recessive lethal mutations, results of the injection experiment were negative. An in vivo mouse bone marrow micronucleus test with chloral hydrate at doses from 125 to 500 mg/kg gave a positive dose trend. In summary, due to the absence of chloral hydrate-induced histopathologic lesions in rats and mice, no-observed-adverse-effect levels (NOAELs) were based on body weights of rats and liver weights of mice. The NOAELs for rats and mice were 200 mg/kg. Chloral hydrate was rapidly metabolized by rats and mice, with trichloroacetic acid occurring as the major metabolite. Peak concentrations of trichloroacetic acid occurred more quickly in mice. Plasma concentrations of chloral hydrate were dose dependent, but metabolic rates were unaffected by dose or sex. Chloral hydrate was mutagenic in vitro and in vivo. Metabolism of chloral hydrate and its metabolites produced free radicals that resulted in lipid peroxidation in liver microsomes of mice, rats, and humans. Induction of cytochrome P(450)2E1 by pyrazole increased the concentrations of lipid peroxidation products; inhibition of cytochrome P(450)2E1 by 2,4-dinitrophenylhydrazine reduced these concentrations. Metabolism of chloral hydrate and its metabolites by mouse, rat, and human liver microsomes formed malondialdehyde, and in the presence of calf thymus DNA formed the DNA adduct MDA-MG-1.