Design and Synthesis of New A2A and A3 Adenosine Receptors Antagonists

The aim of this work was the design and synthesis of new A2A and A3 adenosine receptors antagonists. Two different studies have been performed. The first one based on structural modifications of the pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pirimidine nucleus, reported in literature. In order to identify a new series of A2A or A3 AR antagonists and with the aim to better investigate the role of the nitrogen at the 7- position on the interaction with ARs, it was performed a synthetic strategy for the preparation of the pyrrolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidine nucleus which can be considered the 7-deaza-analogue of the pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine core. In order to complete the SAR studies on this class of compounds, it has been synthesised a novel series of pyrazolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives which can be considered the structural isomers of the parent pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives. From the biological data obtained, we can assert that the concomitant presence of the N7 and N8 is fundamental for the selectivity of these A2A/A3 ligands versus the remaining ARs subtype. Our results confirmed the importance of the presence of the NH at the 5- position of the PTPs nuclus, this could be due to the formation of an essential ligand-receptor hydrogen bond. Most of these compounds, revealed to be non selective ARs antagonists. Derivative 5-{[(4-methoxy-phenyl)carbamoyl]amino}-(2-furan-2-yl)-8-methyl-8H-pyrrolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidine resulted to be the best compound of the series in term of both affinity (hA3Ki = 15nM) and selectivity. The aim of the second project was to obtain A3 antagonists with improve water-solubility. A series of 4-allyl/benzyl-7,8-dihydro-8-methyl/ethyl-2-[(substituted)isoxazol/pyrazol-3/5-yl]-1H-imidazo[2,1-i]purin-5(4H)-one derivatives have been synthesised and evaluated in radioligand binding assays to determine their affinities at the human A1, A2A, and A3 adenosine receptors. Efficacy at the hA2B AR and antagonism of selected ligands at the hA3 AR were also assessed through cAMP experiments. All the synthesised molecules exhibited high affinity at the hA3 AR (Ki values ranging from 1.46 to 44.8 nM) as well as remarkable selectivity versus A1, A2A and A2B AR subtypes. In view of the chirality of the tricycles, for selected compounds both the racemic mixtures and the pure enantiomers have been prepared with the purpose of determine a possible receptor stereoselectivity. Compound (R)-4-allyl-8-ethyl-7,8-dihydro-2-(3-methoxy-1-methyl-1H-pyrazol-5-yl)-1H-imidazo[2,1-i]purin-5(4H)-one showed to be the most potent hA3 AR ligand of the series (hA3Ki = 1.46 nM), in addition with very high selectivity over all the other ARs (hA2AKi/ hA3Ki > 3425; hA2BIC50/ hA3Ki > 3425; hA1Ki/ hA3Ki = 1,729).