Fig. 8

Azithromycin inhibits P. berghei merozoite and Toxoplasma gondii tachyzoite, but not sporozoite, invasion of host cells. (a) Azithromycin inhibited purified P. berghei merozoite invasion at similar concentrations compared to P. falciparum. (b) Treatment of P. berghei-infected erythrocytes immediately after invasion with azithromycin for 30 minutes did not result in a loss of late-stage parasites detected by flow cytometry, confirming that azithromycin inhibited invasion and not parasite growth. (c) Azithromycin was found to have a small but significant effect on the number of P. berghei sporozoites that had entered or traversed host cells. However, using a more specific assay that measures successfully invaded hepatocytes containing a developing parasite (d), there was no significant inhibition in sporozoites that had invaded and formed a parasitophorous vacuole. (e) Azithromycin treatment of T. gondii tachyzoites at concentrations of 250, 125 and 50 μM resulted in a dose-dependent inhibition of host cell invasion. The invasion inhibitory control cytochalasin D (1 μM) was a considerably more potent inhibitor of tachyzoite invasion, while erythromycin A, as was found for P. falciparum, showed no evidence of invasion inhibitory activity at concentrations up to 500 μM. (f) Azithromycin analogue 12e (42 μM) was significantly more inhibitory to Toxoplasma tachyzoite invasion than azithromycin (250 μM, both solubilized in DMSO) even when tested at a 6-fold lower concentration. Data represent the mean and SEM of three or more experiments, significance of differences was tested using an unpaired t-test (*P = 0.01 to 0.05, **P ≤0.01, ***P ≤0.001)