Synthesis and Structure—Activity (Anxiolytic) Relationship Analysis of Leucyltryptophan Ligands of 18-KDA Translocator Protein

Previously, researchers at Zakusov State Institute of Pharmacology used an original strategy for designing pharmacologically active dipeptides based on non-peptide drug structures to prepare the dipeptide ligands of 18-kDa translocator protein (TSPO) N -carbobenzoxy-L-tryptophanyl-L-isoleucine amide (GD-23) and N -phenylpropionyl-L-tryptophanyl-L-leucine amide (GD-102) that showed anxiolytic activity in standard behavioral tests. The present work reports GD-102 analogs with the reverse amino-acid sequence, i.e., N -phenylpropionyl- L -leucyl- L -tryptophan methylamide (GD-140), N -phenylpropionyl- L -leucyl- L -tryptophan (GD-139), N -phenylacetyl- L -leucyl- L -tryptophan amide (GD-141), N -phenylpropionyl- L -leucyl- L -tryptophan methyl ester (GD-138), and N -phenylpropionyl- L -leucyl- L -tryptophan amide (GD-136). Open-field (OF) tests of mice after i.p. administration showed that the newly synthesized dipeptides were less active than GD-102 (0.01 – 0.1 mg/kg). Dipeptide GD-140 showed anxiolytic activity at doses of 0.1, 0.5, and 1 mg/kg; GD-139, 0.5 and 5.0; GD-141, 1 and 5. Dipeptide GD-138 at a dose of 0.1 mg/kg reduced locomotor activity of animals in the OF test. GD-136 was inactive in the dose range 0.1 – 5 mg/kg. Molecular docking studies demonstrated that the studied compounds could be placed at the TSPO binding site of ligand PK 11195 (PDB ID: 2MGY). Also, π-stacking of the phenyl groups of dipeptides GD-136, GD-139, GD-140, and GD-141 with Trp107 of the receptor was revealed. For dipeptides GD-140 and GD-102, π-stacking with Trp95 was also detected. GD-138 did not exhibit π-stacking with either Trp107 or Trp95. It was concluded that the key interaction affecting the manifestation of anxiolytic activity was π-stacking of the dipeptide ligand with Trp95 of the receptor. The Trp-Leu sequence was preferred over the Leu-Trp sequence for the TSPO dipeptide ligands.