Abstract  A novel, standardized geometry of the human nasal cavity was created by aligning and processing 30 sets of computed tomography (CT) scans of nasal airways of healthy subjects. Digital three-dimensional (3-D) geometries of the 60 single human nasal cavities (30 right and 30 mirrored left cavities) were generated from the CT scans and measurements of physical parameters of each single nasal cavity were performed. A methodology was developed to scale, orient, and align the nasal geometries, after which 2-D digital coronal cross-sectional slices were generated. With the use of an innovative image processing algorithm, median cross-sectional geometries were created to match median physical parameters while retaining the unique geometric features of the human nasal cavity. From these idealized 2-D images, an original 3-D standardized median human nasal cavity was created. This new standardized geometry was compared against the original geometries of all subjects as well as limited existing data from the literature. The new model has potential for use as a geometric standard in future experimental and numerical studies of deposition of inhaled aerosols, as well as for use as a reference during diagnosis of unhealthy patients. The specific procedure developed could also be applied to build standard nasal geometries for different identifiable groups within the larger population.

Abstract  Rats of various postnatal ages were utilized to investigate developmental changes in the short-term distribution of [3H]mannitol, [3H]inulin and [51Cr]-tagged erythrocytes in both the cerebellum (CER) and cerebral cortex (CC). As the CNS undergoes maturation the most precipitous decrease in the 1-h radiomannitol (and inulin) space in the CER takes place during the second postnatal week whereas the greatest reduction in the spaces for both radioisotopes in the CC occurs during the earlier period between 0.5 and 1 week. The volume of distribution in immature brain tissue (1 week) into which [3H]mannitol penetrates in 1 h is substantially less (by 75--90%) than that into which this tracer permeates in non-neural tissue such as heart and liver; thus, even prior to the proliferation of glial elements, there is a limited barrier effect. At 3 weeks the permeability of the blood--brain barrier in the CER to mannitol is comparable to that in the CC. The vascularity of the CER (as estimated by residual [51Cr]erythrocyte-volume) is greater than that of the CC at each postnatal age investigated. The uptake of radiomannitol by the choroid plexus (blood--CSF barrier) in immature animals is a reflection primarily of developmental changes in volume of extracellular fluid rather than in permeability of the basolateral face of the choroidal epithelium. Qualitative differences, neonates vs adults, in the distribution of [3H]mannitol between CSF and brain tissue are attributable to maturational changes in secretory and permeability phenomena associated with the blood--CSF and blood--brain barriers.

Abstract  The process of assessing the risk associated with human exposure to environmental chemicals inevitably relies on a number of assumptions, estimates and rationalizations. One of the more challenging aspects of risk assessment involves the need to extrapolate beyond the range of conditions used in experimental animal studies to predict anticipated human risks. The most obvious extrapolation required is that from the tested animal species to humans; but others are also generally required, including extrapolating from high dose to low dose, from one route of exposure to another and from one exposure timeframe to another. Several avenues are available for attempting these extrapolations, ranging from the assumption of strict correspondence of dose to the use of statistical correlations. One promising alternative for conducting more scientifically sound extrapolations is that of using physiologically based pharmacokinetic models that contain sufficient biological detail to allow pharmacokinetic behavior to be predicted for widely different exposure scenarios. In recent years, successful physiological models have been developed for a variety of volatile and nonvolatile chemicals, and their ability to perform the extrapolations needed in risk assessment has been demonstrated. Techniques for determining the necessary biochemical parameters are readily available, and the computational requirements are now within the scope of even a personal computer. In addition to providing a sound framework for extrapolation, the predictive power of a physiologically based pharmacokinetic model makes it a useful tool for more reliable dose selection before beginning large-scale studies, as well as for the retrospective analysis of experimental results.

Abstract  Endometriosis is a disease which afflicts women of reproductive age. It is one of the most common causes of pelvic pain and infertility; an estimated 5 million women in the U.S. are affected.1 Despite its apparent widespread occurrence, there are no national or international surveillance systems to determine how many suffer from this disease. Since there is no noninvasive test that is highly selective and specific for endometriosis, studies of its prevalence and incidence have been limited to women undergoing gynecologic operations.

Abstract  Because many diseases of the Western Hemisphere are hormone-dependent cancers, we have postulated that the Western diet, compared to a vegetarian or semivegetarian diet, may alter hormone production, metabolism, or action at the cellular level by some biochemical mechanisms. Recently, our interest has been mainly focused on the cancer-protective role of some hormonelike diphenolic phytoestrogens of dietary origin, the lignans and the isoflavonoids. The precursors of the biologically active compounds originate in soybean products (mainly isoflavonoids), whole grain cereal food, seeds, and probably berries and nuts (mainly lignans). The plant lignan and isoflavonoid glycosides are converted by intestinal bacteria to hormonelike compounds with weak estrogenic but also antioxidative activity; they have now been shown to influence not only sex hormone metabolism and biological activity but also intracellular enzymes, protein synthesis, growth factor action, malignant cell proliferation, differentiation, and angiogenesis in a way that makes them strong candidates for a role as natural cancer-protective compounds. Epidemiologic investigations strongly support this hypothesis because the highest levels of these compounds in the diet are found in countries or regions with low cancer incidence. This report is a review on recent results suggesting that the diphenolic isoflavonoids and lignans are natural cancer-protective compounds.

Abstract  Female Sprague-Dawley rats were given 0, 168, 840, 2550 or 4200 mg/kg of 1,4-dioxane 21 and 4 h before sacrifice. Hepatic DNA damage (by the alkaline elution technique), ornithine decarboxylase activity (ODC), reduced glutathione content, cytochrome P-450 content and serum alanine aminotransferase activity (ALT) were determined. Treatment with 1,4-dioxane increased hepatic DNA damage and cytochrome P-450 content at doses of 2550 and 4200 mg/kg. Large increases in the activity of hepatic ODC were observed at 840, 2550 and 4200 mg/kg of 1,4-dioxane. Thus the data suggest that 1,4-dioxane is a weak genotoxic carcinogen in addition to being a strong promoter of carcinogenesis (a non-genotoxic carcinogen).

Abstract  Four groups of rats, 60/sex/level, were maintained on drinking water containing 0, 1.0, 0.1, or 0.01% 1,4-dioxane for up to 716 days. Male and female rats receiving 1% dioxane (equivalent to approximately 1015 and 1599 mg/kg/day, respectively) showed decreases in body weight gains, survival rates, and water consumption. Hepatocellular and renal tubular degenerative changes, accompanied by regenerative activity, were similar to those reported in previous studies following exposure to toxic levels of dioxane. Hepatocellular and nasal carcinomas, occurring at this dose level, were considered related to the lifetime exposure to these massive toxic dosages of dioxane. Male and female rats receiving 0.1% dioxane (equivalent to approximately 94 and 148 mg/kg/day, respectively) in the drinking water had variable degrees of renal and hepatic degenerative changes, but there was no indication of treatment-related tumor occurrence. Male and female rats receiving 0.01% dioxane in the drinking water (equivalent to approximately 9.6 and 19.0 mg/kg/day, respectively) showed no evidence of tumor formation or other toxic effects considered to be related to treatment. These data indicate a dose response for the toxicity of dioxane.

Abstract  Dioxane (1,4-dioxane) is a colorless liquid used as a specialty solvent since about 1930. The prinicipal toxic effects of dioxane have long been known to be centrilobular, hepatocellular and renal tubular epithelial degeneration and necrosis (DeNavasquex, 1935; Fairley et al., 1934; Schrenk and Yant, 1936; Kesten et al., 1939). More recent reports by Argus et al. (1965) and Hoch-Ligeti et al.(1970) described nasal and hepatic carcinomas in rats ingesting water containing massive doses (up to 1.8% in the drinking water) of dioxane for over 13 months. Due to the massive doses which the rats received in these more recent studies, it is difficult to use the information to assess the hazard which may be incurred by individuals exposed to lower, more realistic levels of dioxane. Thus, a series of experiments has been conducted in our laboratory. This series has included a study of rats ingesting various dose levels of dioxane in the drinking water for 2 years (kociba et al., 1974), a study of rat exposed to vapors of dioxane for 2 years (Torkelson et al., 1974), and a study of the pharmacokinetics and metabolism of various dose levels of dioxane (Young et al. 1975).

Abstract  A cancer bioassay conducted in 1974 (Kociba et al.) indicated that rats given drinking water containing dioxane at a dose of 1184 mg.kg-1.d-1 produced an increased incidence of liver tumors. Applying the linearized multistage extrapolation model to these data, the administered dose estimated to present a human cancer risk of 1 in 100,000 (10(-5)) was 0.01 mg.kg-1.d-1. As in customary regulatory policy, this estimate assumed that humans were about 5.5 times more sensitive than rats on a mg/kg basis. However, this approach did not consider that the metabolism of dioxane is saturable at high doses. Based on experience with similar chemicals, it is known that the conventional risk extrapolation method may overestimate the most likely human cancer risk. In order to determine more accurately the likely human response following lifetime exposure to dioxane, a physiologically-based pharmacokinetic (PB-PK) model was developed. The objective of this study was to establish a quantitative relationship between the administered dose of dioxane and the internal dose delivered to the target organ. Using this PB-PK model, and assuming that the best dose surrogate for estimating the liver tumor response was the time-weighted average lifetime liver dioxane concentration, the cancer risk for humans exposed to low doses of dioxane was estimated. The dose surrogate in humans most likely to be associated with a tumorigenic response of 1 in 100,000 is 280 mumol/l, equivalent to an administered dose of about 59 mg.kg-1.d-1. The 95% lower confidence limit on the dose surrogate at the same response level is 1.28 mumol/l, equivalent to an administered dose of 0.8 mg.kg-1.d-1. This PB-PK analysis indicated that conventional approaches based on the administered doses in the rodent bioassay, if uncorrected for metabolic and physiological differences between rats and humans, will overestimate the human cancer risk of dioxane by as much as 80-fold.

Abstract  Using 3H-dioxane, the distribution of dioxane among a number of tissues and various subcellular fractions of rat liver was studied. At various times after i.p. injection, dioxane was found to distribute more or less uniformly among various tissues (liver, kidney, spleen, lung, colon and skeletal muscle), consistent with its polar/nonpolar nature. Studies of the nature of dioxane binding, however, revealed that the extent of “covalent” binding (as measured by incorporation into lipid-free, acid-insoluble tissue residues) was significantly higher in the liver (the main carcinogenesis target tissue), spleen and colon than that in other tissues. Investigations of the subcellular distribution in liver indicated that most of the radioactivity was in the cytosol, followed by the microsomal, mitochondrial and nuclear fractions. The binding of dioxane to the macromolecules in the cytosol was mainly noncovalent. The percent covalent binding was highest in the nuclear fraction, followed by mitochondrial and microsomal fractions and the whole homogenate. Pretreatment of rats with inducers of microsomal mixed-function oxidases had no significant effect on the covalent binding of dioxane to the various subcellular fractions of the liver. There was no microsome-catalyzed in vitro binding of 3H- or 14C-dioxane to DNA under conditions which brought about substantial binding of 3H-benzo[a]pyrene.

Abstract  In the preceding communication (I), we summarized our studies on the metabolism in vivo of dioxane which identified p-dioxane-2-one to be the major urinary metabolite. It is well documented that the activity of mixed-function oxidases (MFO) metabolizing foreign compounds can be modulated by various inducers, repressors and inhibitors (2-4). In order to investigate the involvement of MFO in the metabolism in vivo of dioxane, the effect of several such agents on the excretion of E-dioxane-2-one was studied. Male Sprague-Dawley rats (95-130 g) were used throughout. Three typical inducers of MFO were employed: phenobarbital (PB), the polychlorinated biphenyls, Aroclor 1254 (PCB) (Monsanto Chemical Co.), and 3-methylcholanthrene (MC). PB was dissolved in 0. 9% saline and administered i.p. at a dose of 80 mg/kg daily for 4 consecutive days prior to dioxane administration. PCB and MC were dissolved in corn oil; PCB was given at a single dose of 500 mg/kg 4 days prior to dioxane administration and MC at 40 mg/kg 24 hr prior to dioxane administration. Control rats received equivalent amounts of saline or corn oil. To repress the synthesis of cytochrome P-450 (5), cobaltous chloride (60 mg/kg) was injected s. c. 24 hr prior to dioxane administration. Dioxane (3 g/kg) was given i. p. and urine samples were collected at 8- or 12-hr intervals andtreated as previously (1). 2,4-Dichloro-6-phenylphenoxy ethylamine (DPEA), a potent, long-acting inhibitor of MFO (6, 7) (gift of Dr. R. E. McMahon. Eli Lilly Co. ), was dissolved in saline and given at the dose of 15. 9 mglkg 30 min prior to dioxane administration and 8, 16 and 24 hr thereafter. The amount of p-dioxane-2-one present in urine was determined by analytical gas chromatography using purified synthetic reference compound as standard.

Abstract  A six compartment physiologically based pharmacokinetic (PB-PK) model was developed to describe the disposition of diethylene-1,4-dioxide (dioxane) and its principal metabolite beta-hydroxyethoxyacetic acid in rats, mice, and humans. The model was developed from experimentally measured partition coefficients (reported here for the first time) as well as pharmacokinetic data previously reported. The completed PB-PK model adequately described data from gavage and intravenous studies in rats, as well as inhalation studies in rats and humans. Substantial nonlinearities were observed in the kinetic behavior of dioxane under high exposure conditions (water concentrations greater than 0.1% dioxane and atmospheric concentrations greater than 300 ppm dioxane). The PB-PK model was subsequently used to prepare quantitative estimates of the "plausible upper bounds" on carcinogenic risk for human populations exposed to dioxane in air or water. Based on these quantitative estimates, it appears that human populations continuously exposed to 740-3700 ppb dioxane in air or 20,000-120,000 ppb dioxane in water would be unlikely to experience increased frequencies of tumors.

Abstract  The purpose of this Toxicological Review is to provide scientific support and rationale for the hazard and dose-response assessment in IRIS pertaining to chronic exposure to 1,4-dioxane. It is not intended to be a comprehensive treatise on the chemical or toxicological nature of 1,4-dioxane. The intent of Section 6, Major Conclusions in the Characterization of Hazard and Dose Response, is to present the major conclusions reached in the derivation of the reference dose, reference concentration, and cancer assessment, where applicable, and to characterize the overall confidence in the quantitative and qualitative aspects of hazard and dose response by addressing the quality of the data and related uncertainties. The discussion is intended to convey the limitations of the assessment and to aid and guide the risk assessor in the ensuing steps of the risk assessment process.

Abstract  This document presents background information and justification for the Integrated Risk Information System (IRIS) Summary of the hazard and dose-response assessment of 1,4-dioxane. IRIS Summaries may include oral reference dose (RfD) and inhalation reference concentration (RfC) values for chronic and subchronic exposure durations, and a carcinogenicity assessment.

Abstract  A physiologically based pharmacokinetic model which describes the behavior of inhaled styrene in rats accurately predicts the behavior of in baled styrene in humans. The model consists of a series of mass-balance differential equations which quantify the time course of styrene concentration within four tissue groups representing (1) highly perfused organs, (2) moderately perfused tissues such as muscle. (3) slowly perfused fat tissue, and (4) organs with high capacity to metabolize styrene (principally liver). The pulmonary compartment of the model incorporates uptake of styrene controlled by ventilation and perfusion rates and the blood:air partition coefficient The metabolizing tissue group incorporates saturable Michaelis-Menten metabolism controlled by the biochemical constants Vmax and Km. With a single set of physiological and biochemical constants, the model adequately simulates styrene concentrations in blood and fat of rats exposed to 80, 200, 600, or 1200 ppm styrene (data from previously published studies). The simulated behavior of styrene is particularly sensitive to changes in the constants describing the fat tissue group, and to the maximum metabolic rate described by Vmax, The constants used to simulate the fate of stvrene in rats were scaled up to represent humans. Simulated styrene concentrations in blood and exhaled air of humans are in good agreement with previously published data. Model simulations show that styrene metabolism is saturated at inhaled concentrations above approximately 200 ppm in mice, rats, and humans. At inhaled concentrations below 200 ppm, the ratio of styrene concentration in blood to inhaled air is controlled by perfusion limited metabolism. At inhaled concentrations above 200 ppm. This ratio is controlled by the blood:air partition coefficient and is not linearly related to the ratio attained at lower (nonsaturating) exposure concentrations. These results show that physiologically based pharmacokinetic models provide a rational basis with which (1) to explain the relationship between blood concentration and air concentration of an inhaled chemical, and (2) to extrapolate this relationship from experimental animals to humans.

Abstract  In a continuing review of long-term toxicology and carcinogenesis studies in rats and mice, the National Toxicology Program (NTP) is confronted with many problems concerning the interpretation of tumor data. A frequently raised question is: "Should certain neoplasms be combined for overall assessment of rodent carcinogenesis data?" NTP policy is that certain neoplasms may be combined for statistical assessment of tumor data and that hyperplastic responses may be used as supportive evidence. The primary reason for combining neoplastic lesions is to gain more insight into the evidence of the carcinogenicity of a given chemical in that species of animal. This report gives the rationale, criteria, and guidelines used by the NTP for combining neoplasms for the evaluation of long-term rodent toxicology and carcinogenesis studies. The guidelines are based mainly on lesions occurring in the F344/N inbred rat and (C57BL/6 X C3H)F1 mouse and may or may not be appropriate for other strains or species. The concepts of combining neoplasms and sites should be viewed in terms of the study as a whole, since tumor formation is only one of many responses caused by chemicals in mammals. The resulting information becomes part of the "weight of the evidence" for estimating the potential hazard of a given chemical.

Abstract  The mechanisms responsible for ethanol-mediated teratogenesis have not been resolved. However, possible etiologies include the local formation of the teratogen acetaldehyde or oxygen radicals by fetal ethanol-oxidizing enzymes. As alcohol dehydrogenases are expressed at very low concentrations in human embryonic tissues, the ethanol-inducible P450 enzyme, CYP2E1, could be the sole catalyst of fetal ethanol oxidation. With this in mind, we examined the expression of this P450 in liver samples from fetuses ranging in gestational age from 16 to 24 weeks. Immunoblot analysis of fetal liver microsomes revealed the presence of a protein immunoreactive with CYP2E1 antibodies that exhibited a slightly lower molecular weight than that found in adult liver samples. Embryonic CYP2E1 expression was further confirmed by the reverse transcriptase reaction with RNA from a 19-week gestational fetal liver used as template. Catalytic capabilities of human fetal microsomes were assessed by measurement of the rate of ethanol oxidation to acetaldehyde, which were 12-27% of those exhibited by adult liver microsomes. Immunoinhibition studies with CYP2E1 antibodies revealed that the corresponding antigen was the major catalyst of this reaction in both fetal and adult tissues. We then assessed whether embryonic CYP2E1 was, like the adult enzyme, inducible by xenobiotics. Treatment of primary fetal hepatocyte cultures with either ethanol or clofibrate demonstrated a 2-fold increase in CYP2E1 levels compared with untreated cells. Collectively, our results indicate that CYP2E1 is present in human fetal liver, that the enzyme is functionally similar to CYP2E1 from adults, and that fetal hepatocyte CYP2E1 is inducible in culture by xenobiotics, including ethanol. Because fetal CYP2E1 mediates ethanol metabolism, the enzyme may play a pivotal role in the local production of acetaldehyde and free radicals, both of which have potential deleterious effects on the developing fetus.

Abstract  Effect-air concentration regressions of 48 common solvents (aromatic, aliphatic, and chlorinated hydrocarbons, alcohols, ketones, acetates) were determined for 4-hr inhalation exposures in male rats and for 2-hr exposures in female mice. Inhibition of propagation and maintenance of the electrically evoked seizure discharge was used as a criterion of the acute neurotropic effect. The isoeffective concentrations in air were estimated by interpolation on the level of one-third of the maximum effect (ECC). ECC estimates ranged from 90 to 24,000 ppm and were several times lower than concentrations evoking behavioral inhibition and by one to two orders lower than concentrations inducing narcosis. Correlations between corresponding values in both species were high (r > 0.9), indicating a relative independence of the estimates from experimental conditions. The relative potency estimates had only negligible correlation with octanol:water distribution coefficients or other physicochemical predictors for the whole sample of solvents, but moderate to high correlation (r = 0.5 to 0.9) in homogenous groups of nonpolar solvents, permitting cautious predictions. When applied to known effective concentrations of some solvents on human performance and subjective state, the comparative potency procedure suggests that ceiling and STEL values of some solvents may not reliably protect workers from acute nervous depression.

Abstract  Using death as an endpoint, it has been demonstrated that tetrachloroethylene (TCE) produces a greater-than-additive effect when given orally in combination with several other compounds. We have attempted to determine more sensitive indicators which would respond in a synergistic fashion when animals are exposed by inhalation to combinations of organic solvents. Male rats were exposed for 4 hr to various concentrations of TCE, dioxane, butyl ether (BE), acetonitrile (ACN), trichloropropane (TCP), and dichloropropane (DCP). The serum enzymes, glutamic oxalacetic transaminase (SGOT), glutamic pyruvic transaminase (SGPT), glucose-6-phosphatase (G-6-Pase), and ornithine carbamyl transferase (OCT) were measured in rats prior to exposure, immediately after exposure, and at 24 and 48 hr after exposure. The enzymes, SGOT, SGPT and OCT, were markedly elevated as a result of exposure to the above compounds, whereas G-6-Pase was only occasionally altered. Neither TCE in combination with dioxane, BE, or ACN nor TCP with DCP resulted in a greater-than-additive effect. On the contrary, in many instances the effects were significantly less-than-additive when tested for interaction.

Abstract  The fact that the chlorinated dioxins are highly toxic, teratogenic, and acnegenic does not of necessity indicate they are carcinogens as well. However, in view of the widespread use of products that might contain dioxins as contaminants and their extreme stability under environmental conditions, examination of their possible long term effects is imperative. This program was initiated to determine the chronic toxicity and potential carcinogenicity of a series of chlorinated dibenzodioxins by oral administration and skin application. The unsubstituted, 2,7 dichloro- and octachlorodibenzodioxins are relatively innocuous with regard to toxicity. The 2,3,7 trichloro-, 2,3,7,8 tetrachloro- and hexachlorodibenzodioxins, however, are quite toxic and require the use of specialized facilities to minimize the possibility of human exposure. This report describes the authors' results to date with the nontoxic dioxins. Work is being initiated with the toxic dioxins as they become available. These preliminary results are by no means conclusive with regard to the carcinogenicity of the chlorinated dioxins. The oral studies indicate primarily hepatotoxicity, expecially in the case of octachlorodibenzodioxin. The skin carcinogenesis assay for the nontoxic compounds is nearing completion, and no skin tumors have been observed in the complete carcinogenesis studies. The nature of the internal growths must still be determined at necropsy.