Priority Existing Chemical

j.5Reproductive toxicity

MMT-treated females suffered from a slight increase in matting and staining of the anogenital haircoat at 9 mg/kg bw/day. Differences in the number of maternal deaths and pregnant dams between MMT-treated and control groups were not statistically significant. Mean maternal body weights were slightly lower than controls at all dose levels during gestation. Compared to controls, weight loss in animals receiving the highest dose of 9 mg/kg bw/day reached statistical significance (p<0.01) at day 9 after gestation. Mean maternal liver weights for MMT-treated pregnant female rats were not significantly different from controls.

No statistically significant differences were observed between MMT-treated and control groups in the ratio of male to female foetuses, mean number of early resorptions, mean number of viable foetuses, or mean foetal crown-rump lengths. A slight but significant decrease in mean foetal body weight (p<0.05) was observed only in the 6.5 mg/kg bw/day MMT group. A statistically significant increase (p<0.05) in the number of foetuses and litters with malformations was detected at a dosage of 9 mg/kg bw/day MMT. At this dose, 14 foetuses (7 litters) showed malformations out of 163 foetuses (21 litters) examined skeletally. This compares with 5 foetuses (1 litter) showing malformations out of 196 control foetuses (23 litters) examined skeletally.

The significant increase in malformations in the high dose MMT-treated group was due to the presence of bent ribs in this group and not in controls. With the exception of a single foetus showing microphthalmia, all other malformation endpoints were normal. Across doses, the prevalence of bent ribs showed a dose related increase in incidence although a dose relationship was not evident for other malformation endpoints. The authors argue that bent ribs are not regarded as a usual malformation and that the exclusion of other malformation endpoints suggests a lack of a teratogenic response.

Maternal weight losses at the highest MMT dose are evidence of minor maternal toxicity. Historical control data provided in this study show both microphthalmia and rib anomalies as spontaneous events in this species. Given this history of spontaneous rib anomalies, an association between maternal toxicity and delayed ossification and the uncertainty over whether delayed foetal growth in possible association with maternal toxicity constitutes embryofoetotoxicity (Guittin, Eléfant and Saint-Salvi, 2000), data are insufficient to consider the prevalence of bent ribs alone as indicating abnormal enbryogenesis.

The NOAEL for maternal and developmental toxicity was 9 mg/kg bw/day. Twenty-five mated female rats were used in each treatment group (Ethyl Corporation 1979a).

The developmental toxicity of MMT was also examined in Long-Evans rats. Males and female rats were mated nightly at a 1:1 ratio. Vaginal smears were taken each morning and examined for the presence of sperm or a vaginal plug. The day mating was observed was considered day 0 of gestation. Pregnant females (9-59 animals per dose level) were dosed daily by gastric intubation with MMT (0, 5, 10, 20 and 40 mg/kg bw) in corn oil on days 6-15 of gestation.

Maternal body weight gain was significantly reduced during treatment on gestation days 6-15 in all MMT treatment groups. Weight gain was similar for the MMT-treated groups and controls by day 21 of gestation. MMT-treated females suffered from epistaxis, irregular or rapid breathing, and urinary incontinence during the treatment period. In addition to these symptoms, females exposed to 20 or 40 mg/kg bw/day were cachectic, dehydrated, lethargic with some alopecia and pilo-erection. Maternal death rates were comparable to controls at doses of 5 and 10 mg/kg bw/day but were increased significantly at the two highest dose levels, with mortality observed in 41 of 59 rats receiving 20 mg/kg bw/day and mortality after initiation of dosing in rats receiving 40 mg/kg bw/day leading to termination of this dose group. All deaths at 40 mg/kg bw/day occurred within 1-5 days of the treatment (days 7-11 of gestation). Examination of the animals in the high dose group revealed that lungs were mottled (5/9), dark red (6/9) and firm (5/9), the trachea contained a foamy liquid (3/9), and livers were dark red (3/9).

At 20 mg/kg bw/day, a significant decrease in maternal pregnancy rate was observed accompanied by significant decreases in fetal viability. At this dose, fetuses showed significant increases in soft tissue malformations, skeletal malformations and ossification variations.

At 5 mg/kg bw/day, in addition to significantly decreased maternal body weight gains, a significant increase in fetuses with skeletal malformations (total 7.4%, p<0.05) and ossification variations (total 47%, p<0.05) was recorded. This incidence of skeletal malformations at this dose was attributable predominantly to increased observations of curly tail. However, these findings were not supported by external fetal examination at term sacrifice, were not seen at the higher dose of 10 mg/kg bw/day and so were considered of uncertain toxicological significance.

Skeletal malformations observed in MMT-treated fetuses especially at the highest dose of 40 mg/kg bw/day included derangement of ocular pigmentation, curly tail and vertebral defects (missing or fused ribs).

The incidence of soft tissue malformations in MMT-treated fetuses was comparable between controls and 5 and 10 mg/kg bw/day dose groups. Soft tissue malformations observed especially in the 40 mg/kg bw/day dose group included microphthalmia, anophthalmia and distended ureter.

The NOAEL for developmental toxicity was determined as 10 mg/kg bw/day with effects seen at the maternally toxic dose of 20 mg/kg bw/day (Ethyl Corporation 1978a).

In a separate study by Ethyl Corporation (1979b), pregnant female Sprague-Dawley rats (5 per dose group) were treated daily with a single oral dose of MMT (1-30 mg/kg bw/day) on days 6-15 of gestation. Several females treated with 10 or 30 mg/kg bw/day and most treated with 20 mg/kg bw/day showed ocular discharge and matting and staining of the anogenital region. One female receiving 10 mg/kg bw/day died on gestation day 11, four rats treated with 20 mg/kg bw/day and all treated with 30 mg/kg bw/day died between days 7-10 of gestation. Most of the rats that died during the study had congestion of the lungs and liver. A moderate reduction in body weight was observed in females treated with 10 mg/kg bw/day. The reduction in weight gain at 20 mg/kg bw/day was described as severe. Between 1-10 mg/kg bw/day a small increase in the mean number of early resorptions was observed. At 1 and 10 mg/kg bw/day a slight decrease in the mean number of implantations and live fetuses was noted. The one surviving female from the 20 mg/kg bw/day was gravid at day 20 of gestation. No individual animal data or statistics were provided. Insufficient data were provided to establish NOAELs and LOAELs.