Insights into the mechanisms of ifosfamide encephalopathy: Drug metabolites have agonistic effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and induce cellular acidification in mouse cortical neurons

Therapeutic value of the alkylating agent ifosfamide has been limited by ma jor side effects including encephalopathy. Although the underlying biochemi cal processes of the neurotoxic side effects are still unclear, they could be attributed to metabolites rather than to ifosfamide itself. In the prese nt study, the effects of selected ifosfamide metabolites on indices of neur onal activity have been investigated, in particular for S-carboxymethylcyst eine (SCMC) and thiodiglycolic acid (TDGA). Because of structural similarit ies of SCMC with glutamate, the Ca-i(2+) response of single mouse cortical neurons to SCMC and TDGA was investigated. SCMC, but not TDGA, evoked a rob ust increase in Ca-i(2+) concentration that could be abolished by the alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate recepto r antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but only partly d iminished by the N-methyl-D-aspartate receptor antagonist 10,11-dihydro-5-m ethyl-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK = 801). Cyclothiazide (CYZ) , used to prevent AMPA/kainate receptor desensitization, potentiated the re sponse to SCMC. Because activation of AMPA/kainate receptors is known to in duce proton influx, the intracellular pH (pH(i)) response to SCMC was inves tigated. SCMC caused a concentration-dependent acidification that was ampli fied by CYZ. Since H+/monocarboxylate transporter (MCT) activity leads to s imilar cellular acidification, we tested its potential involvement in the p H(i) response. Application of the lactate transport inhibitor quercetin dim inished the pH(i) response to SCMC and TDGA by 43 and 51%, respectively, in dicating that these compounds may be substrates of MCTs. Taken together, th is study indicates that hitherto apparently inert ifosfamide metabolites, i n particular SCMC, activate AMPA/kainate receptors and induce cellular acid ification. Both processes could provide the biochemical basis of the observ ed ifosfamide-associated encephalopathy.