The pharmacokinetics of metabolism of inhalation anaesthetics. A simulation study

Industrial solvents and liver toxicity: risk assessment, risk factors and mechanisms

Organic solvents utilized in various industrial processes may be associated with hepatotoxicity. The hepatotoxicity of some of the solvents was recognized as early as 1887, 1889 and 1904. Factors contributing to the hepatotoxicity of solvents include 1) species differences, 2) liver blood flow, 3) protein binding, 4) point of binding intracellularly, 5) genetic factors, 6) different cellular enzymatic degradation, 7) age, 8) nutritional condition, 9) interaction with alcohol, and 10) interaction with medications of use and abuse. The hepatotoxicity of solvents in general and of carbon tetrachloride, trichloroethylene, tetrachloroethylene, toluene, and 1,1,1-trichloroethene are discussed. Experimental animal data, human data, and in vitro studies are explored. Suggested mechanisms of direct toxicity, indirect toxicity and autoimmune mechanisms are elaborated. The most important message from this review is that laboratory testing that is commonly used by clinicians to detect liver toxicity may not be sensitive enough to detect early liver hepatotoxicity from industrial solvents and new methodologies are being encouraged and utilized in the early recognition and diagnosis of hepatotoxicity for solvents. The final clinical assessment of hepatotoxicity and industrial solvents must take into account synergism with medications, drugs of use and abuse, alcohol, age, and nutrition. Early recognition and reporting will be helpful in further understanding the incidence, cofactors and possible mechanisms.

Consideration of the target organ toxicity of trichloroethylene in terms of metabolite toxicity and pharmacokinetics

Trichloroethylene (TRI) is readily absorbed into the body through the lungs and gastrointestinal mucosa. Exposure to TRI can occur from contamination of air, water, and food; and this contamination may be sufficient to produce adverse effects in the exposed populations. Elimination of TRI involves two major processes: pulmonary excretion of unchanged TRI and relatively rapid hepatic biotransformation to urinary metabolites. The principal site of metabolism of TRI is the liver, but the lung and possibly other tissues also metabolize TRI, and dichlorovinyl-cysteine (DCVC) is formed in the kidney. Humans appear to metabolize TRI extensively. Both rats and mice also have a considerable capacity to metabolize TRI, and the maximal capacities of the rat versus the mouse appear to be more closely related to relative body surface areas than to body weights. Metabolism is almost linearly related to dose at lower doses, becoming dose dependent at higher doses, and is probably best described overall by Michaelis-Menten kinetics. Major end metabolites are trichloroethanol (TCE), trichloroethanol-glucuronide, and trichloroacetic acid (TCA). Metabolism also produces several possibly reactive intermediate metabolites, including chloral, TRI-epoxide, dichlorovinyl-cysteine (DCVC), dichloroacetyl chloride, dichloroacetic acid (DCA), and chloroform, which is further metabolized to phosgene that may covalently bind extensively to cellular lipids and proteins, and, to a much lesser degree, to DNA. The toxicities associated with TRI exposure are considered to reside in its reactive metabolites. The mutagenic and carcinogenic potential of TRI is also generally thought to be due to reactive intermediate biotransformation products rather than the parent molecule itself, although the biological mechanisms by which specific TRI metabolites exert their toxic activity observed in experimental animals and, in some cases, humans are not known. The binding intensity of TRI metabolites is greater in the liver than in the kidney. Comparative studies of biotransformation of TRI in rats and mice failed to detect any major species or strain differences in metabolism. Quantitative differences in metabolism across species probably result from differences in metabolic rate and enterohepatic recirculation of metabolites. Aging rats have less capacity for microsomal metabolism, as reflected by covalent binding of TRI, than either adult or young rats. This is likely to be the same in other species, including humans. The experimental evidence is consistent with the metabolic pathways for TRI being qualitatively similar in mice, rats, and humans. The formation of the major metabolites--TCE, TCE-glucuronide, and TCA--may be explained by the production of chloral as an intermediate after the initial oxidation of TRI to TRI-epoxide.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake and clearance of inhalation anesthetics in man

The uptake, distribution, and clearance of inhaled vapors is governed by rules of partial pressure equilibration in a multicompartmental system. Since halogenated anesthetic agents are not soluble in water, biotransformation is their only clearance pathway during anesthesia. When apparent steady state is reached, the rate of overall metabolism can be determined from the pulmonary uptake rate. As a result of metabolism, pulmonary uptake increases but the concentration of inhaled vapor in blood and tissues decreases, and only a fraction of uptake is exhaled following anesthesia. Uptake and pulmonary clearance of five halogenated anesthetic agents were studied in 45 surgical patients. The susceptibility to biotransformation increases in the following order: isoflurane, enflurane, halothane, fluroxene, methoxyflurane.

Absorption, biotransformation, and storage of halothane

Current knowledge of the quantitative aspects of biotransformation of halothane and the fate of its metabolites are reviewed. Absorbed quantities of the inhalation anesthetic average 12.7 and 18 g during 1 and 2 hr, respectively, of anesthesia. Reported fractions of halothane recovered as urinary metabolites range from 10 to 25%. An analysis of reports of bromide ion accumulation in plasma during and following anesthesia suggests that metabolism of halothane continues for 20-40 hr after exposure and that 22-24% of absorbed halothane is metabolized following 8 hr of anesthesia. Half-times for excretion of trifluoroacetic acid (TFA), a principal urinary metabolite of halothane, tend to confirm that biotransformation proceeds for 2 to 3 days following exposure. Other urinary metabolites which occur in small amounts include a dehydrofluorinated metabolite of halothane conjugated with L-cysteine and N-trifluoroacetyl-n-ethanolamine, both of which are evidence of the occurrence of reactive intermediates during the metabolism of halothane. Support for free radical formation has come from in vivo and in vitro demonstrations of stimulation of lipoperoxidation of polyenoic fatty acids by halothane. Irreversible binding of halothane metabolites to microsomal proteins and phospholipids has been shown to depend on the microsomal P-450 cytochrome system. Irreversible binding is increased by microsomal enzyme induction and by anaerobic conditions. Hypoxia increases irreversible binding to phospholipids, augments the release of inorganic fluoride and is followed by centrilobular hepatic necrosis. It is concluded that one-fourth to one-half of halothane undergoes biotransformation in man. One fraction is excreted as trifluoroacetic acid, chloride and bromide. A second fraction is irreversibly bound to hepatic proteins and lipids. Under anaerobic conditions fluoride is released, binding to phospholipids is increased, and hepatic necrosis may occur.