Pyrimethamine interactions

Drug/Lab test interactions Methotrexate, pyrimethamine, and most antibiotics invalidate folic acid and vitamin Bi2diagnostic microbiological blood assays. 

Both pyrimethamine and sulfonamides are liver enzyme inhibitors, and can cause interactions with drugs that are normally metabolized in the liver. 

Diaminopyrimidines are weak bases. Peak plasma concentrations are reached early and diaminopyrimidines are soon found in high concentrations in tissues. In fact, the tissue concentrations are often higher than the concentrations in serum. When inflammation is present, trimethoprim levels in the CSF may reach 50% of the plasma concentrations. CSF concentrations of pyrimethamine are 25-50% of the plasma concentrations. The Vd for trimethoprim and pyrimethamine is 1.51/kg in horses. The protein binding of trimethoprim is moderate (50%). There is no protein-binding interaction between the sulfonamides and the diaminopyrimidines. 

Clinically important, potentially hazardous interactions with chloroquine, didanosine, furazolidone, ganciclovir, hydroxychloroquine, methotrexate, pyrimethamine, rifabutin, rifampin, sulfonamides, ursodeoxycholic acid... 

Toxicity The toxic effects of sulfonamides include skin rashes, gastrointestinal distress, hemolysis, kidney damage, and drug interactions caused by competition for plasma protein binding sites. Pyrimethamine may cause folic acid deficiency when used in high doses. 

Tan-ariya P, Na-Bangchang K, Ubalee R, Thanavibul A, Thipawangkosol P, Karbwang J. Pharmacokinetic interaction or artemether and pyrimethamine in healthy male Thais. Scmth-... 

Information seems to be limited to the reports cited, but the interaction appears to be established. Its incidence is unknown. Pyrimethamine is usu-... 

Ahmad RA, Rogers HJ. Pharmacokinetics and protein binding interactions of dapsone and pyrimethamine. BrJ Clin Pharmacol (1980) 10, 9-24. 

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