Synergistic killing by combinations of primary and residual inhibitors, but not by combinations from one class only.

(A) Isobologram showing effect of fixed ratio combinations of the primary component inhibitor cpd 1 and the residual transport inhibitor PRT1-20. Each symbol represented the IC50 for a fixed ratio of the two inhibitors, determined from a full dose response experiment with replicates. Error bars, shown for single compound IC50 values (symbols on the x and y axes), represent S.E.M. values. Solid line connecting these intercept values is the expected profile for additive parasite killing. Strong synergistic killing was found for this drug combination as the symbols are markedly below the additive line. (B) Isobologram for two primary component inhibitors, showing additive interaction. (C) Isobologram for two residual component inhibitors, showing additive interaction.