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.
A review of the study records for the "Teratologic Evaluation of Sulfamethazine (CAS No. 57-68-1) in CD Rats" (Wolkowski-Tyl, et al., 1982) was conducted in conjunction with an on-site audit for compliance with the FDA Good Laboratory Practice Regulations (FDA, 1978). Although no formal modification to the final report was requested by the FDA, RTI voluntarily conducted a statistical reanalysis of all endpoints reflecting malformation incidence since some minor errors during initial data processing were noted. The present addendum to the final study report presents (1) a revised study report abstract, (2) a comparison of previously presented and corrected statistical results, and (3) a detailed presentation of the corrected data summaries. Despite minor changes in statistical outcome for some parameters (i.e., number and percentage of male, female or all fetuses malformed per litter), no differences were noted which qualitatively affected the interpretation of the study results.
Revised Study Report Abstract
Sulfamethazine, a widely used antibacterial agent, was evaluated for teratogenicity following maternal exposure. Timed-pregnant CD rats were dosed by gavage on gestational days 6 through 15 with sulfamethazine (0, 545, 685 or 865 mg/kg/day) in distilled water. These dose groups are referred to as SM-0, SM-545, SM-685 and SM-865, respectively. Dams were weighed on gestational days 0, 6-15 (prior to daily dosing) and 20 (immediately following sacrifice), and were also observed for clinical signs of toxicity. At sacrifice on gestational day 20, dams were evaluated for body weight, liver weight, gravid uterine weight and status of uterine implantation sites (i.e., resorptions, dead fetuses, live fetuses). Live fetuses were dissected from the uterus and evaluated for body weight and gross morphological abnormalities. All live fetuses were examined for visceral malformations and identification of sex employing a fresh tissue dissection method. Half of the fetuses were decapitated prior to dissection and the heads were fixed in Bouin's solution for free hand sectioning and examination. All fetal carcasses were prepared with Alizarin Red S stain and examined for skeletal malformations.
The maternal mortality rate in the present study was 3.7% (1 out of 27 dams) for the SM-545 group; all other dams from all dose groups survived to terminal sacrifice on gestational day 20. No significant dose-response trends or differences among dose groups were observed for measures of maternal body weight during treatment (gestational day 11 and 15) or at sacrifice (gestational day 20), gravid uterine weight or maternal liver weight. Maternal weight gain (gestation period and treatment period) and absolute weight gain exhibited a dose-dependent decrease with values for weight gain (gestation period) and absolute weight gain significantly lower for SM-865 dams. Relative maternal liver weight increased with increasing dose. Weight loss of more than 5 grams/day, considered to be a clinical sign of toxicity, was seen in all sulfamethazine dose groups, as were alopecia, rough coat, lightcolored feces, and urine stains and/or wet urogenital area. There were no dose-related differences observed on the following measures: number of implantation sites per dam; number or percent of resorptions, fetal deaths or nonlive (i.e., dead fetuses plus resorptions) conceptuses; and proportion of litters with one or more resorptions, fetal deaths or nonlive conceptuses. There was a significant increased trend for number and percent affected (i.e. , resorbed, dead or malformed) fetuses per litter. The number and percent affected per litter, and numbers of litters with affected fetuses were significantly elevated in the SM-865 group. Among live litters, there were no effects of treatment on number of fetuses, males or females per litter. There were dose-related decreases in body weight per litter for male and female fetuses combined, and for males and females separately; values for these three parameters were significantly reduced below vehicle control in the SM-865 group. There was a significant dose-related increase in the number and percent of fetuses malformed per litter, with only the SM-865 dose group exhibiting a significant increase on these measures relative to controls. When distinguished by sex, the number, but not the percent of males malformed per litter showed a significant dose-related increase with treatment but no significant difference among all groups was observed. These parameters for females showed significant dose-related increases and the SM-865 dose group was significantly higher than controls thus indicating that females were somewhat more susceptible than males. The proportion of litters with gross (external) or visceral malformations in the SM-865 group were significantly elevated relative to controls with the predominant malformations being cleft palate (gross) and hydroureter (visceral) The proportion of litters with visceral malformations (predominantly hydroureter and hydronephrosis) in the SM-685 group was also elevated relative to controls. The proportion of litters with any type of malformation was significantly higher than controls only for the SM-865 group.
In conclusion, sulfamethazine produced an increased incidence of malformations in CD rats exposed by gavage during organogenesis. The combined incidence of all major malformations (primarily cleft palate and hydroureter) was significantly elevated in the SM-865 dose group but not in the SM-545 and SM-685 dose groups; however, the proportion of litters with visceral malformations (predominantly hydroureter and hydronephrosis) was also increased at SM-685 indicating a biologically relevant response at this dose. The increased incidence of malformations in the SM-865 group was accompanied by statistically significant evidence of maternal and fetotoxicity (i.e. , significantly reduced absolute maternal weight gain and average fetal body weight per litter) ; the increased incidence of litters with visceral malformations in the SM-685 dose group was accompanied by a noticeable decrease in maternal absolute weight gain which appeared to contribute to the significant linear trend for this measure, but which was not statistically below controls by pairwise comparison. Therefore, the conclusions drawn from the reanalysis of these data do not differ qualitatively from those in the original report.