Design, Synthesis and Biological Evaluation of Potent and Selective Dopamine or Sigma Receptor Ligands Sharing a Common Pharmacophore

This Ph.D. thesis work has concerned the design and synthesis of new biologically active ligands directed to different receptor systems, including dopaminergic and sigma receptors, and sharing the features of a common pharmacophore, which consists of a lipophilic moiety linked by a spacer to a piperidine or piperazine basic function and an aromatic terminal. Highly selective or multitarget compounds, as well as ligands with a biased profile, were obtained and allowed to shed further light on the involvement of these receptors in investigated pathologies. Specifically, the following research activites have been carried out: -(i) the brain penetrant and dopamine D4 receptor (DRD4)-selective compound 1-(3-(4- phenylpiperazin-1-yl)propyl)-3,4-dihydroquinolin-2(1H)-one has been selected as a lead compound for the discovery of new ligands endowed with high affinity and selectivity for DRD4. Structure-activity relationship (SAR) and molecular modelling studies, highlighted that the quinolinone nucleus of the lead compound can be replaced by an N-indole or an N-tetrahydroquinoline moiety and the propyl linker represents the optimal distance between the lipophilic portion and the basic function. The most DRD4 selective compounds were the ones with para-substituted aromatic terminals, which behaved as DRD4 antagonists. Instead, the 2-pyridyl derivative showed an interesting biased profile, being a partial agonist toward DRD4 G-protein activation and an antagonist toward β- arrestin recruitment. The para-chlorophenyl derivative and the 2-pyridyl compound have been evaluated for their potential in affecting the viability of glioblastoma (GBM) cell lines, and primary GBM stem cells. Both compounds decreased the viability of all the studied cell lines, at 10 μM concentration, eliciting an antiproliferative effect higher than that of temozolomide, the first-choice chemotherapeutic drug in GBM. Such an effect was contrasted by a DRD4 agonist, demonstrating the involvement of DRD4 in the antitumor activity of these compounds. The same lead compound has also been selected as protein-of-interest (POI) ligand in the design of new small-molecule proteolysis targeting chimeras (PROTACs) for DRD4, which will be evaluated as anticancer agents for the treatment of GBM. -(ii) A scaffold hybridization strategy has been used for the discovery of potent dopamine DRD3-selective or multitarget ligands, potentially useful for central nervous system disorders. In particular, novel bitopic ligands bearing the N-(2,3-dichlorophenyl)piperazine nucleus as a primary pharmacophore linked by an unsubstituted or 3-F/3-OH substituted butyl chain to the 6,6- or 5,5-diphenyl-1,4-dioxane-2-carboxamide or the 1,4- benzodioxane-2-carboxamide scaffold as a secondary pharmacophore have been synthesised and studied. The biological results supported by molecular modelling studies highlighted that the ligands characterized by an unsubstituted butyl chain between the pharmacophores showed the highest DRD3 affinity. In particular, the 6,6-diphenyl-1,4- dioxane compound showed a DRD3-preferential profile, while an interesting multitarget profile was highlighted for the 5,5-diphenyl-1,4-dioxane and 1,4-benzodioxane derivatives, both behaving as potent DRD2 antagonists, 5-HT1AR and D4R agonists, as well as potent DRD3 partial agonists. They also behaved as low potency 5-HT2CR partial agonists and 5-HT2AR antagonists. This profile might be a promising starting point for the discovery of novel agents useful for the treatment of schizophrenia. -(iii) A benzo-cracking approach was applied to a potent σ1 receptor antagonist bearing the conformationally constrained 1,3-benzodioxane for the preparation of more flexible 1,3-dioxane compounds. One of the synthesised compounds behaved as a potent σ1 receptor antagonist and showed unprecedented selectivity over σ2 receptor, opioid receptor subtypes, the PCP site of the NMDA receptor, as well as over dopamine transporter. In the in vivo studies, it demonstrated to counteract the overeating of highly palatable food in binging-rats, without affecting palatable food intake in the control group, as well as depression-related and anxiety-like behaviors. These findings strengthened the involvement of σ1 receptor in the compulsive-like eating behavior, suggesting σ1 receptor antagonists as promising candidates to treat binge episodes.