Aminopterin is a folic acid analog and derivative of pterin; it is very similar to methotrexate in structure. Aminopterin exhibits anticancer chemotherapeutic and immunosuppressive activities. Like other folic acid analogs, aminopterin inhibits dihydrofolate reductase, depleting nucleotide pools and inhibiting synthesis of DNA and RNA. Aminopterin was one of the first compounds to successfully induce remission in childhood leukemia and was clinically used to treat rheumatoid arthritis, but has since been replaced by methotrexate.