SUBTYPE-SELECTIVE NEURONAL NICOTINIC ACETYLCHOLINE RECEPTOR AGONISTS AND ANTAGONISTS

This PhD thesis focuses on two specific targets, belonging to the same receptor class of Nicotinic Acetylcholine Receptors (nAChRs): the α4β2 subtype and the α7 subtype. This elaborate is divided in two parts. The aim of the first part is the design and synthesis of α4β2 selective partial agonists as potential smoking-cessation agents. The aim of the second part is the design and synthesis of α7 antagonists with mitocan properties as antitumoral agents. Part 1. In the first project, a series of 3-nitrophenyl ethers and 3-hydroxyphenyl ethers of (S)-N-methylprolinol bearing bulky and lipophilic substituents at the C5 were designed, synthesized and assayed as putative selective α4β2 ligands. Two of them, 5-substituted with a 6-hydroxy-1-hexynyl, had high α4β2 affinity and increased α4β2/α3β4 selectivity when compared with the correspondent unsubstituted parent compounds. In the second project, each -CH= of the unselective antagonist (S,R)-N-methyl-2-pyrrolidinyl-1,4-benzodioxane and of its epimer at the benzodioxane stereocenter, was replaced by a nitrogen. The resulting four diastereomeric pairs of pyrrolidinyl-pyridodioxanes, also designed as the product of rigidification of the flexible scaffolds of pyridyl ethers of N-methyl prolinol, were studied for their nicotinic affinity at the α4β2 and α3β4. The isosteric -CH= to N substitution was detrimental for all the compounds, with the only exception of N-Methyl-pyrrolidinyl 5-pyridodioxane, with the nitrogen at position 5. Indeed, this ligand had similar affinity to its benzodioxane parent compound, but it had high α4β2/α3β4 selectivity and it was shown to be a selective partial agonist. In the third project, the unselective antagonist (S,R)-N-methyl-2-pyrrolidinyl-1,4-benzodioxane and of its epimer at the benzodioxane stereocenter were substituted at position 5 of the benzodioxane moiety, to explore the possibility of introducing selectivity and/or partial agonist as previously done with -CH= to N replacement. Among the synthesized compounds, (S,S)-N-Methyl-pyrrolidinyl-5-amino-benzodioxane had slightly improved affinity at the α4β2 affinity and highly enhanced α4β2/α3β4 selectivity than the unsubstituted parent compound, and it was shown to be a very potent partial agonist. In the fourth project, we applied computational techniques to support the interpretation of the biological results regarding N-Methyl-pyrrolidinyl 5-substituted benzodioxanes and pyridodioxanes. From these findings, we suggested that partial agonism and α4β2/α3β4 selectivity could be achieved when the benzodioxane scaffold is appropriately substituted with an HBA/HBD system, that can displace a water molecule from a small and hydrophilic subpocket of the binding site. Part 2. Adenocarcinoma and glioblastoma cell lines express α7 and α9-α10 nAChRs, whose activation promotes tumor cells growth. On these cells, the triethylammoniumethyl ether of 4-stilbenol MG624, a known selective antagonist of α7 and α9-α10 nAChRs, has antiproliferative activity. The structural analogy of MG624 with the mitocan RDM- 4’BTPI, triphenylphosphoniumbutyl ether of pterostilbene, suggested us that molecular hybridization among their three substructures might result in novel antitumour agents with higher potency and selectivity. We found that replacement of ethylene with butylene in the triethylammonium derivatives results in more potent and selective toxicity towards adenocarcinoma and glioblastoma cells, which was paralleled by increased α7 and α9-α10 nAChR antagonism and improved ability of reducing mitochondrial ATP production. Further elongation to octylene (26) provided a compound with 40-fold and 10-fold increased antiglioblastoma and antiadenocarcinoma activity respectively, when compared to MG624. Elongation of the alkylene linker was greatly advantageous also for the triphenylphosphonium derivatives. RDM- 4’BTPI did not acquire, as expectable, antinicotinic activity by hybridization with MG624 stilbene scaffold, but it was surpassed in glioblastoma cell viability reduction by its stilbene analogues with > 4C alkylene linker. In particular, the analogue with decylene between stilbenoxyl and triphenylphosphonium head (24) was ten- fold and two-fold more potent than RDM-4’BTPI in reducing glioblastoma cell viability and in increasing ROS production respectively. Overall, the ammonium compound 26 reached antiproliferative activities at glioblastoma and adenocarcinoma cells in the same range of the phosphonium 24, but showed good selectivity against neuroblastoma and healthy mouse astrocytes.