N⁶,C8-Disubstituted Adenosine Derivatives as Partial Agonists for Adenosine A₁ Receptors^†

The synthesis and biological evaluation of N⁶,C8-disubstituted derivatives of adenosine as potential partial agonists for adenosine receptors is described. Via three routes, two series of compounds were prepared, viz., N⁶-cyclopentyladenosine derivatives 3a−e and C8-(cyclopentylamino)adenosine analogs 3e and 9a−d, respectively. The X-ray structure determination of one of these compounds, N⁶-ethyl-8-(cyclopentylamino)adenosine (9b), was carried out (orthorhombic, space group P2₁2₁2₁ (No. 19) with a = 11.039(3), b = 8.708(2), and c = 24.815(12) Å, Z = 4, R1 = 0.0974, R2_w = 0.2455). Due to intramolecular hydrogen bonding, the ribose moiety of this compound is in an anti conformation. The compounds were tested in vitro in radioligand binding studies, yielding their affinities for A₁ and A_2a adenosine receptors. All compounds appeared A₁ selective, with affinities in the high nanomolar, low micromolar range. On A₁ receptors the so-called GTP shift was also determined, i.e., the ratio between the affinities measured in the presence and absence of 1 mM GTP. All GTP shifts (values between 1.1 and 3.8) were lower than the GTP shift for CPA (6.0). This GTP shift appeared indicative for partial agonism in vivo, since the N⁶-cyclopentyladenosine derivatives showed lower intrinsic activities than the prototypic full agonist N⁶-cyclopentyladenosine on the decrease in heart rate in conscious, normotensive rats.