Malonaldehyde occurs as a natural metabolic byproduct of prostaglandin biosynthesis and as an end product of polyunsaturated lipid peroxidation. Toxicology and carcinogenesis studies of malonaldehyde were conducted by administering the chemical as malonaldehyde, sodium salt, a stabilized form of malonaldehyde, in distilled water by gavage to groups of F344/N rats and B6C3F1 mice of each sex for 14 days, 13 weeks, and two years. The study material was 63%-79% malonaldehyde, sodium salt, 22%-38% water, and 1% or less other impurities. The water content was taken into account when the dose mixtures were prepared.
Fourteen-day and thirteen-week studies
In the 14-day studies, groups of five rats and five mice of each sex were dosed with 250, 500, 750, 1,000, or 1,500 mg/kg malonaldehyde, sodium salt. Controls were untreated. Rats and mice that received 1,500 mg/kg malonaldehyde, sodium salt, did not survive to the end of the 14-day studies. No compound-related gross lesions were seen in the dosed animals.
In the 13-week studies, groups of 10 males and 10 females of each species were administered 0, 30, 60, 125, 250, or 500 mg/kg malonaldehyde, sodium salt. Nine of 10 male rats, 10/10 female rats, 3/10 male mice, and 1/10 female mice that received 500 mg/kg malonaldehyde, sodium salt, died before the end of the studies. Body weights were reduced by more than 15% in rats receiving 250 or 500 mg/kg and in mice receiving 500 mg/kg.
Compound-related nonneoplastic lesions were present in the stomach, testis, and kidney of rats and in the pancreas, stomach, and testis of mice. Focal and multifocal erosive lesions were observed in the gastric mucosa of the glandular stomach in the 500 mg/kg groups of male and female rats. Dilatation of the gastric glands of the stomach mucosa occurred in the 500 mg/kg male mice. Lesions of the kidney included membranous glomerular nephropathy in the 250 and 500 mg/kg male rats and the 125, 250, and 500 mg/kg female rats and mineralization in the 250 and 500 mg/kg male rats and the 60, 125, 250, and 500 mg/kg female rats. Degeneration of the testicular germinal epithelium was observed in male rats and male mice receiving 250 and 500 mg/kg. Atrophy of the exocrine pancreas was seen in the 125, 250, and 500 mg/kg male and the 250 and 500 mg/kg female mice.
Based on these results, 2-year studies of malonaldehyde, sodium salt, were conducted by exposing groups of 50F344/N rats of each sex at doses of 0, 50, or 100 mg/kg, administered 5 days per week for 103 weeks. Doses of 0, 60, or 120 mg/kg were administered in the same schedule to groups of 50 male and 50 female B6C3F1 mice.
Two-year studies
Body weight and survival
Final mean body weights at the end of the study were reduced by 26% and 36% for high dose male and female rats compared with those for the vehicle controls. The final mean body weight of high dose male mice was 92% that of the vehicle controls. The final mean body weights of low dose male mice, low dose rats, and all groups of female mice were comparable to those of the vehicle controls.
The survival of high dose male and female rats was significantly lower than that of the vehicle controls, with survival declining rapidly after week 76 for high dose males and after week 59 for high dose females (survival-- male: vehicle control, 37/50; low dose, 33/50; high dose, 15/50; female: 37/50; 37/50; 14/50). Survival of all groups of male mice was low (male: 24/50; 20/50; 14/50; female: 41/50; 38/50; 30/50). Survival of the high dose groups of male mice was significantly lower than that of the vehicle controls; no other significant differences in survival were observed between any groups of mice.
Nonneoplastic and neoplastic effects
The incidences of a variety of nonneoplastic lesions were increased in dosed rats of each sex, primarily in the high dose male and female rat groups. These lesions were ulceration and inflammation of the glandular stomach; epithelial hyperplasia of the forestomach; inflammation of the cornea, retinal atrophy, and cataracts of the crystalline lens; focal lipoid degeneration of the adrenal cortex; and diffuse pancreatic atrophy. Cytoplasmic vacuolization and cystic degeneration in the liver occurred at increased incidences in the high dose rat groups; in addition, the incidences of bile duct hyperplasia and bile duct fibrosis were increased in the high dose female and male rat groups, respectively. Bone marrow hematopoietic hyperplasia, hematopoiesis of the spleen, and ultimobranchial cysts of thyroid gland occurred with increased incidences in high dose female rats.
The incidences of thyroid gland follicular cell adenomas or carcinomas (combined) were significantly increased in high dose male (vehicle control, 4/50; low dose, 8/49; high dose, 13/50) and female (2/50; 1/50; 7/50) rats. Follicular cell hyperplasia of the thyroid gland also occurred at an increased incidence in high dose female rats (10/50; 10/50;26/50) but not in male rats (9/50; 7/49; 7/50). The incidence of pancreatic islet cell adenomas was increased in low dose male rats (0/49; 9/50; 1/49). Adenomas and adenomas or carcinomas (combined) of the anterior pituitary gland occurred at significantly lower incidences in high dose rats than those in vehicle controls (combined incidence--male: 20/47; 14/49; 8/49; female: 18/49; 10/49; 2/48).
Nonneoplastic lesions that occurred at increased incidences in dosed mice included atrophy of the pancreatic acinus and dilatation of the uterus. Depigmentation of hair shafts and change of coat color from agouti to gray were observed in high dose mice. No compound-related neoplasms were observed in dosed mice.
Genetic toxicology
Malonaldehyde, sodium salt, was not mutagenic in the Salmonella typhimurium/microsome assay when tested at doses of up to 10,000 ug/plate in a preincubational protocol using the excision-repair deficient strains TA98, TA100, TA1535, and TA1537 with or without S9 metabolic activation. The chemical induced forward mutations in mouse L5178Y lymphoma cells in the absence of S9; it was not tested with S9. Malonaldehyde, sodium salt was not mutagenic in the Drosophila melanogaster sex-linked recessive lethal mutagenicity test in which adult male flies were exposed either by feeding or by abdominal injection. In cytogenetic assays with cultured Chinese hamster ovary (CHO) cells, malonaldehyde, sodium salt, produced a dose-related increase in the frequency of sister-chromatid exchanges both in the presence and absence of rat liver S9; no increase in the number of chromosomal aberrations was observed in CHO cells in the absence or presence of S9.
Audit
The data, documents, and pathology materials from the 2-year studies of malonaldehyde, sodium salt, have been audited. The audit found no special circumstances or significant deficiencies in the conduct or documentation of the studies which needed to be taken into consideration for reporting purposes.
Conclusions
Under the conditions of these 2-year gavage studies, there was clear evidence of carcinogenic activity for male and female F344/N rats administered malonaldehyde, sodium salt, as shown by the increased incidences of follicular cell adenomas or carcinomas (combined) of the thyroid gland. Pancreatic islet cell adenomas were also observed at an increased incidence in low dose male rats. There was no evidence of carcinogenic activity for B6C3F1 mice administered 60 or 120 mg/kg malonaldehyde, sodium salt, in distilled water by gavage 5 days per week for 2 years.
Chemically related increased incidences of nonneoplastic lesions included ulcers and inflammation of the glandular stomach and epithelial hyperplasia of the forestomach; corneal inflammation, retinal atrophy, and cataracts of the crystalline lens; and cystic degeneration of the liver, bile duct fibrosis, and bile duct hyperplasia in rats. Most of these nonneoplastic lesions as well as the thyroid gland follicular cell neoplasms occurred primarily in the high dose rat groups, in which survival and final body weights were reduced in high dose male and female rats. Increased incidences of atrophy of the pancreatic acinus and pigmentation loss in hair shafts were seen in high dose mice.