Inhalation pharmacokinetics based on gas uptake studies. I. Improvement of kinetic models

Pharmacokinetic analysis of the chronic administration of the inert gases Xe and Ar using a physiological based model

Background

New gas therapies using inert gases such as xenon and argon are being studied, which would require chronically administered repeating doses. The pharmacokinetics of this type of administration has not been addressed in the literature.

Methods

A physiologically based pharmacokinetics (PBPK) model for humans, pigs, mice, and rats has been developed to investigate the unique aspects of the chronic administration of inert gas therapies. The absorption, distribution, metabolism and excretion (ADME) models are as follows: absorption in all compartments is assumed to be perfusion limited, no metabolism of the gases occurs, and excretion is only the reverse process of absorption through the lungs and exhaled.

Results

The model has shown that there can be a residual dose, equivalent to constant administration, for chronic repeated dosing of xenon in humans. However, this is not necessarily the case for small animals used in pre-clinical studies.

Conclusions

The use of standard pharmacokinetics parameters such as area under the curve would be more appropriate to assess the delivered dose of chronic gas administration than the gas concentration in the delivery system that is typically reported in the scientific literature because species and gas differences can result in very different delivered doses.

Trichloroacetic acid in urine as biological exposure equivalent for low exposure concentrations of trichloroethene

A urinary trichloroacetic acid (TCA) concentration of 100 mg/l at the end of the last work shift (8 h/day, 5 days/week) of the week has been established in workers as exposure equivalent for the carcinogenic substance trichloroethene (EKA for TRI) at an exposure concentration of 50 ppm TRI. Due to the continuous reduction of atmospheric TRI concentrations during the last years, the quantitative relation given by the EKA for TRI is revised for exposures to low TRI concentrations. A physiological two-compartment model is presented by which the urinary TCA concentrations are calculated that result from inhaled TRI in humans. The model contains one compartment for trichloroethanol (TCE) and one for TCA. Inhaled TRI is metabolized to TCA and to TCE. The latter is in part further oxidized to TCA. Urinary elimination of TCA is modeled to obey first order kinetics. All required model parameters were taken form the literature. In order to evaluate the model performance on the urinary TCA excretion at low exposure concentrations, predicted urinary TCA concentrations were compared with data obtained in two volunteer studies and in one field study. The model was evaluated at exposure concentrations as low as 12.5 ppm TRI. It is demonstrated that the correlation described by the hitherto used EKA for TRI is also valid at low TRI concentrations. For TRI exposure concentrations of 0.6 and 6 ppm, the resulting urinary TCA concentrations at the end of the last work shift of a week are predicted to be 1.2 and 12 mg/l, respectively.

Evaluation of physiologically based pharmacokinetic models for use in risk assessment

Physiologically based pharmacokinetic (PBPK) models are sophisticated dosimetry models that offer great flexibility in modeling exposure scenarios for which there are limited data. This is particularly of relevance to assessing human exposure to environmental toxicants, which often requires a number of extrapolations across species, route, or dose levels. The continued development of PBPK models ensures that regulatory agencies will increasingly experience the need to evaluate available models for their application in risk assessment. To date, there are few published criteria or well-defined standards for evaluating these models. Herein, important considerations for evaluating such models are described. The evaluation of PBPK models intended for risk assessment applications should include a consideration of: model purpose, model structure, mathematical representation, parameter estimation, computer implementation, predictive capacity and statistical analyses. Model purpose and structure require qualitative checks on the biological plausibility of a model. Mathematical representation, parameter estimation, computer implementation involve an assessment of the coding of the model, as well as the selection and justification of the physical, physicochemical and biochemical parameters chosen to represent a biological organism. Finally, the predictive capacity and sensitivity, variability and uncertainty of the model are analysed so that the applicability of a model for risk assessment can be determined. Published in 2007 by John Wiley & Sons, Ltd.

The "Tuebingen desiccator" system, a tool to study oxidative stress in vivo and inhalation toxicokinetics

The "Tuebingen desiccator," a gas-tight all-glass closed chamber system (CCS), has been established in Herbert Remmer's Institute of Toxicology, University of Tuebingen, to investigate the mechanisms underlying the exhalation of endogenous volatile hydrocarbons in rats under oxidative stress. Remmer and associates confirmed the former view that ethane and n-pentane were derived from polyunsaturated fatty acids, and they demonstrated that propane, n-butane and isobutane were released from amino acids. Hydrocarbons exhaled following acute ethanol treatment of rats resulted predominantly from ethanol-dependent inhibition of their metabolism and partly from oxidation of proteins. Exhalation of alkanes in carbon tetrachloride exposed rats did not reflect liver damage, which was, however, directly linked to the amount of carbon tetrachloride metabolized. As has first been shown in Herbert Remmer's institute by investigating the fate of inhaled vinyl chloride in rats, the CSS proved to be also an excellent tool for studying toxicokinetics of inhaled gaseous xenobiotics by means of gas uptake experiments. Based on results gained by such studies, it was recently demonstrated that knowledge of compound-specific physicochemical and species-specific physiological parameters are often sufficient to predict important toxicokinetic properties of inhaled chemicals such as tissue burdens at steady state. By means of the CCS, not only kinetics of a parent gaseous substance but also of gaseous metabolites can be investigated in vivo, as exemplified for ethylene oxide and 1, 2-epoxy-3-butene, metabolites of ethylene and 1,3-butadiene, respectively. Gas uptake studies in closed chamber systems are now worldwide used for determining toxicokinetic parameters relevant for physiological toxicokinetic modeling.

Partition coefficients for the trihalomethanes among blood, urine, water, milk and air

Chloroform, bromodichloromethane, chlorodibromomethane, and bromoform comprise the trihalomethanes, a group of widespread and mildly lipophilic compounds that result from water chlorination and other sources. Many animal studies show the chronic toxicity and carcinogenicity of these compounds, and recent work has demonstrated the importance of both ingestion and inhalation exposure pathways. This study presents partition coefficients describing the equilibrium among biological compartments (air, water, blood, milk, urine) for the four THMs based on results of headspace gas chromatographic analyses performed under equilibrium conditions and at 37 degrees C. The calculated partition coefficients ranged from 2.92 to 4.14 for blood/water, 1.54-2.85 for milk/blood, and 3.41-4.93 for blood/urine, with the lowest being chloroform and the highest being bromoform. Both human and cow milk were tested, with similar results. The available samples of human milk may not fully account for differences in lipid content and possibly other factors that affect estimates of partition coefficients. Simultaneous measurements of milk and blood in exposed individuals are suggested to confirm laboratory results. Partition coefficients are predicted using the octanol-air partition coefficient, also measured in this study, and the octanol-water partition coefficient. Results are similar to literature estimates for liquid/air partitioning of chloroform and chlorodibromomethane, but they differ from predictions based on hydrophobicity and lipid content. High correlations between the derived partitioned coefficients and the molecular structure (number of Br atoms) and physical properties (molecular weight and boiling point) are found for these analogous chemicals. In humans, THMs are both stored and metabolized with relatively rapid clearance rates. The derived partition coefficients can help to interpret results of biological monitoring and predict the potential for the accumulation and transfer of chemicals, specifically by the application of physiologically-based pharmacokinetic models. THM exposures to potentially susceptible populations, e.g. nursing infants, can be predicted using either such models.

Kinetics of propylene oxide metabolism in microsomes and cytosol of different organs from mouse, rat, and humans

Kinetics of the metabolic inactivation of 1,2-epoxypropane (propylene oxide; PO) catalyzed by glutathione S-transferase (GST) and by epoxide hydrolase (EH) were investigated at 37 degrees C in cytosol and microsomes of liver and lung of B6C3F1 mice, F344 rats, and humans and of respiratory and olfactory nasal mucosa of F344 rats. In all of these tissues, GST and EH activities were detected. GST activity for PO was found in cytosolic fractions exclusively. EH activity for PO could be determined only in microsomes, with the exception of human livers where some cytosolic activity also occurred, representing 1-3% of the corresponding GST activity. For GST, the ratio of the maximum metabolic rate (V(max)) to the apparent Michaelis constant (K(m)) could be quantified for all tissues. In liver and lung, these ratios ranged from 12 (human liver) to 106 microl/min/mg protein (mouse lung). Corresponding values for EH ranged from 4.4 (mouse liver) to 46 (human lung). The lowest V(max) value for EH was found in mouse lung (7.1 nmol/min/mg protein); the highest was found in human liver (80 nmol/min/mg protein). K(m) values for EH-mediated PO hydrolysis in liver and lung ranged from 0.83 (human lung) to 3.7 mmol/L (mouse liver). With respect to liver and lung, the highest V(max)/K(m) ratios were obtained for GST in mouse and for EH in human tissues. GST activities were higher in lung than in liver of mouse and human and were alike in both rat tissues. Species-specific EH activities in lung were similar to those in liver. In rat nasal mucosa, GST and EH activities were much higher than in rat liver.

A new technology to measure skin absorption of vapors

Skin vapor absorption is one of the major exposure routes for some widely used chemicals (e.g., 2-methoxy ethanol), but a good apparatus with which exposure can be measured is currently unavailable. In this study, a polished stainless-steel chamber-combined with computer-controlled auto-feedback software and hardware, real-time gas sensors, and an auto-injection microsyringe-was proposed as new technology. In addition, the machines had activated-charcoal tubes and cold traps, both of which simulated the skin uptake and validated the reliability of the proposed system. The exposure concentrations, relative humidity, and temperature were effectively controlled at 25+/-0.5 ppm (or 300+/-10 ppm), 80+/-2%, and 27.5+/-0.5 degrees C, respectively. The relative errors between the quantity of 2-methoxy ethanol collected in either the charcoal tubes or the cold traps and the quantity of ME injected to maintain a constant exposure were less than 5%. The authors also used this new technology to successfully measure skin absorption of ME vapor in 6 volunteers. The authors concluded that this new technology is a direct, continuous, noninvasive, and simple tool with which to measure skin absorption of vapors.

Pharmacokinetics and metabolism of vinyl fluoride in vivo and in vitro

Vinyl fluoride (VF) is an inhalation carcinogen at concentrations of 25 ppm or greater in rats and mice. The main neoplastic lesion induced in rodents was hepatic hemangiosarcomas, and mice were more sensitive than rats. In a first set of experiments, groups of three rats or five mice were exposed to VF in a closed-chamber gas uptake system at starting concentrations ranging from 50 to 250 ppm. Chamber concentrations of VF were measured every 10-12 min by gas chromatography. Partition coefficients were determined by the vial equilibration technique and used as parameters for a physiologically based pharmacokinetic (PBPK) model. Mice showed a higher whole-body metabolic capacity compared to rats (Vmax = 0.3 vs 0.1 mg/hr-kg). Both species had an estimated Km of < or = 0.02 mg/liter. The specificity for the oxidation of VF in vivo was determined by selective inhibition or induction of CYP 2E1. Inhibition with 4-methylpyrazole completely impaired VF uptake in rats and mice, whereas induction with ethanol (rats only) increased the metabolic capacity by two- to threefold. The pharmacokinetics of VF were also investigated in vitro. Microsomes from rat and mouse liver were incubated in a sealed vial with VF and an NADPH-regenerating system. Headspace concentrations (10-300 ppm) were monitored over time by gas chromatography. Consistent with the in vivo data, VF was metabolized faster by mouse microsomes than by rat microsomes (Vmax = 3.5 and 1.1 nmol/hr-mg protein, respectively). The rates of metabolism by human liver microsomes were generally in the same range as those found with rat liver microsomes (Vmax = 0.5-1.3 nmol/hr-mg protein), but one sample was similar to mice (Vmax = 3.3 nmol/ hr-mg protein). Metabolic rates in human microsomes were found to correlate with the amount of CYP 2E1 as determined by Western blotting and by chlorzoxazone 6-hydroxylation. It is concluded that the greater metabolic capacity of mice for VF both in vivo and in vitro may contribute to their greater susceptibility to tumor formation. CYP 2E1 is clearly the main isozyme involved in the oxidation of VF in all species tested. VF pharmacokinetics and metabolism in humans may depend upon the interindividual variability in the expression level of CYP 2E1. The excellent correspondence between in vivo and in vitro kinetics in rodents improves. substantially the degree of confidence for human in vivo predictions from in vitro data.

Pharmacokinetic modeling approaches for describing the uptake, systemic distribution, and disposition of inhaled chemicals

A fundamental relationship in toxicology is that an external chemical exposure leading to an internal tissue dose can result in an adverse biological response. An understanding of these relationships in experimental animals is often used to extrapolate and predict the potential risk to humans following exposure to toxic chemicals. The exposure-dose-response relationships for volatile compounds inhaled by the lungs are complicated by the fact that many toxic effects caused by these chemicals have been identified in tissues and organ systems other than the lungs. Pharmacokinetic modeling approaches have been devised to quantitate the relationships between inhaled concentrations of volatile compounds and the resulting critical tissue doses in experimental animals. These animal models have also been extrapolated to predict chemical disposition in humans for estimation of human health risks. This communication reviews three pharmacokinetic descriptions, each representing different levels of complexity, that have been used to assess chemical disposition of inhaled, volatile chemicals. The mathematical structures, assumptions, data needs, and risk assessment capabilities of each modeling approach are described.

Metabolic transformation of halogenated and other alkenes--a theoretical approach. Estimation of metabolic reactivities for in vivo conditions

Olefinic hydrocarbons are metabolized in vivo by cytochrome P450-dependent monooxygenases to the corresponding epoxides. The maximum in vivo metabolic rate, which is an important toxicokinetic parameter, has been used to define the apparent rate constant (kapp) describing in vivo metabolic reactivity of alkenes. To derive kapp, the metabolic rate normalized per body weight was divided by the corresponding average alkene concentration in the body at saturation conditions of 90%. Toxicokinetic data obtained in rats for 13 compounds (ethene, 1-fluoroethene, 1,1-difluoroethene, 1-chloroethene, 1,1-dichloroethene, cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1,2-trichloroethene, perchloroethene, propene, isoprene, 1,3-butadiene and styrene) have been used to calculate kapp values. A theoretical model, based on the assumption that in vivo epoxidation can be described as a cytochrome P450-mediated electrophilic reaction, has been developed. Using the olefinic hydrocarbons as an example it has been shown that kapp can be explained solely by the following molecular parameters: ionization potential, dipole moment and pi-electron density. These molecular parameters were calculated by a quantum chemical method or were taken from the literature. Furthermore, the model was tested also by predicting kapp for isobutene, an alkene which was not used for the model development. The predicted value of kapp agrees with the one derived experimentally, demonstrating that molecular parameters of halogenated and other alkenes can be used to predict in vivo metabolic reactivity. The model presented here is a first contribution to the ultimate goal to predict toxicokinetic parameters for in vivo conditions based on physicochemical parameters of enzymes and compounds exclusively.

A descriptive and mechanistic study of the interaction between toluene and xylene in humans

This study was undertaken to characterize the mechanism of toxicokinetic interaction between toluene (TOL) and m-xylene (XYL) in the rat using physiologically-based toxicokinetic (PBTK) modeling approach. First, the metabolic rate constants were determined by conducting closed-chamber inhalation exposures with individual solvents (Vmax: TOL = 4.8, XYL = 8.4 mg/hr/kg; Km: TOL = 0.55, XYL = 0.2 mg/l). Then, using the same experimental set-up, rats were exposed to different binary mixtures of TOL and XYL. PBTK analysis of the data showed competitive inhibition as the plausible mechanism of TOL/XYL interaction. This mechanistic modeling study suggests that the interaction between TOL and XYL is likely to be observed when the exposure concentration exceeds 50 ppm of each solvent.

The closed chamber technique--uptake, endogenous production, excretion, steady-state kinetics and rates of metabolism of gases and vapors

The "closed chamber technique" (CCT) is presented. It allows investigation of pharmacokinetics of volatile substances in vivo in animals and in man and in vitro using tissue fractions. During the exposure period only the atmospheric concentrations of the substance are measured. The concentration-time data obtained are pharmacokinetically analyzed by a two compartment model describing uptake, endogenous production and excretion of the unchanged substance and its metabolic elimination. Using this model, pharmacokinetics of ethylene have been determined in rats and man. For both species, the results compared well with an estimation based on an allometric species scaling. Furthermore, the applicability of CCT is demonstrated in vivo on several other gases and vapors of solvents, e.g. trichloroethylene and 1,1,1-trichloroethane, and in vitro on 1,2-epoxybutene-3.

Inhalation exposure technology, dosimetry, and regulatory issues

Inhalation toxicology technology has provided the scientific community with important advances in studies of inhaled toxicants. These advances include new and more efficient exposure systems (e.g., flow-past nose-only exposure systems), and improved approaches to inhalation chamber environmental control (e.g., temperature, humidity, air quality). Practical problems and approaches to testing and operating inhalation exposure systems and the advantages and disadvantages of the major inhalation exposure types (e.g., whole-body, nose-only) are discussed. Important aspects of study design, such as high level particulate exposures resulting in large lung burdens (e.g., greater than or equal to 2 mg/g of lung), slowed pulmonary clearance rates, and nonspecific toxicity are considered, along with practical issues of comparative dosimetry. Regulatory guidelines have continued to present challenges in designing and conducting acute, subchronic, and chronic inhalation studies. The important regulatory issue of performing acute inhalation toxicity studies at high aerosol concentrations and "respirable" particle size distribution is discussed.

Investigation of species differences in isobutene (2-methylpropene) metabolism between mice and rats

Metabolism of isobutene (2-methylpropene) in rats (Sprague Dawley) and mice (B6C3F1) follows kinetics according to Michaelis-Menten. The maximal metabolic elimination rates are 340 mumol/kg/h for rats and 560 mumol/kg/h for mice. The atmospheric concentration at which Vmax/2 is reached is 1200 ppm for rats and 1800 ppm for mice. At steady state, below atmospheric concentrations of about 500 ppm the rate of metabolism of isobutene is direct proportional to its concentration. 1,1-Dimethyloxirane is formed as a primary reactive intermediate during metabolism of isobutene in rats and can be detected in the exhaled air of the animals. Under conditions of saturation of isobutene metabolism the concentration of 1,1-dimethyloxirane in the atmosphere of a closed exposure system is only about 1/15 of that observed for ethene oxide and about 1/100 of that observed for 1,2-epoxy-3-butene as intermediates in the metabolism of ethene or 1,3-butadiene.

Use of linear free energy relationships in toxicology: prediction of partition coefficients of volatile lipophilic compounds

A relationship between the partition coefficient (P) and the boiling point (TBp, expressed in degrees Kelvin) was derived as TBp = alpha *ln(P) + beta. The relationship is based on the Clausius-Clapeyron equation and is valid for volatile lipophilic compounds, when one of the phases is air. Specific tissue/air and whole animal/air partition coefficients, as published in the literature, were used to validate the equation.

Toxicology of fluorine-containing monomers

Fluorine-containing monomers form the basis for production of a large number of commercially important polymers. Most of the polymerization occurs as gas-phase reactions, hence the hazards associated with the monomers arises primarily from inhalation. The chemicals covered in this review include bromotrifluoroethylene (BTFE), chlorotrifluoroethylene (CTFE), hexafluoroacetone (HFA), hexafluoroisobutylene (HFIB), hexafluoropropylene (HFP), perfluorobutylene (PFBE), tetrafluoroethylene (TFE), trichloropropene (TFP), vinyl fluoride (VF), and vinylidene fluoride (VF2). The amount of toxicologic information available on the compounds is relatively small and for certain of these the information consists is short-term or acute, hence the current need to make predictions of biologic activity based on analogy or chemical reactivity is great. In animal models and in man, these monomers may be absorbed into the body at varying rates and the metabolism ranges from extensive to little in a species, dose, and chemical specific fashion. The major toxicologic target of these materials is the kidney, and the degree of involvement depends greatly on the excretion patterns and metabolic profiles of the monomers. However, other target sites exist, such as the reproductive system for HFA, making the use of structure-activity relationships difficult.

Incorporation of biological information in cancer risk assessment: example--vinyl chloride

Vinyl chloride (VC) is used as an example to demonstrate how biological information can be incorporated into quantitative risk assessment. The information included is the pharmacokinetics of VC in animals and humans and the data-generated hypothesis that VC primarily affects the initiation stage of the multistage carcinogenesis. The emphasis in this paper is on the improvement of risk assessment methodology rather than the risk assessment of VC per se. Sufficient data are available to construct physiologically-based pharmacokinetic models for both animals and humans. These models are used to calculate the metabolized dose corresponding to exposure scenarios in animals and in humans. On the basis of the data on liver angiosarcomas and carcinomas in rats, the cancer risk per unit of metabolized dose is comparable, irrespective of routes (oral or inhalation) of exposure. The tumor response from an intermittent/partial lifetime exposure is shown to be consistent with that from a lifetime exposure when VC is assumed to affect the first (initiation) stage of the multistage carcinogenic process. Furthermore, the risk estimates calculated on the basis of animal data are shown to be consistent with the human experience.

Lipid peroxidation as a possible cause of ochratoxin A toxicity

Addition of the mycotoxin ochratoxin A (OA), a nephrotoxic carcinogen, to rat liver microsomes greatly enhanced the rate of NADPH or ascorbate-dependent lipid peroxidation as measured by malondialdehyde formation. NADPH-dependent lipid peroxidation in kidney microsomes was similarly enhanced by OA. The process required the presence of trace amounts of iron but cytochrome P-450 and free active oxygen species appeared not to be involved. The efficiency of several ochratoxins (ochratoxins A, B, C, alpha and O-methyl-ochratoxin C) to enhance lipid peroxidation was related to the presence and reactivity of the phenolic hydroxyl group. Furthermore, the ability of these ochratoxins to enhance lipid peroxidation in microsomes correlated precisely with their known toxicities in chicks. Administration of ochratoxin A to rats also resulted in enhanced lipid peroxidation in vivo as evidenced by a seven-fold increase in the rate of ethane exhalation. These results suggest that lipid peroxidation may play a role in the observed toxicity of ochratoxin A in animals; a mechanism is proposed. (Formula: see text). Ochratoxin A: X = Cl; R1 = R2 = R3 = R4 = H Ochratoxin B: X = H; R1 = R2 = R3 = R4 = H Ochratoxin C: X = Cl; R1 = R2 = R3 = H; = R4 = CH3 O-Methyl-ochratoxin C: X = Cl; R2 = R3 = H; R1 = R4 = CH3 (4R)-4-hydroxyochratoxin A: X = Cl; R1 = R3 = R4 = H; R2 = OH (4S)-4-hydroxyochratoxin A: X = Cl; R1 = R2 = R4 = H; R3 = OH Fig. 1. Chemical structures of the various ochratoxins.

Long-term effects of commercial and congeneric polychlorinated biphenyls on ethane production and malondialdehyde levels, indicators of in vivo lipid peroxidation

Ethane exhalation was increased in male Sprague-Dawley rats following a single intraperitoneal (IP) injection of Aroclor 1254 (500 mg/kg). In the first 2 weeks following Aroclor 1254 treatment, the increase in ethane exhalation was due to an inhibition of metabolism of endogenous ethane rather than to an increase in ethane production. In weeks 3 and 4 following Aroclor 1254 administration, metabolic clearance of ethane returned to and exceeded control levels, while ethane production increased to approximately twice the control rates (day 30). The HPLC determination of in situ hepatic malondialdehyde levels revealed a 2-fold increase in malondialdehyde content on day 30 following the Aroclor 1254 injection. Further, parallel increases in in situ malondialdehyde levels and ethane production rates were also found 30 days following a single IP injection of 3,3',4,4'-tetrachlorobiphenyl, 2,3,4,4',5-pentachlorobiphenyl and 2,2',4,4',5,5'-hexachlorobiphenyl (300 mumol/kg). These effects were not reflected in increased diene conjugation. Redox state of the liver was largely unaffected, as evidenced by the relative concentrations of reduced and oxidized NADPH. However, minor changes in reduced and oxidized glutathione were noted.

Species differences in butadiene metabolism between mouse and rat

Studies on inhalation pharmacokinetics of 1,3-butadiene were conducted in mice (B6C3F1) and rats (Sprague-Dawley) to investigate the considerable differences in the susceptibility of both species to butadiene-induced carcinogenesis. In rats and mice metabolism of 1,3-butadiene to 1,2-epoxybutene-3 follows saturation kinetics. "Linear" (first-order) pharmacokinetics apply at exposure concentrations below 1000 ppm 1,3-butadiene. Saturation of butadiene metabolism is observed at atmospheric concentrations of about 2000 ppm butadiene. In the lower concentration range where first-order metabolism applies, metabolic clearance of inhaled 1,3-butadiene per kg body weight was 7300 ml (gas volume) x hr-1 for mice and 4500 ml x hr-1 for rats. The calculated maximal metabolic elimination rates (Vmax - conditions) were 400 mumol x hr-1 x kg-1 for mice and 220 mumol x hr-1 x kg-1 for rats. This shows that 1,3-butadiene is metabolized by mice at about twice the rate of rats, under conditions of both low and high exposure concentrations.

Inhalation pharmacokinetics of 1,2-epoxybutene-3 reveal species differences between rats and mice sensitive to butadiene-induced carcinogenesis

Comparative investigations of inhalation pharmacokinetics of 1,2-epoxybutene-3 (vinyl oxirane, the primary reactive intermediate of butadiene) revealed major differences in metabolism of this compound between rats and mice. Whereas in rats no indication of saturation kinetics of epoxybutene metabolism could be observed up to exposure concentrations of 5000 ppm, in mice saturation of epoxybutene metabolism becomes apparent at atmospheric concentrations of about 500 ppm. The estimated maximal metabolic rate (Vmax) in mice for epoxybutene was only 350 mumol X h-1 X kg-1 (rats: greater than 2600 mumol X h-1 X kg-1). In the lower concentration range where first order metabolism applies (up to about 500 ppm) epoxybutene is metabolized by mice at higher rates compared to rats (metabolic clearance per kg body weight, mice: 24,900 ml X h-1, rats: 13,400 ml X h-1). Under these conditions the steady state concentration of epoxybutene in the mouse is about 10 times that in the rat. When mice are exposed to high concentrations of butadiene (greater than 2000 ppm; conditions of saturation of butadiene metabolism; closed exposure system) epoxybutene is exhaled by the animals, and its concentration in the gas phase increases with exposure time. At about 10 ppm epoxybutene signs of acute toxicity are observed. When rats are exposed to butadiene under similar conditions, the epoxybutene concentration reaches a plateau at about 4 ppm. Under these conditions hepatic non-protein sulfhydryl compounds are virtually depleted in mice but not in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of the neurotoxin n-hexane in rat and man

The pharmacokinetics of inhaled n-hexane in rat and man were compared. In the rat metabolism was saturable. Up to 300 ppm, the metabolic rate was directly proportional to the concentration in the atmosphere, reaching 47 mumol/(h X kg). Only 17% of n-hexane was exhaled unchanged. Above 300 ppm, the amount of n-hexane in the body rose with increasing atmospheric concentrations from 1.6 up to a limiting value of 9.6, which corresponded to the thermodynamic distribution coefficient of n-hexane between the organism and the atmosphere. Up to 3000 ppm, the rate of metabolism increased to 245 mumol/(h X kg); only a slow further increase was found up to 7000 ppm (285 mumol/(h X kg]. In man the steady-state concentrations of n-hexane were about 1 ppm. The metabolic clearance was 132 1/h, and n-hexane accumulated to a factor of 2.3 in the organism. The thermodynamic distribution coefficient was calculated to be 12. Twenty per cent of n-hexane in the body was exhaled unchanged. At low concentrations the rate of metabolism of n-hexane is limited in both species by transport to the enzyme system. Under these conditions the rate of metabolism of n-hexane should not be influenced by xenobiotics which induce the n-hexane metabolizing enzyme system.

Kinetics and disposition in toxicology. Example: carcinogenic risk estimate for ethylene

Pharmacokinetic analysis of the behaviour of xenobiotics in living organisms is the basis for the understanding of dose (concentration-) response relationships. Risk estimates may lead to erroneous results if pharmacokinetic principles are neglected. The pharmacokinetic behaviour of a xenobiotic should be known before planning long-term experiments on toxicity and carcinogenicity. This is demonstrated using ethylene as an example. On the basis of the pharmacokinetics in rats of ethylene and ethylene oxide, its carcinogenic metabolite, we estimated the theoretical risk for the carcinogenic potential of ethylene. Our results demonstrate that exposure of rats to ethylene concentrations higher than 1000 ppm correspond to a (theoretical) exposure to 5.6 ppm ethylene oxide: exposures to ethylene at 40 ppm are equivalent to ethylene oxide exposures of 1 ppm, which is the (new) TLV for ethylene oxide.

Pharmacokinetics of isoprene in mice and rats

Pharmacokinetic analysis of isoprene inhaled by male Wistar rats and male B6C3F1 mice showed saturation kinetics in both species. Below atmospheric concentrations of 300 ppm in rats and in mice the rate of metabolism is directly proportional to the concentration. The low accumulation of isoprene in the body at low atmospheric concentrations suggests transport limitation of the metabolism. Only small amounts of isoprene taken up are exhaled as unchanged substance (15% in rats and 25% in mice). Its half life in rats is 6.8 min and in mice 4.4 min. At concentrations above 300 ppm the rate of metabolism does not increase further in proportion to the atmospheric concentration. It finally approaches maximal values of 130 mumol/(h X kg) body weight at atmospheric concentrations above 1500 ppm in rats, and 400 mumol/(h X kg) body weight at concentrations above 2000 ppm in mice. This indicates limited production of the two possible mono-epoxides of isoprene at high concentrations. Isoprene is endogenously produced and is systemically available. Its production rate is 1.9 mumol/(h X kg) in rats, and 0.4 mumol/(h X kg) in mice, respectively. Part of the endogenous isoprene is exhaled by the animals but it is metabolized to a greater extent: the rate of metabolism of endogenously produced and systemically available isoprene is 1.6 mumol/(h X kg) (rats) and 0.3 mumol/(h X kg) (mice).

Pharmacokinetic factors and their implication in the induction of mouse liver tumors by halogenated hydrocarbons

The presently available data on pharmacokinetics of halogenated solvents which produce hepatic tumors in B6C3F1 mice, but not in rats, are reviewed. Such compounds are trichloroethylene, perchloroethylene, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, and dichloromethane. It seems likely that higher metabolic rates in mice (compared with other species) may lead to a species-selective toxicity of such compounds. Recurrent cytotoxicity which leads to stimulation of cell replication seems to be a contributing factor in the pathogenesis of mouse liver tumors. However, it is likely that more than one factor contributes to the unique tumor response of the B6C3F1 mouse.

A physiologically based simulation approach for determining metabolic constants from gas uptake data

In vivo metabolic constants were determined in male Fischer rats for five chemicals: 1,1-dichloroethylene (1,1-DCE), diethyl ether (DE), bromochloromethane (BCM), methyl chloroform (MC), and carbon tetrachloride (CCl4). A closed recirculated exposure system was used to collect a series of uptake curves for each chemical at a range of initial concentrations. The shapes of these curves were a function of the tissue partition coefficients and the kinetic characteristics of the metabolism of these chemicals. Tissue:air partition coefficients were experimentally determined for each chemical and incorporated into a physiological kinetic model which was then used to simulate the uptake process. An optimal fit of the family of uptake curves for each chemical was obtained by adjusting the biochemical constants for metabolism of the chemical. Metabolism of both 1,1-DCE and CCl4 was represented by a single saturable process while MC required only a first-order pathway. BCM and DE exhibited a combination of both a saturable and a first-order process. Pyrazole, which blocks oxidative microsomal metabolism, inhibited the saturable pathways of 1,1-DCE, BCM, DE, and CCl4 metabolism and abolished the first-order pathway for MC. The maximum velocity of metabolism for the saturable pathway with 1,1-DCE, BCM, DE, and CCl4 for a 225-g rat was 27.2, 19.9, 26.1, and 0.92 mol/hr, respectively. The simulation approach for analyzing gas uptake data distinguishes between single and multiple metabolic pathways and provides kinetic constants that can be used in predictive toxicokinetic models for describing constant concentration inhalation exposure as well as exposures by other routes of administration.

Interaction between 1,2-dichloroethane and tetraethylthiuram disulfide (disulfiram). II. Hepatotoxic manifestations with possible mechanism of action

The synergistic hepatotoxicity of dietary disulfiram (DSF) with 1,2-dichloroethane (DCE) subchronically administered by inhalation at three concentration levels (150, 300, and 450 ppm) was studied. The criteria for hepatotoxicity were treatment-related increases in serum activities of sorbitol dehydrogenase, 5'-nucleotidase, and alkaline phosphatase, and in liver-to-body weight ratios. DSF alone did not elicit these responses while DCE at the highest concentration level increased liver-to-body weight ratios and the activity of 5'-nucleotidase. Exposure to DSF alone decreased cytochrome P450 levels, but in combination with DCE, the decrement of cytochrome P450 was additive in a DCE concentration-dependent manner. However, depression of cytochrome P450 by DCE alone was not concentration dependent. Although DSF and DSF/DCE combination increased the activity of glutathione S-transferases (GSTs), both DSF and DCE singly and in combination increased the tissue levels of reduced glutathione (GSH). Evidence is presented showing that the potentiation of the hepatotoxicity of DCE observed in the presence of DSF may be due to an inhibition of microsomal mixed-function oxidase-mediated metabolism of DCE and to a compensatory increase in DCE metabolism to reactive metabolites generated by GST-mediated conjugation of DCE with GSH.

Inhalation pharmacokinetics of 1,2-dichloroethane after different dietary pretreatments of male Sprague-Dawley rats

The effect of the pretreatment of male Sprague-Dawley rats with phenobarbital (PB), butylated hydroxyanisole (BHA) and disulfiram (DSF) on the inhalation kinetics of 1,2-dichloroethane [ethylene dichloride (EDC)] was studied by the gas uptake method. A closed recirculating system was constructed and characterized. The rate curves in all the pretreatment regimens showed saturable dependence on EDC concentration. These saturable dependencies (Michaelis-Menten) appeared to be associated with enzymatic metabolism. In general, a two-compartment, steady-state pharmacokinetic model described the uptake data. Data were transformed by Hanes plots to calculate the inhalational Km, the ambient EDC concentration at which uptake proceeded at half maximum rate, and Vmax, the maximum rate of uptake (i.e., maximum rate of metabolism). Although PB and BHA pretreatments did not affect the Km of EDC, PB pretreatment increased the Vmax while DSF pretreatment decreased both the Km and Vmax.

Different pharmacokinetics of dichlorofluoromethane (CFC 21) and chlorodifluoromethane (CFC 22)

Inhalation pharmacokinetics of dichlorofluoromethane (CFC 21) and chlorodifluoromethane (CFC 22) were studied in male Wistar rats by use of a closed inhalation chamber system. CFC 21 was readily eliminated via metabolism. However, CFC 22 underwent no detectable metabolism; pretreatment of the rats with DDT or phenobarbital did not stimulate metabolic transformation of the compound. Hence, formation of biologically relevant amounts of reactive intermediates from CFC 22 as a mechanism of toxicity seems unlikely.

Species differences in butadiene metabolism between mice and rats evaluated by inhalation pharmacokinetics

Metabolism of 1,3-butadiene to 1,2-epoxybutene-3 in rats follows saturation kinetics. Comparative investigation of inhalation pharmacokinetics in mice also revealed a saturation pattern. For both species "linear" pharmacokinetics apply at exposure concentrations below 1000 ppm 1,3-butadiene; saturation of butadiene metabolism is observed at atmospheric concentrations of about 2000 ppm. For mice metabolic clearance per kg body weight in the lower concentration range where first order metabolism applies was 7300 ml X h-1 (rat: 4500 ml X h-1. Maximal metabolic elimination rate (Vmax) was 400 mumol X h-1 X kg-1 (rat: 220 mumol X h-1 X kg-1. This shows that 1,3-butadiene is metabolized by mice at higher rates compared to rats. Based on these investigations, the metabolic elimination rates of butadiene in both species were calculated for the exposure concentrations applied in two inhalation bioassays with rats and with mice. The results show that the higher rate of butadiene metabolism in mice when compared to rats may only in part be responsible for the considerable difference in the susceptibility of both species to butadiene-induced carcinogenesis.

Alkylation of nuclear proteins and DNA after exposure of rats and mice to [1,4-14C]1,3-butadiene

B6C3F1 mice and Wistar rats were exposed to [1,4-14C]1,3-butadiene in a closed exposure system. Based on body weight, mice metabolized the test compound at about twice the rate, compared to rats. Nucleoproteins and DNA were isolated from the livers of the animals and covalent binding of [14C]-butadiene-derived radioactivity was determined. In both species comparable amounts of radioactivity were covalently bound to liver DNA. Covalent binding to mouse-liver nucleoproteins was twice as high as in rats and thus it paralleled the higher metabolic rate for butadiene in this species.

Metabolism of inhaled dihalomethanes in vivo: differentiation of kinetic constants for two independent pathways

Dihalomethanes are metabolized by two major pathways: an oxidative, cytochrome P-450-mediated pathway that has been previously thought to yield only CO, and a glutathione (GSH)-dependent one that yields CO2. Both give 2 mol of halide ion. We studied the kinetic properties of the two pathways in vivo by exposing male rats to various inhaled concentrations of CH2Cl2,CH2F2, CH2FCl, CH2BrCl, and CH2Br2 and determining end-exposure carboxyhemoglobin (HbCO) and plasma bromide (where appropriate). Closed atmosphere gas uptake studies were employed for CH2F2, CH2FCl, CH2Cl2, and CH2BrCl metabolism. A physiologically based kinetic model was used to determine kinetic constants based on gas uptake or plasma bromide data and these constants were used to predict HbCO concentrations. Oxidation was high affinity, low capacity. The maximum metabolic rates for this pathway with CH2Br2, CH2BrCl, and CH2Cl2 were, respectively, 72, 54, and 47 mumol metabolized/kg/hr. CH2FCl did not undergo significant oxidative metabolism and appears more like CH3C1 than a dihalomethane in its metabolic reactivity. The GSH pathway was low affinity, but high capacity and could be described as a single first-order process at all accessible exposure concentrations. The rate constant for this first-order GSH-dependent pathway was related as CH2BrCl greater than CH2Cl2 congruent to CH2FCl greater than CH2Br2 greater than CH2F2. Presumably bromide is a preferred leaving group but steric hindrance in the initial reaction with GSH is important with CH2Br2. We also studied the effects of pyrazole (which inhibits microsomal oxidation) and 2,3-epoxypropanol (which depletes GSH) on dihalomethane metabolism. Pyrazole abolished CO production from CH2Br2, CH2BrCl, and CH2Cl2. GSH depletion did not change the yield of halide ion from the high-affinity pathway; it did increase the steady-state HbCO concentrations with CH2Cl2 and CH2ClBr, but not with CH2Br2. The putative formyl chloride (FC) intermediate from CH2Cl2 or CH2BrCl appears to have a longer life than the formyl bromide from CH2Br2 and a significant portion of the FC (congruent to 20-30%) may react with other cellular nucleophiles instead of spontaneously decomposing to CO. This portion of the oxidative pathway probably yields CO2.

Metabolism and pharmacokinetics of vinyl acetate

The hydrolysis of vinyl acetate (formation of acetic acid) has been studied in vitro with rat liver and lung microsomes, rat and human plasma and purified esterases (such as acetylcholine esterase, butyrylcholine esterase, carboxyl esterase). Characterization of the kinetic parameters revealed that rat liver microsomes and purified carboxyl esterase (from porcine liver) displayed the highest activity. In order to establish the rate of metabolism of vinyl acetate in vivo, rats were exposed in closed desiccator jar chambers, and gas uptake kinetics were studied. The decay of vinyl acetate was dose-dependent, indicating possible saturation of metabolic pathway(s). The maximal clearance (at lower concentrations) of vinyl acetate from the system (30 000 ml/h per kg body weight) was similar to the maximal ventilation rate in this species. This indicated that under conditions when metabolic enzymes are not saturated the metabolic rate is mainly determined by pulmonary uptake. The exposure of rats to vinyl acetate resulted in a transient exhalation of significant amounts of acetaldehyde into to the closed exposure system. This indicates the presence of this metabolic intermediate of vinyl acetate in the organism in vivo.

DNA-binding assay of methyl chloride

Fischer-344 rats and B6C3F1 mice of both sexes were exposed in closed chambers to 14C-labeled methyl chloride. Different clearance values from the gas phase of the system indicated that, based on body weight, mice metabolized the test compound much faster than rats. After isolation of DNA and nucleoproteins from liver and kidneys radioactivity was found in all macromolecular samples; this was ascribed to metabolic C1-incorporation. Radioactivity incorporation was particularly high in DNA of mouse kidneys, suggesting a high turnover to active C1 bodies (formaldehyde, formate) in this tissue. Analyses of DNA samples from kidneys of female and male mice showed neither 7-N-methylguanine nor O6-methylguanine. Hence, the formation of tumors in B6C3F1 mice exposed to high concentrations of methyl chloride is not based on methylation of DNA in this tissue.

Absence of lipid peroxidation as determined by ethane exhalation in rats treated with 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)

The exhalation of ethane is widely used as an indicator of in vivo lipid peroxidation. To test the hypothesis that lipid peroxidative events are involved in the toxicity of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), we administered a lethal dose of TCDD (60 micrograms/kg), IP to male Sprague-Dawley rats (160-180 g) and measured by gas chromatography the exhalation of ethane into the atmosphere of a closed all-glass exposure chamber. TCDD-treated rats exhaled only slightly more ethane than control rats at a single time point 7 days following TCDD administration. Since the exhalation of ethane is the net result of the endogenous production of the gas and its metabolic degradation, the latter was quantified by measuring the clearance of exogenous ethane (initial concentration = 100 ppm) introduced to the atmosphere of the exposure chamber. The clearance of ethane in TCDD-treated rats was markedly decreased, reaching a minimum 7 days following TCDD treatment. Apparently, the slight increase in exhaled ethane was due to an inhibition of ethane metabolism caused by TCDD. However, rats obviously intoxicated and having lost considerable body weight might be impaired in their ability to transport ethane. To bypass this problem we injected ethane (0.2 ml) directly into the rats IP. Here also the metabolic clearance in TCDD-treated rats was diminished. In a further experiment, rats treated with dithiocarb at a dose where ethane metabolism was totally inhibited exhaled more ethane than did TCDD-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhalation pharmacokinetics based on gas uptake studies. V. Comparative pharmacokinetics of ethylene and 1,3-butadiene in rats

The pharmacokinetics of ethylene and 1,3-butadiene were studied in male Sprague-Dawley rats by use of a closed inhalation chamber system. Both compounds showed saturable metabolism when untreated rats were used. "Linear" pharmacokinetics applied at exposure concentrations below 800 ppm ethylene and below 1,000 ppm 1,3-butadiene. A constant elimination rate, indicative of metabolic saturation, occurred at concentrations higher than 1,000 ppm ethylene or 1,500 ppm 1,3-butadiene. Pretreatment with aroclor 1254 (polychlorinated biphenyls) increased Vmax for both compounds. For 1,3-butadiene, no saturation of metabolic capacity was observed with exposure concentrations up to 12,000 ppm when the rats were pretreated with aroclor 1254. A comparison with previous studies on ethane and n-pentane suggested that introduction of a double bond into a saturated aliphatic hydrocarbon increased the rate of metabolism under conditions in vivo.

Inhalation pharmacokinetics based on gas uptake studies. VI. Comparative evaluation of ethylene oxide and butadiene monoxide as exhaled reactive metabolites of ethylene and 1,3-butadiene in rats

When ethylene oxide or butadiene monoxide is added to the atmosphere of a closed inhalation chamber occupied by Sprague-Dawley rats, a first-order elimination pattern is observed. When either of these compounds is IP injected into rats which are subsequently placed in the closed chamber, the course of epoxide in the atmosphere follows Bateman exponential functions. From the experimental data, the kinetic parameters for distribution and metabolic elimination of ethylene oxide and butadiene monoxide can be derived. When ethylene or 1,3-butadiene was added to the closed exposure systems and kept at atmospheric concentrations which assured maximal metabolic turnover of the olefin (i.e., concentrations above 1,000 ppm ethylene or 1,500 ppm 1,3-butadiene), exhalation of the appropriate epoxide occurred and led finally to a constant (plateau) concentration of the reactive metabolite in the system's atmosphere. Although the initial time-course was different between butadiene monoxide and ethylene oxide (with a high initial increase of ethylene oxide and a subsequent decrease) an analysis at steady-state (plateau concentrations) revealed that only 29% of the amounts of both epoxides which in theory are formed as primary metabolites from the parent olefins are systematically available (i.e., distributed in the entire organism). The discrepancy is probably related to first pass elimination of the epoxide.

Pharmacokinetics and toxicity testing

The pharmacokinetic basis for the design of toxicity tests is discussed with reference to the absorption and clearance of drugs. The absorption and clearance of a wide range of drugs by laboratory animals and man has been examined and reviewed to provide a firm basis against which new drugs can be compared. Some pitfalls in either the empirical approach to toxicology or the incorrect interpretation of kinetic data are highlighted. An approach is outlined for the rational application of animal pharmacokinetic data in the assessment of the safety in man of a new therapeutic agent.

Olefinic hydrocarbons: a first risk estimate for ethene

The exhalation of ethene oxide by rats exposed to ethene now allows a first estimate of the genotoxic risk of ethene on a pharmacokinetic basis. The distribution (Keq, Kst) of ethene oxide under conditions of inhalation of ethene oxide, or high levels of ethene and consideration of the saturation behaviour of metabolism of ethene lead to the conclusion that inhalation exposure of rats to 1000 ppm ethene or more would theoretically correspond to about 7.5 ppm ethene oxide exposure. Available carcinogenicity data for inhaled ethene oxide were utilized into a comparison of the genotoxic risks of low levels of ethene oxide and ethene. It has been shown that consideration of ethene as a "simple asphyxiant" is inappropriate. The new intended TLV of 1 ppm for ethene oxide would be consistent with a TLV of 100 ppm for ethene.

Biological activation of 1,3-butadiene to vinyl oxirane by rat liver microsomes and expiration of the reactive metabolite by exposed rats

When 1,3-butadiene is incubated with rat liver microsomes and NADPH both enantiomers of vinyl oxirane are formed, the amount of epoxide being dependent on incubation time, microsomal protein, and substrate concentration. Inhibition by SKF 525 A or dithiocarb as well as induction by pretreatment with phenobarbital or 20-methylcholanthrene suggest participation of cytochrome P-450 in this reaction. The amount of epoxide is enhanced by addition of 1,1,1-trichloropropene oxide and reduced by glutathione, especially in the presence of hepatic cytosol. When rats are exposed to 1,3-butadiene in a closed chamber (conditions of maximal metabolism) vinyl oxirane is exhaled and can be quantitatively determined from the gas phase. The concurrent exhalation of acetone is consistent with the idea of biologic action of a reactive metabolite.

Pulmonary physiology and inhalation dosimetry in rats: development of a method and two examples

Methods were developed to measure simultaneously respiratory frequency, tidal volume, minute volume, and net uptake of an inhaled vapor in rats. During steady state, if metabolism is the only significant route of elimination, net uptake rate of the inhaled vapor is equal to its rate of metabolism. The rates of metabolism of methyl chloride in 50- and 1000-ppm-exposed rats were 0.20 and 3.3 nmol/min/g, respectively; the rates of metabolism of methylene chloride in 50- and 1500-ppm-exposed rats were 0.57 and 2.8 nmol/min/g, respectively. The uptake values obtained for both solvents were consistent with pharmacokinetic and metabolism data that were previously obtained in our laboratory. A pharmacokinetic model incorporating the metabolic rate at steady state, blood concentration versus time, and respiratory minute volume was used to describe the fate of inhaled methyl chloride in F344 rats, and to estimate the inhaled "effective" dose in 50- and 1000-ppm 6-hr-exposed rats (3.8 and 67 mg/kg, respectively). The approach used in these studies appears to be a useful method for the evaluation of metabolic rates and for inhalation dosimetry.

Inhalation pharmacokinetics based on gas uptake studies. IV. The endogenous production of volatile compounds

A pharmacokinetic description of production, distribution and metabolism of endogenous volatile compounds is presented. This description uses the "gas uptake model" of a closed recirculated atmosphere in which experimental animals are exposed. As an example, the production rates of acetone, under different conditions of stimulation by xenobiotics, are calculated from published experimental data. The theoretical descriptions may serve as a basis for treating the problem of hydrocarbon exhalation in toxicological experiments with compounds eliciting lipid peroxidation.

Quantitative evaluation of ethane and n-pentane as indicators of lipid peroxidation in vivo

The use of exhalation of ethane and n-pentane in experimental animals as parameters of lipid peroxidation led to an examination of pharmacokinetics of both compounds in rats. When rats were exposed, in a closed desiccator jar chamber, to a wide range of ethane concentrations, linear elimination pharmacokinetics were observed. n-Pentane, when concentrations higher than 100 ppm were applied, displayed saturation kinetics. These were formally explained by action of two competing metabolizing pathways or enzymes. Application of preexisting models could describe exhalation of both ethane and n-pentane by untreated control rats. Stimulation of lipid peroxidation by ferrous ions or by carbon tetrachloride resulted in dissimilar quantitative behaviours of ethane and n-pentane. Ethane production rates were enhanced after application of both compounds. Because of relatively slow metabolic eliminations this led to markedly elevated concentrations of ethane in the gas phase of the system. Pentane production rates were simultaneously enhanced. However, difficulties in interpretation arise because of rapid metabolic elimination of n-pentane. Compounds that diminish pentane metabolism are shown to evoke higher pentane concentrations in the system than compounds which only enhance the pentane production rate. Determinations of ethane exhalation should provide a more favourable parameter of lipid peroxidation than exhalation of pentane.