Naturally Inspired Privileged Structures in Drug Discovery: Multifunctional Compounds for Alzheimer's Disease Treatment

Polypharmacology-based strategies are gaining ever-increasing attention as useful approaches to develop disease-modifying drug candidates for effective Alzheimer’s disease (AD) treatment. In this scenario, multitarget-directed ligands could increase efficiency by simultaneous modulation of several targets involved in AD pathogenesis. In drug discovery, natural products (NPs) represent an excellent source of evolutionary-chosen “privileged structures”. In this thesis, the polyphenol curcumin, found in Curcuma longa L, encompassing the essential structural elements for the concurrent inhibition of two validated AD targets, BACE-1 and GSK-3β, was rationally identified as lead compound. Aimed at developing well-balanced dual BACE-1/GSK-3β modulators with good BBB permeability, different series of curcumin-based derivatives were designed and synthetized by introducing suitable chemical modifications on the side aryl ring(s) and in the 4-position of the main scaffold. Furthermore, considering the pivotal role of the intramolecular H-bond network of curcumin’s central fragment in establishing appropriate interactions with target binding sites, several complexation and bioisosteric cyclization strategies were performed. Thanks to its strong Michael acceptor reactivity toward critical cysteine residues, curcumin exerts neuroprotection by additional activation of the Keap1-Nrf2-ARE signaling pathway. Thus, aimed at affecting the electrophilicity of its α,β-unsaturated carbonyl fragment, allowing a fine-tuning of its reactivity, diverse electrophilic functions were inserted in different positions of the curcumin scaffold. Furthermore, considering the neuroprotective and antioxidant potentials of simple coumarins, several curcumin-coumarin hybrids were also prepared. Recently, the inhibition of additional AD-correlated protein kinases (PKs), such as CK1 and LRRK2, could offer promises to achieve a successful treatment and indole was envisaged as useful scaffold for both PKs’ inhibition. Thus, a small library of indole-based derivatives was designed and synthetized as valuable BBB permeable pharmacological tools. Finally, chitosan (CS), a natural, nontoxic, biocompatible and biodegradable polysaccharide, was selected to develop CS-based bioconjugates for nanoparticles’ preparation as innovative drug delivery and targeting systems.