Discovery of first-in-class melanostatin-based ago-allosteric modulators of the dopamine D₂ receptors

Melanostatin (MIF-1) is an endogenous tripeptide that acts as a positive allosteric modulator (PAM) of the dopamine D₂ receptors (D₂R), making it a valuable pharmacological lead for the treatment of dopamine-related disorders of the central nervous system (CNS). In this study, a novel series of MIF-1 derivatives was synthesized and pharmacologically evaluated to investigate the influence of ring flexibility and stereochemistry on PAM activity. For this purpose, the alicyclic β-amino acids (1R,2S)-2-aminocyclopentane-1-carboxylic acid (cispentacin) and its unsaturated precursor, (1R,2S)-2-aminocyclopent-3-ene-1-carboxylic acid, were used as proline surrogates. The results obtained showed that cispentacin-based derivatives 8b and 9b displayed PAM activity at 10 pM, significantly reducing the EC₅₀ of dopamine by 4.34- and 4.22-fold, respectively (8b: EC₅₀ = 20.08 ± 3.95 nM; 9b: EC₅₀ = 20.61 ± 4.60 nM). At 1 nM, 8b further decreased the EC₅₀ of dopamine by 9.20-fold (EC₅₀ = 9.47 ± 5.02 nM) and, at higher concentrations (10 and 100 μM), it activated the D₂R in the absence of dopamine, representing the first-in-class MIF-1-based ago-allosteric modulator of the D₂R. Cytotoxicity assays in human dopaminergic-differentiated SH-SY5Y cells showed that neither 8b nor 9b exhibited cytotoxic effects at 100 μM, as assessed by both the MTT reduction and neutral red uptake assays, supporting their favorable toxicological profiles. Overall, these findings demonstrate that a fine-tuned interplay between ring flexibility and stereochemistry is a key determinant in generating effective PAM of the D₂R, providing a framework for the future design of MIF-1-based modulators targeting Parkinson's disease and other dopamine-related CNS disorders.