Monitoring of plasma concentration of pyrimethamine (PYR) in infants with congenital Toxoplasma gondii infection--own observations.

The study objective was to determine plasma concentration of pyrimethamine in 24 infants aged 1-5 months, treated for congenital toxoplasmosis. Pyrimethamine was used in a single daily dose at an amount of 0.35-0.98 mg/kg daily, with sulfadiazine (50-100 mg/kg/day) in divided doses 2-3 times a day, and folinic acid given twice a week (7.5 mg). This regimen was continued for 2-6 months, then Fansidar was administered. Pyrimethamine concentration in plasma was measured using high-performance liquid chromatography method (HPLC). A total of 70 tests were performed. Concentration of pyrimethamine ranged from 0.01 to 1.2 microg/ml. In 14 children (58 tests) the concentration of pyrimethamine achieved therapeutic value. In 7 patients (8 tests) the concentration was below therapeutic level, and in 3 patients (4 tests) above therapeutic level. In 11/24 (46%) children transient moderate neutropenia was observed. Modification of therapy was necessary in 12 patients. Monitoring of pyrimethamine concentration in plasma improves safety and effectiveness of the therapy and is useful in obtaining correct individual dose of the drug. Neutropenia is the most common side-effect of pyrimethamine observed even when using the recommended dose.