Click Modification in the <i>N</i>⁶ Region of A₃ Adenosine Receptor-Selective Carbocyclic Nucleosides for Dendrimeric Tethering that Preserves Pharmacophore Recognition

Abstract

Adenosine derivatives were modified with alkynyl groups on <i>N</i>⁶ substituents for linkage to carriers using Cu­(I)-catalyzed click chemistry. Two parallel series, both containing a rigid North-methanocarba (bicyclo[3.1.0]­hexane) ring system in place of ribose, behaved as A₃ adenosine receptor (AR) agonists: (5′-methyluronamides) or partial agonists (4′-truncated). Terminal alkynyl groups on a chain at the 3 position of a <i>N</i>⁶-benzyl group or simply through a <i>N</i>⁶-propargyl group were coupled to azido derivatives, which included both small molecules and G4 (fourth-generation) multivalent poly­(amidoamine) (PAMAM) dendrimers, to form 1,2,3-triazolyl linkers. The small molecular triazoles probed the tolerance in A₃AR binding of distal, sterically bulky groups such as 1-adamantyl. Terminal 4-fluoro-3-nitrophenyl groups anticipated nucleophilic substitution for chain extension and ¹⁸F radiolabeling. <i>N</i>⁶-(4-Fluoro-3-nitrophenyl)-triazolylmethyl derivative <b>32</b> displayed a <i>K</i>_i of 9.1 nM at A₃AR with ∼1000-fold subtype selectivity. Multivalent conjugates additionally containing click-linked water-solubilizing polyethylene glycol groups potently activated A₃AR in the 5′-methyluronamide, but not 4′ truncated series. <i>N</i>⁶-Benzyl nucleoside conjugate <b>43</b> (apparent <i>K</i>_i 24 nM) maintained binding affinity of the monomer better than a <i>N</i>⁶-triazolylmethyl derivative. Thus, the <i>N</i>⁶ region of 5′-methyluronamide derivatives, as modeled in receptor docking, is suitable for functionalization and tethering by click chemistry to achieve high A₃AR agonist affinity and enhanced selectivity