1,3,4-Thiadiazoles Effectively Inhibit Proliferation of <i>Toxoplasma gondii</i>

Congenital and acquired toxoplasmosis caused by the food- and water-born parasite <i>Toxoplasma gondii</i> (<i>T. gondii</i>) is one of the most prevalent zoonotic infection of global importance. <i>T. gondii</i> is an obligate intracellular parasite with limited capacity for extracellular survival, thus a successful, efficient and robust host cell invasion process is crucial for its survival, proliferation and transmission. In this study, we screened a series of novel 1,3,4-thiadiazole-2-halophenylamines functionalized at the C5 position with the imidazole ring (<b>1b</b>–<b>12b</b>) for their effects on <i>T. gondii</i> host cell invasion and proliferation. To achieve this goal, these compounds were initially subjected to in vitro assays to assess their cytotoxicity on human fibroblasts and then antiparasitic efficacy. Results showed that all of them compare favorably to control drugs sulfadiazine and trimethoprim in terms of <i>T. gondii</i> growth inhibition (IC₅₀) and selectivity toward the parasite, expressed as selectivity index (SI). Subsequently, the most potent of them with <i>meta</i>-fluoro <b>2b</b>, meta-chloro <b>5b</b>, meta-bromo <b>8b</b>, meta-iodo <b>11b</b> and para-iodo <b>12b</b> substitution were tested for their efficacy in inhibition of tachyzoites invasion and subsequent proliferation by direct action on established intracellular infection. All the compounds significantly inhibited the parasite invasion and intracellular proliferation via direct action on both tachyzoites and parasitophorous vacuoles formation. The most effective was <i>para</i>-iodo derivative <b>12b</b> that caused reduction in the percentage of infected host cells by 44% and number of tachyzoites per vacuole by 93% compared to non-treated host cells. Collectively, these studies indicate that 1,3,4-thiadiazoles <b>1b</b>–<b>12b</b>, especially <b>12b</b> with IC₅₀ of 4.70 µg/mL and SI of 20.89, could be considered as early hit compounds for future design and synthesis of anti-<i>Toxoplasma</i> agents that effectively and selectively block the invasion and subsequent proliferation of <i>T. gondii</i> into host cells.