Rescue of experimental intrathecal methotrexate overdose with carboxypeptidase-G₂

The carboxypeptidase G class of enzymes rapidly hydrolyze methotrexate (MTX) into the inactive metabolites 4-deoxy-4-amino-N¹⁰-methylpteroic acid (DAMPA) and glutamate. This study evaluated the use of carboxypeptidase-G₂ (CPDG₂) as a potential intrathecal (IT) rescue agent for massive IT MTX overdose. The CSF pharmacokinetics of MTX with and without CPDG₂ rescue was studied in adult rhesus monkeys (Macaco mulatto) using a nontoxic IT 5 mg dose (equivalent to 50 mg in humans). Without CPDG₂ rescue, peak CSF MTX concentration was 2,904 ± 340 μmol/L. Within 5 minutes of administration of 30 U IT CPDG₂, CSF MIX concentrations decreased greater than 400-fold to 6.55 ± 6.7 μM. Subsequently, groups of three monkeys received either 25 mg IT MTX (equivalent to 250 mg in humans) followed by 150 U IT CPDG₂ or 50 mg IT MTX (equivalent to 500 mg in humans) followed by 300 U IT CPDG₂. All animals survived without neurotoxicity. Our studies suggest that CPDG₂ may prove to be an important addition to the currently recommended strategy for the management of IT MTX overdose.