Histopathological and hemodynamic studies supporting hypoxia and vascular disruption as explanation to phenytoin teratogenicity

Fetal hypoxia and hyperglycemia in the formation of phenytoin-induced cleft lip and maxillary hypoplasia

OBJECTIVE

Phenytoin exposure during the first trimester of pregnancy increases the risk of maxillary hypoplasia and cleft lip. The etiology of phenytoin embryopathy is unknown. Interestingly, phenytoin is also known to induce hyperglycemia in humans as well as rats. This study uses a rat model of fetal phenytoin syndrome to examine the role of hyperoxia, hyperglycemia, and arachidonic acid deficiency in the development of cleft lip and maxillary hypoplasia.

METHODS

Pregnant rats were dosed with phenytoin during the critical period of lip development (day 11 of pregnancy) with or without supplemental oxygen, insulin, or arachidonic acid. The fetuses from all studies were examined at term.

RESULTS

The frequency of cleft lip and maxillary hypoplasia was reduced by treating dams at the time of phenytoin exposure with either increased oxygen or insulin. However, in fetuses from phenytoin-treated dams dosed with arachidonic acid, the incidence of severe lip deformities remained the same although there was an increase in normal and mildly affected fetuses. Interestingly, this occurred in embryos from hyperglycemic dams.

SIGNIFICANCE

Together, the results from these experiments suggest phenytoin-induced malformations may be a multifactorial process as malformations were not solely linked to a hyperglycemic state of the dam.

Experimental modeling of hypoxia in pregnancy and early postnatal life

The important role of equilibrium of environmental factors during the embryo-fetal period is undisputable. Women of reproductive age are increasingly exposed to various environmental risk factors such as hypoxia, prenatal viral infections, use of drugs, smoking, complications of birth or stressful life events. These early hazards represent an important risk for structural and/or functional maldevelopment of the fetus and neonates. Impairment of oxygen/energy supply during the pre- and perinatal period may affect neuronal functions and induce cell death. Thus when death of the newborn is not occurring following intrauterine hypoxia, various neurological deficits, including hyperactivity, learning disabilities, mental retardation, epilepsy, cerebral palsy, dystonia etc., may develop both in humans and in experimental animals. In our animal studies we used several approaches for modeling hypoxia in rats during pregnancy and shortly after delivery, i.e. chronic intrauterine hypoxia induced by the antiepileptic drug phenytoin, neonatal anoxia by decreased oxygen saturation in 2-day-old pups. Using these models we were able to test potential protective properties of natural (vitamin E, melatonin) and synthetic (stobadine) compounds. Based on our results, stobadine was also able to reduce hypoxia-induced hyperactivity and the antioxidant capacity of stobadine exceeded that of vitamin E and melatonin, and contrary to vitamin E, stobadine had no adverse effects on developing fetus and offspring.

The effect of hypoxia in development

There is increasing evidence that the oxygen supply to the human embryo in the first trimester is tightly controlled, suggesting that too much oxygen may interfere with development. The use of hypoxia probes in mammalian embryos during the organogenic period indicates that the embryo is normally in a state of partial hypoxia, and this may be essential to control cardiovascular development, perhaps under the control of hypoxia-inducible factor (HIF). A consequence of this state of partial hypoxia is that disturbances in the oxygen supply can more easily lead to a damaging degree of hypoxia. Experimental mammalian embryos show a surprising degree of resilience to hypoxia, with many organogenic stage embryos able to survive 30-60 min of anoxia. However, in some embryos this degree of hypoxia causes abnormal development, particularly transverse limb reduction defects. These abnormalities are preceded by hemorrhage/edema and tissue necrosis. Other parts of the embryo are also susceptible to this hypoxia-induced damage and include the genital tubercle, the developing nose, the tail, and the central nervous system. Other frequently observed defects in animal models of prenatal hypoxia include cleft lip, maxillary hypoplasia, and heart defects. Animal studies indicate that hypoxic episodes in the first trimester of human pregnancy could occur by temporary constriction of the uterine arteries. This could be a consequence of exposure to cocaine, misoprostol, or severe shock, and there is evidence that these exposures have resulted in hypoxia-related malformations in the human. Exposure to drugs that block the potassium current (IKr) can cause severe slowing and arrhythmia of the mammalian embryonic heart and consequently hypoxia in the embryo. These drugs are highly teratogenic in experimental animals. There is evidence that drugs with IKr blockade as a side effect, for example phenytoin, may cause birth defects in the human by causing periods of embryonic hypoxia. The strongest evidence of hypoxia causing birth defects in the human comes from studies of fetuses lacking hemoglobin (Hb) F. These fetuses are thought to be hypoxic from about the middle of the first trimester and show a range of birth defects, particularly transverse limb reduction defects.

Polytherapy withhERG-blocking antiepileptic drugs: Increased risk for embryonic cardiac arrhythmia and teratogenicity

BACKGROUND

The antiepileptic drugs (AEDs) phenytoin, phenobarbital, dimethadione, and carbamazepine cause a similar pattern of malformations in humans, with an increased risk after polytherapy. The teratogenicity has been linked to cardiac rhythm disturbances and hypoxic damage as a consequence of their common potential to inhibit a specific potassium ion current (IKr). The IKr is of major importance for embryonic cardiac repolarization and rhythm regulation. This study investigated whether these AEDs cause irregular rhythm and if various combinations of AEDs result in higher arrhythmia risk than exposure to a single AED.

METHODS

The effects on heart rhythm of a single AED (monotherapy), and of various combinations (polytherapy) of AEDs, in gestational day 10 C57BL mouse embryos in culture were analyzed and graphically illustrated during a 25 s recording with a digitalization technique.

RESULTS

All of the studied AEDs caused increased intervals between heartbeats (resulting in bradycardia) and large variations in the interval between heartbeats (resulting in irregular rhythm) in a concentration-dependent manner in cultured mouse embryos. Dimethadione caused irregular rhythm at concentrations within and phenytoin slightly above the therapeutic ranges. Polytherapy resulted in more substantial prolongation of the mean interval between heartbeats (>60 ms) than monotherapy at clinically relevant concentrations.

CONCLUSIONS

The results suggest that polytherapy more than monotherapy causes substantial prolongation of the cardiac repolarization, a marker associated with high risk of developing irregular rhythm during longer exposure periods (days to months). This supports the idea that the increased risk for malformations following polytherapy is linked to an increased risk for cardiac rhythm disturbances.

Embryonic cardiac arrhythmia and generation of reactive oxygen species: common teratogenic mechanism for IKr blocking drugs

In the adult organism, it is well established that hypoxia followed by reperfusion may be fatal and result in generation of reactive oxygen species (ROS) and subsequent tissue damage. There is also considerable evidence that temporary decrease or interruption in oxygen supply to the embryo and ROS generation during reperfusion result in tissue damage in embryonic tissues. A wide spectrum of different malformations by transient embryonic hypoxia could be produced, depending on the duration, extent, and timing of the hypoxic event. It is the contention of this paper that drugs that block the potassium channel IKr, either as an intended pharmacologic effect or as an unwanted side-effect, are potentially teratogenic by a common ROS related mechanism. Drugs blocking the IKr channel, such as almokalant, dofetilide, phenytoin, cisapride and astemizole, do all produce a similar pattern of hypoxia-related malformations. Mechanistic studies show that the malformations are preceded by embryonic cardiac arrhythmia and periods of hypoxia/reoxygenation in embryonic tissues. Pretreatment or simultaneous treatment with radical scavengers with capacity to capture ROS, markedly decrease the teratogenicity of different IKr blocking drugs. A second aim of this review is to demonstrate that the conventional design of teratology studies is not optimal to detect malformations caused by IKr blocking drugs. Repeated high doses result in high incidences of embryonic death due embryonic cardiac arrhythmia, thus masking their teratogenic potential. Instead, single dosing on specific days is proposed to be a better way to characterize the teratogenic potential of Ikr blocking drugs.

Phenytoin teratogenicity: Hypoxia marker and effects on embryonic heart rhythm suggest an hERG-related mechanism

BACKGROUND

The antiepileptic drug phenytoin (PHT) is a human and animal teratogen. The teratogenicity has been linked to PHT-induced embryonic cardiac arrhythmia and hypoxic damage during a period when regulation of embryonic heart rhythm is highly dependent on a specific K(+) ion current (I(Kr)). PHT has been shown to inhibit I(Kr). The aims of this study were to investigate whether teratogenic doses cause embryonic hypoxia during and after the I(Kr) susceptible period and to further characterize PHT effects on embryonic heart rhythm.

METHODS

Pregnant C57BL mice were administered the hypoxia marker pimonidazole followed by PHT or saline (controls) on GD 10 or GD 15. The embryos were fixed and sectioned, and the immunostained sections were analyzed with a computer assisted image analysis. Effects of PHT (0-250 microM) on heart rhythm in GD 10 embryos cultured in vitro were videotaped and then analyzed by using a digitalization technique.

RESULTS

PHT dose-dependently increased the hypoxia staining (6- and 11-fold after maternal dosing of 100 and 150 mg/kg, respectively) during the period I(Kr) is expressed and functional (GD 10). In contrast, there were no differences between the PHT doses in hypoxia staining, and much less pronounced hypoxia after this period (GD 15). With increasing PHT concentrations, increased length of the interval (bradycardia) and large variations in length between individual heartbeats (arrhythmia) were recorded.

CONCLUSIONS

PHT induced bradycardia/arrhythmia and severe embryonic hypoxia during the I(Kr) susceptible period, supporting the idea of an I(Kr)-arrhythmia-hypoxia-related teratogenic mechanism.

Teratogenicity of the I(Kr)-blocker cisapride: relation to embryonic cardiac arrhythmia

Cisapride and mosapride are structurally and pharmacologically related prokinetic agents. In contrast to mosapride, cisapride causes embryonic lethality in teratology studies, and has been related to fatal cardiac arrhythmia in the adult. The arrhythmogenic potential of cisapride is linked to its potential to inhibit a specific ion channel (I(Kr)) as a side effect. Mosapride lacks I(Kr)-blocking properties. The aims of this study were (1) to compare the effects of cisapride and mosapride on embryonic heart rhythm in vitro and (2) to investigate if cisapride in vivo, has potential to induce hypoxia-related teratogenic effects as has been shown for selective I(Kr)-blockers. Cisapride induced severe embryonic bradycardia (approximately 60% decrease), and arrhythmia in 94% of the cultured rat embryos at 1000 ng/ml. Mosapride did not induce any bradycardia or arrhythmia up to 2000 ng/ml. In vivo, single dose administration of cisapride to rats on gestational day (GD) 13 caused digital reductions (8/108 fetuses, 4/9 litters) at 75 mg/kg and high incidence of embryonic death (55-82%) at 100-200 mg/kg. Identical developmental toxic effects have been described after temporary interruption of oxygen supply, and after single dose administration of selective I(Kr)-blockers, on the same GD. The results support the idea that all potent I(Kr)-blocking agents have the potential to cause embryolethality and teratogenicity, and that the adverse effects are mediated via hypoxic episodes due to embryonic arrhythmia.

Embryonic arrhythmia by inhibition of HERG channels: a common hypoxia-related teratogenic mechanism for antiepileptic drugs?

PURPOSE

There is evidence that drug-induced embryonic arrhythmia initiates phenytoin (PHT) teratogenicity. The arrhythmia, which links to the potential of PHT to inhibit a specific potassium channel (Ikr), may result in episodes of embryonic ischemia and generation of reactive oxygen species (ROS) at reperfusion. This study sought to determine whether the proposed mechanism might be relevant for the teratogenic antiepileptic drug trimethadione (TMO).

METHODS

Effects on embryonic heart rhythm during various stages of organogenesis were examined in CD-1 mice after maternal administration (125-1,000 mg/kg) of dimethadione (DMO), the pharmacologically active metabolite of TMO. Palatal development was examined after administration of a teratogenic dose of DMO and after simultaneous treatment with DMO and a ROS-capturing agent (alpha-phenyl-N-tert-butyl-nitrone; PBN). The Ikr blocking potentials of TMO and DMO were investigated in HERG-transfected cells by using voltage patch-clamping tests.

RESULTS

DMO caused stage-specific (gestation days 9-13 only) and dose-dependent embryonic bradycardia and arrhythmia at clinically relevant maternal plasma concentrations (3-11 mM). Hemorrhage in the nasopharyngeal part of the embryonic palate (within 24 h) preceded cleft palate in fetuses at term. Simultaneous treatment with PBN significantly reduced the incidence of DMO-induced cleft palate, from 40 to 13%. Voltage patch-clamping studies showed that particularly DMO (70% inhibition), but also TMO, had Ikr blocking potential at clinically relevant concentrations.

CONCLUSIONS

TMO teratogenicity, in the same way as previously shown for PHT, was associated with Ikr-mediated episodes of embryonic cardiac arrhythmia and hypoxia/reoxygenation damage.

Transposition of the great arteries and hypocalcemia in a patient with fetal hydantoin syndrome

We report a patient with fetal hydantoin syndrome (FHS) with associated d-transposition of the great arteries (d-TGA) and persistent hypocalcemia. d-TGA and hypocalcemia have each been individually reported once in association with FHS, but these patients were also prenatally exposed to phenobarbital. To our knowledge, this is the first report of these problems occurring after prenatal exposure to hydantoin alone. The combination of congenital heart disease and hypocalcemia in our patient raises the possibility of a hydantoin effect on neural crest migration.

Phenytoin-induced cleft palate: evidence for embryonic cardiac bradyarrhythmia due to inhibition of delayed rectifier K+ channels resulting in hypoxia-reoxygenation damage

BACKGROUND

Phenytoin (PHT) teratogenicity has been related to embryonic arrhythmia due to the capacity of PHT to block I(K) channels pharmacologically, resulting in hypoxia-reoxygenation damage. The aim of this study was to further elucidate the proposed mechanism.

METHODS

Pregnant CD-1 mice were given PHT (85 mg/kg) or saline intraperitoneally on gestational days 10-11. Embryonic heart rhythm and presence of hemorrhage in orofacial region was recorded on day 12, fetuses were examined for malformations on day 18. Embryonic heart rate was also recorded on individual days after dosing days 9-16. In addition, PHT was given at doses of 10, 25, or 85 mg/kg on day 12 for analysis of plasma concentrations.

RESULTS

PTH-induced bradycardia and arrhythmia in approximately 20% of the embryos, 48% showed hemorrhage in the orofacial region; 39% of the fetuses had cleft palate. The region in which hemorrhages were visible in the embryo corresponded with the region where tissue deficiency (cleft palate) was visible in the fetus at term. None of the controls showed hemorrhages, dysrhythmia, or cleft palate. PHT affected embryonic heart rates on days 9-13, but not on days 14-16. Single dose administration on day 12, the most sensitive day, resulted in a dose-dependent decrease in embryonic heart rate (12-34%). Embryonic arrhythmia occurred at 25 and 85, but not at 10 mg/kg or in the controls. Mean maternal free plasma concentrations were 6 and 14 micromol/L in the 10- and 25-mg/kg groups, respectively.

CONCLUSIONS

PHT-induced cleft palate was preceded by embryonic dysrhythmia and hemorrhage in the orofacial region. Embryonic heart rhythm was phase specifically affected, as described for selective I(Kr) channel blockers, at clinically relevant concentrations. The results support the idea that PHT teratogenicity is a consequence of pharmacologically induced dysrhythmia and hypoxia-related damage.

The rabbit as a model for reproductive and developmental toxicity studies

The rabbit has many advantages as a nonrodent and second model for assessing the effects of toxic agents on semen quality, fertility, developmental toxicity, and teratology. The male and female reproductive systems of the rabbit are described, and data on growth, sexual development and reproduction are compared with mice, rats, and humans. Techniques for semen collection and evaluation in the male, and artificial insemination, superovulation, embryo culture, and embryo transfer in the female are included as useful procedures in toxicity testing. Examples of the use of rabbits and experimental replication for toxicity testing are given. Special features of the visceral yolk sac and development of the chorioallantoic placenta of the rabbit are compared with rodents. The rabbit extraembryonic membranes more closely resemble the human than do the rodents, in some respects. The use of the rabbit in developmental toxicity and teratology studies is discussed.

Pharmacokinetic data support pharmacologically induced embryonic dysrhythmia as explanation to Fetal Hydantoin Syndrome in rats

New studies suggest that the teratogenicity of phenytoin (PHT) is linked to its membrane-stabilizing pharmacological action via the rapid component of the delayed rectified potassium channel (lkr), resulting in embryonic cardiac dysrhythmia during a restricted sensitive period. In order to further elucidate this theory, PHT was administered to Sprague-Dawley rats on gestation day (GD) 11 with either a single dose of 150 or 100 mg/kg ip or 150 mg/kg po and developmental toxicity at term (GD 21) was studied. In satellite animals blood samples were withdrawn (0.5-24 h after dose) and total and free maternal plasma concentrations of PHT were measured. Pharmacokinetic data correlated well with pregnancy outcome data. At 150 mg/kg ip high concentrations of long duration (C(max) 240 microM and AUC 5300 microMhl(-1) - total) and marked developmental toxicity (embryonic death, decreased fetal weights, and orofacial clefts) were observed. After 100 mg/kg ip (C(max) 150 microM, AUC 2600 microMhl(-1) - total) only slight developmental toxicity (decreased fetal weights) was recorded and after 150 mg/kg po the plasma concentrations were even lower (C(max) 63 microM and AUC 1100 microMhl(-1) - total) and no adverse effects at all were observed. In separate experiments the effect of different concentrations of PHT on the embryonic heart was studied by adding PHT to GD 11 rat embryos cultured in vitro or by culturing GD 11 embryos from exposed dams. The decrease in heart rates was 3, 16, and 32% after culture with 50, 100, and 200 microM of PHT, respectively. After maternal administration of 150 mg/kg ip or po, the embryonic heart rate in vitro decreased by 25 and 7%, respectively, compared to controls. Altogether the results suggest that the development toxicity of PHT is caused by concentration-dependent induction of embryonic dysrhythmia and hypoxia related damage.

Thalidomide inhibits angiogenesis in embryoid bodies by the generation of hydroxyl radicals

Thalidomide is a teratogen with anti-angiogenic properties and causes stunted limb growth (dysmelia) during human embryogenesis. The molecular mechanisms of thalidomide action in embryopathy are currently unknown. Using the endothelial-specific antigen platelet endothelial cell adhesion molecule-1 and confocal laser scanning microscopy we have demonstrated that thalidomide exerts anti-angiogenic effects on the development of capillary structures in embryoid bodies differentiated from murine embryonic stem cells. Consequently, in thalidomide-treated embryoid bodies the diffusion properties of the tissue were deteriorated. Thalidomide raised reactive oxygen species (ROS), as revealed using 2'7'-dichlorodihydrofluorescein diacetate (H(2)DCF-DA) as an indicator. A comparable ROS generation was achieved with the thalidomide hydrolysis product phthaloyl glutamic acid (PGA), but not with phthalimide (PI), the major component of thalidomide. ROS formation by thalidomide was inhibited by the hydroxyl radical scavengers mannitol and 2-mercaptoethanol. After coadministration of either 2-mercaptoethanol or mannitol with thalidomide the anti-angiogenic effects of thalidomide were abolished and the diffusion properties of the tissue were restored to the control values. In summary, our data suggest that thalidomide exerts its anti-angiogenic properties via the generation of toxic hydroxyl radicals, which impair vasculogenesis and angiogenesis during embryoid body development.

A case of prenatal diagnosis of fetal hydantoin syndrome by ultrasound

Fetal hydantoin syndrome (FHS) is a set of disruptions occasionally present in fetuses exposed in utero to phenytoin or other anticonvulsants. Administration of phenytoin in early pregnancy may impair proper psychomotor performance expected for children's development. Several combined phenotypic markers delineate the syndrome, but the presence of single clinical signs is more common. There is controversy about the etiology of FHS. Associated disruptions may be related to a deficiency in a detoxifying enzyme (epoxide hydrolase), vascular problems, and/or factors not yet known. Genetic causes are believed to influence susceptibility to the drug. This text reports an unusual pattern of malformations detected in an ultrasound scan (gastroschisis, sacral meningomyelocele, and absence of the right lower limb) and in the anatomopathological study (left-side gastroschisis, sacral meningomyelocele, scoliosis, left clubfoot, absence of the right lower limb, and pectus carinatum) of a fetus whose mother took phenytoin. These defects may have been provoked by exposure to the drug during embryogenesis. In view of similar malformations observed in cases of prenatal exposure to cocaine, a recognized vasoconstrictor, it is suggested that vascular disruptions of hemodynamic origin constituted the event leading to some of the anomalies caused in the developing embryo. A complication of the chorionic villus sampling procedure, used for cytogenetic analysis, is another possibility.

Teratogenicity of the class III antiarrhythmic drug almokalant. Role of hypoxia and reactive oxygen species

The class III antiarrhythmic drug almokalant (ALM) was given to pregnant rats on Gestation Day 11 (125 micromol/kg) or 13 (25 micromol/kg). Other groups were pretreated with alpha-phenyl-N-t-butylnitrone, (PBN; 850 micromol/kg intraperitoneally) 1 h before administration of ALM or given (-)-2-oxo-4-thiazolidine carboxylic acid (OTC; 250 micromol/kg subcutaneously) 4 h before administration of ALM. PBN is a spin-trapping agent that can capture reactive oxygen species (ROS), and OTC is an antioxidant. Controls received tap water only. All groups (eight in total) consisted of 7 to 10 pregnant rats. ALM induced cardiovascular defects, orofacial clefts, and tail defects after administration on Day 11, and reduced the size of digits on Day 13. Pretreatment with PBN prevented induction of all the above-mentioned malformations by ALM. The results also indicated that OTC may have some protective effect against ALM-induced teratogenicity but not to the same extent as PBN. The results support the hypothesis that almokalant induces malformations via induction of episodes of embryonic arrhythmia/cardiac arrest, which result in hypoxia followed by reoxygenation and generation of ROS.

Limb reduction defects in the A/J mouse strain associated with maternal blood loss

We previously described a high incidence of digit/limb anomalies in the offspring of A/J mice subjected to surgery on day 12.5 postconception (p.c.), but not in the offspring of untreated control mice. To investigate the cause of these defects, we compared the offspring of mice in experimental groups involving adrenalectomy, sham adrenalectomy, blood sampling, and anesthesia with the offspring of control mice. All treatments significantly reduced fetal weight and increased resorptions as compared with the controls. The highest incidence of digit anomalies occurred in the offspring of dams from which blood samples had been drawn on days 12.5, 14.5, and 15.5 p.c. The incidence of isolated cleft palate was also increased in the offspring of mice that had been subjected to blood sampling. We conclude that digit anomalies in the offspring of A/J mice result from fetal vascular disruptive phenomena subsequent to maternal blood loss induced hypovolemia and hypoperfusion to the uterus and placenta as has been suggested for uterine vascular clamping, misoprostol, chorionic villus sampling, and cocaine teratogenesis. The etiology for cleft lip in these mice may involve mechanisms unrelated to uterine/placental hypoperfusion.

Retinoic acid-induced caudal regression syndrome in the mouse fetus

Caudal regression syndrome (CRS) comprises developmental anomalies of the caudal vertebrae, neural tube, urogenital and digestive organs, and hind limbs, the precursors of all of which are derived from the caudal eminence. Although the syndrome is well recognized, the etiology and pathogenetic mechanisms are poorly understood. Genetic and experimental models may provide some important clues to the early events that precede the dysmorphogenesis in CRS. The objectives of this study were to determine the susceptible stages for induction of CRS and to ascertain the early events that precede the development of this syndrome in a mouse model. Single oral doses of 100, 150, or 200 mg/kg retinoic acid (RA) were administered to TO mice on one of Gestation Days (GD) 8 to 12, and fetuses were observed on GD 18. All doses administered on GD 8 or 9 resulted in CRS in a large number of survivors. Agenesis of the tail, caudal vertebral defects, spina bifida occulta/aperta, imperforate anus, rectovesicle or rectourethral fistula, renal malformations, cryptorchidism, gastroschisis, and limb malformations, including the classical mermaid syndrome (sirenomelia), were characteristic features of this animal model. Several craniofacial malformations accompanied CRS in the GD 8 treatment group. Chronologic examination of treated embryos at early stages revealed pronounced cell death in the caudal median axis, hindgut, and neural tube and consequently, failure of development of the tail bud in the high-dose groups. In the 100 mg/kg RA group, patches of hemorrhage occurred initially that subsequently coalesced into large hematomas and the tail progressively regressed. Histologic examination revealed the onset and progression of hemorrhage, edema, and cell death in these embryos. Transillumination and histologic preparations also revealed dilation of the caudal neural tube in the prospective CRS embryos. Thus, a combination of cell death, vascular disruption, and tissue deficiency appears to be the highlight of caudal regression in this model. Symmelia appeared to be due to failure of fission or due to the merger of limb fields rather than a result of fusion of two limb buds. The data are also indicative of caudal agenesis in the high-dose RA groups and caudal regression due to a combination of vascular disruption, edema, and cell death in the lower dose groups of TO mouse embryos.

Pharmacologically induced embryonic dysrhythmia and episodes of hypoxia followed by reoxygenation: a common teratogenic mechanism for antiepileptic drugs?

Antiepileptic drugs (AEDs), such as phenytoin (PHT), carbamazepine (CBZ), trimethadione (TMD), and phenobarbital (PB), have all been associated with a similar pattern of malformations, as well as growth retardation and developmental delay. Valproic acid (VPA) has been associated with a different pattern of malformations. Recent studies suggest that PHT's fetal adverse effect is related to its membrane stabilizing pharmacological properties (blockage of voltage-dependent ion channels). During a restricted sensitive period, this results in induction of concentration-dependent bradyarrhythmia in the embryo and episodes of hypoxia/reoxygenation. The aim of this study was to compare the potential of PHT, CBZ, PB, TMD, and dimethadione (DMD; the active metabolite of TMD) to cause bradyarrhythmias. All of these AEDs exert mainly their pharmacological effect via blockage of ion channels. VPA and vigabatrin (VGB), which are pharmacologically active mainly by other mechanisms, were also tested. C57 Bl/6J mouse embryos were cultured in vitro on gestation day 10 in vitro (in 20% rat serum). The drugs were suspended in either water or dimethylsulfoxide and administered into the culture medium in increasing concentrations up to 20 times the human therapeutic plasma concentration. A scoring system was employed in order to rank the drugs based on their potential to cause bradycardia, ventricular arrhythmia, and cardiac arrest in relation to human therapeutic concentrations. Based on this system, the drugs were ranked as follows: DMD = PHT >> PB = CBZ > TMD = VPA >> VGB (no potential). The results correlate well with the available clinical/experimental data of the tested AED's potential to induce hypoxia-related fetal adverse effects, such as oral clefts, distal limb defects, growth retardation, and developmental delay. The results support the idea that adverse fetal effects after in utero exposure to PHT, PB, CBZ, and TMD (via the active metabolite DMD) are initiated via a common pharmacological mechanism: blockage of ion channels in the developing heart in the early embryo resulting in bradyarrhythmias, hemodynamic alterations, and hypoxia/reoxygenation damage.

Initiation of phenytoin teratogenesis: pharmacologically induced embryonic bradycardia and arrhythmia resulting in hypoxia and possible free radical damage at reoxygenation

The aim of this study was to investigate if phenytoin has the capacity to induce embryonic hypoxia mediated via adverse effects on the embryonic heart. Mouse embryos of different strains (CD-1, C57B1/6J and A/J) as well as Sprague Dawley (SD) rat embryos were cultured in vitro (in 75-80% rat serum) by the whole embryo technique. Effects on the heart were examined on gestational day 10 for mouse embryos and days 11 and 13 for rat embryos. Phenytoin was dissolved in water to give concentrations of 50-800 microM. In the mouse embryo studies, phenytoin caused a concentration-dependent decrease in embryonic heart rate in all three strains, with a slight decrease at 100 microM (2-7%) and a more pronounced effect at 200 microM (approximately 20%). Temporary or permanent cardiac arrest occurred in 86% of the CD-1 embryos at 500 microM, in 67% of the C57B1/6JM at 400 microM, and in all A/J embryos at 300 microM. Arrhythmias was observed in 8% in CD-1 embryos at 200 microM, in 18% at 150 microM in C57B1/6J embryos, and in 67% of the A/J embryos at 100 microM (lowest tested concentrations where arrhythmias occurred). In rat embryos, a concentration-dependent decrease in heart rate was observed on both days 11 and 13 at similar concentrations as in the mouse embryo studies. In a separate experiment, the effects on the heart rate of free phenytoin (not serum protein bound) were examined in rat embryos cultured in serum-free medium. Already at 12 microM a significant decrease in heart rate was observed. Altogether, the results support the hypothesis that phenytoin teratogenicity is initiated by pharmacologically induced embryonic hypoxia. A genetic susceptibility to the adverse effects of phenytoin on the embryonic heart may be of importance to explain strain and species differences in phenytoin teratogenicity.

Phenytoin-induced teratogenesis: a molecular basis for the observed developmental delay during neurulation

PURPOSE

We wished to determine whether chronic phenytoin (PHT) exposure could impair neural development and if any morphological alterations could be linked to changes in gene expression.

METHODS

Pregnant SWV mice were chronically administered PHT 40 mg/kg/day from gestational day (GD) 0:12 (day:h) until they were killed at various timepoints throughout neural tube closure (NTC). At each timepoint, embryos from both treated and control dams were collected and scored for their progression through NTC. The neural tubes were then isolated and subjected to in situ transcription (IST) and antisense RNA amplification procedures. Using these techniques, we examined the expression of 10 genes: N-cadherin (Ncad), collagen type IV (col-IV), bcl-2, c-jun, PAX-3, collular retinol binding protein-2 (CRBP-2), retinoic acid receptor alpha (RAR alpha), transforming growth factor(beta2) (TGF(beta2)), wee-1, and EMX-2.

RESULTS

Chronic PHT exposure not only caused a delay in NTC whereby exposed embryos lagged behind the controls at each collection timepoint, but also significantly altered the expression of specific genes at distinct times during NTC. Early in NTC, PHT induced a significant reduction in the expression of N-cad, col-IV, and c-jun in exposed embryos as compared with controls. In contrast, during the midstages of NTC, the only significant molecular alterations observed in the PHT-exposed embryos was the continued decreased expression of col-IV and an increase in CRBP-2 expression. Finally, in the latter stages of NTC, PHT caused a significant reduction in the expression of bcl-2, RAR alpha, TGF(beta2), EMX-2, and PAX-3.

CONCLUSIONS

These results show that although the effects of PHT are morphologically subtle, causing a delay in the development of the neural tube, this delay is accompanied by alterations in critical genes at crucial times of neural development that may account for the observed neurological deficits often associated with PHT exposure.

Phenytoin causes phalangeal hypoplasia in the rabbit fetus at clinically relevant free plasma concentrations

New Zealand White rabbits were treated orally with 0 (controls), 50, 100, or 150 mg/kg phenytoin on days 14-16 of pregnancy. Total and free plasma concentrations of phenytoin were determined in maternal plasma 2, 6, and 24 hr after the final dose in all animals. In addition, after administration of 150 mg/kg maternal plasma concentrations were also determined after 12 and 48 hr, the concentrations in amniotic fluid after 6 hr, and those in fetal tissue 6 and 24 hr after the final treatment. A high degree of plasma protein binding was observed in maternal blood. Treatment with 50 mg/kg resulted in free plasma concentrations of up to 5.0 mumol/l during the 24 hr period following the final dose. Significantly higher free plasma concentrations were observed at the two higher dose levels; up to 9.7 mumol/l at 100 mg/kg and 12.7 mumol/l at 150 mg/kg. Digital hypoplasia was not seen in the control group or the animals treated with 50 mg/kg. However, treatment with 100 mg/kg resulted in hypoplasia in a single or a few digits in approximately 50% of the fetuses, and 150 mg/kg provoked hypoplasia in almost all digits in all fetuses. These results show that even though the doses which caused digital defects in rabbits are much higher than those used therapeutically, the resulting free concentrations of phenytoin are similar to those which are associated with the same type of defects in humans. These data indicate that the pharmacologically induced fetal hypoxia/ischemia and vascular disruption preceding malformations of this type, which were observed in a previous study in rabbits, may be of human relevance.

Aspirin-alcohol interaction in the production of cleft palate and limb malformations in the TO mouse

Our objective in the present study was to determine the effects of alcohol on stages when the limb buds and renal primordia develop in the TO mouse and to see if aspirin pretreatment would prevent these organ systems from being malformed as was shown by Randall et al. ('91) in the C57 mice. On one of days 9-12 of gestation, groups of TO mice were injected intraperitoneally (IP) with a single dose of 200 mg/kg of aspirin, or a proportionate volume of physiological saline. An hour later, half of the aspirin-treated animals received a single dose of 0.03 ml/g of freshly prepared 25% (v/v) solution of absolute alcohol and the other half received a proportionate volume of saline. Half of the saline-treated animals received a single dose of 0.03 ml/g of saline or a proportionate volume of alcohol solution. All animals were killed on day 18 of gestation. Alcohol significantly increased embryonic resorption and caused remarkable intrauterine growth retardation (IUGR). It also induced arched palate, cleft palate and deformities of the digits with haematomas in a modest number of embryos. Aspirin alone did not have any teratogenic effects. Pretreatment with aspirin significantly augmented alcohol-induced resorption, IUGR, cleft palate and digital malformations associated with haematomas. Chronological observations on the development of the treated limbs showed the occurrence of vascular stasis, haematomas, edema and cell death at early stages. Subsequently, digital rays were either destroyed (ectrodactyly) or remained hypoplastic (brachydactyly). It appears that limb development in the aspirin- and alcohol-treated TO mouse embryos is largely affected by vascular disruption. These data provide further evidence to our earlier observation that alcohol and aspirin interact in the production of malformations and that the teratogenic effects of alcohol in the TO mouse are possibly not mediated via treatment related prostaglandin elevation.

Chemical teratogenesis

This review has briefly summarized what is currently known concerning the mechanisms whereby several groups of chemicals regarded as "recognized" human teratogens elicit their respective teratogenic effects. It is evident that the extent of our understanding of mechanisms for individual chemicals varies dramatically from that of a reasonably good understanding for methotrexate and other folic acid antagonists to that of virtually no understanding for the most widely recognized human teratogen, thalidomide. Even with methotrexate, however, much remains to be learned pertaining to mechanisms--i.e., the critical links in the chain of events between dihydrofolate reductase inhibition and the manifestation of specific abnormalities. Nevertheless, we can take some comfort in being able to say that we understand the primary causative mechanism. For thalidomide, as well as several others the chemical represents both a shame and a challenge--a challenge that should be addressed with our most serious efforts.