NTP report on the toxicology and carcinogenesis study of benzene (CAS No. 71-43-2) in genetically modified haploinsufficient p16 Ink4a/p19 Arf mice (gavage study).

Unlabelled: Benzene is used primarily as a solvent in the chemical and pharmaceutical industries, as a starting material and intermediate in the synthesis of numerous chemicals, and in gasoline. The major United States source of benzene is petroleum. Benzene has been previously evaluated in 2-year carcinogenicity studies by the National Toxicology Program (1986). In this study, the carcinogenic effects of benzene were studied in the haploinsufficient p16 Ink4a/p19 Arf mouse model as part of an ongoing NTP effort to seek improved model systems for toxicology and carcinogenesis studies, especially those that can provide mechanistic information relative to understanding an agent's mode of action. Male and female haploinsufficient p16 Ink4a/p19 Arf mice were administered benzene (greater than 99% pure) by gavage for 27 weeks. Genetic toxicology studies were conducted in mouse peripheral blood erythrocytes. 27-WEEK STUDY in MICE: Groups of 15 male and 15 female haploinsufficient p16 Ink4a/p19 Arf mice were administered 0, 25, 50, 100, or 200 mg benzene/kg body weight in corn oil by gavage 5 days per week for 27 weeks. All animals survived until the end of the study except one male administered 200 mg/kg. Mean body weights of males administered 50 mg/kg or greater were generally less than those of the vehicle controls throughout the study, and those of 25 mg/kg males were less after week 13. Mean body weights of 200 mg/kg females were less than those of the vehicle controls after week 17. Treatment-related clinical findings in 25 mg/kg or greater males and 50 mg/kg or greater females included black, brown, or gray discoloration (pigmentation) of the feet. The thymus weights of all dosed groups of males were significantly decreased. At weeks 13 and 27, a dose-related decrease in the erythron occurred in males and females. The erythron decrease was shown by decreases in the hematocrit, hemoglobin, and erythrocyte count values in all dosed males and in the 100 mg/kg or greater females. Decreased leukocyte counts, primarily lymphocyte counts, resulted in a dose-related leukopenia in males and females. In males, segmented neutrophil counts were also decreased. The incidence of malignant lymphoma was significantly increased in 200 mg/kg males compared to the vehicle controls. In the bone marrow, significantly increased incidences of minimal to mild atrophy occurred in the 100 and 200 mg/kg males compared to the vehicle controls. In the spleen, there were significantly increased incidences of lymphoid follicle atrophy in 100 and 200 mg/kg male mice. The incidence of hematopoietic cell proliferation was significantly increased in 200 mg/kg males. The incidences of atrophy of the thymus in the 100 and 200 mg/kg males were significantly greater than those in the vehicle controls. In the lymph nodes, significantly increased incidences of atrophy (mandibular, mediastinal, and mesenteric) occurred in 100 and 200 mg/kg males, and the incidence of atrophy of the mediastinal lymph node was significantly increased in the 100 mg/kg females. The incidences of skin pigmentation were significantly increased in all dosed groups of males and in females dosed with 50 mg/kg or greater.

Genetic toxicology: The frequency of micronucleated erythrocytes was assessed at four timepoints during the 27-week study. Significant increases in micronucleated cells were observed at all timepoints, and the magnitude of the response correlated with duration of treatment.

Conclusions: Under the conditions of this 27-week gavage study, there was clear evidence of carcinogenic activity of benzene in male haploinsufficient p16 Ink4a/p19 Arf mice based on the occurrence of malignant lymphoma. There was no evidence of carcinogenic activity of benzene in haploinsufficient p16 Ink4a/p19 Arf female mice administered 25, 50, 100, or 200 mg/kg. Treatment of male and female haploinsufficient p16 Ink4a/p19 Arf mice with benzene was associated with toxicity to the hematopoietic system, lymphoid atrophy, and the accumulation of pigment in the extremities.