The present work reports on the synthesis, anti-Trypanosoma cruzi activities and docking studies of a novel series of 2-(pyridin-2-yl)-1,3-thiazoles derived from 2-pyridine thiosemicarbazone. The majority of these compounds are potent cruzain inhibitors and showed excellent inhibition on the trypomastigote form of the parasite, and the resulting structure-activity relationships are discussed. Together, these data present a novel series of thiazolyl hydrazones with potential effects against Chagas disease and they could be important leads in continuing development against Chagas disease.

gThe pressing need for better drugs against Chagas disease, African sleeping sickness, and schistosomiasis motivates the search for inhibitors of cruzain, rhodesain, and Schistosoma mansoni CB1 (SmCB1), the major cysteine proteases from Trypanosoma cruzi, Trypanosoma brucei, and S. mansoni, respectively. Thiosemicarbazones and heterocyclic analogues have been shown to be both antitrypanocidal and inhibitory against parasite cysteine proteases. A series of compounds was synthesized and evaluated against cruzain, rhodesain, and SmCB1 through biochemical assays to determine their potency and structure-activity relationships (SAR). This approach led to the discovery of 6 rhodesain, 4 cruzain, and 5 SmCB1 inhibitors with 50% inhibitory concentrations (IC 50 s) of <10 M. Among the compounds tested, the thiosemicarbazone derivative of peracetylated galactoside (compound 4i) was discovered to be a potent rhodesain inhibitor (IC 50 ‫؍‬ 1.2 ؎ 1.0 M). The impact of a range of modifications was determined; removal of thiosemicarbazone or its replacement by semicarbazone resulted in virtually inactive compounds, and modifications in the sugar also diminished potency. Compounds were also evaluated in vitro against the parasites T. cruzi, T. brucei, and S. mansoni, revealing active compounds among this series. N ew drugs for parasitic diseases are urgently needed, but these globally important infections are often "neglected" because they most commonly afflict poor and marginalized communities. Current therapies are limited by poor efficacy, toxicity, high costs, and parasite resistance. Chagas disease, African sleeping sickness, and schistosomiasis are examples of diseases for which new therapies are needed (1, 2). Among the most studied and exploited molecular targets for these diseases are cysteine proteases. These enzymes have essential roles in parasite nutrition, immune evasion, host cell invasion, and metacyclogenesis (3-6). Indeed, the cysteine proteases cruzain, rhodesain, and Schistosoma mansoni CB1 (SmCB1) from Trypanosoma cruzi, Trypanosoma brucei, and S. mansoni, respectively, are validated molecular targets and have been the subject of numerous medicinal chemistry projects (7-17) that have yielded trypanocidal inhibitors, both in parasite culture and in animal models of infection (13,15,(18)(19)(20)(21).The diverse inhibitors of these enzymes comprise compound classes which bind noncovalently (11, 12) and scaffolds containing a "warhead" that binds covalently to the catalytic cysteine. Within the latter category, vinylsulfones (8,(22)(23)(24)(25), oxy-methyl ketones (7, 26), nitriles (16), epoxides, and thiosemicarbazones (13-15, 27-29) have been described previously. Thiosemicarbazones present as advantages their low molecular weight, low cost of synthesis, and nonpeptidic nature (27). Greenbaum and coworkers synthesized and evaluated the cysteine protease inhibitory and antiparasitic activities of a library of thiosemicarbazones, with promising results (13). According to those authors, the thiosemicarbazones are...

Pharmacological treatment of Chagas disease is based on benznidazole, which displays poor efficacy when administered during the chronic phase of infection. Therefore, the development of new therapeutic options is needed. This study reports on the structural design and synthesis of a new class of anti-Trypanosoma cruzi thiazolidinones (4 a-p). (2-[2-Phenoxy-1-(4-bromophenyl)ethylidene)hydrazono]-5-ethylthiazolidin-4-one (4 h) and (2-[2-phenoxy-1-(4-phenylphenyl)ethylidene)hydrazono]-5-ethylthiazolidin-4-one (4 l) were the most potent compounds, resulting in reduced epimastigote proliferation and were toxic for trypomastigotes at concentrations below 10 μM, while they did not display host cell toxicity up to 200 μM. Thiazolidinone 4 h was able to reduce the in vitro parasite burden and the blood parasitemia in mice with similar potency to benznidazole. More importantly, T. cruzi infection reduction was achieved without exhibiting mouse toxicity. Regarding the molecular mechanism of action, these thiazolidinones did not inhibit cruzain activity, which is the major trypanosomal protease. However, investigating the cellular mechanism of action, thiazolidinones altered Golgi complex and endoplasmic reticulum (ER) morphology, produced atypical cytosolic vacuoles, as well as induced necrotic parasite death. This structural design employed for the new anti-T. cruzi thiazolidinones (4 a-p) led to the identification of compounds with enhanced potency and selectivity compared to first-generation thiazolidinones. These compounds did not inhibit cruzain activity, but exhibited strong antiparasitic activity by acting as parasiticidal agents and inducing a necrotic parasite cell death.