Teratologic potential of 2-methoxyethanol and transplacental distribution of its metabolite, 2-methoxyacetic acid, in non-human primates

Methoxyacetic acid inhibits histone deacetylase and impairs axial elongation morphogenesis of mouse gastruloids in a retinoic acid signaling-dependent manner

BACKGROUND

Teratogenic potential has been linked to various industrial compounds. Methoxyacetic acid (MAA) is a primary metabolite of the widely used organic solvent and plasticizer, methoxyethanol and dimethoxyethyl phthalate, respectively. Studies using model animals have shown that MAA acts as the proximate teratogen that causes various malformations in developing embryos. Nonetheless, the molecular mechanisms by which MAA exerts its teratogenic effects are not fully understood.

METHODS

Gastruloids of mouse P19C5 pluripotent stem cells, which recapitulate axial elongation morphogenesis of gastrulation-stage embryos, were explored as an in vitro model to investigate the teratogenic action of MAA. Morphometric parameters of gastruloids were measured to evaluate the morphogenetic effect, and transcript levels of various developmental regulator genes were examined to assess the impact on gene expression patterns. The effects of MAA on the level of retinoic acid (RA) signaling and histone deacetylase activity were also measured.

RESULTS

MAA reduced axial elongation of gastruloids at concentrations comparable to the teratogenic plasma level (5 mM) in vivo. MAA at 4 mM significantly altered the expression profiles of developmental regulator genes. In particular, it upregulated the RA signaling target genes. The concomitant suppression of RA signaling using a pharmacological agent alleviated the morphogenetic effect of MAA. MAA at 4 mM also significantly reduced the activity of purified histone deacetylase protein.

CONCLUSIONS

MAA impaired axial elongation morphogenesis in a RA signaling-dependent manner in mouse gastruloids, possibly through the inhibition of histone deacetylase.

Special Report: Reproductive and Developmental Toxicity of Ethylene Glycol and Its Ethers

Polymers of Ethylene Glycol are linked via ether linkages with various alcohols or via ester linkages to various fatty acids in many cosmetic ingredients. Ethylene Glycol, when reacted with an alkyl alcohol, forms an ethylene glycol monoalkyl ether. These compounds are metabolized in the human body by alcohol dehydrogenase and aldehyde dehydrogenase to form corresponding acetaldehyde and acetic acid derivatives. Data are presented that show reproductive and developmental toxicity is associated with metabolites of ethylene glycol monoalkyl ethers, but not with the monoalkyl ethers themselves. Further, it is suggested that the toxicity of these metabolites is inversely proportional to the length of the alkyl chain in the original alkyl ether. In the case of the compounds used in cosmetics, most have alcohols or fatty acids linked to polyethylene glycol chains, not a single Ethylene Glycol moiety. Where Ethylene Glycol is linked to a fatty acid by an ester linkage, the resulting compound is chemically different from the monoalkyl ethers. Where Ethylene Glycol is linked to an alcohol via an ether linkage, the alkyl chain is large and complex, suggesting little or no potential toxicity. Overall, it was found that metabolites of ethylene glycol monoalkyl ethers are reproductive and developmental toxins. In general, however, the metabolites of concern are not expected to be formed in cosmetic formulations that contain polymers of ethylene glycol.

Existence of vimentin and GFAP protein expressions as a result of 2-Methoxyethanol administration in cerebral cortex tissue of Swiss Webster mice (Mus musculus): an immunohistochemical analysis

Une of the plastic-based materials widely used in the plastics industry in various countries is ester phthalate. This compound will be oxidized in the body into 2-methoxyethanol (2-ME). The effect of 2-ME on human health and environment depends on the number, duration and the frequency of exposure. Recently, the incidence of brain damage tends to increase. In the last decade, it has been widely reported the negative effects of chemical pollutants to the environment. The aim of this study were to know the existence of the expression of Vimentin and GFAP proteins caused by 2-ME on the histological structure of the cerebral cortex of mice fetal during the prenatal period on gestation day 14 (GD 14) and day 18 (GD 18). The 2-ME compound was injected intraperitoneally with a dose of 7.5 mmol kg(-1) of body weight at GD-10. The result showed that there is a change in existence Vimentin protein in the cerebral cortex fetal of treated mice at GD 14, which is very conspicuous. Meanwhile, a change in existence of GFAP protein in cerebral cortex fetal of treated mice at GD 14, have relatively no difference from controls and no impact on histological structure changes of the cerebral corteks at GD 14. The change in existence of Vimentin protein in the cerebral cortex fetal of treated mice at GD 14 have an impact on histological structure of the cerebral cortex of mice treated at GD 18. It is believed that the impact is due to the effects of 2-methoxyethanol.

Effects of ethylene glycol monomethyl ether and its metabolite, 2-methoxyacetic acid, on organogenesis stage mouse limbs in vitro

Exposure to ethylene glycol monomethyl ether (EGME), a glycol ether compound found in numerous industrial products, or to its active metabolite, 2-methoxyacetic acid (2-MAA), increases the incidence of developmental defects. Using an in vitro limb bud culture system, we tested the hypothesis that the effects of EGME on limb development are mediated by 2-MAA-induced alterations in acetylation programming. Murine gestation day 12 embryonic forelimbs were exposed to 3, 10, or 30 mM EGME or 2-MAA in culture for 6 days to examine effects on limb morphology; limbs were cultured for 1 to 24 hr to monitor effects on the acetylation of histones (H3K9 and H4K12), a nonhistone protein, p53 (p53K379), and markers for cell cycle arrest (p21) and apoptosis (cleaved caspase-3). EGME had little effect on limb morphology and no significant effects on the acetylation of histones or p53 or on biomarkers for cell cycle arrest or apoptosis. In contrast, 2-MAA exposure resulted in a significant concentration-dependent increase in limb abnormalities. 2-MAA induced the hyperacetylation of histones H3K9Ac and H4K12Ac at all concentrations tested (3, 10, and 30 mM). Exposure to 10 or 30 mM 2-MAA significantly increased acetylation of p53 at K379, p21 expression, and caspase-3 cleavage. Thus, 2-MAA, the proximate metabolite of EGME, disrupts limb development in vitro, modifies acetylation programming, and induces biomarkers of cell cycle arrest and apoptosis

Assessing the toxic effects of ethylene glycol ethers using Quantitative Structure Toxicity Relationship models

Experimental determination of toxicity profiles consumes a great deal of time, money, and other resources. Consequently, businesses, societies, and regulators strive for reliable alternatives such as Quantitative Structure Toxicity Relationship (QSTR) models to fill gaps in toxicity profiles of compounds of concern to human health. The use of glycol ethers and their health effects have recently attracted the attention of international organizations such as the World Health Organization (WHO). The board members of Concise International Chemical Assessment Documents (CICAD) recently identified inadequate testing as well as gaps in toxicity profiles of ethylene glycol mono-n-alkyl ethers (EGEs). The CICAD board requested the ATSDR Computational Toxicology and Methods Development Laboratory to conduct QSTR assessments of certain specific toxicity endpoints for these chemicals. In order to evaluate the potential health effects of EGEs, CICAD proposed a critical QSTR analysis of the mutagenicity, carcinogenicity, and developmental effects of EGEs and other selected chemicals. We report here results of the application of QSTRs to assess rodent carcinogenicity, mutagenicity, and developmental toxicity of four EGEs: 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol, and 2-butoxyethanol and their metabolites. Neither mutagenicity nor carcinogenicity is indicated for the parent compounds, but these compounds are predicted to be developmental toxicants. The predicted toxicity effects were subjected to reverse QSTR (rQSTR) analysis to identify structural attributes that may be the main drivers of the developmental toxicity potential of these compounds.

Diethylene glycol monomethyl ether, ethylene glycol monomethyl ether and the metabolite, 2-methoxyacetic acid affect in vitro chondrogenesis

Diethylene glycol monomethyl ether (DEGME), ethylene glycol monomethyl ether (EGME) and their common metabolite, methoxyacetic acid (MAA) have been associated with adverse reproductive effects. The objective of this research is to investigate the effects of DEGME, EGME and MAA on in vitro chondrogenesis and the mechanisms by which these effects occur. Micromass cultures were exposed to DEGME, EGME or MAA for 5 days and proteoglycan abundance and cell proliferation determined. Longer-term 9- and 14-day cultures were exposed to MAA and apoptosis analyzed. All three chemicals decreased proteoglycan abundance and cell proliferation at the highest dose tested (100 microL/mL). However, only MAA showed a dose-dependent effect for both parameters at 0.01, 10, and 100 microL/mL. Furthermore, micromass cultures show an increase in apoptotic cells which when treated with MAA suggest that cell death could result from induced apoptosis. These results suggest that effects of DEGME and EGME are the result of generalized toxicity, but their metabolite MAA induces mitochondrial-mediated apoptosis during in vitro chondrogenesis.

Prolonged menstrual cycles in female workers exposed to ethylene glycol ethers in the semiconductor manufacturing industry

BACKGROUND

It has been shown that female workers exposed to ethylene glycol ethers (EGEs) in the semiconductor industry have higher risks of spontaneous abortion, subfertility, and menstrual disturbances, and prolonged waiting time to pregnancy.

AIMS

To examine whether EGEs or other chemicals are associated with long menstrual cycles in female workers in the semiconductor manufacturing industry.

METHODS

Cross-sectional questionnaire survey during the annual health examination at a wafer manufacturing company in Taiwan in 1997. A three tiered exposure-assessment strategy was used to analyse the risk. A short menstrual cycle was defined to be a cycle less than 24 days and a long cycle to be more than 35 days.

RESULTS

There were 606 valid questionnaires from 473 workers in fabrication jobs and 133 in non-fabrication areas. Long menstrual cycles were associated with workers in fabrication areas compared to those in non-fabrication areas. Using workers in non-fabrication areas as referents, workers in photolithography and diffusion areas had higher risks for long menstrual cycles. Workers exposed to EGEs and isopropanol, and hydrofluoric acid, isopropanol, and phosphorous compounds also showed increased risks of a long menstrual cycle.

CONCLUSIONS

Exposure to multiple chemicals, including EGEs in photolithography, might be associated with long menstrual cycles, and may play an important role in a prolonged time to pregnancy in the wafer manufacturing industry; however, the prevalence in the design, possible exposure misclassification, and chance should be considered.

Prolonged time to pregnancy in female workers exposed to ethylene glycol ethers in semiconductor manufacturing

BACKGROUND

Previous research on reproductive effects of working in the semiconductor industry is limited and has produced conflicting results.

METHODS

This retrospective cohort study was designed to investigate the risk factors for female fertility in a wafer-manufacturing company of Taiwan in 1997. Waiting time to pregnancy and potential confounders were collected by face-to-face interview. Exposure was assessed by directors and senior engineers in manufacturing, administrative, and safety and health departments according to safety records, personnel records, and job histories from questionnaires, using a tiered exposure-assessment approach. Of a total 842 female workers, 720 (85.5%) participated. There were 292 pregnancies from 173 workers eligible for analysis. Relative fecundability (fecundability ratio, or FR) was calculated using Cox's proportional hazard models to evaluate the effect of occupational exposure to ethylene glycol ethers on female fertility.

RESULTS

Waiting time to pregnancy of female workers in the photolithography area was longer than that of those in the nonfabrication area (FR = 0.77; 95% CI = 0.45-1.32), and those who were potentially exposed to ethylene glycol ethers showed longer time to pregnancy compared with those not exposed (FR = 0.59; 95% CI = 0.37-0.94).

CONCLUSION

This study provides further evidence that ethylene glycol ethers may cause female subfertility.

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.

Toxicity review of ethylene glycol monomethyl ether and its acetate ester

Ethylene glycol monomethyl ether (EGME) and its acetate ester (EGMEA) are highly flammable, colorless, moderately volatile liquids with very good solubility properties. They are used in paints, lacquers, stains, inks and surface coatings, silk-screen printing, photographic and photo lithographic processes, for example, in the semiconductor industry, textile and leather finishing, production of food-contact plastics, and as an antiicing additive in hydraulic fluids and jet fuel. EGME and EGMEA are efficiently absorbed by inhalation as well as via dermal penetration. Dermal absorption may contribute substantially to the total uptake following skin contact with liquids or vapours containing EGME or EGMEA. EGMEA is rapidly converted to EGME in the body and the two substances are equally toxic in animals. Therefore, the two substances should be considered as equally hazardous to man. Effects on peripheral blood, testes, and sperm have been reported at occupational exposure levels ranging between 0.4 and 10 ppm EGME in air, and with additional, possibly substantial, dermal exposure. Severe malformations and disturbed hematopoiesis have been linked with exposure to EGME and EGMEA at unknown, probably high, levels. Embryonic deaths in monkeys and impaired spermatogenesis in rabbits have been reported after daily oral doses of 12 and 25 mg per kg body weight, respectively. In several studies, increased frequency of spontaneous abortions, disturbed menstrual cycle, and subfertility have been demonstrated in women working in the semiconductor industry. The contribution of EGME in relation to other exposure factors in the semiconductor industry is unclear.

Development of a physiologically based pharmacokinetic model of 2-methoxyethanol and 2-methoxyacetic acid disposition in pregnant rats

An accurate description of developing embryos' exposure to a xenobiotic is a desirable component of mechanism-based risk assessments for humans exposed to potential developmental toxicants during pregnancy. 2-Methoxyethanol (2-ME), a solvent used in the manufacture of semiconductors, is embryotoxic and teratogenic in all species tested including nonhuman primates. 2-Methoxyacetic acid (2-MAA) is the primary metabolite of 2-ME and the proximate embryotoxic agent. The objective of the work described here was to adapt an existing physiologically based pharmacokinetic (PBPK) model for 2-ME and 2-MAA kinetics during midorganogenesis in mice to rats on gestation days (GD) 13 and 15. Blood and tissue data were analyzed using the extrapolated PBPK model that was modified to simulate 2-ME and 2-MAA kinetics in maternal plasma and total embryo tissues in pregnant rats. The original mouse model was simplified by combining the embryos and placenta with the richly perfused tissue compartment. The model includes a description of the growth of the developing embryo and changes in the physiology of the dam during pregnancy. Biotransformation pathways of 2-ME to either ethylene glycol (EG) or to 2-MAA were described as first-order processes based on the data collected from rats by Green et al., (Occup. Hyg. 2, 67-75, 1996). Tissue partition coefficients (PCs) for 2-ME and 2-MAA were determined for a variety of maternal tissues and the embryos. Model simulations closely reflected the biological measurement of 2-ME and 2-MAA concentrations in blood and embryo tissue following gavage or iv administration of 2-ME or 2-MAA. The PBPK model for rats as described here is well suited for extrapolation to pregnant women and for assessment of 2-MAA dosimetry under various conditions of possible human exposure to 2-ME.

Subchronic inhalation toxicity of diglyme

Diglyme [1,1'-oxybis(2-methoxyethane)] is an organic solvent belonging to the glycol ether class of compounds. To assess the inhalation toxicity of diglyme, groups of 20 male and 10 female rats were exposed by nose-only inhalation 6 hours/day, 5 days/week for 2 weeks to either 0 (control), 110, 370 or 1100 ppm diglyme. To compare potency, 2-methoxyethanol was also tested at 300 ppm. Rats were sacrificed either immediately following exposure, after a 14-day recovery period, or after 42 and 84 days of recovery (males only). Parameters investigated included in-life observations and body weights, clinical pathology, and histopathology with organ weights. Exposure to diglyme produced a variety of concentration-related haematological, clinical chemical and histopathological changes in both sexes. The most striking effect produced in all test groups was cellular injury involving the testes, seminal vesicles, epididymides and prostate. Although these effects were more severe at the higher concentrations tested, partial or complete recovery was seen by 84 days post-exposure. Changes in the haematopoietic system occurred in both sexes and involved the bone marrow, spleen, thymus, leucocytes and erythrocytes. The testicular effects of diglyme were somewhat less pronounced than those seen with 2-methoxyethanol. The no-observed-effect level (NOEL) for repeated inhalation exposure to diglyme in female rats is 370 ppm. For males, all concentrations tested produced effects to the reproductive system, hence a no-observed-effect level could not be demonstrated.

Association of spontaneous abortion and other reproductive effects with work in the semiconductor industry

This study tested the hypothesis that fabrication room (fab) work in the silicon-based semiconductor industry is associated with an increased risk of spontaneous abortion (SAB). The study was conducted nationwide at 14 companies representing a spectrum of large to small manufacturers. A small increase in risk of SAB was observed among fab workers compared with nonfabrication room (nonfab) workers in two cohorts, historical (adjusted RR = 1.43, 95% confidence interval [CI] 0.95-2.09) and prospective (adjusted RR = 1.25, 95% CI = 0.63-1.76). Analysis of specific fab exposures in the historical cohort showed a consistent, dose-response association of SAB with photoresist and developer solvents, whose major component was ethylene-based glycol ethers. The consistency of our findings and the toxicological data for these agents suggest that this is a causal association. Independent associations of SAB with self-reported stress and with etching fluorides were observed and require further research. No significant decrease in fertility was observed among men or women working in fabs, but reduced fecundability was suggested for some women fab workers.

Historical cohort study of spontaneous abortion among fabrication workers in the Semiconductor Health Study: agent-level analysis

Risk of spontaneous abortion (SAB) was examined in relation to chemical and physical agents in a retrospective study of employees of 14 seminconductor manufacturers: After screening over 6,000 employees, 506 current and 385 former workers were eligible. If a woman had multiple eligible pregnancies, one was selected at random. Telephone interviews provided data on demographics and occupational and other exposures during the first trimester. Two groups of chemicals accounted for the 45% excess risk of SAB among fabrication-room (fab) workers: photoresist and developed solvents (PDS), including glycol ethers, and fluoride compounds used in etching. Women exposed to high levels of both these agents were at greater risk (RR = 3.21, 95% confidence interval [CI] = 1.29-5.96). In fab workers without these exposures, SAB rates were not elevated (adjusted relative risk [RR] = 0.98, 95% CI = 0.55-1.69). An association was seen with workplace stress, which was not limited to women exposed to PDS or fluoride, nor did stress explain the associations between these chemicals and SAB.

Physiologically based pharmacokinetics of methoxyacetic acid: dose-effect considerations in C57BL/6 mice

Methoxyacetic acid (MAA), a weak acid with a pKa of 3.57, was used to test the broad hypothesis that distribution of weak acids in maternal and fetal tissues is determined principally by the pKa of the acid and the pH values of tissue and fluid compartments and to examine tissue dose-teratogenesis relationships, as well as administered dose-teratogenesis relationships. Five related experimental studies were conducted in pregnant C57BL/6CrIBR mice: a conventional dose-response study of developmental toxicity and transplacental pharmacokinetics in mice, a second dose-response study in which reproductive outcomes in litters from individual dams were related to individual pharmacokinetic behavior, a protein-binding experiment, an embryo tissue localization study, and determination of pH in maternal and embryonic compartments after exposure to MAA. MAA was administered intraperitoneally at 9:00 a.m. on day 10 of gestation, at doses ranging from 88 to 164 mg/kg. Localization within the forelimb bud of the embryo, an MAA target site, was determined by computerized image analysis of the distribution of radiolabeled MAA. The kinetic predictions of a physiologically based model incorporating tissue pH values and MAA pKa agreed well with observed concentrations at the lowest dose. However, at intermediate and higher doses, concentrations in both maternal and embryonic tissues were consistently underestimated. MAA was bound neither to maternal plasma proteins nor to embryonic proteins. Intermediate and higher doses of MAA caused dose-dependent transient depressions in tissue pH, but these were not of sufficient duration to bring predicted tissue concentrations into congruence with the concentrations observed. Distribution of MAA within the forelimb bud was broadly consistent with the pH hypothesis, but MAA concentration was not increased in the distal postaxial sector that is the site of the precursor cells of the missing digits. Internal exposure to MAA, defined as the area under the maternal plasma or embryo concentration curve (AUC), was not proportional to administered dose, but AUC-response relationships generated by the group and individual dose-response studies were comparable. While AUC may be a useful measure of effective MAA dose, it cannot be accurately predicted at teratogenic doses of this agent by the model as it is presently structured.

Developmental phase alters dosimetry-teratogenicity relationship for 2-methoxyethanol in CD-1 mice

The industrial solvent 2-methoxyethanol (2-ME) elicits phase-specific terata in mice through its primary metabolite and proximate toxicant, 2-methoxyacetic acid (2-MAA). Recent pharmacokinetic studies indicate that the incidence and severity of digit malformations induced in CD-1 mice by 2-ME exposure on gestation day (gd) 11 (copulation plug = gd 0) correlate better with the total 2-MAA exposure over time (= area under the curve; AUC) than with its peak concentrations (Cmax) in maternal plasma, embryo and extraembryonic fluid. In this study, the phase specificity of exencephaly induction by 2-ME was investigated to ascertain whether the 2-ME/2-MAA dosimetry-teratogenicity relationship remains consistent throughout organogenesis. Following a single intravenous (iv) bolus dose of 250 mg 2-ME/kg given to pregnant mice, exposure on gd 8 was decidedly the gestation day that best balanced low embryo lethality and high malformation incidence as recorded in near-term fetuses. Concentrations of 2-MAA were measured during distribution and elimination in maternal plasma and conceptuses following iv bolus doses of 175, 250, and 325 mg 2-ME/kg, as well as during and after termination of subcutaneous (sc) constant-rate infusion (4, 6, and 8 hr; 8 microliters/hr) of 277, 392, and 606 mg 2-ME/kg total doses. For all administration regimens, exencephaly incidence rates were determined in fetuses on gd 18. Similar plasma 2-MAA Cmax values (approximately 5 mmol/l) and fetal malformation frequencies (approximately 12%) were induced by sc infusion of 392 mg 2-ME/kg or a bolus dose of 250 mg 2-ME/kg. However, the AUC produced by infusion was significantly larger than that following the iv bolus dose (38 vs. 26 mmol.hr/l, respectively). In both maternal plasma and conceptuses, the correlation coefficients between Cmax and exencephaly rates, as well as developmental toxicity, were higher than they were for AUC and those end points. This outcome suggests that dosimetry-teratogenicity determinants may be quite specific for a distinct developmental phase during which a particular organ differentiates and a specific chemical acts upon the embryo.

Electron paramagnetic resonance studies of the effects of methoxyacetic acid, a teratologic toxin, on human erythrocyte membranes

Methoxyacetic acid (MAA), a teratogenic toxin, is the major metabolite of ethylene glycol monomethyl ether (EGME, also referred to as 2-methoxyethanol, 2-ME). MAA causes a wide range of toxic effects in laboratory animals including reproductive and developmental toxicity, as well as hematotoxicity, by mechanisms that are not clear. In this study, we employed electron paramagnetic resonance (EPR) spin-labeling techniques in conjunction with spin labels specific for cytoskeletal proteins, bilayer lipids, cell-surface sialic acid, or cell-surface galactose and N-acetylgalactosamine residues of human erythrocyte membranes in order to gain insight into the mechanism of MAA toxicity. The major findings are: (1) MAA significantly increases the protein-protein interactions of skeletal proteins in a concentration-dependent manner (P < 0.001), while 2-ME has no effect (at even a 2.5-fold higher concentration). (2) Addition of MAA leads to significant increase in the rotational motion of spin-labeled terminal galactose and N-acetylgalactosamine residues (2.0 mM MAA, 38% decrease of the apparent rotational correlation time tau a, P < 0.01). (3) The rotational motion of spin-labeled sialic acid, 70% of which is on the major transmembrane sialoglycoprotein (glycophorin A or PAS 1), was not affected by MAA treatment. (4) MAA has no effect on the lipid bilayer fluidity, since no change in the motion of a lipid bilayer specific spin label (5-NS) in the erythrocyte membrane was observed. These results suggest that MAA may lead to teratologic toxicity by interacting not with lipid components but with certain, perhaps specific, protein components, i.e., transport proteins, cytoskeleton proteins or neurotransmitter receptors.

Mouse placenta: hemodynamics in the main maternal vessel and histopathologic changes induced by 2-methoxyethanol and 2-methoxyacetic acid following maternal dosing

The two main maternal vessels that are a major, if not the entire, source of maternal blood for the mouse placenta are unique in possessing intraluminal valvular projections. The morphologic configuration of these projections suggests their potential to converge, diverge, and rotate blood currents flowing under systolic pressure. The intravascular occurrence of circular fibrin bodies composed of concentric fibrin strands coagulated from the plasma and almost no blood cellular elements in these strands lends credence to this concept. Histopathologic changes in the extraembryonic and embryonic tissues induced by an intraperitoneal injection of 250 or 500 mg/kg of 2-methoxyethanol, or its metabolite, 2-methoxyacetic acid, via oral gavage were determined 48 hr after dosing CD-1 mice on day 11 of pregnancy. Both compounds caused 1) marked congestion and dilatation, associated with or without fibrinous occlusions, of the main maternal vessel of the placenta, 2) serosanguinous exudation and maternal hemorrhages from the placental periphery, 3) necrosis and desquamation involving the mesometrial surface or peripheral edge of the placenta, 4) translabyrinthine embryonic hemorrhage into the maternal circulation, and 5) embryonic hemorrhages into the exocoelomic, amniotic, and pericardial cavities. These lesions signify a disordered maternal circulation in the placenta suggestive of potentially serious pathologic effects. These lesions may play a role in the resorption, reduction in fetal body weight, and syndactyly or oligodactyly attributed to 2-methoxyethanol and 2-methoxyacetic acid.

2-Methoxyacetic acid dosimetry-teratogenicity relationships in CD-1 mice exposed to 2-methoxyethanol

The teratogen 2-methoxyethanol (2-ME), an industrial solvent, was administered to pregnant CD-1 mice either as a single subcutaneous (sc) bolus dose (100-250 mg/kg) or via constant-rate infusion from sc implanted osmotic minipumps (34.7 or 69.4 mg/kg/hr for up to 12 hr) on gestation Day 11, when embryonic paw development is maximally sensitive to perturbation by this agent. The sc entry route most closely reflects likely human exposures via dermal penetration, while bolus and constant-rate infusion administrations were contrasted to mimic potential occupational exposure scenarios. The pharmacokinetic profiles of 2-methoxyacetic acid (2-MAA), the proximate toxic metabolite of 2-ME, were quantitated, generating peak concentration (Cmax) and total 2-MAA exposure values (24-hr area under the concentration-time curve; AUC) in the maternal plasma, extraembryonic fluid, and embryo. The total 2-ME dose (mg/kg) required to achieve similar 2-MAA levels (Cmax or AUC) in these compartments was 2- to 3-fold higher by constant-rate infusion than by bolus injection; therefore, no simple association existed between 2-MAA levels and the total 2-ME dose, when the dose rate was not considered. Similarly, there was no good correlation between the combined total 2-ME doses and the fetal malformation rate, although clear dose-response patterns for paw malformations were observed in litters and fetuses for each individual dosing regimen. However, the combined 2-MAA pharmacokinetic data from each of the dosing regimens demonstrated that during the phase of maximum susceptibility of paw morphogenesis to disruption by 2-MAA (from gd 11 to gd 11.5), a strong linear correlation existed between fetal malformation incidence and 2-MAA AUC levels in either maternal plasma or embryonic compartments (linear correlation coefficient, r2 0.91-0.92). The correlation with Cmax was less favorable (r2 0.74-0.81) over the dose range studied. In a further experiment designed to investigate the importance of AUC vs Cmax regarding 2-ME teratogenicity, infusion of 2-ME (34.7 mg/kg/hr for 8 hr) beginning 2.5 hr after bolus loading (175 mg/kg) provided an increased 24-hr 2-MAA AUC without increased Cmax. This resulted in greater than 70% of the fetuses having various digit malformations (micro-, syn-, ectro-, and polydactyly), compared to only 32-35% of fetuses with mostly stunted digits when either dose was applied singularly. These data support total 2-MAA exposure (AUC levels), rather than peak 2-MAA concentrations, as the principle determinant of teratogenesis following exposure to 2-ME.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Methoxyethanol metabolism in pregnant CD-1 mice and embryos

Upon oxidation to 2-methoxyacetic acid (2-MAA), 2-methoxyethanol (2-ME) causes malformations in all animal species that have been examined. Commonly, 2-MAA is thought to be the proximate toxicant. However, our previous studies with [1,2-14C]2-ME and the present data obtained with [1-14C]2-MAA, [2-14C]2-ME and [methoxy-14C]2-ME revealed that metabolism beyond 2-MAA occurs. Regardless of the 14C position, dams exhaled approximately 5% of the radioactivity administered as a single teratogenic oral dose (3.3 mmol/kg on Gestation Day [gd] 11) as 14CO2. With all isotopic variants urine contained 70-80% of the dose within 24 hr after administration and 13-18% in the next 24 hr. Three labeled products were resolved using HPLC: an unidentified Peak A (12-18% of dose), 2-MAA (approximately 50%), and the glycine conjugate of 2-MAA (approximately 25%). Short-term (4 hr) whole embryo culture on gd 11 with 3 mM 2-MAA and a tracer dose of [1-14C]2-MAA, [2-14C]2-MAA, or [methoxy-14C]2-MAA showed that 14CO2 evolved from the former two substrates, while there was none detectable from the latter. The data indicate that dams metabolized [methoxy-14]2-MAA to 14CO2, while embryos apparently did not. The production of labeled CO2 from [2-14C]2-ME suggests that 2-methoxyacetyl approximately CoA (the precursor for amino acid conjugation with glycine) entered into the tricarboxylic acid (TCA) cycle. This interpretation is supported by the inhibition of 14CO2 evolution elicited by fluoroacetate (0.1 or 1.0 mM) and sodium acetate (5 mM). It is not yet clear whether entry of 2-methoxyacetyl approximately CoA as a "false substrate" in the TCA cycle is of significance for the embryotoxic effects of 2-ME/2MAA.

The possible role of one-carbon moieties in 2-methoxyethanol and 2-methoxyacetic acid-induced developmental toxicity

The ethylene glycol ether, 2-methoxyethanol (2-ME), is rapidly (less than 1 hr) oxidized to 2-methoxyacetic acid (2-MAA). Both agents are selectively embryotoxic and equipotent in causing digit malformations when given to CD-1 mice on gestation Day 11. Previous observations have shown that simple physiological compounds such as formate, acetate, glycine, and glucose ameliorate the embryotoxicity of 2-ME. A common link for all of the attenuating agents may be oxidation pathways involving tetrahydrofolic acid (THF) as a catalyst for one-carbon transfer into purine and pyrimidine bases. In the present study serine at 16.5 mmol/kg, which reacts directly with THF, was as effective as formate in almost completely eliminating digit malformations resulting from treatment with 2-ME. Unlike formate, serine was equally effective against 2-MAA-induced dysmorphogenesis and the attenuating efficacy remained unchanged when serine administration was delayed for up to 8 hr after 2-ME or 2-MAA exposure. The protective effect of sarcosine, which is an intermediate in a pathway leading from choline to glycine and a structural analog of 2-MAA, was also determined. Both concomitant (43, 16.5, or 3.3 mmol/kg) and delayed (16.5 mmol/kg at 6 hr) sarcosine administration resulted in significantly less 2-ME-induced paw dysmorphogenesis. In addition, acetate administration was delayed for increasing intervals after 2-ME to determine the time at which attenuation would no longer occur, and acetate was effective for as long as 12 hr after 2-ME. These results support our hypothesis that 2-MAA, which has a long biological half-life, may interfere with the availability of one-carbon units for incorporation into purine and pyrimidine bases. Alterations in availability of these precursors might be expected to affect DNA and/or RNA synthesis and thereby influence normal cellular proliferation and differentiation in the developing embryo.

Effects of 2-methoxyethanol on fetal development, postnatal behavior, and embryonic intracellular pH of rats

The industrial solvent 2-methoxyethanol (2ME) is a reproductive and developmental toxicant when administered by inhalation, gavage, and IP injection. The present research established that this solvent can produce teratogenicity in rats when administered in liquid diet. Groups of 10 Sprague-Dawley rats were given various percentages of 2ME in liquid diet on gestation days 7-18. Day 20 fetuses were examined for visceral or skeletal malformations. Concentrations above 0.025% 2ME (approximately 73 mg/kg/day) produced total embryo-mortality. Cardiovascular malformations were produced at lower levels. The teratogenic no-effect level was 0.006% 2ME (16 mg/kg). In a second experiment, groups of 12 Sprague-Dawley rats were given 0, 0.006 and 0.012% of 2ME as above. Litters were culled to 8 pups, and tested for auditory and tactile startle and conditioned lick suppression, and for performance in figure-8 activity and the Cincinnati water maze on postnatal days 48-65. The high dose of 2ME produced approximately 50% mortality in the offspring and increased the number of errors in the Cincinnati maze. No other behavioral effects were observed at either dose. An interaction study was conducted to determine if simultaneous exposure to 2ME and ethanol would reduce the teratogenicity of 2ME, but no reduction was observed. The hypothesis that 2ME acts by altering embryonic intracellular pH was tested by injecting 0.33 ml/kg of 2ME into rats on gestation day 13, and determining embryonic intracellular pH at 2, 4, 8, and 24 hours thereafter. There was an increase in pH at 4 hours, but not at later time points. Another group of rats was given 2ME along with amiloride, which blocks the sodium/hydrogen antiporter. The combined 2ME-amiloride exposure produced an incidence of cardiovascular malformations in fetuses twice that of 2ME alone. These studies confirmed the structural teratogenicity of 2ME even when given in liquid diet, as it was given for the first time in the present study. At nonteratogenic doses, developmental toxicity (e.g., postnatal deaths) persisted, but only limited evidence of behavioral teratogenicity was observed. The pH data are consistent with the concept that 2ME may alter embryonic intracellular pH at critical stages of organogenesis.