Structure-activity relationships and molecular modeling of 3,5-diacyl-2,4-dialkylpyridine derivatives as selective A3 adenosine receptor antagonists

The structure-activity relationships of 6-phenyl-1,4-dihydropyridine derivatives as selective antagonists at human A(3) adenosine receptors have been explored (Jiang et al. J. Med. Chem. 1997, 39, 4667-4675). In the present study, related pyridine derivatives have been synthesized and tested for affinity at adenosine receptors in radioligand binding assays. K-i values in the nanomolar range were observed for certain 3,5-diacyl-2,4-dialkyl-6-phenylpyridine derivatives in displacement of [I-125]AB-MECA (N-6-(4-amino-3-iodobenzyl)-5'-N-methylcarbamoyladenosine) at recombinant human A(3) adenosine receptors. Selectivity for As adenosine receptors was determined vs radioligand binding at rat brain A(1) and A(2A) receptors. Structure-activity relationships at various positions of the pyridine ring (the 3- and 5-acyl substituents and the 2- and 4-alkyl substituents) were probed. A 4-phenylethynyl group did not enhance A(3) selectivity of pyridine derivatives, as it did for the 4-substituted dihydropyridines. At the 2- and 4-positions ethyl was favored over methyl. Also, unlike the dihydropyridines, a thioester group at the 3-position was favored over an ester for affinity at As adenosine receptors, and a 5-position benzyl ester decreased affinity. Small cycloalkyl groups at the 6-position of 4-phenylethynyl-1,4-dihydropyridines were favorable for high affinity at human A(3) adenosine receptors, while in the pyridine series a 6-cyclopentyl group decreased affinity. 5-Ethyl 2,4-diethyl-3 -(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate, 38, was highly potent at human A(3) receptors, with a K-i value of 20 nM. A 4-propyl derivative, 39b, was selective and highly potent at both human and rat A(3) receptors, with K-i values of 18.9 and 113 nM, respectively. A 6-(3-chlorophenyl) derivative, 44, displayed a K-i value of 7.94 nM at human A(3) receptors and selectivity of 5200-fold. Molecular modeling, based on the steric and electrostatic alignment (SEAL) method, defined common pharmacophore elements for pyridine and dihydropyridine structures, e.g., the two ester groups and the 6-phenyl group. Moreover, a relationship between affinity and hydrophobicity was found for the pyridines.