Tritiated pralatrexate, pemetrexed (both generally labeled), and [3′,5′,7-3H]MTX were obtained from Moravek Biochemicals (Brea, CA). Pemetrexed and MTX purity was established and monitored by high-performance liquid chromatography (HPLC) as described previously [24 (link)]. Pralatrexate purity was monitored using a 5 mm OSD2 4.6 × 250 mm reversed phase high-performance liquid chromatography column (Waters Spherisorb), by isocratic elution with 100 mM sodium acetate pH 5.5 (solvent A) and 15% acetonitrile (solvent B). The mobile phase was delivered at 1 ml/min, reaching 100% solvent B in 30 min. Nonlabeled MTX was obtained from Sigma-Aldrich (St. Louis, MO), nonlabeled (6S)5-formyltetrahydrofolate (5-formylTHF) from Schircks Laboratories (Jona, Switzerland) and nonlabeled pemetrexed from LC Laboratories (Woburn, MA). Pralatrexate was obtained from Spectrum Pharmaceuticals (Irvine, CA).