Repurposing of a Nucleoside Scaffold from Adenosine Receptor Agonists to Opioid Receptor Antagonists

While screening off-target
effects of rigid (N)-methanocarba-adenosine 5′-methylamides
as A3 adenosine
receptor (AR) agonists, we discovered μM binding hits at the
δ-opioid receptor (DOR) and translocator protein (TSPO). In
an effort to increase OR and decrease AR affinity by structure activity
analysis of this series, antagonist activity at κ-(K)­OR appeared
in 5′-esters (ethyl 24 and propyl 30), which retained TSPO interaction (μM). 7-Deaza modification
of C2-(arylethynyl)-5′-esters but not 4′-truncation
enhanced KOR affinity (MRS7299 28 and 29, Ki ≈ 40 nM), revealed μ-OR
and DOR binding, and reduced AR affinity. Molecular docking and dynamics
simulations located a putative KOR binding mode consistent with the
observed affinities, placing C7 in a hydrophobic region. 3-Deaza modification
permitted TSPO but not OR binding, and 1-deaza was permissive to both;
ribose-restored analogues were inactive at both. Thus, we have repurposed
a known AR nucleoside scaffold for OR antagonism, with a detailed
hypothesis for KOR recognition.