Discovery of Potent Melanostatin-Based Positive Allosteric Modulators of the Dopamine D₂ Receptors Using Heteroaromatic Scaffolds

Introduction: Parkinson’s disease (PD), the most common ataxia in the world, occurs as a consequence of the loss of dopaminergic neurons. First-line treatment for PD is focused on dopamine (DA) potentiation, with the administration of levodopa, a DA precursor, and co-administration of inhibitors of the catechol-O-methyl transferase and monoamine oxidase B enzymes. However, prolonged use leads to significant health complications, highlighting the need for alternative pharmacological strategies. Melanostatin (MIF-1), an endogenous hypothalamic neuropeptide, acts as a positive allosteric modulator (PAM) of the dopamine D₂ receptors (D₂R). By enhancing these receptor's affinity for DA, MIF-1 enables their activation at lower DA levels, making it a promising candidate for the treatment of dopamine-related disorders such as PD.

Methodology: In previous studies, our research group developed promising MIF-1 mimetics by bioisosteric substitution of L-Proline (Pro) residue by 2-furoic acid (2-Fu). To study the influence of the position of the oxygen atom in the furanyl motif on PAM activity, eight novel MIF-1 analogs were synthesized by incorporation of 3-furoic acid (3-Fu) as a 2-Fu surrogate, using a previously established protocol. Preliminary pharmacological functional assays were conducted in Chinese hamster ovary cells transfected with human D₂R to assess cyclic adenosine monophosphate (cAMP) mobilization.

Results: Peptidomimetics 4a and 6a statistically increased dopamine potency (EC₅₀ = 0.53 ± 0.17 μM), with EC₅₀ values of 0.28 ± 0.10 and 0.13 ± 0.07 μM, respectively, at 0.01 nM, being comparable with the parent neuropeptide (EC50 = 0.17 ± 0.07 μM). Both peptidomimetics also displayed no agonism effect in the absence of dopamine, confirming their activity as PAMs.

Conclusions: Overall, the remarkable pharmacological profiles of these peptidomimetics validate 3-Fu as a promising Pro bioisostere, paving the way for the rational development of novel PAMs for application in PD.

Acknowledgments: This work received support and help from FCT/MCTES (2023.14440.PEX DOI: 10.54499/2023.14440.PEX, 2022.01175.PTDC DOI: 10.54499/2022.01175.PTDC, LA/P/0008/2020 DOI 10.54499/LA/P/0008/2020, UIDP/50006/2020 DOI 10.54499/UIDP/50006/2020, and UIDB/50006/2020 DOI 10.54499/UIDB/50006/2020), through national funds. I.E.S.-D. thanks FCT for funding through the Individual Call to Scientific Employment Stimulus with Grant 2020.02311.CEECIND/CP1596/CT0004 (DOI: 10.54499/2020.02311.CEECIND/CP1596/CT0004). B.L.P.-L., X.C.C., and H.F.C.-A. thank FCT for the Ph.D. grants 2022.14060.BD, 2024.02245.BD, and UI/BD/154888/2023, respectively. X.G.-M. thanks Xunta de Galicia for financial funding with reference GPC2020/GI1597.