Application of an automated HS-GC method in partition coefficient determination for xylenes and ethylbenzene in rat tissues

An automated static head space-gas chromatography method was used in the determination of partition coefficients (Kd) for the xylene isomers and ethylbenzene in blood, brain, muscle, kidney, liver and fat of Sprague Dawley rats. Since homogenization resulted in the potential loss of analytes from tissue samples, unhomogenized samples were used. With a few exceptions, tissue:air Kd values were independent of the concentrations of the analytes, singly or as a mixture. The tissue:blood Kd values were determined. For each tissue and analyte, the value obtained for each analyte concentration was within +/- 10% of the mean value calculated for the entire concentration range.

Bioaccumulation of musk xylene (MX) in developing and adult rats of both sexes

Bioaccumulation of musk xylene (MX) was measured by GC-ECD in adult and developing Long Evans rats. Males and females were fed with MX-containing chow (0.001, 0.01, 0.033, 0.1 g MX/kg food pellets) for 10 weeks before mating. Treatment continued during pregnancy and lactation. Offspring exhibited dose-dependent MX accumulation with 1/2-3/4 of adult female or 3-4 times adult male body fat levels (at 0.1 mg/kg food) at days 1 and 14. Milk levels were comparable to adult female adipose tissue. Data indicate significant transplacental passage and exposure via maternal milk. In rats fed with MX in adulthood, levels were highest in adipose tissue with significant amounts in other organs (ovary, adrenal). Female tissue levels were 3.7-6.8 times higher. This unexplained sex difference was unrelated to lipid content and was absent in offspring.

Physiologically based pharmacokinetic modeling of a ternary mixture of alkyl benzenes in rats and humans

The objective of the present study was to develop a physiologically based pharmacokinetic (PBPK) model for a ternary mixture of alkyl benzenes [toluene (TOL), m-xylene (XYL), and ethylbenzene (EBZ)] in rats and humans. The approach involved the development of the mixture PBPK model in the rat and extrapolation to humans by substituting rat physiological parameters and blood:air partition coefficients in the model with those of humans, scaling maximal velocity for metabolism on the basis of body weight0.75 and keeping all other model parameters species-invariant. The development of the PBPK model for the ternary mixture in the rat was accomplished by initially validating or refining the existing PBPK models for TOL, XYL, and EBZ and linking the individual chemical models via the hepatic metabolism term. Accordingly, the Michaelis-Menten equation for each solvent was modified to test four possible mechanisms of metabolic interaction (i.e., no interaction, competitive inhibition, noncompetitive inhibition, and uncompetitive inhibition). The metabolic inhibition constant (Ki) for each binary pair of alkyl benzenes was estimated by fitting the binary chemical PBPK model simulations to previously published data on blood concentrations of TOL, XYL, and EBZ in rats exposed for 4 hr to a binary combination of 100 or 200 ppm of each of these solvents. Competitive metabolic inhibition appeared to be the most plausible mechanism of interaction at relevant exposure concentrations for all binary mixtures of alkyl benzenes in the rat (Ki,TOL-XYL = 0.17; Ki,TOL-EBZ = 0.79; Ki,XYL-TOL = 0.77; Ki,XYL-EBZ = 1.50; Ki,EBZ-TOL = 0.33; Ki,EBZ-XYL = 0.23 mg/L). Incorporating the Ki values obtained with the binary chemical mixtures, the PBPK model for the ternary mixture simulated adequately the time course of the venous blood concentrations of TOL, XYL, and EBZ in rats exposed to a mixture containing 100 ppm each of these solvents. Following the validation of the ternary mixture model in the rat, it was scaled to predict the kinetics of TOL, XYL, and EBZ in blood and alveolar air of human volunteers exposed for 7 hr to a combination of 17, 33, and 33 ppm, respectively, of these solvents. Model simulations and experimental data obtained in humans indicated that exposure to atmospheric concentrations of TOL, XYL, and EBZ that remain within the permissible concentrations for a mixture would not result in biologically significant modifications of their pharmacokinetics. Overall, this study demonstrates the utility of PBPK models in the prediction of the kinetics of components of chemical mixtures, by accounting for mechanisms of binary chemical interactions.

Mechanism-based inactivation of mouse hepatic cytochrome P4502B enzymes by amine metabolites of musk xylene

Musk xylene (2,4,6-trinitro-1-t-butylxylene; MX) is a synthetic nitromusk perfume ingredient that induces and inhibits mouse cytochrome P4502B (CYP2B) enzymes in vivo. The purpose of the present work was to determine whether amine metabolites of MX contributed to the enzyme inhibition and, if so, to define the nature and kinetics of this inhibition. When dosed orally to phenobarbital (PB)-treated mice, MX (200 mg/kg) inhibited > 90% of the PB-induced O-dealkylation of 7-pentoxyresorufin (PROD), and [14C]MX equivalents bound covalently to microsomal proteins. However, when this experiment was repeated in mice pretreated with antibiotics to eliminate the gastrointestinal flora, no decrease in PB-induced PROD activity and no covalent binding to microsomal proteins were observed. Thus, the ability of antibiotic treatment to eliminate the enzyme inhibition and covalent binding implicated amine metabolites of MX formed by nitroreduction in anaerobic intestinal flora as obligatory for these effects. Two monoamine metabolites of MX were synthesized to study enzyme inhibition directly. These metabolites were 2-amino-4,6-dinitro-1-t-butyl-xylene and 4-amino-2,6-dinitro-1-t-butylxylene, referred to as o-NH2-MX and p-NH2-MX, respectively, reflecting the position of the amine substitution relative to the t-butyl function. In the in vitro studies with PB-induced mouse liver microsomes, both amines inhibited PROD activity when preincubated in the absence of NADPH. However, only p-NH2-MX caused a time- and NADPH-dependent loss of PROD activity, and the inactivation rate was a pseudo-first-order process that displayed saturation kinetics. These results indicate that p-NH2-MX is a mechanism-based inactivator of mouse CYP2B enzymes. From kinetic analyses, the Ki was calculated to be 10.5 microM and the Kinact was 1.2 min-1. As final confirmation of the inhibitory effects of p-NH2-MX on mouse CYP2B enzymes, the amine (0.67 mmol/kg) was dosed orally to PB-induced mice. At 2 hr after dosing, p-NH2-MX inhibited essentially all of the PB-induced PROD activity, whereas an equimolar dosage of parent MX had no effect at this early time. Thus, although MX is an inducer of mouse CYP2B enzymes, an amine metabolite of MX is a mechanism-based inactivator of mouse CYP2B10. Furthermore, it is likely that the amine is responsible for the lack of functional CYP2B enzyme activity associated with induction of this enzyme by MX.

Induction and inhibition of mouse cytochrome P-450 2B enzymes by musk xylene

Musk xylene (MX) (1,3,5-trinitro-2-t-butylxylene) is a nitromusk perfume ingredient that although uniformly negative in a battery of genotoxicity tests, produces a high incidence of liver tumors in mice. The purpose of this work was to characterize the profile and dose-response relationship of microsomal enzyme induction following exposure to MX. MX was dosed by gavage to male B6C3F1 mice for 7 days at 0, 1, 5, 10, 20, 50, 100, and 200 mg/kg after which microsomes were prepared. At 200 mg/kg, MX increased liver weight by about 65% and increased microsomal cytochrome P-450 content 2-fold over control. MX increased microsomal activity for O-dealkylation of 7-ethoxy and 7-methoxyresorufin 4- and 2-fold, respectively, and increased the N-demethylation of erythromycin approximately 2-fold. These results were generally consistent with increased CYP1A1, 1A2, and 3A protein levels determined by Western blotting. In contrast, whereas no increase in O-dealkylation of 7-pentoxyresorufin (PROD) was observed, MX treatment increased CYP2B protein levels about 25-fold over control at 200 mg/kg. Furthermore, a single dosage of MX (200 mg/kg) increased Cyp2b-10 mRNA to a maximal level and with a time course similar to phenobarbital (PB). To study inhibition of CYP2B enzymes in vivo, mice were treated with PB (0.05% in drinking water for 5 days), then given a single dosage of corn oil or MX (200 mg/kg) at 2 or 18 hr before necropsy. PB treatment increased PROD activity 25-fold, and at 2 hr after MX treatment (associated with peak plasma levels of MX), there was no change in the PB-induced PROD activity. However, at 18 hr, MX treatment decreased PROD activity by 90%. Despite the in vivo inhibition, in vitro studies indicated that MX did not cause mechanism-based inactivation of CYP2B enzymes. The potential for nitroreduction of MX (catalyzed by anaerobic intestinal bacteria) to contribute to the inhibition of CYP2B enzyme activity was evaluated in a separate group of PB-induced mice that were dosed orally with a regimen of broad spectrum antibiotics (neomycin, tetracyline, and bacitracin) to reduce gut flora prior to administration of MX. In these animals, MX (200 mg/kg) did not inhibit PB-induced PROD activity. In summary, MX treatment produced general hepatic changes consistent with induction of CYP2B enzymes in mice and caused a large increase in CYP2B protein and mRNA levels. These data indicate that MX is a PB-like inducer of cytochrome P-450 enzymes and may cause liver tumors in a manner analogous to PB. However, no increase in CYP2B enzyme activity was observed, suggesting that MX or metabolites of MX also inhibit this enzyme. When the intestinal flora was eliminated by antibiotic treatment, MX no longer inhibited the CYP2B enzyme, indicating that anaerobic bacteria are capable of metabolizing MX, and suggesting that amine metabolites formed by nitroreduction are involved in the inhibition of mouse CYP2B enzymes.

Ethnic differences in biological monitoring of several organic solvents. II. A simulation study with a physiologically based pharmacokinetic model

To improve the reliability of biological monitoring and the development of biological limit values, ethnic differences in the biological monitoring of several organic solvents were studied in Orientals and Caucasians. Six Caucasian and six Oriental volunteers were exposed to each organic solvent in an exposure chamber for 6 h at rest. The exposure concentrations were 50 ppm for perchloroethylene, 50 ppm for styrene, and 100 ppm for m-xylene, respectively. Experimental results were compared with simulation results of a physiologically based pharmacokinetic (PB-PK) model. Differences between Orientals and Caucasians under occupational exposure were also estimated by extrapolation. The simulation results obtained for the Caucasian group showed good agreement with the experimental results. However, the Oriental group did not show good agreement when the same metabolic parameters values applied to Caucasians were used in the PB-PK model. By modification of the metabolic parameters it was possible to get a good fit between the model and the results of the Oriental group. The simulation results obtained for occupational exposure also showed differences in biological levels between the two ethnic groups. Implications of these differences between experimental and simulation results are discussed in the context of the application of biological monitoring and in the development of biological limit values.

Partition coefficients between human blood or adipose tissue and air for aromatic solvents

OBJECTIVES

The partitioning of lipophilic toxicants into blood and into adipose tissue plays an important role in the physiological distribution and toxicology of these substances. The partition coefficients between blood and air and adipose tissue and air were determined for widely used aromatic solvents in an in vitro test system using human tissue samples.

METHODS

Samples of whole venous blood (N = 35) were drawn from 10 subjects. In addition, samples of perirenal and epididymal adipose tissue were obtained from F344 rats, along with subcutaneous, omental, or inguinal adipose tissue from 43 patients who had undergone surgery. Portions of each tissue were injected into vials for equilibration with atmospheres containing deuterated and nondeuterated organic solvents. Gas chromatographic headspace analysis was then used to determine the partition coefficients between blood and air and adipose tissue and air.

RESULTS

The mean partition coefficients between human blood and air or adipose tissue and air were 334 (SE 11) (adipose tissue) for benzene; 1764 (SE 49) (adipose tissue) for ethylbenzene; 3184 (SE 84) (adipose tissue) for styrene; 18.3 (SE 0.24) (blood) and 962 (SE 32) (adipose tissue) for toluene; 35.2 (SE 0.45) (blood) and 2460 (SE 63) (adipose tissue) for O-xylene; 31.9 (SE 0.45) (blood) and 1919 (SE 53) (adipose tissue) for m-xylene; and 39.0 (SE 0.70) (blood) and 2019 (SE 102) for p-xylene. Regression analyses revealed coefficients of determination of 0.88 (human) and 0.98 (rat) between blood and air and log tissue and air. A value of 0.98 was found for partition coefficients between rat and human adipose tissue.

CONCLUSIONS

The partition coefficients between blood and air and adipose tissue and air were strongly correlated. The partitioning of aromatic solvents into rat adipose tissue is predictive of partitioning into human adipose tissue.

Physiologically-based pharmacokinetic modeling of a mixture of toluene and xylene in humans

A physiologically-based pharmacokinetic (PBPK) model for a mixture of toluene (TOL) and xylene (XYL), developed and validated in the rat, was used to predict the uptake and disposition kinetics of TOL/XYL mixture in humans. This was accomplished by substituting the rat physiological parameters and the blood:air partition coefficient with those of humans, scaling the maximal velocity for hepatic metabolism on the basis of body weight0.75, and keeping all other model parameters species-invariant. The human TOL/XYL mixture PBPK model, developed based on the quantitative biochemical mechanism of interaction elucidated in the rat (i.e., competitive metabolic inhibition), simulated adequately the kinetics of TOL and XYL during combined exposures in humans. The simulations with this PBPK model indicate that an eight hour co-exposure to concentrations that remain within the current threshold limit values of TOL (50 ppm) and XYL (100 ppm) would not result in significant pharmacokinetic interferences, thus implying that data on biological monitoring of worker exposure to these solvents would be unaffected during co-exposures.

Kinetic disposition of xylene-based or aqueous formulations of deltamethrin applied to the dorsal mid-line of sheep and their effect on lice

A xylene-based topical formulation of the synthetic pyrethroid insecticide deltamethrin was applied to the dorsal mid-line of 2 groups of 5 Merino sheep within 24 h after shearing. One group was free of ectoparasites and the second was infested with sheep body lice, Bovicola (Damalinia) ovis. A water-based deltamethrin formulation was applied to the dorsal mid-line of a third group of 5 Merinos which were infested with lice. Insecticide concentrations on the wool of the back, upper and lower body and in the skin and bloodstream were measured at regular intervals between 1 and 98 days after treatment. Movement of the deltamethrin from the back to the lower body occurred within 24 h on all 3 groups, but maximum concentrations took 4-5 days to develop on the fleece in sheep treated with the xylene-based formulation and 11 days in sheep treated with the water-based formulation. There was a significant difference in concentration of deltamethrin close to the dorsal mid-line between the groups treated with the different formulations. Overall, concentrations of deltamethrin were lower at all sites examined on the sheep treated with the water-based formulation. Levels of deltamethrin in the tip of the fleece were significantly greater than those in the base and there was little movement of deltamethrin down the staple. The concentration of deltamethrin in the skin was significantly lower than the concentration in the wool and fell below the amount required to kill lice fully susceptible to deltamethrin after 12 days. Deltamethrin levels in the blood were near the limits of detection in all groups at all observations, indicating that there was little absorption or re-distribution of deltamethrin via the bloodstream. Most lice were killed after 20 h of exposure in vitro, to wool samples collected between 1 and 14 days after treatment. However, many lice survived in samples containing the same concentration of deltamethrin, but collected between 16 and 98 days after treatment. Numbers of lice surviving increased with the sampling time after treatment, suggesting that the bio-availability of the deltamethrin changed as the insecticide aged in the fleece.

An automated head space analysis method for xylenes and ethylbenzene in blood and water

A sensitive, reliable method of analysis was established for water and blood samples containing xylene isomers (m-,o- & p-) and ethylbenzene by means of an automated head space sampler connected to a GC equipped with a flame ionization detector. Minimum detection limits (MDLs) were ca. 1 and 6 ng/mL, respectively, for each of the four target compounds in water and blood samples. Practical quantitation limits (PQLs) with precision values better than +/- 4% for duplicate samples were 40 and 240 ng/mL, respectively, for the individual organic compounds in water and blood. The analytical precision was < +/- 4% for concentrations above the PQL and up to 50 micrograms/mL. Calibration curves for the C2-benzene isomers in water and blood samples were linear (r2 > 0.9999) for individual analyte concentration up to ca. 50 micrograms/mL. Blank values were below the MDLs. The effect of cocontaminants on head space analyte concentration was insignificant for the anticipated range of sample composition.

m-xylene toxicokinetics in phenobarbital-treated rats: comparison among inhalation exposure, oral administration, and intraperitoneal administration

Rats, pretreated with phenobarbital (PB) for 3 days (80 mg/kg/day), were challenged with m-xylene orally or intraperitoneally at a small (0.01 ml/kg or 0.081 mmol/kg) or a large (0.10 ml/kg or 0.814 mmol/kg) dose, or by 6-hr inhalation exposure at a low (40 ppm) or a high (400 ppm) concentration. The concentrations of m-xylene and its major metabolite, m-methyl hippuric acid (m-MHA), were measured over time in the blood and urine, respectively. PB treatment, which increased the hepatic metabolism of m-xylene in vitro about sixfold, had a significant effect on the metabolism of inhaled m-xylene (decreased blood m-xylene concentration together with increased urinary excretion of m-MHA) only at a high exposure concentration (400 ppm). On the other hand, the enzyme induction had a significant effect on the metabolism of orally administered m-xylene either at a small (0.081 mmol/kg) or a large (0.814 mmol/kg) dose, due to the first-pass metabolism that plays a great role in this route. When m-xylene was administered intraperitoneally, the effect of enzyme induction was shown only at the large dose, a finding which suggests that intraperitoneal administration is more similar to inhalation exposure than oral administration. However, in agreement with oral administration but in contrast to inhalation exposure, PB treatment increased the urinary excretion of m-MHA only shortly after intraperitoneal administration of m-xylene, with no significant increase occurring in the total amount of m-MHA excreted in the urine, despite the great difference in the blood m-xylene concentration between PB-treated and control rats.

The rate of percutaneous permeation of xylene, measured using the "perfused pig ear" model, is dependent on the effective protein concentration in the perfusing medium

In order to study the dermal permeation of compounds through the skin, an in vitro model was developed which utilized pig ears perfused with autologous pig blood (de Lange, J., van Eck, P., Elliott, G. R., de Kort, W. L. A. M., and Wolthuis, O. L. (1992). J. Pharmacol. Toxicol. Methods 27, 71-77). In the present article we investigated to what extent the rate of permeation of xylene through pig ear skin is dependent on the perfusion medium used. Pig ears were exposed to xylene (10 cm2 area) for a 4-hr period (30 degrees C, relative humidity of 40-60%) and the perfusate was analyzed for xylene using gas chromatography. The rates of permeation of xylene for whole blood, blood depleted of white blood cells, and a buffer containing 4.5% albumin were similar (+/- 300 ng/min/cm2). The rate of penetration was fivefold higher when pig plasma was used and ninefold lower when albumin was excluded from the buffer. Using the buffer, we found that the rate of permeation of xylene was proportional to flow (constant protein concentration) and protein concentration (constant flow). Our data demonstrate that the measured permeation rate for xylene is, to a large degree, dependent on the effective protein concentration (mg/min) passing through the ear. Differences in this parameter could explain the variations in rates of permeation found using the different perfusion media. To avoid problems associated with the choice of receptor fluid for permeation experiments, we suggest that full blood remains the vehicle of choice, although the practical perfusion period is limited to about 6 hr. If longer perfusion periods are required, then it should be possible to reproduce results obtained with whole blood by choosing an appropriate buffer.

Xylene: its toxicity, measurement of exposure levels, absorption, metabolism and clearance

Xylene is an aromatic hydrocarbon widely used in industry and medical technology as a solvent. Health and safety authorities in most countries, including Australia, recommend a threshold limit value (TLV) of 100 ppm in the working environment. Recently, the amount of the major metabolite of xylene, methylhippuric acid (MHA), in urine has been recommended as a better indicator of exposure. The American Conference of Governmental Industrial Hygienists has recommended an upper limit for this indicator, called a biological exposure index (BEI), of 2.0 g MHA/L urine (SG 1.016). Xylene vapour is absorbed rapidly from the lungs, and xylene liquid and vapour are absorbed slowly through the skin. Of the xylene absorbed, about 95% is metabolised in the liver to MHA and 70 to 80% of metabolites are excreted in the urine within 24 hours. However, the many variables which affect the absorption, metabolism and clearance of xylene include exercise, alcohol intake, cigarette smoking, co-exposure to other solvents, gender, and gastrointestinal, hepatic and renal pathology. Xylene in high concentrations acts as a narcotic, inducing neuropsychological and neurophysiological dysfunction. Respiratory tract symptoms are also frequent. More chronic, occupational exposure has been associated with anemia, thrombocytopenia, leukopenia, chest pain with ECG abnormalities, dyspnea and cyanosis, in addition to CNS symptoms. Concomitant exposure to xylene and other solvents, including toluene, affected hematological parameters, liver size, liver enzymes, auditory memory, visual abstraction, and vibration threshold in the toes. Normal metabolic pathways were altered and significant increases in some serum bile acids may reflect early liver damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentrations of phenol, o-cresol, and 2,5-xylenol in the urine of workers employed in the distillation of the phenolic fraction of tar

Phenol (87.3 mg/l), p-cresol (58.6 mg/l), o-cresol (76.9 mg/l), and 2,5-xylenol (36.7 mg/l) were detected in the urine of workers employed in the distillation of the high temperature phenolic fraction of tar (carbolic oil). The concentrations of these compounds in the urine of non-exposed male workers was 11.7 mg/l, 25.7 mg/l, 68.1 micrograms/l, and 69 micrograms/l respectively. The excretion rates were 4.20 mg/h for phenol, 2.4 mg/h for p-cresol, 3.3 mg/h for o-cresol; and 1.5 mg/h for 2,5-xylenol. The highest concentrations of the mentioned compounds were detected in urine collected between eight and 10 hours from the beginning of exposure. The kinetics of excretion are considered.

Ethanol induced modification of m-xylene toxicokinetics in humans

This study was undertaken to determine whether previous subacute treatment with ethanol could modify the kinetics of m-xylene in humans. A group of six volunteers was exposed twice to either 100 or 400 ppm of m-xylene during two hours (between 0800 and 1000). Ethanol was given orally in the early evening on each of two consecutive days before exposures (total ethanol intake of 137 g). Such ethanol pretreatment affected the kinetics of m-xylene but only at the high exposure (400 ppm). The modifications were: (1) decreased concentration of m-xylene in blood and alveolar air during and after exposure; (2) increased urinary excretion of m-methylhippuric acid at the end of exposure. Ethanol treatment also enhanced the elimination of antipyrine in saliva. Overall, this study showed that the effect of enzyme induction on the metabolism of m-xylene, after ethanol ingestion, depends on the exposure concentration and is not likely to occur as long as the exposure concentrations remain under the current maximum allowable concentration (100 ppm) in the workplace.

Acute effects of m-xylene inhalation on body sway, reaction times, and sleep in man

Nine male volunteers were exposed to either a constant or a fluctuating exposure pattern of m-xylene with a time-weighted average exposure concentration of 200 ppm in both cases. The subjects remained sedentary throughout the exposure, or the exercised at 100 W for 10 min at the beginning of the morning and the afternoon sessions. In another experiment, 12 sedentary male volunteers were exposed to a fixed 200 ppm of m-xylene. The effects of m-xylene on body sway, reaction time performance, and overnight sleep were measured. Body balance was stabilized after exposure to the peaks of 400 ppm of m-xylene in both sedentary and exercising subjects. Simple visual reaction times were prolonged after the peak exposures at rest whereas auditive choice reaction times were prolonged after peaks combined with exercise. Exposure to m-xylene at a constant level of 200 ppm did not affect the ratio of "active" to "quiet" sleep in the volunteers as measured with the static charge sensitive bed recording, but decreased slightly the number of body movements in bed. On the next morning no changes were found in body sway and reaction time performance as compared to the morning before exposure.

Enzyme induction by ethanol consumption affects the pharmacokinetics of inhaled m-xylene only at high levels of exposure

The experimental study with rats was undertaken to verify the working hypothesis that enzyme induction caused by ethanol consumption affects the kinetics of m-xylene only at a high level of exposure. m-Xylene was administered to ethanol-treated rats either perorally (0.01, 0.02 or 0.1 ml/kg) or by inhalation (50, 100 or 500 ppm each for 6 h) and the concentration of m-xylene in the blood and the urinary excretion of a m-xylene metabolite (m-methyl hippuric acid or m-MHA) were measured with time. The ethanol consumption, which increased the in vitro m-xylene metabolism about 5-fold, had no effect on the metabolism of inhaled m-xylene in vivo until the exposure concentration was raised to 500 ppm. On the other hand, metabolism of m-xylene after oral administration was markedly enhanced at any dose by the consumption, as evidenced by a decrease in the blood concentration of m-xylene together with an increase in the urinary excretion of m-MHA. These findings indicate that enzyme induction does not affect the pharmacokinetics of inhaled m-xylene when its exposure concentration is low. This may be because the hepatic blood flow, rather than the enzyme activity, rate-limits the metabolism of m-xylene, which is highly metabolized in the liver.

Glue sniffing deaths in Singapore--volatile aromatic hydrocarbons in post-mortem blood by headspace gas chromatography

Over a period from 1983 to 1991, of a total of 19,000 post-mortems, 33 were found to have at least one aromatic hydrocarbon (benzene, toluene or xylenes) in the blood. Of the 33 deceased, 22 had a history of toluene or petrol abuse while most of the remaining 11 were suspected to be glue sniffers through evidence found at the scene. This number, which represented 0.17 per cent of all the unnatural deaths, is considered small for a nation having a glue sniffing epidemic. The low death rate, as compared to 2.1 per cent through drug and chemical poisoning during the same period, is attributed to the timely intervention by the Government who outlawed glue sniffing and the effectiveness of compulsory rehabilitation. The male gender predominates (81.8 per cent) among the 33 deceased with a mean age of 20.1 years (range 15 to 33). The mean age for the female gender is 17.7 years (range 16 to 20). The blood toluene levels were found to be in the range 0.2 to 92 micrograms per ml blood. The causes of death are: 63.6 per cent due to falling or suicide by jumping; 18.2 per cent drowning; 6.1 per cent hanging; 6.1 per cent homicide; and 6.1 per cent acute toluene poisoning. The high proportion of traumatic deaths are discussed. Headspace gas chromatography with a suitable GC column was used for the analysis. Calibration blood standards were prepared in situ or in bulk stabilized by 10 per cent (v/v) methanol to overcome the hydrophobic and volatile nature of the aromatic hydrocarbons.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiologically based modeling of the toxicokinetic interaction between toluene and m-xylene in the rat

The present study was undertaken to investigate the mechanism of toxicokinetic interaction between toluene (TOL) and m-xylene (XYL) in vivo in the male Sprague-Dawley rat by physiologically based toxicokinetic (PBTK) modeling. First, the metabolic constants (Vmax and Km) were determined for TOL and XYL individually by conducting a series of closed-chamber inhalation exposures of three rats to starting concentrations of 500 to 4000 ppm. The values of Km (TOL, 0.55 mg/liter; XYL, 0.20 mg/liter) and Vmax (TOL, 4.8 mg/hr/kg; XYL, 8.4 mg/hr/kg) were obtained following best visual fit of PBTK model simulations to experimental data. Then using the same experimental set-up, rats were exposed to three different mixtures of both solvents (500 ppm TOL + 1000 ppm XYL; 1000 ppm TOL + 1000 ppm XYL; 1000 ppm TOL + 500 ppm XYL). The data from the time course of chamber solvent concentrations were analyzed with a binary chemical mixture PBTK model that had four mechanistic hypotheses of metabolic interaction (i.e., no interaction, competitive inhibition, noncompetitive inhibition, and uncompetitive inhibition) quantitatively defined in the liver compartment. The validity of the various model descriptions was verified with open-chamber inhalation exposure data on toxicokinetics of TOL and XYL. Overall, the results of this combined experimental and modeling approach are consistent with a competitive metabolic inhibition between XYL and TOL in the rat.

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.

Comparative evaluation of urinalysis and blood analysis as means of detecting exposure to organic solvents at low concentrations

One hundred and forty-three workers exposed to one or more of toluene, xylene, ethylbenzene, styrene, n-hexane, and methanol at sub-occupational exposure limits were examined for the time-weighted average intensity of exposure by diffusive sampling, and for biological exposure indicators by means of analysis of shift-end blood for the solvent and analysis of shift-end urine for the corresponding metabolite(s). Urinalysis was also performed in 20 nonexposed control men to establish the "background level." Both solvent concentrations in blood and metabolite concentrations in urine correlated significantly with solvent concentrations in air. Comparison of blood analysis and urinalysis as regards sensitivity in identifying low solvent exposure showed that blood analysis is generally superior to urinalysis. It was also noted that estimation of exposure intensity on an individual basis is scarcely possible even with blood analysis. Solvent concentration in whole blood was the same as that in serum in the case of the aromatics, except for styrene. It was higher in blood than in serum in the case of n-hexane, and lower in the cases of styrene and methanol.

Sex differences in the bioavailability of soil-adsorbed m-xylene in orally exposed rats

The bioavailability of soil-adsorbed m-xylene was assessed in male and female rats gavaged with an aqueous suspension of 14C-m-xylene alone or adsorbed to sandy or clay soil. Sex-related differences were observed in the rate and the amount of m-xylene-derived radioactivity absorbed and excreted in the presence of the soils. A higher peak plasma concentration of radioactivity was observed in females following treatment with sandy soil-adsorbed m-xylene. Further, sandy and clay soil-adsorbed chemicals demonstrated significantly longer absorption half-lives (t1/2), while sandy soil produced a shorter elimination t1/2 vs. m-xylene alone in female rats. Increased bioavailability of sandy soil-adsorbed m-xylene in females was evidenced by a significantly increased area under the plasma concentration time curve (AUC). Neither of the soils altered the maximum plasma concentration, the rate at which xylene-derived radioactivity was absorbed or eliminated, or the AUC in male rats. Fat contained the highest tissue concentration of xylene-derived radioactivity in all treatment groups of both sexes. Further, in all male and female treatment groups m-xylene was primarily metabolized and excreted in urine with methyl hippuric acid identified as the main urinary metabolite. Sandy soil slightly delayed urinary excretion in females while both soils increased expired air excretion in males compared to m-xylene alone. Methylhippuric acid was the main urinary metabolite in all groups.

Pharmacokinetic and metabolic disposition of p-chloro-m-xylenol (PCMX) in dogs

The pharmacokinetic and metabolic profile of p-chloro-m-xylenol (PCMX) was studied in healthy mongrel dogs after intravenous and oral administration of single doses of 200 and 2000 mg of PCMX, respectively. Calculation of pharmacokinetic parameters was based on compartmental and noncompartmental methods. The mean pharmacokinetic parameters of elimination half-life and mean residence time were 1.84 and 1.69 hr, respectively. The apparent volume of distribution at steady state was estimated to be 22.4 liters, and the plasma clearance was 14.6 liters/hr. The bioavailability of PCMX was 21%, indicating low absorption for this drug. PCMX's metabolite data show that a presystemic elimination process (first-pass effect) is also occurring. PCMX plasma concentrations after intravenous administration of 500-, 200-, and 100-mg doses were found to be proportional to the dose given, demonstrating that the pharmacokinetic profile of PCMX is linear over the dose range studied. Biotransformation studies showed that urinary excretion was not the major route for rapid elimination of unchanged PCMX and almost all material excreted in urine was associated with the conjugated species (glucuronides and sulfates). Statistical significant differences were not found (P greater than 0.05) between the percentages excreted in urine of PCMX and its conjugated metabolites after intravenous and oral administration. The percentages excreted in urine after iv and oral doses of unchanged PCMX were, respectively, 0.45 and 0.37; total conjugates, 46.3 and 43.3; sulfates, 38.1 and 33.2; and glucuronides, 8.2 and 10.2.