Structure activity relationship of 3-nitro-2-(trifluoromethyl)-2H-chromene derivatives as P2Y₆ receptor antagonists

Various 6-alkynyl analogues of a known 3-nitro-2-(trifluoromethyl)-2H-chromene antagonist 3 of the G_q-coupled P2Y₆ receptor (P2Y₆R) were synthesized using a Sonogashira reaction to replace a 6-iodo group. The analogues were tested in a functional assay consisting of inhibition of calcium mobilization in P2Y₆R-expressing astrocytoma cells elicited by native P2Y₆R agonist UDP. 6-Ethynyl and 6-cyano groups were installed, and the alkynes were extended through both alkyl and aryl spacers. The most potent antagonists, with IC₅₀ of ~1 µM, were found to be trialkylsilyl-ethynyl 7 and 8 (3-5 fold greater affinity than reference 3), t-butyl prop-2-yn-1-ylcarbamate 14 and p-carboxyphenyl-ethynyl 16 derivatives, and 3 and 8 displayed surmountable antagonism of UDP-induced production of inositol phosphates. Other chain-extended terminal carboxylate derivatives were less potent than the corresponding methyl ester derivatives. Thus, the 6 position in this chromene series is suitable for derivatization with flexibility of substitution, even with sterically extended chains, without losing P2Y₆R affinity. However, a 3-carboxylic acid or 3-ester substitution did not serve as a nitro bioisostere, as the affinity was eliminated. These compounds provide additional ligand tools for the underexplored P2Y₆R, which is a target for inflammatory, neurodegenerative and metabolic diseases.