Pyrimethamine concentrations in serum during treatment of acute murine experimental toxoplasmosis

Central nervous system toxoplasmosis is a major opportunistic infection in patients with acquired immunodeficiency syndrome. The standard therapy for this infection is pyrimethamine (PYR) and sulfonamides. To assess in vivo if PYR alone could adequately treat toxoplasmosis, a murine model of acute toxoplasmosis was used. The CD1 strain of mice was infected intraperitoneally with 10⁴ parasites of the RH strain of Toxoplasma gondii. Pyrimethamine was administered in mouse chow at concentrations of 0, 0.03125, 0.0625, 0.125, 0.25, or 1.0 mg of PYR/g of food, which provides the following daily PYR dosages: 0, 6.25, 12.5, 25, 50, and 200 mg/kg/day. No sulfonamides were administered. Serum PYR levels proved more accurate than mg of PYR/g of food in predicting survival. Mice with serum PYR levels ≥ 500 ng/ml (2 μM) survived and had no parasites present on peritoneal lavage. Mice with serum PYR levels < 100 ng/ml (0.4 μM) had a 100% mortality rate and the average parasite count was 3 x 10⁷ organisms in the lavage fluid. At a PYR level of 370 ng/ml, six of 11 mice survived and the lavage fluid contained 2.5 x 10⁵ organisms. Previously, using ³H-uracil in an in vitro assay, PYR at a concentration of 500 ng/ml was shown to be as effective in inhibiting Toxoplasma growth as the combination of PYR (100 ng/ml) and sulfonamides 25 μg/ml). These data suggest the potential usefulness of PYR for monotherapy of toxoplasmosis and are consistent with previously described in vitro assays.