5’-Carbamoyl Derivatives of 2'-C-Methyl-purine Nucleosides as Selective A1 Adenosine Receptor Agonists: Affinity, Efficacy and Selectivity for A1 Receptor from Different Species.

A series of 5'-carbamoyl and 5'-thionocarbamoyl derivatives of 2'-C-methyl analogues of the A, adenosine receptor (AIAR) full agonists N-6-cyclopentyladenosine (CPA), 2-chloro-N-6-cyclopentyladenosine (CCPA), N-6-[3-(R)-tetrahydrofuranyl]adenosine (tecadenoson), and 2-chloro analogue (2-Cl-tecadenoson) was synthesized and evaluated for their affinity for adenosine receptor subtypes from bovine, porcine, and human species. In the N-6-cyclopentylamino series, the 5'-substituted derivatives showed a reduced affinity at the bovine A(1)AR compared to the parent compounds; however, the selectivity for A(1) versus A(2A) receptor was retained or increased. The corresponding N-6-3-(R)-tetrahydrofuranylamino analogues displayed a very low affinity toward the bovine AIAR. The 5'-methylthionocarbamoyl derivative of 2'-Me-CCPA showed the best affinity at porcine AIAR with a K-i value of 13 nM. At human AR subtypes tecadenoson derivatives showed 2.3- to 5-fold lower affinity at AIAR and very low affinity at the other subtypes (A(2A), A(2B), and A(3) compared to the corresponding N-6-cyclopentyl analogues. The 5'-carbamoyl and 5'-thionocarbamoyl derivatives of 2'-Me-CCPA 3, 4, 7 and tecadenoson derivative 12 were found to be partial A, agonists at the porcine receptor. Docking studies explained the lower affinity of N-6-3-(R)-tetrahydrofuranyl-substituted compounds at bovine AIAR compared to that of N-6-cyclopentyl analogues, showing that the oxygen of the tetrahydrofuranyl ring establishes unfavorable electrostatic interactions with the CO oxygen of Asn254. The low binding affinity of the 2'-C-methyl-N-6-3-(R)-tetrahydrofuranyl adenosine analogues at human AIAR may be ascribed to the presence of unfavorable interactions between the hydrophilic tetrahydrofuranyl ring and the surrounding hydrophobic residues Leu250 (TM6) and Ile274 (TM7).