Disposition of inhaled isoprene in B6C3F1 mice

Prediction of isoprene diepoxide levels in vivo in mouse, rat and man using enzyme kinetic data in vitro and physiologically-based pharmacokinetic modelling

The present study was designed to explain the differences in isoprene toxicity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the human situation was attempted. For this purpose, enzyme kinetic parameters K(m) and V(max) were determined in vitro in mouse, rat and human liver microsomes/cytosol for the cytochrome P450-mediated formation of isoprene mono- and diepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diepoxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences regarding isoprene diepoxide levels were forecasted. Almost similar isoprene diepoxide liver and lung concentrations were predicted in mouse and rat, while predicted levels in humans were about 20-fold lower. However, when interindividual variation in enzyme activity was introduced in the human model, the levels of isoprene diepoxide changed considerably. It was forecasted that in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentrations in the liver increased to similar concentrations as predicted for the mouse. However, the interpretation of the latter finding for human risk assessment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxide concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining the extent of isoprene toxicity. In order to confirm this, in vivo experiments are required in which isoprene epoxide concentrations will be established in rats and mice.

Metabolism and molecular toxicology of isoprene

Isoprene (2-methylbuta-1,3-diene) is a large-scale petrochemical used principally in the manufacture of synthetic rubbers. It is also produced by plants and trees and is the major endogenous hydrocarbon formed by mammals, probably from mevalonic acid. Isoprene is metabolised by mammals in processes that involve epoxidation by cytochrome P450-dependent monooxygenases to the isomeric mono-epoxides, (1-methylethenyl)-oxirane and 2-ethenyl-2-methyloxirane. Further metabolism of the mono-epoxides to mutagenic isoprene di-epoxides, (2, 2')-2-methylbioxiranes, can also occur. The oxidations to the mono- and di-epoxides occur enantioselectively and diastereoselectively. The mono-epoxides are hydrolysed enantioselectively to vicinal diols under catalysis by epoxide hydrolase. 2-Ethenyl-2-methyloxirane is also readily hydrolysed non-enzymatically. Because of the stereochemical possibilities for metabolites, the metabolism of isoprene is complex. The metabolism of isoprene by liver microsomes in vitro from a range of species including rat, mouse and human shows significant differences between species, strains and gender in respect of the diastereoselectivity and enantioselectivity of the metabolic oxidation and hydrolysis reactions. The impact of the extra methyl in isoprene on di-epoxide reactivity also appears to be critically important for the resulting biological effects. Isoprene di-epoxides may exhibit a lower cross-linking potential in vivo compared to butadiene di-epoxides. Differences in metabolism and reactivity of metabolites may be factors contributing to the significant differences in toxicological response to isoprene observed between species.

Toxicokinetics of inhaled and endogenous isoprene in mice, rats, and humans

Isoprene (IP) is ubiquitous in the environment and is used for the production of polymers. It is metabolized in vivo to reactive epoxides, which might cause the tumors observed in IP exposed rodents. Detailed knowledge of the body and tissue burden of inhaled IP and its intermediate epoxides can be gained using a physiological toxicokinetic (PT) model. For this purpose, a PT-model was developed for IP in mouse, rat, and human. Experimentally determined partition coefficients were taken from the literature. Metabolic parameters were obtained from gas-uptake experiments. The measured data could be described by introducing hepatic and extrahepatic metabolism into the model. At exposure concentrations up to 50 ppm, the rate of metabolism at steady-state is 14 times faster in mice and about 8 times faster in rats than in humans (2.5 micromol/h/kg at 50 ppm IP in air). IP does accumulate only barely due to its fast metabolism and its low thermodynamic partition coefficient whole body:air. IP is produced endogenously. This production is negligible in rodents compared to that in humans (0.34 micromol/h/kg). About 90% of IP produced endogenously in humans is metabolized and 10% is exhaled unchanged. The blood concentration of IP in non-exposed humans is predicted to be 9.5 nmol/l. The area under the blood concentration-time curve (AUC) following exposure over 8 h to 10 ppm IP is about 4 times higher than the AUC resulting from the unavoidable endogenous IP over 24 h. A comparison of such AUCs can be used for establishing workplace exposure limits. For estimation of the absolute risk, knowledge of the body burden of the epoxide intermediates of IP is required. Unfortunately, such data are not yet available.

Species differences in the stereochemistry of the metabolism of isoprene in vitro

1. Comparative studies on the stereochemistry of the metabolism of isoprene in vitro have been carried out using liver microsomes from rats, mice, monkeys, dogs, rabbits and humans. Differences between strains and gender were also investigated. 2. In the production of the isoprene monoepoxides, microsomes from the livers of the male Sprague-Dawley or Wistar rat showed an approximately 2:1 preference for the formation of (S)-2-(1-methylethenyl)oxirane compared with the (R)-enantiomer. No enantioselectivity was observed for mouse or rabbit. In contrast, liver microsomes from dog, monkey or male human preferentially formed (R)-2-(1-methylethenyl)oxirane. There was no enantioselectivity observed with microsomes from female human liver. 3. The significant differences between species in the in vitro metabolism of isoprene indicate that stereochemical and mechanistic data should be taken into account when evaluating the results of animal studies designed to assess the carcinogenic risks to humans that may be associated with exposure to isoprene.

Metabolism of isoprene in vivo

Available data suggest that: (1) the greater uptake of isoprene in mice relative to rats may not contribute to differences in sensitivity because the blood levels of total metabolites are similar in the two species; (2) in rats, saturation of blood levels of isoprene monoxide and the hydrocarbon could not be demonstrated at inhaled isoprene concentrations up to 8200 ppm, but saturation of blood levels of diols/diepoxide and of non-volatile metabolites occurred at approximately 1480 ppm (similar data are not reported for mice); (3) reaction of reactive isoprene metabolites with blood components in mice and rats is similar to reaction of butadiene metabolites in rats and much greater than that of butadiene metabolites in mice. This may contribute to the greater sensitivity of mice to butadiene carcinogenicity relative to isoprene carcinogenicity; (4) reported data do not conclusively show that differences in tissue concentrations of isoprene metabolites contribute to the differences in the relative potencies of isoprene and butadiene in mice or to the greater carcinogenic potency of isoprene in mice relative to that in rats.

Toxicokinetics of isoprene in rodents and humans

A physiological toxicokinetic model (PT model) was developed for inhaled isoprene in mouse, rat and man. Partition coefficients blood:air and tissue:blood were determined in vitro by a headspace method. Parameters of a saturable isoprene metabolism in B6C3F1 mice, Sprague-Dawley rats and volunteers were obtained from gas uptake experiments in closed systems, analyzed by means of a two-compartment model. Incorporation of these parameters into the PT model revealed that isoprene was metabolized not only in the liver but also in extrahepatic organs. Endogenous production of isoprene in man was quantified from experiments with volunteers breathing into a closed system. The PT model was validated for mice, rats and humans by comparing simulated values with data determined by other authors.

Chronic inhalation oncogenicity study of isoprene in B6C3F1 mice

The oncogenic potential of isoprene as affected by concentration, length of daily exposure, and weeks of exposure over the life-span of the animal, as independent variables, was evaluated. Ten groups were exposed for 8 h/day, 5 days/week as follows (ppm-weeks): 0-80, 10-80, 70-40, 70-80, 140-40, 280-20, 280-80, 700-80, 2200-40, 2200-80. Two groups were exposed for 4 h/day: 2200-20, 2200-80. Groups were held until 96 or 105 weeks on study. The concentration x time (duration of exposure) values provided a series of theoretically equivalent exposure hazards. There was an exposure-related increased incidence of liver, lung, Harderian gland and forestomach tumors, and hemangiosarcomas and histiocytic sarcomas. The LOEL appeared to be 70 ppm. These results are similar to the profile of tumors seen in 1,3-butadiene (BD)-exposed mice without the early onset of T-cell lymphoma as seen with BD. Isoprene appears to be about one order of magnitude less potent than BD in mice. Statistical analyses indicated that the product of isoprene concentration, and length/duration of exposure was not a sufficient basis for predicting tumor risk at any site. Extrapolation of tumor probability between the high and low doses based on cumulative exposure was not appropriate and could not be justified by statistical models. A threshold effect level and strong nonlinearities with respect to concentration appeared to exist for tumor development in this study.