Fig. 6
The mechanism of invasion inhibition is unlikely to target the apicoplast ribosome. (a) Clindamycin targets the same subunit of the apicoplast ribosome but was found to have a much higher IC80 for apparent merozoite invasion inhibition (2,972 μM). There was evidence of non-specific inhibition of invasion as pretreatment of erythrocytes with clindamycin gave significant inhibition, which was not seen for azithromycin (prepared in ethanol; mean and SEM of four or more experiments; significance of differences tested with an unpaired t-test; **P ≤0.01) (38 μM). (b) The D10-AZRr line showed up to a 57-fold higher tolerance of azithromycin in 2 cycle (delayed death) apicoplast-targeting drug inhibition assays compared to D10 parental line. In contrast, the IC50 for purified merozoite invasion inhibitory activity differed by less than 2.5-fold between the D10-AZRr line and the D10-PfPHG line for (c) azithromycin (IC50: PfPHG 10 μM; D10-AZRr 25 μM), (d) erythromycin A (IC50: PfPHG 420 μM; D10-AZRr 732 μM) and (e) clindamycin (IC50: PfPHG 743 μM; D10-AZRr 557 μM). Data represent the mean of two or more experiments in at least duplicate. D10-AZRr, D10 azithromycin-resistant