Nicotinic acid has been used clinically for decades to control serum lipoproteins. Nicotinic acid lowers very low-density lipoprotein (VLDL)-cholesterol, low-density lipoprotein (LDL)-cholesterol, and lipoprotein-a (LPa), and it is also effective in raising high-density lipoprotein (HDL)-cholesterol. However, nicotinic acid has several side effects in clinical use. The most notable is intense cutaneous vasodilation "flushing" on the upper body and face. We discovered a pyranopyrimidinedione series to be nicotinic acid receptor agonists. A potent nicotinic acid receptor agonist from this series {5-(3-cyclopropylpropyl)-2-(difluoromethyl)-3H-pyrano[2,3-d]pyrimidine-4,7-dione}with reduced flushing side effect in dogs was identified.KEYWORDS Nicotinic acid, agonist, flushing, dyslipidemia, VLDL, HDL N icotinic acid (NA) has been used as a drug to lower very low-density lipoprotein (VLDL)-cholesterol, lowdensity lipoprotein (LDL)-cholesterol, and lipoprotein-a (LPa), and it is also effective in raising high-density lipoprotein (HDL)-cholesterol in plasma. [1][2][3][4][5][6][7] Studies have demonstrated that nicotinic acid is able to reduce the rate of nonfatal myocardial infarction (MI) and decrease total mortality of patients with a previous history of heart disease. 8,9 In combination with statins, nicotinic acid provided better therapeutic benefit than taking statins alone. 10,11 Despite all of these encouraging clinical benefits, nicotinic acid is known to have side effects that limit its clinical use. The most notable is a severe cutaneous flushing sensation on the upper body and face. 12,13 Other issues include gastrointestinal (GI) side effects 14 and hepatotoxicity. [15][16][17] Additionally, patients need to take large quantities of the drug (grams per day) due to its short half-life, which also leads to compliance issues. To alleviate this associated flushing side effect, an extended release formulation of nicotinic acid (NIASPAN) is available by prescription. 18 More recently, a combination of extended release nicotinic acid and an antagonist of the prostaglandin D2 receptor (DP) (Tredaptive) has been introduced in Europe. Studies have shown that patients taking this combination of drugs experienced reduced flushing symptoms in comparison with taking nicotinic acid alone. 19 Nicotinic acid binds with high affinity to a G-proteincoupled receptor, GPR109a, expressed in human adipose tissue. A highly related G-protein-coupled receptor, GPR109b, that shares 95% identity and is only expressed in human and chimpanzee, was also identified. [20][21][22][23] GPR109b is a low affinity receptor for nicotinic acid. It has been hypothesized that activation of GPR109a by nicotinic acid decreases intracellular cAMP levels in adipocytes. As a downstream effect, protein kinase A activity is reduced, leading to a decrease in hormone sensitive lipase activity, eventually causing the reduction of intracellular triglyceride (TG) hydrolysis and free fatty acid (FFA) secretion. This decrease of FFA production from adipo...