Folate and methotrexate polyglutamate tissue levels in rhesus monkeys following chronic low-dose methotrexate.

Methotrexate (MTX), a mainstay in the treatment of acute lymphoblastic leukemia, is associated with both hepatic and neurologic toxicity. Like a folate, MTX is metabolized to polyglutamated derivatives (MTXGlun) with long intracellular half-lives. These metabolites may contribute to MTX toxicity through a direct effect on cellular metabolism or indirectly through a perturbation of folate homeostasis. To better define the effects of chronic MTX treatment, tissue levels of MTX, MTXGlun, and folate were measured in three monkeys treated with weekly intramuscular MTX for 1 year. Greater than 80% of the total tissue MTX found was in the form of polyglutamated derivatives. Most of these derivatives were MTXGlu3-5 but Glu6-7 were easily detectable. Total tissue folates were measured in liver, kidney, brain and testis with MTX treated animals having a 90% loss of total folate in brain tissue. This is of special interest since inborn errors of folate metabolism are often associated with severe neurologic abnormalities.