Evaluating the immunomodulatory and antileishmanial effects of thymol-meglumine antimoniate combination against Leishmania major: Computational and experimental analyses.

Cutaneous leishmaniasis (CL) remains a major health challenge due to drug resistance, treatment toxicity, and lack of effective vaccines. This study investigated the antileishmanial and immunomodulatory activities of thymol, alone and in combination with meglumine antimoniate (MA), against Leishmania major using in silico and in vitro approaches. Molecular docking indicated strong binding of thymol to IFN-γ and IL-12p40, suggesting immunostimulatory potential. In vitro assays showed dose-dependent inhibition of parasite viability. For promastigotes, IC50 values were 378.3 ± 2.6 µg/mL (thymol) and 350.7 ± 1.7 µg/mL (MA), while the combination reduced IC50 to 292.0 ± 3.2 µg/mL. For intracellular amastigotes, IC50 values were 45.2 ± 2.1 µg/mL (thymol) and 51.9 ± 1.4 µg/mL (MA), with the combination achieving 33.2 ± 2.6 µg/mL. Cytotoxicity assays showed lower host toxicity for thymol (CC50 = 633.0 ± 2.7 µg/mL) than MA (582.9 ± 3.4 µg/mL); the combination had CC50 = 541.8 ± 2.8 µg/mL. Selectivity indices were 13.9, 11.2, and 16.3 for thymol, MA, and the combination, respectively, indicating improved parasite selectivity. Although combination index analysis revealed mild antagonism at the 1:1 ratio tested (CI > 1), effective doses decreased. Gene expression analysis demonstrated significant upregulation of Th1 cytokines (IL-12p40, IFN-γ) and downregulation of Th2 cytokines (IL-4, IL-10), reflecting a shift toward protective cellular immunity. In conclusion, thymol exhibits direct leishmanicidal and immunomodulatory effects with acceptable safety. Optimizing dosing ratios may enhance therapeutic outcomes. These findings highlight the potential of thymol as an adjunct therapy and support further studies on optimized formulations and in vivo efficacy.