Design of New Sigma1 Agonists: a Structure-Based Approach for Huntington’s Disease Treatment.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation in the HTT gene, leading to the progressive degeneration and death of neurons in specific brain regions. Despite extensive research, no effective therapies are currently available to halt or prevent the onset of this debilitating condition. In recent years, however, a growing body of evidence has highlighted the potential role of the sigma-1 receptor (σ1R) in modulating various neurodegenerative processes, including those associated with HD1. The σ1R is a small and poorly understood membrane receptor expressed in the central nervous system, whose 3D structure has been recently determined by X-ray crystallography and responding to different synthetic ligands such as (+)-pentazocine (agonist) and haloperidol (antagonist). Substantial evidence has shown that agonists have neuroprotective activity in neurodegenerative diseases. Nevertheless, the structural basis for agonism or antagonism on σ1R is largely unknown. In general, the overall conformation of the receptor bound to the agonist crystallizes similarly to that bound to the antagonist, except for a shift of about 1.8Å in the α4 helix2. Probably, this shift is responsible for the tendency of agonists to decrease the oligomeric state of the protein and can be used as a discriminator for classification into agonist. Through structure-based computational methods, we designed new Iloperidone analogues as potential σ1R agonists. Indeed, very recently, a high binding affinity for σ1R of the antipsychotic Iloperidone has been demonstrated3,4. From our computational studies, including cross-docking procedures and molecular dynamics simulations, the pharmacophoric groups have emerged. Starting from these data, we synthesized new small molecules that retained the piperidine core and replaced the benzoisoxazole ring (responsible for a generic π-π interaction) with oximes. Subsequently, we functionalized the oxygen atom of the oxime group to increase the steric hindrance between the α5 and α4 helices, thus promoting the shift of the latter. The most promising compounds were further analyzed by molecular dynamics simulations, which highlighted significant conformational differences in the α4 helix between agonist- and antagonist-bound forms. These simulations also suggested that agonists may destabilize key structural motifs involved in maintaining the trimeric (inactive) organization of the receptor, which is instead stabilized by antagonists. Finally, the newly synthesized compounds were evaluated through a series of biological assays: in vitro for their binding affinity to σ1R, in cellulo to assess their agonistic activity, and in vivo to investigate their potential neuroprotective effects. The data coming from these studies will be shown and discussed.