Discovery of New Inhibitors of Toxoplasma gondii via the Pathogen Box

<i>Toxoplasma gondii</i> is a cosmopolitan protozoan parasite which affects approximately 30% of the population worldwide. The drugs currently used against toxoplasmosis are few in number and show several limitations, such as drug intolerance, poor bioavailability, or drug resistance mechanism developed by the parasite. Thus, it is important to find new compounds able to inhibit parasite invasion or proliferation. In this study, the 400 compounds of the open-access Pathogen Box, provided by the Medicines for Malaria Venture (MMV) foundation, were screened for their anti-<i>Toxoplasma gondii</i> activity. A preliminary <i>in vitro</i> screening performed over 72 h by an enzyme-linked immunosorbent assay (ELISA) revealed 15 interesting compounds that were effective against <i>T. gondii</i> at 1 μM. Their cytotoxicity was estimated on Vero cells, and their 50% inhibitory concentrations (IC₅₀) were further calculated. As a result, eight anti-<i>Toxoplasma gondii</i> compounds with an IC₅₀ of less than 2 μM and a selectivity index (SI) value of greater than 4 were identified. The most active was MMV675968, showing an IC₅₀ of 0.02 μM and a selectivity index value equal to 275. Two other compounds, MMV689480 and MMV687807, also showed a good activity against <i>T. gondii</i>, with IC₅₀s of 0.10 μM (SI of 86.6) and 0.15 μM (SI of 11.3), respectively. Structure-activity relationships for the eight selected compounds also were discussed on the basis of fingerprinting similarity measurements using the Tanimoto method. The anti-<i>Toxoplasma gondii</i> compounds highlighted here represent potential candidates for the development of new drugs that could be used against toxoplasmosis.