Recently a G protein coupled receptor GPR a

Recently, a G-protein-coupled receptor, GPR109a, was identified as a molecular target for niacin. Mechanistic studies have suggested that the benefits of niacin therapy may result from the activation of GPR109a located on adipocytes. Recent experiments have shown that niacin activation of GPR109a results in a reduction of intracellular cAMP and it is believed that this reduction in cAMP levels effectively inhibits lypolysis by the negative modulation of hormone sensitive lipase (HSL). As a result, a concomitant decrease in plasma free fatty KPT-276 (pFFA) levels is observed and it is thought that this reduction in pFFA is crucial to the lipid modulation and resulting therapeutic value of niacin. In control experiments, GPR109a knockout mice, when treated with niacin therapy, failed to show the characteristic reduction in plasma FFA that is observed in wild type mice. Furthermore, these mice also failed to show a niacin-induced cutaneous flush. As a result, it has been suggested that the niacin-associated flush and anti-lipolytic effects are both mediated through GPR109a. In light of these recent findings, we initiated a drug discovery program focused on the identification of a ‘flush-free’ agonist of the niacin receptor. Our lead identification effort identified pyrazole–tetrazole as a promising candidate., , While pyrazole displayed only modest in vitro affinity for GPR109a (), its in vivo pharmacology was intriguing (see ). In a mouse plasma free-fatty acid (pFFA) assay, mice treated with 10mpk of pyrazole showed a net reduction in plasma free fatty acid levels comparable to that observed upon treatment with 100mpk of niacin. Furthermore, in a mouse vasodilation-assay (mVD), mice treated with 30mpk of failed to show any signs of the characteristic cutaneous flushing response observed with niacin treatment. This putative decoupling of the free fatty acid and cutaneous flushing effect characteristic of niacin treatment captivated out attention. As a result, pyrazole became the focal point of our medicinal chemistry effort. The initial goal of the program focused on improving the in vitro profile of pyrazole . With this goal in mind, we proceeded to explore the SAR around the framework of pyrazole . Early in our efforts, we discovered that both the pyrazole and tetrazole moieties were essential in retaining affinity for the receptor. In addition, it was discovered that substituents in either the C4 or C6 positions of the saturated carbon backbone of were poorly tolerated. In light of these observations, we focused our attention around the C5 position of the molecule. In order to explore the C5 position of the molecule most judiciously, we utilized triflate as a late stage synthetic intermediate that allowed for the installation of both alkyl and aryl substituents into the framework of the molecule. As outlined in , our synthesis toward compound began with acylation of the commercially available cyclopentenone followed by a condensation reaction with benzyl hydrazine to install the pyrazole. Hydrolysis of the enol-ether and -butyl ester revealed ketone which was further elaborated to the nitrile. Finally, the C5 ketone was converted to the desired enol-triflate intermediate . Enol-triflate was converted to the desired C5-alkyl or C5-aryl substituted pyrazole via Suzuki or Stille couplings with the appropriate boronic acid or organostannane. With the desired C5 substituent in place, the tetrazole functionality was furnished through a [3+2] cycloaddition with sodium azide to afford . Finally, hydrogenation provided the saturated carbon backbone with concomitant removal of the benzyl protecting group to afford the C5-pyrazole derivatives – and – (see ). As a means of identifying compounds suitable for in vivo analysis, we explored the in vitro activity of this class of pyrazole–tetrazoles. As illustrated in , the C5-alkyl-pyrazoles showed a markedly improved affinity for both the human (GPR109a) and murine (Puma-G) variants of the receptor. Ethyl derivative (1.3μM and 4.6μM) demonstrated nearly comparable in vitro affinity to pyrazole , while propyl derivative (0.26μM and 0.69μM) provided a modest boost in potency with both the mouse and human receptors. Further extension of the carbon side-chain, as in -butyl derivative (0.21μM and 0.34μM), showed an incremental boost in affinity as the racemate; however, a chiral resolution of , yielding the active stereoisomer (0.06μM and 0.08μM) provided the most potent agonist within the C5-alkyl series. Additional elongation of the alkyl chain, as in pentane derivative (3.6μM and 4.8μM) and hexane derivative (0.8μM and 0.87μM) showed a precipitous drop in affinity, when compared to .