METHOTREXATE PHARMACOKINETICS FOLLOWING ADMINISTRATION OF RECOMBINANT CARBOXYPEPTIDASE-G(2) IN RHESUS-MONKEYS

Purpose: Carboxypeptidase-G2 (CPDG2) is a bacterial enzyme that rapidly hydrolyzes methotrexate (MTX) into inactive metabolites. As an alternative form of rescue after high-dose MTX (HDMTX), CPDG2 has more potential advantages than standard leucovorin (LV) rescue. In this study, the plasma pharmacokinetics of MTX with and without CPDG2 were evaluated in adult rhesus monkeys. Materials and Methods: The plasma pharmacokinetics of MTX were determined in groups of animals that had received a 300-mg/m2 loading dose of MTX followed by a 60-mg/m2/h infusion during an 18-hour period. One group received CPDG2 at the end of the infusion, and the other group served as a control. Two additional animals with high titers of anti-CPDG2 antibody also were studied. Results: During infusion, the steady-state MTX plasma concentration was 11.3 ±4.8 μmol/L. Without CPOG2, the postinfusion plasma MTX concentration remained above 0.1 μmol/L for more than 6 hours. After the administra-tion of 50 U/kg of CPDG2, plasma MTX concentrations decreased to nontoxic levels (< 0.05 μmol/L) within 30 minutes. The initial half-life (t1/2α) of MTX decreased from 5.8 ± 2.1 minutes to 0.7 ± 0.02 minutes after enzyme administration. The postinfusion area under the plasma concentration time curve of MTX was 301 ± 171 μmol/L/ min without CPDG2 compared with 19.6 ± 6.1 μmol/L/ min with CPDG2. The immunogenicity studies performed indicated that although animals developed anti-CPDG2 antibodies, none of them manifested allergic symptoms. The effectiveness of CPDG2 was diminished but not eliminated in animals with high titers of anti-CPDG2 antibody. Conclusions: CPDG2 is capable of rapidly decreasing plasma MTX concentrations to nontoxic levels. The administration of CPDG2 seems safe, well tolerated, and it may be useful as an alternative to LV rescue.