Interactions of carboxypeptidase G2 with 6S-leucovorin and 6R-leucovorin in vitro: implications for the application in case of methotrexate intoxications.

Carboxypeptidase G2 (CPG2) is used when unexpected toxicity or renal failure occurs during high-dose methotrexate therapy. Leucovorin is also administered to antagonise the effects of methotrexate on purine anabolism. To investigate the effects of CPG2 on leucovorin rescue, we incubated the enzyme with both stereoisomers and analysed the degradation. A method for separating the stereoisomers of leucovorin, the internal standard aminopterin and the degradation products by capillary electrophoresis with 2.6-dimethyl-beta-cyclodextrin as a chiral selector has been developed. The active 6S-leucovorin is degraded much faster than the inactive 6R-isomer. The maximum observed degradation velocity was 31 microM/min for 6S-leucovorin and 20 microM/min for 6R-leucovorin, respectively, with an initial concentration of each stereoisomer of 250 microM. Similar results were obtained at lower concentrations of leucovorin isomers. Thus, the selectivity of CPG2 for methotrexate in comparison to leucovorin is not as high as anticipated in the literature as only the active 6S-leucovorin and not the mixture of the diastereomers should be taken into account. We conclude that the protective effects of leucovorin are antagonized by CPG2. Therefore, CPG2 should be administered to patients with caution.