Discovery of bridged melanostatin derivatives as potent allosteric modulators of the dopamine D2 receptors with improved permeability

Melanostatin (MIF-1) is an endogenous positive allosteric modulator (PAM) of dopamine D 2 receptors (D 2 R) 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 (1 R ,3 S ,4 S )-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 D 2 R 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 D 2 R PAM ligands with favorable pharmacokinetic and safety profiles.