Inhibitors of folic acid synthesis in the treatment of experimental Pneumocystis carinii pneumonia.

Inhibitors of folic acid synthesis were compared alone and in different combinations in the therapy of pneumocystosis in immunosuppressed rats. Sulfonamides (sulfamethoxazole, sulfadiazine, and sulfadoxine) and sulfones (dapsone) used alone were very active against Pneumocystis carinii, as judged by histologic examination of the lungs and by organism quantitation. Improved efficacy could not be demonstrated by the addition of an inhibitor of dihydrofolate reductase to the regimen. Dihydrofolate reductase inhibitors (trimethoprim, diaveridine, and pyrimethamine) used alone were ineffective against P. carinii. All drugs were well tolerated except pyrimethamine, which caused bone marrow depression; folinic acid ameliorated this adverse reaction but did not interfere with P. carinii treatment. These data have potential clinical implications but need to be interpreted with caution and in light of other systems of P. carinii drug evaluation.