Proline Homologation in Melanostatin Neuropeptide: Discovery of Potent Modulators of the Dopamine D ₂ Receptors

Melanostatin (MIF-1) is a naturally occurring neuropeptide acting as a positive allosteric modulator (PAM) of dopamine D₂ receptors (D₂R), underscoring its potential for therapeutic use in central nervous system disorders associated with dopaminergic dysregulation, including depression, drug addiction, restless legs syndrome, tardive dyskinesia, and Parkinson's disease. In this work, a new series of MIF-1 analogs using l-pipecolic acid as an l-proline surrogate was synthesized and pharmacologically evaluated by functional assays at the D₂R. In this series, methyl l-pipecolyl-l-leucylglycinate (<b>9</b>) was found to exhibit superior performance compared to MIF-1 by promoting a 4.1- and 4.2-fold increase of dopamine potency at 0.01 and 1 nM, respectively. <i>In silico</i> conformational studies demonstrate that <b>9</b> preferentially adopts a γ-turn, corroborating that neither the C-terminal carboxamide nor the postulated type II β-turn conformation is required for PAM activity. Toxicological assays in human dopaminergic SH-SY5Y neuronal cells show that this compound exhibits no significant toxicity up to 100 μM in the MTT reduction and neutral red uptake assays.