Employing a simple synthetic protocol, a series of highly effective halogen-substituted imidazole-thiosemicarbazides with anti-Toxoplasma gondii effects against the RH tachyzoites, much better than sulfadiazine, were obtained (IC50s 10.30—113.45 µg/mL vs. ~2721.45 µg/mL). The most potent of them, 12, 13, and 15, blocked the in vitro proliferation of T. gondii more potently than trimethoprim (IC50 12.13 µg/mL), as well. The results of lipophilicity studies collectively suggest that logP would be a rate-limiting factor for the anti-Toxoplasma activity of this class of compounds.

One of the key stages in the development of new therapies in the treatment of toxoplasmosis is the identification of new non-toxic small molecules with high specificity to Toxoplasma gondii. In the search for such structures, thiosemicarbazide-based compounds have emerged as a novel and promising leads. Here, a series of imidazole-thiosemicarbazides with suitable properties for CNS penetration was evaluated to determine the structural requirements needed for potent anti-Toxoplasma gondii activity. The best 4-arylthiosemicarbazides 3 and 4 showed much higher potency when compared to sulfadiazine at concentrations that are non-toxic to the host cells, indicating a high selectivity of their anti-toxoplasma activity.

dThis article reports anti-Toxoplasma gondii activity of 3-(thiophen-2-yl)-1,2,4-triazole-5-thione. The compound displayed significant and reproducible antiparasitic effects at nontoxic concentrations for the host cells, with an experimentally determined 50% inhibitory concentration (IC 50 ) at least 30 times better than that of the known chemotherapeutic agent sulfadiazine. Purine nucleoside phosphorylase was defined as the probable target for anti-Toxoplasma activity of the tested compound. These results provide the foundation for future work to develop a new class of medicines to better treat toxoplasmosis. Standard chemotherapy for the treatment of Toxoplasma infections relies on the inhibition of folate metabolism. The protocol recommends synergistic combination of diaminopyrimidines with sulfonamides, supplemented with folinic acid to mitigate the toxic effects of pyrimethamine on bone marrow. For patients with sensitivity to sulfonamines, macrolides and lincosamides are a second class of medications with anti-Toxoplasma activity. The third class of anti-Toxoplasma drugs, which are only occasionally used as a potential substitute, comprises electron transport inhibitors such as atovaquone (1). In all of these situations, drug resistance, high cost, limited efficacy, and side effects of these drugs often result in discontinuation of therapy (2-5). Therefore, new agents with better activity profiles and that are less expensive are needed. One possible class of drugs are s-triazole derivatives, and in this article, we present a newly found triazole-based candidate to develop novel medicines for more effective treatment of toxoplasmosis.The search for agents that are potent and selective against Toxoplasma continues in several laboratories. Numerous inhibitors with activities in the nanomolar range with no appreciable in vitro toxicity to human cells have been identified. Examples are pyrimidines, oryzalines, thiazolidinones, berberines, tryptanthrines, thiocyanates, and bisphosphonates (6, 7). Our attention has been focused on the role of s-triazole series as potential new toxoplasmosis therapeutics. We found that 3-(thiophen-2-yl)-1,2,4-triazole-5-thione (compound 1) showed a potent and reproducible antiparasitic effect with no appreciable toxicity to human cells, while 4-ethyl-3-(4-methyl-1,2,3-thiadiazol-5-yl)-1,2,4-triazole-5-thione (compound 2) was inactive.The procedure for synthesis of 3-(thiophen-2-yl)-1,2,4-triazole-5-thione (compound 1) and 4-ethyl-3-(4-methyl-1,2,3-thiadiazol-5-yl)-1,2,4-triazole-5-thione (compound 2), the effects of tested compounds and sulfadiazine on the viability of L929 (ATCC no. CCL-1) and HeLa (ATCC no. CCL-2) cells, and inhibition of Toxoplasma (RH strain; ATCC no. 50174) growth were described elsewhere (8). The effect of tested compounds on the intensity of Toxoplasma gondii proliferation (%) was measured by 2 methods: incorporation of [ 3 H]uracil (Fig. 1) into the T. gondii DNA (9) and quantitative real-time PCR (qRT-PCR) (Fig. 1) (8). The 50% inhibitory concentration (I...

Congenital and acquired toxoplasmosis caused by the food- and water-born parasite Toxoplasma gondii (T. gondii) is one of the most prevalent zoonotic infection of global importance. T. gondii 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 (1b–12b) for their effects on T. gondii 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 T. gondii growth inhibition (IC50) and selectivity toward the parasite, expressed as selectivity index (SI). Subsequently, the most potent of them with meta-fluoro 2b, meta-chloro 5b, meta-bromo 8b, meta-iodo 11b and para-iodo 12b 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 para-iodo derivative 12b 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 1b–12b, especially 12b 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-Toxoplasma agents that effectively and selectively block the invasion and subsequent proliferation of T. gondii into host cells.

We report herein anti-proliferation effects of 4-arylthiosemicarbazides, with a cyclopentane substitution at N1 position, on highly virulent RH strain of Toxoplasma gondii. Among them, the highest in vitro anti-Toxoplasma activity was found with the meta-iodo derivative. Further experiments demonstrated inhibitory effects of thiosemicarbazides on tyrosinase (Tyr) activity, and good correlation was found between percentage of Tyr inhibition and IC 50Tg . To confirm the concept that thiosemicarbazides are able to disrupt tyrosine metabolism in Toxoplasma tachyzoites, the most potent Tyr inhibitors were tested for their efficacy of T. gondii growth inhibition. All of them significantly reduced the number of tachyzoites in the parasitophorous vacuoles (PVs) compared to untreated cells, as well as inhibited tachyzoites growth by impeding cell division. Collectively, these results indicate that compounds with the thiosemicarbazide scaffold are able to disrupt tyrosine metabolism in Toxoplasma tachyzoites by deregulation of their crucial enzyme tyrosine hydroxylase (TyrH).

A safer treatment for toxoplasmosis would be achieved by improving the selectivity profile of novel chemotherapeutics compared to the standard therapy pyrimethamine (PYR) and sulfadiazine (SDZ). We previously reported on the identification of the compounds with imidazole-thiosemicarbazide scaffold as potent and selective anti- Toxoplasma gondii ( T. gondii) agents. In our current research, we report on the optimisation of this chemical scaffold leading to the discovery cyclic analogue 20 b with s -triazole core structure. This compound displayed prominent CC 30 to IC 50 selectivity index (SI) of 70.72, making it 160-fold more selective than SDZ, 11-fold more selective than PYR, and 4-fold more selective than trimethoprim (TRI). Additionally, this compound possesses prerequisite drug-like anti- Toxoplasma properties to advance into preclinical development; it showed ability to cross the BBB, did not induce genotoxic and haemolytic changes in human cells, and as well as it was characterised by low cellular toxicity.