SIGMA 1 RECEPTOR (S1R) MODULATORS AS A THERAPEUTIC STRATEGY FOR PROMOTING NEUROPLASTICITY. DESIGN AND SYNTHESIS OF NOVEL MONO- AND BI-VALENT LIGANDS FOR SRS

During my three-years-project I focused on the development of novel mono- and bi-valent Sigma1 Receptor (S1R) modulators to address two main objectives: (i) the obtainment of multitarget-directed ligands (MTDLs) endowed with therapeutic potential for the treatment of neurodegenerative diseases; (ii) the preparation of a series of bivalent compounds to be used for the study of S1R oligomerization process. These two major topics are briefly discussed hereafter. (i) Neurodegeneration is a key event in many challenging disorders (e.g. Alzheimers diseases, Parkinsons disease, multiple sclerosis). Such pathologies involve the alteration of several molecular pathways, making the multi-target paradigm a promising strategy for new effective therapies. Among the numerous molecular targets that have been correlated with neurodegenerative disorders, S1R has gained great attention from the scientific community, and S1R agonists are considered viable pharmacological tools for their neuroprotective activity. Hence, we reasoned that coupling S1R agonism with modulation of other selected targets might afford new molecular entities more effective in counteracting neuropathies. The additional targets of our MTDLs include N-Methyl-D-Aspartate (NMDA) receptor, which plays a relevant role in synaptic plasticity, and acetylcholinesterase (AChE), which regulates acetylcholine levels in central nervous system. A structurally focused compound library was prepared through a divergent synthesis. The so-obtained compounds were tested for a preliminary biological evaluation, evaluating their affinity and selectivity towards S1R and NMDA receptor, the AChE inhibition and their antioxidant properties, since oxidative stress is considered a hallmark of neurodegeneration. A number of promising compounds, endowed with effective multitarget profile, was identified. These results will pave the way for further biological investigation and structure optimization in order to achieve viable tools for the treatment of neurodegenerative diseases. (ii) In the last decade numerous studies have supported the hypothesis that S1R can exist in multiple oligomeric forms. In detail, agonists seem to stabilize S1R monomers and dimers that act as chaperones, whereas antagonists bind to higher oligomer complexes, maintaining them in repository forms. Moreover, the recently disclosed crystal of S1R was obtained as a trimer. Nevertheless, the mechanism of generation, as well as the precise biological function of S1R oligomers, are still unknown. Accordingly, a series of homo- and hetero-bivalent S1R ligands was designed and synthetized to investigate S1R oligomerization process. Since S1R agonists are known to exert neuroprotective effects, and S1R can form homo-dimeric structures upon interaction with agonists, we reasoned that promoting dimerization through bivalent agonists might enhance ligands activity. The designed bivalent compounds consist in two units of (R)-RC-33 (a potent and selective S1R agonist developed by our group) joined by a linker. Different lengths, polarities and spatial constraints were explored for the linker. The key precursor of the synthesis is (R)-RC-33A, an aminic derivative of RC-33. For the obtainment of enantiopure (R)-RC-33A, three different synthetic approaches have been explored, resulting in the identification of an efficient pathway to access (R)-RC-33 derivatives with high yield and chiral purity. Once the designed ligands were obtained in sufficient amount and purity, they were tested in binding assays to assess their S1R affinity. Moreover, computational studies were performed on both mono- and bi-valent S1R modulators. In detail, docking into the crystals binding pocket served as basis for the development of a 3D-QSAR model and for the rationalization of experimental results. Molecular dynamics studies are ongoing, and future functional assays will contribute to shed light on the S1R oligomeric states.