Synthesis and study of 5'-ester prodrugs of N6-cyclopentyladenosine, a selective A1 receptor agonist.

A series of 5'-esters of N6-cyclopentyladenosine (CPA) were prepared with the aim to improve stability and bioavailability of selective A1 agonists. Log P values, stability, affinity, and activity toward human adenosine A1 receptors were evaluated. An appropriate synthetic procedure was adopted to avoid concomitant deamination at position 6. Log P values were obtained by the Mixxor system. The stability of CPA and its 5'-ester was evaluated in human plasma and whole blood and analyzed with high-performance liquid chromatography. The affinities to human A1 receptor expressed by N6-cyclohexyladenosine cells were obtained by binding experiments. The activities were evaluated by measurements of the inhibition of forskolin stimulated 3'-5'-cyclic adenosine monophosphate, performing competitive binding assays. All prodrugs were more lipophilic than CPA, and their hydrolysis, in whole blood and in plasma, was found related, respectively, to the length and hindrance of 5'-substituents. Affinity and activity values indicated a very weak interaction toward adenosine A1 receptor of the intact prodrugs. We propose 5'-esters of CPA, characterized by suitable lipophilicity and elevated degree of stability in physiological fluids, as possible candidates for CPA prodrugs.