Design-Oriented Synthesis and Biological Evaluation of Melanostatin Neuropeptide Derivatives with Improved Pharmacokinetic Profiles

Melanostatin (MIF-1) is a short endogenous neuropeptide that acts as a highly selective positive allosteric modulator (PAM) of human dopamine D₂ receptors (hD₂R), exhibiting significant clinical potential for the treatment of dopamine-related central nervous system (CNS) disorders, including depression, drug addiction, obesity, restless legs syndrome, tardive dyskinesia, and Parkinson’s disease. Preliminary studies have shown that substitution of the L-proline residue with L-pipecolic acid generates bioactive derivatives, highlighting the importance of this residue for PAM activity.

In this work, we further explored the role of the L-proline residue by fusing the structural features of L-proline and L-pipecolic acid into a bridged chimera scaffold, (1R,3S,4S)-2-azanorbornane-3-carboxylic acid, and designed a series of 16 bridged melanostatin derivatives to enhance PAM activity and improve pharmacokinetic properties. A concise stereoselective synthetic route afforded the target compounds in high overall yields, with absolute stereochemistry confirmed by X-ray crystallography.

Functional assays at hD₂R identified four compounds exhibiting potent PAM activity at 0.01 nM, producing a 5- to 6.6-fold increase in dopamine potency. Toxicological profiling in HepG2 and differentiated SH-SY5Y neuronal cells revealed no hepatotoxicity (up to 100 µM) and generally favorable neurotoxicological profiles (up to 200 µM). Permeability studies demonstrated negligible P-glycoprotein interaction and favorable Caco-2 transport, suggesting improved CNS penetration relative to the parent neuropeptide.

Collectively, these findings establish 2-azanorbornane as a privileged scaffold for the development of potent, selective, and brain-penetrant PAMs targeting hD₂R signaling. This work provides a strong basis for the discovery of novel therapeutics for dopamine-related neurological disorders, combining enhanced pharmacokinetic properties, safety, and high PAM efficacy.