Pyrimethamine and Sulfadiazine

**Pyrimethamine and sulfadiazine** is a synergistic antimicrobial and antiprotozoal combination primarily used in veterinary medicine to treat **Equine Protozoal Myeloencephalitis (EPM)** caused by *Sarcocystis neurona*. * **Synergistic Action:** By combining a sulfonamide with a diaminopyrimidine, the formulation achieves a sequential blockade of the folate synthesis pathway, which is lethal to susceptible protozoa and bacteria. * **Off-label Uses:** While FDA-approved specifically for horses, this combination (or similar folate antagonist combinations) is sometimes utilized off-label in small animal medicine to treat severe protozoal infections such as **Toxoplasmosis** in cats and **Neosporosis** in dogs. * **Clinical Considerations:** Prolonged therapy (often 3-9 months) is required for EPM. Clinicians must monitor for bone marrow suppression due to systemic folate deficiency.

กลไกการออกฤทธิ์: This combination exerts its antiprotozoal and antibacterial effects through a **sequential blockade** of the folic acid synthesis pathway, which is essential for nucleic acid (DNA/RNA) production in the parasite: 1. **Sulfadiazine** acts as a structural analogue of para-aminobenzoic acid (**PABA**). It competitively inhibits the enzyme **dihydropteroate synthase**, blocking the conversion of PABA → **dihydrofolic acid (DFA)**. 2. **Pyrimethamine** specifically targets and inhibits the enzyme **dihydrofolate reductase**, blocking the conversion of DFA → **tetrahydrofolic acid (THFA)** (the active form of folate). > **Pharmacological Pearl:** Pyrimethamine has a much higher affinity for protozoal dihydrofolate reductase compared to bacterial or mammalian forms, making it highly effective against protozoal parasites like *Sarcocystis neurona* and *Toxoplasma gondii*.

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