Conjugable A3 adenosine receptor antagonists for the development of functionalized ligands and their use in fluorescent probes

Compounds able to simultaneously bind a biological target and be conjugated to a second specific moiety are attractive tools for the development of multi-purpose ligands useful as multi-target ligands, receptor probes or drug delivery systems, with both therapeutic and diagnostic applications. The human A(3) adenosine receptor is a G protein-coupled receptor involved in many physio-pathological conditions, e.g. cancer and inflammation, thus representing a promising research target. In this work, two series of conjugable hA(3)AR antagonists, based on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine nucleus, were developed. The introduction of an aromatic ring at the 5 position of the scaffold, before (phenylacetamido moiety) or after (1,2,3-triazole obtained by click chemistry) the conjugation is aimed to increase affinity and selectivity towards the hA(3)AR receptor. As expected, conjugable compounds showed good affinity towards the hA(3)AR. In order to prove their potential in the development of hA(3)AR ligands for different purposes, compounds were also functionalized with fluorescent probes. Unfortunately, conjugation decreased affinity and selectivity for the target as compared to the hA(2A)AR. Computational studies identified specific non-conserved residues of the extracellular loops which constitute a structural barrier able to discriminate between ligands, giving insights into the rational development of new highly selective ligands. (C) 2019 Elsevier Masson SAS. All rights reserved.