Effects of calcium channel blockers on cultured rat embryos

Tocolyse par les inhibiteurs calciques et hémorragies intraventriculaires

OBJECTIVE

To determine the possible association between intraventricular hemorrhage (IVH) in very premature infants and calcium-channel blockers used as tocolytics.

MATERIALS AND METHODS

We performed a case-control study (from October 1999 to December 2002) including 51 premature infants under 30 weeks with IVH (all grade) and 112 premature infants under 30 weeks without IVH. In this study only premature infants issued from spontaneous prematurity were included. The exposure frequency to calcium-channel blockers and to other tocolytics were compared between the two groups by univariate analysis and by logistic regression analysis.

RESULTS

Calcium-channel blockers were used in monotherapy before birth in 16% of infants without IVH and in 20% of infants with IVH (P=0.55). An exposure to a bitherapy or a tritherapy with a calcium-channel blocker and one or several other tocolytics has been found in 43% of infants with IVH and in 26% of infants without IVH (P<0.05). However this association disappears after adjustment for gestational age.

CONCLUSION

We did not find a significant association between calcium-channel blockers used as tocolytics and an increased risk of intraventricular hemorrhage in premature infants less than 30 weeks.

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.

A calcium agonist, Bay k 8644, suppresses the embryotoxic effects induced by dihydropyridines calcium channel blockers in cultured rat embryos

Day 9 rat embryos were exposed to 1,4-dihydropyridine calcium channel blockers; nifedipine (NIF), nicardipine (NIC) or nitrendipine (NIT), for 48 hr in the whole embryo culture system. There were dose-dependent growth retardation and abnormalities, predominantly in cardiovascular system. The three compounds exhibited very similar pattern of dysmorphogenic effects, but the potency of these compounds were quantitatively different. The incidences of embryos with the abnormalities were 100%, 100% and 85% following either exposure of NIF, NIC or NIT at concentration of 300, 8 and 15 microM, respectively. This study was to investigate whether these blocker-induced embryotoxicity was due to calcium channel blocking properties themselves in the embryos. Day 9 rat embryos were co-exposed to 1,4-dihydropyridine calcium channel agonist, Bay k 8644 (BAY) and each calcium channel blocker under the same culture condition. The retarded embryonic growth induced by 200 or 300 microM of NIF, 8 microM of NIC and 15 microM of NIT nearly of completely ameliorated when embryos were co-exposed with BAY at one-third or half concentration of each calcium channel blocker. Supplementation of BAY reduced the incidence of abnormalities by NIF-, NIC- and NIT-alone. These results suggested that one of mechanisms for embryotoxicity induced by calcium channel blocker was directly related to channel blocking property of the chemicals.

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.