Aminopterin is primarily used in medical research and treatment, particularly in the field of oncology. It functions as a potent inhibitor of dihydrofolate reductase, an enzyme crucial for DNA synthesis and cell division. This property makes it effective in targeting rapidly dividing cells, such as cancer cells. It has been utilized in chemotherapy regimens for certain types of leukemia and other malignancies. Additionally, Aminopterin has been employed in the treatment of autoimmune diseases like rheumatoid arthritis, where it helps suppress the overactive immune response. In research, it is often used to study folate metabolism and to create animal models of folate deficiency for experimental purposes. Its application, however, requires careful monitoring due to its high toxicity and potential side effects.