Rational Design of Sulfonated A3 Adenosine Receptor-Selective Nucleosides as Pharmacological Tools To Study Chronic Neuropathic Pain

(N)-Methanocarba(bicyclo[3.1.0]hexane)adenosine derivatives were probed for sites of charged sulfonate substitution, which precludes diffusion across biological membranes, e.g., blood-brain barrier. Molecular modeling predicted that sulfonate groups on C2-phenylethynyl substituents would provide high affinity at both mouse (m) and human (h) A(3) adenosine receptors (ARs), while a N-6-p-sulfophenylethyl substituent would determine higher hA(3)AR vs mA(3)AR affinity. These modeling predictions, based on steric fitting of the binding cavity and crucial interactions with key residues, were confirmed by binding/efficacy studies of synthesized sulfonates. N-6-3-Chlorobenzyl-2-(3-sulfophenylethynyl) derivative 7 (MRS5841) bound selectively to h/m A(3)ARs (K-i(hA(3)AR) = 1.9 nM) as agonist, while corresponding p-sulfo isomer 6 (MRS5701) displayed mixed A(1)/A(3)AR agonism. Both nucleosides administered ip reduced mouse chronic neuropathic pain that was ascribed to either A(3)AR or A(1)/A(3)AR using A(3)AR genetic deletion. Thus, rational design methods based on A(3)AR homology models successfully predicted sites for sulfonate incorporation, for delineating adenosine's CNS vs peripheral actions.