Mirex (95% pure), formerly used a systemic insecticide and as a fire retardant, was studied for toxicologic and carcinogenic effects by administering diets containing 0, 0.1, 1.0, 10, 25, or 50 ppm mirex to groups of 52 F344/N rats of each sex for 104 weeks. Doses selected for the 2-year studies were based primarily on the effects on body weights and survival of rats in a 26-week study. During the first 6 months of the 2-year study, because of good survival and the absence of observable toxic effects in female rats, additional groups (termed second study) of 52 F344/N female rats were started at higher dietary concentrations of 0, 50, and 100 ppm mirex. Based on feed consumption data, the estimated average intake per day was 0, 0.007, 0.075, 0.75, 1.95, and 3.85 mg mirex/kg body weight for male rats and female rats in the first study, and 0, 3.9, and 7.7 mg/kg for female rats in the additional study.
Body weights, feed consumption, and survival
Mean body weights of male rats that received 25 or 50 ppm mirex were 5%-18% lower than those of the controls throughout most of the study; mean body weights of female rats that received 50 or 100 ppm mirex were 4%-18% lower than those of the controls after week 40; mean body weights of groups receiving 0.1, 1.0, or 10 ppm were similar to those of controls. Feed consumption by dosed male rats was 83%-91% that by controls, and that by dosed female rats was 86%-99% that by controls. The top dietary exposure groups of rats received the equivalent of 3.85 mg mirex/kg body weight, whereas the 100-ppm group of female rats (second study) averaged 7.7 mg/kg. At the end of the study, survival of male rats that received 25 or 50 ppm of mirex was lower than that of controls, whereas survival of all dosed groups of female rats was similar to that of controls (male: control, 44/52; 0.1 ppm, 37/52; 1 ppm, 36/52; 10 ppm, 37/52; 25 ppm, 19/52; 50 ppm, 15/52; female-- first study: 38/52; 38/52; 35/52; 41/52; 35/52; female-- second study: control, 44/52; 50 ppm, 44/52; 100 ppm, 39/52).
Nonneoplastic and neoplastic effects
The most notable compound-related effects were observed in the liver of male and female rats. Fatty metamorphosis, cytomegaly, angiectasis (males only), and necrosis of the liver were observed at increased incidences in dosed rats. The incidences of neoplastic nodules of the liver were dose related, and in the 10-, 25-, and 50-ppm groups of males and the 50- and 100-ppm groups of females (second study), they were markedly greater than those in controls (52/group-- male: control, 3; 0.1 ppm, 5; 1 ppm, 5;10 ppm, 14; 25 ppm, 15; 50 ppm, 26; female (second study): control, 2; 50 ppm, 23; 100 ppm, 30). In the first study in female rats, the incidences of neoplastic nodules were not significantly different between control and dosed groups (10; 5; 4; 5; 9; 7). The 10 neoplastic nodules of the liver seen in the control group (19%) was significantly greater than the mean incidence observed historically (57/2,015; 2.8%). The incidences of hepatocellular carcinomas in control and dosed groups were relatively low and were not significantly different between groups.
The incidences of pheochromocytomas of the adrenal gland occurred with a positive trend in male rats (8/51; 7/52; 13/52; 11/52; 18/51, 19/51); the incidences in the 25- and 50-ppm male rats were greater than that in controls; malignant pheochromocytomas were observed in 2 controls and in 2 mirex-exposed male rats. The incidence of pheochromocytomas in 50-ppm female rats in the first study was marginally greater than that in controls (control, 1/51; 50 ppm, 6/52); this borderline increase was not observed in the second female rat study and thus is not considered to be due to the dietary administration of mirex.
Nephropathy occurred at similar incidences in control and mirex-exposed groups of male and female rats; however, the severity of this nonneoplastic lesion was judged to be slightly greater in the groups given 25, 50, or 100 ppm mirex (male: severe vs. moderate in controls; female: moderate to severe vs. moderate). Hyperplasia of the transitional epithelium of the kidney pelvis was observed in dosed male rats (0/51; 2/51; 2/52; 5/52; 14/51; 9/52). Transitional cell papillomas of the renal pelvis in male rats occurred with a positive trend (P<0.02) (0/51; 0/51; 0/52; 1/51; 3/52). The highest incidence previously observed in untreated male F344/N rats in NTP studies is 1/48, and the mean historical incidence is 5/1,968 (0.3%).
In both the first and second studies in female rats, the incidence of mononuclear cell leukemia showed dosed-related increases (first study: 8/52; 8/52; 11/52; 14/52; 18/52; 18/52; second study: 6/52; 9/52; 14/52). When the data from both studies are combined, the incidences are significantly increased in the 10-, 25-, 50-, and 100-ppm groups. The mean historical incidence is 19% (375/2,021).
For the thyroid gland, there was a positive trend for follicular cell neoplasms in male rats (0/51; 1/50; 0/47; 1/47; 0/35; 4/49) and a negative trend for C-cell neoplasms in male rats (8/51; 6/50; 4/47; 7/47; 3/35; 0/49) and in female rats in the first study (12/50; 13/50; 7/48; 9/47; 6/48; 2/46). Neither observation is considered to be associated with the dietary administration of mirex.
Mirex was not mutagenic in the Salmonella typhimurium-microsome assay when tested in a preincubation protocol in the presence or absence of exogenous metabolic activation in strains TA98, TA100, TA1535, or TA1537. Mirex did not induce either sister chromatid exchanges or chromosomal aberrations in Chinese hamster ovary cells in the presence or absence of S9.
Under the conditions of these 2-year feed studies of mirex, there is clear evidence of carcinogenic activity for male and female F344/N rats, as primarily indicated by marked increased incidences of benign neoplastic nodules of the liver, as well as by increased incidences of pheochromocytomas of the adrenal gland and transitional cell papillomas of the kidney in males and by increased incidences of mononuclear cell leukemia in females.
Nonneoplastic effects induced by mirex include cytomegaly, fatty metamorphosis, angiectasis (males only), and cellular necrosis in the liver.