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Pralatrexate, an injectable DHFR inhibitor, was launched for the treatment of patients with relapsed or refractory PTCL. PTCL is an aggressive form of non-Hodgkin’s lymphoma (NHL) characterized by the proliferation of abnormal T-lymphocytes that circulate in the peripheral bloodstream. The inhibition of the folate enzymes DHFR and thymidylate synthase is a well-validated method of cancer treatment. In vitro, pralatrexate is slightly less potent than MTX in inhibiting DHFR derived from murine leukemia L1210 cells (K_i = 18.2 pM vs. 5.75 pM) and human leukemia CCRF-CEM cells (K_i = 13.4 pM vs. 5.4 pM). However, it is transported into both types of cells with 10-fold higher efficiency than MTX, thereby providing a more potent inhibition of cell growth as compared with MTX. In vivo, intraperitonally administered pralatrexate at 60 mg/ kg twice weekly for three or four doses caused complete lymphoma regressions in 89, 56, and 30% of HT, RL, and SKI-DLBCL-1 xenografted mice, respectively, whereas a similar dosing of MTX at 40 mg/kg twice weekly did not produce complete regression. The posttreatment tumor diameter was also smaller in pralatrexate-treated animals.