Toxoplasma gondiiis a widely distributed protozoan pathogen that causes devastating ocular and central nervous system disease. We show that the endochin-like quinolone (ELQ) class of compounds contains extremely potent inhibitors ofT. gondiigrowth in vitro and is effective against acute and latent toxoplasmosis in mice. We screened 50 ELQs againstT. gondiiand selected two lead compounds, ELQ-271 and ELQ-316, for evaluation. ELQ-271 and ELQ-316, have in vitro IC₅₀values of 0.1 nM and 0.007 nM, respectively. ELQ-271 and ELQ-316 have ED₅₀values of 0.14 mg/kg and 0.08 mg/kg when administered orally to mice with acute toxoplasmosis. Moreover, ELQ-271 and ELQ-316 are highly active against the cyst form ofT. gondiiin mice at low doses, reducing cyst burden by 76–88% after 16 d of treatment. To investigate the ELQ mechanism of action againstT. gondii, we demonstrate that endochin and ELQ-271 inhibit cytochromecreduction by theT. gondiicytochromebc₁complex at 8 nM and 31 nM, respectively. We also show that ELQ-271 inhibits theSaccharomyces cerevisiaecytochromebc₁complex, and an M221Q amino acid substitution in the Q_isite of the protein leads to >100-fold resistance. We conclude that ELQ-271 and ELQ-316 are orally bioavailable drugs that are effective against acute and latent toxoplasmosis, likely acting as inhibitors of the Q_isite of theT. gondiicytochromebc₁complex.

The goal for developing new antimalarial drugs is to find a molecule that can target multiple stages of the parasite’s life cycle, thus impacting prevention, treatment, and transmission of the disease. The 4(1H)-quinolone-3-diarylethers are selective potent inhibitors of the parasite’s mitochondrial cytochrome bc₁ complex. These compounds are highly active against the human malaria parasites Plasmodium falciparum and Plasmodium vivax. They target both the liver and blood stages of the parasite as well as the forms that are crucial for disease transmission, that is, the gametocytes, the zygote, the ookinete, and the oocyst. Selected as a preclinical candidate, ELQ-300 has good oral bioavailability at efficacious doses in mice, is metabolically stable, and is highly active in blocking transmission in rodent models of malaria. Given its predicted low dose in patients and its predicted long half-life, ELQ-300 has potential as a new drug for the treatment, prevention, and, ultimately, eradication of human malaria.

The historical antimalarial compound endochin served as a structural lead for optimization. Endochin-like quinolones (ELQ) were prepared by a novel chemical route and assessed for in vitro activity against multidrug resistant strains of Plasmodium falciparum and against malaria infections in mice. Here we describe the pathway to discovery of a potent class of orally active antimalarial 4(1H)-quinolone-3-diarylethers. The initial prototype, ELQ-233, exhibited low nanomolar IC₅₀ values against all tested strains including clinical isolates harboring resistance to atovaquone. ELQ-271 represented the next critical step in the iterative optimization process, as it was stable to metabolism and highly effective in vivo. Continued analoging revealed that the substitution pattern on the benzenoid ring of the quinolone core significantly influenced reactivity with the host enzyme. This finding led to the rational design of highly selective ELQs with outstanding oral efficacy against murine malaria that is superior to established antimalarials chloroquine and atovaquone.

It is believed that at no time in the history of the human race malaria has been absent. This disease, which is caused by protozoa of the genus Plasmodium, in all likelihood has been responsible for the death of about half of all people who ever lived. Even today, after attempts at intervention on a worldwide scale, malaria remains the most significant parasitic disease in the tropics and sub-tropics, where it causes at least 500 million clinical episodes and claims 1.5 million lives each year, mostly young children and pregnant women. Widespread resistance to the best and least expensive antimalarials, chloroquine and S/P (i.e., a combination of sulfadoxine and pyrimethamine), combined with an increasing tolerance to insecticides in the mosquito vector, threaten a global malaria tragedy unless new countermeasures are developed. For malaria therapy, the great panacea would be the development of a long-lasting vaccine, but until this becomes a reality, people living in and traveling to endemic regions must rely on a dwindling cache of more expensive drugs; many beyond the economic reach of impoverished people living in malarious regions of the world. Our course to recognition of xanthones as potential antimalarial agents took a rather circuitous route, involving both serendipity and empiricism, and is described together with mechanistic details of drug action. From a chance encounter with a sea urchin collected near the city of Cannon Beach on the Oregon coast to naturally occurring and functionalized xanthones, it is revealed how these compounds target the Plasmodium parasites most vulnerable feature - the digestive vacuole.