Methylphenidate hydrochloride is a drug used in the treatment of narcolepsy and attention deficit hyperactivity disorders. This drug was nominated for study by the Food and Drug Administration and the National Cancer Institute because of its widespread use in human medicine and because of lack of data on its potential carcinogenicity. Oral administration is the most common route of human exposure. Toxicology and carcinogenicity studies were conducted by administering methylphenidate hydrochloride (USP grade) ad libitum in feed to groups of male and female F344/N rats and B6C3F1, mice for 14 days, 13 weeks, or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium and in cultured Chinese hamster ovary cells.
Fourteen-day study in rats
Groups of five male and five female F344/N rats were fed diets containing 0, 16, 62, 250, 1,000, or 4,000 ppm methylphenidate hydrochloride for 14 days. All rats survived to the end of the study. The final mean body weights of 4,000 ppm male and female rats were 9% lower than those of the controls. Absolute and relative liver weights of 4,000 ppm males and females were significantly greater than those of the controls. Clinical findings during the first week of the study included hyperactivity in 4,000 ppm males and females, but these animals appeared to be normal during the second week of treatment. No treatment-related gross lesions were observed; however, centrilobular hypertrophy was observed in 4,000 ppm males and females.
Fourteen-day study in mice
Groups of five male and five female B6C3F1, mice were fed diets containing 0, 16, 62, 250, 1,000, or 4,000 ppm methylphenidate hydrochloride for 14 days. Three 4,000 ppm males died during the second week of the study; all other mice survived to the end of the study. The final mean body weight of 4,000 ppm females was 11% lower than that of the controls, and the mean body weight gains of 1,000 and 4,000 ppm males and females were also significantly lower than those of the controls. Absolute and relative liver weights of all exposed groups of males and of 4,000 ppm females were significantly greater than those of the controls. Hyperactivity was observed during the second week of the study in some 4,000 ppm males. Degeneration and necrosis of the renal tubule epithelium were observed in two 4,000 ppm males. Hepatocellular hypertrophy was observed in males and females exposed to 1,000 or 4,000 ppm and in males exposed to 250 ppm.
Thirteen-week study in rats
Groups of 10 male and 10 female F344/N rats were fed diets containing 0, 125, 250, 500, 1,000, or 2,000 ppm methylphenidate hydrochloride for 13 weeks. There were no chemical-related effects on survival. Mean body weight gains of 500, 1,000, and 2,000 ppm males and females and of 250 ppm females were significantly lower than those of the controls. Final mean body weights of exposed males and females were similar to those of the controls. During the first week of the study, feed consumption by 2,000 ppm rats was less than that by controls, but during the remainder of the study feed consumption by exposed and control groups was similar. Rats exposed to 125, 250, 500, 1,000, or 2,000 ppm received approximate doses of 8, 15, 30, 70, or 130 mg methylphenidate hydrochloride per kilogram body weight per day (males) or 9, 18, 30, 70, or 150 mg/kg per day (females). Clinical findings in 1,000 and 2,000 ppm females included slight hypersensitivity to touch, hyperactivity, and increased vocalization during handling periods.
Absolute and relative liver weights of 2,000 ppm males and females were significantly greater than those of the controls, as were the relative liver weights of 1,000 ppm males and females. No chemical-related differences in bone length, bone density, or nose-to-rump lengths were noted in males or females, nor were there treatment related histopathologic lesions.
Thirteen-week study in mice
Groups of 10 male and 10 female B6C3F1, mice were fed diets containing 0, 125, 250, 500, 1,000, or 2,000 ppm methylphenidate hydrochloride for 13 weeks. There were no chemical-related effects on survival. Final mean body weights of males exposed to 250, 500, 1,000, or 2,000 ppm and of 2,000 ppm females were significantly lower than those of the controls. The final mean body weights of other exposed male and female groups were similar to those of the controls. During the first week of the study, feed consumption by 2,000 ppm mice was less than that by controls; feed consumption by exposed groups was similar to that by the controls throughout the remainder of the study. Mice exposed to 125, 250, 500, 1,000, or 2,000 ppm received approximate doses of 15, 30, 70, 115, or 230 mg/kg per day (males) or 15, 30, 70, 125, or 260 mg/kg per day (females). No chemical-related clinical findings were observed.
Absolute and relative liver weights of 1,000 and 2,000 ppm males and females were significantly greater than those of the controls, as were the relative liver weights of 125, 250, and 500 ppm males. Centrilobular hypertrophy and hepatocellular degeneration or necrosis were observed in males exposed to 500, 1,000, or 2,000 ppm methylphenidate hydrochloride.
Two-year study in rats
Based on the increased liver weights and lower body weight gains in 2,000 ppm rats in the 13-week study, the high dose selected for the 2-year rat study was 1,000 ppm. Groups of 70 male and 70 female F344/N rats were fed diets containing 0, 100, 500, or 1,000 ppm methylphenidate hydrochloride for up to 2 years. As many as 10 male and 10 female rats per exposure group were evaluated at 9 or 15 months.
Survival, body weights, feed and compound consumption, and clinical findings
Survival of exposed rats was similar to that of the controls at the end of the study. Mean body weights of 500 and 1,000 ppm males were 3% to 10% lower than those of the controls from week 30 to the end of the study; during the same time period, mean body weights of 500 and 1,000 ppm females were 4% to 24% less than those of the controls. Final mean body weights of rats exposed to 100, 500, or 1,000 ppm were 102%, 95%, or 90% (males) and 96%, 89%, or 78% (females) those of the controls. Rats exposed to 100, 500, or 1,000 ppm methylphenidate hydrochloride in feed received approximate doses of 5, 25, or 50 mg/kg per day (males and females). The only chemical-related clinical finding was an increased incidence of fighting among group-housed males exposed to 1,000 ppm.
Hematology and clinical chemistry
No biologically significant differences in hematology or clinical chemistry parameters occurred at 9 or 15 months.
Pathology findings
In female rats exposed to 500 or 1,000 ppm, the incidence of mammary gland fibroadenomas was decreased (0 ppm, 15/49; 100 ppm, 13/50; 500 ppm, 6/ 48; 1,000 ppm, 5/50), and the decrease was considered to be related to chemical administration. No significant chemical-related increases in neoplasm incidences were observed in male or female rats.
Two-year study in mice
Based on the liver toxicity and lower body weight gains observed in 1,000 and 2,000 ppm mice in the 13-week study, the high dose selected for the 2-year study was 500 ppm. Groups of 70 male and 70 female B6C3F1 mice were fed diets containing 0, 50, 250, or 500 ppm methylphenidate hydrochloride for 2 years. As many as 10 male and 10 female mice per exposure group were evaluated at 9 or 15 months.
Survival, body weights, feed and compound consumption, and clinical findings
Survival of exposed mice was similar to that of the controls at the end of the study. Mean body weights of mice exposed to 250 or 500 ppm were 3% to 11% lower than those of the controls throughout much of the study; during the same time period, mean body weights of 250 ppm females were 3% to 7% lower than those of the controls. Final mean body weights of mice exposed to 50, 250, or 500 ppm were 97%, 89%, or 93% (males) and 98%, 93%, or 97% (females) that of the controls. Mice exposed to 50, 250, or 500 ppm methylphenidate hydrochloride in feed were estimated to have received 6, 30, or 60 mg/kg body weight per day (males) or 8, 40, or 80 mg/kg per day (females). There were no chemical related clinical findings.
Hematology and clinical chemistry
No biologically significant differences in hematology or clinical chemistry parameters occurred at 9 or 15 months.
Pathology findings
The principal lesions associated with the administration of methylphenidate hydrochloride occurred in the liver. A few hepatocellular neoplasms were observed in control and exposed male mice at the 9-and 15-month interim evaluations, but the incidences in exposed groups were not significantly increased. At the end of the 2-year study, incidences of eosinophilic foci were increased in 500 ppm males and females. Increased incidences of hepatoblastoma occurred in 500 ppm males (0 ppm, 0/50; 50 ppm, 1/50; 250 ppm, 1/50; 500 ppm, 5/50). Increased incidences of hepatocellular adenoma also occurred in 500 ppm males (18/50, 18/50, 16/50, 29/50) and females (6/49, 10/48, 10/49, 28/50). The incidences of hepatocellular carcinoma were similar among control and exposed mice.
Genetic toxicology
Methylphenidate hydrochloride was not mutagenic in Salmonella typhimurium strains TA97, TA98, TA100, TA1535, or TA1537, with or without exogenous metabolic activation (S9). Methylphenidate hydrochloride was also tested for induction of sister chromatid exchanges and chromosomal aberrations in cultured Chinese hamster ovary cells. In the chromosomal aberrations tests, positive results were not consistently dependent upon the presence or absence of S9 activation. Sister chromatid exchanges were not increased in the presence of S9, but one laboratory did obtain a positive response without S9 by testing higher doses than were used in tests With S9.
Conclusions
Under the conditions of these 2-year feed studies, there was no evidence of carcinogenic activity of methylphenidate hydrochloride in male or female F344/ N rats receiving 100, 500, or 1,000 ppm. There was some evidence of carcinogenic activity of methylphenidate hydrochloride in male and female B6C3F1 mice based on the occurrence of hepatocellular neoplasms.
Treatment of female rats with methylphenidate hydrochloride was associated with a decrease in the incidence of mammary gland fibroadenomas. Administration of methylphenidate hydrochloride to male and female mice resulted in increased incidences of eosinophilic foci.