Primidone is used alone or with other anticonvulsants in the control of grand mal, psychomotor, and focal epileptic seizures. It may control grand mal seizures refractory to other anticonvulsant therapy. Primidone was nominated by the National Cancer Institute for 2-year toxicology and carcinogenicity studies due to its human use as an anticonvulsant. Male and female F344/N rats and B6C3F1 mice received primidone (greater than 99% pure) in feed for 14 days, 14 weeks, or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium, cultured Chinese hamster ovary cells, and mouse bone marrow cells.
Fourteen-day study in rats
Five male and five female rats were exposed to 0, 1,250, 2,500, 5,000, 10,000 or 20,000 ppm primidone (equivalent to average daily doses of approximately 120, 240, 500, 970, or 1,100 mg primidone/kg body weight to males and 120, 240, 500, or 900 mg/kg to females) in feed for 14 days. All 20,000 ppm females died before the end of the study as did one 10,000 ppm male and two 20,000 ppm males. The mean body weights of 10,000 ppm males and females and 20,000 ppm males were significantly less than those of the controls. Feed consumption by all exposed rats was generally similar to that by the controls. Males and females in the 10,000 and 20,000 ppm groups were observed to have eye discharge, ataxia, and abnormal posture and were thin and lethargic.
Fourteen-day study in mice
Five male and five female mice were exposed to 0, 625, 1,250, 2,500, 5,000 or 10,000 ppm primidone (equivalent to average daily doses of approximately 100, 200, 400, or 800 mg/kg body weight to males and 100, 250, 500, or 900 mg/kg to females) in feed for 14 days. All mice in the 10,000 ppm groups and one male and one female mouse in the 5,000 ppm groups died on day 3 of the study. The mean body weights of mice in the 625, 1,250, 2,500, and 5,000 ppm groups were similar to those of the controls. Feed consumption by all exposed mice was generally similar to that by the controls. Males and females in the 10,000 ppm groups were observed to have abnormal posture, ataxia, and lethargy.
Fourteen-week study in rats
Groups of 10 male and 10 female rats were exposed to 0, 300, 600, 1,300, 2,500, or 5,000 ppm primidone (equivalent to average daily doses of approximately 20, 40, 100, 200, or 400 mg/kg) in feed for 14 weeks. All rats survived to the end of the study. The mean body weights of male and female rats in the 2,500 and 5,000 ppm groups were significantly less than those of the controls. Feed consumption by all exposed rats was generally similar to that by the controls.
A minimal to mild exposure-related thrombocytosis occurred on day 22 and at week 14 in all exposed groups of male rats and in females in the 1,300 ppm or greater groups. A minimal decrease in hemoglobin concentration occurred in 2,500 and 5,000 ppm male and female rats on day 22 and at week 14.
The incidences of centrilobular hepatocyte hypertrophy in male rats exposed to 600 ppm or greater and in female rats exposed to 1,300 ppm or greater were significantly greater than those in the controls. The severity of chronic nephropathy in male rats exposed to 1,300 ppm or greater increased with increasing exposure concentration.
Fourteen-week study in mice
Groups of 10 male and 10 female mice were exposed to 0, 300, 600, 1,300, 2,500, or 5,000 ppm primidone (equivalent to average daily doses of approximately 50, 100, 200, 400, or 1,000 mg/kg to males and 60, 120, 220, 440, or 1,100 mg/kg to females) in feed for 14 weeks. Three male and two female mice in the 5,000 ppm group died during week 1 of the study. The final mean body weights of all exposed groups were similar to those of the controls. Feed consumption by male mice in the 5,000 ppm group was slightly greater than that by the controls; this may have been due to feed spillage. Male and female mice in the 5,000 ppm groups were ataxic and lethargic.
Compared to controls, the estrous cycle lengths of females exposed to 1,300, 2,500, or 5,000 ppm were significantly longer. The liver weights of male and female mice exposed to 600 ppm or greater were significantly greater than those of the controls. The incidences of centrilobular hepatocyte hypertrophy in all exposed males and in females exposed to 600 ppm or greater and the incidences of cytoplasmic alteration of the adrenal gland and hematopoietic cell proliferation of the spleen in 2,500 and 5,000 ppm males and in 5,000 ppm females were significantly greater than in the controls.
Two-year study in rats
Groups of 50 male and 50 female F344/N rats were exposed to 0, 600, 1,300, or 2,500 ppm primidone (equivalent to average daily doses of approximately 25, 50, or 100 mg/kg) in feed for 2 years.
Survival, body weights, and feed consumption
Survival of the 1,300 and 2,500 ppm males was significantly less than that of the controls. The mean body weights of males and females in the 2,500 ppm groups were less than those of the controls, beginning at week 29 for males and week 17 for females; the mean body weights of 1,300 ppm males and females were less than those of the controls during the second year of the study. Feed consumption by all exposed groups of rats was generally similar to that by the controls.
Pathology findings
Male rats exposed to primidone had increased incidences of thyroid gland follicular cell neoplasms (adenoma and/or carcinoma). All exposed groups of male rats had follicular cell adenomas or carcinomas (combined) at incidences above the historical control range, with the highest incidence in the 1,300 ppm group.
Hepatocyte cytoplasmic vacuolation and centrilobular hypertrophy were associated with primidone exposure in male and female rats. These changes were more severe in females than in males and the incidences in all exposed groups of females were significantly greater than those in the controls. Females in the 2,500 ppm group had an increased incidence of hepatocellular eosinophilic foci.
In 2,500 ppm males, the incidence of renal tubule hyperplasia was greater than that in the controls in the standard evaluation. Additional hyperplasias were found in the extended evaluation, and the incidences in exposed groups of males were significantly greater than that in the controls. In the extended evaluation, the incidence of renal tubule adenoma in 2,500 ppm males was significantly increased. The incidence of adenoma or carcinoma (combined) in 2,500 ppm males in the combined standard and extended evaluations were marginally increased over those in the controls. Male rats had an exposure-related increase in the severity of chronic nephropathy, which probably accounted for the reduced survival in the 1,300 and 2,500 ppm groups. The incidences of kidney cysts were increased in 1,300 and 2,500 ppm males. Hyperparathyroidism, secondary to the loss of renal function, was present in many exposed male rats. The incidences of parathyroid gland hyperplasia in all groups of exposed males were significantly greater than that in the controls.
Two-year study in mice
Groups of 50 male and 50 female mice were exposed to dietary levels of 0, 300, 600, or 1,300 ppm primidone (equivalent to average daily doses of approximately 30, 65, or 150 mg/kg to males and 25, 50, or 100 mg/kg to females) in feed for 2 years.
Survival, body weights, feed consumption, and clinical findings
Survival of the 1,300 ppm males was significantly less than that of the controls. During the second year of the study, the mean body weights of 1,300 ppm male and female mice were less than those of the controls. The final mean body weights of 600 ppm males and females were less than those of the controls. Feed consumption by all exposed groups of mice was similar to that by the controls. During the latter part of the study, a treatment-related increase in the number of animals with swelling of the abdominal area was observed; necropsy revealed that the swelling was due to liver nodules/masses.
Pathology findings
The liver was a target organ in both male and female mice. The incidences and multiplicities of hepatocellular neoplasms (hepatocellular adenoma, hepatocellular carcinoma, and hepatoblastoma) in all exposed groups of males and females (except hepatoblastoma in females) were significantly greater than those in the controls. The incidences of hepatocellular adenoma or carcinoma (combined) and hepatocellular adenoma, hepatocellular carcinoma, or hepatoblastoma (combined) in all exposed groups exceeded the historical control ranges in 2-year NTP studies. The incidences of centrilobular hepatocyte hypertrophy were increased in exposed groups of males and females, and the severities increased with increasing exposure concentration. The incidences of cytoplasmic vacuolization were increased in all exposed groups of females and in 300 ppm males. Incidences of eosinophilic focus in all exposed groups of females were significantly greater than those in the controls.
Proliferative changes occurred in the thyroid gland in an exposure-related manner in male and female mice. Incidences of follicular cell hyperplasia were increased in all exposed groups of males and in 600 and 1,300 ppm females, but incidences of follicular cell adenomas were increased only in male mice.
Genetic toxicology
Primidone was mutagenic in Salmonella typhimurium strain TA1535 in the absence of S9 activation only; no mutagenicity was detected in strain TA98, TA100, or TA1537, with or without S9. Primidone did not induce sister chromatid exchanges or chromosomal aberrations in cultured Chinese hamster ovary cells, with or without S9. The single in vivo study with primidone, a mouse bone marrow micronucleus test, also gave negative results.
Conclusions
Under the conditions of these 2-year feed studies, there was equivocal evidence of carcinogenic activity of primidone in male F344/N rats based on a marginal increase in thyroid gland follicular cell neoplasms, primarily adenomas, and a marginal increase in renal tubule neoplasms. There was no evidence of carcinogenic activity of primidone in female F344/N rats exposed to 600, 1,300, or 2,500 ppm. There was clear evidence of carcinogenic activity of primidone in male B6C3F1 mice based on the increased incidences of hepatocellular neoplasms, and the increased incidence of thyroid gland follicular cell adenomas was also considered to be chemical related. There was clear evidence of carcinogenic activity of primidone in female B6C3F1 mice based on the increased incidences of hepatocellular neoplasms.
Exposure of rats to primidone resulted in increased incidences of hepatocyte cytoplasmic vacuolization and centrilobular hypertrophy in males and females and eosinophilic foci in females. The increased severity of nephropathy and increased incidence of renal tubule hyperplasia in male rats were related to primidone exposure. Exposure of male mice to primidone resulted in hepatocyte centrilobular hypertrophy and thyroid gland follicular cell hyperplasia. Exposure of female mice to primidone resulted in hepatocyte centrilobular hypertrophy and cytoplasmic vacuolization, eosinophilic focus, and thyroid gland follicular cell hyperplasia.