Effects of valproic acid, some of its metabolites and analogues on prenatal development of rats in vitro and comparison with effects in vivo

Integrating in vitro chemical transplacental passage into a generic PBK model: A QIVIVE approach

With the increasing application of cell culture models as primary tools for predicting chemical safety, the quantitative extrapolation of the effective dose from in vitro to in vivo (QIVIVE) is of increasing importance. For developmental toxicity this requires scaling the in vitro observed dose-response characteristics to in vivo fetal exposure, while integrating maternal in vivo kinetics during pregnancy, in particular transplacental transfer. Here the transfer of substances across the placental barrier, has been studied using the in vitro BeWo cell assay and six embryotoxic compounds of different kinetic complexity. The BeWo assay results were incorporated in an existing generic Physiologically Based Kinetic (PBK) model which for this purpose was extended with rat pregnancy. Finally, as a "proof of principle", the BeWo PBK model was used to perform a QIVIVE based on developmental toxicity as observed in various different in vitro toxicity assays. The BeWo results illustrated different transport profiles of the chemicals across the BeWo monolayer, allocating the substances into two distinct groups: the 'quickly-transported' and the 'slowly-transported'. BeWo PBK exposure simulations during gestation were compared to experimentally measured maternal blood and fetal concentrations and a reverse dosimetry approach was applied to translate in vitro observed embryotoxicity into equivalent in vivo dose-response curves. This approach allowed for a direct comparison of the in vitro dose-response characteristics as observed in the Whole Embryo Culture (WEC), and the Embryonic Stem Cell test (cardiac:ESTc and neural:ESTn) with in vivo rat developmental toxicity data. Overall, the in vitro to in vivo comparisons suggest a promising future for the application of such QIVIVE methodologies for screening and prioritization purposes of developmental toxicants. Nevertheless, the clear need for further improvements is acknowledged for a wider application of the approach in chemical safety assessment.

In vitro effects of folate derivatives on valproate-induced neural tube defects in mouse and rat embryos

The anticonvulsant drug valproic acid (VPA), produces neural tube defects in mouse and rat embryos treated in vivo or in vitro. The mechanism for the drug's embryotoxic effect is unknown, but 5-formyltetrahydrofolate has been reported to decrease the incidence of VPA-induced neural tube defects in mice treated in vivo. In the present study we have examined the ability of 5-formyltetrahydrofolate, tetrahydrofolate, 5-methyltetrahydrofolate and folic acid to protect against VPA-induced neural tube defects in CD-1 mouse or CD rat embryos grown in a whole embryo culture system. Mouse embryos with 2-5 somite pairs were cultured for 48 hr beginning on gestation day 8; presomite stage rat embryos were cultured beginning on gestation day 9 (for both species gestation day 0 was taken as the day a vaginal sperm plug was found). VPA at 1.2 mm (rats) or 1.8 mm (mice) produced a high incidence of open neural tubes. None of the folate derivatives in concentrations up to 100 mug/ml was able to decrease the incidence of VPA-induced defects in either species. These data suggest that folate is not involved in the mechanism of VPA-induced neural tube defects.

Analysis of reproductive toxicity and classification of glufosinate-ammonium

CONCLUSION REGARDING CLASSIFICATION OF GLUFOSINATE-AMMONIUM: Science Partners' Evaluation Group (Evaluation Group) has conducted an independent analysis of the herbicide glufosinate-ammonium (GA) relative to its potential to cause reproductive toxicity in humans. Further, the Evaluation Group has evaluated the implementation of Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC) and Council Directive 91/414/EEC, with respect to classification of chemicals posing potential reproductive hazards. After consideration of all information available to us relevant to the potential of glufosinate-ammonium (GA) to cause reproductive toxicity, the Science Partners Evaluation Group concludes that no classification of GA is justified. The following form the basis of this conclusion. There are no human data to suggest that GA causes reproductive toxicity in women or in their conceptus. The issue concerning possible reproductive hazard to humans is raised solely on the basis of positive animal test results that show GA to cause preimplantation or implantation losses in rats. SPECIFICALLY: a. Daily treatment with GA had no detectable effect on the earliest stages of the reproductive sequence including gametogenesis, ovulation, mating and conception; b. Treatment with GA interfered with rat gestation before and at the stage when the conceptus implants into the uterus. This effect occurred at doses of 360 ppm in the feed (corresponding to daily doses of 27.8 mg/kg bw) and above; and c. After implantation, no further effect of GA on prenatal and post-natal development was recognized. Previous concerns that GA might be toxic to embryonic stages after implantation were not supported by the data. Abortions and stillbirth seen were associated with, and regarded as secondary to, maternal toxicity. There was no evidence suggesting the induction of malformations in the offspring. The mechanism underlying this adverse effect in experimental laboratory animals is identified-inhibition of glutamine synthetase. Glutamine is essential to the viability of the embryo. The embryo is dependent on a maternal source of the amino acid. For embryo lethality to occur, a significant reduction of maternal glutamine is required. Such reduction in maternal glutamine depends on a significant inhibition of glutamine synthetase by GA. This can only occur when the mother is exposed to very high levels of GA. SPECIFICALLY: a. The reproductive toxicity of GA is confined to very short, early stages of reproduction, during which the conceptus is dependent on maternal glutamine; and b. In order for the effect to occur, significant reduction in maternal blood glutamine level is required, which in turn depends on a significant inhibition of glutamine synthetase, induced by high levels of GA in the maternal system. There is no evidence for accumulation of GA in the mammalian organism beyond a factor of two and no evidence for its metabolic toxification. To raise a concern in humans, women would have to be exposed to GA during the very limited time frame of preimplantation or implantation and the exposure would have to be to the exceedingly high levels necessary to alter the maternal metabolism and, correspondingly, result in glutamine levels in maternal tissue and blood plasma being drastically reduced. There is no basis to suggest that such exposures would occur under conditions of normal handling and use. SPECIFICALLY: a. Under conditions of normal handling and use, operators would never be exposed to GA levels that could potentially inhibit glutamine synthetase to the extent that this inhibition could impair preimplantation or implantation. b. All acceptable exposure measurements and predictive calculations confirm this conclusion, and in fact demonstrate that reasonably foreseeable exposure of workers would be to levels significantly below the AOEL. c. The evidence is also clear that there is no reproductive toxicity hazard to workers upon reentry tosprayed fields, bystanders, consumers or toddlers. The safety margin compared to the NOAEL in animal studies is sufficiently large to assure protection of the health of workers using GA as well as bystanders, consumers, and toddlers. Pursuant to Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC), to justify a classification of category 2 there must be sufficient evidence to produce a strong presumption that human exposure to the substance may result in impaired fertility in humans. It is the conclusion of the Science Partners Evaluation Group that there is no reasonable evidence to suggest a strong presumption of impairment. To the contrary, there is clear evidence demonstrating a strong presumption that exposure to GA would not cause the adverse effect demonstrated in rats. Pursuant to Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC), to justify a classification of category 3, there must be sufficient evidence to provide a strong suspicion of impaired fertility in humans. There is no basis to conclude that the animal data demonstrating impaired preimplantation or implantation has any relevance to humans in that the effect found in rats only occurs at levels which would never be experienced by workers under conditions of normal handling and use or by bystanders, consumers, or toddlers.

Morphological alterations induced by sodium valproate on somites and spinal nerves in rat embryos

The antiepileptic drug valproic acid is a well-known teratogenic agent; its main target organ is the neural tube, though skeletal malformations have also been described. In our recent work, respecifications of vertebrae were described in rat fetuses after treatment with 400 mg/kg of sodium valproate at specific somitogenic stages. The observed malformations were stage-dependent. Morphological segmental respecification was observed at the level of segments in formation at the moment of exposure and at the level of more posterior segments. Recently, specific alterations in the development of cranial nerves and ganglia were described in mouse embryos after in vitro exposure to VPA. The aim of the present work was to analyze dysmorphogenetic effects of VPA on embryonic metameric structures: somites, spinal and cranial nerves, and ganglia. Sodium valproate (400 mg/kg) was subcutaneously injected at specific gestational times corresponding to embryonic stages: presomitic or at about 2, 6, 10, 14, 18, or 22 somites. Females were sacrificed on the day 12 post coitum, and embryos were examined. Morphological examination of somites was performed by staining with acridine orange. Morphological examination of nerves and ganglia was performed by immunostaining, using monoclonal antibodies to the 160-kD neurofilament protein. No abnormalities were observed in the cranial nerves and ganglia. Specific and stage-dependent alterations were observed both at the level of the somites and at the level of the spinal nerves. The following characteristic malformations were observed: fusions, duplications, and reductions of somites and corresponding spinal nerves and ganglia. Our morphological data suggest a morphogenetic action of VPA at the level of the axial segments, with a possible respecification of the identity of the interested segments and their derivatives.

Rat embryo culture using rabbit serum as a medium for developmental toxicity studies

The usefulness of rabbit serum as a culture medium for postimplantation rat embryos was examined. Rat embryos at day 9.5 of gestation were cultured in a mixture of rat and rabbit sera at various ratios (v/v) for 48 h. In 100% rat serum, a usual medium, rat embryos grew well. On the contrary, rat embryos died with little growth in 100% rabbit serum. In 75% rabbit and 25% rat sera, rat embryos grew but were morphologically abnormal. In 50% rabbit and 50% rat serum, however, rat embryos grew well showing no morphological abnormalities, as in 100% rat serum. It was concluded from these results that rabbit serum could be used at a proportion up to 50% as a medium for postimplantation rat embryo culture in a mixed form with rat serum. The rat embryo culture using rabbit serum as a medium would be useful in developmental toxicity studies, especially those involving species differences and toxicokinetics.

Stereoselective dysmorphogenicity of the enantiomers of the valproic acid analogue 2-N-propyl-4-pentynoic acid (4-yn-VPA): cross-species evaluation in whole embryo culture

We previously reported the in vitro differential stereoselective dysmorphogenic potential of the R(+) and S(-) enantiomers of 2n-propyl-4-pentynoic acid (4-yn-VPA) in mice. To determine whether this stereoselectivity is species specific, we evaluated the dysmorphogenic potential of these isomers as well as valproic acid (VPA) to gestational day 9 rat embryos using whole embryo culture (WEC). Aqueous solutions of the sodium salts of R-4-yn-VPA, S-4-yn-VPA, 50%R/ 50%S-4-yn-VPA or VPA were added to the culture medium to give 0, 0.075, 0.15, 0.3, 0.6, or 1.2 mmol/L and embryos were evaluated 48 hr later. The S-4-yn-VPA enantiomer gave clear concentration-dependent dysmorphology as well as effects on developmental score, somite number, crown rump length, and head length. Effects on rotation and defects of the neural tube, somites and heart were observed. Embryolethality was observed only at 1.2 mmol/L concentration. The R-4-yn-VPA enantiomer was neither embryo toxic nor dysmorphogenic at any concentration. VPA significantly reduced all parameters and was dysmorphogenic at the highest concentration but was not embryo lethal. The 50/50 mixture of R- and S-isomers appeared to elicit a degree of embryolethality and dysmorphology similar to VPA. The potency order for the four chemicals was S(-) > S(-)/R(+) = VPA > > > R(+), comparable to that observed in mice by either in vivo or in vitro exposure. These data demonstrate that the stereoselective dysmorphology for these enantiomers can be observed across species and is not related to maternal metabolism.

Effect of supplemental folic acid on valproic acid-induced embryotoxicity and tissue zinc levels in vivo

Valproic acid (VPA) is an anti-convulsant drug known to cause spina bifida in humans. Administration of the vitamin, folic acid, has been shown to decrease the recurrence and possibly also the occurrence of neural tube defects, primarily spina bifida, in humans. Additionally, treatment with a derivative (folinic acid) of folic acid has been reported to decrease the frequency of VPA-induced exencephaly in mice treated with the drug in vivo. A protective effect by folinic acid has not been observed in vitro. The purpose of this investigation was to reexamine the ability of folinic acid to decrease the incidence of VPA-induced neural tube defects in vivo. We also examined the effect of increased intake of folic acid on zinc levels in various maternal and embryonic tissues. Folinic acid, whether administered by intraperitoneal injection or in osmotic mini-pumps, did not decrease the number of mouse fetuses with VPA-induced exencephaly. Dietary supplementation with 10-20 times the daily required intake of folic acid in rodents also failed to decrease the embryotoxicity of VPA. Such dietary supplementation had no effect on zinc levels in maternal liver, brain, or kidney, nor in embryonic tissues. These results indicate that folic acid is not able to reverse the embryotoxicity induced by the anticonvulsant, that there is no apparent effect of high dietary folate intake on maternal or embryonic zinc levels and suggest that folate is probably not involved in the mechanism of VPA-induced embryotoxicity.

Mouse Whole-embryo Culture in Serum Diluted with Waymouth Medium: A Study of Valproic Acid Teratogenicity

The culture of whole post-implantation rodent embryos has become an important tool in developmental biology and toxicology. Since the establishment of this system, rat serum has been the main culture medium used. In this study, we demonstrate that medium composed of 50% serum and 50% chemically defined medium (Waymouth 705/1) permits satisfactory development of mouse embryos over a 26-hour period. In addition, our data demonstrate that, with less than 50% serum, the frequency of malformations increases and growth and differentiation decrease with dilution in a dose-dependent way. Little information is available in the literature on the types of abnormalities induced by inadequate dilutions of serum. This study shows that a chemically defined medium, supplemented with amounts of serum that are below threshold levels, interferes mainly with the normal development of the head (neural tube, eyes and maxillary processes) and with the growth of the embryo. The second part of this work compares the teratogenicity of valproic acid (VPA) in this new medium to that in undiluted serum. We show that the effects of VPA are similar in terms of the type of alterations observed, the reduction of growth and the differentiation. However, the concentrations necessary to induce these effects in Waymouth/human serum/rat serum are half of those needed in human serum/rat serum.

In vitro neuroteratogenicity of valproic acid and 4-en-VPA

Mouse embryos displaying 8 to 9 pairs of somites were cultured during 26 h in presence of 0.75 mM of VPA, or of 1 mM of 4-en-VPA. These concentrations induced approximately 50% of dysmorphogenic embryos. Irregular suture of caudal neural tube, abnormal head shape, cranial neural tube defects, and deformed optic vesicles were the most common defects observed with both compounds. The main differences in the types of dysmorphogeneses detected between the two compounds concerned the suture of the caudal neural tube and the telencephalic region. Other macroscopic effects induced by the two compounds were similar. Several of the observed abnormalities can be correlated with defects reported after in vivo exposure. The major alteration of the histological structure of the neural tube concerned a specific area in the hindbrain : VPA and 4-en-VPA induced an abnormal and irregular budding of the neuroepithelium at this level. Immunohistology with an antibody specific for radial glial fibers (RC-2) as well as SEM analysis showed a moderate effect on glial development, mainly after exposure to VPA.

Trans-2-ene-valproic acid is less behaviorally teratogenic than an equivalent dose of valproic acid in rats

Although animal experiments have shown the trans-2-ene metabolite (t-2-ene-VPA) of valproic acid (VPA) to be pharmacologically equivalent to the parent compound in terms of anticonvulsant activity, it is considerably less teratogenic in studies which have examined prenatally exposed fetuses for morphological defects. This has made t-2-ene-VPA an attractive potential antiepileptic agent. However, while neurobehavioral alterations have also been observed in rats prenatally exposed to VPA, even at doses below those which produce malformations, the developmental neurotoxicity of t-2-ene-VPA had not previously been examined. The current study evaluated the long-term behavioral effects of prenatal exposure to t-2-ene-VPA. Pregnant CD rats were treated with 300 or 400 mg/kg t-2-ene-VPA by gavage on days 7-18 of gestation, doses previously shown to produce no teratogenicity. A VPA group was administered 300 mg/kg for comparison. The pharmacokinetic profiles of the two compounds were similar. Behavioral findings in offspring prenatally exposed to VPA were consistent with previous findings, i.e., VPA offspring exhibited decreased locomotor activity, increased swimming maze errors, and reduced tactile startle responding compared to controls. In the 400 mg/kg t-2-ene-VPA group, Cincinnati maze errors and auditory startle reactivity were increased, while no significant behavioral effects were detected in the 300 mg/kg t-2-ene group. These results indicate that the developmental neurotoxicity of t-2-ene-VPA is lower than that of VPA but is still significant.

Effect of lithium on rat embryos in culture: growth, development, compartmental distribution and lack of a protective effect of inositol

Lithium chloride (LiCl) was tested at various concentrations (50, 100, 150 and 200 micrograms/ml) using a rat whole-embryo culture system beginning on gestation day 9.5 (headfold stage) in order to establish a concentration-response relationship. Open neural tubes--as described in former publications (Tesh 1988)--were not induced by lithium. A significant reduction of embryonic growth and development occurred at the lowest concentration tested (50 micrograms/ml). Clear-cut dysmorphogenetic effects (absence of the eye cup, kink in the spinal anlage, "bleb" at the rostral head region) occurred at a concentration of 150 micrograms/ml LiCl. LiCl concentrations in the embryo, visceral yolk sac, exocoelomic fluid and medium were determined after the embryos had been cultured for 48 h in the presence of a moderately embryotoxic dose of LiCl (3.5 mEq/l or 150 micrograms/ml). Medium supplementation with myo-inositol in different concentrations was ineffective in antagonizing the embryotoxicity induced by LiCl.

Methionine decreases the embryotoxicity of sodium valproate in the rat: in vivo and in vitro observations

Methionine provided in the drinking water of pregnant rats injected with sodium valproate reduced the frequency of resorptions but did not improve embryo growth. Rats drinking methionine supplemented water had approximately twice the level of serum-free methionine and consumed only one-half the volume of water of controls. Using whole rat embryo cultures, the simultaneous addition of methionine and sodium valproate to the medium provided no protection from neural tube defects, nor did the addition of methionine to a medium of serum obtained from rats previously dosed with sodium valproate. However, protection from the teratogenic effects of sodium valproate was afforded by methionine when the culture medium was sera from rats consuming methionine and was particularly striking when embryos for culture were taken from pregnant rats that had been consuming methionine. These observations along with those of others indicated the importance of dietary and culture media methionine levels in evaluating experimental and regulatory teratology studies and suggested the possibility that methionine may play an important role in human teratology where multifactorial causes have been implicated in problems such as neural tube closure defects.

Valproic acid-induced placental and teratogenic effects in rats

Studies on teratogenicity and pathology of the cenceptus were conducted in Sprague-Dawley rats treated with 600, 800, and 1,000 mg/kg valproic acid po on day 13 of pregnancy. Each of the three doses was maternotoxic and caused (1) resorptions and/or abortions, reduction in the number of live fetuses per litter and mean fetal weight, and defects of the tail, rib and phalanx; and (2) degenerative changes in the labyrinth (thrombosis, angiectasis in the maternal lacunar network, necrosis of cytotrophoblasts and suppressed proliferation of fetal capillaries), reduced diameter nearing obliteration of umbilical vessels, with or without karyorrhexis of embryonic tissues. The lesions in the placental labyrinth were specific but, in the embryonic tissues, they were generalized. It was postulated that the vascular lesions in the labyrinth and umbilicus may have influenced embryonic development by reducing maternoembryonic gaseous and nutritional exchange.

Valproic acid induced abnormal development of the central nervous system of three species of amphibians: Implications for neural tube defects and alternative experimental systems

Embryos of Ambystoma mexicanum, Xenopus laevis, and Hyperolius viridiflavus taeniatus were exposed to various concentrations of valproic acid (VPA: 0.1, 1.5, 10 mM) from blastula stage (S) 9 on up to advanced gastrulation of control embryos (S 11 1/2-12). At 10 and 5 mM VPA early development was affected in all species tested. However, the most pronounced effects occurred in Ambystoma: the neural folds appeared delayed and showed a flattened and wavy shape; the neural tube was not formed and embryos successively died. In Xenopus and Hyperolius (10, 5 mM VPA) the beginning of gastrulation was delayed up to neurulation of control embryos. In Xenopus many of the embryos completed neurulation, whereas some embryos exposed to 10 mM VPA showed neural tube defects (NTDs) of different type and degree (open neural tube at different regions of the dorsum). In Hyperolius neural folds arose around the blastoporus and fused later on (earlier in embryos treated with 5 mM VPA), but the shape of these embryos was abnormal and the development was not continued (pronounced effect at 10 mM VPA). Comparing the three species, Xenopus proved to be the least sensitive species (at 5 mM VPA 14.2% NTDs of total malformations compared to 100% in the other species). The most sensitive species, Ambystoma, developed head-oedema at 1 mM VPA, whereas the anurans were not affected. Our results suggest a similar mechanism of VPA-induced NTDs in mammals and amphibians.