Physiologically based pharmacokinetic modeling of ethyl acetate and ethanol in rodents and humans

A physiologically based pharmacokinetic (PBPK) model was developed and applied to a metabolic series approach for the ethyl series (i.e., ethyl acetate, ethanol, acetaldehyde, and acetate). This approach bases toxicity information on dosimetry analyses for metabolically linked compounds using pharmacokinetic data for each compound and toxicity data for parent or individual compounds. In vivo pharmacokinetic studies of ethyl acetate and ethanol were conducted in rats following IV and inhalation exposure. Regardless of route, ethyl acetate was rapidly converted to ethanol. Blood concentrations of ethyl acetate and ethanol following both IV bolus and infusion suggested linear kinetics across blood concentrations from 0.1 to 10 mM ethyl acetate and 0.01-0.8 mM ethanol. Metabolic parameters were optimized and evaluated based on available pharmacokinetic data. The respiratory bioavailability of ethyl acetate and ethanol were estimated from closed chamber inhalation studies and measured ventilation rates. The resulting ethyl series model successfully reproduces blood ethyl acetate and ethanol kinetics following IV administration and inhalation exposure in rats, and blood ethanol kinetics following inhalation exposure to ethanol in humans. The extrapolated human model was used to derive human equivalent concentrations for the occupational setting of 257-2120 ppm ethyl acetate and 72-517 ppm ethyl acetate for continuous exposure, corresponding to rat LOAELs of 350 and 1500 ppm.

Computational modeling of nanoscale and microscale particle deposition, retention and dosimetry in the mouse respiratory tract

Comparing effects of inhaled particles across rodent test systems and between rodent test systems and humans is a key obstacle to the interpretation of common toxicological test systems for human risk assessment. These comparisons, correlation with effects and prediction of effects, are best conducted using measures of tissue dose in the respiratory tract. Differences in lung geometry, physiology and the characteristics of ventilation can give rise to differences in the regional deposition of particles in the lung in these species. Differences in regional lung tissue doses cannot currently be measured experimentally. Regional lung tissue dosimetry can however be predicted using models developed for rats, monkeys, and humans. A computational model of particle respiratory tract deposition and clearance was developed for BALB/c and B6C3F1 mice, creating a cross-species suite of available models for particle dosimetry in the lung. Airflow and particle transport equations were solved throughout the respiratory tract of these mice strains to obtain temporal and spatial concentration of inhaled particles from which deposition fractions were determined. Particle inhalability (Inhalable fraction, IF) and upper respiratory tract (URT) deposition were directly related to particle diffusive and inertial properties. Measurements of the retained mass at several post-exposure times following exposure to iron oxide nanoparticles, micro- and nanoscale C60 fullerene, and nanoscale silver particles were used to calibrate and verify model predictions of total lung dose. Interstrain (mice) and interspecies (mouse, rat and human) differences in particle inhalability, fractional deposition and tissue dosimetry are described for ultrafine, fine and coarse particles.

Derivation of a human equivalent concentration for n-butanol using a physiologically based pharmacokinetic model for n-butyl acetate and metabolites n-butanol and n-butyric acid

The metabolic series approach for risk assessment uses a dosimetry-based analysis to develop toxicity information for a group of metabolically linked compounds using pharmacokinetic (PK) data for each compound and toxicity data for the parent compound. The metabolic series approach for n-butyl acetate and its subsequent metabolites, n-butanol and n-butyric acid (the butyl series), was first demonstrated using a provisional physiologically based pharmacokinetic (PBPK) model for the butyl series. The objective of this work was to complete development of the PBPK model for the butyl series. Rats were administered test compounds by iv bolus dose, iv infusion, or by inhalation in a recirculating closed chamber. Hepatic, vascular, and extravascular metabolic constants for metabolism were estimated by fitting the model to the blood time course data from these experiments. The respiratory bioavailability of n-butyl acetate (100% of alveolar ventilation) and n-butanol (50% of alveolar ventilation) was estimated from closed chamber inhalation studies and measured ventilation rates. The resulting butyl series PBPK model successfully reproduces the blood time course of these compounds following iv administration and inhalation exposure to n-butyl acetate and n-butanol in rats and arterial blood n-butanol kinetics following inhalation exposure to n-butanol in humans. These validated inhalation route models can be used to support species and dose-route extrapolations required for risk assessment of butyl series family of compounds. Human equivalent concentrations of 169 ppm and 1066 ppm n-butanol corresponding to the rat n-butyl acetate NOAELs of 500 and 3000 ppm were derived using the models.

Genome-wide loss of heterozygosity analysis of chemically induced rat hepatocellular carcinomas reveals elevated frequency of allelic imbalances on chromosomes 1, 6, 8, 11, 15, 17, and 20

Neoplastic development is a multistep process that involves the stochastic accumulation of heritable genetic alterations in proto-oncogenes, DNA repair genes, and tumor suppressor genes. Loss of heterozygosity (LOH) analysis has been used successfully to identify the genetic determinants of neoplastic development, including tumor suppressor genes, in several species and organs but not in the rat liver. We report the results of a sensitive genome-wide LOH analysis of rat hepatocellular carcinomas (HCCs). Heterozygous rats (Wistar-Furth x Fisher 344) were subjected to an Initiation-Promotion-Progression (IPP) protocol of hepatocarcinogenesis. Two weeks after initiation (by partial hepatectomy, 10 mg/kg diethylnitrosamine), the rats were placed on a diet containing 0.05% phenobarbital (PB). After 24 wk of PB promotion, the rats received either 100 or 1 50 mg/kg ethylnitrosourea. Hepatocellular tumors were resected after a total of 76wk of PB promotion. LOH analysis was completed on 26 HCCs by using 60 microsatellite markers covering all 20 rat autosomes and chromosome X. While 85% of the HCCs had one or more allelic imbalances, the average HCC had 3.3 allelic imbalances (range 0-9). A conditional hypothesis-testing method called the Hot-Cold model was used to determine the location of statistically significant elevations in the frequency of allelic imbalances. Elevated allelic imbalances were observed on chromosomes 1q, 6, 8, 11, 15, 17, and 20p. Together, these allelic imbalances suggest that the retinoblastoma and insulin-like growth factor genes as well as the resistance to chemical carcinogenesis (rcc) locus may be involved in HCC development in the rat but that LOH of the p53 gene is not. The elevated rate of allelic imbalances on chromosomes 8,11, and 17 may indicate the location of undiscovered tumor suppressor genes important to neoplastic development in rat liver. Microdissection-based LOH analysis of HCC revealed that contamination of non-neoplastic and nonhepatocellular tissue was not masking LOH in the whole-tumor analysis. There were no statistically significant differences in the frequency of allelic imbalances between HCC of any differentiation state (histological grade). To the degree that it does not reflect differences in etiological factors, the absence of allelic imbalances in chromosomal regions containing the p53 and mamose-6-phosphate/insulin-like growth factor II receptor tumor suppressor genes and the generally low frequency of allelic imbalances in these tumors, suggests that LOH and allelic imbalances play a less significant role in the molecular pathogenesis of HCC in rats than humans.

Genome-wide loss of heterozygosity analysis of chemically induced rat hepatocellular carcinomas reveals elevated frequency of allelic imbalances on chromosomes 1, 6, 8, 11, 15, 17, and 20

Neoplastic development is a multistep process that involves the stochastic accumulation of heritable genetic alterations in proto-oncogenes, DNA repair genes, and tumor suppressor genes. Loss of heterozygosity (LOH) analysis has been used successfully to identify the genetic determinants of neoplastic development, including tumor suppressor genes, in several species and organs but not in the rat liver. We report the results of a sensitive genome-wide LOH analysis of rat hepatocellular carcinomas (HCCs). Heterozygous rats (Wistar-Furth x Fisher 344) were subjected to an Initiation-Promotion-Progression (IPP) protocol of hepatocarcinogenesis. Two weeks after initiation (by partial hepatectomy, 10 mg/kg diethylnitrosamine), the rats were placed on a diet containing 0.05% phenobarbital (PB). After 24 wk of PB promotion, the rats received either 100 or 1 50 mg/kg ethylnitrosourea. Hepatocellular tumors were resected after a total of 76wk of PB promotion. LOH analysis was completed on 26 HCCs by using 60 microsatellite markers covering all 20 rat autosomes and chromosome X. While 85% of the HCCs had one or more allelic imbalances, the average HCC had 3.3 allelic imbalances (range 0-9). A conditional hypothesis-testing method called the Hot-Cold model was used to determine the location of statistically significant elevations in the frequency of allelic imbalances. Elevated allelic imbalances were observed on chromosomes 1q, 6, 8, 11, 15, 17, and 20p. Together, these allelic imbalances suggest that the retinoblastoma and insulin-like growth factor genes as well as the resistance to chemical carcinogenesis (rcc) locus may be involved in HCC development in the rat but that LOH of the p53 gene is not. The elevated rate of allelic imbalances on chromosomes 8,11, and 17 may indicate the location of undiscovered tumor suppressor genes important to neoplastic development in rat liver. Microdissection-based LOH analysis of HCC revealed that contamination of non-neoplastic and nonhepatocellular tissue was not masking LOH in the whole-tumor analysis. There were no statistically significant differences in the frequency of allelic imbalances between HCC of any differentiation state (histological grade). To the degree that it does not reflect differences in etiological factors, the absence of allelic imbalances in chromosomal regions containing the p53 and mamose-6-phosphate/insulin-like growth factor II receptor tumor suppressor genes and the generally low frequency of allelic imbalances in these tumors, suggests that LOH and allelic imbalances play a less significant role in the molecular pathogenesis of HCC in rats than humans.

Hazard assessment of chemical carcinogens: the impact of hormesis

The recent report of reductions in the number and area of preneoplastic hepatic lesions in response to low doses of the tumor promoter phenobarbital provides important new support for the existence of hormetic responses to carcinogens. The presence of hormetic responses to carcinogenic agents and the corollary that beneficial doses of these compounds can be determined have several implications for the bioassay and hazard assessment of carcinogens as well as for public policy regulating exposure to these agents. To be adequately sensitive to detect and quantify hormetic or other non-linear dose-response functions, current study designs must be modified to include lower doses and sufficiently large numbers of animals. Short- or medium-term animal studies are a cost-effective means of addressing these needs and have been used recently to describe a classical hormetic response to the non-genotoxic carcinogen phenobarbital. These basic changes should be supported by a continuing emphasis on mechanistic research and the development of biologically based quantitative models of toxicant action. Linking these models with physiologically based pharmacokinetic model descriptions of target dose holds the greatest promise for improving the description of the dose-response curve at low doses. These approaches are generally encouraged by the USEPA in the form of The 1996 Proposed Carcinogen Risk Assessment Guidelines. However, there remain substantial questions regarding integration of the concept of hormesis into hazard testing and public policy that require careful consideration. Herein, we explore the issues that surround testing for hormetic responses and the implications for public policy.

Quantitative analysis of dose- and time-dependent promotion of four phenotypes of altered hepatic foci by 2,3,7,8-tetrachlorodibenzo-p-dioxin in female Sprague-Dawley rats

Determining both the mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) acts as a tumor promoter and the shape of the dose-response curve at low doses remains an important goal of risk-assessment-directed research. In this report, we extend previous mechanistic and descriptive work done on the effect of TCDD on promotion in the two-stage model of hepatocarcinogenesis, to include lower, more clinically relevant doses. After initiation [PH + 10 mg diethylnitrosamine (DEN)/kg], groups of female Sprague-Dawley rats were administered TCDD in one of four doses: 0.01, 0.1, 1.0, or 10 ng/kg/day for 1, 3, or 6 months. Early increases in liver weight (19-69%) due to hepatocyte hypertrophy were resolved after 3- or 6-months exposures to TCDD, and were not associated with the effects of TCDD on promotion. Non-focal cell proliferation in DEN-treated groups was significantly reduced after 1 or 3 months of exposure to 0.1 ng/kg/day TCDD, leading to U-shaped dose-response curves. TCDD effects on non-focal cell proliferation were not associated with effects on promotion. GSTP-positive AHF represented approximately 97% of the total AHF. Significant increases in both the volume fraction and the number of altered hepatic foci (AHF) were observed at the highest dose (10 ng/kg/day) for GSTP-positive AHF in DEN-treated groups. Increases in the number of G6Pase- and ATPase-deficient AHF/cm3 were observed at TCDD doses as low as 0.01 ng/kg/day. This is the lowest tumor-promoting dose of TCDD reported to date. This study represents an unusually complete data set for further dose-response analysis and simulation or mathematical modeling of TCDD-mediated promotion in the rat liver.

Mutational analysis of three tumor suppressor genes in two models of rat hepatocarcinogenesis

An albumin-simian virus 40 (SV40) large T-antigen (T-Ag) transgenic model and a chemically induced model of multistage hepatocarcinogenesis were created in our laboratory to study the molecular mechanisms involved in the genesis and progression of neoplasia in the rat liver. In the study presented here, these two models of rat hepatocarcinogenesis were used to perform a comparative mutational analysis of three tumor suppressor genes involved in hepatic neoplastic growth. By using polymerase chain reaction-single strand conformation polymorphism analysis and sequencing, exons 5-8 of the p53 tumor suppressor gene and a region between nt 4325 and 4479 of the rat mannose 6-phosphate/insulin-like growth factor 2 receptor (M6p/Igf2r) coding sequence were screened. The latter is homologous to the human M6P/IGF2r coding sequence which is mutated in human hepatocellular carcinoma. A complete single strand conformation polymorphism analysis of the entire coding region of the rat adenomatous polyposis coli (Apc) gene was also performed for the first time in rat tumorigenic samples. Twenty-six chemically induced rat hepatocellular carcinomas, 21 neoplasms from the livers of SV40 T-Ag animals, and five immortalized hepatic cell lines from the transgenic rats were evaluated. None of the hepatic tumors exhibited mutations in the regions analyzed. The albumin-SV40 T-Ag transgenic cell line L-60, derived from normal hepatic tissue, had two mutations in contiguous codons of exon 5 of the p53 gene: a GGT --> GTT missense transversion in codon 183 and a silent mutation in codon 184. The transversion, which may affect the DNA binding domain of the p53 protein, probably originated during cell culture and may have been positively selected because it gave a growth advantage to the mutated cells. The studied region of the M6p/Igf2r gene was not found to be mutated in these two models of rat hepatocarcinogenesis. Although M6p/Igf2r, Apc, and p53 have been shown to be mutated in a variety of human hepatic proliferative diseases, our results indicate that aberrations in these genes may not be necessary for liver carcinogenesis in the rat.

Mutational analysis of three tumor suppressor genes in two models of rat hepatocarcinogenesis

An albumin-simian virus 40 (SV40) large T-antigen (T-Ag) transgenic model and a chemically induced model of multistage hepatocarcinogenesis were created in our laboratory to study the molecular mechanisms involved in the genesis and progression of neoplasia in the rat liver. In the study presented here, these two models of rat hepatocarcinogenesis were used to perform a comparative mutational analysis of three tumor suppressor genes involved in hepatic neoplastic growth. By using polymerase chain reaction-single strand conformation polymorphism analysis and sequencing, exons 5-8 of the p53 tumor suppressor gene and a region between nt 4325 and 4479 of the rat mannose 6-phosphate/insulin-like growth factor 2 receptor (M6p/Igf2r) coding sequence were screened. The latter is homologous to the human M6P/IGF2r coding sequence which is mutated in human hepatocellular carcinoma. A complete single strand conformation polymorphism analysis of the entire coding region of the rat adenomatous polyposis coli (Apc) gene was also performed for the first time in rat tumorigenic samples. Twenty-six chemically induced rat hepatocellular carcinomas, 21 neoplasms from the livers of SV40 T-Ag animals, and five immortalized hepatic cell lines from the transgenic rats were evaluated. None of the hepatic tumors exhibited mutations in the regions analyzed. The albumin-SV40 T-Ag transgenic cell line L-60, derived from normal hepatic tissue, had two mutations in contiguous codons of exon 5 of the p53 gene: a GGT --> GTT missense transversion in codon 183 and a silent mutation in codon 184. The transversion, which may affect the DNA binding domain of the p53 protein, probably originated during cell culture and may have been positively selected because it gave a growth advantage to the mutated cells. The studied region of the M6p/Igf2r gene was not found to be mutated in these two models of rat hepatocarcinogenesis. Although M6p/Igf2r, Apc, and p53 have been shown to be mutated in a variety of human hepatic proliferative diseases, our results indicate that aberrations in these genes may not be necessary for liver carcinogenesis in the rat.

Implications of hormesis on the bioassay and hazard assessment of chemical carcinogens

Hormesis has been defined as a dose-response relationship which depicts improvement in some endpoint (increased metabolic rates, reduction in tumor incidence, etc.) at low doses of a toxic compound followed by a decline in the endpoint at higher doses. The existence of hormetic responses to carcinogenic agents has several implications for the bioassay and hazard assessment of carcinogens. To be capable of detecting and statistically testing for hormetic or other nonlinear dose-response functions, current study designs must be modified to include lower doses and sufficiently large numbers of animals. In addition, improved statistical methods for testing nonlinear dose-response relationships will have to be developed. Research integrating physiologically-based pharmacokinetic model descriptions of target dose with mechanistic data holds the greatest promise for improving the description of the dose-response curve at low doses. The 1996 Proposed Carcinogen Risk Assessment Guidelines encourage the use of mechanistic data to improve the descriptions of the dose-response curve at low doses, but do not distinguish between the types of nonlinear dose-response curves. Should this refined approach lead to substantial support for hormesis in carcinogenic processes, future guidelines will need to provide guidance on establishing safe doses and communicating the results to the public.