3,5,5-trimethylcyclohex-3-en-1-one

Carcinogenicity

Description of key information

There was some evidence of carcinogenicity of isophorone in male rats (kidney tumors, preputial gland carcinomas). The kidney tumors can be attributed to an α2u-globulin associated mechanism. The observed nephropathy in male rats is therefore irrelevant to other species. As the preputium is only investigated histopathologically when gross lesions are found, neither true tumor incidences from this study nor from historical controls are available. Therefore, the higher incidence of preputial gland tumors in high dose male rats cannot be put into perspective. There was equivocal evidence of carcinogenicity for male mice (liver tumors, mesenchymal tumors of the integumentary system). There was no evidence of carcinogenicity of isophorone in female rats and mice.

Justification for classification or non-classification

Based on the results of the carcinogenicity tests, isophorone should be classified with Carc cat 3, R40 according to Directive 67/548/EEC and Carc 2, H351 according to the EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008.

Additional information

In an NTP 2-year study (NTP, 1986) Fischer-344 rats (50 animals/dose/sex) were exposed by gavage to daily doses (5 days/week) of 0, 250, or 500 mg/kg isophorone in corn oil. The overall survival rate was low (males 33/50, 33/50, 14/50; females 30/50, 23/50, 20/50). Increased mortality was observed after week 98 in males in the 500 mg/kg bw dose group. Compound related clinical signs were not observed. Throughout the study, the mean body weights of high dose males averaged 5 % lower than vehicle controls. During the second year, the mean body weights of high dose females averaged 8 % lower than vehicle controls.

Dosed males showed proliferative lesions of the kidney (tubular cell hyperplasia: 0/50, 1/50, 4/50; tubular cell adenoma: 0/50, 0/50, 2/50; tubular cell adenocarcinoma: 0/50, 3/50, 1/50; epithelial hyperplasia of the renal pelvis: 0/50, 5/50, 5/50) and an increased mineralization of the medullary collecting ducts (1/50, 31/50, 20/50). Low dose males revealed more severe nephropathy than is commonly observed in aging F344 rats. Female rats only showed a statistically significant increase in nephropathy (21/50, 39/50, 32/50) with no further findings in the kidneys.

Carcinomas of the preputial gland were increased in high dose males only (0/50, 0/50, 5/50). The preputium is only investigated histopathologically when gross lesions are found, therefore neither true tumor incidences from this study nor from historical controls are available. As also two clitoral gland tumors were found in low dose females, a treatment related effect cannot be discounted.

 

Investigations on α2u-globulin induced nephropathy:

Various chemicals are known to induce nephropathy in male rats only. Only 60 % of α2u-globulin is reabsorbed in the kidney of male rats. 40 % of this low molecular weight protein remains in the filtrate and is excreted to the urine. The amount of α2u-globulin in female rats is 120 times lower and the protein is nearly absent in mice and in men. Chemicals that induce α2u-globulin nephropathy bind to the protein in the liver of the animals; these conjugates are difficult to hydrolyse and induce the formation of hyaline droplets which accumulate in the tubules (Charbonneau and Swenberg, 1988; Swenberg et al., 1989).

Isophorone as well as the proposed metabolites isophorol and dihydroisophorone form protein complexes with α2u-globulin in vivo, (SDS-page and immunoblotting after 14 d isophorone administration; GC/MS determination of isolated protein complexes) (Saito et al., 1992; Strasser et al., 1988). The degradation of α2u-globulin via lysosomal proteinases was decreased by 33 % (Lehman-McKeeman et al., 1990). In a further experiment with male NCI-Black-Reiter rats (strain is unable to synthesize the hepatic form of the low molecular weight protein α2u -globulin) isophorone administration, which clearly induced kidney lesions in male F344 rats, failed to induce nephropathy, α2u-globulin or the formation of hyaline droplets (Dietrich and Swenberg, 1991). It is thus concluded that the observed nephropathy in male rats is caused via the α2u-globulin mechanism and therefore irrelevant to other species.

In a parallel NTP 2-year gavage study with B6C3F1 mice (50 animals/dose/sex) animals were dosed daily (5 days/week) with 0, 250, or 500 mg/kg isophorone in corn oil. The survival of male mice was low (13/50, 13/50, 18/50) in contrast to female mice, where there was a trend towards increased survival of dosed animals relative to the controls (24/50, 33/50, 34/50). In high dose males, there was an increased incidence of hepatocellular adenomas and carcinomas (18/48, 18/50, 29/50). Increased incidences of coagulative liver necrosis (3/48, 10/50, 11/50) and hepatocytomegaly (23/48, 39/50, 37/50) were found in low and high dose males.

Mesenchymal tumors of the integumentary system (6/48, 8/50, 14/50) were increased in high dose males. An increased incidence of lymphomas or leukemias was noted in low dose males only (8/48, 18/50, 5/50). These results were interpreted as chemically related marginal increases in number of neoplasms (equivocal evidence).

Compound related non-neoplastic or neoplastic lesions associated with isophorone exposure were not seen in female mice.