Hyperphalangeal bones induced in rat pups by maternal treatment with nifedipine

Considerations for the development of guidance on dose level selection for developmental and reproductive toxicity studies

In 2022, the European Chemicals Agency issued advice on the selection of high dose levels for developmental and reproductive toxicity (DART) studies indicating that the highest dose tested should aim to induce clear evidence of reproductive toxicity without excessive toxicity and severe suffering in parental animals. In addition, a recent publication advocated that a 10% decrease in body weight gain should be replaced with a 10% decrease in bodyweight as a criterion for dose adequacy. Experts from the European Centre for Ecotoxicology and Toxicology of Chemicals evaluated these recent developments and their potential impact on study outcomes and interpretation and identified that the advice was not aligned with OECD test guidelines or with humane endpoints guidance. Furthermore, data analysis from DART studies indicated that a 10% decrease in maternal body weight during gestation equates to a 25% decrease in body weight gain, which differs from the consensus of experts at a 2010 ILSI/HESI workshop. Dose selection should be based on a biological approach that considers a range of other factors. Excessive dose levels that cause frank toxicity and overwhelm homeostasis should be avoided as they can give rise to effects that are not relevant to human health assessments.

Maternal toxicity

Although demonstration of some degree of maternal toxicity is required in regulatory developmental toxicology studies, marked maternal toxicity may be a confounding factor in data interpretation. Reduction in maternal body weight gain is the far most frequently used endpoint of toxicity, but alternative endpoints, like organ toxicity or exaggerated pharmacological response, can also be taken into consideration. The following conclusions are based on literature data and discussions at maternal toxicity workshops attended by representatives from regulatory agencies, academia, and industry: (1) Available results do not support that maternal toxicity (defined as clinical signs, decreased body weight gain or absolute body weight loss of up to 15% in rats or 7% in rabbits) can be used to explain the occurrence of major malformations. (2) There is clear evidence that substantial reductions in maternal weight gain (or absolute weight loss) are linked with other manifestations of developmental toxicity. Among these can be mentioned decreased fetal weight, and skeletal anomalies (e.g., wavy ribs) in rats and decreased fetal weights, post implantation loss, abortions, and some skeletal anomalies in rabbits. (3) There are several examples of misinterpretation among companies, where it was incorrectly expected that regulatory authorities would not label chemicals/drugs as "teratogens/developmental toxicants" because embryo fetal adverse effects were only observed at doses also causing signs of maternal toxicity. (4) Similarly, even if mechanistic studies indicate that a substance causes developmental toxicity via exaggerated pharmacological effects in the mother, such a mechanism does not automatically negate the observed fetal adverse effects.From a regulatory perspective, an observed developmental toxic finding is considered to be of potential human relevance (even if it is mediated via maternal pharmacological effects or occur at doses causing signs of maternal toxicity) unless the company can provide appropriate mechanistic and/or other convincing evidence to the contrary.

Risks of congenital malformations and perinatal events among infants exposed to calcium channel and beta-blockers during pregnancy

PURPOSE

Calcium channel blockers and beta-blockers (BBs) are widely used during pregnancy, but data on their safety for the developing infant are scarce. We used population-based data from 5 HMOs to study risks for perinatal complications and congenital defects among infants exposed in-utero.

METHODS

We studied women older than 15 years delivering an infant between 1/1/96 and 12/31/00, who had been continuously enrolled with prescription drug coverage for ≥ 1 year prior to delivery. Information on prescription drug dispensings, inpatient, and outpatient diagnoses and procedures was obtained from automated databases at each HMO.

RESULTS

There were 584 full-term infants exposed during pregnancy to BBs and 804 full-term infants exposed to calcium-channel blockers, and over 75,000 unexposed mother-infant pairs with ≥ 30 days follow-up. Infants exposed to BBs in the third trimester of pregnancy had over threefold increased risk for hypoglycemia (RR 3.1; 95% CI 2.2, 4.2) and an approximately twofold increased risk for feeding problems (RR 1.8; 95% CI 1.3, 2.5). Infants exposed to calcium-channel blockers in the third trimester had an increased risk for seizures (RR 3.6 95% CI 1.3, 10.4). Chart review confirmed the majority of the exposed seizure and hypoglycemia cases. There were no increased risks for congenital anomalies among either group of infants, except for the category of upper alimentary tract anomalies; this increased risk was based on only two exposed cases.

CONCLUSIONS

Infants whose mothers receive BBs are at increased risk for neonatal hypoglycemia, while those whose mothers take calcium-channel blockers are at increased risk for neonatal seizures.

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.

Calcium channel blockers in the management of preterm labor and hypertension in pregnancy

Calcium channel blockers (CCBs) have the ability to inhibit contractility in smooth muscle cells. CCBs have an already established role in the treatment of non-pregnant hypertension and angina pectoris. Some epidemiological studies found an association between the use of CCBs and an increase in cardiovascular mortality, malignancy, and gastrointestinal bleeding. More recent studies with many more patients and a longer follow-up did not find these associations. In obstetrics CCBs have become increasingly popular for the management of preterm labor and pregnancy-induced hypertensive disorders. Meta-analysis shows that use of nifedipine in comparison with betamimetics is associated with a more frequent successful prolongation of pregnancy in case of preterm labor, resulting in significantly fewer admissions of newborns to the neonatal intensive care unit (NICU), and is associated with a lower incidence of respiratory distress syndrome. No adverse fetal side effects in humans have been reported with the use of nifedipine for obstetric indications. Nifedipine is an effective and safe drug to use when tocolytic therapy is indicated for preterm labor. In preeclampsia nifedipine effectively lowers blood pressure and can be a good alternative for (di) hydralazine.

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.

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

The limb plates and craniofacial regions in rabbit fetuses were examined shortly after the last dose of phenytoin on day 16 after daily administration by gavage with either 150 mg/kg on days 14-16 or 300 mg/kg on days 15-16. Both treatment regimens resulted in similar changes. Histologically, the digital areas of the limb plates showed extensive edema and dilated blood vessels within 2 h. After 8 h, vascular disruption occurred with hemorrhages. At 24-48 h after dosing, mesenchymal necrosis and, on some occasions, amputation of digits was observed. In the craniofacial region, well-defined superficial hemorrhage was seen in the frontal and nasal region at 8 h. Histologically, subectodermal hemorrhage caused by vascular disruption and microfocal mesenchymal necrosis was observed. At 48 h, some fetuses showed severe diffuse intracranial and superficial hemorrhage, resulting in massive tissue damage, also in the central nervous system (CNS). Maternal heart rate, blood pressure, PO2, and PCO2 were also measured in awake pregnant rabbits 6 h after the last dose on day 16 after daily administration with 150 mg/kg during gestational days 14-16. An attempt was also made to measure fetal heart rate in anesthetized rabbits. The maternal heart rate and blood pressure decreased with about 15% in phenytoin-treated animals, resulting in a decrease in PO2 (approximately 15%) and an increase in PCO2 (approximately 15%). A decrease in fetal heart rate was also registered. The results thus indicate that phenytoin exerts its teratogenic effects by inducing fetal hypoxia, leading to vascular disrupture and necrosis of existing and developing structures.

Identical phalangeal defects induced by phenytoin and nifedipine suggest fetal hypoxia and vascular disruption behind phenytoin teratogenicity

In previous experimental studies in rabbits, we have shown that vasodilating drugs (including nifedipine) cause distal digital defects. These defects were preceded by edema, hemorrhage, and finally necrosis of the developed cartilage in the phalanges. The underlying mechanism is most likely a fetal hypoxic response, secondary to maternal hypotension and decreased uteroplacental blood flow. Since phenytoin is known to cause distal digital defects both in man and rabbits, we decided to compare the defects provoked by oral administration of phenytoin (100 mg/kg) versus nifedipine (8.3 mg/kg) to New Zealand White rabbits on days 6-18 of gestation. In order to investigate phase-specificity, phenytoin (150 mg/kg) was given on days 14-17. The result of single dose administration on day 16 of phenytoin (300 mg/kg) versus nifedipine (33.2 mg/kg) was also studied. In this latter experiment maternal heart rate was measured up to 21 hours after phenytoin administration. Phenytoin induced digital defects identical with those produced by nifedipine and caused marked maternal cardiodepression. The defects consisted of a reduction, absence, or abnormal structure of the distal phalanges. The distal phalanx of the fourth digit on the hindpaw was the first to be affected, with inclusion of other phalanges, both on the hind- and forepaws, with increasing dose. The sensitive period for induction and histological appearance of these defects was identical for phenytoin and nifedipine. These results suggest that vascular disruption due to a fetal hypoxic response lies behind phenytoin teratogenicity, as has been shown for vasodilators. A cardiodepressive action on the maternal and fetal hearts, possibly in combination with decreased uteroplacental blood flow, is discussed as a probable factor behind phenytoin teratogenicity.

Effects of calcium channel blockers on the development of early rat postimplantation embryos in culture

Rat embryos (9.5-day-old) were cultured for 48 h in the presence of nifedipine (NIF), nimodipine (NIM), nitrendipine (NIT), gallopamil HCl (GAL), verapamil HCl (VER) and diltiazem HCl (DIL). The effects on growth and morphogenetic differentiation in vitro were monitored. Dose-response relationships were evaluated, including an assessment of the "no-observed-effect-level" (NOEL) or the "lowest-observed-effect-level" (LOEL), and the lowest concentration tested inducing abnormalities in 100% of the embryos ("100% EL"). The morphological alterations observed at the highest concentrations were very similar for all six drugs. The abnormalities concerned yolk sac circulation and morphology, as well as heartbeat, the morphology of the heart, head, neural tube, or forelimbs, and the shape of the embryo. The abnormal embryos were also growth retarded (decrease in protein content and crown-rump length). Interference with calcium channel functions seems to represent an interesting model for studying a special kind of abnormal prenatal development, especially the differentiation of certain mesenchymal structures. The concentration ranges between NOELs and 100% ELs were found to be: NIM = 0.1-1 microgram/ml; NIT and VER = 1-10 micrograms/ml; DIL = 1-30 micrograms/ml, and LOELs-100% ELs were: GAL = 1-10 micrograms/ml; NIF = 10-30 micrograms/ml.

Histological and in vitro studies supporting decreased uteroplacental blood flow as explanation for digital defects after administration of vasodilators

In a recent study, the vasodilating drugs nifedipine, nitrendipine, felodipine, and hydralazine induced phalangeal defects in rabbits, when given on day 16 of pregnancy. Histologically, the changes were characterized by disturbed chondrogenesis. In order to elucidate mechanisms behind the defects, the fetal concentration of felodipine was measured, and the fetal limb plates were examined histologically, at 0, 2, 4, 8, 12, and 24 hours after single oral administration of felodipine (12 mumol/kg) on day 16 in pregnant rabbits. The effects of nifedipine, nitrendipine, and felodipine were also investigated in an in vitro system, in which chick embryonic mesenchymal limb bud cells differentiated into chondrocytes. In this system, no inhibition of chondrogenesis was observed below concentrations 3 x 10(5) M. At this concentration, unspecific cytotoxicity was found. The highest fetal concentrations of felodipine were more than 500 times lower than what was required for in vitro toxicity. Histologically, the digital areas of the limb plates showed extensive edema and dilatation of marginal sinus within 2 hours. After 8 hours, rupture of the thin-walled vessels occurred with hemorrhages. Finally, small necroses and blisters were observed. Similar early changes have been reported in experiments where digital defects were induced by clamping uterine vessels. This study thus indicates that the phalangeal defects after administration of high doses of vasodilators are secondary to pharmacological action (associated with a significant reduction in the uteroplacental blood flow), and not a direct effect on fetal chondrogenesis.