M.J. McKenna

Comparative metabolism and disposition of ethylene glycol monomethyl ether and propylene glycol monomethyl ether in male rats

Male Fischer 344 rats were given a single po dose of approximately 1 or 8.7 mmol/kg of [14C]EGME (ethylene glycol monomethyl ether) or [14C]PGME (propylene glycol monomethyl ether). After dosing, expired air, excreta, and tissues were analyzed for 14C; metabolites in urine were isolated and identified. There were pronounced differences in the metabolism and disposition of [14C]EGME and [14C]PGME. Approximately 50 to 60% of the administered 14C was excreted in urine, and about 12% was eliminated as 14CO2 within 48 hr after a single po dose of [14C]EGME. For PGME, only 10 to 20% of the administered 14C was excreted in urine, while 50 to 60% was eliminated as 14CO2 within 48 hr. Methoxyacetic acid was identified as the primary urinary metabolite of EGME, accounting for 80 to 90% of the total 14C in urine. PGME, propylene glycol(1,2-propanediol), and the sulfate and glucuronide conjugates of PGME were identified in urine of rats given PGME. Since methoxyacetic acid causes the same spectrum of toxicity as EGME in male rats, it is likely that the adverse effects of EGME are the result of its in vivo bioactivation to methoxyacetic acid. Hence, differences in routes of metabolism and types of metabolites appear to be the underlying basis for the remarkably different toxicologic properties of EGME and PGME, respectively.

Comparative short-term inhalation toxicity of ethylene glycol monomethyl ether and propylene glycol monomethyl ether in rats and mice

Abstract Male and female Fischer 344 rats and B6C3F1 mice were exposed to 0, 100, 300, or 1000 ppm ethylene glycol monomethyl ether (EGME) or to 0, 300, 1000, or 3000 ppm propylene glycol monomethyl ether (PGME) 6 hr/day for a total of 9 days during an 11-day interval. Although structurally similar, the biological activities of the two materials were dramatically different. The high concentration of EGME (1000 ppm) had pronounced adverse effects on body weight gain, peripheral blood counts, bone marrow, testes, and lymphoid tissues. Similar but less pronounced changes also occurred in some animals in the 300 ppm EGME group. Exposure to 3000 ppm PGME resulted in increased liver weights in male rats as well as central nervous system depression and decreases in specific gravity of urine of both male and female rats. However, there were no gross or histopathologic changes in either rats or mice which could be attributed to exposure to PGME. Hence the treatment-related changes which occurred in rats and mice exposed to PGME vapors, even at the highest concentration (3000 ppm), would constitute, at most, a minimal effect. Although PGME and EGME have comparable vapor pressures, the potential hazard of exposure to PGME vapors appears to be distinctly less than to EGME vapors.

Physiologically Based Pharmacokinetic Modeling with Dichloromethane, Its Metabolite, Carbon Monoxide, and Blood Carboxyhemoglobin in Rats and Humans'

Dichloromethane (methylene chloride, DCM) and other dihalomethanes are metabolized to carbon monoxide (CO) which reversibly binds hemoglobin and is eliminated by exhalation. We have developed a physiologically based pharmacokinetic (PB-PK) model which describes the kinetics of CO, carboxyhemoglobin (HbCO), and parent dihalomethane, and have applied this model to examine the inhalation kinetics of CO and of DCM in rats and humans. The portion of the model describing CO and HbCO kinetics was adapted from the Coburn-Forster-Kane equation, after modification to include production of CO by DCM oxidation. DCM kinetics and metabolism were described by a generic PB-PK model for volatile chemicals (RAMSEY AND ANDERSEN, Toxicol. Appl. Pharmacol. 73, 159-175, 1984). Physiological and biochemical constants for CO were first estimated by exposing rats to 200 ppm CO for 2 hr and examining the time course of HbCO after cessation of CO exposure. These CO inhalation studies provided estimates of CO diffusing capacity under free breathing and for the Haldane coefficient, the relative equilibrium distribution ratio for hemoglobin between CO and O2. The CO model was then coupled to a PB-PK model for DCM to predict HbCO time course behavior during and after DCM exposures in rats. By coupling the models it was possible to estimate the yield of CO from oxidation of DCM. In rats only about 0.7 mol of CO are produced from 1 mol of DCM during oxidation. The combined model adequately represented HbCO and DCM behavior following 4-hr exposures to 200 or 1000 ppm DCM, and HbCO behavior following 1/2-hr exposure to 5160 ppm DCM or 5000 ppm bromochloromethane. The rat PB-PK model was scaled to predict DCM, HbCO, and CO kinetics in humans exposed either to DCM or to CO. Three human data sets from the literature were examined: (1) inhalation of CO at 50, 100, 250, and 500 ppm; (2) seven 1/2-hr inhalation exposures to 50, 100, 250, and 500 ppm DCM; and (3) 2-hr inhalation exposures to 986 ppm DCM. An additional data set from human volunteers exposed to 100 or 350 ppm DCM for 6 hr is reported here for the first time. Endogenous CO production rates and the initial amount of CO in the blood compartment were varied in each study as necessary to give the baseline HbCO value, which varied from less than 0.5% to greater than 2% HbCO. The combined PB-PK model gave a good representation of the observed behavior in all four human studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Toxicity of Methoxyacetic Acid in Rats

Male Fischer 344 rats were given eight daily doses of 0, 30, 100 or 300 mg/kg methoxyacetic acid by gavage. The high dose resulted in decreased body weight, severe degeneration of testicular germinal epithelium, decreased size of the thymus with depletion of thymic cortical lymphoid elements, and reductions in bone marrow cellularity resulting in depressions of red blood cell counts, hemoglobin concentration, packed cell volume, and white blood cell counts. Some of these observations were apparent to a lesser degree in rats given 100 mg/kg. The low dose produced no apparent effects during the course of the study. These toxicological properties of methoxyacetic acid are remarkably similar to ethylene glycol monomethyl ether (EGME), and the adverse effects of EGME in rats are probably the result of in vivo bioactivation of EGME to methoxyacetic acid.

Methylene chloride: a two-year inhalation toxicity and oncogenicity study in rats and hamsters.

Abstract Methylene Chloride: A Two-Year Inhalation Toxicity and Oncogenicity Study in Rats and Hamsters. BUREK, J. D., NITSCHKE, K. D., BELL, T. J., WACKERLE, D. L., CHILDS, R. C., BEYER, J. E., DITTENBER, D. A., RAMPY, L. W., AND MCKENNA, M. J. (1984). Fundam. Appl. Toxicol. 4, 30–47. A long-term study was conducted to determine the possible chronic toxicity and oncogenicity of methylene chloride. Rats and hamsters were exposed by inhalation to 0, 500, 1500, or 3500 ppm of methylene chloride for 6 hr per day, 5 days a week, for 2 years. No exposure-related cytogenetic effects were present in male or female rats exposed to 500, 1500, or 3500 ppm. Females rats exposed to 3500 ppm had an increased mortality rate while female hamsters exposed to 1500 or 3500 ppm had decreased mortality rates. Carboxyhemoglobin values were elevated in rats and hamsters exposed to 500, 1500, or 3500 ppm with the percentage increase in hamsters greater than in rats. Minimal histopathologic effects were present in the livers of rats exposed to 500, 1500, or 3500 ppm. Decreased amyloidosis was observed in the liver and other organs in hamsters exposed to 500, 1500, or 3500 ppm. While the number of female rats with a benign tumor was not increased, the total number of benign mammary tumors was increased in female rats in an exposure-related manner. This effect was also evident in male rats in the 1500- and 3500-ppm exposure groups. Finally, male rats exposed to 1500 or 3500 ppm had an increased number of sarcomas in the ventral neck region located in or around the salivary glands. Therefore, in this 2-year study, some effects were observed in male and female rats exposed to 500, 1500, or 3500 ppm of methylene chloride. In contrast, hamsters exposed to the same exposure concentrations had less extensive spontaneous geriatric changes, decreased mortality (females), and lacked evidence of definite target organ toxicity.

The comparative absorption and excretion of chemical vapors by the upper, lower, and intact respiratory tract of rats.

Upper respiratory tract (URT) absorption of several compounds with differing water solubilities and potentials to cause lesions of the nasal mucosa were studied in rats. Absorption of propylene glycol monomethyl ether (PGME), PGME acetate (PGMEAc), ethyl acrylate (EA), epichlorohydrin (EPI), styrene (STY), nitroethane (NE), ethylene dibromide (EDB), and methylene chloride (MeCl2) vapors by the isolated URT was compared to that by the isolated lower respiratory tract (LRT) and the intact animal. Nearly all PGME and PGMEAc and 30-70% of EA, EPI, STY, NE, and EDB were absorbed when passed through the URT. In general, similar levels were absorbed by both the isolated LRT and intact animal. It was estimated that intact animals received more than 90% of their total dose of PGME and PGMEAc, and 50% of EA, NE, EPI, and EDB via the URT. Further, the dosage per unit of surface area in the URT may be 5000-6000 times that of the LRT. However, the extent of URT absorption was not related to the ability to cause lesions of the nasal mucosa. Absorption of compounds by the URT was not a simple function of water solubility or of blood or water/air partitioning coefficients suggesting that a more complex mechanism for controlling absorption may exist. In one case, it was demonstrated that URT enzymatic activity could influence the absorption of certain compounds by the URT.

Saturable Metabolism and its Relationship to Toxicity

Metabolism plays a central role in regulating the toxicity of a variety of chemicals. Relatively innocuous substances can be converted to highly toxic metabolites. Conversely, toxic substances can be biotransformed to less harmful metabolites or be excreted, thus limiting their duration of biological action. Virtually all metabolism and many excretory processes utilize specific binding proteins, i.e., enzymes and carrier proteins. These metabolic and carrier-mediated excretory clearance pathways are capacity-limited, becoming saturated at sufficiently high substrate concentrations. Saturable metabolic clearance processes lead to dose-dependent pharmacokinetics for many chemicals. When dose-dependent pharmacokinetics prevail, internally significant parameters, such as area under the curve for concentration of toxicant at active sites and the amount of metabolite formed during inhalation exposure, are not linearly related to externally significant parameters such as administered dose or inspired concentration. Dose-response curves should relate observed effects to some internally significant parameter. Toxic response should often be indexed to area under the curve relationships or total amount metabolized, instead of dose or inspired concentration. The former parameters are complexly related to the latter. The nature of the relationship depends on the kinetic constants for metabolic and excretory clearance. Pharmacokinetic analyses of dose-dependent clearance mechanisms provide an understanding of how one transforms externally significant parameters to internally significant parameters under various exposure conditions. Consideration of metabolic clearance at the organ level illuminates the importance of physiological factors, showing unequivocally that blood flow may be rate-limiting for metabolism under many exposure conditions. Recognition of the potential for this behavior is essential to the proper design and evaluation of certain toxicological experimentation. Development of comprehensive pharmacokinetic descriptions of the influence of saturable clearance on delivery of active chemical to target sites augurs well for improving both intraspecies and interspecies extrapolation of toxicity data. This is a critical area of contemporary toxicology. Dose selection for chronic studies could also be improved by knowledge of the dose-dependence of pharmacokinetic parameters in proposed test species. The field of toxicology reviewed here represents an interface between pharmacokinetic research and studies on basic mechanisms of toxic action. It entails utilization of quantitative concepts to better understand the physiological and biochemical controls which regulate the expression of the toxicity of various chemicals. Much work remains to be accomplished in this exciting area of toxicological research. Some of the predictions of the pharmacokinetic analyses are still tentative and require more definitive experimentation...

Inhalation Toxicity of Acrylic Acid

Male and female Fischer 344 rats and B6C3F1 mice were exposed to 0, 5, 25 or 75 ppm acrylic acid vapors 6 hours per day, 5 days per week, for 13 weeks. These exposure levels were selected after conducting a 2-week probe study in which 225 ppm caused pronounced growth retardation and nasal lesions in both rats and mice. The 13-week exposures had no adverse effect on the growth of male and female rats and male mice. However, mean body weight gains of female mice in the 25 and 75 ppm exposure groups were statistically significantly lower than for controls after 12 weeks of exposure. There were no pronounced treatment related effects on organ weights, hematologic parameters, clinical chemistry parameters or urinary parameters. Histopathologic examinations revealed lesions of the nasal mucosa in rats in the 75 ppm exposure group, and in some or all mice at each treatment level. The nasal lesions were primarily localized to the olfactory epithelium; the respiratory epithelium was relatively unaffected. The histopathologic observations in both rats and mice included degeneration, and inflammatory cell infiltration in the olfactory mucosa. In mice there were also instances of hyperplasia of the submucosal glands and, replacement of olfactory epithelium by respiratory epithelium. These effects were attributed to the irritant properties of acrylic acid vapors.

Metabolism and pharmacokinetic profile of vinylidene chloride in rats following oral administration

Abstract Male rats (normally fed or previously fasted for 18 hr) were given a single oral dose of 1 or 50 mg/kg of [ 14 C]vinylidene chloride (VDC) in corn oil and the routes and rates of elimination of 14 C activity were then followed for 72 hr. After a single oral dose of 1 mg/kg of [ 14 C]VDC, 78% of the dose was metabolized and excreted in urine and feces as nonvolatile metabolites of VDC. The remainder was exhaled as 14 CO 2 (21%) and unchanged [ 14 C]VDC (1–3%). Fasting prior to [ 14 C]VDC administration did not significantly affect the fate of the 1-mg/kg dose of [ 14 C]VDC. Conversely, after a 50-mg/kg dose of [ 14 C]VDC, excretion of the parent compound via the lungs accounted for 19 and 29% of the dose in fed and fasted rats, respectively. Elimination of nonvolatile metabolites of VDC was slightly greater in fed than in fasted rats indicating a reduced capacity for metabolism of VDC in fasted rats. Fasting also resulted in an increased concentration of covalently bound [ 14 C]VDC metabolites in the livers of rats given 50 mg/kg of [ 14 C]VDC. Urinary radioactivity was separated by high-pressure liquid chromatography into four major metabolites. Two of the four urinary metabolites were identified as S -(2-hydroxyethyl)- N -acetylcysteine and thiodiglycolic acid by gas chromatography-mass spectrometry, indicating that a major pathway for detoxification of VDC is via conjugation with glutathione (GSH). The fate of VDC following oral administration to rats is depedent upon both the dose administered and the nutritional status of the animal. The diminished ability of fasted animals to metabolize the high dose of VDC correlates well with the previously observed enhancement of VDC-induced hepatotoxicity in fasted animals. Both the hepatotoxic response to VDC and the extent of its detoxification appear to be dependent on the concentration of GSH in the liver. When hepatic GSH is depleted (i.e., in fasted animals or at higher doses of VDC) a toxic response to VDC is elicited.

Effects of vinylidene chloride on DNA synthesis and DNA repair in the rat and mouse: A comparative study with dimethylnitrosamine

Abstract Exposure to vinylidene chloride (VDC) vapor has been reported to induce tumors in mice, but rats are apparently insensitive to this effect of VDC. This species difference has been correlated with the greater capacity of mice to activate VDC to a reactive electrophile which can react with macromolecules. To increase our understanding of the molecular events associated with this species difference, we have investigated the potential of VDC to cause DNA alkylation, DNA repair, and DNA replication in the liver and kidneys of rats and mice. For comparative purposes, the potent carcinogen dimethylnitrosamine (DMN) was also studied. Male Sprague-Dawley rats and CD-1 mice were exposed to 10 and 50 ppm VDC for 6 hr. DNA alkylation after 50 ppm [ 14 C]VDC was minimal in liver and kidney of both rats and mice (one or two orders of magnitude less than reported for DMN in rats). Similarly, DNA repair in the kidney of mice exposed to 50 ppm VDC was only 38% higher than control values, while DNA repair in the liver of mice injected with 20 mg/kg DMN was elevated 637%. However, tissue damage and increased DNA replication (25-fold) were seen in the kidneys of mice exposed to 50 and 10 ppm VDC. Comparable effects were not seen in the liver of mice exposed to VDC (50 or 10 ppm) or in the liver or kidneys of rats exposed to 10 ppm VDC. Thus an important distinction between DMN and VDC has been demonstrated. Tumorigenic doses of DMN produced relatively little tissue damage, but were associated with a high degree of DNA alkylation and DNA repair synthesis. In contrast, exposure to tumorigenic doses of VDC resulted in massive tissue damage but induced minimal DNA alkylation or DNA repair synthesis. This suggests that the tumors observed in mice exposed to VDC arise primarily through effects of the chemical on nongenetic components of the cells. Consequently protection of humans from levels of VDC sufficient to cause tissue damage should also serve to preclude any carcinogenic activity of VDC.