The present study aimed to design, synthesize, and evaluate a new series of thieno[3,2-d]pyrimidin-4(3H)-one-based Schiff bases as potential antileishmanial agents against Leishmania major (L. major). A series of twenty thieno[3,2-d]pyrimidine Schiff base derivatives were synthesized and characterized using FTIR, NMR, and HRMS techniques. Their antipromastigote activities were evaluated in vitro against L. major, while cytotoxic effects were assessed on HUVECs to determine selectivity indices. The most active compound was further investigated using molecular docking against several L. major proteins. Among the tested compounds, compound 12, bearing a 2-hydroxy-5-bromophenyl moiety, exhibited the most potent activity against L. major promastigotes with an IC₅₀ value of 13.7 µM, along with a favorable selectivity index (SI = 17.5), outperforming the reference drug miltefosine (IC₅₀ = 31 µM and SI = 0.2). Docking studies demonstrated that compound 12 showed the strongest binding affinity toward phosphodiesterase B1, supported by a docking score of −9.042 kcal/mol and an MM-GBSA value of −67.21 kcal/mol. This study highlights thieno[3,2-d]pyrimidin-4(3H)-one as a promising scaffold in the context of in vitro antileishmanial screening and suggests the role of ortho-phenolic substitution in enhancing activity and selectivity. Compound 12 emerges as a promising lead, warranting further optimization and biological evaluation in future studies.

Background/Objectives: Leishmaniasis is a neglected tropical disease and the toxicity, limited efficacy and emerging drug resistance of current therapies underscore the urgent need for new antileishmanial agents. This study aimed to experimentally assess the in vitro antileishmanial activity, cytotoxicity and selectivity of quinazolin-4(3H)-one derivatives against Leishmania tropica and Leishmania infantum and to computationally investigate the potential interaction of compound E9 with trypanothione reductase. Methods: The in vitro antileishmanial activities of sixteen quinazolin-4(3H)-one derivatives (E1-E16) against the promastigote forms of L. tropica and L. infantum were evaluated. Additionally, the most active compounds were subjected to an evaluation of their cytotoxicity in human umbilical vein endothelial cells (HUVECs) and their selectivity indices (SIs) were subsequently calculated. Molecular docking and molecular mechanics/generalized Born surface area (MM/GBSA) analyses were performed for compound E9 against L. infantum trypanothione reductase. Results: Compounds E2, E9 and E11 showed activity against L. tropica, with half-maximal inhibitory concentration (IC₅₀) values of 23.2, 22.4 and 76.0 μM, respectively. In contrast, compounds E2, E3, E9 and E11 were active against L. infantum, with IC₅₀ values of 38.1, 69.7, 28.2 and 86.7 μM, respectively, suggesting species-dependent activity. Furthermore, among the tested compounds, E2 showed the most favorable selectivity profile against L. tropica and L. infantum, with SI values of 8.8 and 5.4, respectively. Moreover, molecular docking of E9 produced a docking score of −8.070 kcal/mol, with MM/GBSA analysis indicating a binding free energy of −82.44 kcal/mol. Conclusions: These findings support further investigation of quinazolin-4(3H)-one derivatives as potential antileishmanial agents, with E2 emerging as the most selective compound. Furthermore, they suggest that the biological activity of this compound series may differ among Leishmania species. The molecular modeling findings suggest a potential interaction between E9 and trypanothione reductase, supporting the need for further evaluation of these compounds in intracellular amastigote models and subsequent in vivo studies.

Aims: This study aimed to evaluate the in vitro antileishmanial activity of five quinazolinone derivatives against Leishmania major (L. major) promastigotes and to determine their cytotoxicity toward human umbilical vein endothelial cells (HUVECs) and corresponding selectivity indices (SI).Methods: The quinazolinone derivatives (K1–K5) were tested against logarithmic-phase L. major promastigotes over a concentration range of 3.125–200 µM, with miltefosine used as the reference drug. Antileishmanial activity was assessed after 24 h using the cell viability detection kit-8 (CVDK-8). Cytotoxicity was evaluated in HUVECs, and half-maximal inhibitory concentration (IC50) values were determined by nonlinear regression. SI was calculated as the ratio of the HUVEC IC50 to the L. major IC50.Results: All five quinazolinone derivatives exhibited concentration-dependent antileishmanial activity against L. major, with IC50 values ranging from 29.66 to 41.11 µM. Their HUVEC IC50 values ranged from 69.73 to 91.53 µM. Miltefosine showed greater antileishmanial activity (IC50=21.54 µM) but higher cytotoxicity toward HUVECs (IC50=9.93 µM). Accordingly, K1–K5 showed SI values ranging from 2.15 to 2.38, whereas the SI of miltefosine was 0.46.Conclusion: The five quinazolinone derivatives exhibited in vitro antileishmanial activity against L. major promastigotes and higher SI values than miltefosine under the present experimental conditions. However, the SI values of 2.15–2.38 indicate modest selectivity and should be interpreted cautiously. Further evaluation in intracellular amastigote models is required to determine whether the observed antileishmanial activity is maintained in a host-cell-associated system.