Atovaquone

The target active site in this study is the Qo site at which ubiquinol is oxidized. The Qo site is located at the interface of two four-helical bundles in the cytochrome b subunit, residing in-between the bL heme and the 2Fe–2S cluster of the iron-sulphur protein when the latter is docked in ISP docking crater. This mainly hydrophobic pocket of approximately 15 A in length, which has the shape of a saddle, is also believed to be the target site a number of pesticides15 as well as that of atovaquone, as evidenced by mutation studies in model organisms, e.g., identification of point mutations in atovaquone/Malarone resistant parasites and molecular modelling studies. The Qo site is also known to be the site of action of the natural bc₁ inhibitor stigmatellin 3.

Atovaquone (ATO) is a quinone based, orally administered, pharmaceutical effective against several disease-causing protozoa including Plasmodium falciparum (malaria), Pneumocystis jiroveci (pneumonia), and Toxoplasma gondii (toxoplasmosis). The activity of ATO is tracked to its ability to competitively inhibit (Ki, 9 nM) Coenzyme Q10 (CoQ10; ubiquinol) binding to the parasite cytochrome bc1 complex, with a myriad of down-stream consequences to the parasite's mitochondrial respiratory chain.

Noteworthy feature of 183 was its 30-fold more potency than atovaquone against murine malaria. It was found to be $ 20 000-fold more selective for parasite bc1 complex over human enzyme with EC50 value of 0.56 nM thus exhibiting superiority over atovaquone (IC₅₀ = 2.0 nM). Preclinical animal studies on 183 demonstrated nonlinear pharmacokinetics i.e. good to excellent oral bioavailability at low doses needed for therapeutic effect but fell off rapidly at higher doses. Further studies need to be carried out to identify clinical formulation in order to improve exposure at higher doses. Compound 183 displayed potential that might be carried forward for clinical studies.