NTP Toxicology and Carcinogenesis Studies of Dichloromethane (Methylene Chloride) (CAS No. 75-09-2) in F344/N Rats and B6C3F1 Mice (Inhalation Studies).
Dichloromethane is widely used in industrial processes, food preparation, and agriculture. In industry, dichloromethane is used as a solvent in paint removers, degreasing agents, aerosol propellants, and triacetate solutions; as a blowing agent in flexible urethane foams; and as a process solvent in the manufacture of steroids, antibiotics, vitamins, and tablet coatings. The use of dichloromethane as an extraction solvent for spice oleoresins, hops, and caffeine from coffee has been approved by the U.S. Food and Drug Administration. Dichloromethane has been used as an inhalation anesthetic and as a fumigant for grain and strawberries. Toxicology and carcinogenesis studies of dichloromethane (99% pure) were conducted by inhalation exposure of groups of 50 male and 50 female F344/N rats and B6C3F1 mice 6 hours per day, 5 days per week, for 102 weeks. The exposure concentrations used (0, 1,000, 2,000, or 4,000 ppm for rats and 0, 2,000, or 4,000 ppm for mice) were selected on the basis of results from 13-week inhalation studies in which groups of 10 rats and 10 mice of each sex were exposed to dichloromethane at concentrations of 525-8,400 ppm 6 hours per day, 5 days per week. During the 2-year studies in rats, body weight gains for exposed males and females were comparable to those of the chamber controls. The survival of exposed male rats was comparable to that of the chamber controls; however, the survival of all groups of males at the termination of the study was low (control, 16/50; low dose, 16/50; mid dose, 17/50; high dose, 9/50). Most of the early deaths among male rats occurred during the final weeks of the study; the survival of male rats through week 86 of the study was 36/50, 39/50, 37/50, and 33/50. This decreased survival is believed to be related to the high incidence of leukemia (34/50; 26/50; 32/50; 35/50). Survival of female rats exposed at 4,000 ppm was reduced relative to that of the chamber controls (30/50; 22/50; 22/50; 15/50); leukemia occurred frequently in all female rat groups. Final mean body weights of high dose male mice and low and high dose female mice were 10%-17% lower than those of the chamber controls; these reductions occurred during the last 16 weeks of the study. The survival of dosed male mice and high dose female mice was reduced relative to that of the chamber controls (male: control, 39/50; low dose, 24/50; high dose, 11/50; female: 25/50; 25/50; 8/50). This reduced survival may have been due to the chemically induced development of liver and lung neoplasia in male and female mice. Increased incidences of benign mammary gland lesions (adenomas and fibroadenomas) occurred in male and female rats exposed to dichloromethane (male: 0/50; 0/50; 2/50; 5/50; female: 5/50; 11/50; 13/50; 23/50). The incidence of malignant mammary gland neoplasms was not increased in female rats (2/50; 2/50; 2/50; 0/50); none was observed in male rats. In addition, integumentary system tumors in the area of the mammary chain occurred with a positive trend in male rats (subcutaneous tissue fibroma or sarcoma: 1/50; 1/50; 2/50; 5/50); the combined incidence of all tumors in the mammary area in male rats was 1/50, 1/50, 4/50, and 9/50. Exposure to dichloromethane was associated with increased incidences of hepatic hemosiderosis, cytomegaly, cytoplasmic vacuolization, necrosis, granulomatous inflammation, and bile duct fibrosis in both male and female rats. There was a positive but marginal trend in the incidence of hepatocellular neoplastic nodules or hepatocellular carcinomas (combined) in female rats (2/50; 1/50; 4/50; 5/50). The incidence of squamous metaplasia of the nasal cavity was increased in female rats exposed at 4,000 ppm (1/50; 2/50; 3/50; 9/50) but not in males (4/50; 5/50; 3/50; 3/50). No nasal cavity tumors were observed in rats. The increased incidences of mononuclear cell leukemia in mid dose and high dose female rats (17/50; 17/50; 23/50; 23/50) were statistically significant by age-adjusted analyses. In male rats, mesotheliomas (arising primarily from the tunica vaginalis) occurred at increas) occurred at increased incidences (0/50; 2/50; 5/50; 4/50). Lung tumors occurred at increased incidences in male and female mice exposed to dichloromethane (alveolar/bronchiolar adenomas: male - 3/50; 19/50; 24/50; female - 2/50; 23/48; 28/48; alveolar/bronchiolar carcinomas: male - 2/50; 10/50; 28/50; female - 1/50; 13/48; 29/48). Cytologic degeneration of the liver was observed at increased incidences in high dose male and dosed female mice (male: 0/50; 0/49; 22/49; female: 0/50; 23/48; 21/48). Incidences of hepatocellular adenomas or hepatocellular carcinomas (combined) were increased in high dose male and dosed female mice (male: 22/50; 24/49; 33/49; female: 3/50; 16/48; 40/48). There were also dose-related increases in the numbers of mice bearing multiple lung or liver neoplasms. Dose-related increases were observed in the incidences of testicular atrophy in male mice and uterine and ovarian atrophy in female mice; these effects are considered to be secondary responses to neoplasia. An audit of the experimental data was conducted for the 2-year studies of dichloromethane. No data discrepancies were found that influenced the final interpretations. Under the conditions of these inhalation studies, there was some evidence of carcinogenicity of dichloromethane for male F344/N rats as shown by an increased incidence of benign neoplasms of the mammary gland. There was clear evidence of carcinogenicity of dichloromethane for female F344/N rats as shown by increased incidences of benign neoplasms of the mammary gland. There was clear evidence of carcinogenicity of dichloromethane for male and female B6C3F1 mice, as shown by increased incidences of alveolar/bronchiolar neoplasms and of hepatocellular neoplasms. Synonyms: DCM; methylene chloride