Glabridin and Glabrene from Glycyrrhiza glabra L. as Natural Co-operative Modulators of the Human Serotonin Transporter: Evidence from Docking, Molecular Simulations and Machine-Learning QSAR Models.

Serotonin transporter (SERT), encoded by SLC6A4, is a key regulator of serotonergic neurotransmission and a primary target for selective serotonin reuptake inhibitors (SSRIs). Dysregulation of SERT is implicated in neuropsychiatric and neurodevelopmental disorders, including autism spectrum disorder (ASD). SSRIs, while widely used, have limitations such as adverse effects, drug-drug interactions, and interindividual variability, necessitating safer and mechanistically distinct SERT modulators. This study utilized an integrative in- silico approach to evaluate the SERT inhibitory potential of glabridin, glabrene, and liquiritigenin the phytoestrogens from Glycyrrhiza glabra (licorice), known for neuroprotective properties. Blind and site-specific molecular docking, molecular dynamics simulations, binding free energy calculations, ADMET profiling, and machine learning-based QSAR modeling were employed to characterize ligand-transporter interactions. The compounds glabridin and glabrene exhibited favourable binding affinity for the central S1 substrate-binding site of human SERT, with a comparable or superior binding free energy (-50.35 kcal/mol and -52.41 kcal/mol, respectively) to the reference SSRI paroxetine (-53.04 kcal/mol). The 100 ns MD simulations confirmed stable ligand occupancy and persistent interactions with key residues involved in extracellular gate regulation, stabilizing the SERT outward-open inhibitory conformation. ADMET predictions indicated favorable pharmacokinetic properties, including blood-brain barrier permeability and acceptable toxicity profiles of glabridin and glabrene. QSAR-derived IC50 estimates and combination modeling revealed nonantagonistic, mild positive cooperativity between glabridin and glabrene, supported by dual-ligand docking and MM-GBSA analyses, suggesting complementary orthosteric engagement. These findings position glabridin and glabrene as putative natural SERT modulators, providing a mechanistic basis for their potential use in serotonergic modulation therapeutics, warranting further experimental validation.