Discovery of Potent Positive Allosteric Modulators of the D₂ Receptors through Conjugation of Melanostatin Neuropeptide with Amantadine

Introduction: Melanostatin is a short neuropeptide with the amino acid sequence Pro-Leu-Gly-NH₂ that acts as a positive allosteric modulator (PAM) of the dopamine D₂ receptors (D₂R) in the central nervous system (CNS). Its therapeutic potential for dopamine-related disorders, such as Parkinson's disease, is hampered by its poor gastrointestinal absorption and susceptibility to CNS proteases due to its intrinsic peptide nature. This project aims to overcome these limitations by developing novel melanostatin-based derivatives using a "lipophilic bullet" approach to enhance lipophilicity and PAM potency.

Methodology: Adamantane-1-amine (Am) was selected for conjugation via an amide bond at the C-terminal of melanostatin. The conjugates were synthesised in solution-phase, starting with the coupling between Am and Boc-Gly-OSu, followed by linear C-to-N synthesis using classical reactions in peptide chemistry. Functional assays were conducted in Chinese hamster ovary cells expressing human D₂R. The PAM activity of the conjugates was evaluated in terms of potency (EC₅₀) and efficacy (E_max) by measuring cAMP mobilization in response to D₂R dopamine-mediated activation using quantification by homogeneous time-resolved fluorescence.

Results: Conjugates 1 and 2 exhibited a statistically significant increase in dopamine potency (EC₅₀ = 87.08 ± 24.87 nM), with EC₅₀ values of 26.39 ± 3.37 and 17.82 ± 4.24 nM, respectively, at 0.01 nM, comparing favorably to the parent neuropeptide. Both conjugates showed no agonistic effect in the absence of dopamine, confirming their PAM mechanism. Using the shake-flask method, conjugates also showed and increased log P_o/w when compared to melanostatin.

Conclusions: These findings suggest that conjugation with adamantane-1-amine effectively enhances the therapeutic potential of melanostatin, offering a promising strategy for treating dopamine-related disorders.