A linker length of and atoms

A linker length of 4 and 5 atoms was found to be optimal ( and ) compared to the only weakly active compounds with longer linkers ( and ) or the inactive compounds with shorter linker chains (–). In a next step incorporation of hetero-atoms in the linker was explored (). An amide linker as in and different ether linkers improved affinity compared to and with the most interesting compounds being and . To further improve the activity of compounds and the influence of substitution at the phenyl ring was investigated with small substituents. Whereas the introduction of methoxy-groups was not beneficial, chlorine or fluorine substitution in or position led to compounds with very high affinity to the GPR109A-receptor such as --Cl and --Cl (, ). These compounds also exhibited very good functional activity at the human and rat receptor. Unfortunately, their in vitro microsomal clearance was not improved compared to (). Therefore, the slightly less potent but metabolically more stable compound --F was selected for in vivo PK profiling. Surprisingly, the half-life after iv dosing was even shorter than that of , despite lower clearance in microsomes. Partly, this can be explained by a fivefold lower volume of distribution of compound --F compared to compound . Another reason however is a mismatch of microsomal and in vivo clearance that was later found to originate from efficient extra-hepatic metabolism; the elucidation of this elimination mechanism, and the further optimization of metabolic stability, will be the subject of a later publication. Representative syntheses of pyrido pyrimidinones are outlined in , , , and follow two general routes. On one hand, side chains can be introduced by N-acylation of 2-amino-nicotinonitrile with the appropriate Tranilast Sodium receptor derivatives. The resulting amide derivatives , , or can be cyclized to the corresponding pyrido pyrimidinones , , and --F using hydrogen peroxide under basic conditions. On the other hand, oxygen-linked side chains can be introduced by nucleophilic substitution reaction using known chloro-derivative and appropriate alcohols such as to obtain compounds as --Cl. In summary, a novel class of agonists for the human GPCR, GPR109A (HM74A), the pyrido pyrimidinones has been discovered and optimized. These molecules show no activity on the highly homologous low affinity receptor GPR109B (HM74). Based on the high throughput screening hit and supported by receptor-based modeling the key pharmacophore moieties of the pyrido pyrimidinones have been identified and a first comprehensive SAR study of such molecules was established making use of human GPR109A binding and GTPγS assays. Elongation at the 2-position of the pyrido pyrimidinone led to molecules with significantly improved in vitro activity. Although selected compounds of this class, for example, --F, show an improved microsomal clearance compared to the HTS hit compound , this is not mirrored by clearance and half-life in rat pharmacokinetic studies. Progress made to address this issue in order to come up with GPR109A agonists which avoid high values, have low / ratios and therefore might be able to dissect desired dyslipidemic effects from the undesired flushing effect will be reported in due course. Acknowledgments
Nicotinic acid is a group B vitamin that in high doses reduces morbidity and mortality from cardiovascular disease., These benefits presumably arise via alterations in the serum lipid profile including elevated high density lipoprotein cholesterol (HDL-C), and reduced total plasma cholesterol, very low-density lipoprotein cholesterol (VLDL-C), low-density lipoprotein cholesterol (LDL-C), triglyceride (TG), and lipoprotein a (Lp(a)). However, the adverse effect of cutaneous vasodilation (flushing) often limits patient compliance to therapy. GPR109A (also called PUMA-G in mice; protein-upregulated in macrophages by interferon γ) is a G-protein coupled receptor for nicotinic acid that is highly expressed in adipocytes and myeloid-lineage immune cells., Binding of agonists to GPR109A induces reduction of intracellular cAMP levels which, in adipocytes, is thought to impair triglyceride lipolysis and thus reduce levels of circulating free fatty acid (FFA). Decreased plasma FFA is thought to be responsible for the other observed changes in serum lipids (e.g., reduction in LDL-C and increase of HDL-C). GPR109A expressed in epidermal Langerhans’ cells has also been shown to mediate cutaneous flushing.