The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings may not have been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. For more information, see the Explanation of Levels of Evidence for Developmental Toxicity. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
Alpha-Methyldopa, a widely used antihypertensive, was evaluated for toxic and teratogenic effects in timed-pregnant CD rats exposed to MD on gestational days 6 through 20 and sacrificed on gd 20. Prior to the initiation of the teratology study, a preliminary study was conducted in order to establish appropriate doses for use in the teratology study. In the preliminary dose range-finding study, MD, suspended in corn oil, was administered by gavage to timed-pregnant CD rats at doses of 0, 100, 250, 500, 750, and 1000 mg/kg/day (6 animals/group) on gd 6 through 20. Animals were sacrificed and evaluated on gd 20. The results of the preliminary dose range-finding study indicated that the 1000 mg/kg/day dose caused 33% mortality of confirmed-pregnant dams, and was therefore unsuitable for use in the teratology study. A probit plot analysis of the mortality data from the preliminary study suggested an LD10 of 581 mg/kg/day. Accordingly, doses of 0, 50, 100, 250, and 500 mg/kg/day MD were chosen for administration in the teratology study.
In the teratology study, MD suspended in corn oil (0, 50, 100, 250 and 500 mg/kg/day) was administered by gavage to timed-pregnant CD rats on gd 6 through 20. The study was conducted using a two-replicate design, with 10-20 animals assigned to each dose group in each replicate. In each replicate, females were weighed and observed during daily treatment for clinical signs of toxicity. At sacrifice on gd 20, the gravid uterus of each dam was weighed. Following uterine dissection the number and status of uterine implantation sites was recorded. Each live fetus was weighed, sexed, and examined for external, visceral, and skeletal malformations. Fetal heart, whole head, and liver weight were recorded. A total of 19-22 dams (i.e., confirmed-pregnant females) per treatment group were evaluated in the study.
During MD treatment, dams in the teratology study exhibited observable signs of toxicity including lethargy, chromodacryorrhea, blue discoloration of the nose, diarrhea, rough coat, vaginal bleeding, and urogenital staining and wetness. Clinical signs of toxicity were observed most frequently at doses of 100 mg/kg/day or more. The maternal mortality was 0%, 0%, 0%, 8.7%, and 34.5% for animals in the vehicle through high dose group, respectively. The unexpected high mortality in the 500 mg/kg/day dose group was observed predominantly in the first replicate; examination of the study records did not provide an explanation for this occurrence. MD at doses of 100 mg/kg/day or more significantly reduced maternal body weight, maternal weight gain, gravid uterine weight and absolute maternal liver weight. MD significantly increased embryotoxicity at the 500 mg/kg/day dose level, as evidenced by a significant increase in the percent resorptions per litter, the percent nonlive implants per litter (resorptions and dead fetuses) and the percent adversely affected implants per litter (nonlive implants plus malformed live fetuses). A trend toward an increase in the percent dead fetuses per litter, percent litters with dead fetuses, percent litters with nonlive implants, and percent litters with adversely affected implants was observed. However, this trend was not strictly dose-related and was determined primarily by an increase in these parameters at the high dose. For litters with live fetuses, the number of live fetuses per litter exhibited a trend toward a decrease that did not appear to be biologically significant. However, average fetal body weight per litter (male, female, or sexes combined) decreased as the dose of MD increased, with the 100, 250, and 500 mg/kg/day dose groups significantly below controls. The percent fetuses malformed per litter and percent male fetuses malformed per litter exhibited a trend toward an increase that was determined primarily by marginal increases occurring in the 250 and 500 mg/kg/day dose groups. This trend was not observed for the percent female fetuses malformed per litter. MD was not associated with any specific malformation or group of malformations. For gd 20 rat fetuses, relative female fetal heart weight per litter was significantly greater than controls in the 500 mg/kg/day dose group, and mean absolute fetal head weight per litter for both sexes combined and for male fetuses was significantly reduced in the 250 and 500 mg/kg/day dose groups. No clearcut effects of treatment on mean absolute fetal heart weight per litter for males, females or for the sexes combined, relative fetal heart weight per litter for males or for the sexes combined, mean absolute female fetal head weight per litter or relative fetal head weight (males, females, or sexes combined) per litter were observed.
In conclusion, in the teratology study, exposure of timed-pregnant CD rats to MO suspended in corn oil (0, 50, 100, 250, and 500 mg/kg/day) by gavage on gestational day 6-20, produced the following results:
- A no effect level for maternal toxicity at 50 mg/kg/day, but significant maternal toxicity at 100, 250, and 500 mg/kg/day, with maternal mortality at both the 250 and 500 mg/kg/day dose levels.
- A no effect level for embryo and fetal toxicity at 50 and 100 mg/kg/day, significant embryo toxicity at 500 mg/kg/day, and significant fetotoxicity at 250 and 500 mg/kg/day.
- No clearcut teratogenic effect at any dose level, but a trend toward an increase in the incidence of malformations as a result of a marginal increase in the percent malformed fetuses per litter, observed at the 250 and 500 mg/kg/day dose levels.
In addition to the teratology study, a preliminary evaluation of perinatal survival and postnatal growth to postnatal day 22 of rat pups exposed in utero to MD on gd 6-20 and fostered to untreated mothers was also conducted. For this preliminary perinatal and postnatal evaluation, MD was administered in corn oil by gavage at doses of 0, 100, 250, 500 or 750 mg/kg/day to timed-mated females (6-7 per group) on gd 6 through 20. During treatment, dams exposed to MD exhibited observable signs of toxicity, predominantly at doses of 250 mg/kg/day and greater; no treatment-related deaths occurred. Exposure to MD significantly reduced maternal body weight and absolute liver weight at doses of 250 mg/kg/day and above, and reduced maternal weight gain at doses of 500 mg/kg/day and above, but had no effect on relative maternal liver weight. Interestingly, MD treatment significantly increased the length of the gestational period in the 500 and 750 mg/kg/day dose groups. On postnatal day 1, pups from vehicle or MD-treated litters were examined for external malformations, weighed, sexed, and observed for clinical signs of toxicity and immediately reassigned by litter to untreated foster dams. MD had no significant effect on live litter size on pnd 1, but by pnd 4 perinatal survival exhibited a decreasing trend. In addition, when prenatal and perinatal mortality were considered together, the percent cumulative mortality for both pnd 1 and 4 exhibited an increasing trend. MD caused no evidence of malformation in pnd 1 pups. However, average pup body weight per litter exhibited a trend toward a decrease on pnd 1 that was evident but no longer significant after pnd 4. MD treatment had no significant effect on absolute liver, brain or heart weight or on relative liver, brain, or heart weight of pnd 22 pups. No overt differences in demeanor were observed in MD-treated pups.