Targeted Structure–Activity Analysis of Endochin-like Quinolones Reveals Potent Qi and Qo Site Inhibitors of Toxoplasma gondii and Plasmodium falciparum Cytochrome bc1 and Identifies ELQ-400 as a Remarkably Effective Compound against Acute Experimental Toxoplasmosis

Abstract

<p class="first" id="P1">Cytochrome <i>bc</i> ₁ inhibitors have been broadly studied as human and veterinary medicines, and agricultural fungicides. For the most part, cytochrome <i>bc</i> ₁ inhibitors compete with ubiquinol at the ubiquinol oxidation (Qo) site or with ubiquinone at the quinone reduction (Qi) site. 4(1 <i>H</i>)-Quinolones with 3-position substituents may inhibit either site based on quinolone ring substituents. 4(1 <i>H</i>)-Quinolones that inhibit the Qi site are highly effective against toxoplasmosis, malaria and babesiosis and do not inhibit human cytochrome <i>bc</i> ₁. We tested a series of 4(1 <i>H</i>)-Quinolones against wild-type and drug resistant strains of <i>Toxplasma gondii</i> and <i>Plasmodium falciparum.</i> These experiments identified very potent compounds that inhibit <i>T. gondii</i> proliferation at picomolar concentrations. The most potent compounds target the Qo site, and for these compounds, an alkyl side chain confers potency against <i>T. gondii</i> greater than that of bulkier side chains. Our experiments also show that substituents on the quinolone ring influenced selectivity between <i>T. gondii</i> and <i>P. falciparum</i> and between Qo and Qi site-mediated activity. Comparison of the parasite cytochrome <i>b</i> sequences identified amino acids that are associated with drug resistance in <i>P. falciparum</i> that exist naturally in wild-type <i>T. gondii.</i> These underlying differences may influence drug susceptibility. Finally, a Qo site active 4(1 <i>H</i>)-quinolone-3-diarylether tested in a murine model of toxoplasmosis was superior to atovaquone, resulting in survival from Type I strain <i>T. gondii</i> infection. These experiments identify highly effective compounds for toxoplasmosis and provide valuable insight into the structure-activity relationship of cytochrome <i>bc</i> ₁ inhibitors. </p><p id="P2"> <div class="figure-container so-text-align-c"> <img alt="" class="figure" src="/document_file/3dcaf3fb-1399-42f9-877b-b35e942a3c82/PubMedCentral/image/nihms-1056610-f0006.jpg"/> </div> </p>