Recently a G protein coupled receptor GPR

Recently, a G-protein-coupled receptor, GPR109a, was identified as a molecular target for niacin. Following this breakthrough, our group initiated a drug discovery program focused on the development of a high affinity ‘flush-free’ niacin-like agonist. Previously, we reported on the identification of a pyrazole-tetrazole agonist , a compound that showed potential as a ‘flush-free’ agonist of GPR109a. The intriguing pharmacology of compound inspired continued interests within this structure class. Subsequent work in the group showed that C5-alkyl- and aryl-substituted pyrazole-tetrazole derivatives had improved in vitro affinities and similar pharmacology to . With a continued interest in this structure class, we proceeded to explore further structure based improvements. In this effort, we discovered that the tetrazole moiety of compound could be suitably replaced with a carboxylic PPDA group. Curious as to the effect of this functional group exchange on the pharmacological profile for the class, we proceeded to develop this class of pyrazole-carboxylic acid derivatives. Initially, we focused our effort on synthesis of the C5-alkyl and aryl derived pyrazole–acids. In this regard, generation of the series of C5-alkyl pyrazole-acids – and the -methyl pyrazole derivative was accomplished in a linear fashion from the commercially available ethoxy-cyclopentenone (). Addition of the desired alkyl lithium or the lithiated -methyl pyrazole to installed the desired C5-alkyl and -methyl pyrazole moiety. Further elaboration of the core to the pyrazole derivative was achieved via acylation with diethyl oxalate followed by condensation with hydrazine–HCl. Transfer hydrogenation reduced the olefin and saponification successfully converted the ester to the desired carboxylic acid to afford the C5-pyrazole derivatives – and . Syntheses of the C5-aryl-pyrazole-acids – were accomplished via the readily available iodocyclopentenone (). Suzuki coupling of the corresponding aryl boronic acid to enone afforded the desired aryl-substituted cyclopentenone . Conversion of enone to the desired pyrazole-acids was achieved, following previously outlined conditions, via acylation of the enone with diethyl oxalate, condensation with hydrazine–HCl, transfer hydrogenation, and a final saponification. The preparation of N-linked pyrazole and triazole derivatives and is outlined in . Treatment of commercially available cyclopentenone with either commercially available pyrazole or triazole afforded the desired adduct . Following previously described conditions, ketone was further elaborated to the desired class of C5-substituted pyrazole-acids. As illustrated in , the in vitro affinities for this pyrazole-acid series compared favorably with the similar class of pyrazole-tetrazoles. In this regard, -propyl derivative (0.25μM and 0.38μM) showed good affinity on both the human receptor (GPR109a, hNBA) and murine receptor (PUMA-G, mNBA), while the branched alkyl derivatives (0.99μM and 1.7μM) and (1.1μM and 1.3μM) showed only modest affinity for the niacin receptor. Further studies into the C5 phenyl series of molecules showed parallel trends to the pyrazole-tetrazole class. As demonstrated in , the phenyl substituted pyrazole-acid (0.15μM and 0.20μM) showed improved affinity for both mouse and human receptors. The 2,3,5-trifluoro analog (0.03μM and 0.02μM), showed excellent intrinsic affinity for GPR109a, and a marked improvement over the pyrazole-tetrazole derivative . However, despite the promising intrinsic activity of compound , when screened in our competition binding assay in the presence of 4% human serum, compound showed an eightfold reduction in affinity for the receptor. The intrinsic affinity of pyrazole-tetrazole derivative was similarly affected by the presence of serum. Concerned with the role this serum shift effect may have on the in vivo efficacy, our focus turned towards identifying a agonist.