Aminopterin (4-aminopteroic acid) is the 4-amino derivative of folic acid. Aminopterin is a potent competitive inhibitor of dihydrofolate reductase therby blocking tetrahydrofolate synthesis. Aminopterin was used clinically against pediatric acute lymphoblastic leukemia and some solid tumors. Aminopterin was withdrawn from clinical development in favor of its less toxic methylated derivative, methotrexate. Conjugates of aminopterin have immunosuppressive activity.