5′-N-substituted carboxamidoadenosines as agonists for adenosine receptors

Novel as well as known 5'-N-substituted carboxamidoadenosines were prepared via new routes that provided shorter reaction times and good yields. Binding affinities were determined for rat A(1) and A(2A) receptors and human A(3) receptors. EC50 values were determined for cyclic AMP production in CHO cells expressing human A(2B) receptors. On all receptor subtypes relatively small substituents on the carboxamido moiety were optimal. Selectivity for the Ag receptor was found for several analogues (1a, 1d, 1h, and 1k). On A(1) receptors a number of compounds, but not 5'-N-ethylcarboxamidoadenosine (NECA, 1b), showed small GTP shifts, which could be indicative of lower intrinsic activities at the A(1) receptor. At the A(2B) receptor, derivatives 1i-k with modified ethyl substituents had reduced activities compared to the A(2B) reference agonist NECA (1b). Thiocarboxamido derivatives (8b and 8c) displayed considerable although decreased A(2B) receptor activity. The X-ray structure determination of compound 8b was carried out. Due to intramolecular hydrogen bonding between the carboxamido NH and the purine N3 in the crystal structure, the ribose moiety of this compound is in a syn conformation. However, theoretical calculations support that NECA (Ib), and less so 8b, can readily adopt both the syn and the anti conformation, therefore not excluding the proposed anti mode of binding to the receptor.