Reversibly inhibits dihyroorotate dehydrogenase, the rate-limiting step in the de novo synthesis of pyrimidines, preventing DNA synthesis of activated lymphocytes (IC50s = 0.09, 3.5, and 12.5 μM for rat, mouse, and human, respectively); also inhibits tyrosine kinases responsible for early T cell and B cell cycling and the production of PGE2, MMP-1, and IL-6 in immune-activated synoviocytes