In vitro assessment of the pharmacodynamic properties of DB75, piperaquine, OZ277 and OZ401 in cultures of Plasmodium falciparum

Objectives: Using synchronized cultures of Plasmodium falciparum, the time-and concentration-dependent growth changes of erythrocytic parasite stages to DB75, piperaquine, OZ277 and OZ401 were investigated in vitro over a concentration range of similar to 1-100x the IC(50) of piperaquine, OZ277 and OZ401 and similar to 10-1000x the IC(50) of DB75. Methods: The effects of timed in vitro exposure (1, 6, 12 or 24 h) were monitored by the incorporation of [(3)H] hypoxanthine into the parasite nucleic acids. Results: After 1 h of exposure to the highest concentration of the compound followed by removal of the compound, the growth of all stages of P. falciparum was reduced to < 34% for DB75 and 15% for piperaquine, OZ277 and OZ401 compared with untreated control parasites. At this time point, no stage-specific effects were observed at any of the concentrations. Strong inhibition (<= 10% growth) of all parasite stages was observed when the parasites were exposed to 103 or 1003 the IC50 of OZ277 and OZ401 for >= 6 h. At the 6 h incubation time point, DB75 was more active against mature parasite stages, with the IC(50)s of young ring forms elevated up to 7-fold. This trend was observed up to 12 h, but was only statistically significant at the lowest concentration. Interestingly, the stage-specific effect of DB75 on ring forms was not detectable when washing procedures were omitted. This indicates a cytostatic action of DB75 on P. falciparum ring forms. Conclusions: The current study suggests that P. falciparum ring stages are less susceptible to DB75. A milder and often statistically insignificant stage-specific trend was observed for piperaquine, whereas OZ277 and OZ401 were equally active against the erythrocytic parasite stages.