<i>In Vitro</i> Leishmanicidal Effect of Silibinin: Disrupting Redox Balance via Trypanothione Reductase Inhibition

Visceral leishmaniasis, caused by the parasite <i>Leishmania infantum</i>, is a life-threatening disease with limited therapeutic options that are often associated with toxicity and resistance. In this study, we investigated the <i>in vitro</i> leishmanicidal effects of silibinin, a key flavonolignan from <i>Silybum marianum</i>, against both the promastigote and intracellular amastigote forms of <i>L. infantum</i>. Mechanistically, silibinin inhibits trypanothione reductase (TR), disrupting the redox balance in the parasite and causing cell death. Silibinin concentration-dependently inhibited promastigote proliferation (IC₅₀ of 416.7 μM) with potent activity against intracellular amastigotes (EC₅₀ of 0.7 μM) and a high selectivity index (242), indicating its strong therapeutic potential and nontoxicity to macrophages. Importantly, silibinin disrupts the <i>L. infantum</i> redox balance by inhibiting TR activity, which increases the reactive oxygen species (ROS) levels to kill the parasite. ROS accumulation and the inhibition of parasite proliferation were significantly correlated (Pearson correlation coefficient of 0.9895). Molecular docking confirmed that silibinin binds to the catalytic site of TR, corroborating its role in ROS-mediated parasite death. Furthermore, <i>in silico</i> ADMET analysis revealed that silibinin has favorable pharmacokinetic properties for oral administration. Finally, <i>in vitro</i> and <i>in silico</i> studies indicated that silibinin inhibits TR, a key enzyme in <i>Leishmania</i> redox homeostasis, to exert antileishmanial effects.