Genetic Evidence for Cytochrome b Qi Site Inhibition by 4(1H)-quinolone-3-diarylethers and Antimycin in Toxoplasma gondii [PublishAheadOfPrint]

Toxoplasma gondii is an apicomplexan parasite that causes fatal and debilitating brain and eye disease. Endochin-like-quinolones (ELQs) are preclinical compounds that are efficacious against apicomplexan-caused diseases including toxoplasmosis, malaria and babesiosis. Of the ELQs, ELQ-316 has demonstrated the greatest efficacy against acute and chronic experimental toxoplasmosis. Although genetic analyses in other organisms have highlighted the importance of the cytochrome bc₁ complex Qi site for ELQ sensitivity, the mechanism of action of ELQs against T. gondii and the mechanism of ELQ-316 remains unknown. Here we describe the selection and genetic characterization of T. gondii clones resistant to ELQ-316. A T. gondii strain selected under ELQ-316 drug pressure was found to possess a Thr222-Pro amino acid substitution that confers 49-fold resistance to ELQ-316 and 19-fold resistance to antimycin, a well-characterized Qi site inhibitor. These findings provide further evidence for ELQ Qi site inhibition in T. gondii and greater insight into the interaction of Qi site inhibitors with the apicomplexan cytochrome bc₁ complex.

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