Synthesis and Structure–Activity Relationship of Thiourea Derivatives Against Leishmania amazonensis

<b>Background:</b> Leishmaniasis, caused by <i>Leishmania</i> protozoa and transmitted by vectors, presents varied clinical manifestations based on parasite species and host immunity. The lack of effective vaccines or treatments has prompted research into new therapies, including thiourea derivatives, which have demonstrated antiprotozoal activities. <b>Methods:</b> We synthesized two series of <i>N</i>,<i>N</i>'-disubstituted thiourea derivatives through the reaction of isothiocyanates with amines. These compounds were evaluated in vitro against promastigote and amastigote forms of <i>L. amazonensis</i>, alongside cytotoxicity assessments on macrophages. In silico studies were conducted to analyze structure-activity relationships (SARs) and drug-likeness. <b>Results:</b> A total of fifty thiourea derivatives were synthesized and tested. Compound <b>3e</b> from the first generation exhibited significant anti-leishmanial activity with an IC₅₀ of 4.9 ± 1.2 µM and over 80-fold selectivity compared to that of miltefosine (IC₅₀ = 7.5 ± 1.2 µM). The introduction of a piperazine ring in the second-generation thioureas enhanced potency and selectivity, with compound <b>5i</b> achieving an IC₅₀ of 1.8 ± 0.5 µM and a selectivity index of approximately 70. Pharmacokinetic predictions indicated favorable profiles for the active compounds. <b>Conclusions:</b> SAR and ADMET analyses identified compound <b>5i</b> as the most promising candidate for further preclinical evaluation, suggesting that piperazine thiourea derivatives represent a novel class of anti-leishmanial agents.