Apicoplast-Targeting Antibacterials Inhibit the Growth of Babesia Parasites

The apicoplast housekeeping machinery, specifically apicoplast DNA replication, transcription, and translation, was targeted by ciprofloxacin, thiostrepton, and rifampin, respectively, in the in vitro cultures of four Babesia species. Furthermore, the in vivo effect of thiostrepton on the growth cycle of Babesia microti in BALB/c mice was evaluated. The drugs caused significant inhibi-tion of growth from an initial parasitemia of 1 % for Babesia bovis, with 50 % inhibitory concentrations (IC50s) of 8.3, 11.5, 12, and 126.6M for ciprofloxacin, thiostrepton, rifampin, and clindamycin, respectively. The IC50s for the inhibition of Babesia bigemina growth were 15.8M for ciprofloxacin, 8.2M for thiostrepton, 8.3M for rifampin, and 206M for clindamycin. The IC50s for Babesia caballiwere 2.7M for ciprofloxacin, 2.7M for thiostrepton, 4.7M for rifampin, and 4.7M for clin-damycin. The IC50s for the inhibition of Babesia equi growth were 2.5M for ciprofloxacin, 6.4M for thiostrepton, 4.1M for rifampin, and 27.2M for clindamycin. Furthermore, an inhibitory effect was revealed for cultures with an initial parasitemia of either 10 or 7 % for Babesia bovis or Babesia bigemina, respectively. The three inhibitors caused immediate death of Babesia bo-vis and Babesia equi. The inhibitory effects of ciprofloxacin, thiostrepton, and rifampin were confirmed by reverse transcrip-tion-PCR. Thiostrepton at a dose of 500 mg/kg of body weight resulted in 77.5 % inhibition of Babesia microti growth in BALB/c mice. These results implicate the apicoplast as a potential chemotherapeutic target for babesiosis. Babesia, a tick-born protozoan parasite, is one of the majorpathogens that infect erythrocytes in a wide range of wild and economically valuable animals, such as cattle and horses. The clin