Selective A(3) adenosine receptor antagonists: Water-soluble 3,5-diacyl-1,2,4-trialkylpyridinium salts and their oxidative generation from dihydropyridine precursors

A(3) adenosine receptor antagonists are sought for their potential antiinfl ammatory, antiasthmatic, and antiischemic properties. We have found that 3, 5-diacyl-1,2,4-trialkyl-6-phenylpyridinium derivatives constitute a novel c lass of selective A(3) adenosine receptor antagonists. The structure-activi ty relationships of this class of antagonists, incorporating the 3-thioeste r, have been explored. The most potent analogue in this group was 2,4-dieth yl-1-methyl-3-(ethylsulfanylcarbonyl)-5-ethyloxycarbon-6-phenylpyridinium i odide (11), which had an equilibrium inhibition constant (K-i) value of 219 nM at human A(3) receptors (binding of [I-125]-AB-MECA (N-6-(4-amino-3-iod obenzyl)-5'-N-methylcarbamoyladenosine)) expressed in Chinese hamster ovary (CHO) cells and > 10 mu M at rat brain A(1) and A(2A) receptors and at rec ombinant human A(2B) receptors. Compound 11 could be generated through oxid ation of the corresponding 3,5-diacyl-1,2,4-trialkyl-6-phenyl-1,4-dihydropy ridine, 24, with iodine or in the presence of rat brain homogenates. A 6-cy clopentyl analogue was shown to increase affinity at human A(3) receptors u pon oxidation from the 1-methyl-1,4-dihydropyridine analogue, 25, to the co rresponding pyridinium derivative, 23 (K-i 695 nM), suggesting a prodrug sc heme. Homologation of the M-methylpyridinium derivatives to N-ethyl and N-p ropyl at the 1-position caused a progressive reduction in the affinity at A (3) receptors. Modifications of the alkyl groups at the 2-, 3-, 4-, and 5-p ositions failed to improve potency in binding at A(3) receptors. The pyridi nium antagonists are not as potent as other recently reported, selective A( 3) receptor antagonists; however, they display uniquely high water solubili ty (43 mM for 11). Compound 11 antagonized the inhibition of adenylate cycl ase elicited by IB-MECA in CHO cells expressing the human A(3) adenosine re ceptor, with a K-B value of 399 nM, and did not act as an agonist, demonstr ating that the pyridinium salts are pure antagonists.