Modulation of phenytoin teratogenicity and embryonic covalent binding by acetylsalicylic acid, caffeic acid, and alpha-phenyl-N-t-butylnitrone: implications for bioactivation by prostaglandin synthetase

Phenytoin induces connective tissue growth factor (CTGF/CCN2) production through NADPH oxidase 4-mediated latent TGFβ1 activation in human gingiva fibroblasts: Suppression by curcumin

OBJECTIVE AND BACKGROUND

Gingival overgrowth (GO) is a common side effect of some drugs such as anticonvulsants, immunosuppressant, and calcium channel blockers. Among them, the antiepileptic agent phenytoin is the most common agent related to this condition due to its high incidence. Transforming growth factor β (TGFβ) importantly contributes to the pathogenesis of GO. Connective tissue growth factor (CTGF or CCN2) is a key mediator of tissue fibrosis and is positively associated with the degree of fibrosis in GO. We previously showed that Src, c-jun N-terminal kinase, and Smad3 mediate TGFβ1-induced CCN2 protein expression in human gingival fibroblasts (HGFs). This study investigates whether phenytoin can induce CCN2 synthesis through activated latent TGFβ in HGFs and its mechanisms.

METHODS

CCN2 synthesis, latent TGFβ1 activation, and cellular reactive oxygen species (ROS) generation in HGFs were studied using western blot analysis, a TGFβ1 Emax® ImmunoAssay System, and 2',7'-dichlorodihydrofluorescein diacetate (an oxidation-sensitive fluorescent probe), respectively.

RESULTS

Phenytoin significantly stimulated CCN2 synthesis, latent TGFβ1 activation, and ROS generation in HGFs. Addition of an TGFβ-neutralizing antibody, TGFβ receptor kinase inhibitor SB431542, and Smad3 inhibitor SIS3 completely inhibited phenytoin-induced CCN2 synthesis. General antioxidant N-acetylcysteine, NADPH oxidase (NOX) inhibitor diphenylene iodonium, and specific NOX4 inhibitor plumbagin almost completely suppressed phenytoin-induced total cellular ROS and latent TGFβ1 activation. Curcumin dose-dependently decreased phenytoin-induced TGFβ1 activation and CCN2 synthesis in HGFs.

CONCLUSIONS

Our findings indicated that NOX4-derived ROS play pivotal roles in phenytoin-induced latent TGFβ1 activation. Molecular targeting the phenytoin/NOX4/ROS/TGFβ1 pathway may provide promising strategies for the prevention and treatment of GO. Curcumin-inhibited phenytoin-induced CCN2 synthesis is caused by the suppression of latent TGFβ1 activation.

The effect of phenytoin on embryonic heart rate in Vivo

Phenytoin is a known human teratogen with unknown etiology. Several mechanisms have been proposed including disturbances in folate metabolism, induction of embryonic hypoxia following phenytoin-induced bradycardia, free radical formation following re-oxygenation and phenytoin-induced maternal hyperglycemia. Using high frequency ultrasound, we demonstrated that phenytoin induced a dramatic decrease in the heart rate of embryos. This coincided with a moderate transient decrease in maternal heart rate and blood glucose levels. Embryonic heart rate had not fully recovered 24 h later in some embryos despite normal maternal physiological parameters. In a separate study, extent of hypoxia was measured using the marker pimonidazole. Phenytoin-exposed embryos did not demonstrate increased hypoxia compared to control embryos at 2, 4, 8 or 24 h dosing. Together our results show that phenytoin induces malformations as a result of a combination of insults: embryonic bradycardia, maternal bradycardia and maternal hyperglycemia. However, this does not appear to result in measurable embryonic hypoxia in our animal model.

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.

Cognitive impairment in childhood onset epilepsy: up-to-date information about its causes

Cognitive impairment associated with childhood-onset epilepsy is an important consequence in the developing brain owing to its negative effects on neurodevelopmental and social outcomes. While the cause of cognitive impairment in epilepsy appears to be multifactorial, epilepsy-related factors such as type of epilepsy and underlying etiology, age at onset, frequency of seizures, duration of epilepsy, and its treatment are considered important. In recent studies, antecedent cognitive impairment before the first recognized seizure and microstructural and functional alteration of the brain at onset of epilepsy suggest the presence of a common neurobiological mechanism between epilepsy and cognitive comorbidity. However, the overall impact of cognitive comorbidity in children with epilepsy and the independent contribution of each of these factors to cognitive impairment have not been clearly delineated. This review article focuses on the significant contributors to cognitive impairment in children with epilepsy.

Oxidative DNA damage in the in utero initiation of postnatal neurodevelopmental deficits by normal fetal and ethanol-enhanced oxidative stress in oxoguanine glycosylase 1 knockout mice

Studies in mice with deficient antioxidative enzymes have shown that physiological levels of reactive oxygen species (ROS) can adversely affect the developing embryo and fetus. Herein, DNA repair-deficient progeny of oxoguanine glycosylase 1 (ogg1)-knockout mice lacking repair of the oxidative DNA lesion 8-oxo-2'-deoxyguanosine (8-oxodGuo) exhibited enhanced postnatal neurodevelopmental deficits, revealing the pathogenic potential of 8-oxodGuo initiated by physiological ROS production in fetal brain and providing the first evidence of a pathological phenotype for ogg1-knockout mice. Moreover, when exposed in utero to ethanol (EtOH), ogg1-knockout progeny exhibited higher levels of 8-oxodGuo in fetal brain and more severe postnatal neurodevelopmental deficits than wild-type littermates, both of which were blocked by pretreatment with the free radical trapping agent phenylbutylnitrone. These results suggest that ROS-initiated DNA oxidation, as distinct from altered signal transduction, contributes to neurodevelopmental deficits caused by in utero EtOH exposure, and fetal DNA repair is a determinant of risk.

The free radical spin trapping agent phenylbutylnitrone reduces fetal brain DNA oxidation and postnatal cognitive deficits caused by in utero exposure to a non-structurally teratogenic dose of ethanol: a role for oxidative stress

Reactive oxygen species (ROS), although implicated in morphological birth defects caused by ethanol (EtOH) during pregnancy, have not been directly linked to its behavioral deficits. To determine this, a pathogenic oxidative DNA lesion was measured in fetal brain, and a passive avoidance learning test was assessed postnatally in the progeny of CD-1 mice treated once on gestational day 17 with 4g/kg EtOH or its saline vehicle, with or without pretreatment with the free radical spin trapping agent α-phenyl-N-tert-butylnitrone (PBN; 40mg/kg). EtOH-exposed CD-1 progeny, unlike C57BL/6 progeny, had no morphological birth defects, but exhibited a learning deficit at 12 weeks of age (p<0.001), which continued to 16 weeks in males (p<0.01). Peak blood EtOH concentrations were 2.5-fold higher in C57BL/6 mice compared to CD-1 mice given the same dose. PBN pretreatment of CD-1 dams blocked both EtOH-initiated DNA oxidation in fetal brain (p<0.05) and postnatal learning deficits (p<0.01), providing the first direct evidence for ROS in the mechanism of EtOH-initiated neurodevelopmental deficits.

Phenytoin-induced gingival overgrowth

Gingival overgrowth is a common adverse effect of therapy with Phenytoin, having important medical and cosmetic implications. Poor periodontal hygiene is an important risk factor for severity of Phenytoin-induced gingival overgrowth (PIGO), which is a time-dependent process. There is complex interplay of altered fibroblast biology, connective tissue turnover, inflammatory processes, and growth factors on a background of genetic susceptibility to produce increase in various components of interstitial matrix in PIGO tissue. Treatment options have included change of PHT to another anti-seizure drug, measures to improve periodontal hygiene and gingivectomy. There is conclusive evidence that folic acid supplementation significantly decreases the incidence of PIGO.

Increased Susceptibility to Phenytoin Teratogenicity: Excessive Generation of Reactive Oxygen Species or Impaired Antioxidant Defense?

Phenytoin is a human and animal teratogen. Accumulating evidence suggests that the teratogenicity is associated with a potential of phenytoin to cause embryonic cardiac arrhythmia and resultant generation of toxic reactive oxygen species via hypoxia-reoxygenation mechanisms. The A/J mouse is more susceptible to phenytoin teratogenicity than other mouse strains. The aim of this study was to investigate whether A/J mice have other antioxidant enzyme activities than C57BL/6J and CD-1 mice. Also, strain differences in phenytoin effects on embryonic heart rate and rhythm were determined. Another objective was to determine whether a spin trapping agent with capacity to capture reactive oxygen species alter the developmental toxicity of phenytoin. Treatment with this agent resulted in a marked decrease in phenytoin teratogenicity, which supports the idea that reactive oxygen species are important mediators for the teratogenic action of phenytoin. The A/J mice embryos were most susceptible to the adverse cardiac effects of phenytoin and had the highest activity of superoxide dismutase and glutathione peroxidase, while the activity of catalase was the same in embryos of the three different strains. The high activities of antioxidant enzymes in the A/J stain indicate that the sensitivity to develop malformations is caused by excessive arrhythmia-related generation of reactive oxygen species rather than impaired antioxidant defense.

Maternal antioxidant supplementation does not reduce the incidence of phenytoin-induced cleft lip and related malformations in rats

There is considerable evidence that phenytoin-induced birth defects in the rat are a consequence of a period of bradycardia and hypoxia in the embryos. Experiments were designed to test the hypothesis that phenytoin-induced birth defects result from free-radical damage to the embryos during the reoxygenation period posthypoxia. Female rats (>9 per group) were fed either a control diet or a diet high in antioxidants (vitamins C and E and coenzyme Q(10)) both before and during pregnancy and were then given a teratogenic dose of phenytoin (180 mg/kg) on GD 11. The rats were killed on GD 20 and the fetuses were examined for malformations. The initial results showed that the antioxidant diet had a significant protective effect, with far fewer antioxidant-group fetuses showing cleft lip or maxillary hypoplasia compared with the control group. However, this result was confounded by reduced food intake by the rats fed the antioxidant diet and a significantly lower maternal body weight at the time of phenytoin administration. Since the phenytoin was administered by intraperitoneal injection (i.p.) the control rats received higher absolute doses of phenytoin and it is speculated that this results in higher fetal exposure. A second experiment, in which the rats were pair-fed, failed to demonstrate any protective effect of the high antioxidant diet. These results do not support the reoxygenation hypothesis for phenytoin teratogenesis. An alternative explanation would be hypoxia-induced transcription-related changes resulting in cell cycle arrest and apoptosis.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Additional investigation on the potentiation of phenytoin teratogenicity by fluconazole

Fluconazole (FCZ) is a potent inhibitor of the cytochrome P450 (CYP)-mediated metabolism of the anti-epileptic agent phenytoin (PHT), a well-known human and animal teratogen. It has been postulated that phenytoin must be bioactivated via the CYP system to initiate teratogenesis. In contrast with this view, FCZ pretreatment has been previously shown to result in a potentiation of PHT teratogenesis. The current study was initiated to determine the impact of FCZ pretreatment on PHT exposure levels in maternal and embryonal compartments. HPLC analysis revealed that under a co-dosing FCZ-PHT regimen resulting in enhanced PHT teratogenesis, statistically significant higher PHT levels are detectable in maternal plasma and embryonic tissue in comparison to controls. These results further argue against a role for CYP system in teratogenic bioactivation of PHT.

Involvement of multiple UDP-glucuronosyltransferase 1A isoforms in glucuronidation of 5-(4'-hydroxyphenyl)-5-phenylhydantoin in human liver microsomes

In humans, orally administered phenytoin, 5,5-diphenylhydantoin, is mainly excreted as 5-(4'-hydroxyphenyl)-5-phenylhydantoin (4'-HPPH) O-glucuronide. Phenytoin is oxidized to 4'-HPPH by CYP2C9 and to a minor extent by CYP2C19, and then 4'-HPPH is metabolized to 4'-HPPH O-glucuronide by UDP-glucuronosyltransferase (UGT). In the present study, 4'-HPPH O-glucuronidation in human liver microsomes was investigated. The metabolite formed by incubation with human liver microsomes, 4'-HPPH, and UDP-glucuronic acid was identified as 4'-HPPH O-glucuronide by liquid chromatography-tandem mass spectrometry analysis. The 4'-HPPH O-glucuronosyltransferase activity in human liver microsomes was not saturated at concentrations up to 500 microM of 4'-HPPH. Any commercially available recombinant human UGTs (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7, and UGT2B15) expressed in baculovirus-infected insect cells did not show detectable 4'-HPPH O-glucuronide. The 4'-HPPH O-glucuronidation in pooled human liver microsomes was inhibited by beta-estradiol as a typical substrate for UGT1A1 (IC(50) = 21.1 microM) and imipramine as a typical substrate for UGT1A4 (IC(50) = 57.7 microM). The inhibitory effects of propofol as a specific substrate for UGT1A9 (IC(50) = 167.1 microM) and emodin as a substrate for UGT1A8 and UGT1A10 (IC(50) = 287.6 microM) were not prominent. The interindividual difference in the 4'-HPPH O-glucuronidation in 14 human liver microsomes was 28.5-fold (0.023-0.656 nmol/min/mg of protein). The 4'-HPPH O-glucuronosyltransferase activity in 11 human liver microsomes was significantly (r = 0.609, P < 0.05) correlated with the 4-nitrophenol glucuronosyltransferase activity, which is catalyzed by UGT1A6 and UGT1A9. These results suggest that multiple UGT1As such as UGT1A1, UGT1A4, UGT1A6, and UGT1A9 are involved in 4'-HPPH O-glucuronidation in human liver microsomes, although the percentage contribution of each UGT1A could not be estimated. Large interindividual differences in the glucuronidation of 4'-HPPH might be responsible for the nonlinearity of the phenytoin plasma concentration or adverse reactions in humans.

Identification of catalase in human livers as a factor that enhances phenytoin dihydroxy metabolite formation by human liver microsomes

We have reported previously that the formation of a 3',4'-dihydroxylated metabolite of phenytoin (3',4'-diHPPH) by human liver microsomal cytochrome P450 (P450) is enhanced by the addition of human liver cytosol [Komatsu et al., Drug Metab Dispos 2000;28:1361-8]. The enhancing factor was determined in this study. The addition of cytosolic proteins precipitated by 50% ammonium sulfate to incubation mixtures increased the rate of microsomal 3',4'-diHPPH formation. This fraction was separated further by diethylaminoethyl-, carboxymethyl-, and hydroxyapatite-column chromatography. The amino acid sequence of the purified protein of approximately 55kDa by electrophoresis revealed this protein to be a catalase. The addition of purified or authentic catalase to the incubation mixtures increased the rates of microsomal 3',4'-diHPPH formation from 3'- and 4'-hydroxylated metabolites and from phenytoin in a concentration-dependent manner. In reconstituted systems containing CYP2C9, CYP2C19, and CYP3A4, the formation of 3',4'-diHPPH was also enhanced by catalase to different extents. This is the first report that catalase in livers enhances drug oxidation activities catalyzed by P450 in human liver microsomes.

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.

Antioxidant and GSH-related enzyme response to a single teratogenic exposure to the anticonvulsant phenytoin: temporospatial evaluation

BACKGROUND

It has been proposed that the anticonvulsant drug phenytoin (PHT) requires bioactivation to reactive intermediate(s) to achieve its recognized teratogenic potential and that embryonal detoxification power may play a fundamental role in the teratogenic response. On this basis, we sought to investigate the potential effects of a teratogenic exposure to PHT on the activities of antioxidant and GSH-related detoxifying enzymes in gestational murine tissues.

METHODS

Pregnant Swiss mice were injected intraperitoneally with 0 (vehicle) or 65 mg/kg of PHT on gestation day (GD) 12 (plug day = GD 1). Biochemical determinations, including activities of glutathione transferase, glutathione peroxidase, glutathione reductase, glyoxalase I, glyoxalase II, catalase, and superoxide dismutase, were carried out on maternal and embryonic/fetal livers and in placentas on GD 14 and 19.

RESULTS

The major findings of this study show that (1) organogenesis-stage conceptal tissues have detectable levels of all the tested enzymes; (2) most of the embryonic liver and placental enzymes investigated undergo a significant induction within 48 hr (GD 14) after PHT administration; and (3) in the same tissues a down-regulation of enzyme activities is noted near term (GD 19).

CONCLUSIONS

Overall, these findings show that teratogenic exposure to PHT is associated with a modulation of reactive-intermediates-scavenging enzyme activities, and provide further support for role of generation of reactive intermediates in PHT-induced teratogenesis.

Temporal and spatial expression of Hoxa-2 during murine palatogenesis

1. Mice homozygous for a targeted mutation of the Hoxa-2 gene are born with a bilateral cleft of the secondary palate associated with multiple head and cranial anomalies and these animals die within 24 hr of birth (Gendron-Maguire et al., 1993; Rijli et al., 1993; Mallo and Gridley, 1996). We have determined the spatial and temporal expression of the Hoxa-2 homeobox protein in the developing mouse palate at embryonic stages E12, E13, E13.5, E14, E14.5, and E15. 2. Hoxa-2 is expressed in the mesenchyme and epithelial cells of the palate at E12, but is progressively restricted to the tips of the growing palatal shelves at E13. 3. By the E13.5 stage of development, Hoxa-2 protein was found to be expressed throughout the palatal shelf. These observations correlate with palatal shelf orientation and Hoxa-2 protein may play a direct or indirect role in guiding the palatal shelves vertically along side the tongue, starting with the tips of the palatal shelves at E13, followed by the entire palatal shelf at E13.5. 4. As development progresses to E14, the stage at which shelf elevation occurs, Hoxa-2 protein is downregulated in the palatal mesenchyme but remains in the medial edge epithelium. Expression of Hoxa-2 continues in the medial edge epithelium until the fusion of opposing palatal shelves. 5. By the E15 stage of development, Hoxa-2 is downregulated in the palate and expression is localized in the nasal and oral epithelia. 6. In an animal model of phenytoin-induced cleft palate, we report that Hoxa-2 mRNA and protein expression were significantly decreased, implicating a possible functional role of the Hoxa-2 gene in the development of phenytoin-induced cleft palate. 7. A recent report by Barrow and Capecchi (1999), has illustrated the importance of tongue posture during palatal shelf closure in Hoxa-2 mutant mice. This along with our new findings of the expression of the Hoxa-2 protein during palatogenesis has shed some light on the putative role of this gene in palate development.

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.

Teratological interaction between the bis-triazole antifungal agent fluconazole and the anticonvulsant drug phenytoin

Previous studies implicated the cytochrome P450 (CYP) system as critical in the teratogenic bioactivation of phenytoin (PHT). Fluconazole (FCZ) is an antifungal bis-triazole with potent inhibitory effect on the principal CYP-dependent metabolic pathway of PHT. In this study an in vivo experimental model was used to evaluate the potential ability of FCZ (2, 10, or 50 mg/kg intraperitoneally) to modulate PHT (65 mg/kg intraperitoneally) teratogenesis on day 12 (plug day = day 1) Swiss mice. PHT alone elicited embryocidal and malformative effects, with cleft palate as the major malformation. Pretreatment with the nonembryotoxic dosage of 10 mg FCZ/kg potentiated PHT-induced teratogenesis, as indicated by a twofold (from 6.2% to 13.3%) increment of cleft palate incidence (P < 0.05). Combined treatment with 50 mg FCZ/kg plus PHT resulted in a statistically significant (P < 0.05) increment of the resorption incidence recorded after PHT-alone exposure, but possibly as a consequence of the increased embryolethality, in the loss of the potentiative effect on PHT teratogenesis. Although the mechanistic nature of teratological interaction between FCZ and PHT remains to be established, these results may not support CYP system-mediated metabolic conversion as the mechanistic component of PHT teratogenesis.

Phenytoin-induced alterations in craniofacial gene expression

In utero exposure to the anticonvulsant drug phenytoin has been shown to alter normal embryonic development, leading to a pattern of dysmorphogenesis known as the Fetal Hydantoin Syndrome. This embryopathy is characterized by growth retardation, microcephaly, mental deficiency, and craniofacial malformations, although the precise mechanism(s) by which phenytoin alters normal developmental pathways remains unknown. To better understand the molecular events involved in the pathogenesis of phenytoin-induced congenital defects, alterations in gene expression were examined during critical periods of craniofacial development. Pregnant SWV mice were administered phenytoin (60 mg/kg/day) from gestational day 6.5 until they were sacrificed at selected developmental time points. Tissue from the craniofacial region of control and exposed embryos was isolated, and samples were subjected to in situ transcription, antisense RNA amplification, and hybridization on reverse Northern blots to quantitatively assess expression of 36 candidate genes. Chronic phenytoin exposure significantly altered expression of several genes at distinct times during morphogenesis. Results of these studies show that expression of the retinoic acid receptors (RAR) alpha, beta, and gamma were significantly increased by phenytoin exposure. Elevations in gene expression of laminin beta 1, and the growth factors IGF-2, TGF alpha, and TGF beta 1, were also demonstrated in the craniofacial region of phenytoin-exposed embryos. As several of these genes are transcriptionally regulated by retinoic-acid-responsive elements in their promoter regions, phenytoin-induced alterations in expression of the RAR isoforms may have severe downstream consequences in the regulation of events necessary for normal craniofacial development. Such alterations occurring coordinately at critical times during craniofacial development may account for the dysmorphogenesis often associated with phenytoin exposure.

Evidence for embryonic prostaglandin H synthase-catalyzed bioactivation and reactive oxygen species-mediated oxidation of cellular macromolecules in phenytoin and benzo[a]pyrene teratogenesis

A mouse embryo culture model was used to determine whether embryonic prostaglandin H synthase (PHS)-catalyzed bioactivation and resultant oxidative damage to embryonic protein and DNA may constitute a molecular mechanism mediating phenytoin and benzo[a]pyrene teratogenesis. Embryos were explanted from CD-1 mouse dams on gestational day 9.5 (vaginal plug = day 1) and incubated for either 4 h (biochemistry) or 24 h (embryotoxicity) at 37 degrees C in medium containing either phenytoin (20 micrograms/ml, 80 microM), benzo[a]pyrene (10 microM), or their respective vehicles. As previously observed with phenytoin (Mol. Pharmacol.48: 112-120, 1995), embryos incubated with benzo[a]pyrene showed decreases in anterior neuropore closure, turning, yolk sac diameter, and somite development (p < .05). Addition of the antioxidative enzyme superoxide dismutase (SOD) substantially enhanced embryonic SOD activity (p < .05) and completely inhibited benzo[a]pyrene embryotoxicity (p < .05). Substantial PHS was detected in day 9.5 embryos using SDS/PAGE, anti-PHS antibody, and alkaline phosphatase-conjugated donkey anti-goat IgG. Embryonic protein oxidation was detected by the reaction of 0.5 mM 2,4-dinitrophenylhydrazine with protein carbonyl groups. This method was first validated by using a known hydroxyl radical-generating system consisting of vanadyl sulfate and H2O2, with bovine serum albumin or embryonic protein as the target. Embryonic proteins were characterized by SDS/PAGE, anti-dinitrophenyl antisera, and peroxidase-labeled goat anti-donkey IgG. Using enhanced chemiluminescence, the number and content of oxidized protein bands detected between 25 and 200 kDa were substantially increased by both phenytoin and benzo[a]pyrene. Addition of the reducing agent dithiothreitol, or SOD or catalase, decreased protein oxidation in phenytoin-exposed embryos. Both phenytoin (Mol. Pharmacol.48: 112-120, 1995) and benzo[a]pyrene enhanced embryonic DNA oxidation, determined by the formation of 8-hydroxy-2'-deoxyguanosine, as measured by high-performance liquid chromatography (HPLC) (p < .05). Phenytoin also enhanced the oxidation of embryonic glutathione (GSH) to its GSSG disulfide, as measured by HPLC (p < .05). These results provide direct evidence that, in the absence of maternal or placental processes, embryonic PHS-catalyzed bioactivation and reactive oxygen species-mediated oxidation of embryonic protein, thiols, and DNA may constitute a molecular mechanism mediating phenytoin and benzo[a]pyrene teratogenesis.

In vitro embryotoxicity of carbamazepine and carbamazepine-10, 11-epoxide

Carbamazepine (Tegretol, CBZ) is an anticonvulsant drug that is very effective in the treatment of tonic-clonic seizures and is gaining acceptance as a treatment for various psychiatric disorders. The drug is embryotoxic in rodents and has been reported to produce neural tube defects in approximated 1% of prenatally exposed human offspring. It is metabolized by the cytochrome P-450 system to a stable, pharmacologically active epoxide intermediate, carbamazepine-10, 11-epoxide. It is currently unknown whether the parent compound, the epoxide intermediate or some other metabolite is the embryotoxic agent. The present study was designed to determine the embryotoxicity of CBZ and its epoxide intermediate (CBZ-E) in a rodent whole embryo culture system. Rat embryos were cultured beginning on day 9 of gestation (GD 9), and mouse embryos were cultured beginning in GD 8. All embryos were cultured for 48 hr in medium containing various concentrations of either CBZ or CBZ-E. Mice were more sensitive to the effects of CBZ than were rats. The parent compound was embryotoxic to mouse embryos at concentrations as low as 12 micrograms, but it was only embryotoxic at 60 micrograms/ml to rat embryos. CBZ-E was not embryotoxic to either species at concentrations as high as 48 micrograms/ml. These results suggest that the parent compound is the embryotoxic agent and that the epoxide intermediate plays no role in the drug's embryotoxic mechanism.

Biochemical toxicology of chemical teratogenesis

Although exposure during pregnancy to many drugs and environmental chemicals is known to cause in utero death of the embryo of fetus, or initiate birth defects (teratogenesis) in the surviving offspring, surprisingly, little is known about the underlying biochemical and molecular mechanisms, or the determinants of teratological susceptibility, particularly in humans. In vitro and in vivo studies based primarily on rodent models suggest that many potential embryotoxic xenobiotics are actually proteratogens that must be bioactivated by enzymes such as the cytochromes P450 and peroxidases such as prostaglandin H synthase to teratogenic reactive intermediary metabolites. These reactive intermediates generally are electrophiles or free radicals that bind covalently (irreversibly) to, or directly of indirectly oxidize, embryonic cellular macromolecules such as DNA, protein, and lipid, irreversibly altering cellular function. Target oxidation, known as oxidase stress, often appears to be mediated by reactive oxygen species (ROS) such as hydroxyl radicals. The precise nature of the teratologically relevant molecular targets remains to be established, as do the relative conditions of the various types of macromolecular lesions. Teratological suseptibility appears to be determined in part by a balance among pathways of maternal xenobiotic elimination, embryonic xenobiotic bioactivation and detoxification of the xenobiotic reactive intermediate, direct and indirect pathways for the detoxification of ROS (cytoprotection), and repair of macromolecular lesions. Due largely to immature or otherwise compromised embryonic pathways for detoxification, Cytoprotection, and repair, the embryo is relatively susceptible to reactive intermediates, and teratogenesis via this mechanism can occur from exposure to therapeutic concentrations of drugs, or supposedly safe concentrations of environmental chemicals. Greater insight into the mechanisms involved in human reactive intermediate-mediated teratogenicity, and the determinants of individual teratological susceptibility, will be necessary to reduce the unwarranted embryonic attrition from xenobiotic exposure.

Phenytoin covalent binding and embryopathy in mouse embryos co-cultured with maternal hepatocytes from mouse, rat, and rabbit

The anticonvulsant drug phenytoin is teratogenic in a variety of species including humans. Traditional embryo culture studies have employed the addition of 9000 g supernatant (S-9) or microsomal fractions from induced rat or mouse liver as an exogenous bioactivating system to approximate a maternal contribution. However, cellular fractions, unlike cultured intact hepatocytes, may themselves be embryotoxic, and do not reflect the in vivo balance of bioactivation and detoxification. To evaluate in vitro the known in vivo differential species susceptibility to phenytoin teratogenesis, day 9.5 (day of plug = day 1) mouse embryos either were cultured alone for 24 hr or were co-cultured with hepatocytes from maternal mice, rats or male rabbits, thereby exposing the embryos to the effects of potential species-specific phenytoin metabolism. In the absence of hepatocytes, phenytoin embryotoxicity was concentration dependent (0, 10, 20 and 60 micrograms/mL), with decreases in embryonic growth, reflected by reduced yolk sac diameter and crown rump length, apparent within the maternal therapeutic range (20 micrograms/mL). Covalent binding of the radiolabeled drug to live embryonic tissue was significantly higher than in control embryos previously killed by fixation, suggesting that the embryo can bioactivate phenytoin to a toxic reactive intermediate. Mouse embryos grew equally well with hepatocytes from all three species, indicating interspecies tissue compatibility. The addition of rat and rabbit hepatocytes, but not mouse hepatocytes, significantly enhanced the phenytoin-induced impairment of mouse embryonic development, as demonstrated by reductions in somite number. The phenytoin-induced impairment of mouse embryonic growth was not enhanced by the addition of rat or rabbit hepatocytes, while mouse hepatocytes conferred protection. The covalent binding of phenytoin to extracellular proteins in the culture medium was not enhanced by the addition of mouse hepatocytes. These results suggest that mouse embryos intrinsically can bioactivate phenytoin to a toxic reactive intermediate, with embryopathic consequences. The protection conferred by maternal mouse hepatocytes suggests a species-specific maternal biochemical balance favouring detoxification that is not shared by rat and rabbit hepatocytes, which enhanced phenytoin embryopathy. Thus, while phenytoin teratogenicity likely involves embryonic bioactivation, maternal determinants may contribute variably to teratologic susceptibility in a species-specific manner.

DNA oxidation as a potential molecular mechanism mediating drug-induced birth defects: phenytoin and structurally related teratogens initiate the formation of 8-hydroxy-2'-deoxyguanosine in vitro and in vivo in murine maternal hepatic and embryonic tissues

A considerable number of teratogens, including the anticonvulsant drug phenytoin and structurally related drugs and environmental chemicals, may be bioactivated by peroxidases, such as prostaglandin H synthase (PHS) and lipoxygenases (LPOs), to a reactive free radical intermediate that initiates birth defects. However, the molecular targets of the reactive free radical intermediates mediating chemical teratogenesis, and hence the fundamental determinants of susceptibility, are poorly understood. In these studies, a teratogenic dose of phenytoin (65 mg/kg), when injected into pregnant CD-1 mice during organogenesis on gestational day 12, initiated the oxidation of DNA in maternal hepatic and embryonic nuclei, forming 8-hydroxy-2'-deoxyguanosine. Significant maternal and embryonic DNA oxidation occurred at 6 and 3 h, respectively, suggesting relative embryonic deficiencies in free radical-related cytoprotective enzymes, although the rates appeared similar. Maximal DNA oxidation in both maternal and embryonic tissues occurred at 6 h, presumably reflecting the balance of DNA oxidation and repair, the latter of which appeared similar in both tissues. Inhibition of phenytoin-initiated embryonic DNA oxidation by the free radical spin trapping agent alpha-phenyl-N-t-butylnitrone (41.5 mg/kg), and by acetylsalicylic acid (10 mg/kg), an inhibitor of the cyclooxygenase component of PHS, was consistent with the previously reported reduction by these inhibitors of phenytoin-initiated murine birth defects. In vitro studies using a horseradish peroxidase (0.5 mg/ml)-H2O2 (5.45 micrograms/ml) bioactivating system for drug-initiated oxidation of 2'-deoxyguanosine (3.74 mM), indicated that the potency of xenobiotic-initiated formation of 8-hydroxy-2'-deoxyguanosine for the structurally related drugs and metabolites phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin, trimethadione, dimethadione, l-mephenytoin, l-nirvanol, d-nirvanol (80 microM each), or thalidomide (64 microM), reflected their murine teratogenic potency. Given the relatively low activities of cytochromes P450, compared to PHS and LPOs, in human and rodent embryonic tissues, these data support the potential teratological importance of peroxidase-catalysed bioactivation of xenobiotics with structural similarities to phenytoin. These studies provide the first evidence that peroxidase-catalysed embryonic DNA oxidation may constitute a critical molecular mechanism mediating the teratogenicity of phenytoin and related drugs and environmental chemicals, and suggest the potential teratological importance of additional embryonic processes, such as DNA repair and tumor suppressor genes, as determinants of susceptibility.

Metabolism of phenytoin and covalent binding of reactive intermediates in activated human neutrophils

ontivation of neutrophils by phorbol-12-myristate-13-acetate (PMA) causes rapid production of superoxide radical (O2-), leading to the formation of additional reactive oxygen species, including hydrogen peroxide (H2O2), hypochlorous acid (HOCl), and possibly hydroxyl radical (.OH). These reactive oxygen species have been associated with the oxidation of some drugs. We investigated the metabolism of phenytoin (5,5-diphenylhydantoin) and the covalent binding of reactive intermediates to cellular macromolecules in activated neutrophils. In incubations with 100 microM phenytoin, PMA-stimulated neutrophils from six human subjects produced p-, m-, and o-isomers of 5-(hydroxyphenyl)-5-phenylhydantoin (HPPH) in a ratio of 1.0:2.1:2.8, respectively, as well as unidentified polar products. Analysis of cell pellets demonstrated that phenytoin was bioactivated to reactive intermediates that bound irreversibly to macromolecules in neutrophils. Glutathione, catalase, superoxide dismutase, azide, and indomethacin all diminished the metabolism of phenytoin and the covalent binding of its reactive intermediates. The iron-inactivating chelators desferrioxamine and diethylenetriaminepentaacetic acid had little or no effect on the metabolism of phenytoin by neutrophils, demonstrating that adventitious iron was not contributing via Fenton chemistry. In an .OH-generating system containing H2O2 and Fe2+ chelated with ADP, phenytoin was oxidized rapidly to unidentified polar products and to p-, m-, and o-HPPH (ratio 1.0:1.7:1.5, respectively). Reagent HOCl and human myeloperoxidase (MPO), in the presence of Cl- and H2O2, both formed the reactive dichlorophenytoin but no HPPH. However, no chlorinated phenytoin was detected in activated neutrophils, possibly because of its high reactivity. These findings, which demonstrated that activated neutrophils biotransform phenytoin in vitro to hydroxylated products and reactive intermediates that bind irreversibly to tissue macromolecules, are consistent with phenytoin hydroxylation by .OH generated by a transition metal-independent process, chlorination by HOCl generated by MPO, and possibly cooxidation by neutrophil hydroperoxidases. Neutrophils activated in vivo may similarly convert phenytoin to reactive intermediates, which could contribute to some of the previously unexplained adverse effects of the drug.

n-3 fatty acids inhibit defects and fatty acid changes caused by phenytoin in early gestation in mice

Our previous work has shown that n-3 fatty acids exert a protective effect against phenytoin-induced cleft palate when phenytoin was administered midgestation [gestational days (GD) 12 and 13] to CD-1 mice. The effects of dietary n-3 fatty acids on phenytoin teratogenicity were investigated at an earlier gestational period (GD 9) to examine whether n-3 fatty acids could exert protective action against other teratogenic effects of phenytoin apart from cleft palate. The effect of phenytoin exposure on maternal hepatic polyunsaturated fatty acid composition was also studied since delta 6 desaturase activity has been shown to be modified by pharmacological action. Female CD-1 mice were fed a standard laboratory diet (SLD), safflower oil (SAFF) or a cod liver/linseed oil (CLO/LO)-based diet for three weeks prior to impregnation and throughout pregnancy. Pregnant mice were administered a single i.p. dose of phenytoin on GD 9, and teratological assessments were performed on GD 19. Tissues were harvested on GD 10 for maternal hepatic phospholipid fatty acid analysis from another group of phenytoin-treated mice. The CLO/LO and the SLD mice, as compared to the SAFF-fed animals, showed a reduction in total malformations and fetal growth retardation due to phenytoin. Open eye defect was the only anomaly induced by phenytoin in the CLO/LO fetuses while phenytoin produced a variety of malformations in the SAFF fetuses such as tail defects, cleft palate, open eye and absence or blockage of the ureter.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic disorders of embryogenesis

Prevention of major physical malformations would represent a significant reduction in the burden of mortality and morbidity in infants and young children. However, preventive and therapeutic approaches must be based on a clear understanding of underlying pathogenic mechanisms. While it is estimated that single gene defects account for up to 10% of cases of major malformation, relatively few of these have been identified and analysed in detail. The recognition of characteristic patterns of developmental anomalies associated with specific enzyme defects has highlighted the important role of the metabolic environment in normal development and offers the possibility of correlating biochemical abnormalities with particular teratogenic effects. Once it is generally appreciated that some forms of structural malformation have a specific biochemical basis, metabolic studies should be performed more often in patients with major developmental anomalies. This should lead to identification of other examples of diseases of this type and the elucidation of molecular mechanisms of human teratogenesis.

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.

Applications of molecular physics ‘biotechnology’ to the rational design of an improved phenytoin analogue

This study exploits molecular physics, in conjunction with a large scale computing environment, as a tool for understanding the clinical phenomenology of phenytoin (PHT) toxicology at a molecular level and for employing this understanding in an attempt to design improved drugs. The application of molecular physics techniques, such as quantum mechanics and molecular force field calculations, to the process of rational anticonvulsant drug design remains virtually unexplored. A 3-step strategy for applying these techniques to the design of an improved PHT molecule is presented. Step 1 employs quantitative structure-activity relationship calculations on 80 PHT analogues to ascertain the portion of the PHT molecule necessary for bioactivity (i.e. the 'bioactive face' of PHT); the N3-C4(O)-C5-R fragment of PHT was identified as the bioactive face. Step 2 employs molecular modelling studies to determine the portion of the PHT molecule necessary for the teratogenic, mutagenic and connective tissue toxicities of PHT (i.e. the 'biotoxic face'); the C2(O)-N3 fragment of PHT was identified as the biotoxic face. Step 3 experiments design an 'improved' PHT analogue, which maintains the bioactive face while eliminating the integrity of the biotoxic face; 2-deoxy-5,5-diphenylhydantoin was designed and synthesized as the improved PHT analogue. This compound had biological activity equivalent to PHT, but was unable to bind to nucleic acids or to chelate metals involved in connective tissue metabolism.

Inhibition of phenytoin bioactivation and teratogenicity by dietary n-3 fatty acids in mice

Evidence suggests that the teratogenicity of the anticonvulsant drug phenytoin (DPH) can result from its bioactivation via embryonic prostaglandin synthase and/or maternal cytochromes P450. This study examined whether DPH bioactivation and teratogenicity could be reduced by dietary n-3 fatty acids. Female CD-1 mice were fed diets containing 2 wt% safflower oil and 10 wt% of either hydrogenated coconut oil, safflower oil, or a cod liver oil/linseed oil mixture (CLO/LO) for three weeks prior to impregnation and throughout gestation. DPH (55 or 65 mg/kg) was administered via intraperitoneal injections to pregnant mice at 0900 on gestational days 12 and 13, and on day 19 fetuses were given teratologic assessments. A similar dietary study evaluated in vivo covalent binding of radiolabeled DPH administered on day 12, and dams were killed 24 h later. A reduction in DPH-induced cleft palates and a decrease in DPH covalent binding to embryonic protein was observed in the CLO/LO group. Feeding CLO/LO enhanced incorporation of n-3 fatty acids into embryos and inhibited embryonic prostaglandin synthase activity. No differences in maternal hepatic cytochromes P450 activities were observed among dietary treatments. These data indicate that dietary n-3 fatty acids could reduce DPH teratogenicity via inhibition of embryonic prostaglandin synthase bioactivation of DPH.

Phenytoin potentiates interleukin-1-induced prostaglandin biosynthesis in human gingival fibroblasts

1. The effect of phenytoin (PHT) on prostaglandin E2 (PGE2) biosynthesis in human gingival fibroblasts stimulated by interleukin-1 (IL-1 alpha, IL-1 beta) or by tumour necrosis factor alpha (TNF alpha) was studied. 2. IL-1 alpha (1.5-6.0 ng ml-1) and IL-1 beta (30-300 pg ml-1), dose-dependently, stimulated PGE2 formation, in 24 h cultures, with IL-beta being the most potent agonist. 3. PHT (2.5-20 micrograms ml-1) did not induce PGE2 formation itself but potentiated IL-1 alpha- and IL-1 beta-induced PGE2 formation in the gingival fibroblasts in a manner dependent on the concentrations of both IL-1 and PHT. 4. IL-1 beta (0.1-1.0 ng ml-1) induced release of [3H]-arachidonic acid ([3H]-AA) from prelabelled fibroblasts that was potentiated by PHT (20 micrograms ml-1). 5. TNF-alpha (greater than or equal to 0.01 micrograms ml-1) significantly stimulated the biosynthesis of PGE2 by a process that was potentiated by PHT. 6. Addition of exogenous arachidonic acid (AA) (greater than or equal to 1 microM) caused an increase of PGE2 formation in the fibroblasts that was not potentiated by PHT (20 micrograms ml-1). 7. The results indicate that treatment with PHT results in upregulation of prostaglandin biosynthesis in gingival fibroblasts challenged with IL-1 or TNF alpha, at least partly due to enhanced level of phospholipase A2 activity.

Inhibition of isotretinoin teratogenicity by acetylsalicylic acid pretreatment in mice

Although isotretinoin (ITR) has been suggested to cause malformations via cytopathic effects on embryonic cells, the molecular mechanisms of ITR cytotoxicity in teratogenesis are not clear. Since ITR undergoes metabolism by prostaglandin synthase to a potentially cytotoxic peroxyl free radical, the possible role of prostaglandin synthase metabolism as a modulator of ITR teratogenicity was evaluated. Craniofacial and limb abnormalities were noted in fetuses on day 18.5 of gestation following administration of ITR to pregnant CD-1 mice in a three dose regimen of 100 mg/kg at 4 hr intervals on day 10.5 of gestation (plug day = day 0.5 of gestation). Mice were also treated with acetylsalicylic acid (ASA), an irreversible inhibitor of the cyclooxygenase component of prostaglandin synthase, at doses of 20 and 60 mg/kg body weight 2 hr prior to each ITR dose. ASA pretreatment of mice receiving ITR treatment showed a dose-dependent decrease in the overall incidence of malformations, number of defects per fetus, and the incidence of specific craniofacial and limb defects. Equivalent doses of ASA given to control mice did not cause malformations or alter the incidence of resorptions. These results demonstrate that ASA is able to ameliorate the teratogenic effects of ITR observed in fetal mice near term and indicate that prostaglandin metabolism could play a mechanistic role in ITR teratogenicity.

Risks of pregnancy in women with epilepsy

Pregnant women with epilepsy are at increased risk of seizures and complications. An increase in seizure frequency is seen in 25-30% of pregnant women with epilepsy; the offspring of mothers who experienced seizures during pregnancy are at a 2.5 times higher risk for seizures later in life. One of the main reasons for the increase in seizures during pregnancy is a decline in plasma concentrations of antiepileptic drug (AED) that occurs as pregnancy progresses, largely as a result of marked alterations in plasma protein binding. It is well known that epilepsy represents a risk for a variety of adverse pregnancy outcomes or malformations, especially in polytherapy. The adverse outcomes range from dysmorphic features to hemorrhagic disorders resulting from a deficiency of vitamin K-dependent clotting factors or to spina bifida. Folic acid supplements appear to reduce the risk of spina bifida. A strong genetic link seems to exist for many of the malformations that occur, and more research is required in this field. In the meantime, there are interventions that clinicians can already make to reduce the risk of adverse outcomes, such as seizure control without toxicity, monotherapy, and preconceptual use of vitamins with folate.

Genetic differences in susceptibility to anticonvulsant drug-induced developmental defects

The teratogenicity associated with gestational exposure to anticonvulsant medications remains a significant clinical therapeutic concern. Although most effective antiepileptic agents have been implicated as potential teratogens, the adverse developmental effects associated with phenytoin, trimethadione and valproic acid remain unequivocal. Although approximately 12,000 infants are exposed in utero annually in the United States to these compounds, only 10% will present with the clinical features associated with anticonvulsant drug embryopathies, clearly suggesting the presence of a genetic basis of susceptibility that certain infants inherit. In this article, the genetic basis for such variability in expression, with emphasis on the biochemical and morphological differences that might predominate, are examined in light of recent experimental evidence.

Bioactivation of xenobiotics by prostaglandin H synthase

Prostaglandin H synthase (PHS) catalyzes the oxidation of arachidonic acid to prostaglandin H2 in reactions which utilize two activities, a cyclooxygenase and a peroxidase. These enzymatic activities generate enzyme- and substrate-derived free radical intermediates which can oxidize xenobiotics to biologically reactive intermediates. As a consequence, in the presence of arachidonic acid or a peroxide source, PHS can bioactivate many chemical carcinogens to their ultimate mutagenic and carcinogenic forms. In general, PHS-dependent bioactivation is most important in extrahepatic tissues with low monooxygenase activity such as the urinary bladder, renal medulla, skin and lung. Mutagenicity assays are useful in the detection of compounds which are converted to genotoxic metabolites during PHS oxidation. In addition, the oxidation of xenobiotics by PHS often form metabolites or adducts to cellular macromolecules which are specific for peroxidase- or peroxyl radical-dependent reactions. These specific metabolites and/or adducts have served as biological markers of xenobiotic bioactivation by PHS in certain tissues. Evidence is presented which supports a role for PHS in the bioactivation of several polycyclic aromatic hydrocarbons and aromatic amines, two classes of carcinogens which induce extrahepatic neoplasia. It should be emphasized that the toxicities induced by PHS-dependent bioactivation of xenobiotics are not limited to carcinogenicity. Examples are given which demonstrate a role for PHS in pulmonary toxicity, teratogenicity, nephrotoxicity and myelotoxicity.

Parental epilepsy, anticonvulsant drugs, and reproductive outcome: epidemiologic and experimental findings spanning three decades; 1: Animal studies

In conclusion, it is clear that the experimental animal literature has been extremely beneficial in validating the teratogenicity of selected anticonvulsant drugs such as phenytoin and valproic acid, and in providing much needed information on pharmacokinetic parameters that are involved in altering normal embryogenesis. Continued efforts are needed to further elucidate the mechanism of teratogenic action for these drugs. It is clear from the work on phenytoin that reactive intermediates are important, and care must be taken to either avoid drug therapies that promote the formation of or inhibit the rapid degradation of toxic oxidative metabolites. For valproic acid and carbamazepine the pathogenesis of congenital defects remains much less defined. Until adequate information is ascertained on just how antiepileptic drugs disrupt normal development, it will be difficult, if not impossible, to develop either alternative medications or treatment strategies that maximize clinical effectiveness without the risk of an adverse pregnancy outcome. Such information emanating from animal studies shall, hopefully, be available in the not-too-distant future.

Phenytoin embryotoxicity: role of enzymatic bioactivation in a murine embryo culture model

A murine embryo culture model was developed to study the potential contribution of enzymatic bioactivation to the teratogenicity of phenytoin. To assess the relative embryonic and maternal contributions to bioactivation, embryos were cultured respectively alone or in the presence of an exogenous source of cytochromes P-450 (P-450), which are thought to bioactivate phenytoin to a teratogenic reactive intermediate. Embryological development from gestational day 9 to day 10 was assessed, and bioactivation was quantified by the irreversible binding of radiolabeled phenytoin to embryonic protein. Embryos cultured with phenytoin and an exogenous P-450 bioactivating system showed a significant decrease in the incidence of turning and closure of the anterior neuropore, yolk sac diameter, and protein content as well as growth retardation. In the absence of an exogenous P-450 system, phenytoin did not decrease the incidence of turning or anterior neuropore closure but did cause growth retardation and a lesser but significant reduction in yolk sac diameter and embryonic protein content. An exogenous P-450 system enhanced the bioactivation of phenytoin, although significant activity also was detectable in embryos cultured without an exogenous bioactivating system. These results suggest that the embryo itself can enzymatically bioactivate embryotoxically significant amounts of phenytoin, and that bioactivation and embryotoxicity can be further enhanced, qualitatively and quantitatively, by an exogenous P-450 system, implicating a possible maternal contribution to phenytoin teratogenicity.

Enhancement of murine phenytoin teratogenicity by the gamma-glutamylcysteine synthetase inhibitor L-buthionine-(S,R)-sulfoximine and by the glutathione depletor diethyl maleate

The teratogenicity of phenytoin may result from its enzymatic bioactivation to a reactive intermediate, which, if not detoxified, can interact with embryonic tissues and alter development. Glutathione (GSH) is an important cofactor/substrate for many physiological processes and for the detoxification of xenobiotic reactive intermediates. This study examined the effects of the GSH depletor diethyl maleate (DEM) and the GSH synthesis inhibitor L-buthionine-(S,R)-sulfoximine (BSO) on phenytoin embryopathy. Phenytoin, 55 mg/kg, was administered intraperitoneally (ip) to pregnant CD-1 mice at 0900 hr on gestational days 12 and 13. Pretreatment with DEM, 150 or 300 mg/kg ip, enhanced the incidence of phenytoin-induced cleft palates by 3.3-fold and 2.3-fold, respectively (P less than 0.05), without affecting the incidence of resorptions, postpartum death, or mean fetal weight. BSO, 1,800 mg/kg ip, given 0.5 hr prior to phenytoin, resulted in a 2.4-fold increase in postpartum lethality and a 5-fold increase in fetal weight loss (P less than 0.05), without altering the incidence of resorptions or cleft palates. In two subsequent studies, BSO, 680-1,018 mg/kg/day, was given in the drinking water on gestational days 9 to 13 in the first study and on days 10 to 14 in the second study. Phenytoin, 55 mg/kg ip, was given on days 11 and 12 and on days 11 to 13 in the respective studies. In the first drinking water study, BSO enhanced the incidence of phenytoin-induced fetal resorptions 3.8-fold and cleft palates 3.3-fold (P less than 0.05) but did not affect postpartum death. In the second study, BSO enhanced the incidence of resorptions, cleft palates, and postpartum death by 2-fold, 2.6-fold, and 1.7-fold, respectively (P less than 0.05). In both of the latter two studies, phenytoin-induced fetal weight loss was altered by BSO treatment (P less than 0.05). BSO alone had no embryopathic effects. These results suggest that GSH may be involved in the detoxification of a reactive intermediate of phenytoin and/or in fetal cytoprotection.

Effects of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate on phenytoin-induced embryopathy in mice

The anticonvulsant drug phenytoin may be cooxidized during prostaglandin biosynthesis to a reactive free radical intermediate capable of exerting embryopathic effects. Since 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter, activates the enzymatic release of arachidonic acid, thereby initiating prostaglandin synthesis, the potential teratological synergism between these two compounds was investigated. Pregnant CD-1 mice were given a fixed dose of phenytoin, 65 mg/kg intraperitoneally (ip), on Gestational Days 12 and 13, followed 2 hr later by varying doses of TPA, 0.2 to 2000 micrograms/kg ip. The dams were killed on Day 19 and fetuses were examined for anomalies. TPA post-treatment with doses of 2 to 200 micrograms/kg produced a TPA dose-related increase in maternal lethality of up to 100%, compared with no lethality in dams given TPA or phenytoin alone. Subsequent studies therefore were restricted to TPA doses between 0.2 and 20 micrograms/kg. TPA 20 micrograms/kg caused a threefold increase (p = 0.11) in the incidence of phenytoin-induced cleft palates, which likely was an underestimate due to the extremely high (97%) incidence of fetal resorptions. With lower TPA doses of 0.2 to 20 micrograms/kg, the incidence of fetal resorptions in animals treated with both phenytoin and TPA increased in a TPA dose-related manner, to over twofold (p less than 0.05), compared with phenytoin controls. Postpartum fetal lethality was enhanced similarly by the combination treatment, reaching a maximum of 100% (p less than 0.05). Lower doses of TPA also significantly enhanced the fetal weight loss in phenytoin-treated mice, while in contrast, TPA alone significantly increased fetal body weight compared with vehicle-treated controls. In animals treated with only TPA, the incidence of embryopathy generally was either comparable to controls or significantly less than that in phenytoin controls. These data indicate that TPA can potentiate phenytoin-induced embryopathy, possibly through a mechanism involving bioactivation by prostaglandin synthetase.

Inhibition of trimethadione and dimethadione teratogenicity by the cyclooxygenase inhibitor acetylsalicylic acid: a unifying hypothesis for the teratologic effects of hydantoin anticonvulsants and structurally related compounds

Teratogenicity of the anticonvulsant phenytoin may be due in part to its bioactivation by prostaglandin synthetase, forming a reactive free radical intermediate. We examined whether teratogenicity of the structurally similar oxazolidinedione anticonvulsants, trimethadione and its N-demethylated metabolite dimethadione, could be inhibited by the prostaglandin synthetase inhibitor acetylsalicylic acid (ASA). Trimethadione, 700 or 1000 mg/kg intraperitoneally (ip), was given to pregnant CD-1 mice during (Gestational Days 12 and 13) or before (Days 11 and 12) the critical period of susceptibility to phenytoin-induced fetal cleft palates. Dimethadione was given similarly on Days 11 and 12, or 12 and 13, in a dose (900 mg/kg ip) that was equimolar to 1000 mg/kg of trimethadione. ASA, 10 or 1 mg/kg ip, was given 2 hr before trimethadione or dimethadione on Days 11 and 12, and before trimethadione on Day 11 only. Dams were killed on Day 19 and fetuses were examined for anomalies. Either dose of trimethadione given on Days 12 and 13 was negligibly teratogenic, as evidenced by a non-dose-related, 1.1% mean incidence of fetal cleft palates. However, when given earlier on Days 11 and 12, trimethadione 1000 mg/kg caused an 8.9% incidence of cleft palates (p less than 0.05). Similarly, dimethadione caused a 3.9-fold higher incidence of cleft palates when given earlier on Days 11 and 12 (17.3-34.9%) than on Days 12 and 13 (4.4%) (p less than 0.05). At equimolar doses, dimethadione caused a 1.9- to 3.9-fold higher incidence of cleft palates compared to trimethadione (p less than 0.05), suggesting that dimethadione may be the proximate teratogen. Either dose of ASA given on both days before trimethadione totally prevented cleft palates, and ASA 10 mg/kg given only on Day 11 reduced the incidence of trimethadione-induced cleft palates to 1.1% (p less than 0.05). ASA reduced the incidence of cleft palates caused by dimethadione given on Days 11 and 12 from 34.9 to 20.3% (p less than 0.05). These results suggest that the teratogenic potential of trimethadione may depend at least in part upon its prior N-demethylation to dimethadione, which then can be bioactivated by prostaglandin synthetase to a teratogenic reactive intermediate, possibly involving a free radical located in the oxazolidinedione ring. This would provide a unifying hypothesis for the teratogenicity of hydantoins, as well as structurally related teratogens like trimethadione, which lack the molecular configuration necessary for the formation of a teratogenic arene oxide intermediate.