Chronic toxicity of methotrexate in mice.

Daily (five times/week) administration of 0.25-2 mg methotrexate (MTX)/kg to 5- to 6-week-old male C57BL/6, DBA/2, and C3H mice for 12-18 months was well tolerated, apart from minimal cellular suppression in the lymphoid tissues, testes, and skin. Larger doses of MTX (3-6 mg/kg) given to 5- to 6-week-old mice produced well-known acute to subacute hematopoietic and gastrointestinal damage that leads to early death. These young mice did not develop other lesions that were described in humans after long-term MTX administration, nor was the toxicity cumulative. A large difference was observed in the ability of mice of different ages to withstand the toxic effects of MTX; 16-week-old mice were able to survive daily doses of 3-6 mg/kg up to 18 months. Histologic studies of these mice showed a more pronounced cellular depression of the lymphoid tissues, testes, and skin. Osteoporosis was also observed in these older mice that tolerated the drug for 10 months or longer, thus providing a laboratory animal model for further study of this MTX-induced lesion.