FIG 1.

Imidazopyridazine compounds have two modes of action. (A) Imidazopyridazine compounds in this study are characterized by an invariant core, an aromatic linker group (A), and variable R1 and R2 groups. (B) P. falciparum 3D7 parasites approximately 21 h after merozoite invasion of erythrocytes were incubated for 48 h with a concentration equal to 10 times the EC₅₀ of each compound (determined by fluorescence-activated cell sorting [23]). In mock-treated cultures (0.05% DMSO), newly invaded erythrocytes with ring stage parasites are seen. In the presence of compound C (CpdC; 0.3 μM), parasites developed until very late schizogony and then arrested prior to merozoite egress, whereas with compound D (CpdD; 4 μM), parasites failed to develop beyond late rings/early trophozoites. (C) EC₅₀ determination using a SYBR green I assay. P. falciparum cultures were incubated for either 48 h or 96 h with serial dilutions of inhibitors. For compound A, the EC₅₀ with 48 h of incubation was not able to be determined with any accuracy, whereas with 96 h of incubation the EC₅₀ was 0.023 μM (standard deviation, 0.007). The EC₅₀ for compound E was 0.298 μM (standard deviation, 0.066) or 0.142 μM (standard deviation, 0.024) with 48 h or 96 h of incubation, respectively. The graphs shown are representative examples of triplicate experiments. Error bars show the standard errors of the means for duplicate samples.