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.
Isoeugenol is a widely used perfumery and flavoring ingredient. This study was performed due to the potential for human exposure to ISOG and the lack of pertinent developmental toxicity data.
Dose selection was based on a screening study in which CD rats were treated by gavage with 0, 200, 400, 600, 800, or 1000 mg ISOG/kg body weight/day on gestational days 6 through 19 (NTP, 1998). Developmental toxicity was limited to a decreasing dose-response trend for average fetal body weight per litter. Minimal, reversible maternal toxicity was noted at 200 and 400 mg/kg/day, with mild, persistent effects at greater than or equal to 600 mg/kg/day (NTP, 1998).
In this study, female Sprague-Dawley-derived (CD) rats were dosed by gavage with ISOG (250, 500, or 1000 mg/kg/day) or its vehicle (corn oil) on gd 6 through 19. The dose volume was 5 ml/kg. Twenty-five timed-mated rats were assigned to each group. Dams were monitored at regular intervals throughout gestation for clinical signs, food and water intake, and body weight. At necropsy on gd 20, the following were recorded: maternal clinical condition; body, liver and gravid uterine weights; pregnancy status; and number of corpora lutea. In the gravid uterus, the numbers of resorbed, dead, or live fetuses were recorded. All live fetuses were weighed, sexed, and examined for external morphological anomalies. Approximately one-half of the fetuses were examined for visceral anomalies, including internal head structures, and the remaining fetuses were examined for skeletal anomalies.
Confirmed pregnancy rates were 92-100% per group. No treatment-related maternal deaths occurred in this study. Clinical signs associated with ISOG exposure included dose-related evidence of sedation at the high dose throughout treatment. The incidence of piloerection was increased (greater than or equal to 500 mg/kg/day), and aversion to treatment (rooting behavior) was noted in all ISOG groups.
Maternal body weight and body weight gain were decreased in a dose-related manner. Maternal body weight was decreased at the high and mid doses on gd 9, 12, 15, 18, 19, and 20. Decreases in body weight were observed at the low dose on gd 12, 18, 19, and 20. Decreased maternal weight gain was observed at the high dose (gd 6-9, 15-18, and 6-20), mid dose (gd 15-18, 6-20), and low dose (gd 6-20). Maternal body weight gain during gestation (absolute and corrected for gravid uterine weight) was reduced at all three dose levels. Gravid uterine weight was decreased at the mid and high doses. Maternal absolute liver weight showed a decreasing trend, but relative liver weight was significantly increased at all dose levels.
Maternal relative food intake (g/kg/day) was significantly decreased in the high dose group (gd 6-9, 9-12, 12-15, 6-20, and 0-20). In the low dose, relative food consumption was elevated on gd 12-15. Maternal relative water intake in the high dose group was reduced on gd 6-9, but was significantly elevated in the high dose group throughout the treatment and gestation periods. In the mid dose group, relative maternal water consumption was elevated during every time period from gd 9-20 with the exception of gd 18-19.
Average fetal body weight per litter exhibited a decreasing dose-response trend, influenced primarily by an 7-9% decrease in fetal body weight at 1000 mg/kg/day that was statistically significantly different from controls. However, ISOG did not affect any indices of prenatal viability. Live litter size in ISOG-treated groups was 94-98% of the control mean. Incidence of fetal morphological anomalies were statistically equivalent among groups, except for an increase in the incidence of unossified sternebra(e), a skeletal variation, at the high dose.
In summary, maternal toxicity and aversion to dosing were noted at greater than or equal to 250 mg ISOG/kg/day on gd 6 through 19. Thus, the maternal toxicity lowest-observed-adverse-effect level was 250 mg/kg/day and the maternal toxicity no-observed-adverse-effect level was not determined in this study. Reduced fetal body weight and an increased incidence of unossified sternebra(e) were found at 1000 mg ISOG/kg/day. Thus, the developmental toxicity LOAEL was 1000 mg/kg/day based on intrauterine growth retardation and mildly delayed skeletal ossification. The developmental toxicity no observed adverse effect level was 500 mg/kg/day.