Nitrofurazone, a widely used antimicrobial agent, was evaluated for toxic and teratogenic effects in timed-pregnant CD-1 mice. NF (0%, 0.0038%, 0.0075%, 0.0250% or 0.0500%) was administered continuously in the feed (i.e., ground rodent chow) on gestational days 6 through 15. Availability of NF in the diet resulted in the ingestion of average doses of 0, 6.28, 13.62, 41.18 and 81.70 mg/kg/day NF for the control through highest concentration, respectively. Females were weighed and observed during daily treatment for clinical signs of toxicity. At sacrifice on gd 17, a total of 20-26 dams (i.e., confirmed pregnant females) per group were evaluated. The gravid uterus for each dam was weighed. Following uterine dissection the number and status of uterine implantation sites was recorded (i.e., resorptions, and live or dead fetuses). Each live fetus was weighed, sexed and examined for external, visceral and skeletal malformations.
During NF treatment, dams exhibited limited clinical signs of toxicity and no unscheduled deaths occurred during this investigation. Notably, the results of the dose range finding study indicated that exposure to a higher dietary concentration (0.0750%) produced mortality or morbidity in 15% of the females on study, and was therefore considered too toxic to be included in the teratology study. In the teratology study, maternal body weight did not differ significantly among groups prior to initiation of NF exposure (gd O and 6). NF exposure produced no treatment-related effects on the following parameters: maternal body weight on gd 11, 15 or 17; maternal weight gain during gestation (gd 0-20); maternal weight gain during treatment (gd 6-15); absolute maternal weight gain (i.e., gestational weight gain minus gravid uterine weight), gravid uterine weight, maternal liver weight (g) or relative maternal liver weight (% body weight).
Indices of reproductive competence prior to the initiation of treatment on gd 6 revealed no statistically significant differences among groups for the number of corpora lutea per dam or for the number of implantation sites per dam. However, the percent preimplantation loss was significantly decreased below the control group for dams in the 0.0038%, 0.0250% and 0.0500% NF-exposed groups. Since the number of implantation sites per dam did not differ among groups, subjects were considered to be reasonably matched across treatment groups for reproductive status at the time NF exposure was initiated.
With regard to post-implantation viability, NF exposure was not associated with an increased incidence of resorptions. Neither the percent resorptions per litter nor the percent litters with resorptions showed statistically significant differences among groups. However, both the percent dead fetuses per litter and the percent litters with dead fetuses exhibited a significant increasing trend as dietary NF increased in concentration. No statistically significant treatment-related effects were observed for the percent nonlive implants (i.e., resorptions plus dead fetuses) per litter, the percent litters with nonlive implants, the percent adversely affected implants (i.e., nonlive implants plus malformed live fetuses) per litter, or the percent litters with adversely affected implants. In litters containing at least one live fetus (herein referred to as "live litters"), no statistically significant treatment-related effects were observed for the number of live fetuses per live litter, the percent male fetuses per live litter, the percent fetuses (male, female or both) malformed per live litter, or the percent live litters with malformed fetuses. Average fetal body weight (males, females or both) per live litter declined as dietary concentration of NF increased and the 0.0500% NF group was significantly below the vehicle control group for average body weight of male fetuses, female fetuses or both sexes combined.
In conclusion, exposure of timed-pregnant CD-l mice to NF (0%, 0.0038%, 0.0075%, 0.0250% or 0.0500%) in the diet during major organogenesis (gd 6-15) produced no evidence of a teratogenic effect in fetuses evaluated on gd 17. Selective embryotoxicity - expressed as an increased risk of late fetal death and intrauterine growth retardation - was observed at exposure levels which were only marginally toxic to the exposed dams.