Identification of a class of sulfonamides highly active against dihydropteroate synthase form Toxoplasma gondii, Pneumocystis carinii, and Mycobacterium avium.

Sulfanilanilides with 3',5'-halogen substitutions had Ki values 6- to 57-fold lower than the Ki of sulfamethoxazole when tested against dihydropteroate synthase from Toxoplasma gondii. The compounds acted as competitive inhibitors. These compounds were also active against dihydropteroate synthase from Pneumocystis carinii, Mycobacterium avium, and Escherichia coli but were not significantly more active than sulfamethoxazole. The compounds were significantly more active in culture than were standard agents. Against T. gondii in culture, 50% inhibitory concentrations were 7- to 30-fold lower than that of sulfadiazine; against P. carinii in culture, a concentration of 100 microM caused 33 to 95% inhibition of growth, compared with 9% inhibition with 100 microM sulfamethoxazole.