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

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 IC 50 for F 32 was: PYR 6·1 × 10 −9 m(mol/litre), TMP 1·3 × 10 −7 m, Fansidar ® ( SDX PYR 80:1) < 10 −8 to 1·3 × 10 −10 m and cotrimoxazole ( SMZ TMP 20:1) 2·6 × 10 −7 to 1·3 × 10 −8 m. The IC 50 for K 1 was: PYR > 10 −6 m, TMP 8·2 × 10 −7 m, Fansidar ® 4·1 × 10 −7 to 1·1 × 10 −9 m and cotrimoxazole 1·8 × 10 −6 to 9·0 × 10 −8 m. The difference in IC 50 between F 32 and K 1 against TMP and cotrimoxazole is much less than the difference between the IC 50 values against PYR and Fansidar ®, indicating that cross-resistance between PYR and TMP exists, but is not complete. A method for calculating the IC 50 by linear regression analysis is described.