A folate analog antimetabolite with antineoplastic activity. As the 6R diastereomer of 5,10-dideazatetrahydrofolate, lometrexol inhibits glycinamide ribonucleotide formyltransferase (GARFT), the enzyme that catalyzes the first step in the de novo purine biosynthetic pathway, thereby inhibiting DNA synthesis, arresting cells in the S phase of the cell cycle, and inhibiting tumor cell proliferation. The agent has been shown to be active against tumors that are resistant to the folate antagonist methotrexate. or A substance that is being studied as a treatment for cancer. It belongs to the family of drugs called antifolates.