The doses selected for the 16-day studies ranged from 413 to 6,600 mg/kg for both rats and mice; deaths and reduction in body weight gain occurred at the two highest doses. No compound-related clinical signs or histopathologic lesions were observed in any of the surviving dose groups.
In the 13-week studies, doses of d-limonene ranged from 150 to 2,400 mg/kg for rats and from 125 to 2,000 mg/kg for mice. Deaths occurred in the high dose group of each species and sex. Greater than 10% reductions in body weight gain were observed in the two highest dose groups of male rats and male mice and the high dose female rats. Rough hair coats and decreased activity were observed at the two highest doses in both rats and mice. There were no chemical-related histopathologic lesions in female rats or in mice of either sex. A compound-related increased severity of nephropathy was observed in the kidney of male rats. This lesion was characterized by degeneration of epithelial cells in the convoluted tubules, granular casts in the outer stripe of the outer medulla, and epithelial regeneration. These lesions have been described as reasonably characteristic of the hyaline droplet nephropathy that is associated with an accumulation of liver-generated a2u-globulin in the cytoplasm of tubular epithelial cells.
Two-year studies of d-limonene were conducted by administering 0, 75, or 150 mg/kg d-limonene in corn oil by gavage to groups of 50 F344/N male rats, 5 days per week for 103 weeks; groups of 50 female F344/N rats were administered 0, 300, or 600 mg/kg. These doses were selected based on compound-related, potentially life-threatening kidney lesions observed in males at 300 mg/kg and higher and on the large number of deaths of female rats at 2,400 mg/kg. Groups of 50 male B6C3F1 mice were administered 0, 250, or 500 mg/kg according to the same schedule; groups of 50 female B6C3F1 mice were administered 0, 500, or 1,000 mg/kg. These doses were selected based on the deaths observed for both male and female mice at 2,000 mg/kg during the 13-week studies and the body weight depression in male mice at 1,000 mg/kg and higher.
Mean body weights of rats dosed with d-limonene were similar to those of vehicle controls throughout the studies. Survival of the high dose female rats after week 39 and of the vehicle control male rats after week 81 was significantly reduced (survival at week 104-- male: vehicle control, 29/50; low dose, 33/50; high dose, 40/50; female: 42/50; 40/50; 26/50). Mean body weights of dosed and vehicle control male mice were similar throughout the studies. Mean body weights of high dose female mice were notably lower than those of the vehicle controls after week 28. Survival of the low dose group of male mice was significantly lower than that of vehicle controls at the end of the study (33/50; 24/50; 39/50). No difference in survival was observed between vehicle control and dosed female mice (43/50; 44/50; 43/50).
In the 2-year studies, the kidney was confirmed as the primary target organ for chemically related lesions. No lesions were observed in female rats. For males, the nonneoplastic lesions included exacerbation of the age-related nephropathy, linear deposits of mineral in the renal medulla and papilla, and focal hyperplasia of the transitional epithelium overlying the renal papilla. Uncommon tubular cell adenomas and adenocarcinomas of the kidney also occurred in dosed male rats, and this effect was supported by a dose-related increased incidence of tubular cell hyperplasia, as shown in the table below.
Incidences of Male Rats with Renal Lesions in the Two-year Gavage Study of d-Limonene
| Site/Lesion | Vehicle Control | 75 mg/kg | 150 mg/kg |
|---|---|---|---|
| Mineralization | 7/50 | 43/50 | 48/50 |
| Epithelial hyperplasia | 0/50 | 35/50 | 43/50 |
| Site/Lesion | Vehicle Control | 75 mg/kg | 150 mg/kg |
|---|---|---|---|
| Tubular cell hyperplasia | 0/50 | 4/50 | 7/50 |
| Tubular cell adenoma | 0/50 | 4/50 | 8/50 |
| Tubular cell adenocarcinoma | 0/50 | 4/50 | 3/50 |
In subsequent 21-day studies, male and female F344/N rats were administered d-limonene at doses ranging from 75 to 1,200 mg/kg. Microscopic examination of the kidney sections from these rats indicated a compound-related increase in intracytoplasmic granules in the proximal convoluted tubules of dosed male rats but not of female rats. The granules were shown to contain a2u-globulin by an immunohistochemical strain. a2u-Globulin was shown to be increased in kidney homogenates from dosed male rats by an ELISA test.
In mice, no chemically related increases in neoplasms were observed. The incidence of neoplasms of the anterior pituitary gland in high dose female mice was lower than that in vehicle controls (adenomas or carcinomas, combined:vehicle control, 12/49; high dose, 2/48). Cells with an abnormal number of nuclei (8/49; 32/50) and cytomegaly (23/49; 38/50) were observed in the liver of high dose male mice.
Genetic toxicology
d-Limonene was not mutagenic in four strains of S. typhimurium(TA98, TA100, TA1535, or TA1537), did not significantly increase the number of trifluorothymidine (Tft)-resistant cells in the mouse L5178Y/TK+/- assay, and did not induce chromosomal aberrations or sister chromatid exchanges (SCEs) in cultured CHO cells. All assays were conducted in the presence and absence of exogenous metabolic activation.
Conclusions
Under the conditions of these 2-year gavage studies, there was clear evidence of carcinogenic activity of d-limonene for male F344/N rats, as shown by increased incidences of tubular cell hyperplasia, adenomas, and adenocarcinomas of the kidney. There was no evidence of carcinogenic activity of d-limonene for female F344/N rats that received 300 or 600 mg/kg. There was no evidence of carcinogenic activity of d-limonene for male B6C3F1 mice that received 250 or 500 mg/kg. There was no evidence of carcinogenic activity of d-limonene for female B6C3F1 mice that received 500 or 1,000 mg/kg.
An increased severity of spontaneous nephropathy, increased incidences of linear mineralization of the renal medulla and papilla, and hyperplasia of the transitional epithelium of the renal papilla were present in dosed male rats.