A possible mechanism of toxicity of trifluoroethanol and other halothane metabolites

Effect of pyrazole, 4-methylpyrazole, 4-bromopyrazole and 4-iodopyrazole on brain noradrenaline levels of mice and rats

Four daily doses of pyrazole (50 mg/kg), caused a reduction in rat brain noradrenaline (NA) of over 20% when determined 24 hrs after the final injection. Neither 4-methylpyrazole (10-50 mg/kg), nor 4-iodopyrazole (10-50 mg/kg) had any effect. In mice treated similarly, pyrazole (50-400 mg/kg) caused a dose-dependent decrease in brain NA. Neither 4-methylpyrazole, 4-bromopyrazole nor 4-iodopyrazole caused any significant change in the levels. However if the brain NA levels were examined 6 hrs after a single dose, then in addition to pyrazole, 4-methylpyrazole showed a dose-dependent ability to lower brain NA. 4-bromopyrazole and 4-iodopyrazole, given acutely, caused a dose-dependent decrease in rectal temperature and exploratory behaviour. 4-methylpyrazole in high doses (200-400 mg/kg) showed similar properties but they did not correlate with the decrease in brain NA. Pyrazole, after acute treatment, showed little ability to change rectal temperature of exploratory behaviour. It is concluded that the NA-depleting effect of pyrazole is not related to inhibition of alcohol dehydrogenase, since other 4-substituted pyrazoles which are more potent inhibitors of the enzyme have little or no effect on brain NA levels.

19F nuclear magnetic resonance analysis of trifluoroethanol metabolites in the urine of the Sprague-Dawley rat

2,2,2-Trifluoroethanol (TFE) is a common industrial solvent and a known metabolite of the inhalation anesthetics fluroxene (2,2,2-trifluoroethyl vinyl ether) and halothane (2-bromo-2-chloro-1,1,1-trifluoroethane). The water-soluble metabolites of TFE were identified in the urine of Sprague-Dawley rats using 19F NMR spectroscopy. In rats dosed with 0.21 g TFE/kg body weight, approximately one-half of the administered TFE was excreted as the trifluoroethyl glucuronide. The remaining TFE was oxidized, primarily to trifluoroacetaldehyde hydrate, with a small percentage of the aldehyde oxidized further to trifluoroacetate. One additional fluorinated compound was found; after investigation, this was identified as a Schiff's base compound resulting from the addition of trifluoroacetaldehyde to urea. The time-dependent excretion of TFE metabolites was measured as a function of ethanol induction of hepatic enzymes. This study demonstrates the utility of 19F NMR for the analysis of drug metabolism in laboratory animals. In addition, the resistance of trifluoroacetaldehyde hydrate to further oxidation, coupled with its reactivity with common cellular amines, indicates the potential toxicity of this metabolite to mammalian tissues.

Effects of trifluoroacetic acid, a halothane metabolite, on C6 glioma cells

Effects of trifluoroacetic acid (TFA) on cell growth, DNA, glycoprotein, and dolichol-linked oligosaccharides synthesis and ribonucleotide triphosphate concentrations were examined in exponentially growing C6 murine glioma cells. One day of treatment with TFA caused a slight concentration-dependent enhancement of cell growth and [3H]thymidine incorporation. Exposure for 1 or 5 d to TFA (0.5-7.0 mM) elevated the [3H]leucine incorporation in a dose- and time-dependent manner. The results suggested that TFA stimulated cell growth and enhanced protein synthesis. TFA also affected [3H]mannose incorporation into glycoproteins and dolichol-linked oligosaccharides in a dose-dependent fashion. In addition, it was found that TFA accelerated lectin-induced cell agglutination. These data suggest that TFA, the principle halothane metabolite, alters plasmalemmal glycoprotein synthesis. These findings should form a basis for further understanding on the mechanism underlying halothane-associated neurotoxicity.

2,2,2-Trifluoroethanol intestinal and bone marrow toxicity: the role of its metabolism to 2,2,2-trifluoroacetaldehyde and trifluoroacetic acid

2,2,2-Trifluoroethanol (TFE) produces bone marrow and small intestine toxicity resulting in leukopenia, loss of intestinal dry weight, and consequent lethal septicemia in male Wistar rats. Its metabolic pathway, based on serum and small intestine time courses of substrate and metabolites, was determined to be TFE in equilibrium 2,2,2-trifluoroacetaldehyde (TFAld)----trifluoroacetic acid (TFAA). Administered TFE and TFAld were not toxic per se, since their toxicity and metabolism were inhibited by pyrazole. TFE and TFAld were equipotent at equimolar doses thus precluding the oxidative reaction, TFE to TFAld, from being the toxic step. Since equimolar TFAA exhibited no toxic effects, an oxidative intermediate on the pathway from TFAld to TFAA, most likely F3C-C+(OH)2, must thus be the toxic moiety. The intermediate TFAld is stable in serum, as determined by a novel assay developed for its analysis in biological systems, and can be transported to the target tissues, bone marrow, and small intestine, after formation probably in the liver. On the basis of the more rapid metabolism of TFE to higher levels of TFAld in the small intestine and bone marrow than in the serum, the closer correspondence of bone marrow and small intestine metabolite ratios than serum ratios at high and low doses of TFE to the corresponding ratios of toxicity, and the decreased toxicity of TFAld when administered ig versus ip, the formation of the toxic metabolic intermediate of TFE probably occurs in the target tissues.

Metabolism of 2,2,2-trifluoroethanol and its relationship to toxicity

2,2,2-Trifluoroethanol (TFE), the toxic metabolite of the anesthetic agent fluoroxene, is further metabolized to trifluoroacetic acid, which accumulated to maximum serum concentrations 16 to 24 hr after TFE administration to rats. To determine how the metabolic pathways of TFE are related to its toxicity, male Wistar rats were pretreated with various metabolic inhibitors and inducers of cytochrome P-450 metabolism, and TFE toxicity and metabolism were assessed. Pyrazole, disulfiram, isoniazid, diethyldithiocarbamate, and 2-allyl-2-isopropyl-acetamide pretreatment significantly decreased the in vivo metabolism of TFE by 53-100% and decreased the toxicity (as assessed by mortality and leukocyte count alterations). Ethanol induction of rats increased metabolism of TFE by 65% 24 hr after TFE administration but not the toxicity. We conclude that hepatic ethanol-inducible cytochrome P-450 catalyzes the metabolism of most of the TFE but this metabolism is not associated with TFE toxicity, which probably arises by extrahepatic metabolism of TFE by another cytochrome P-450 form.

Mechanism of toxicity of 2,2,2-trifluoroethanol in rats

2,2,2-Trifluoroethanol (TFE) is the toxic metabolite of the anesthetic agent fluroxene. TFE treatment (0.21 g/kg, ip) of male Wistar rats significantly reduced peripheral white blood cell count, bone marrow nucleated cellularity, and dry weight of the small intestine. These toxic effects of TFE were first observed at 8 to 16 hr after treatment, persisted for 96 hr, and were accompanied by severe diarrhea and edema of the small intestine. A non-lethal dose of TFE increased the sensitivity of rats to bacterial endotoxin lethality by approximately 1000-fold. Antibiotic and antiendotoxin pretreatment reduced the lethality of TFE from 80 to 20% of the rats, but did not prevent the other toxic effects of TFE. In vitro experiments with serum from TFE-pretreated rats (0.13 g/kg) supported the growth of an average of 65% fewer cultured bone marrow cell colonies compared to the number of colonies produced when serum from control rats was used. This suggests that TFE-induced bone marrow depression and leukopenia are related to a decrease in colony stimulating factor activity. Taken together these results explain the rapid development of lethal bacterial infections in TFE-treated rats. TFE-mediated damage to the small intestine combined with prolonged leukopenia decreases the resistance of the rat to endogenous pathogens leading to systemic bacterial infection. In addition, the increased sensitivity to endotoxin induced by TFE leads to lethal endotoxemia.

A comparative study on the irreversible binding of labeled halothane trichlorofluoromethane, chloroform, and carbon tetrachloride to hepatic protein and lipids in vitro and in vivo

1) After intraperitoneal injection of labeled CCl4, CHCl3, and halothane in mice, 14C is preferentially bound to liver endoplasmic protein and lipid. A considerable activity is also associated with mitochondrial constituents. Maximal protein binding (nmol/mg): CCl4: 2.8 (0.5 hrs); CHCl3: 11.5 (6 hrs); halothane: 5 (6 hrs). Lipid binding: CCl4: 6.4 (5 min); CHCl3: 8 (4 hrs); halothane: 13.5 (2 hrs). The form of the binding curves in microsomal and mitochondrial protein and lipid differed with the individual haloalkanes. 2) The irreversible (covalent) binding of 14C from labeled haloalkanes in anaerobic suspensions of isolated rabbit liver microsomes and NADPH after 30 min was for protein (lipid) (nmol/mg): CCl4: 15 (58); CHCl3: 3.4 (3.2); halothane: 2.3 (10); trichlorofluoromethane: 6.5 (30). Anerobic incubation favored dehalogenation, but CHCl3 metabolism and irreversible binding requires oxygen. The greatest differences in the in vitro "covalent" binding rates were observed with CHCl3 in rat, mouse, and rabbit. 3) Altered microsomal cytochrome P-450 concentrations in newborn animals, or produced by pretreatment of rats with phenobarbital, 3-methylcholanthrene (MC), or CoCl2 effected similar, but not proportional changes in the rates of irreversible protein and lipid binding. Upon addition of CCl4 the difference of light absorption of reduced liver microsomes from MC-pretreated rats containing cytochrome P-448 appeared at 452 nm. The irreversible binding rate in these microsomes was also increased. The small accleration in irreversible binding in liver microsomes from rats pretreated with isopropanol is not proportional to the high increase of CCl4 toxicity. 4) Practically no binding to added, soluble albumin or RNA was observed in microsomal incubates. However, 14C is bound to the nicotine-adenine dinucleotides of the NADPH system. All haloalkanes produced a similar increase of NADPH oxidation in incubates of rabbit liver microsomes and NADPH.