Applications of physiologic pharmacokinetic modeling in carcinogenic risk assessment.

The use of physiologically based pharmacokinetic (PBPK) models has been proposed as a means of estimating the dose of the reactive metabolites of carcinogenic xenobiotics reaching target tissues, thereby affording an opportunity to base estimates of potential cancer risk on tissue dose rather than external levels of exposure. In this article, we demonstrate how a PBPK model can be constructed by specifying mass-balance equations for each physiological compartment included in the model. In general, this leads to a system of nonlinear partial differential equations with which to characterize the compartment system. These equations then can be solved numerically to determine the concentration of metabolites in each compartment as functions of time. In the special case of a linear pharmacokinetic system, we present simple closed-form expressions for the area under the concentration-time curves (AUC) in individual tissue compartments. A general relationship between the AUC in blood and other tissue compartments is also established. These results are of use in identifying those parameters in the models that characterize the integrated tissue dose, and which should therefore be the primary focus of sensitivity analyses. Applications of PBPK modeling for purposes of tissue dosimetry are reviewed, including models developed for methylene chloride, ethylene oxide, 1,4-dioxane, 1-nitropyrene, as well as polychlorinated biphenyls, dioxins, and furans. Special considerations in PBPK modeling related to aging, topical absorption, pregnancy, and mixed exposures are discussed. The linkage between pharmacokinetic models used for tissue dosimetry and pharmacodynamic models for neoplastic transformation of stem cells in the target tissue is explored.

Pharmacokinetics of inhaled pyrene in rats.

Male Wistar rats were exposed to micronized aerosol concentrations of a 14C-labeled model polycyclic aromatic hydrocarbon (pyrene) at 200, 500, and 800 mg/m3 for a period of 95 min. Both the 14C label and free pyrene were monitored in the blood, urine, and feces. At the termination of the blood sampling, three of the six rats per dose group were killed and the distribution of [14C]pyrene to eight major tissues was analyzed. The analysis of blood concentration data using a one-compartment pharmacokinetic model revealed that the uptake and elimination kinetic parameters were dose dependent, for both total radioactivity (pyrene plus metabolites) and for pyrene per se, over the range of exposures used in this study. The ratio of the percent excreted via the urinary and fecal routes, collected over a 5-d period postexposure was about 1.0 at each exposure level.

Percutaneous uptake, distribution, and excretion of pyrene in rats.

Groups of 12 male Wistar rats, of about 400 g body weight, were dosed with 2, 6, or 15 mg/kg of 14C-labeled pyrene, dissolved in acetone, applied to 4 cm2 of a shaved area of the mid back. Three animals in each dose group were killed at 1, 2, 4, and 6 d post-dosing, and their principal organs were removed and analyzed for pyrene and [14C]pyrene equivalents. Urine and feces, as well as the area of skin to which the dose was applied, were also analyzed for [14C]pyrene equivalents. The rate of uptake from the skin was rapid (t1/2 0.5-0.8 d) relative to rate processes for the other organs, and about 50% of the applied dose was excreted over the 6 d of the study. The significant decrease in the fraction of the dose excreted and in the normalized amounts distributed to the various organs and tissues, as the dose increased for all chemical species measured, was strongly suggestive of nonlinear kinetics, as has been observed in previous studies. Levels of pyrene were highest in the liver, kidneys, and fat. Levels of metabolites were also high in the lung. It was evident that the dermal route of uptake was not insignificant for this model polycyclic aromatic hydrocarbon and may represent a significant exposure route for exposed humans.

Distribution of benzo[a]pyrene in pregnant rats following inhalation exposure and a comparison with similar data obtained with pyrene.

Eight pregnant rats were exposed, on the 17th day of gestation, for 95 min to a microcondensation aerosol of benzo[a]pyrene at five different atmospheric concentrations between 200 and 800 mg m-3 in a 'head-only' inhalation chamber. Five rats were killed immediately following the exposure and three were killed at 6 h post-dosing. Concentrations of the radiolabel and 'free' benzo[a]pyrene were measured in the individual fetuses and in the maternal blood, fat, kidney, liver and lung. Distribution to the fetus did not appear to be related to its position on the uterine horn and the uptake of benzo[a]pyrene was non-linear with increasing exposure concentrations, which was similar to the observations previously reported for pyrene. The levels of benzo[a]pyrene were much higher in the fetus and, especially, the lung than those observed in the pyrene study; so also were the levels of total metabolites in these tissues, which might, in part, account for the carcinogenic potency of benzo[a] pyrene.

Physiological pharmacokinetics and cancer risk assessment.

There has been considerable progress in recent years in developing physiological models for the pharmacokinetics of toxic chemicals and in the application of these models in cancer risk assessment. Physiological pharmacokinetic models consist of a number of individual compartments, based on the anatomy and physiology of the mammalian organism of interest, and include specific parameters for metabolism, tissue binding, and tissue reactivity. Because of the correspondence between these compartments and specific tissues or groups of tissues, these models are particularly useful for predicting the doses of biologically active forms of toxic chemicals at target tissues under a wide variety of exposure conditions and in different animal species, including humans. Due to their explicit characterization of the biological processes governing pharmacokinetic behaviour, these models permit more accurate predictions of the dose of active metabolites reaching target tissues in exposed humans and hence of potential cancer risk. In addition, physiological models also permit a more direct evaluation of the impact of parameter uncertainty and inter-individual variability in cancer risk assessment. In this article, we review recent developments in physiologic pharmacokinetic modeling for selected chemicals and the application of these models in carcinogenic risk assessment. We examine the use of these models in integrating diverse information on pharmacokinetics and pharmacodynamics and discuss challenges in extending these pharmacokinetic models to reflect more accurately the biological events involved in the induction of cancer by different chemicals.

Distribution to the fetus and major organs of the rat following inhalation exposure to pyrene.

Concentrations of pyrene and total metabolites were determined for individual fetuses and selected maternal organs and tissues immediately and 6 h following a 95-min head-only exposure of pregnant Wistar rats, on gestation day 17, to five levels of pyrene over the range 200-800 mg m-3 as a microcondensation aerosol. The influence of uterine horn, side and position, on distribution to the fetus was assessed. The concentration of both pyrene and its metabolites increased more over the exposure range (eightfold) than did those in the fetus. Concentrations of pyrene or its metabolites in fetal tissues were not found to be related to its position on the uterine horn. There was an unexplained and significant (P less than 0.01) higher concentration of pyrene in fetuses on the right side relative to the left side of the uterine horn for the animals killed immediately following exposure. A comparison of the levels in maternal tissues measured immediately following the exposure and 6 h later showed that there was some redistribution of pyrene and its metabolites to the fat tissues, i.e. levels in the fat increased over the 6 h period following the exposure. Levels in the other tissues diminished during this period. In general, concentrations of pyrene and its metabolites were lowest in the fetal tissues relative to those in the sampled maternal organs and tissues.

Pharmacokinetics and bioavailability of pyrene in the rat.

Groups of 6 male Wistar rats, of about 400 g body weight, were dosed with 14C-labeled pyrene, dissolved in an Emulphor/water solvent vehicle, at 5 different dose levels by the intravenous or oral routes. Appropriate mathematical models were fitted to blood concentration-time data for [14C]pyrene and pyrene per se and dose-trend analyses were carried out. Areas under these curves were used to assess the bioavailability of the orally administered doses. Tissue concentrations, measured at the termination of the blood sampling period, gave a quantitative measure of the distribution of the administered dose. Attempts to repeat these studies with similar doses of tritium-labeled benzo[a]pyrene were frustrated by the lack of meaningful blood-level data. Dose trends for the derived pharmacokinetic parameters for pyrene revealed that the kinetics were nonlinear and strongly suggestive of enterohepatic recycling. Biliary excretion, measured in a separate experiment, gave support to this hypothesis. The bioavailability of the orally administered doses was between 50 and 60%. Over a 6-d period postdosing, some 45 and 40% of the administered dose was excreted via the urine and feces, respectively, irrespective of the route of administration. Distribution to the tissues of the 14C-label was highest in the perirenal fat, intermediate in the liver, kidneys, and lungs, and lowest in the heart, testes, spleen, and brain.

Recent developments in carcinogenic risk assessment.

In this paper, recent developments in the quantitative assessment of carcinogenic risks based on toxicological and epidemiological data are reviewed. In particular, model-free approaches to low-dose risk assessment which involve only the assumption of low-dose linearity are considered. Measures of carcinogenic potency which avoid the need to extrapolate to low doses are also described. The allometric bases for converting risk estimates between species are then discussed. Pharmacokinetic models for determining the dose delivered to the target tissue are examined, and the implications of using such models in extrapolating between doses, of exposure, and species are examined. The application of these concepts in chemical and radiation carcinogenesis is illustrated by means of brief case studies of methylene chloride and Rn. Biologically motivated cancer models based on the initiation-promotion-progression theory of carcinogenesis are discussed and compared with the classical multistage model. The estimation of risks with time-dependent exposure patterns is considered, and conditions under which the use of a time-weighted average dose is appropriate are identified. Finally, the estimation of carcinogenic risks posed by exposure to complex mixtures is explored.

Methods for route-to-route extrapolation of dose.

Results of acute toxicity studies for a variety of chemicals have indicated that, in most cases, although the inhalation route was more effective than the IG route, wide variations in toxicity occurred between these two routes. The major factors that may result in variations in toxicity between routes include: differences in absorption efficiency; differences in systemic effects; occurrence of critical toxicological effects at the portal of entry; first-pass effects resulting in inactivation or activation of the chemical agent before it reaches the target organ; and variations in temporal patterns of target organ concentrations. Extrapolation to determine safe exposure levels during chronic exposure becomes less reliable, not only as information relating to these factors decreases, but also as the quality or length of exposure decreases in the available toxicologic studies. VDC is an example of one of a few chemicals for which both chronic inhalation and oral toxicity data are available, along with detailed pharmacokinetic information. On the basis of the pharmacokinetic data, differences in toxicity between the two routes did not appear to be very likely for this chemical. This conjecture was supported by the results of chronic toxicity studies. Finally, assuming sufficient data and pharmacokinetic parameters are available, this paper presents a useful and practical approach to route extrapolation.

Fetal distribution of styrene in rats after vapor phase exposures.

Pregnant rats were subjected to a 5-hour exposure of either 2,000 or 1,000 ppm atmospheres of styrene. Concentrations of styrene in the maternal blood and in each fetus were measured. The fetal concentrations in the 2,000 ppm exposure group were significantly more than double that in the 1,000 ppm group. Fetal weight and distribution of styrene appear to be related to the fetal position on the uterine horn. Those at the ovarian and cervical ends of the uterine horn were of lowest weight and those at the ovarian end had the highest concentration of styrene. Concentrations of styrene appeared to be much lower in the fetuses than in maternal organs and tissues after similar exposures.

The fetal distribution of some aliphatic chlorinated hydrocarbons in the rat after vapor phase exposure.

Rats, on the 17th day of pregnancy, were exposed for five h to several different concentrations, ranging from about 100 to 3,000 ppm, of methylene chloride, 1,2-dichloroethane, chloroform and trichloroethylene. Immediately following exposure, the concentrations of these compounds were determined in each fetus and in the maternal blood and characterized as a function of the administered dose. Fetal weights and fetal concentrations were related to their position on the two horns of the uterus. Fetal weight distribution conformed with observations previously reported [Withey and Karpinski 1983, Barr et al. 1969]. Fetuses at either end of each branch of the uterine horns had the lowest concentration of trichloroethylene. The data for methylene chloride and dichloroethane revealed a linear decrease in fetal concentration with the location of the fetus from the ovarian to the cervical end of the uterine horns. These relationships were consistent across doses. Fetal chloroform concentrations did not appear to be related to fetal position. Good linear relationships between the mean fetal concentrations and the maternal blood concentrations with exposure level were observed for the four compounds used in this study.

Effect of vehicle on the pharmacokinetics and uptake of four halogenated hydrocarbons from the gastrointestinal tract of the rat.

A variety of low molecular weight aliphatic chlorinated hydrocarbons have a history of use in the processing of foods, and their presence in potable water supplies has also been demonstrated. In this study, the relative uptake of four different aliphatic chlorinated hydrocarbons (methylene chloride, dichloroethane, chloroform and trichloroethylene) was studied after administration, by intragastric intubation, at the same dose level as corn-oil or aqueous solutions. Serial blood samples were collected over a 5-h period and uptake was assessed as the calculated area under blood concentration-time curves. A significant decrease in the rate and extent of uptake was observed for the compounds when administered as an oil solution as compared with an aqueous solution. Lower peak concentrations and an increase in the time taken to reach peak concentration, after dosing with corn oil solutions, was also observed.

Chlorinated aliphatic hydrocarbons used in the foods industry: the comparative pharmacokinetics of methylene chloride, 1,2 dichloroethane, chloroform and trichloroethylene after I.V. administration in the rat.

the pharmacokinetics of methylene chloride, CH2Cl2, 1,2-dichloroethane, CH2ClCH2Cl, chloroform, CHCl3 and trichloroethylene, CHCl.CCl2 were determined, after i.v. administration to male Wistar rats, at five dose levels ranging from 3 to 15 mg/kg. The elimination of chloroform followed a three compartment model at all dose levels. The remaining compounds gave data that fitted a two compartment model at low doses which changed to a three compartment model at higher dose levels. All data were fitted to a two and three compartment model and the model that gave the best fit was selected on the basis of the smallest residual mean squared error. Very few differences, apart from the change in the nature of the pharmacokinetic model, were seen in the rate coefficients over the dose range tested. At the highest dose level, the rates of uptake and elimination for the major tissues were similar to those observed for the blood but were much slower for the perirenal fat.

The distribution and pharmacokinetics of styrene monomer in rats by the pulmonary route.

Exposures (5 hr) of individual rats to atmospheres having styrene concentrations of from 50 to 2,000 ppm yield uptake blood profiles which show a continued and increasing absorption of styrene, proportional to the styrene atmospheric concentration, over the entire absorption period. Post-exposure elimination follows a dose-dependent two compartment model, similar to that observed after I.V. administration, although the initial stage of the elimination is more rapid and the terminal stage much slower than is observed in the I.V. case. A two-compartment pharmacokinetic model, with zeroth-order input, is proposed to interpret the uptake kinetics. Tissue concentrations of styrene in the heart, liver, lung, kidney, spleen, brain and perirenal fat show a different pattern of distribution as the dose increases. At all exposure concentrations the concentration of styrene in the perirenal fat is ten times that in any organ. At the lowest exposure concentration the kidney registered the highest concentration while a higher level of styrene is found in the liver, brain and kidney as the exposure level increases and for these tissues the concentration of styrene is always greater than that in the blood.

Pharmacokinetics and distribution of styrene monomer in rats after intravenous administration.

An interest in the pharmacokinetics of styrene monomer in the rat, arising from the presence of the monomer in the industrial work place and in foods, necessitated an investigation of the dose dependency of the kinetics of styrene monomer when administered by the iv route. A rapid distribution of the monomer to the major organs was observed, and all of the rate coefficients describing the rates of distribution and elimination decreased with increasing dose. No change in the apparent volume of distribution with dose was observed. Some evidence for the involvement of saturable metabolic pathways was obtained.

A statistical assessment of the quantitative uptake of vinyl chloride monomer from aqueous solution.

The presence of vinyl chloride monomer (VCM) in foodstuffs and its demonstrated carcinogenic potential when administered by the oral route has raised questions concerning the quantitative estimation of the safety of the use of food packaging fabricated from rigid polyvinyl chloride. A statistical model, which was tested by curve-fitting data obtained from an oral uptake study, has been demonstrated to be of predictive value. Ninety-five percent condifence limits were also calculated, and the data from this study were compared with those from a previous gas phase exposure study. It was concluded that if the total daily liquid intake contained 20 ppm of VCM then the area generated under the blood level-time curve, for rats, would be equivalent to an inhalation exposure of about 2 ppm for 24 hr.