The potent Toxoplasma gondii growth inhibitor QQ-437 does not bind to its predicted target, parasite adaptin3β, in yeast three-hybrid assays

New drugs are needed to treat infections with Toxoplasma gondii, a ubiquitous protozoan parasite that can cause miscarriage, blindness, and life-threatening encephalitis in its human hosts. A novel N-benzoyl-2-hydroxybenzamide named QQ-437 was recently shown to be a potent inhibitor of T. gondii growth in vitro (EC50 = 16nM), and to reduce parasite load in a mouse model of infection. Intriguingly, mutations in the parasite protein TgAP3{beta}, which is thought to play an important role in intracellular protein trafficking, confer resistance to QQ-437. In this report, we use yeast three-hybrid analysis to test the hypothesis that QQ-437 inhibits parasite growth via a direct effect on TgAP3{beta}. We see no evidence for the ability of QQ-437 to bind directly to TgAP3{beta}, suggesting that the mutation in TgAP3{beta} that confers resistance does so through an indirect mechanism. Further studies are needed to identify the direct molecular target(s) of this promising class of compounds.