Persistent cognitive deficits, induced by intrathecal methotrexate, are associated with elevated CSF concentrations of excitotoxic glutamate analogs and can be reversed by an NMDA antagonist.

For patients with acute lymphoblastic leukemia or non-Hodgkin lymphoma, intrathecal (IT) methotrexate (MTX) significantly reduces the risk of relapse within the central nervous system, but is associated with neurotoxic sequelae. We established a rat model of MTX-induced cognitive deficits to further investigate the underlying pathophysiology and to develop protective therapeutic interventions. IT MTX 0.5 mg/kg was administered to 10-week old male Long Evans rats. Cerebrospinal fluid (CSF) was collected for measurement of folate, homocysteine, and excitotoxic glutamate analogs. Recognition and spatial memory were tested in the novel object recognition (NOR) task and the object placement (OP) task, respectively. Four doses of IT MTX in a two-week period induced cognitive deficits persisting at least three months after the final injection. CSF concentrations of the excitotoxic glutamate analogs homocysteic acid and homocysteine sulfinic acid were increased relative to baseline for the same three-month period. Dextromethorphan, a noncompetitive antagonist at the N-methyl-D-aspartate receptor, administered at a dose of 2 mg/kg intraperitoneally twice daily for a total of four doses, improved cognitive function among the MTX-treated rats, with no effect on control rats. Although this improvement was transient, each repeated treatment with dextromethorphan was followed by normalization of cognitive function. In conclusion, IT MTX induces persistent alterations in glutaminergic tone that may contribute to persistent cognitive deficits. Treatment with a glutamate receptor antagonist such as dextromethorphan may ameliorate the negative cognitive outcomes observed among patients with leukemia or lymphoma treated with repeated doses of prophylactic IT MTX.