Discovery of bridged melanostatin derivatives as potent allosteric modulators of the dopamine D<sub>2</sub> receptors with improved permeability.

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Melanostatin (MIF-1) is an endogenous positive allosteric modulator (PAM) of dopamine D2 receptors (D2R) with recognized therapeutic potential in central nervous system (CNS) disorders associated with dopaminergic dysfunction, particularly Parkinson's disease. However, the therapeutic development of MIF-1 has been limited by poor pharmacokinetic properties. Building on previous studies of conformationally constrained MIF-1 analogs, the present work describes the design, synthesis, and comprehensive functional, toxicological, and permeability evaluation of new bridged derivatives incorporating the enantiopure scaffold (1R,3S,4S)-2-azanorbornane-3-carboxylic acid as a hybrid prolyl/pipecolyl surrogate. The target MIF-1 derivatives were synthesized through a concise and stereoselective route, and their absolute configuration was unequivocally established by X-ray crystallography. Pharmacological assays at human D2R identified four active derivatives (7b, 8d, 9c, and 9d) that potentiated dopamine responses by 5.3- to 6.6-fold at concentrations as low as 10 pM, consistent with PAM-like activity. Toxicological profiling in HepG2 (100 µM) and differentiated SH-SY5Y neuronal cells (up to 200 µM) confirmed that these compounds are well tolerated at supra-pharmacological concentrations, except for 9c. Additionally, these compounds exhibited reduced P-gp-mediated efflux and improved Caco-2 permeability compared to MIF-1. Overall, these findings identify 2-azanorbornane as a promising scaffold for the development of D2R PAM ligands with favorable pharmacokinetic and safety profiles.

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