Toxoplasma gondii (T. gondii) causes one of the most prevalent parasitic infections worldwide – toxoplasmosis. The risk to humans is substantial with an estimated one third of the world’s population being infected.1 Infection can cause severe neurological, congenital and ocular pathology1 and is a major cause of abortion storms in small ruminants in the agricultural industry.2 Despite being one of the most extensively studied parasites,3 few prevention and treatment options exist. Current livestock vaccines are often ineffective due to T. gondii’s complex genome, strain diversity and life cycle.4 No human T. gondii vaccine exists.
In screening studies, we identified compound NCL139 as active against T. gondii with an IC50 of 74 nM. Preliminary evaluation of metabolic stability, however, revealed rapid hepatic clearance in mice (CLint, in vitro = 245 µL/min/mg protein; half-life = 6 min). NCL139, with its notable potency and ease of synthesis, was viewed as an excellent development lead but required improved stability.
To minimise unnecessary chemical synthesis towards metabolically stable analogues, density functional theory was employed. Fukui indices calculate atom-specific changes in electron density when introducing or removing an electron. Fukui indices were generated and used to predict molecular sites in NCL139 that may be susceptible to nucleophilic, electrophilic or radical attack, and hence metabolic attack.5 Analogues were designed with metabolic blocking groups installed at these predicted sites of metabolism.
We discuss the synthesis and biological activity of selected designed analogues, and the overall outcomes of this relatively novel computational approach to designing metabolically stable compounds.