Vol. 294, Issue 3, 894-901, September 2000

Pharmacokinetics and Metabolism of the Methotrexate Metabolite 2,4-Diamino-N¹⁰-methylpteroic Acid

Brigitte C. Widemann, Edward Sung, Lawrence Anderson, Wanda L. Salzer¹ , Frank M. Balis, Karen S. Monitjo, Cynthia McCully, Mary Hawkins and Peter C. Adamson²

Pharmacology and Experimental Therapeutics Section, Pediatric Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland (B.C.W., E.S., W.L.S., F.M.B., K.S.M., C.M., M.H., P.C.A.); and United States Food and Drug Administration, Rockville, Maryland (L.A.)

The novel methotrexate (MTX) rescue agent carboxypeptidase-G₂ (CPDG₂) converts >98% of plasma MTX to 2,4-diamino-N¹⁰-methylpteroic acid (DAMPA) and glutamate in patients with MTX-induced renal failure and delayed MTX excretion. DAMPA is eliminated more rapidly than MTX in these patients, suggesting nonrenal elimination. The pharmacokinetics and metabolism of DAMPA were studied in four nonhuman primates with reverse-phase HPLC with UV, photodiode array detection, and mass spectroscopy. The mean peak plasma DAMPA concentration was 51 µM and the plasma disposition was described by a three-compartment open model with first order elimination. The mean clearance of DAMPA was 1.9 l/kg/h and the mean terminal half-life was 51 min. Forty-six percent of the dose was excreted in the urine as parent compound. Three DAMPA metabolites, hydroxy-DAMPA, DAMPA-glucuronide, and hydroxy-DAMPA-glucuronide, were identified in plasma and urine. These metabolites also were identified in plasma from patients who received CPDG₂ as an MTX rescue agent. The cytotoxicity of DAMPA and its effect on MTX cytotoxicity were assessed in the Molt-4 human leukemic cell line. DAMPA was not cytotoxic and did not significantly alter the cytotoxicity of MTX. In nonhuman primates metabolism of DAMPA is a major route of DAMPA elimination, and metabolism underlies the more rapid elimination of DAMPA versus MTX in patients with MTX-induced renal dysfunction after administration of CPDG₂.