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Pyrimethamine-Sulfadoxine is a combination product containing pyrimethamine and sulfadoxine.  Pyrimethamine is a competitive inhibitor of dihydrofolate reductase (DHFR).  DHFR is a key enzyme in the redox cycle for production of tetrahydrofolate, a cofactor that is required for the synthesis of DNA and proteins.  Sulfadoxine competitively antagonizes paraaminobenzoic acid (PABA), resulting in disruption of folic acid synthesis and ultimately DNA synthesis. Sulfadoxine acts synergistically with pyrimethamine. (NCI04)