Discovery of Indole–Thiourea Derivatives as Tyrosinase Inhibitors: Synthesis, Biological Evaluation, Kinetic Studies, and In Silico Analysis

Tyrosinase, a key enzyme in melanin synthesis, represents a crucial therapeutic target for hyperpigmentation disorders due to excessive melanin production. This study aimed to design and evaluate a series of indole-thiourea derivatives by conjugating thiosemicarbazones with strong tyrosinase inhibitory activity to indole. Among these derivatives, compound <b>4b</b> demonstrated tyrosinase inhibitory activity with an IC₅₀ of 5.9 ± 2.47 μM, outperforming kojic acid (IC₅₀ = 16.4 ± 3.53 μM). Kinetic studies using Lineweaver-Burk plots confirmed competitive inhibition by compound <b>4b</b>. Its favorable ADMET and drug-likeness properties make compound <b>4b</b> a promising therapeutic candidate with a reduced risk of toxicity. Molecular docking revealed that the compounds bind strongly to mushroom tyrosinase (mTYR) and human tyrosinase-related protein 1 (TYRP1), with compound <b>4b</b> showing superior binding energies of -7.0 kcal/mol (mTYR) and -6.5 kcal/mol (TYRP1), surpassing both kojic acid and tropolone. Molecular dynamics simulations demonstrated the stability of the mTYR-<b>4b</b> complex with low RMSD and RMSF and consistent Rg and SASA values. Persistent strong hydrogen bonds with mTYR, along with favorable Gibbs free energy and MM/PBSA calculations (-19.37 kcal/mol), further support stable protein-ligand interactions. Overall, compound <b>4b</b> demonstrated strong tyrosinase inhibition and favorable pharmacokinetics, highlighting its potential for treating pigmentary disorders.