DESIGN AND SYNTHESIS OF "BIVALENT" LIGANDS AS NEW MODULATORS OF ENDOCANNABINOID SYSTEM

The Endocannabinoid system (ECS) is a neuromodulator endogenous system. It consists of cannabinoid receptors (CB1R and CB2R), their main endogenous lipid-based ligands called endocannabinoids (ECs), amongst which the most studied are Anandamide (AEA) and 2-arachidonoylglycerol (2-AG) and the enzymes for the synthesis, degradation and transport of ECs. CBRs belong to the G protein-coupled receptor family (GPCR). CB1R is one of the most abundandt receptor of the central nervous system (CNS)and it is found mostly in the mammalian brain whereas CB2R is expressed mainly in immune cells and hematopoietic systems but it is also expressed by some neurons, playing an active role in neurological activities. The ECS is involved in a wide range of pathological conditions such as mood and anxiety disorders, movement disorders (Parkinson’s and Huntington’s disease), neuropathic pain, neurodegenerative diseases, cancer, glaucoma and obesity. For these reasons, since its discovery, research studies focused on the synthesis and development of ligands of the CBRs. However, despite all these potential benefits, the direct modulation of these receptors led to numerous side effects such as depression, suicidal ideation, psychotropic effects which also limited the clinical development. In order to overcome these adverse effects, mainly due to the CB1R modulation, one of the first strategies consisted in the development of selective CB2R ligands which resulted to be effective in the treatment of neuropathic and inflammatory pain. Recently, a new medicinal chemistry approach, considered very promising, comprises the synthesis of CBR allosteric modulators. Indeed, it has been reported that CBRs have spatially distinct allosteric sites that can be targeted by exogenous/endogenous ligands able in this way to modify the receptor’s functional state, modulating the affinity and/or efficacy of orthosteric ligands. Apart from the lack of side effects, allosteric modulators present many other advantages such as greater subtype selectivity thanks to the less conserved domains compared to the orthosteric ones and enhanced tissue selectivity since they exert their effects only in the presence of orthosteric ligands. Yet, because of their dependence on endogenous/orthosteric ligands, their effect is saturable and, especially in pathological state, the entity and the duration of their effect will depend on the endocannabinoid tone at that time and in that district. Moreover, allosteric ligands present the ability to modulate selected signaling pathways depending on the orthosteric ligand bound (biased signaling and probe-dependence). Allosteric modulation can be achieved through the action of positive allosteric modulators (PAMs) which increase the response of a certain orthosteric ligand-driven signaling of receptor or negative allosteric modulators (NAMs) which reduce receptor responsiveness through a decrease of agonist affinity or efficacy or yet neutral allosteric ligands (NALs) which instead bind to the allosteric sites without affecting the response of the orthosteric agonist. One of the newest approach in the medicinal chemistry field is represented by a multi-target strategy. Multi-target approach represents a promising pharmacological modulation of the ECS, since the simultaneous modulation of two or more targets might offer a safer and more effective pharmacological strategy if compared to a single target modulation. Consequently, bivalent compounds have been developed. They are hybrid compounds which combined two different functional pharmacophores, linked by a spacer, in one new ligand. The aim of my thesis was to synthesize bivalent compounds able to bind CB1R allosteric site and CB2R orthosteric site at the same time. For this reason, a portion derived from a well-known CB1R PAM has been combined through an alkylic spacer of different length and a triazole ring to a portion of a CB2R selective agonist, synthesized and already studied in the lab where I performed my thesis. These new compounds will be tested for their binding and functional activity at CBRs at the University of Aberdeen in Prof. Pertwee’s lab to evaluate their allosteric or bivalent behavior