Structural Sweet Spot
for A<sub>1</sub> Adenosine
Receptor Activation by Truncated (N)-Methanocarba Nucleosides: Receptor
Docking and Potent Anticonvulsant Activity
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Abstract
A<sub>1</sub> adenosine receptor (AR) agonists display
antiischemic
and antiepileptic neuroprotective activity, but peripheral cardiovascular
side effects impeded their development. SAR study of <i>N</i><sup>6</sup>-cycloalkylmethyl 4′-truncated (N)-methanocarba-adenosines
identified <b>10</b> (MRS5474, <i>N</i><sup>6</sup>-dicyclopropylmethyl, <i>K</i><sub>i</sub> = 47.9 nM) as
a moderately A<sub>1</sub>AR-selective full agonist. Two stereochemically
defined <i>N</i><sup>6</sup>-methynyl group substituents
displayed narrow SAR; groups larger than cyclobutyl greatly reduced
AR affinity, and those larger or smaller than cyclopropyl reduced
A<sub>1</sub>AR selectivity. Nucleoside docking to A<sub>1</sub>AR
homology model characterized distinct hydrophobic cyclopropyl subpockets,
the larger “A” forming contacts with Thr270 (7.35),
Tyr271 (7.36), Ile274 (7.39), and carbon chains of glutamates (EL2)
and the smaller subpocket “B” forming contacts between
TM6 and TM7. <b>10</b> suppressed minimal clonic seizures (6
Hz mouse model) without typical rotarod impairment of A<sub>1</sub>AR agonists. Truncated nucleosides, an appealing preclinical approach,
have more druglike physicochemical properties than other A<sub>1</sub>AR agonists. Thus, we identified highly restricted regions for substitution
around <i>N</i><sup>6</sup> suitable for an A<sub>1</sub>AR agonist with anticonvulsant activity