Common mechanism for teratogenicity of antiepileptic drugs Drug-induced embryonic arrhythmia and hypoxia-reoxygenation damage

Abstract: The Antiepilptic drugs (AEDs) phenytoin (PHT), carbamazepine (CBZ), phenobarbital (PB), tri- and dimethadione (TMD and DMD) are known teratogens having a common malformation pattern in human and animal studies. This thesis was designed chiefly to test a hypothesis correlating the teratogenicity of these AEDs to episodes of pharmacologically induced embryonic arrhythmia and hypoxia-reoxygenation damage.Effects on the embryonic heart were studied both after maternal administration in mice and inmouse embryos cultured in vitro. Only AEDs, correlated with the same type of malformation as could be induced by episodes of interrupted oxygen supply to the embryo (e.g. cleft palate) caused concentration dependent bradycardia and arrhythmia. PHT and DMD had the highest potential and affected embryonic heart at clinically relevant concentration, followed by CBZ, TMD and PB. Valproate and vigabatrin not associated with hypoxia-related malformations caused neither arrhythmia nor severe bradycardia.The results showed that the embryonic heart is extremely susceptible to PHT and DMD onlyduring a restricted period of development, between gestational days 9-13 (weeks 5-9 of human pregnancy).An observed genetic susceptibility to react with arrhythmia at low concentrations when exposed to PHT or to external stress, could explain why A/J strain of mice is more susceptible to develop cleft palate compared to other strains. High activities of reactive oxygen species (ROS) capturing antioxidant enzymes observed in untreated A/J embryos supported this assumption. The potential to cause embryonic arrythmia by an AED was related to the potential to inhibit the rapid component of the delayed rectifier potassium channel (I kr ).A marked I kr blocking activity (70%)of DMD in voltage clamping studies was observed. The I kr inhibition occurred at similar concentrations, which causes severe arrhythmia.The idea of a relation between teratogenicity and arrhythmia, resulting in ischemia followed by reperfusion and generation of ROS was supported by mechanistic studies. Pre-treatment with the spin-trapping agent PBN, which has the capacity to capture ROS, markedly reduced the incidence of PHT and DMD-induced cleft palate. In utero exposure to teratogenic doses of DMD and PHT resulted in hemorrhages in the embryonic palatal region. The same type of haemorrhage in the palatal region precedes orofacial clefts induced by episodic hypoxia.