Thymol's antileishmanial activity and its impact on host cytokine profiles: In vitro and ex vivo studies on Leishmania tropica.

Cutaneous leishmaniasis (CL) is a neglected tropical disease associated with significant morbidity, primarily due to chronic skin lesions, scarring, and psychosocial consequences. This study aimed to investigate the in vitro and ex vivo antileishmanial effects of thymol (1-500 μM) against Leishmania tropica (MHOM/TR/2012/CBCL-LT) infection. Thymol's in vitro efficacy was assessed on both promastigote (Haemocytometry and CellTiter-Glo assays) and amastigote (Giemsa staining and Parasite Rescue Transformation Assay) forms of L. tropica. Additionally, its immunomodulatory effects were evaluated by analyzing cytokine secretion (IFN-γ, IL-12, IL-10, and IL-4) and infectivity in THP-1 macrophages using ELISA. Cytotoxicity was determined by calculating the 50 % cytotoxic concentration (CC₅₀) in THP-1 cells. The in vitro inhibitory concentration (IC₅₀) value against L. tropica promastigotes was determined as 79.41 μM, while the ex vivo IC₅₀ value against amastigotes was 105.2 μM. Incubation of infected macrophages with thymol resulted in a dose-dependent increase in IFN-γ and IL-12 levels, along with a significant reduction in IL-10 and IL-4 secretion (p < 0.05). The CC₅₀ value of thymol in THP-1 cells was 160.7 μM, indicating low cytotoxicity. Moreover, the selectivity index (SI) values greater than 1 confirmed the compound's preferential action against amastigotes while exhibiting minimal toxicity toward macrophages. These findings highlight thymol's potential as an antileishmanial agent by effectively eliminating and controlling Leishmania parasites in both in vitro and ex vivo models. Due to its immunomodulatory properties and low cytotoxicity, thymol represents a promising starting point for the development of novel antileishmanial agents and alternative therapeutic strategies against CL caused by L. tropica.