Use of quantitative in vitro to in vivo extrapolation (QIVIVE) for the assessment of non-combustible next-generation product aerosols.

With the use of in vitro new approach methodologies (NAMs) for the assessment of non-combustible next-generation nicotine delivery products, new extrapolation methods will also be required to interpret and contextualize the physiological relevance of these results. Quantitative in vitro to in vivo extrapolation (QIVIVE) can translate in vitro concentrations into in-life exposures with physiologically-based pharmacokinetic (PBPK) modelling and provide estimates of the likelihood of harmful effects from expected exposures. A major challenge for evaluating inhalation toxicology is an accurate assessment of the delivered dose to the surface of the cells and the internalized dose. To estimate this, we ran the multiple-path particle dosimetry (MPPD) model to characterize particle deposition in the respiratory tract and developed a PBPK model for nicotine that was validated with human clinical trial data for cigarettes. Finally, we estimated a Human Equivalent Concentration (HEC) and predicted plasma concentrations based on the minimum effective concentration (MEC) derived after acute exposure of BEAS-2B cells to cigarette smoke (1R6F), or heated tobacco product (HTP) aerosol at the air liquid interface (ALI). The MPPD-PBPK model predicted the in vivo data from clinical studies within a factor of two, indicating good agreement as noted by WHO International Programme on Chemical Safety (2010) guidance. We then used QIVIVE to derive the exposure concentration (HEC) that matched the estimated in vitro deposition point of departure (POD) (MEC cigarette = 0.38 puffs or 11.6 µg nicotine, HTP = 22.9 puffs or 125.6 µg nicotine) and subsequently derived the equivalent human plasma concentrations. Results indicate that for the 1R6F cigarette, inhaling 1/6th of a stick would be required to induce the same effects observed in vitro, in vivo. Whereas, for HTP it would be necessary to consume 3 sticks simultaneously to induce in vivo the effects observed in vitro. This data further demonstrates the reduced physiological potency potential of HTP aerosol compared to cigarette smoke. The QIVIVE approach demonstrates great promise in assisting human health risk assessments, however, further optimization and standardization are required for the substantiation of a meaningful contribution to tobacco harm reduction by alternative nicotine delivery products.

Creatinine-Based Renal Function Assessment in Pediatric Drug Development: An Analysis Using Clinical Data for Renally Eliminated Drugs.

The estimated glomerular filtration rate (eGFR) equations based on serum creatinine (SCR) have been used for pediatric dose adjustment in drug labeling. This study evaluated the performance of those equations in estimating individual clearance of drugs that are predominantly eliminated by glomerular filtration, using clinical data from the renally eliminated drugs gadobutrol, gadoterate, amikacin, and vancomycin. The eGFR was compared with the observed drug clearance (CL) in 352 pediatric patients from birth to 12 years of age. Multiple eGFR equations overestimated the drug CL on average, including the original and bedside Schwartz equations, which showed an average eGFR/CL ratio between 1 and 3. Further analysis with bedside Schwartz equation showed a higher eGFR/CL ratio in the subjects with a lower SCR or CL. Supraphysiological eGFR as high as 380 mL/min/1.73 m2 was obtained using the bedside Schwartz equation for some of the subjects, most of whom are children < 2 years of age with SCR < 0.2 mg/dL. Excluding the subjects with supraphysiological eGFR from the analysis did not change the overall trend of overestimation. In conclusion, Schwartz equations led to an overestimation of drug clearance for the drugs evaluated. When greater precision is required in predicting eGFR for pediatric patients, such as in drug dosing, revised k constants for the Schwartz equation or new methods of glomerular filtration rate estimation may be necessary.

Use of a physiologically-based pharmacokinetic model to explore the potential disparity in nicotine disposition between adult and adolescent nonhuman primates.

The widespread use and high abuse liability of tobacco products has received considerable public health attention, in particular for youth, who are vulnerable to nicotine addiction. In this study, adult and adolescent squirrel monkeys were used to evaluate age-related metabolism and pharmacokinetics of nicotine after intravenous administration. A physiologically-based pharmacokinetic (PBPK) model was created to characterize the pharmacokinetic behaviors of nicotine and its metabolites, cotinine, trans-3'-hydroxycotinine (3'-OH cotinine), and trans-3'-hydroxycotinine glucuronide (3'-OH cotinine glucuronide) for both adult and adolescent squirrel monkeys. The PBPK nicotine model was first calibrated for adult squirrel monkeys utilizing in vitro nicotine metabolic data, plasma concentration-time profiles and cumulative urinary excretion data for nicotine and metabolites. Further model refinement was conducted when the calibrated adult model was scaled to the adolescents, because adolescents appeared to clear nicotine and cotinine more rapidly relative to adults. More specifically, the resultant model parameters representing systemic clearance of nicotine and cotinine for adolescent monkeys were approximately two- to three-fold of the adult values on a per body weight basis. The nonhuman primate PBPK model in general captured experimental observations that were used for both model calibration and evaluation, with acceptable performance metrics for precision and bias. The model also identified differences in nicotine pharmacokinetics between adolescent and adult nonhuman primates which might also be present in humans.

Metabolism and disposition of arsenic species from oral dosing with sodium arsenite in neonatal CD-1 mice. IV. Toxicokinetics following gavage administration and lactational transfer.

Arsenic is a ubiquitous contaminant, with typical human dietary intake below 1 µg/kg bw/d and extreme drinking water exposures up to ∼50 µg/kg bw/d. The formation and binding of trivalent metabolites are central to arsenic toxicity and strong human evidence suggests special concern for early life exposures in the etiology of adult diseases, especially cancer. This study measured the metabolism and disposition of arsenite in neonatal mice to understand the role of maturation in metabolic activation and detoxification of arsenic. Many age-related differences were observed after gavage administration of arsenite, with consistent evidence in blood and tissues for higher exposures to trivalent arsenic species in neonatal mice related to the immaturity of metabolic and/or excretory functions. The evidence for greater tissue binding of arsenic species in young mice is consistent with enhanced susceptibility to toxicity based on metabolic and toxicokinetic differences alone. Lactational transfer from arsenite-dosed dams to suckling mice was minimal, based on no dosing-related changes in the levels of arsenic species in pup blood or milk collected from the dams. Animal models evaluating whole-life exposure to inorganic arsenic must use direct dosing in early neonatal life to predict accurately potential toxicity from early life exposures in children.

A biokinetic model for nickel released from cardiovascular devices.

Many alloys used in cardiovascular device applications contain high levels of nickel, which if released in sufficient quantities, can lead to adverse health effects. While nickel release from these devices is typically characterized through the use of in-vitro immersion tests, it is unclear if the rate at which nickel is released from a device during in-vitro testing is representative of the release rate following implantation in the body. To address this uncertainty, we have developed a novel biokinetic model that combines a traditional toxicokinetic compartment model with a physics-based model to estimate nickel release from an implanted device. This model links the rate of in-vitro nickel release from a cardiovascular device to serum nickel concentrations, an easily measured endpoint, to estimate the rate and extent of in-vivo nickel release from an implanted device. The model was initially parameterized using data in the literature on in-vitro nickel release from a nickel-containing alloy (nitinol) and baseline serum nickel levels in humans. The results of this first step were then used to validate specific components of the model. The remaining unknown quantities were fit using serum values reported in patients following implantation with nitinol atrial occluder devices. The model is not only consistent with levels of nickel in serum and urine of patients following treatment with the atrial occluders, but also the optimized parameters in the model were all physiologically plausible. The congruity of the model with available data suggests that it can provide a framework to interpret nickel biomonitoring data and use data from in-vitro nickel immersion tests to estimate in-vivo nickel release from implanted cardiovascular devices.

Application of physiologically-based pharmacokinetic modeling to explore the role of kidney transporters in renal reabsorption of perfluorooctanoic acid in the rat.

Renal elimination and the resulting clearance of perfluorooctanoic acid (PFOA) from the serum exhibit pronounced sex differences in the adult rat. The literature suggests that this is largely due to hormonally regulated expression of organic anion transporters (OATs) on the apical and basolateral membranes of the proximal tubule cells that facilitate excretion and reabsorption of PFOA from the filtrate into the blood. Previously developed PBPK models of PFOA exposure in the rat have not been parameterized to specifically account for transporter-mediated renal elimination. We developed a PBPK model for PFOA in male and female rats to explore the role of Oat1, Oat3, and Oatp1a1 in sex-specific renal reabsorption and excretion of PFOA. Descriptions of the kinetic behavior of these transporters were extrapolated from in vitro studies and the model was used to simulate time-course serum, liver, and urine data for intravenous (IV) and oral exposures in both sexes. Model predicted concentrations of PFOA in the liver, serum, and urine showed good agreement with experimental data for both male and female rats indicating that in vitro derived physiological descriptions of transporter-mediated renal reabsorption can successfully predict sex-dependent excretion of PFOA in the rat. This study supports the hypothesis that sex-specific serum half-lives for PFOA are largely driven by expression of transporters in the kidney and contribute to the development of PBPK modeling as a tool for evaluating the role of transporters in renal clearance.

The hypothalamic-pituitary-thyroid axis in infants and children: protection from radioiodines.

Potassium iodide (KI) is recommended as an emergency treatment for exposure to radioiodines, most commonly associated with nuclear detonation or mishaps at nuclear power plants. Protecting the thyroid gland of infants and children remains a priority because of increased incidence of thyroid cancer in the young exposed to radioiodines (such as (131)I and (133)I). There is a lack of clinical studies for KI and radioiodines in children or infants to draw definitive conclusions about the effectiveness and safety of KI administration in the young. In this paper, we compare functional aspects of the hypothalamic-pituitary-thyroid (HPT) axis in the young and adults and review the limited studies of KI in children. The HPT axis in the infant and child is hyperactive and therefore will respond less effectively to KI treatment compared to adults. Research on the safety and efficacy of KI in infants and children is needed.

Comparison of life-stage-dependent internal dosimetry for bisphenol A, ethinyl estradiol, a reference estrogen, and endogenous estradiol to test an estrogenic mode of action in Sprague Dawley rats.

Bisphenol A (BPA) was administered by gavage (2.5-300,000 µg/kg body weight (bw)/day) to pregnant Sprague Dawley dams, newborn pups, and continuing into adulthood. Aglycone (i.e., unconjugated and active) and conjugated (i.e., inactive) BPA were evaluated by liquid chromatography electrospray tandem mass spectrometry (LC-ES/MS/MS) in serum to better interpret toxicological endpoints measured in the study. Ethinyl estradiol (EE2, 0.5 and 5 µg/kg bw/day) and the endogenous hormones, 17ß-estradiol (E2) and testosterone, were similarly evaluated. Mean BPA aglycone levels in vehicle and naïve control rat serum (0.02-0.5 ng/ml) indicated sample processing artifact, consistent with literature reports of a propensity for postexposure blood contamination by BPA. Direct measurements of BPA-glucuronide in vehicle and naïve control serum (2-10nM) indicated unintentional exposure and metabolism at levels similar to those produced by 2.5 µg/kg bw/day BPA (7-10nM), despite careful attention to potential BPA inputs (diet, drinking water, vehicle, cages, bedding, and dust) and rigorous dosing solution certification and delivery. The source of this exposure could not be identified, but interpretation of the toxicological effects, observed only at the highest BPA doses, was not compromised. Internal exposures to BPA and EE2 aglycones were highest in young rats. When maximal serum concentrations from the two highest BPA doses and both EE2 doses were compared with concurrent levels of endogenous E2, the ERα binding equivalents were similar to or above those of endogenous E2 in male and female rats of all ages tested. Such evaluations of estrogenic internal dosimetry and comprehensive evaluation of contamination impact should aid in extrapolating risks from human BPA exposures.

Are typical human serum BPA concentrations measurable and sufficient to be estrogenic in the general population?

Mammalian estrogen receptors modulate many physiological processes. Chemicals with structural features similar to estrogens can interact with estrogen receptors to produce biological effects similar to those caused by endogenous estrogens in the body. Bisphenol A (BPA) is a structural analogue of estrogen that binds to estrogen receptors. Exposure to BPA in humans is virtually ubiquitous in industrialized societies, but BPA is rapidly detoxified by metabolism and does not accumulate in the body. Whether or not serum concentrations of BPA in humans are sufficiently high to disrupt normal estrogen-related biology is the subject of intense political and scientific debate. Here we show a convergence of robust methods for measuring or calculating serum concentrations of BPA in humans from 93 published studies of more than 30,000 individuals in 19 countries across all life stages. Typical serum BPA concentrations are orders of magnitude lower than levels measurable by modern analytical methods and below concentrations required to occupy more than 0.0009% of Type II Estrogen Binding Sites, GPR30, ERα or ERß receptors. Occupancies would be higher, but ≤0.04%, for the highest affinity receptor, ERRγ. Our results show limited or no potential for estrogenicity in humans, and question reports of measurable BPA in human serum.

Association of brominated proteins and changes in protein expression in the rat kidney with subcarcinogenic to carcinogenic doses of bromate.

The water disinfection byproduct bromate (BrO3(-)) produces cytotoxic and carcinogenic effects in rat kidneys. Our previous studies demonstrated that BrO3(-) caused sex-dependent differences in renal gene and protein expression in rats and the elimination of brominated organic carbon in their urine. The present study examined changes in renal cell apoptosis and protein expression in male and female F344 rats treated with BrO3(-) and associated these changes with accumulation of 3-bromotyrosine (3-BT)-modified proteins. Rats were treated with 0, 11.5, 46 and 308 mg/L BrO3(-) in drinking water for 28 days and renal sections were prepared and examined for apoptosis (TUNEL-staining), 8-oxo-deoxyguanosine (8-oxoG), 3-BT, osteopontin, Kim-1, clusterin, and p-21 expression. TUNEL-staining in renal proximal tubules increased in a dose-related manner beginning at 11.5mg BrO3(-)/L in female rats and 46 mg/L in males. Increased 8-oxoG staining was observed at doses as low as 46 mg/L. Osteopontin expression also increased in a dose-related manner after treatment with 46 mg/L, in males only. In contrast, Kim-1 expression increased in a dose-related manner in both sexes, although to a greater extent in females at the highest dose. Clusterin and p21 expression also increased in a dose-related manner in both sexes. The expression of 3-BT-modified proteins only increased in male rats, following a pattern previously reported for accumulation of α-2u-globulin. Increases in apoptosis in renal proximal tubules of male and female rats at the lowest doses suggest a common mode of action for renal carcinogenesis for the two sexes that is independent of α-2u-globulin nephropathy.

Radioactive iodide (131 I-) excretion profiles in response to potassium iodide (KI) and ammonium perchlorate (NH4ClO4) prophylaxis.

Radioactive iodide ((131)I-) protection studies have focused primarily on the thyroid gland and disturbances in the hypothalamic-pituitary-thyroid axis. The objective of the current study was to establish (131)I- urinary excretion profiles for saline, and the thyroid protectants, potassium iodide (KI) and ammonium perchlorate over a 75 hour time-course. Rats were administered (131)I- and 3 hours later dosed with either saline, 30 mg/kg of NH(4)ClO(4) or 30 mg/kg of KI. Urinalysis of the first 36 hours of the time-course revealed that NH(4)ClO(4) treated animals excreted significantly more (131)I- compared with KI and saline treatments. A second study followed the same protocol, but thyroxine (T(4)) was administered daily over a 3 day period. During the first 6-12 hour after (131)I- dosing, rats administered NH(4)ClO(4) excreted significantly more (131)I- than the other treatment groups. T(4) treatment resulted in increased retention of radioiodide in the thyroid gland 75 hour after (131)I- administration. We speculate that the T(4) treatment related reduction in serum TSH caused a decrease synthesis and secretion of thyroid hormones resulting in greater residual radioiodide in the thyroid gland. Our findings suggest that ammonium perchlorate treatment accelerates the elimination rate of radioiodide within the first 24 to 36 hours and thus may be more effective at reducing harmful exposure to (131)I- compared to KI treatment for repeated dosing situations. Repeated dosing studies are needed to compare the effectiveness of these treatments to reduce the radioactive iodide burden of the thyroid gland.

Absorption and disposition of bromate in F344 rats.

Bromate (BrO(3)(-)) is a ubiquitous by-product of using ozone to disinfect water containing bromide (Br(-)). The reactivity of BrO(3)(-) with biological reductants suggests that its systemic absorption and distribution to target tissues may display non-linear behavior as doses increase. The intent of this study is to determine the extent to which BrO(3)(-) is systemically bioavailable via oral exposure and broadly identify its pathways of degradation. In vitro experiments of BrO(3)(-) degradation in rat blood indicate a rapid initial degradation immediately upon addition that is >98% complete at concentrations up to 66µM in blood. As initial concentrations are increased, progressively lower fractions are lost prior to the first measurement. Secondary to this initial loss, a slower and predictable first order degradation rate was observed (10%/min). Losses during both phases were accompanied by increases in Br(-) concentrations indicating that the loss of BrO(3)(-) was due to its reduction. In vivo experiments were conducted using doses of BrO(3)(-) ranging from 0.077 to 15.3mg/kg, administered intravenously (IV) or orally (gavage) to female F344 rats. The variable nature and uncertain source of background concentrations of BrO(3)(-) limited derivation of terminal half-lives, but the initial half-life was approximately 10min for all dose groups. The area under the curve (AUC) and peak concentrations (C(t=5')) were linearly related to IV dose up to 0.77mg/kg; however, disproportionate increases in the AUC and C(t=5') and a large decrease in the volume of distribution was observed when IV doses of 1.9 and 3.8mg/kg were administered. The average terminal half-life of BrO(3)(-) from oral administration was 37min, but this was influenced by background levels of BrO(3)(-) at lower doses. With oral doses, the AUC and C(max) increased linearly with dose up to 15.3mgBrO(3)(-)/kg. BrO(3)(-) appeared to be 19-25% bioavailable without an obvious dose-dependency between 0.077 and 1.9mg/kg. The urinary elimination of BrO(3)(-) and Br(-) was measured from female F344 rats for four days following administration of single doses of 8.1mgKBrO(3)/kg and for 15 days after a single dose of 5.0mgKBr/kg. BrO(3)(-) elimination was detected over the first 12h, but Br(-) elimination from BrO(3)(-) over the first 48h was 18% lower than expected based on that eliminated from an equimolar dose of Br(-) (15.5±1.6 vs. 18.8±1.2µmol/kg, respectively). The cumulative excretion of Br(-) from KBr vs. KBrO(3) was equivalent 72h after administration. The recovery of unchanged administered BrO(3)(-) in the urine ranged between 6.0 and 11.3% (creatinine corrected) on the 27th day of treatment with concentrations of KBrO(3) of 15, 60, and 400mg/L of drinking water. The recovery of total urinary bromine as Br(-)+BrO(3)(-) ranged between 61 and 88%. An increase in the fraction of the daily BrO(3)(-) dose recovered in the urine was observed at the high dose to both sexes. The deficit in total bromine recovery raises the possibility that some brominated biochemicals may be produced in vivo and more slowly metabolized and eliminated. This was supported by measurements of dose-dependent increases of total organic bromine (TOBr) that was eliminated in the urine. The role these organic by-products play in BrO(3)(-)-induced cancer remains to be established.

Extrapolation of hypothalamic-pituitary-thyroid axis perturbations and associated toxicity in rodents to humans: case study with perchlorate.

Functional aspects of the Hypothalamic-Pituitary-Thyroid (HPT) axis in rats and humans are compared, exposing why extrapolation of toxicant-induced perturbations in the rat HPT axis to the human HPT axis cannot be accomplished using default risk assessment methodology. Computational tools, such as biologically based dose response models for the HPT axis, are recommended to perform complex animal to human extrapolations involving the HPT axis. Experimental and computational evidence are presented that suggest perchlorate acts directly on the thyroid gland in rats. The apparent escape from perchlorate-induced inhibition of thyroidal uptake of radioactive iodide in humans is discussed along with "rebound" or increased thyroidal uptake of radioactive iodide observed after discontinued clinical treatment with perchlorate.

Highly parallel genome-wide expression analysis of single mammalian cells.

BACKGROUND: We have developed a high-throughput amplification method for generating robust gene expression profiles using single cell or low RNA inputs. METHODOLOGY/PRINCIPAL FINDINGS: The method uses tagged priming and template-switching, resulting in the incorporation of universal PCR priming sites at both ends of the synthesized cDNA for global PCR amplification. Coupled with a whole-genome gene expression microarray platform, we routinely obtain expression correlation values of R(2)~0.76-0.80 between individual cells and R(2)~0.69 between 50 pg total RNA replicates. Expression profiles generated from single cells or 50 pg total RNA correlate well with that generated with higher input (1 ng total RNA) (R(2)~0.80). Also, the assay is sufficiently sensitive to detect, in a single cell, approximately 63% of the number of genes detected with 1 ng input, with approximately 97% of the genes detected in the single-cell input also detected in the higher input. CONCLUSIONS/SIGNIFICANCE: In summary, our method facilitates whole-genome gene expression profiling in contexts where starting material is extremely limiting, particularly in areas such as the study of progenitor cells in early development and tumor stem cell biology.

Pharmacokinetics of bisphenol A in neonatal and adult rhesus monkeys.

Bisphenol A (BPA) is a high-production volume industrial chemical used in the manufacture of polycarbonate plastic products and epoxy resin-based food can liners. The presence of BPA in urine of >90% of Americans aged 6-60 is controversial because of the potential for endocrine disruption, particularly during perinatal development, as suggested by in vitro, experimental animal, and epidemiological studies. The current study used LC/MS/MS to measure serum pharmacokinetics of aglycone (active) and conjugated (inactive) BPA in adult and neonatal rhesus monkeys by oral (PND 5, 35, 70) and intravenous injection (PND 77) routes using d6-BPA to avoid sample contamination. The concentration-time profiles observed in adult monkeys following oral administration of 100 µg/kg bw were remarkably similar to those previously reported in human volunteers given a similar dose; moreover, minimal pharmacokinetic differences were observed between neonatal and adult monkeys for the receptor-active aglycone form of BPA. Circulating concentrations of BPA aglycone were quite low following oral administration (< 1% of total), which reflects the redundancy of active UDP-glucuronosyl transferase isoforms in both gut and liver. No age-related changes were seen in internal exposure metrics for aglycone BPA in monkeys, a result clearly different from developing rats where significant inverse age-related changes, based on immaturity of Phase II metabolism and renal excretion, were recently reported. These observations imply that any toxicological effect observed in rats from early postnatal exposures to BPA could over-predict those possible in primates of the same age, based on significantly higher internal exposures and overall immaturity at birth.

Pharmacokinetics of bisphenol A in neonatal and adult Sprague-Dawley rats.

Bisphenol A (BPA) is an important industrial chemical used in the manufacture of polycarbonate plastic products and epoxy resin-based food can liners. The presence of BPA in urine of >90% of Americans aged 6-60 suggests ubiquitous and frequent exposure. The current study used LC/MS/MS to measure serum pharmacokinetics of aglycone (active) and conjugated (inactive) BPA in adult and neonatal Sprague-Dawley rats by oral and injection routes. Deuterated BPA was used to avoid issues of background contamination. Linear pharmacokinetics were observed in adult rats treated orally in the range of 0-200 microg/kg bw. Evidence for enterohepatic recirculation of conjugated, but not aglycone, BPA was observed in adult rats. Significant inverse relationships were observed between postnatal age and measures of internal exposures to aglycone BPA and its elimination. In neonatal rats treated orally, internal exposures to aglycone BPA were substantially lower than from subcutaneous injection. The results reinforce the critical role for first-pass Phase II metabolism of BPA in gut and liver after oral exposure that attenuates internal exposure to the aglycone form in rats of all ages. The internal exposures to aglycone BPA observed in adult and neonatal rats following a single oral dose of 100 microg/kg bw are inconsistent with effects mediated by classical estrogen receptors based on binding affinities. However, an impact on alternative estrogen signaling pathways that have higher receptor affinity cannot be excluded in neonatal rats. These findings emphasize the importance of matching aglycone BPA internal dosimetry with receptor affinities in experimental animal studies reporting toxicity.

Interpreting NHANES biomonitoring data, cadmium.

Cadmium (Cd) occurs naturally in the environment and the general population's exposure to it is predominantly through diet. Chronic Cd exposure is a public health concern because Cd is a known carcinogen; it accumulates in the body and causes kidney damage. The National Health and Nutritional Examination Survey (NHANES) has measured urinary Cd; the 2003-2004 NHANES survey cycle reported estimates for 2257 persons aged 6 years and older in the Fourth National Report on Human Exposure to Environmental Chemicals. As part of translational research to make computerized models accessible to health risk assessors we re-coded a cadmium model in Berkeley Madonna simulation language. This model was used in our computational toxicology laboratory to predict the urinary excretion of cadmium. The model simulated the NHANES-measured data very well from ages 6 to 60+ years. An unusual increase in Cd urinary excretion was observed among 6-11-year-olds, followed by a continuous monotonic rise into the seventh decade of life. This observation was also made in earlier studies that could be life stage-related and a function of anatomical and phsysiological changes occurring during this period of life. Urinary excretion of Cd was approximately twofold higher among females than males in all age groups. The model describes Cd's cumulative nature in humans and accommodates the observed variation in exposure/uptake over the course of a lifetime. Such models may be useful for interpreting biomonitoring data and risk assessment.

Novel characterization of the aerosol and gas-phase composition of aerosolized jet fuel.

Few robust methods are available to characterize the composition of aerosolized complex hydrocarbon mixtures. The difficulty in separating the droplets from their surrounding vapors and preserving their content is challenging, more so with fuels, which contain hydrocarbons ranging from very low to very high volatility. Presented here is a novel method that uses commercially available absorbent tubes to measure a series of hydrocarbons in the vapor and droplets from aerosolized jet fuels. Aerosol composition and concentrations were calculated from the differential between measured total (aerosol and gas-phase) and measured gas-phase concentrations. Total samples were collected directly, whereas gas-phase only samples were collected behind a glass fiber filter to remove droplets. All samples were collected for 1 min at 400 ml min(-1) and quantified using thermal desorption-gas chromatography-mass spectrometry. This method was validated for the quantification of the vapor and droplet content from 4-h aerosolized jet fuel exposure to JP-8 and S-8 at total concentrations ranging from 200 to 1000 mg/m(3). Paired samples (gas-phase only and total) were collected every approximately 40 min. Calibrations were performed with neat fuel to calculate total concentration and also with a series of authentic standards to calculate specific compound concentrations. Accuracy was good when compared to an online GC-FID (gas chromatography-flame ionization detection) technique. Variability was 15% or less for total concentrations, the sum of all gas-phase compounds, and for most specific compound concentrations in both phases. Although validated for jet fuels, this method can be adapted to other hydrocarbon-based mixtures.

Trichloroethylene risk assessment: a review and commentary.

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

Quantitative evaluation of dichloroacetic acid kinetics in human--a physiologically based pharmacokinetic modeling investigation.

Dichloroacetic acid is a common disinfection by-product in surface waters and is a probable minor metabolite of trichloroethylene. Dichloroacetic acid (DCA) liver carcinogenicity has been demonstrated in rodents but epidemiological evidence in humans is not available. High doses of DCA ( approximately 50mg/kg) are used clinically to treat metabolic acidosis. Biotransformation of DCA by glutathione transferase zeta (GSTzeta) in the liver is the major elimination pathway in humans. GSTzeta is also inactivated by DCA, leading to slower systemic clearance and nonlinear pharmacokinetics after multiple doses. A physiologically based pharmacokinetic (PBPK) model was developed to quantitatively describe DCA biotransformation and kinetics in humans administered DCA by intravenous infusion and oral ingestion. GSTzeta metabolism was described using a Michaelis-Menten equation coupled with rate constants to account for normal GSTzeta synthesis, degradation and irreversible covalent binding and inhibition by the glutathione-bound-DCA intermediate. With some departures between observation and model prediction, the human DCA PBPK model adequately predicted the DCA plasma kinetics over a 20,000-fold range in administered doses. Apparent inhibition of GSTzeta mediated metabolism of DCA was minimal for low doses of DCA (microg/kg day), but was significant for therapeutic doses of DCA. Plasma protein binding of DCA was assumed to be an important factor influencing the kinetics of low doses of DCA (microg/kg day). Polymorphisms of GSTzeta may help explain inter-individual variability in DCA plasma kinetics and warrants evaluation. In conclusion, using a previously published rodent DCA PBPK model (Keys, D.A., Schultz, I.R., Mahle, D.A., Fisher, J.W., 2004. A quantitative description of suicide inhibition of dichloroacetic acid in rats and mice. Toxicol. Sci. 82, 381-393) and this human DCA PBPK model, human equivalent doses (HEDs) were calculated for a 10% increase in mice hepatic liver cancer (2.1mg/kg day). The HEDs for the dosimetrics, area-under-the-concentration-curve (AUC) for total and free DCA in plasma, AUC of DCA in liver and amount of DCA metabolized per day were 0.02, 0.1, 0.1 and 1.0mg/kg day, respectively. Research on the mechanism of action of DCA and the relevance of mouse liver cancer is needed to better understand which dosimetric may be appropriate for extrapolation from animal studies to human.