Brequinar sodium is primarily used as an immunosuppressive agent in research and experimental therapies, particularly in organ transplantation and autoimmune disease models. It works by inhibiting the enzyme dihydroorotate dehydrogenase (DHODH), which is crucial for de novo pyrimidine synthesis. By blocking this pathway, brequinar sodium selectively targets rapidly dividing cells such as activated lymphocytes, thereby suppressing immune responses.

It has been investigated in preclinical and clinical studies to prevent graft-versus-host disease (GVHD) and to improve transplant outcomes. Additionally, due to its antiproliferative effects, brequinar sodium has shown potential in antiviral research, including studies on cytomegalovirus and other viral infections where host cell pyrimidine pools are critical for viral replication.

More recently, it has gained attention in oncology research for its ability to disrupt nucleotide metabolism in cancer cells, especially in combination with other agents that increase cellular dependence on de novo pyrimidine synthesis. Its application remains largely experimental, but it holds promise in targeted therapies where immune modulation or inhibition of cell proliferation is required.