Toxicokinetics of inhaled propylene in mouse, rat, and human

Argon pharmacokinetics: measurements in pigs and analysis in humans using a physiologically based pharmacokinetics model

The primary objective of this study was to investigate the pharmacokinetics of inhaled argon in young pigs using mechanical ventilation. Also a physiologically based model of argon pharmacokinetics (PBPK) is validated with human data for xenon from the literature and the new data from juvenile pigs. The inherent difficulty in performing pharmacokinetics studies of argon makes the use of the PBPK model especially relevant. The model is used to investigate argon pharmacokinetics for adult and neonate applications. Juvenile pigs (n = 4) were anesthetized, submitted to endotracheal intubation, and mechanical ventilation using a conventional ventilator. Argon inhalation was achieved by switching the animal from the first mechanical ventilator (with air/oxygen) to a second one that was supplied with 75% argon and 25% oxygen from premixed gas cylinders. This administration yielded blood samples that were analyzed using a quadrupole based technique for determining argon concentration. The range of blood:gas partition coefficient corresponding to the average measured Cmax of 190-872 μM is 0.005-0.022. Based on the average curve, T1/2= 75 seconds. The PBPK is shown to be in general agreement with the experimental data in pigs. Inhaled argon administration exhibited an on-off nature such that AUC was proportional to administration time. Confidence in the PBPK model and the remarkably robust and stable on-off nature of argon pharmacokinetics, notwithstanding intersubject variability and comorbidity, suggests that inhaled argon could readily be applied to any treatment regime.

Understanding the importance of low-molecular weight (ethylene oxide- and propylene oxide-induced) DNA adducts and mutations in risk assessment: Insights from 15 years of research and collaborative discussions

The interpretation and significance of DNA adduct data, their causal relationship to mutations, and their role in risk assessment have been debated for many years. An extended effort to identify key questions and collect relevant data to address them was focused on the ubiquitous low MW N7-alkyl/hydroxyalkylguanine adducts. Several academic, governmental, and industrial laboratories collaborated to gather new data aimed at better understanding the role and potential impact of these adducts in quantifiable genotoxic events (gene mutations/micronucleus). This review summarizes and evaluates the status of dose-response data for DNA adducts and mutations from recent experimental work with standard mutagenic agents and ethylene oxide and propylene oxide, and the importance for risk assessment. This body of evidence demonstrates that small N7-alkyl/hydroxyalkylguanine adducts are not pro-mutagenic and, therefore, adduct formation alone is not adequate evidence to support a mutagenic mode of action. Quantitative methods for dose-response analysis and derivation of thresholds, benchmark dose (BMD), or other points-of-departure (POD) for genotoxic events are now available. Integration of such analyses of genetox data is necessary to properly assess any role for DNA adducts in risk assessment. Regulatory acceptance and application of these insights remain key challenges that only the regulatory community can address by applying the many learnings from recent research. The necessary tools, such as BMDs and PODs, and the example datasets, are now available and sufficiently mature for use by the regulatory community. Environ. Mol. Mutagen. 60: 100-121, 2019. © 2018 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Pharmacokinetic analysis of the chronic administration of the inert gases Xe and Ar using a physiological based model

Background

New gas therapies using inert gases such as xenon and argon are being studied, which would require chronically administered repeating doses. The pharmacokinetics of this type of administration has not been addressed in the literature.

Methods

A physiologically based pharmacokinetics (PBPK) model for humans, pigs, mice, and rats has been developed to investigate the unique aspects of the chronic administration of inert gas therapies. The absorption, distribution, metabolism and excretion (ADME) models are as follows: absorption in all compartments is assumed to be perfusion limited, no metabolism of the gases occurs, and excretion is only the reverse process of absorption through the lungs and exhaled.

Results

The model has shown that there can be a residual dose, equivalent to constant administration, for chronic repeated dosing of xenon in humans. However, this is not necessarily the case for small animals used in pre-clinical studies.

Conclusions

The use of standard pharmacokinetics parameters such as area under the curve would be more appropriate to assess the delivered dose of chronic gas administration than the gas concentration in the delivery system that is typically reported in the scientific literature because species and gas differences can result in very different delivered doses.

The prediction of blood-tissue partitions, water-skin partitions and skin permeation for agrochemicals

BACKGROUND

There is considerable interest in the blood-tissue distribution of agrochemicals, and a number of researchers have developed experimental methods for in vitro distribution. These methods involve the determination of saline-blood and saline-tissue partitions; not only are they indirect, but they do not yield the required in vivo distribution.

RESULTS

The authors set out equations for gas-tissue and blood-tissue distribution, for partition from water into skin and for permeation from water through human skin. Together with Abraham descriptors for the agrochemicals, these equations can be used to predict values for all of these processes. The present predictions compare favourably with experimental in vivo blood-tissue distribution where available. The predictions require no more than simple arithmetic.

CONCLUSIONS

The present method represents a much easier and much more economic way of estimating blood-tissue partitions than the method that uses saline-blood and saline-tissue partitions. It has the added advantages of yielding the required in vivo partitions and being easily extended to the prediction of partition of agrochemicals from water into skin and permeation from water through skin.

Ethylene oxide in blood of ethylene-exposed B6C3F1 mice, Fischer 344 rats, and humans

The gaseous olefin ethylene (ET) is metabolized in mammals to the carcinogenic epoxide ethylene oxide (EO). Although ET is the largest volume organic chemical worldwide, the EO burden in ET-exposed humans is still uncertain, and only limited data are available on the EO burden in ET-exposed rodents. Therefore, EO was quantified in blood of mice, rats, or 4 volunteers that were exposed once to constant atmospheric ET concentrations of between 1 and 10 000 ppm (rodents) or 5 and 50 ppm (humans). Both the compounds were determined by gas chromatography. At ET concentrations of between 1 and 10 000 ppm, areas under the concentration-time curves of EO in blood (µmol × h/l) ranged from 0.039 to 3.62 in mice and from 0.086 to 11.6 in rats. At ET concentrations ≤ 30 ppm, EO concentrations in blood were 8.7-fold higher in rats and 3.9-fold higher in mice than that in the volunteer with the highest EO burdens. Based on measured EO concentrations, levels of EO adducts to hemoglobin and lymphocyte DNA were calculated for diverse ET concentrations and compared with published adduct levels. For given ET exposure concentrations, there were good agreements between calculated and measured levels of adducts to hemoglobin in rats and humans and to DNA in rats and mice. Reported hemoglobin adduct levels in mice were higher than calculated ones. Furthermore, information is given on species-specific background adduct levels. In summary, the study provides most relevant data for an improved assessment of the human health risk from exposure to ET.

Quantitative Property-Property Relationship for Screening-Level Prediction of Intrinsic Clearance of Volatile Organic Chemicals in Rats and Its Integration within PBPK Models to Predict Inhalation Pharmacokinetics in Humans

The objectives of this study were (i) to develop a screening-level Quantitative property-property relationship (QPPR) for intrinsic clearance (CL_int) obtained from in vivo animal studies and (ii) to incorporate it with human physiology in a PBPK model for predicting the inhalation pharmacokinetics of VOCs. CL_int, calculated as the ratio of the in vivo V_max (μmol/h/kg bw rat) to the K_m (μM), was obtained for 26 VOCs from the literature. The QPPR model resulting from stepwise linear regression analysis passed the validation step (R² = 0.8; leave-one-out cross-validation Q² = 0.75) for CL_int normalized to the phospholipid (PL) affinity of the VOCs. The QPPR facilitated the calculation of CL_int (L PL/h/kg bw rat) from the input data on log P_ow, log blood: water PC and ionization potential. The predictions of the QPPR as lower and upper bounds of the 95% mean confidence intervals (LMCI and UMCI, resp.) were then integrated within a human PBPK model. The ratio of the maximum (using LMCI for CL_int) to minimum (using UMCI for CL_int) AUC predicted by the QPPR-PBPK model was 1.36 ± 0.4 and ranged from 1.06 (1,1-dichloroethylene) to 2.8 (isoprene). Overall, the integrated QPPR-PBPK modeling method developed in this study is a pragmatic way of characterizing the impact of the lack of knowledge of CL_int in predicting human pharmacokinetics of VOCs, as well as the impact of prediction uncertainty of CL_int on human pharmacokinetics of VOCs.

Propylene oxide: genotoxicity profile of a rodent nasal carcinogen

Propylene oxide (PO) is a DNA-reactive genotoxic agent; that is, it reacts with DNA to produce lesions in the genetic material. PO also induces tumors in rodents, although only at high concentrations and at portals of entry. This review of PO's genotoxicity profile is organized according to endpoints measured, that is, nonmutational or mutational endpoints, and as to whether the results were from in vitro or in vivo studies. In addition to results of experimental studies, PO's genotoxicity for humans is assessed by reviewing results of published biomarker studies. The weight of evidence indicates that although it is genotoxic, PO's potency as a DNA-reactive mutagen is weak. Other aspects of PO's overall tissue toxicities are also reviewed, with attention to glutathione (GSH) depletion and its consequences, that is, cell proliferation, death, and necrosis. These toxic tissue responses occur in the same anatomical regions in rodents as do the PO-induced tumors. Furthermore, some of these tissue toxicities can produce effects that may either augment PO's DNA-reactive mutagenicity or be genotoxic in themselves, not dependent on PO's DNA reactivity. Although its DNA reactivity may be a necessary component of PO's overall genotoxicity and rodent carcinogenicity, it is likely not sufficient, and the associated tissue toxicities, which are rate-limiting, also seem to be required. This complex mode of action has implications for estimations of PO's cancer potential in humans, especially at low exposure concentrations.

Air to liver partition coefficients for volatile organic compounds and blood to liver partition coefficients for volatile organic compounds and drugs

Values of in vitro air to liver partition coefficients, K(liver), of VOCs have been collected from the literature. For 124 VOCs, application of the Abraham solvation equation to logK(liver) yielded a correlation equation with R(2)=0.927 and SD=0.26 log units. Combination of the logK(liver) values with logK(blood) values leads to in vitro blood to liver partition coefficients, as logP(liver) for VOCs; an Abraham solvation equation can correlate 125 such values with R(2)=0.583 and SD=0.23 log units. Values of in vivo logP(liver) for 85 drugs were collected, and were correlated with R(2)=0.522 and SD=0.42 log units. When the logP(liver) values for VOCs and drugs were combined, an Abraham solvation equation could correlate the 210 compounds with R(2)=0.544 and SD=0.32 log units. Division of the 210 compounds into a training set and a test set, each of 105 compounds, showed that the training equation could predict logP(liver) values with an average error of 0.05 and a standard deviation of 0.34 log units.

Air to fat and blood to fat distribution of volatile organic compounds and drugs: Linear free energy analyses

Partition coefficients, K(fat), from air to human fat and to rat fat have been collected for 129 volatile organic compounds, VOCs. A linear free energy relationship, LFER, correlates the 129 values of log K(fat) with R(2)=0.958 and a standard deviation, S.D., of 0.194 log units. Use of training and test sets gives a predictive assessment of around 0.20 log units. Combination of log K(fat) with our previously listed values of log K(blood) enables blood/plasma to fat partition coefficients, as log P(fat), to be obtained for 126 VOCs. These values can be correlated with R(2)=0.847, S.D.=0.304 log units; the latter is also our assessment of the predictive capability of the LFER. Values of log P(fat) have been collected for 46 drugs, and can be fitted to an LFER with R(2)=0.811 and S.D.=0.355 log units. Unlike partition into brain or muscle, the data for VOCs and drugs cannot be combined. There are marked discrepancies for PCBs for which partition from blood/plasma into fat is very much less than that calculated from the data on VOCs or from the data on drugs.

Air to Muscle and Blood/Plasma to Muscle Distribution of Volatile Organic Compounds and Drugs: Linear Free Energy Analyses

Distribution coefficients, K(mus), from the gas phase to the muscle have been collected for volatile organic compounds (VOCs). For 114 VOCs, a linear free energy relationship (LFER) yields an equation for log K(mus) with R(2) = 0.944 and SD = 0.267; construction of a training and test set shows that the LFER can predict further values to around 0.30 log units. The combination of the log K(mus) values with values for air to blood yields distribution coefficients from blood to muscle, log P(mus), for 110 VOCs; the corresponding LFER has R(2) = 0.537 and SD = 0.207 and a predictive capability of 0.22 log units. We also collected data on the distribution of drugs from blood or plasma to muscle and showed that the two sets of data can be combined. A LFER for blood/plasma to muscle for 59 drugs has R(2) = 0.745 and SD = 0.253 and a predictive capability of 0.25 log units. Finally, we show that the in vitro data on VOCs and the in vivo data on drugs can be combined; a LFER on the total data for 163 compounds has R(2) = 0.595, SD = 0.220, and a predictive capability of about 0.25 log units.

Air to brain, blood to brain and plasma to brain distribution of volatile organic compounds: linear free energy analyses

Partition coefficients, K(brain), for volatile organic compounds, VOCs, from air to brain have been collected for 81 compounds (air to human brain and air to rat brain). For the 81 VOCs a linear free energy equation (LFER) correlates log K(brain) with R(2) = 0.923 and S.D. = 0.346 log units. Use of training and test sets gives a predictive assessment of 0.35-0.40 log units. Combination of log K(brain) with our previously listed values of log K(blood) enables blood to brain partition, as log P(b-brain), to be obtained for 78 VOCs. These values can be correlated with R(2) = 0.725 and S.D. = 0.203 log units; use of training and test sets allows a predictive assessment for log P(b-brain) of 0.16-0.20 log units. Values for air to plasma were available for 21 VOCs. When these data were combined with the data on air to blood and air to brain, values for partition between (blood or plasma) to brain, P(bp-brain), were available for 99 VOCs. A LFER correlates this data with R(2) = 0.703 and S.D.=0.197 log units; use of training and test sets allows a predictive assessment for log P(bp-brain) of 0.15-0.20 log units.

QSAR modeling of blood:air and tissue:air partition coefficients using theoretical descriptors

Human blood:air, human and rat tissue (fat, brain, liver, muscle, and kidney):air partition coefficients of a diverse set of organic compounds were correlated and predicted using structural descriptors by employing CODESSA-PRO and ISIDA programs. Four and five descriptor regression models developed using CODESSA-PRO were validated on three different test sets. Overall, these models have reasonable values of correlation coefficients (R(2)) and leave-one-out correlation coefficients (R(cv)(2)): R(2) = 0.881-0.983; R(cv)(2) = 0.826-0.962. Calculations with ISIDA resulted in models based on atom/bond sequences involving two to three atoms with statistical parameters that were similar to those of models obtained with CODESSA-PRO (R(2) = 0.911-0.974; R(cv)(2) = 0.831-0.936). A mixed pool of molecular and fragment descriptors did not lead to significant improvement of the models.

Air to Blood Distribution of Volatile Organic Compounds: A Linear Free Energy Analysis

Partition coefficients, K(blood), for volatile organic compounds from air to blood have been collected for 155 compounds (air to human blood) and 127 compounds (air to rat blood). For 86 common compounds, the average error, AE, between the two sets of log K(blood) values is 0.12 log units, somewhat smaller than our estimated interlaboratory average SD value of around 0.16 log units. We conclude that with regard to experimental errors, there is no significant difference between K(blood) values in human blood and in rat blood. There are 196 compounds for which either or both K(blood) (human) and K(blood) (rat) are available. A training set of 98 compounds could be fitted with the Abraham solvation parameters with R(2) = 0.933 and SD = 0.34 log units. The training equation was then used to predict the test set of values with AE = 0.04 log units, SD = 0.33 log units, and an average absolute error, AAE, of 0.25 log units. A second training and test set yielded similar values: AE = 0.01, SD = 0.39, and AAE = 0.29 log units. It is concluded that it is possible to construct an equation capable of predicting further values of log K(blood) to around 0.30 log units. Because the descriptors used in the correlation equations can be predicted from structure, it is now possible to predict log K(blood) for any chemical structure.

The "Tuebingen desiccator" system, a tool to study oxidative stress in vivo and inhalation toxicokinetics

The "Tuebingen desiccator," a gas-tight all-glass closed chamber system (CCS), has been established in Herbert Remmer's Institute of Toxicology, University of Tuebingen, to investigate the mechanisms underlying the exhalation of endogenous volatile hydrocarbons in rats under oxidative stress. Remmer and associates confirmed the former view that ethane and n-pentane were derived from polyunsaturated fatty acids, and they demonstrated that propane, n-butane and isobutane were released from amino acids. Hydrocarbons exhaled following acute ethanol treatment of rats resulted predominantly from ethanol-dependent inhibition of their metabolism and partly from oxidation of proteins. Exhalation of alkanes in carbon tetrachloride exposed rats did not reflect liver damage, which was, however, directly linked to the amount of carbon tetrachloride metabolized. As has first been shown in Herbert Remmer's institute by investigating the fate of inhaled vinyl chloride in rats, the CSS proved to be also an excellent tool for studying toxicokinetics of inhaled gaseous xenobiotics by means of gas uptake experiments. Based on results gained by such studies, it was recently demonstrated that knowledge of compound-specific physicochemical and species-specific physiological parameters are often sufficient to predict important toxicokinetic properties of inhaled chemicals such as tissue burdens at steady state. By means of the CCS, not only kinetics of a parent gaseous substance but also of gaseous metabolites can be investigated in vivo, as exemplified for ethylene oxide and 1, 2-epoxy-3-butene, metabolites of ethylene and 1,3-butadiene, respectively. Gas uptake studies in closed chamber systems are now worldwide used for determining toxicokinetic parameters relevant for physiological toxicokinetic modeling.

QSPR treatment of rat blood:air, saline:air and olive oil:air partition coefficients using theoretical molecular descriptors

A QSPR treatment has been applied to a data set that consists of 100 diverse organic compounds to relate the logarithmic function of rat blood:air, saline:air and olive oil:air partition coefficients (denoted by log K(b:a), log K(s:a), and log K(o:a), respectively), with theoretical molecular and fragment descriptors. Three QSPR models with squared correlation coefficients of 0.881, 0.926, and 0.922, respectively, were obtained. The verification of the predictive power of these models on a test set of 33 organic chemicals that were not included in the training set gave satisfactory squared correlation coefficients: 0.791 for rat blood:air, 0.794 for saline:air and 0.846 for olive oil:air.

Hprt mutant frequencies in splenic T-cells of male F344 rats exposed by inhalation to propylene

Propylene is a major industrial intermediate and atmospheric pollutant to which humans are exposed by inhalation. In this study, 6-week-old male F344 rates were exposed to 0, 200, 2000, or 10,000 ppm propylene by inhalation for 4 weeks (6 h/day, 5 days/week), and mutant frequencies were determined in the Hprt gene of splenic T-lymphocytes. Twenty milligrams of cyclophosphamide monohydrate (CPP)/kg bw, given on the penultimate day of propylene exposure, was used as a positive control mutagen. Rats (n = 8/group) were necropsied for isolation of T-cells 8 weeks after the last dose, a sampling time that produced peak spleen Hprt mutant frequencies (Mfs) in a preliminary mutant manifestation study using CCP treatment. Hprt Mfs were measured via the T-cell cloning assay, which was performed without knowledge of the animal treatment groups. Mean Hprt Mfs were significantly increased over control values (mean Mf = 5.24 +/- 1.55 (SD) x 10(-6)) in CPP-treated rats (10.37 +/- 4.30 x 10(-6), P = 0.007). However, Hprt Mfs in propylene-exposed rats were not significantly increased over background, with mean Mfs of 4.90 +/- 1.84 x 10(-6) (P = 0.152), 5.05 +/- 3.70 x 10(-6) (P = 0.895), and 5.95 +/- 2.49 x10(-6) (P = 0.500) for animals exposed to 200, 2000, or 10,000 ppm propylene, respectively. No significant increase in F344 rat or B6C3F1 mouse cancer incidence was reported in the National Toxicology Program carcinogenicity studies of propylene across this same exposure range. Taken together, these findings support the conclusion that inhalation exposure of rats to propylene does not cause mutations or cancer.

Estimation of a possible tumorigenic risk of styrene from daily intake via food and ambient air

Concerns of a tumorigenic risk of styrene (ST) originate from the findings that styrene (ST) is metabolized to the genotoxic intermediate styrene-7,8-oxide (SO). Therefore, it was hypothesized that results of animal long-term studies with ST and SO together with the SO tissue burden are sufficient for conducting a 'worst case' estimate of the tumorigenic risk of ST. On this basis we predicted the excess human lifetime risk for lung tumors (p(EXL)) and the highest possible risk for other systemic tumors (p(HPS)) resulting from daily intake of ST via food and ambient air. As measures for p(EXL) the mean lifetime concentration of SO in the transitional zone of the lung and for p(HPS) the mean lifetime concentration of SO in blood were calculated using a physiological toxicokinetic model. For a daily oral intake of 12 microST, p(EXL) was obtained to be between 5x10(-9) and 2x10(-8) and p(HPS) to be between 7x10(-9) and 2x10(-8). Lifetime risks calculated for continuous exposure to 3 microg/m(3) ST in ambient air were between 8x10(-7) and 3x10(-6) (p(EXL)) and between 2x10(-8) and 4x10(-8) (p(HPS)). Although these values indicate very low risks, the actual risks are expected to be even by far smaller. This is discussed in detail for lung tumorigenesis.

Toxicokinetics of inhaled and endogenous isoprene in mice, rats, and humans

Isoprene (IP) is ubiquitous in the environment and is used for the production of polymers. It is metabolized in vivo to reactive epoxides, which might cause the tumors observed in IP exposed rodents. Detailed knowledge of the body and tissue burden of inhaled IP and its intermediate epoxides can be gained using a physiological toxicokinetic (PT) model. For this purpose, a PT-model was developed for IP in mouse, rat, and human. Experimentally determined partition coefficients were taken from the literature. Metabolic parameters were obtained from gas-uptake experiments. The measured data could be described by introducing hepatic and extrahepatic metabolism into the model. At exposure concentrations up to 50 ppm, the rate of metabolism at steady-state is 14 times faster in mice and about 8 times faster in rats than in humans (2.5 micromol/h/kg at 50 ppm IP in air). IP does accumulate only barely due to its fast metabolism and its low thermodynamic partition coefficient whole body:air. IP is produced endogenously. This production is negligible in rodents compared to that in humans (0.34 micromol/h/kg). About 90% of IP produced endogenously in humans is metabolized and 10% is exhaled unchanged. The blood concentration of IP in non-exposed humans is predicted to be 9.5 nmol/l. The area under the blood concentration-time curve (AUC) following exposure over 8 h to 10 ppm IP is about 4 times higher than the AUC resulting from the unavoidable endogenous IP over 24 h. A comparison of such AUCs can be used for establishing workplace exposure limits. For estimation of the absolute risk, knowledge of the body burden of the epoxide intermediates of IP is required. Unfortunately, such data are not yet available.