Exploring Directionality of 5-Substitutions in a New Series of 5-Alkyamino-pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine as a Strategy to Design Novel Human A3 Adenosine Receptors Antagonists.

The activation of A3 adenosine receptors (ARs) is related to various second messenger systems: in particular, they have been shown to stimulate phospholipase C and D and to inhibit adenylyl cyclase.[1] In the rat, the activation of A3 AR results in hypotension through the promotion of the release of inflammatory mediators from mast cells.[2] It has also been suggested that the A3 AR plays an important role in brain ischemia, immunosuppression and cellular growth.[1] On the basis of these pharmacological observations, many efforts have been made in search of highly selective A3 AR antagonists as potential antiasthmatic, antinflammatory and cerebroprotective agents and for the treatment of glaucoma.[1,3] For this purpose, in the last few years, different classes of heterocyclic derivatives have emerged as A3 AR antagonists.[1] The structure activity relationship (SAR) of new 5-alkylamino-pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidines as antagonists of the A3 AR was explored with the principal aim to establish the directionality of 5-subtitutions inside the orthosteric binding site of the A3 AR. All the synthesized compounds showed affinity for the hA3 AR from nanomolar to sub-nanomolar range. In particular, the most potent and selective antagonist presents an (S) α-phenylethylamino decoration at the 5 position (Ki hA3 = 0.3 nM). Using an in silico receptor-driven approach, we have determined the most favorable orientation of the substitutions at the 5-position of the pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine scaffold, opening at the possibility to further derivatization directing the N5 position toward the extracellular environment. [1] Cheong, S.L.; Federico, S.; Venkatesan, G.; Mandel, A.L.; Shao, Y.M.; Moro, S.; Spalluto,G.; Pastorin, G. The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry and in silico approaches. Med. Res. Rev. In press, DOI 10.1002/med.20254. [2] Ramkumar, V.; Stiles, G.L.; Beaven, M.A.; Ali, H. The A3 adenosine receptors is the unique adenosine receptor which facilitates release of allergic mediators in mast cells. J. Biol. Chem. 1993, 268, 16887-16890. [3] Jacobson, K. A.; Gao, Z. G. Adenosine receptors as therapeutic targets. Nat. Rev. Drug Discov. 2006, 5, 247-264.