Physiologically based pharmacokinetics and the dermal absorption of 2-butoxyethanol vapor by humans

Suitability of the Limit Dose in Evaluating Reproductive Toxicity of Substances and Preparations

An oral dose of 1000 mg/kg body weight/day is mentioned in Organisation for Economic Cooperation and Development (OECD) and European Union (EU) guidelines as a default maximum dose in limit tests for studies on reproductive toxicity. This paper investigated whether upper range human exposure data from the workplace are supportive of this limit dose as an upper limit of possible human exposure. To this end, published exposure data as well as data from the database MEGA of the German “Berufsgenossenschaften” were evaluated. These data indicate that exposure concentrations in the range of 500 to 2000 mg/m3 (time-weighted averages) can be considered high human exposures to volatile compounds. Inhalation exposure to aerosols and dermal exposure result in lower dose levels. By applying suitable extrapolation factors, it was concluded that occupational exposures up to 325 mg/m3 can reliably be assessed with limit tests using a dose level of 1000 mg/kg/day. The limit dose has been proposed for use in the EU as a starting point to derive specific concentration limits for hazard classification of preparations containing reproductive toxicants, with the objective to consider the potency of the substances. This analysis shows that for some groups of chemicals, instead of the limit dose, the putative maximum levels of human exposure should be taken into account when deriving concentration limits for the classification of preparations. Furthermore, possible deviations from a linear correlation between concentration in the preparation and exposure should be considered.

Modeling Aggregate Exposures to Glycol Ethers from Use of Commercial Floor Products

Computer modeling of aggregate exposure provides the capability to estimate the range of doses that can occur from product use and to understand the relative importance of different routes of exposure. This paper presents an assessment of aggregate occupational exposure to two glycol ethers used as solvents in floor maintenance products for industrial and institutional facilities, using a simulation tool named PROMISE. Three commercial floor-care products were assumed to be applied in sequence—a floor stripper, then a floor cleaner, and lastly a protective coating. The glycol ethers modeled were ethylene glycol butyl ether (EGBE) in the floor stripper and in the floor cleaner, and dipropylene glycol methyl ether (DPGME) in the coating. Modeling uncertainty was assessed through a comparison of the PROMISE inhalation exposure estimates with those from an independent model (MCCEM), and parameter uncertainty was investigated using PROMISE software’s Monte Carlo simulation capabilities. Modeling results indicated that inhalation is the dominant exposure route. The predicted average air concentration and inhalation dose from PROMISE agreed with the second model (MCCEM) within 10%. Monte Carlo simulation indicated that the upper end of the aggregate-dose distribution for the scenario was more than 50% higher than the value of the point estimate. The modeled 8-h TWA concentrations for EGBE and DPGME were lower than the corresponding permissible exposure limits American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLV) by at least a factor of 20, indicating that under the assumed conditions work-place exposures to glycol ethers are below levels of concern.

Dermal uptake of organic vapors commonly found in indoor air

Transdermal uptake directly from air is a potentially important yet largely overlooked pathway for human exposure to organic vapors indoors. We recently reported (Indoor Air 2012, 22, 356) that transdermal uptake directly from air could be comparable to or larger than intake via inhalation for many semivolatile organic compounds (SVOCs). Here, we extend that analysis to approximately eighty organic compounds that (a) occur commonly indoors and (b) are primarily in the gas-phase rather than being associated with particles. For some compounds, the modeled ratio of dermal-to-inhalation uptake is large. In this group are common parabens, lower molecular weight phthalates, o-phenylphenol, Texanol, ethylene glycol, and α-terpineol. For other compounds, estimated dermal uptakes are small compared to inhalation. Examples include aliphatic hydrocarbons, single ring aromatics, terpenes, chlorinated solvents, formaldehyde, and acrolein. Analysis of published experimental data for human subjects for twenty different organic compounds substantiates these model predictions. However, transdermal uptake rates from air have not been measured for the indoor organics that have the largest modeled ratios of dermal-to-inhalation uptake; for such compounds, the estimates reported here require experimental verification. In accounting for total exposure to indoor organic pollutants and in assessing potential health consequences of such exposures, it is important to consider direct transdermal absorption from air.

Dermal absorption of chemicals: estimation by IH SkinPerm

This article describes the IH SkinPerm mathematical tool for estimating dermal absorption. The first part provides the scientific background of the IH SkinPerm model, including the QSARs and the developed differential equations. Then the practical value of the tool is demonstrated through example dermal absorption assessments for substances with skin notations. IH SkinPerm simulates three types of dermal absorption scenarios relevant to occupational environments. The first is dermal absorption from instantaneous splash type exposures onto bare skin for pure liquids. The second estimates dermal absorption from the deposition of pure liquids over time. The third enables estimation of dermal uptake from an airborne vapor concentration. A simulation with IH SkinPerm was made using vapor absorption data published from volunteer exposure studies. Comparison of measured and estimated dermal absorbed dose showed IH SkinPerm estimated dermal absorbed dose was within a factor of 3 compared to the reported study values. IH SkinPerm accounts for substance volatility and evaporated mass and provides real-time description of dermal absorption with graphical displays and numerical outputs. To assess absorption resulting from dermal exposure scenarios, the mass of the substance loaded onto the skin, substance physical chemical properties, exposure duration, and the skin surface area affected are the only required input parameters.

Case study illustrating the WHO IPCS guidance on characterization and application of physiologically based pharmacokinetic models in risk assessment

The World Health Organization (WHO) International Programme on Chemical Safety (IPCS) Guidance on Characterization and Application of Physiologically Based Pharmacokinetic Models in Risk Assessment (IPCS, 2010) describes key principles for risk assessors and model developers. In the WHO Guidance, a template for model documentation was developed and a case study included. Here the WHO Guidance, including the template, is summarized and an additional case study is presented to illustrate its application, based upon an existing risk assessment for 2-butoxyethanol (CAS NO. 111-76-2). The goal of the WHO Guidance and the current paper is to increase regulatory acceptance of complex biologically descriptive pharmacokinetic (or toxicokinetic) models, such as PBPK models, by facilitating communication and successful interaction between modelers and risk assessors.

Exposure of German residents to ethylene and propylene glycol ethers in general and after cleaning scenarios

Glycol ethers are a class of semi-volatile substances used as solvents in a variety of consumer products like cleaning agents, paints, cosmetics as well as chemical intermediates. We determined 11 metabolites of ethylene and propylene glycol ethers in 44 urine samples of German residents (background level study) and in urine samples of individuals after exposure to glycol ethers during cleaning activities (exposure study). In the study on the background exposure, methoxyacetic acid and phenoxyacetic acid (PhAA) could be detected in each urine sample with median (95th percentile) values of 0.11 mgL(-1) (0.30 mgL(-1)) and 0.80 mgL(-1) (23.6 mgL(-1)), respectively. The other metabolites were found in a limited number of samples or in none. In the exposure study, 5-8 rooms were cleaned with a cleaner containing ethylene glycol monobutyl ether (EGBE), propylene glycol monobutyl ether (PGBE), or ethylene glycol monopropyl ether (EGPE). During cleaning the mean levels in the indoor air were 7.5 mgm(-3) (EGBE), 3.0 mgm(-3) (PGBE), and 3.3 mgm(-3) (EGPE), respectively. The related metabolite levels analysed in the urine of the residents of the rooms at the day of cleaning were 2.4 mgL(-1) for butoxyacetic acid, 0.06 mgL(-1) for 2-butoxypropionic acid, and 2.3 mgL(-1) for n-propoxyacetic acid. Overall, our study indicates that the exposure of the population to glycol ethers is generally low, with the exception of PhAA. Moreover, the results of the cleaning scenarios demonstrate that the use of indoor cleaning agents containing glycol ethers can lead to a detectable internal exposure of residents.

Predicting the absorption of chemical vapours

The focus of this review is on the systemic absorption of vapours via skin, including experimental data as well as regression and pharmacokinetic models. Dermal contribution ratios (DCR), i.e. amount absorbed through skin relative to total intake (skin and inhalation) at specified conditions, could be identified or calculated from published data for 33 chemical vapours. The ratios vary from ~0.0002 (vinyl chloride) to ~0.8 (2-butoxyethanol), with hydrophilic chemicals having a higher ratio than lipophilic ones. Multiple regression analysis of these data suggests that the DCR is largely explained by the octanol:water partition coefficient, vapour pressure and molecular weight (R(2)=0.69). Several physiologically-based pharmacokinetic models were identified; however, all describe the absorption of single substances. Regarding predictive models, only two models were found. In conclusion, dermal uptake of chemical vapours needs more attention, as such exposures are common, data are scarce and few predictive models exist.

Development of PK- and PBPK-based modeling tools for derivation of biomonitoring guidance values

There are numerous programs ongoing to analyze environmental exposure of humans to xenobiotic chemicals via biomonitoring measurements (e.g.: EU ESBIO, COPHES; US CDC NHANES; Canadian Health Measures Survey). The goal of these projects is to determine relative trends in exposure to chemicals, across time and subpopulations. Due to the lack of data, there is often little information correlating biomarker concentrations with exposure levels and durations. As a result, it can be difficult to utilize biomonitoring data to evaluate if exposures adhere to or exceed hazard/exposure criteria such as the Derived No-Effect Level values under the EU REACH program, or Reference Dose/Concentration values of the US EPA. A tiered approach of simple, arithmetic pharmacokinetic (PK) models, as well as more standardized mean-value, physiologically-based (PBPK) models, have therefore been developed to estimate exposures from biomonitoring results. Both model types utilize a user-friendly Excel spreadsheet interface. QSPR estimations of chemical-specific parameters have been included, as well as accommodation of variations in urine production. Validation of each model's structure by simulations of published datasets and the impact of assumptions of major model parameters will be presented.

A generic, cross-chemical predictive PBTK model with multiple entry routes running as application in MS Excel; design of the model and comparison of predictions with experimental results

AIM

Physiologically based toxicokinetic (PBTK) models are computational tools, which simulate the absorption, distribution, metabolism, and excretion of chemicals. The purpose of this study was to develop a physiologically based pharmacokinetic (PBPK) model with a high level of transparency. The model should be able to predict blood and urine concentrations of environmental chemicals and metabolites, given a certain environmental or occupational exposure scenario.

MODEL

The model refers to a reference human of 70 kg. The partition coefficients of the parent compound and its metabolites (blood:air and tissue:blood partition coefficients of 11 organs) are estimated by means of quantitative structure-property relationship, in which five easily available physicochemical properties of the compound are the independent parameters. The model gives a prediction of the fate of the compound, based on easily available chemical properties; therefore, it can be applied as a generic model applicable to multiple compounds. Three routes of uptake are considered (inhalation, dermal, and/or oral) as well as two built-in exercise levels (at rest and at light work). Dermal uptake is estimated by the use of a dermal diffusion-based module that considers dermal deposition rate and duration of deposition. Moreover, evaporation during skin contact is fully accounted for and related to the volatility of the substance. Saturable metabolism according to Michaelis-Menten kinetics can be modelled in any of 11 organs/tissues or in liver only. Renal tubular resorption is based on a built-in algorithm, dependent on the (log) octanol:water partition coefficient. Enterohepatic circulation is optional at a user-defined rate. The generic PBTK model is available as a spreadsheet application in MS Excel. The differential equations of the model are programmed in Visual Basic. Output is presented as numerical listing over time in tabular form and in graphs. The MS Excel application of the PBTK model is available as freeware.

EXPERIMENTAL

The accuracy of the model prediction is illustrated by simulating experimental observations. Published experimental inhalation and dermal exposure studies on a series of different chemicals (pyrene, N-methyl-pyrrolidone, methyl-tert-butylether, heptane, 2-butoxyethanol, and ethanol) were selected to compare the observed data with the model-simulated data. The examples show that the model-predicted concentrations in blood and/or urine after inhalation and/or transdermal uptake have an accuracy of within an order of magnitude.

CONCLUSIONS

It is advocated that this PBTK model, called IndusChemFate, is suitable for 'first tier assessments' and for early explorations of the fate of chemicals and/or metabolites in the human body. The availability of a simple model with a minimum burden of input information on the parent compound and its metabolites might be a stimulation to apply PBTK modelling more often in the field of biomonitoring and exposure science.

Percutaneous absorption of vapors in human skin

CONTEXT

The absorption of vapors through the skin is an important issue because exposure of skin to chemicals in the ambient air occurs at all times. In regards to occupational health, accurately quantifying percutaneous absorption is crucial for worker health and safety.

OBJECTIVE

Review the available data regarding the percutaneous absorption of vapors in humans.

METHODS

We conducted a systematic search in Scopus(®) and PubMed using keywords "vapor" and "percutaneous absorption" up until September 23, 2010.

RESULTS

Eleven articles document the absorption of vapors in human skin in vivo. Seven articles utilized aromatic solvents including xylene and toluene, two tested 2-methoxyethanol, and two tested solely 2-butoxyethanol. Of the 11 articles, eight estimated the percentage of skin absorption compared with whole body exposure. Of the eight articles, four concluded that percutaneous absorption of aromatic solvent vapors from the air is likely to be insignificant and four concluded that dermal uptake of alcohol solvents caused significant absorption.

CONCLUSION

Skin absorption of vapors is an important and relevant topic that has not been studied extensively. Further investigation of percutaneous vapor absorption is needed to ensure safe usage of solvent vapors in the workplace, and possibly elsewhere.

Quantitative risk analysis for N-methyl pyrrolidone using physiologically based pharmacokinetic and benchmark dose modeling

Establishing an occupational exposure limit (OEL) for N-methyl pyrrolidone (NMP) is important due to its widespread use as a solvent. Based on studies in rodents, the most sensitive toxic end point is a decrease in fetal/pup body weights observed after oral, dermal, and inhalation exposures of dams to NMP. Evidence indicates that the parent compound is the causative agent. To reduce the uncertainty in rat to human extrapolations, physiologically based pharmacokinetic (PBPK) models were developed to describe the pharmacokinetics of NMP in both species. Since in utero exposures are of concern, the models considered major physiological changes occurring in the dam or mother over the course of gestation. The rat PBPK model was used to determine the relationship between NMP concentrations in maternal blood and decrements in fetal/pup body weights following exposures to NMP vapor. Body weight decrements seen after vapor exposures occurred at lower NMP blood levels than those observed after oral and dermal exposures. Benchmark dose modeling was used to better define a point of departure (POD) for fetal/pup body weight changes based on dose-response information from two inhalation studies in rats. The POD and human PBPK model were then used to estimate the human equivalent concentrations (HECs) that could be used to derive an OEL value for NMP. The geometric mean of the PODs derived from the rat studies was estimated to be 350 mg h/l (expressed in terms of internal dose), a value which corresponds to an HEC of 480 ppm (occupational exposure of 8 h/day, 5 days/week). The HEC is much higher than recently developed internationally recognized OELs for NMP of 10-20 ppm, suggesting that these OELs adequately protect workers exposed to NMP vapor.

Physicochemical and Biological Data for the Development of Predictive Organophosphorus Pesticide QSARs and PBPK/PD Models for Human Risk Assessment

A search of the scientific literature was carried out for physiochemical and biological data [i.e., IC50, LD50, Kp (cm/h) for percutaneous absorption, skin/water and tissue/blood partition coefficients, inhibition ki values, and metabolic parameters such as Vmax and Km] on 31 organophosphorus pesticides (OPs) to support the development of predictive quantitative structure-activity relationship (QSAR) and physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) models for human risk assessment. Except for work on parathion, chlorpyrifos, and isofenphos, very few modeling data were found on the 31 OPs of interest. The available percutaneous absorption, partition coefficients and metabolic parameters were insufficient in number to develop predictive QSAR models. Metabolic kinetic parameters (Vmax, Km) varied according to enzyme source and the manner in which the enzymes were characterized. The metabolic activity of microsomes should be based on the kinetic activity of purified or cDNA-expressed cytochrome P450s (CYPs) and the specific content of each active CYP in tissue microsomes. Similar requirements are needed to assess the activity of tissue A- and B-esterases metabolizing OPs. A limited amount of acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and carboxylesterase (CaE) inhibition and recovery data were found in the literature on the 31 OPs. A program is needed to require the development of physicochemical and biological data to support risk assessment methodologies involving QSAR and PBPK/PD models.

Approaches for applications of physiologically based pharmacokinetic models in risk assessment

Physiologically based pharmacokinetic (PBPK) models are particularly useful for simulating exposures to environmental toxicants for which, unlike pharmaceuticals, there is often little or no human data available to estimate the internal dose of a putative toxic moiety in a target tissue or an appropriate surrogate. This article reviews the current state of knowledge and approaches for application of PBPK models in the process of deriving reference dose, reference concentration, and cancer risk estimates. Examples drawn from previous U.S. Environmental Protection Agency (EPA) risk assessments and human health risk assessments in peer-reviewed literature illustrate the ways and means of using PBPK models to quantify the pharmacokinetic component of the interspecies and intraspecies uncertainty factors as well as to conduct route to route, high dose to low dose and duration extrapolations. The choice of the appropriate dose metric is key to the use of the PBPK models for the various applications in risk assessment. Issues related to whether uncertainty factors are most appropriately applied before or after derivation of human equivalent dose (or concentration) continue to be explored. Scientific progress in the understanding of life stage and genetic differences in dosimetry and their impacts on variability in susceptibility, as well as ongoing development of analytical methods to characterize uncertainty in PBPK models, will make their use in risk assessment increasingly likely. As such, it is anticipated that when PBPK models are used to express adverse tissue responses in terms of the internal target tissue dose of the toxic moiety rather than the external concentration, the scientific basis of, and confidence in, risk assessments will be enhanced.

On the incorporation of chemical-specific information in risk assessment

This paper describes the evolution of chemical risk assessment from its early dependence on generic default approaches to the current situation in which mechanistic and biokinetic data are routinely incorporated to support a more chemical-specific approach. Two methodologies that have played an important role in this evolution are described: mode-of-action evaluation and physiologically based biokinetic (PBBK) modelling. When used together, these techniques greatly increase the opportunity for the incorporation of biokinetic and mechanistic data in risk assessment. The resulting risk assessment approaches are more appropriately tailored to the specific chemical and are more likely to provide an accurate assessment of the potential hazards associated with human exposures. The appropriate application of PBBK models in risk assessment demands well-formulated statements about the chemical mode of action. It is this requirement for an explicit, mechanistic hypothesis that gives biologically motivated models their power, but at the same time serves as the greatest impediment to the acceptance of a chemical-specific risk assessment approach by regulators. The chief impediment to the regulatory acceptance and application of PBBK models in risk assessment is concern about uncertainties associated with their use. To some extent such concerns can be addressed by the development of generally accepted approaches for model evaluation and quantitative uncertainty analysis. In order to assure the protection of public health while limiting the economic and social consequences of over-regulation, greater dialogue between researchers and regulators is crucially needed to foster an increased use of emerging scientific information and innovative methods in chemical risk assessments.

Biomonitoring as a tool in the human health risk characterization of dermal exposure

Dermal exposure is an important factor in risk characterization. In occupational settings it becomes relatively more important because of the continuous reduction in inhalation exposure. In the public health arena, dermal exposure may also form a significant contribution to the total exposure. Dermal exposure, however, is difficult to assess directly because it is determined by a host of factors, which are difficult to quantify. As a consequence, dermal exposure is often estimated by application of models for external exposure. In combination with modeled or measured data for percutaneous penetration, these provide an estimate for the internal exposure that is directly related to the systemic effects. The advantages and drawbacks of EASE (Estimation and Assessment of Substance Exposure) and RISKOFDERM (Risk Assessment of Occupational Dermal Exposure), two models for external exposure that are mentioned in the Technical Guidance Document for the European Union risk assessments performed under the Existing Substances Regulation (EEC/793/93), are discussed. Although new chemicals regulation (REACh, 1907/2006/EC) is now in place in the European Union, the principles applied under the previous legislation do not change and the same models will continue to be used. The results obtained with these models for styrene, 2-butoxyethanol, and 1-methoxy-2-propanol in specific exposure scenarios are compared with an alternative method that uses biomonitoring data to assess dermal exposure. Actual external exposure measurements combined with measured or modeled percutaneous penetration data give acceptable results in risk assessment of dermal exposure, but modeled data of external dermal exposure should only be used if no other data are available. However, if available, biomonitoring should be considered the method of choice to assess (dermal) exposure.

The application of non-default uncertainty factors in the U.S. EPA's Integrated Risk Information System (IRIS). Part I: UF(L), UF(S), and "other uncertainty factors"

The United States Environmental Protection Agency's Integrated Risk Information System (IRIS) includes hazard identification and dose-response assessment values developed by Agency scientists. Uncertainty factors (UFs) are used in the development of IRIS values to address the lack of information in five main areas. The standard UFs account for interspecies uncertainty (UF(A)) and intraspecies variability (UF(H)). The UF(A) addresses uncertainty related to the extrapolation of data from animals to humans, whereas the UF(H) addresses variability amongst individuals (i.e., intrahuman). Additional UFs have been employed to account for database incompleteness, extrapolations from a lowest-observed-adverse-effect level in the absence of a no-observed-adverse-effect level (UF(L)), and subchronic-to-chronic extrapolation (UF(S)). A sixth UF designated as "other uncertainty factors" (UF(O)) has also been applied in place of the UF(L) to account for uncertainty with the adversity of points of departure obtained using benchmark dose modeling. This review will discuss how UF(L), UF(S), and UF(O) have been applied in IRIS assessments, along with the rationale used to describe the choice of UF values that deviate from the standard default of 10.

Physiologically based pharmacokinetic modelling of human exposure to 2-butoxyethanol

A physiologically based pharmacokinetic (PBPK) model describing the disposition of 2-butoxyethanol (2-BE) was developed in order to predict the urinary concentration of its major metabolite, butoxyacetic acid (BAA) under a range of exposure scenarios. Based on Corley et al. [Corley, R.A., Bormett, G.A., Ghanayem, B.I., 1994. Physiologically based pharmacokinetics of 2-butoxyethanol and its major metabolite, 2-butoxyacetic acid, in rats and humans. Toxicol. Appl. Pharmacol. 129, 61-79], the model included such features as multiple entry routes into the body, varying workload conditions, metabolism in the liver and elimination of free BAA in urine by glomerular filtration and acid transport. A bladder compartment simulating the fluctuations in metabolite concentration in urine caused by micturition formed a novel aspect of the model. Good agreement between model predictions and existing experimental data of total BAA levels in the blood and urine over various exposure conditions were observed. The mechanistically based PBPK model allowed comparison of disparate studies and also enabled the prediction of urinary concentrations of BAA post-shift. By calculating the total amount of BAA, any inter-individual variability in conjugation is taken into account. This led us to conclude that a biological monitoring guidance value should be proposed for total rather than free BAA with a value of 250 mmol/mol of creatinine (post-shift), based on an 8h exposure to 25 ppm 2-BE at resting working conditions.

U.S. EPA's IRIS assessment of 2-butoxyethanol: the relationship of noncancer to cancer effects

U.S. EPA's integrated risk information system (IRIS) assessment of 2-butoxyethanol (EGBE) indicates that the human carcinogenic potential of EGBE cannot be determined at this time, but that "suggestive evidence" for cancer exists from laboratory animal studies (hemangiosarcoma of the liver in male mice and forestomach squamous cell papilloma or carcinoma in female mice [National Toxicology Program (NTP), 2000a. Toxicology and carcinogenesis studies of 2-butoxyethanol (CAS no. 111-76-2) in F344/N rats and B6C3F1 mice (inhalation studies). National Toxicology Program Technical Report Series No. 484. U.S. Department of Health and Human Services, National Institutes of Health, Washington, DC]). Since the last EGBE IRIS assessment, a number of studies have provided evidence that the carcinogenic effects observed in mice are nonlinear in their mode of action and may be dependent on threshold events such as EGBE-induced hemolytic effects. EPA is in the process of considering several questions relating to this issue. First, can a plausible mode of action be determined for the two types of tumors observed in mice? Second, are the mechanisms involved applicable to humans? If so, should the mode of action be considered to result in a linear or nonlinear dose-response? These questions will be addressed within the context of the agency's new cancer guidelines and with regard to how the answers might affect a revised IRIS assessment for EGBE.

International industry initiatives to improve the glycol ether health effects knowledge base

Following the recognition in the early 1980s of the potential reproductive hazards of certain glycol ethers, industry organizations were formed in the US and Europe having a number of stated goals to: (1) provide hazard information by expanding the toxicity database for glycol ethers; (2) promote cooperation among scientists, governmental authorities, and industry; and (3) promote scientifically sound regulatory actions and to assist in the setting of scientifically defensible safety standards. This effort led to early recommendations that EGME, EGEE, and their acetates be removed from consumer products. Also, studies conducted by industry under US EPA test rules have led to a better understanding of the hazards associated with glycol ether constituents of brake fluids, paints, and other products. Industry-provided information has greatly assisted the setting of occupational and public safety standards in a number of countries. Hazard assessments for a number of large-volume glycol ethers have been performed under the OECD SIDS program. This work continues with the industry-funded ICCA/HPV testing initiative. To provide sound risk assessment data, industry continues to sponsor basic research aimed at better understanding human versus mouse versus rat sensitivities to certain glycol ethers. Industry has also prepared and supported the publication of toxicological data compendia for glycol ethers.

Development of a physiologically based pharmacokinetic model for propylene glycol monomethyl ether and its acetate in rats and humans

Propylene glycol monomethyl ether (PM), along with its acetate, is the most widely used of the propylene glycol ether family of solvents. The most common toxic effects of PM observed in animal studies include sedation, very slight alpha(2u)-globulin mediated nephropathy (male rats only) and hepatomegally at high exposures (typically > 1000 ppm). Sedation in animal studies usually resolves within a few exposures to 3000 ppm (the highest concentration used in subchronic and chronic inhalation studies) due to the induction of metabolizing enzymes. Data from a variety of pharmacokinetic and mechanistic studies have been incorporated into a PBPK model for PM and its acetate in rats and mice. Published controlled exposure and workplace biomonitoring studies have also been included for comparisons of the internal dosimetry of PM and its acetate between laboratory animals and humans. PM acetate is rapidly hydrolyzed to PM, which is further metabolized to either glucuronide or sulfate conjugates (minor pathways) or propylene glycol (major pathway). In vitro half-lives for PM acetate range from 14 to 36 min depending upon the tissue and species. In vivo half-lives are considerably faster, reflecting the total contributions of esterases in the blood and tissues of the body, and are on the order of just a few minutes. Thus, very little PM acetate is found in vivo and, other than potential portal of entry irritation, the toxicity of PM acetate is related to PM. Regardless of the source for PM (either PM or its acetate), rats were predicted to have a higher Cmax and AUC for PM in blood than humans, especially at concentrations greater than the current ACGIH TLV of 100 ppm. This would indicate that the major systemic effects of PM would be expected to be less severe in humans than rats at comparable inhalation exposures.

Determination of age and gender differences in biochemical processes affecting the disposition of 2-butoxyethanol and its metabolites in mice and rats to improve PBPK modeling

2-Butoxyethanol (BE) is the most widely used glycol ether solvent. BEs major metabolite, butoxyacetic acid (BAA), causes hemolysis with significant species differences in sensitivity. Several PBPK models have been developed over the past two decades to describe the disposition of BE and BAA in male rats and humans to refine health risk assessments. More recent efforts by Lee et al. [Lee, K.M., Dill, J.A., Chou, B.J., Roycroft, J.H., 1998. Physiologically based pharmacokinetic model for chronic inhalation of 2-butoxyethanol. Toxicol. Appl. Pharmacol. 153, 211-226] to describe the kinetics of BE and BAA in the National Toxicology Program (NTP) chronic inhalation studies required the use of several assumptions to extrapolate model parameters from earlier PBPK models developed for young male rats to include female F344 and both sexes of B6C3F1 mice and the effects of aging. To replace these assumptions, studies were conducted to determine the impact of age, gender and species on the metabolism of BE, and the tissue partitioning, renal acid transport and plasma protein binding of BAA. In the current study, the Lee et al. PBPK model was updated and expanded to include the further metabolism of BAA and the salivary excretion of BE and BAA which may contribute to the forestomach irritation observed in mice in the NTP study. The revised model predicted that peak blood concentrations of BAA achieved following 6 h inhalation exposures are greatest in young adult female rats at concentrations up to 300 ppm. This is not the case predicted for old (> or =18 months) animals, where peak blood concentrations of BAA in male and female mice were similar to or greater than female rats. The revised model serves as a quantitative tool for integrating an extensive pharmacokinetic and mechanistic database into a format that can readily be used to compare internal dosimetry across dose, route of exposure and species.

Use of mode of action in risk assessment: past, present, and future

The evolution of chemical risk assessment has been marked by a steadily increasing expectation for the use of chemical-specific dosimetric and mechanistic information to tailor the risk assessment approach. The information to be used can range from the broad physical properties of the chemical to detailed information on the mechanism by which it causes a particular toxic outcome, and the risk assessment decisions effected can in turn range from how to define equivalent exposures across species to whether a particular animal outcome is relevant to a human health assessment. A concept that has proven useful in support of these considerations is the "mode of action," a term coined by the USEPA in their new guidelines for carcinogen risk assessment. This paper describes the increasing use of mode-of-action considerations in risk assessment, beginning with early examples involving quantitative dosimetry on the one hand, and qualitative relevance on the other, which foreshadowed the current interest in mode of action. It then describes more recent developments regarding the use of the mode-of-action concept for the selection of a low-dose extrapolation approach, for harmonization of cancer and noncancer risk assessment approaches, and for cross-chemical evaluations. Finally, examples of recent controversies associated with the use of mode-of-action information in risk assessment are provided to demonstrate the challenges that must be overcome to assure the continued viability of the mode-of-action approach.

Is the permeability coefficient Kp a reliable tool in percutaneous absorption studies?

In percutaneous absorption studies the potency of penetration of chemical substances is often described by the permeability coefficient Kp. The experimentally determined Kp is characterized according to Fick's first law of diffusion by the ratio of flux and the concentration of the test compound (Kp=Flux/C). This equation implies that in percutaneous absorption studies Kp is theoretically a more reliable parameter than flux taking the concentration into consideration, and should remain constant for each compound independent from the grade of dilution. In our study we evaluated the course of the percutaneous absorption parameters flux and Kp of neat and of 50% aqueous solution of 2-butoxyethanol (BE). An infinite dose of neat and 50% aqueous solution of non-radiolabeled BE were applied on excised human skin from two donors in static diffusion cells in parallel (for each test setting n=21). The flux of 50% aqueous BE (0.704+/-0.152 mg/cm2/h) was about 15-fold higher than that of neat BE (0.045+/-0.014 mg/cm2/h). The comparison of the Kp values of both test settings showed with a factor of about 31 (Kp=1.563 x 10(-3) cm/h) much higher values for 50% aqueous BE and Kp=0.050 x 10(-3) cm/h for neat BE. Although the flux does not consider the chemical concentration, it showed a smaller difference in both test settings as Kp; however, the flux remains a non-specific parameter for the description of percutaneous absorption. The results of our experiments showed that the permeability coefficient Kp was not able to adjust the flux of BE to the concentration. This is in agreement with the evaluation of Kp from BE data described in the literature.

Free and total urinary 2-butoxyacetic acid following dermal and inhalation exposure to 2-butoxyethanol in human volunteers

OBJECTIVES

To assess excretion kinetics of free and total (free + conjugated) 2-butoxyacetic acid (BAA) following dermal and inhalation exposure to butoxyethanol (BE).

METHODS

Six male volunteers were dermally exposed for 4 h to a 50% aqueous solution of BE on an area of 40 cm(2) of the volar forearm. Six other male volunteers were exposed by inhalation (mouth only) to 93 mg m(-3) BE for 30 min. As biological indices of exposure, BE in blood and total and free BAA in urine were measured.

RESULTS

Following inhalation exposure, the 24-h cumulative excretion of free and total BAA in urine amounted to 5.5 +/- 2.7 and 12.8 +/- 4.0 mg, respectively. After dermal exposure, 147.1 +/- 61.0 and 346 +/- 52 mg, respectively, of free and total BAA were excreted in urine up to 48 h after the onset of exposure. The proportion of conjugated BAA in single urine samples increased after dermal exposure in time from 45+/-30% in the first collection period to 92+/-2% after 48 h. The elimination half-life of total BAA following dermal exposure was longer than that of free BAA (5.1 +/- 0.6 and 3.8 +/- 0.4 h, respectively). The interindividual variation in the cumulative excreted amount after inhalatory exposure was higher (49%) for free BAA than for total BAA (31%). The average dermal flux amounted to 3.5 mg cm(-2) h(-1) independently of whether free or total BAA was used for the calculation, and, again, the interindividual variation in the estimated fluxes was higher for free BAA than for total BAA (41% and 15%, respectively).

CONCLUSION

The interindividual variation in the extent of conjugation is large, and the degree of conjugation increases with time. Due to lower interindividual variability, total BAA is superior to free BAA as a biomarker of exposure.

A human exposure study to investigate biological monitoring methods for 2-butoxyethanol

2-Butoxyethanol is a glycol ether widely used in printing inks, varnishes and cleaning fluids. As skin absorption can be significant, biological monitoring is useful in monitoring worker exposure. A number of analytes and matrices have been used previously, including 2-butoxyethanol in blood and free and total 2-butoxyacetic acid in urine. Using a combination of a volunteer study and samples from exposed workers, we compared the applicability of some of the biological monitoring markers available. We conclude that 2-butoxyethanol in blood is not a suitable marker for biological monitoring due to sampling problems. In view of the low-level exposures reported in occupational surveys, 2-butoxyethanol in breath is also unsuitable because of a lack of sensitivity. Measuring 2-butoxyacetic acid in blood is possible, although non-invasive urine samples are preferred. Free 2-butoxyacetic acid in urine has previously been widely used; however, we found that the extent of conjugation of 2-butoxyacetic acid in urine varied from 0 to 100% both within and between individuals and is not related to time, concentration or urine pH. Data from 48 exposed workers suggested that an estimated 57% (95% confidence interval 44-70%) of the total 2-butoxyacetic acid is excreted in the conjugated form, and that conjugation may be activated above a certain exposure level. Using total 2-butoxyacetic acid significantly reduced inter-individual variation. Elimination half-lives for free and total 2-butoxyacetic acid were similar ( approximately 6 h) and there was no delay in excretion of the conjugated metabolite (peak excretion for both free and total was between 6 and 12 h after the end of exposure). In conclusion, we propose that total butoxyacetic acid (after acid hydrolysis) in urine is the biomarker of choice for monitoring exposure to 2-butoxyethanol. Urine samples should be collected post-shift towards the end of the working week.

Amino acid pharmacokinetics and safety assessment

Tracer kinetic studies of amino acid metabolism during periods of high amino acid intake should allow insights into adaptive or maladaptive regulatory mechanisms controlling amino acid catabolic or disposal events before clinically evident effects. The principles of amino acid tracer kinetics have been well defined, but their application to establishing upper safe intake levels has been essentially nonexistent. Similarly, the pharmacology field has well-established disciplines of toxicokinetics (the relationship of toxicant dose and delivery to its site of action) and toxicodynamics (the relationship of toxicant at its site of action and downstream functional consequences), but these principles have not been transferred to the field of amino acid metabolism. In this context, a theoretical framework is presented for tracer kinetic experiments to help establish upper tolerable levels of amino acid infusion and/or ingestion. In addition, experiments to couple specific amino acid intake levels with their consequent physiological dynamic effects are suggested to lead to the construction of benefit-risk curves that may permit definition of safe amino acid intake ranges for the population.

Methods for assessing risks of dermal exposures in the workplace

The skin as a route of entry for toxic chemicals has caused increasing concern over the last decade. The assessment of systemic hazards from dermal exposures has evolved over time, often limited by the amount of experimental data available. The result is that there are many methods being used to assess safety of chemicals in the workplace. The process of assessing hazards of skin contact includes estimating the amount of substance that may end up on the skin and estimating the amount that might reach internal organs. Most times, toxicology studies by the dermal route are not available and extrapolations from other exposure routes are necessary. The hazards of particular chemicals can be expressed as "skin notations", actual exposure levels, or safe exposure times. Characterizing the risk of a specific procedure in the workplace involves determining the ratio of exposure standards to an expected exposure. The purpose of this review is to address each of the steps in the process and describe the assumptions that are part of the process. Methods are compared by describing their strengths and weaknesses. Recommendations for research in this area are also included.

PBPK modeling of the percutaneous absorption of perchloroethylene from a soil matrix in rats and humans

Perchloroethylene (PCE) is a widely used volatile organic chemical. Exposures to PCE are primarily through inhalation and dermal contact. The dermal absorption of PCE from a soil matrix was compared in rats and humans using real-time MS/MS exhaled breath technology and physiologically based pharmacokinetic (PBPK) modeling. Studies with rats were performed to compare the effects of loading volume, concentration, and occlusion. In rats, the percutaneous permeability coefficient (K(P)) for PCE was 0.102 +/- 0.017, and was independent of loading volume, concentration, or occlusion. Exhaled breath concentrations peaked within 1 h in nonoccluded exposures, but were maintained over the 5 h exposure period when the system was occluded. Three human volunteers submerged a hand in a container of PCE-laden soil for 2 h and their exhaled breath was continually monitored during and for 2.5 h following exposure. The absorption and elimination kinetics of PCE were slower in these subjects than initially predicted based upon the PBPK model developed from rat dermal kinetic data. The resulting K(P) for humans was over 100-fold lower than for the rat utilizing a single, well-stirred dermal compartment. Therefore, two additional PBPK skin compartment models were evaluated: a parallel model to simulate follicular uptake and a layered model to portray a stratum corneum barrier. The parallel dual dermal compartment model was not capable of describing the exhaled breath kinetics, whereas the layered model substantially improved the fit of the model to the complex kinetics of dermal absorption through the hand. In real-world situations, percutaneous absorption of PCE is likely to be minimal.

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.

Comparison of the acute hematotoxicity of 2-butoxyethanol in male and female F344 rats

Administration of 2-butoxyethanol (BE) to rodents causes acute hemolytic anemia, and metabolic activation of BE to butoxyacetic acid (BAA) is required for the development of this effect. Recent studies have shown that female rats treated with BE exhibit a variety of histopathologic lesions that are absent in males and many of these lesions are attributed to the hemolytic effects of BE. Current studies were designed to compare the acute hematotoxicity of BE in male and female F344 rats. Rats were treated with 250 mg BE/kg body weight or water (control; 5 ml/kg) by gavage. At 4, 8, or 24 h after dosing, rats were anesthetized, blood was collected by cardiac puncture, and various blood parameters were measured. BE resulted in a time-dependent swelling of erythrocytes as evidenced by an early increase in hematocrit (Hct) and mean cell volume (MCV) in male rats. In contrast, increased Hct in female rats did not accompany an increase in MCV. It is likely that hemolysis was so severe at 4 h that Hct exhibited a decline in female rats at that time point. Subsequently, red blood cell (RBCs), hemoglobin concentration (Hgb), and Hct declined as hemolysis progressed. However, the onset of BE-induced hemolysis was faster in female compared to male rats. These effects were also associated with a significant increase in the spleen weight to body weight ratio. Blood smears were also prepared and morphological changes evaluated by light microscopy included stomatocytosis, spherocytosis, and schistocytosis. Furthermore, aggregation of RBCs in female rats as evidenced by increased formation of rouleaux was observed at 24 h after BE administration. These effects were observed earlier and more frequently in female rats. No differences in the sensitivity of RBCs obtained from male and female rats and exposed to butoxyacetic acid (BAA) in vitro was observed as determined by measuring the packed cell volume. In conclusion, these data suggest that female rats are more sensitive to hemolysis and morphological alterations of erythrocytes induced by BE during the first 24 h after exposure compared to males. It is likely that the greater sensitivity of female rats to BE effects on RBCs may account for the reported development of thrombosis and tissue infarction in female rats.

Improved method to measure urinary alkoxyacetic acids

OBJECTIVES

To simplify the current preparation of samples, and to improve the specificity and reliability of the conventional analytical methods to measure urinary alkoxyacetic acids.

METHODS

Samples containing alkoxyacetic acids including methoxy, ethoxy, and butoxyacetic acids (MAA, EAA, and BAA) were acidified with HCl and extracted with a mixed solvent of methylene chloride and isopropyl alcohol, then analysed by gas chromatography/mass spectrometry (GC/MS).

RESULTS

Optimal results were obtained when pH was 1.05-1.45, the ratio of methylene chloride and isopropyl alcohol was 2:1, and when extraction time was 10 minutes. Over the concentration range 0.3-200 micrograms/ml, MAA, EAA, and BAA could be determined with a pooled coefficient of variation (nine concentrations, six replicate samples) of 5.55%, 6.37%, and 6.41%, respectively. Urine samples were stable for at least 5 months and 3 freeze-thaw cycles at -20 degrees C. The limits of detection of MAA, EAA, and BAA were 0.055, 0.183, and 0.009 microgram/ml, respectively. The matrix effect of urine samples was negligible for MAA and EAA, but were marginally significant for BAA. The average recoveries of alkoxyacetic acids were 99%-101%. In urine samples MAA from 15 exposed workers showed a strong linear correlation (r = 0.999, slope = 1.01) between the new GC/MS method and Sakai's GC method.

CONCLUSIONS

The simplified non-derivatisation pretreatment of samples coupled with GC/MS can provide a specific, sensitive, simple, safe, and reliable method for the biological monitoring of occupational exposure of ethylene glycol ethers.

Physiologically based pharmacokinetic model for chronic inhalation of 2-butoxyethanol

2-Butoxyethanol (2BE) is used extensively in the production of cleaning agents and as a general solvent. It is primarily metabolized in the liver to 2-butoxyacetic acid (2BAA), which is excreted in urine. The objective of this study was to develop a physiologically based pharmacokinetic (PBPK) model describing the toxicokinetic behavior of 2BE and 2BAA in different species following repeated, long-term exposures. The PBPK model was first developed for short-term 2BE exposure to male rats. Allometric scaling was employed to estimate physiological and biochemical model parameters based on body weight. To accommodate differences in 2BE toxicokinetics in female rats, a higher Vmax for 2BE metabolism to 2BAA, higher plasma protein binding sites for 2BAA, and lower Vmax for 2BAA excretion through the kidney were incorporated into the model. For mice, a higher Vmax for 2BE metabolism to 2BAA for both sexes and higher plasma protein binding sites for 2BAA for female mice were also incorporated into the model. Subsequently, the model was expanded to simulate 2BE and 2BAA toxicokinetics for long-term, repeated exposures by incorporating time-dependent changes in model parameters. To reflect physiological/biochemical changes in animals during a chronic exposure, parameters for cardiac output, body composition, metabolic capacity, protein binding, or capacity of renal excretion were adjusted over time depending on species and sex. Sensitivity analysis was performed to better understand how sensitive model responses were to uncertainties in input parameters. The resulting PBPK model was used to simulate toxicokinetic data acquired during a 2-year inhalation toxicity and carcinogenicity study in male and female F344/N rats and B6C3F1 mice.

Toxicokinetics of inhaled 2-butoxyethanol and its major metabolite, 2-butoxyacetic acid, in F344 rats and B6C3F1 mice

2-Butoxyethanol (2BE) is used extensively in the production of cleaning agents and solvents. It is primarily metabolized in the liver to 2-butoxyacetic acid (2BAA), which is believed to be responsible for 2BE toxicities associated with hemolysis of red blood cells. The objective of the study was to characterize the systemic disposition of 2BE and 2BAA in rats and mice during 2-year 2BE inhalation toxicity studies. Male and female F344 rats and B6C3F1 mice (6-7 weeks old) were exposed to target 2BE concentrations of 0, 31.2, 62.5, or 125 ppm (rats), or 0, 62.5, 125, or 250 ppm (mice), by whole-body inhalation for 6 h/day, 5 days/week for up to 18 months. Postexposure blood samples were collected after 1 day, 2 weeks, and 3, 6, 12, and 18 months of exposure. Postexposure 16-h urine samples were collected after 2 weeks and 3, 6, 12, and 18 months of exposure. A separate set of mice was kept in the control chamber and exposed to 2BE for 3 weeks when they were approximately 19 months old. Postexposure blood samples were collected after 1 day and 3 weeks of exposure and 16-h urine samples were collected after 2 weeks of exposure from these aged mice. Blood samples were analyzed for both 2BE and 2BAA and urine samples were analyzed for 2BAA using GC/MS, and their kinetic parameters were estimated through the curve-fitting method using SAS. Systemically absorbed 2BE was rapidly cleared from blood (t1/2-RAT < 10 min; t1/2-MOUSE < 5 min after the 1-day exposure) independent of exposure concentration. Proportional increases in AUC2BE relative to increases in exposure concentration indicated linear 2BE kinetics. In contrast, the rate of 2BAA elimination from blood decreased as the exposure concentration increased. Nonproportional increases in AUC2BAA also indicated that 2BAA is eliminated following dose-dependent, nonlinear kinetics. Overall, mice eliminated both 2BE and 2BAA from blood faster than rats. Sex-related differences in 2BAA elimination were most significant with rats, in that females were less efficient in clearing 2BAA from the blood. Differences in renal excretion of 2BAA are possibly responsible for the sex-related difference in the 2BAA blood profiles in rats. As exposure continued, the rates of elimination for both 2BE and 2BAA decreased in both species, resulting in longer residence times in the blood. When 19-month-old naive mice were exposed to 125 ppm, 2BE was rapidly cleared from the systemic circulation, exhibiting clearance profiles similar to young mice. However, old mice eliminated 2BAA from blood > 10 times slower than young mice after 1-day of exposure. This delayed elimination of 2BAA in old mice was less obvious after 3 weeks of exposure, suggesting that there might be other factors in addition to the age of animals that could influence the apparent difference in 2BAA kinetics between old and young mice. It was concluded that the elimination kinetics of 2BE and 2BAA following repeated 2BE exposure appear to be dependent on species, sex, age, time of exposure, as well as the exposure concentration.

The pharmacokinetics and metabolism of 14C/13C-labeled ortho-phenylphenol formation following dermal application to human volunteers

1. The pharmacokinetics and metabolism of uniformly labeled 14C/13C-ortho-phenylphenol (OPP) were followed in six human male volunteers given a single 8 h dermal dose of 6 microg OPP/kg body weight formulated as a 0.4% (w/v) solution in isopropyl alcohol. The application site was covered with a non-occlusive dome allowing free movement of air, but preventing the loss of radioactivity due to physical contact. At 8 h post-exposure the non-occlusive dome was removed, the dose site was wiped with isopropyl alcohol containing swabs and the skin surface repeatedly stripped with tape. Blood specimens, urine, and feces were collected from each volunteer over a 5 day post-exposure period and were analyzed for radioactivity and metabolites (urine only). 2. Following dermal application, peak plasma levels of radioactivity were obtained within 4 h post-exposure and rapidly declined with virtually all of the absorbed dose rapidly excreted into the urine within 24 h post-exposure. A one-compartment pharmacokinetic model was used to describe the time-course of OPP absorption and clearance in male human volunteers. Approximately 43% of the dermally applied dose was absorbed through the skin with an average absorption half-life of 10 h. Once absorbed the renal clearance of OPP was rapid with an average half-life of 0.8 h. The rate limiting step for renal clearance was the relatively slower rate of dermal absorption; therefore the pharmacokinetics of OPP in humans was described by a 'flip-flop' single compartment model. Overall, the pharmacokinetics were similar between individuals, and the model parameters were in excellent agreement with the experimental data. 3. Approximately 73% of the total urinary radioactivity was accounted for as free OPP, OPP-sulfate and OPP-glucuronide conjugates. The sulfate conjugate was the major metabolite (approximately 69%). Therefore, total urinary OPP equivalents (acid-labile conjugates+free OPP) can be used to estimate the systemically absorbed dose of OPP. 4. The rapid excretion of OPP and metabolites into the urine following dermal exposure indicates that OPP is unlikely to accumulate in humans upon repeated exposure. Based on these data, blood and/or urinary OPP concentration (acid-labile conjugates) could be utilized to quantify the amount of OPP absorbed by humans under actual use conditions.