Triamterene is a potassium-sparing diuretic used in the treatment of edema associated with congestive heart failure, cirrhosis of the liver, and other diseases in which edema may occur. Toxicity and carcinogenicity studies were conducted by administering triamterene (greater than 99% pure) in feed to groups of male and female F344/N rats and B6C3F1 mice for 15 days, 13 weeks, or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium and Chinese hamster ovary cells.
Fifteen-day studies
Groups of five male and five female rats were fed diets containing 0, 1,000, 3,000, 10,000, 30,000, or 60,000 ppm triamterene. The diets containing 10,000 ppm or more were unpalatable, and feed consumption by the 3,000 ppm groups was reduced. Rats exposed to 1,000 or 3,000 ppm triamterene received approximate doses of 80 or 60 mg/kg body weight per day (males) or 70 or 50 mg/kg per day (females). One male rat and two female rats receiving 3,000 ppm died during the second week of the study. The final mean body weights of 3,000 ppm male and female rats were significantly lower than those of controls. Rats in the 3,000 ppm groups had renal tubule regeneration and cytoplasmic vacuolization of the zona glomerulosa of the adrenal gland.
Groups of five male and five female mice were fed diets containing 0, 300, 1,000, 3,000, 10,000, or 30,000 ppm triamterene, but the diets containing 10,000 or 30,000 ppm were unpalatable. All mice receiving 3,000 ppm died by day 6. Mice exposed to 300 or 1,000 ppm triamterene received approximate doses of 40 or 155 mg/kg body weight per day (males) or 45 or 170 mg/kg body weight per day (females). The final mean body weights of mice in the 300 and 1,000 ppm groups were similar to those of the controls. Renal tubule degeneration and necrosis were observed in the kidney of 3,000 ppm mice.
Thirteen-week studies
Groups of 10 male and 10 female rats were fed diets containing 0, 150, 300, 600, 1,200, or 2,400 ppm triamterene. All rats receiving 2,400 ppm died before the end of the study; all other rats survived to the end of the study. Rats exposed to 150, 300, 600, or 1,200 ppm triamterene received approximate doses of 10, 20, 40, or 70 mg/kg body weight per day (males) or 10, 20, 40, or 80 mg/kg per day (females). Body weight gains and final mean body weights of rats in the 1,200 ppm groups were significantly lower than those of controls. There were no biologically significant differences in hematologic, clinical chemistry, or urinalysis parameters among exposed and control rats. Calculi were observed in the renal pelvis of four male rats in the 1,200 ppm group. Chemical-related lesions were observed in the kidney and adrenal gland of rats in the 1,200 and 2,400 ppm groups. These consisted of degeneration and regeneration of the renal tubule epithelium and cytoplasmic vacuolization of cells of the zona glomerulosa of the adrenal cortex. Depletion of hematopoietic cells from the bone marrow and of lymphocytes from the spleen and thymus of rats in the 2,400 ppm groups may have been related to debilitation and reduced feed consumption rather than chemical exposure.
Groups of 10 male and 10 female mice were fed diets containing 0, 100, 200, 400, 800, or 1,600 ppm triamterene. All mice receiving 1,600 ppm, one 800 ppm female, one 200 ppm male, and four 100 ppm males died before the end of the study. Mice exposed to 100, 200, 400, or 800 ppm triamterene received approximate doses of 15, 25, 50, or 90 mg/kg body weight per day (males) or 15, 25, 50, or 115 mg/kg per day (females). The body weight gain and final mean body weight of male mice receiving 800 ppm were significantly lower than those of the controls. The total leukocyte and lymphocyte counts of males receiving 800 ppm and of females receiving 100, 400, or 800 ppm were significantly lower than those of controls. No other differences in hematologic, clinical chemistry, or urinalysis parameters were considered to be biologically significant. Necrosis of Lymphocytes was observed in the lymph node, spleen, and thymus of mice in the 800 and 1,600 ppm groups.
Two-year studies
The doses selected for the 2-year studies were based on lower body weights, mortality, and chemical-related lesions observed in exposed animals during the 13-week studies. Groups of 70 male and 70 female rats were fed diets containing 0, 150, 300, or 600 ppm triamterene and groups of 70 male and 70 female mice were fed diets containing 0, 100, 200, or 400 ppm. Ten animals from each group were included for interim evaluations at 3 and 15 months. Because of a dosing error involving the high-dose mice at week 40, a second study was conducted with groups of 60 male and 60 female mice fed diets containing 0 or 400 ppm triamterene.
In the 2-year studies, rats exposed to 150, 300, or 600 ppm triamterene received approximately 5,10, or 25 mg/kg body weight per day (males) and 5, 15, or 30 mg/kg (females) and mice exposed to 100, 200, or 400 ppm received approximately 10, 25, or 45 mg/kg (males) and 15, 30, or 60 mg/kg (females) per day.
Three-month and fifteen-month interim evaluations
There were no biologically significant differences in hematologic, clinical chemistry, or urinalysis parameters between exposed and control rats or mice at the 3- or 15-month interim evaluations. At necropsy, the mean body weights of exposed rats and mice were similar to those of the controls. There were no chemical-related lesions in exposed rats at 3 months or in exposed mice at 3 or 15 months. At the 15-month evaluation, basophilic, clear cell, and mixed cell foci of the liver occurred in exposed male rats. No chemical-related lesions were observed in female rats at 15 months.
Survival, body weights, clinical findings, and feed consumption
Survival of exposed rats was similar to that of controls (males: 0 ppm, 25/47; 150 ppm, 25/50; 300 ppm, 19/50; 600 ppm, 27/50; females: 29/50, 34/50, 34/50, 29/50). The mean body weights of 600 ppm rats were consistently lower than, but within 5% of, those of controls after week 49. Feed consumption by male and female rats was similar among exposed and control groups throughout the studies. There were no clinical findings of toxicity.
Survival of 400 ppm male mice in the first study was lower than that of controls because of the dosing accident at week 40. Survival of 100 and 200 ppm male mice and of all exposed groups of female mice in the first study and of exposed males and females in the second study was similar to controls (males: first study, 0 ppm, 47/50; 100 ppm, 45/50; 200 ppm, 46/50; 400 ppm, 46/60; second study, 0 ppm, 43/50; 400 ppm, 39/50; females: first study, 38/50; 43/50; 43/50; 43/60; second study, 40/50; 38/51). Mean body weights of exposed mice were similar to those of controls throughout the first study with one exception; in the week following the dosing error, the mean body weight of 400 ppm males was 16% lower than that of controls. In the second study, mean body weights of 400 ppm mice were slightly lower than those of controls during the final 8 weeks. Feed consumption by exposed mice was similar to that by controls throughout the studies. There were no clinical findings of toxicity in exposed mice.
Neoplasms and nonneoplastic lesions
The incidences of mixed cell foci and focal hyperplasia of the liver were significantly increased in 300 and 600 ppm male rats, and the incidences of clear cell and mixed cell foci were significantly increased in 300 and 600 ppm female rats. Hepatocellular adenomas occurred in all groups of exposed male rats, but none occurred in controls; the incidence of hepatocellular adenoma in the 150 ppm males was significantly higher than that of controls (O ppm, 0/50; 150 ppm, 6/50; 300 ppm, 4/50; 600 ppm, 3/49). Hepatocellular adenomas were observed in two 600 ppm female rats, but not in the lower exposure groups or in controls. No hepatocellular carcinomas were seen in exposed or control rats.
The incidences of nephropathy in exposed rats were similar to those of controls, but the average severity of the lesion was marginally increased in male rats receiving 300 ppm and in female rats receiving 600 ppm (males: 47/50, 2.4; 49/50, 2.7; 50/50, 3.0; 49/50, 2.8; females: 38/50, 1.1; 45/50, 1.2; 45/50, 1.3; 45/50, 1.4).
Although in the first study the incidences of hepatocellular adenoma in exposed male mice were similar to that of controls, the incidences of multiple adenomas were greater in the exposed groups, and the incidence of hepatocellular carcinoma in the 400 ppm group was marginally greater (hepatocellular adenoma: 0 ppm, 17/50; 100 ppm, 22/50; 200 ppm, 19/50; 400 ppm, 20/60; hepatocellular carcinoma: 5/50; 7/50; 3/50; 13/60). In the second study, the incidence of hepatocellular adenoma in the 400 ppm males was significantly higher than that of controls (hepatocellular adenoma: 0 ppm, 21/50; 400 ppm, 36/50; hepatocellular carcinoma: 9/50; 11/50).
The incidences of hepatocellular adenoma in exposed female mice in the first and second studies were significantly greater than those of controls (hepatocellular adenoma, first study: 10/50; 22/50; 23/50; 36/60; second study: 7/50; 28/51). The incidences of multiple adenoma were also increased in the exposed groups. Although the incidences of hepatocellular carcinoma were similar among exposed and control female mice in the first study, the incidence of hepatocellular carcinoma in the 400 ppm females in the second study was marginally greater than that of controls (hepatocellular carcinoma, first study: 4/50; 4/50; 3/50; 8/60; second study: 5/50; 11/50). In both studies, hepatocellular foci (basophilic, eosinophilic, clear cell, or mixed cell) also occurred more frequently in exposed female mice than in controls.
The incidences of thyroid gland follicular cell hyperplasia in the 200 and 400 ppm males and in all exposed groups of females were significantly greater than those of controls in the first study. These findings were confirmed in the second study (follicular cell hyperplasia: males, first study, 3/50, 8/50, 16/50, 20/60; second study, 0/50,16/50; females, first study, 4/49,17/49,18/50, 28/60; second study, 9/50, 32/51). The incidences of follicular cell neoplasms were similar among exposed and control mice in both studies.
The incidences (28/50, 36/50, 43/50, 49/60) and average severity (0.56, 0.80, 1.00, 1.07) of nephropathy were marginally higher in exposed female mice than in controls in the first study. In the second study, the differences in incidence (15/50, 21/50) and severity (0.38, 0.55) were not as great. It is uncertain if these increases were related to the ingestion of triamterene. The incidences and severity of nephropathy were similar among exposed and control male mice in both studies.
Genetic toxicology
Triamterene was not mutagenic in Salmonella typhimurium strains TA98, TA100, TA1535, or TA1537 with or without exogenous metabolic activation (S9). It did not induce chromosomal aberrations in Chinese hamster ovary cells, with or without S9. Positive results were obtained for induction of sister chromatid exchanges in Chinese hamster ovary cells with and without S9.
Conclusions
Under the conditions of these 2-year feed studies, there was equivocal evidence of carcinogenic activity of triamterene in male F344/N rats based on a marginal increase in the incidence of hepatocellular adenoma. There was no evidence of carcinogenic activity of triamterene in female F344/N rats administered 150, 300, or 600 ppm. There was some evidence of carcinogenic activity of triamterene in male B6C3F1 mice based on a marginal increase in the incidence of hepatocellular carcinoma in the first study and a significantly increased incidence of hepatocellular adenoma in the second study. There was some evidence of carcinogenic activity of triamterene in female B6C3F1 mice based on significantly increased incidences of hepatocellular adenoma and of adenoma and carcinoma (combined).
Exposure to triamterene was associated with an increased incidence of hepatocellular foci, primarily mixed cell type, and an increase in the severity of nephropathy in female rats. In mice, exposure to triamterene was associated with an increased incidence of hepatocellular foci in females and an increased incidence of thyroid gland follicular cell hyperplasia in males and females.