In vitro susceptibility of Plasmodium falciparum malaria to pyrimethamine, sulfadoxine, trimethoprim and sulfamethoxazole, singly and in combination

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Abstract
Two isolates of Plasmodium falciparum (F 32 and K 1) were tested against sulfadoxine (SDX), sulfamethoxazole (SMZ), pyrimethamine (PYR) and trimethoprim (TMP), using a 48 h microtest, with RPMI-1640 low in PABA and folic acid. The IC50 for F 32 was: PYR 6·1 × 10−9m(mol/litre), TMP 1·3 × 10−7m, Fansidar® ( 80:1) < 10−8 to 1·3 × 10−10m and cotrimoxazole ( 20:1) 2·6 × 10−7 to 1·3 × 10−8m. The IC50 for K 1 was: PYR > 10−6m, TMP 8·2 × 10−7m, Fansidar® 4·1 × 10−7 to 1·1 × 10−9m and cotrimoxazole 1·8 × 10−6 to 9·0 × 10−8m. The difference in IC50 between F 32 and K 1 against TMP and cotrimoxazole is much less than the difference between the IC50 values against PYR and Fansidar®, indicating that cross-resistance between PYR and TMP exists, but is not complete. A method for calculating the IC50 by linear regression analysis is described.