A₃ Adenosine Receptor Antagonists with Nucleoside Structures and Their Anticancer Activity

The overexpression of the A₃ adenosine receptor (AR) in a number of cancer cell types makes it an attractive target for tumor diagnosis and therapy. Hence, in the search for new A₃AR ligands, a series of novel 2,N⁶-disubstituted adenosines (Ados) was synthesized and tested in radioligand binding and functional assays at ARs. Derivatives bearing a 2-phenethylamino group in the N⁶-position were found to exert higher A₃AR affinity and selectivity than the corresponding N⁶-(2,2-diphenylethyl) analogues. 2-Chloro-N⁶-phenylethylAdo (15) was found to be a potent full A₃AR agonist with a Ki of 0.024 nM and an EC₅₀ of 14 nM, in a cAMP accumulation assay. Unlike 15, the other ligands behaved as A₃AR antagonists, which concentration-dependently reduced cell growth and exerted cytostatic activity on the prostate cancer cell line PC3, showing comparable and even more pronounced effects with respect to the ones elicited by the reference full agonist Cl-IB-MECA. In particular, the N⁶-(2,2-diphenylethyl)-2-phenylethynylAdo (12: GI₅₀ = 14 µM, TGI = 29 µM, and LC₅₀ = 59 µM) showed the highest activity proving to be a potential antitumor agent. The cytostatic effect of both A₃AR agonist (Cl-IB-MECA) and antagonists (12 and other newly synthesized compounds) confirm previous observations according to which, in addition to the involvement of A₃ARs, other cellular mechanisms are responsible for the anticancer effects of these ligands.