sulfadiazine

Sulfadiazine is a folate synthesis inhibitor that competitively inhibits dihydropteroate synthase, preventing bacteria and certain parasites (e.g., Toxoplasma gondii) from synthesizing folic acid, which they require for nucleic acid synthesis since they cannot use preformed folate like human cells. It is most notably used in combination with pyrimethamine (a dihydrofolate reductase inhibitor) and leucovorin (folinic acid, to rescue human cells) for treatment of toxoplasmosis encephalitis in HIV/AIDS patients and congenital toxoplasmosis. This combination produces sequential blockade of the folate pathway, synergistically enhancing antimicrobial activity while leucovorin protects host bone marrow from toxicity. Sulfadiazine is also historically used for nocardiosis and as a topical silver sulfadiazine formulation for burn wound infection prophylaxis.

Pharmacologically, sulfonamides are structurally similar to para-aminobenzoic acid (PABA), acting as competitive antagonists at dihydropteroate synthase. Adverse effects are characteristic of the sulfonamide class: hypersensitivity reactions (rash, Stevens-Johnson syndrome, photosensitivity), hemolytic anemia in G6PD-deficient patients (due to oxidative stress), kernicterus risk in neonates (displaces bilirubin from albumin, contraindicated in pregnancy near term and in infants), crystalluria/nephrotoxicity requiring adequate hydration and urine alkalinization, and bone marrow suppression (megaloblastic anemia, leukopenia) exacerbated when combined with pyrimethamine. Sulfonamides can also potentiate warfarin and sulfonylureas via displacement from albumin and CYP450 interactions.

Clinically, sulfadiazine-pyrimethamine is first-line therapy for toxoplasmic encephalitis and ocular toxoplasmosis, and is used for primary prophylaxis in HIV patients with CD4 <100 and positive Toxo IgG. It is important for Step 1/2 exams to associate sulfadiazine with folate pathway inhibition, toxoplasmosis treatment, and its sulfa drug adverse effect profile.