Hepatotoxicity in a rat model caused by orally administered methotrexate.

We undertook a dose-response study in Wistar rats to develop an animal model for methotrexate hepatotoxicity. Rats were given oral methotrexate in 300, 200, 150 and 100 micrograms/kg/day doses for variable lengths of time. The 300 micrograms/kg/day dose produced systemic toxicity; the animals needed to be killed early, and hepatotoxicity was not observed. The lower doses of methotrexate were tolerated for longer durations and were associated with hepatotoxicity in five of the five rats receiving 200 micrograms/kg/day, four of the five rats receiving 150 micrograms/kg/day and five of the five rats on 100 micrograms/kg/day. Within each treatment group the liver injury ranged in severity from focal necrosis of some zone 3 hepatocytes to confluent necrosis of zone 3. All five rats that received 100 micrograms/kg/day methotrexate for 6 wk showed continuing liver injury in the form of focal necrosis, cell lysis and enlarged Kupffer cells. In addition, three of the rats showed evidence of early hepatic fibrosis. We believe that this is the first experimental model in which oral methotrexate administration has been associated with hepatotoxicity. Further development of this model should provide valuable insights into the pathogenesis of methotrexate hepatotoxicity.