Methotrexate-d3diammonium is the deuterated form of Methotrexate diammonium. Methotrexate (Amethopterin) functions as an antimetabolite and antifolate by inhibiting dihydrofolate reductase, which prevents the conversion of folic acid to tetrahydrofolate, thereby inhibiting DNA synthesis. It also acts as an immunosuppressant and antineoplastic agent, used in treating rheumatoid arthritis and in researching various cancers, such as acute lymphoblastic leukemia.