leflunomide + orotic acid

Leflunomide + orotic acid is a combination of two agents. Leflunomide is an immunomodulatory small molecule prodrug that, after oral administration, is rapidly converted to its active metabolite A77 1726. This metabolite inhibits dihydroorotate dehydrogenase, a key mitochondrial enzyme in the de novo synthesis of pyrimidine ribonucleotides (specifically uridine monophosphate), leading to decreased DNA and RNA synthesis and cell cycle arrest in activated lymphocytes. This results in cytostatic effects on autoimmune T-cell proliferation and B-cell autoantibody production[1][4][6]. Orotic acid is a naturally occurring intermediate in pyrimidine biosynthesis; it can bypass the block induced by dihydroorotate dehydrogenase inhibition by providing substrate for downstream steps of pyrimidine synthesis. The rationale for combining leflunomide with orotic acid would be to modulate the immunosuppressive effect or manage toxicity related to excessive inhibition of pyrimidine biosynthesis, though this specific combination does not appear as an approved pharmaceutical product nor as a standard therapy in clinical guidelines.