A toxicokinetic study of inhaled ethylene glycol monomethyl ether (2-ME) and validation of a physiologically based pharmacokinetic model for the pregnant rat and human

Exposures to sufficiently high doses of ethylene glycol monomethyl ether (2-methoxyethanol, 2-ME) have been found to produce developmental effects in rodents and nonhuman primates. The acetic acid metabolite of 2-ME, 2-methoxyacetic acid (2-MAA), is the likely toxicant, and, as such, an understanding of the kinetics of 2-MAA is important when assessing the potential risks to humans associated with 2-ME. A previously described physiologically based pharmacokinetic (PBPK) model of 2-ME/2-MAA kinetics for rats exposed via oral or iv administration was extended and validated to inhalation exposures. Pregnant Sprague-Dawley rats were exposed for 5 days (gestation days 11-15), 6 h/day, to 2-ME vapor at 10 and 50 ppm. Validation consisted of comparing model output to maternal blood and fetal 2-ME and 2-MAA concentrations during and following 5 days of exposure (gestation days 11-15). These concentrations correspond to a known no observed effect level (NOEL) and a lowest observed effect level (LOEL) for developmental effects in rats. The rat PBPK model for 2-ME/2-MAA was scaled to humans and the model (without the pregnancy component) was used to predict data collected by other investigators on the kinetics of 2-MAA excretion in urine following exposures to 2-ME in human volunteers. The partially validated human model (with the pregnancy component) was used to predict equivalent human exposure concentrations based on 2-MAA dose measures (maximum blood concentration, C(max), and average daily area under the 2-MAA blood concentration curve, AUC, during pregnancy) that correspond to the concentrations measured at the rat NOEL and LOEL exposure concentrations. Using traditional PBPK scale-up techniques, it was calculated that pregnant women exposed for 8 h/day, 5 days/week, for the duration of pregnancy would need to be exposed to 12 or 60 ppm 2-ME to produce maternal 2-MAA blood concentrations (C(max) or average daily AUC) equivalent to those in rats exposed to the NOEL (10 ppm) or LOEL (50 ppm), respectively.

A toxicokinetic study of inhaled ethylene glycol ethyl ether acetate and validation of a physiologically based pharmacokinetic model for rat and human

The solvents ethylene glycol monoethyl ether acetate (EGEEA) and ethylene glycol monoethyl ether (EGEE), at sufficiently high doses, are known to be rodent developmental toxicants, exerting their toxic effects through the action of their metabolite 2-ethoxyacetic acid (2-EAA). Thus risks associated with exposure to these compounds are best evaluated based on a measure of the internal dose of 2-EAA. The goals of the work reported here were to develop physiologically based pharmacokinetic (PBPK) models of EGEEA and EGEE for pregnant rats and humans. These models were used to identify human exposure levels (ppm in air) equivalent to the rat no observed effect level (NOEL) and lowest observed effect level (LOEL) for developmental effects (Hanley et al., 1984). We exposed pregnant Sprague-Dawley rats to concentrations of EGEEA corresponding to the NOEL and LOEL. Maternal blood, urine, and fetal tissue concentrations of EGEE and 2-EAA measured in these experiments were used to validate the rat EGEEA and EGEE models. Data collected by other researchers were used to validate the capabilities of the rodent EGEEA and EGEE models to predict the kinetics in humans. The models for estimating circulating blood concentrations of 2-EAA were considered valid based on the ability of the model to accurately predict 2-EAA concentrations in rat blood, urine, and fetal tissue. The human inhaled concentration equivalent to the rat NOEL for EGEEA (50 ppm) was predicted to be 25 ppm using the maternal blood average daily area under the curve (AUC) and 40 ppm using the maximum concentration achieved in maternal blood (C(max)). The human inhaled concentration equivalent to the rat LOEL for EGEEA (100 ppm) was determined to be 55 ppm using the maternal blood average daily AUC and 80 ppm using the maternal blood C(max).

Evaluation of the potential for developmental toxicity in rats and mice following inhalation exposure to tetrahydrofuran

Sprague-Dawley rats and Swiss (CD-1) mice were exposed to 0, 600, 1800, or 5000 ppm THF (a four-carbon cyclic ether, widely used as an industrial solvent) vapors, 6 hr/day, 7 days/week (6-19 days of gestation (DG) for rats; 6-17 DG for mice). Body weights of pregnant rats in the 5000 ppm group were reduced at euthanization. There were no effects on the percentage of live rat fetuses/litter or on the fetal sex ratio. Fetal body weight was significantly reduced for the 5000 ppm group, but the incidence of abnormalities was not increased. Mice in the 1800 and 5000 ppm groups were sedated during exposure; approximately 27% of the mice in the 5000 ppm group died. Mean body and uterine weights of mice were reduced for the 1800 and 5000 ppm groups at euthanization (18 DG), but adjusted maternal weight gain was not affected at 1800 ppm. There was a reduction in the percentage of live fetuses/litter for the mice in 1800 and 5000 ppm groups (95% resorptions in the 5000 ppm group). Fetal weight and sex ratio in mice were not affected. An increase in the incidence of reduced sternebral ossifications was correlated to THF concentration, although differences between groups were not statistically significant. There were no increases in the incidences of other malformations or variations. These results suggest that THF may be embryotoxic in mice, but if the conceptus survives, development as assessed by this experimental design continues in a normal fashion. The no-observable-adverse-effect level (NOAEL) for maternal toxicity was 1800 ppm in both rats and mice. The NOAEL for developmental toxicity was 1800 ppm in rats and 600 ppm in mice.

Developmental toxicity of inhaled methyl ethyl ketone in Swiss mice

Methyl ethyl ketone (MEK) is a widely used industrial solvent to which there is considerable human exposure. To assess the potential for MEK to cause developmental toxicity in rodents, groups of Swiss (CD-1) mice were exposed to 0, 400, 1000, or 3000 ppm MEK vapors 7 hr/day on Days 6-15 of gestation. Groups consisted of about 30 bred females each. Exposure of pregnant mice to these concentrations of MEK did not result in overt maternal toxicity although there was a slight, treatment-related increase in relative liver weight which was statistically significant in the 3000 ppm group. Mild developmental toxicity was observed in the 3000 ppm group in the form of a reduction in mean fetal body weight. This reduction was statistically significant for the males only, although the relative decrease from the control values was the same for both sexes. There was no increase in the incidence of resorptions or the number of litters with resorptions among mice exposed to MEK. There was no significant increase in the incidence of any single malformation, but several malformations which were not observed in the concurrent control group or the controls of contemporary studies were present at a low incidence--cleft palate, fused ribs, missing vertebrae, and syndactyly. There was also a significant trend for increased incidence of misaligned sternebrae, a developmental variation. In summary, pregnant Swiss (CD-1) mice were relatively insensitive to the toxic effects of MEK at the inhaled concentrations used in this study. However, the offspring of the mice exhibited significant signs of developmental toxicity at the 3000 ppm exposure level.(ABSTRACT TRUNCATED AT 250 WORDS)

Overview of reproductive and developmental toxicity studies of 1,3-butadiene in rodents

A series of studies to further evaluate the developmental and reproductive toxicity of inhaled 1,3-butadiene was sponsored by the National Toxicology Program. Pregnant Sprague-Dawley rats (24-28/group) and Swiss (CD-1) mice (18-22/group) were exposed to atmospheric concentrations of 0, 40, 200, or 1000 ppm 1,3-butadiene for 6 hr/day on days 6 through 15 of gestation (dg) and killed on dg 18 (mice) or dg 20 (rats). Subsequently, the uterine contents were evaluated; individual fetal body weights were recorded; and external, visceral, and skeletal examinations were performed. In rats, maternal toxicity was observed in the 1000-ppm group in the form of reduced extragestational weight gain and, during the first week of treatment, decreased body weight gain. Under these conditions, there was no evidence of developmental toxicity in rats. In contrast, results of the mouse developmental toxicity study indicated that the fetus may be more susceptible than the dam to inhaled 1,3-butadiene. Maternal toxicity was observed in mice at the 200- and 1000-ppm 1,3-butadiene exposure levels, whereas 40 ppm and higher concentrations of 1,3-butadiene caused significant exposure-related reductions in the mean body weights of male fetuses. Mean body weights of female fetuses were also reduced at the 200- and 1000-ppm exposure levels. No increased incidence of malformations was observed in either study. Other studies addressing male reproductive and mutagenesis end points were performed with B6C3F1 mice (sperm-head morphology) and Swiss (CD-1) mice (dominant lethal study).(ABSTRACT TRUNCATED AT 250 WORDS)

Predicting the human teratogenic potential of the anticonvulsant, valproic acid, from a non-human primate model

The anticonvulsant, valproic acid (VPA) is a suspected human teratogen. This study, employing the rhesus monkey as an animal model, demonstrates that VPA has a significant teratogenic potential in the monkey. Timed pregnant monkeys were exposed orally to VPA at approx. 1X, 10X, and 30X (20, 200, and 600 mg/kg/day, respectively) the human therapeutic dose, daily, during organogenesis (gestation days 21-50). All fetuses of mothers exposed to greater than 1X exhibited some form of embryotoxicity. The highest dose, 30X, was 100% embryolethal, while offspring of the 10X dose group exhibited craniofacial and skeletal defects, and low body weights. Maternal pharmacokinetic parameters and plasma metabolites were determined for VPA on the first and last day of dosing for the 10X dose group. Comparison of the kinetic and metabolite data with that obtained for man indicates that the rhesus monkey is a good model for predicting the teratogenic potential of VPA in the human.