Commentary on "Penetration of benzene, toluene and xylenes contained in gasolines through human abdominal skin in vitro"

In this commentary we refer to the new data recently published by Adami et al. [Adami, G., Larese, F., Venier, M., Barbieri, P., Lo Coco, F., Reisenhofer, E., 2006. Penetration of benzene, toluene and xylenes contained in gasolines through human abdominal skin in vitro. Toxicol. In Vitro 20, 1321-1330], which we acknowledge as a reliable basis for the retrospective assessment of percutaneous benzene absorption at the workplace. The data from Adami et al. (2006) are supported by the literature and by a German approach for calculating the contribution of the dermal uptake of benzene to the total body burden. This knowledge is important for the judgment of leukaemia suspected to be an occupational disease.

Desorption kinetics of benzene in a sandy soil in the presence of powdered activated carbon

Desorption kinetics of benzene was investigated with a modified biphasic desorption model in a sandy soil with five different powdered activated carbon (PAC) contents (0, 1, 2, 5, 10% w/w) as sorbents. Sorption experiments followed by series dilution desorption were conducted for each sorbent. Desorption of benzene was successively performed at two stages using deionized water and hexane. Modeling was performed on both desorption isotherm and desorption rate for water-induced desorption to elucidate the presence of sorption-desorption hysteresis and biphasic desorption and if present to quantify the desorption-resistant fraction (q (irr)) and labile fraction (F) of desorption site responsible for rapid process. Desorption isotherms revealed that sorption-desorption exhibited a severe hysteresis with a significant fraction of benzene being irreversibly adsorbed onto both pure sand and PAC, and that desorption-resistant fraction (q (irr)) increased with PAC content. Desorption kinetic modeling showed that desorption of benzene was biphasic with much higher (4-40 times) rate constant for rapid process (k (1)) than that for slow process (k (2)), and that the difference in the rate constant increased with PAC content. The labile fraction (F) of desorption site showed a decreasing tendency with PAC. The experimental results would provide valuable information on remediation methods for soils and groundwater contaminated with BTEX.

Physiologically-based pharmacokinetic modeling of benzene in humans: a Bayesian approach

Benzene is myelotoxic and leukemogenic in humans exposed at high doses (>1 ppm, more definitely above 10 ppm) for extended periods. However, leukemia risks at lower exposures are uncertain. Benzene occurs widely in the work environment and also indoor air, but mostly below 1 ppm, so assessing the leukemia risks at these low concentrations is important. Here, we describe a human physiologically-based pharmacokinetic (PBPK) model that quantifies tissue doses of benzene and its key metabolites, benzene oxide, phenol, and hydroquinone after inhalation and oral exposures. The model was integrated into a statistical framework that acknowledges sources of variation due to inherent intra- and interindividual variation, measurement error, and other data collection issues. A primary contribution of this work is the estimation of population distributions of key PBPK model parameters. We hypothesized that observed interindividual variability in the dosimetry of benzene and its metabolites resulted primarily from known or estimated variability in key metabolic parameters and that a statistical PBPK model that explicitly included variability in only those metabolic parameters would sufficiently describe the observed variability. We then identified parameter distributions for the PBPK model to characterize observed variability through the use of Markov chain Monte Carlo analysis applied to two data sets. The identified parameter distributions described most of the observed variability, but variability in physiological parameters such as organ weights may also be helpful to faithfully predict the observed human-population variability in benzene dosimetry.

Penetration of benzene, toluene and xylenes contained in gasolines through human abdominal skin in vitro

Few studies are available in literature on the risk for humans from skin exposure to gasolines. This work is focused on the in vitro skin penetration of benzene (carcinogenic substance), toluene and xylenes. We examined three commercial gasolines using the Franz diffusion cells and human abdominal full thickness skin. Gasoline composition was determined using a multi-dimensional gas chromatographic (MDGC) technique. Aromatic compounds into the receptor fluid, consisting of saline solution were quantitated by a gas chromatography technique equipped with a flame ionization detector (GC-FID) and coupled with a headspace-solid phase micro extraction system (HS-SPME). Among the three substances, benzene showed the highest average apparent permeability coefficient (K(p)=43.8x10(-5)cmh(-1)) compared to toluene (K(p)=6.48x10(-5)cmh(-1)) and xylenes (K(p)=0.84x10(-5)cmh(-1)). This value could be explained by the lower boiling point and higher water solubility of benzene. Lag times were about 1h for benzene and 2h for toluene and xylenes. Averaged total recoveries in the receptor fluid were 0.43% of dose for benzene, 0.06% for toluene and 0.008% for xylenes. A statistical significative difference (Student's t-test, P<0.05) between the fluxes calculated for the three gasolines are noted only for xylene and for toluene between gasolines #1 (richer in aromatic compounds) and #3. The obtained apparent permeability coefficient are useful for determining the permeability of these aromatics components from gasolines of a different composition. Hands exposure risk, calculated using RfD and RfC as defined by US EPA, is critical for benzene. The risk of skin permeation of gasoline, and, in particular, of benzene, should be better evaluated for those workers who have a large potential for exposure. Adequate personal protective equipment should be used in the high exposure jobs, mainly for hands and forearms.

Measurements of exposure concentrations of benzene, toluene and xylene, and amounts of respiratory uptake

With respect to benzene, toluene, and o-, m- and p-xylene contained in indoor air, this study determined the amounts of their uptake through the human respiratory system using the difference between concentrations in inhalation and exhalation, and examined their relationship to concentrations in blood and urine measured before and after exposure. At relatively high concentrations, respiratory absorption of these compounds tended to increase rapidly in the early stage of exposure but decrease after several hours. It was also confirmed that concentrations of these compounds in both blood and urine increased during the first 3 hours of exposure. These results suggested that measurements of concentrations in inhalation and exhalation may provide a simple method for estimating the extent of respiratory exposure to these substances.

Adsorption and diffusion of benzene in the nanoporous catalysts FAU, ZSM-5 and MCM-22: A molecular dynamics study

Molecular dynamics (MD) simulations of benzene in siliceous zeolites (FAU, ZSM-5, and MCM-22) were performed at loadings of 1, 2, 4, 8, and 16 molecules per supercell. The potential energy functions for these simulations were constructed in a semi-empirical way from existing potentials and experimental energetic data. The MD simulations were employed to analyze the dynamic properties of the benzene-zeolite systems. The adsorption energies of benzene/siliceous zeolite complexes increase with increasing loading number, due to the intermolecular attraction between benzene molecules. The self-diffusion coefficient of benzene in siliceous zeolites decreases with increasing loading due to the steric hindrance between the sorbates passing each other. From the zeolite-benzene radial distribution functions it was found that the benzene molecules are relatively far from each other, about 5.2A for siliceous FAU, 5.2A for siliceous ZSM-5, and 4.8A for siliceous MCM-22. In the case of FAU, the benzene molecules prefer to be adsorbed parallel to the surface of the sodalite cage above the six-membered-ring. In ZSM-5, we found a T-structure of the benzene molecules at loadings 2, 4, and 8 molecules per supercell. At loadings of 16 molecules per supercell, the molecules are lined up along the straight channel and their movement is highly correlated. For MCM-22 we found adjacent benzene molecules at a loading of 4 molecules with an orientation similar to the stacked conformation of benzene dimer in the gas phase.

Unexpected cause of raised benzene absorption in coke oven by-product workers

BACKGROUND

Urinary biological monitoring for benzene (by measuring benzene metabolites) in coke oven by-product workers produced the unexpected result that 2 out of 10 employees had significantly raised urinary S-phenylmercapturic acid (S-PMA). However, simultaneous personal air sampling showed no excessive airborne exposure.

METHODS

Possible causes for this finding were investigated having excluded inhalation as the route of uptake. It was suspected that skin absorption via contaminated overalls was the possible mechanism and a standard frequency for overall change was introduced.

RESULTS

Changing overalls after every four shifts reduced uptake levels to less than the equivalent of 1 ppm inhaled dose for all employees.

CONCLUSION

Skin absorption of benzene in coke oven by-product workers from contaminated overalls can be significant and therefore overalls should be changed on a regular and frequent basis.

Modeling natural attenuation of benzene with analytical and numerical models

Natural attenuation of benzene is a much-accepted technology for remediating low risk sites. To date, numerous protocols have been developed for assessing natural attenuation and measuring indicator parameters. Many models have additionally been developed to describe the advection, dispersion, sorption and biodegradation processes involved. It is evident that while there is extensive guidance in natural attenuation protocols for field sampling methodologies, less emphasis is placed on analyzing natural attenuation data for supporting appropriate model development. This paper presents methodologies for data analysis and interpretation that may be undertaken to achieve data reduction for modeling purposes. A case study is presented to illustrate the use of an analytical and a numerical natural attenuation model at the same site for predicting the time required to achieve the remedial goal at the site.

Dermal exposure assessment to benzene and toluene using charcoal cloth pads

Charcoal cloth pads have been used to assess volatile chemicals on the skin in a laboratory setting; however, they have not yet been applied to measure dermal exposure in occupational settings. This study aimed at evaluating whether charcoal pads can be used to assess dermal exposure to benzene and toluene in workers of a petrochemical plant. Inhalation and dermal exposure levels to benzene and toluene were assessed for workers of a petrochemical plant performing different jobs. Benzene uptake was assessed by determining S-phenylmercapturic acid in workers' urine samples. Dermal exposure levels on the charcoal pads were adjusted for ambient air levels of benzene and toluene by subtracting the amount of benzene or toluene measured in personal air from the amount of benzene or toluene measured on the charcoal pad. In general, measured external and internal exposure levels were low. The estimated contribution of the dermal route to internal benzene exposure levels was less than 0.06% for all jobs. Toluene personal air concentrations and benzene and toluene dermal exposure levels differed statistically significantly between job titles. For benzene, differences between jobs were larger for adjusted dermal exposures (maximum 17-fold, P = 0.02) than for inhalation exposures (maximum two-fold, P = 0.08). Also for toluene, although less clear, differences between jobs were larger for adjusted dermal exposures (maximum 23-fold, P = 0.01) as compared to inhalation exposures (maximum 10-fold, P = 0.01). Charcoal pads appeared to measure dermal exposures to benzene and toluene in addition to ambient air levels. Future studies applying charcoal cloth pads for the dermal exposure assessment at workplaces with higher dermal exposure to organic solvents may provide more insight into the biological relevance of dermal exposure levels measured by charcoal cloth pads. In addition, the design of the dermal sampler might be improved by configuring a dermal sampler, where part of the sampler is protected against direct contact and splashes, but still permeable for the gas phase. This design would most likely result in a better ability to correct for airborne concentrations at a given body location.

On the Importance of Exposure Variability to the Doses of Volatile Organic Compounds

The connection between occupational exposure to volatile organic compounds (VOCs) and the resulting internal doses is complicated by variability in air levels from day to day and by nonlinear kinetics of metabolism. We investigated long-term liver doses of VOCs and their metabolites using a physiologically based toxicokinetic model, to which 10,000 random 8-h exposures were inputted. Three carcinogenic VOCs were studied (i.e., benzene, perchloroethylene, and acrylonitrile); these compounds are all bioactivated in the liver and represent a wide range of an important toxicokinetic parameter Vmax/QL x KM. For each VOC, simulations were performed using mean air concentrations (muX) between 0.0003 and 1 mg/l (which covers both linear and saturated metabolism) and using coefficients of variation of exposure (CVX) between 0.23 and 2.18 (which includes most occupational settings). Two long-term measures of internal dose were examined, i.e., the area under the liver concentration-time curve (AUCL) and the area under the metabolic rate-time curve (AURC). Interestingly, both AUCL and AURC were linear functions of cumulative exposure (CE, mg x h/l air) even when metabolism was saturated and CVX was large. Yet, at a given CE, both AUCL and AURC were affected by CVX, with the magnitude of the effect increasing with Vmax/QL x KM (i.e., perchloroethylene < benzene < acrylonitrile). Nonetheless, the effects of CVX were typically only a few percent and should be of little consequence unless a VOC has large values of Vmax/QL x KM, muX,and CVX. We conclude that CE should be a sufficient predictor of the dose of either the parent chemical (VOC) or its metabolite in the liver, even when metabolism is nonlinear. We also observed that AUCL and AURC were sensitive to changes in values of model parameters in the high-variability scenarios, suggesting that (when CVX is large) the population variability of AUCL and AURC can be quite large at a fixed CE.

Simultaneous determination of trans, transmuconic acid and s-phenylmercapturic acid by high pressure liquid chromatography and its application

The simultaneous determination of urinary trans,trans-muconic acid (t,t-MA) and S-phenylmercapturic acid (S-PMA) was performed by liquid extraction with ethyl acetate and reversed-phase high performance liquid chromatography (RP-HPLC) on a Hypersil-ODS column using the gradient mobile phase of methanol and 0.0012 N perchloric acid and diode array detection at 205 and 264 nm for S-PMA and t,t-MA, respectively. The retention times for t,t-MA and S-PMA were 3.8 and 12.3 minutes, respectively. The recoveries of t,t-MA and S-PMA were > 97%; between-day precisions were all within 8% RSD (100x SD/mean). The method was applied to analyze the urinary t,t-MA and S-PMA of 59 service station attendants exposed to average benzene concentrations in the air of 0.20+/-0.18 ppm. Significant differences in pre-shift and post-shift urinary t,t-MA between smokers and non-smokers were found.

Kinetic factors involved in the metabolism of benzene in mouse lung and liver

Benzene is an occupational and environmental toxicant. The main human health concern associated with benzene exposure is acute myelogenous leukemia. Benzene produces lung tumors in mice, while its effects on human lung are not clear. The adverse effects of benzene are dependent on its metabolism by the cytochrome P-450 enzyme system. The isozymes CYP2E1 and CYP2F2 play roles in the metabolism of benzene at low, environmentally relevant concentrations. Previous studies indicate that the mouse lung readily metabolizes benzene and that CYP2F2 plays a role in this biotransformation. The significance of CYP2E1 and CYP2F2 in benzene metabolism was determined by measuring their apparent kinetic parameters K(m) and V(max). Use of wild-type and CYP2E1 knockout mice and selective inhibitors allowed the determination of the individual importance of both CYP2E1 and CYP2F2 in mouse liver and lung. A simple Michaelis-Menten relationship involving Lineweaver-Burk plots for the microsomal metabolism of benzene shows the apparent kinetic factors are different between the wild-type (K(m): 30.4 microM, V(max): 25.3 pmol/mg protein/min) and knockout (K(m): 1.9 microM, V(max): 0.5 pmol/mg protein/min) mouse livers. Wild-type lung has a K(m) of 2.3 microM and V(max) of 0.9 pmol/mg protein/min. CYP2E1 knockout lung has similar affinity and metabolic activity with a K(m) of 3.7 microM and V(max) of 1.2 pmol/mg protein/min. These data suggest CYP2E1 is less important in the lung than liver, and that it has a lower affinity for benzene but higher rate of hydroxylated metabolite production than does CYP2F2, which plays the predominant role in metabolizing benzene in mouse lung.

Detection of Benzene, Toluene, Ethyl Benzene, and Xylenes (BTEX) Using Toluene Dioxygenase-Peroxidase Coupling Reactions

We have developed a simple, whole-cell bioassay for the detection of bioavailable benzene, toluene, ethyl benzene, and xylenes (BTEX) and similar compounds. A genetically engineered E. coli strain expressing toluene dioxygenase (TDO) and toluene dihydrodiol dehydrogenase (TodD) was constructed, enabling the conversion of BTEX into their respective catechols, which were quickly converted into colored products by a horseradish peroxidase (HRP)-coupled reaction. The intensity of the color formation was correlated to concentrations of the BTEX compounds. Under the optimized conditions, a detection limit (defined as three times the standard deviation of the response obtained from the blank) of 10, 10, 20, and 50 microM was observed for benzene, toluene, ethyl benzene, and xylene, respectively. The bioassay was selective toward BTEX-related compounds with no interference observed with commonly used pesticides, herbicides, and organic solvent. The bioassay was very stable with little change in response over a 10-week period. The excellent stability suggests that the reported bioassay may be suitable for field monitoring of BTEX to identify and track contaminated water and follow the bioremediation progress.

Assessing the dose-dependency of allometric scaling performance using physiologically based pharmacokinetic modeling

The performance of allometric scaling of dose as a power of body weight under a variety of extrapolation conditions with respect to species, route, exposure intensity, and mechanism/mode of action, remains untested in many cases. In this paper, animal-human internal dose ratio comparisons have been developed for 12 chemicals (benzene, carbon tetrachloride, chloroform, diisopropylfluorophosphate, ethanol, ethylene oxide, methylene chloride, methylmercury, styrene, tetrachloroethene, trichloroethene, and vinyl chloride). This group of predominantly volatile and lipophilic chemicals was selected on the basis that their kinetics have been well-studied and can be predicted in mice, rats, and humans using physiologically based pharmacokinetic (PBPK) models. PBPK model predictions were compared to the allometric scaling predictions for interspecies extrapolation. Recommendations for the application of the allometric scaling are made with reference to internal dose measure (mode of action) and concentration level. The results of this assessment generally support the use of scaling factors recommended in the published literature, which includes scaling factors of 1.0 for risk assessments in which toxicity is attributed to the parent chemical or stable metabolite, and -0.75 for dose-response assessments in which toxicity is attributed to the formation of a reactive metabolite from an inhaled compound. A scaling factor of 0.75 is recommended for dose-response assessments of orally administered compounds in which toxicity is attributed to the parent chemical or stable metabolite and 1.0 for risk assessments in which toxicity is attributed to the formation of a reactive metabolite from a compound administered by the oral route. A dose-dependency in the results suggests that the scaling factors appropriate at high exposures may differ from those at low exposures, primarily due to the impact of saturable metabolism.

Modeling VOCs adsorption onto activated carbon

The activated carbon adsorption process is affected by the characteristics of adsorbent, adsorbate and environmental conditions. In this study, both adsorption and desorption processes are assumed to occur simultaneously and a numerical model was developed with a non-linear driving force in conjunction with the Langmuir model for predicting the overall adsorption process. The numerical model provides both adsorption and desorption rate constants and activation energies. The resultant equilibrium constants are of the same order of magnitude as reported by other studies. Results show that the model could well predict the adsorption isotherms and breakthrough curves under various conditions.

Use of stable isotopically labeled benzene to evaluate environmental exposures

The use of stable, isotopically labeled compounds in controlled exposure experiments at environmentally relevant levels allows for the distinguishing of urinary metabolites associated with known exposure from background levels generally present in the urine. Exposures of volunteers to (13)C-benzene for 2 h at 40+/-10 p.p.b. were conducted after obtaining informed consent, and urinary phenol, catechol, hydroquinone and trans,trans- muconic acid were measured. Each isotopically labeled urinary metabolite was determined in the presence of significantly higher concentrations of the unlabeled metabolite. Following exposure, free and acid hydrolyzed phenol, acid hydrolyzed catechol and hydroquinone, and free trans,trans-muconic acid were determined by GC/MS. The percentage of trans,trans-muconic acid excreted was higher than reported following exposure at occupational levels. The use of isotopically labeled compounds has the potential to investigate the metabolism of common environmental contaminants for validation of toxicokinetic models and improve risk extrapolation from high concentration occupational exposures and animal studies to environmentally relevant pollutant levels.

Environmental benzene exposure assessment for parent-child pairs in Rouen, France

There is a lack of data on environmental benzene exposure in children. In this study, we compared personal benzene exposure and inhalation uptake in a group of children to those of their parents. We also compared levels of urinary benzene metabolites, trans,trans-muconic acid (MA) and hydroquinone (HQ), for those two groups, and assessed the correlation between personal benzene exposure and urinary MA and HQ concentrations. The study was performed on 21, 2-3-year-old children and their parents recruited on a voluntary basis among non-smokers from the three largest day-care centers of the town of Rouen in France. Average benzene concentrations were measured over 5 consecutive days with diffusive samplers. The following simultaneous measurements were carried out: personal exposure of the parents, concentrations inside and outside the day care centers, and inside the volunteer's bedrooms. Morning and evening urine samples were collected during the same period. Benzene personal exposure levels were 14.4+/-7.7 microg/m(3) and 11.09+/-6.15 microg/m(3) in parents and children, respectively. Benzene inhalation uptake estimates were 2.51+/-1.23 microg/kg/day in the group of parents and 5.68+/-3.17 microg/kg/day in the group of children. Detectable levels of MA and HQ were found in 85% and 100% of the samples, respectively. Intra-individual variation of urinary MA and HQ concentrations expressed as a coefficient of variation (CV) ranged from 63 to 232% and from 13 to 144%, respectively. Mean values of MA and HQ (in mg/g creatinine) were 1.6- and 1.8-fold higher in the group of children than in the group of parents (P=0.008 and P<0.0001, respectively). Significant correlations between metabolites levels and benzene were not found.

Male mice deficient in microsomal epoxide hydrolase are not susceptible to benzene-induced toxicity

Enzymes involved in benzene metabolism are likely genetic determinants of benzene-induced toxicity. Polymorphisms in human microsomal epoxide hydrolase (mEH) are associated with an increased risk of developing leukemia, specifically those associated with benzene. This study was designed to investigate the importance of mEH in benzene-induced toxicity. Male and female mEH-deficient (mEH-/-) mice and background mice (129/Sv) were exposed to inhaled benzene (0, 10, 50, or 100 ppm) 5 days/week, 6 h/day, for a two-week duration. Total white blood cell counts and bone marrow cell counts were used to assess hematotoxicity and myelotoxicity. Micronucleated peripheral blood cells were counted to assess genotoxicity, and the p21 mRNA level in bone marrow cells was used as a determinant of the p53-regulated DNA damage response. Male mEH-/- mice did not have any significant hematotoxicity or myelotoxicity at the highest benzene exposure compared to the male 129/Sv mice. Significant hematotoxicity or myelotoxicity did not occur in the female mEH-/- or 129/Sv mice. Male mEH-/- mice were also unresponsive to benzene-induced genotoxicity compared to a significant induction in the male 129/Sv mice. The female mEH-/- and 129/Sv mice were virtually unresponsive to benzene-induced genotoxicity. While p21 mRNA expression was highly induced in male 129/Sv mice after exposure to 100-ppm benzene, no significant alteration was observed in male mEH-/- mice. Likewise, p21 mRNA expression in female mEH-/- mice was not significantly induced upon benzene exposure whereas a significant induction was observed in female 129/Sv mice. Thus mEH appears to be critical in benzene-induced toxicity in male, but not female, mice.

Genetic susceptibility to benzene-induced toxicity: role of NADPH: quinone oxidoreductase-1

Enzymes that activate and detoxify benzene are likely genetic determinants of benzene-induced toxicity.NAD(P)H: quinone oxidoreductase-1 (NQO1) detoxifies benzoquinones, proposed toxic metabolites of benzene. NQO1 deficiency in humans is associated with an increased risk of leukemia, specifically acute myelogenous leukemia, and benzene poisoning. We examined the importance of NQO1 in benzene-induced toxicity by hypothesizing that NQO1-deficient (NQO1-/-) mice are more sensitive to benzene than mice with wild-type NQO1 (NQO1+/+; 129/Sv background strain). Male and female NQO1-/- and NQO1+/+ mice were exposed to inhaled benzene (0, 10, 50, or 100 ppm) for 2 weeks, 6 h/day, 5 days/week. Micronucleated peripheral blood cells were counted to assess genotoxicity. Peripheral blood counts and bone marrow histology were used to assess hematotoxicity and myelotoxicity. p21 mRNA levels in bone marrow cells were used as determinants of DNA damage response. Female NQO1-/- mice were more sensitive (6-fold) to benzene-induced genotoxicity than the female NQO1+/+ mice. Female NQO1-/- mice had a 9-fold increase (100 versus 0 ppm) in micronucleated reticulocytes compared with a 3-fold increase in the female NQO1+/+ mice. However, the induced genotoxic response in male mice was similar between the two genotypes (> or = 10-fold increase at 100 ppm versus 0 ppm). Male and female NQO1-/- mice exhibited greater hematotoxicity than NQO1+/+ mice. p21 mRNA levels were induced significantly in male mice (>10-fold) from both strains and female NQO1-/- mice (> 8-fold), which indicates an activated DNA damage response. These results indicate that NQO1 deficiency results in substantially greater benzene-induced toxicity. However, the specific patterns of toxicity differed between the male and female mice.

The use of non-tumor data in cancer risk assessment: reflections on butadiene, vinyl chloride, and benzene

The estimation and characterization of a cancer risk is grounded in the observation of tumors in humans and/or experimental animals. Increasingly, however, other kinds of data (non-tumor data) are finding application in cancer risk assessment. Metabolism and kinetics, adduct formation, genetic damage, mode of action, and biomarkers of exposure, susceptibility, and effects are examples. While these and other parameters have been studied for many important chemicals over the past 30-40 years, their use in risk assessments is more recent, and new insights and opportunities are continuing to unfold. To provide some perspective on this field, the ILSI Risk Science Institute asked a select working group to characterize the pertinent non-tumor data available for 1,3-butadiene, benzene, and vinyl chloride and to comment on the utility of these data in characterizing cancer risks. This paper presents the findings of that working group and concludes with 15 simple principles for the use of non-tumor data in cancer risk assessment.

Determination of dose coefficients and urinary excretion function for inhalation of carbon-14-labelled benzene

Based on existing pharmacokinetic models for benzene, the distribution and retention of activity after inhalation of 14C-labelled benzene in humans were studied. Six different benzene concentrations from 0.1 to 10,000 ppm (corresponding to activity concentrations between 9.6 x 10(6) and 9.6 x 10(11) Bq m(-3)) and five exposure times from 0.1 to 1000 min were considered. The cumulated activities in the different organs and tissues and the urinary excretion rates were observed to depend non-linearly on the activity intake. The fraction of activity removed via urine varies between 52 and 10% of the intake. Nevertheless, for times that are long compared to the exposure duration the urinary excretion rate is determined by the activity clearance from adipose tissue and thus decreases at a constant rate. This decrease is common for all exposure conditions examined and thus allowed determining a mean urinary excretion rate and corresponding dose coefficients for committed equivalent doses as well as for the effective dose. The uncertainty of the dose coefficients is estimated to be about 50% for the exposure range covered. A 14-day interval for the incorporation monitoring by urine activity counting seems to be reasonable.

Partition coefficients for benzene in human skin

The contribution of benzene to body burden after skin absorption compared with that due to inhalation absorption is of potential interest in the setting and interpretation of benzene (inhalation) exposure standards. However, an understanding of the quantitative relationship between skin and inhalation absorption, under different exposure conditions, is required. Such knowledge may be gained through physiological based pharmacokinetic (PBPK) modeling. The intake of benzene to the body via inhalation has been studied extensively. Physiological parameters enabling the calculation of amounts of benzene entering the blood stream per unit time are readily available for use in a PBPK model. Unfortunately, some data (i.e., partition coefficients) that would enable biologically plausible calculation of amounts of benzene entering the blood stream via skin absorption in a PBPK model are not available. Hence, the aim of this research was to determine partition coefficients across the epidermal and dermal layers of human skin so that these could be used within a PBPK model to determine quantitatively the flow rate of benzene per unit time through intact skin into the blood stream. The partition coefficients found for blood substitute: viable epidermis and blood substitute: dermis were, respectively, 2.4 and 11.2. Partition coefficients for benzene : stratum corneum (4.2), whole skin : blood substitute (2.2), benzene : water (109/126), and benzene : blood substitute (55/59) also were determined for the purposes of validating the blood substitute: viable epidermis and blood substitute : dermis partition coefficients.

Imaging Abeta plaques in living transgenic mice with multiphoton microscopy and methoxy-X04, a systemically administered Congo red derivative

The identification of amyloid deposits in living Alzheimer disease (AD) patients is important for both early diagnosis and for monitoring the efficacy of newly developed anti-amyloid therapies. Methoxy-X04 is a derivative of Congo red and Chrysamine-G that contains no acid groups and is therefore smaller and much more lipophilic than Congo red or Chrysamine-G. Methoxy-X04 retains in vitro binding affinity for amyloid beta (Abeta) fibrils (Ki = 26.8 nM) very similar to that of Chrysamine-G (Ki = 25.3 nM). Methoxy-X04 is fluorescent and stains plaques, tangles, and cerebrovascular amyloid in postmortem sections of AD brain with good specificity. Using multiphoton microscopy to obtain high-resolution (1 microm) fluorescent images from the brains of living PSI/APP mice, individual plaques could be distinguished within 30 to 60 min after a single i.v. injection of 5 to 10 mg/kg methoxy-X04. A single i.p. injection of 10 mg/kg methoxy-X04 also produced high contrast images of plaques and cerebrovascular amyloid in PSI/APP mouse brain. Complementary quantitative studies using tracer doses of carbon- 11-labeled methoxy-X04 show that it enters rat brain in amounts that suggest it is a viable candidate as a positron emission tomography (PET) amyloid-imaging agent for in vivo human studies.

Construction of a database of benzene biological monitoring

Biological monitoring of occupational exposure to benzene has been conducted in the petroleum, steel and chemical industries. The urinary benzene-specific biomarker, S-phenylmercapturic acid (PMA), was quantified in post-shift samples using a sensitive enzyme-linked immunosorbent assay (ELISA) and expressed as a function of urinary creatinine concentration. The assay, based on a PMA-specific antiserum, is sufficiently sensitive to measure PMA levels in non-occupationally exposed control subjects. The assay delivers batch results in a timely manner which may be as short as 3 h. Samples were analysed from groups of workers engaged in coke oven combustion processes, petroleum refining and decontamination of a benzene land spill. The construction of a database of results provides an index of benzene uptake as a consequence of the respective work processes and tasks and readily enables benchmarking exercises aimed at comparing degrees of exposure across segments of industry.