Brequinar sodium is primarily used as an immunosuppressive agent in research and clinical settings, particularly in organ transplantation. It works by inhibiting the enzyme dihydroorotate dehydrogenase (DHODH), which is crucial for de novo pyrimidine synthesis. By blocking this pathway, brequinar sodium suppresses lymphocyte proliferation, thereby reducing immune response. It has been investigated for preventing graft-versus-host disease (GVHD) and improving transplant outcomes due to its potent effect on T-cell activation. Additionally, brequinar sodium has shown antiviral and anticancer potential in preclinical studies, as rapidly dividing cells—such as those in tumors or virus-infected cells—rely heavily on pyrimidine synthesis. Its application in combination therapies with other immunosuppressants or antineoplastics is also being explored to enhance efficacy and reduce resistance.