New substituted 9-alkylpurines as adenosine receptor ligands

The structure-activity relationships in 9-ethylpurine derivs., aimed at prepg. A1, A2A, A2B, and A3 selective adenosine receptor antagonists, was undertaken. The synthetic approach was to introduce various substituents (amino, alkoxy and alkynyl groups) into the 2-, 6-, or 8-positions of the purine ring. The resp. starting compds. for each series of derivs. were 2-iodo-9-ethyladenine, obtained from 2-amino-6-chloropurine; 9-ethyl-6-iodo-9H-purine, 8-bromo-9-ethyladenine and 8-bromo-9-ethyl-6-iodo-9H-purine, obtained from 9-ethyladenine. The prepd. compds. were tested in in vitro radioligand binding assays at A1, A2A, and A3 human adenosine receptor subtypes. Because of the lack of a suitable radioligand the affinity of the 9-ethyladenine derivs. at A2B adenosine receptors was detd. in adenylyl cyclase expts. The series of 9-ethylpurine derivs. exhibited a similar pharmacol. profile at A1 and A2A receptors while some differences were found for the A3 and the A2B subtypes. 8-Bromo-9-ethyladenine showed higher affinity for all receptors in comparison to 9-ethyladenine, and the highest affinity in the series for the A2A and A2B subtypes (Ki=0.052 and 0.84 μM, resp.). Analyzing the different substituents, a phenethoxy group in 2-position I (R = O(CH2)2Ph, R1 = NH2, R2 = H) gave the highest A2A vs. A2B selectivity (near 400-fold), while 2-phenethylamino in I (R = NH(CH2)2Ph, R1 = NH2, R2 = H) and 6-phenethylamino I (R = H, R1 = NH(CH2)2Ph, R2 = H) improved the affinity at A2B receptors, compared to 9-ethyladenine. The presence of a hexynyl substituent in the 8-position led to a compd. with good affinity at the A3 receptor I (R = H, R1 = NH2, R2 = C≡CBu) (Ki=0.62 μM), while (ar)alkynyl groups are detrimental for the potency at the A2B subtype. These differences raise the possibility that further modifications will result in the development of currently unavailable leads with good affinity and selectivity for A2B adenosine receptors.