Expanding the drug discovery space with predicted metabolite-target interactions

Metabolites produced in the human gut are known modulators of host immunity. However, large-scale identification of metabolite-host receptor interactions remains a daunting challenge. Here, we employed computational approaches to identify 983 potential metabolite-target interactions using the Inflammatory Bowel Disease (IBD) cohort dataset of the Human Microbiome Project 2 (HMP2). Using a consensus of multiple machine learning methods, we ranked metabolites based on importance to IBD, followed by virtual ligand-based screening to identify possible human targets and adding evidence from compound assay, differential gene expression, pathway enrichment, and genome-wide association studies. We confirmed known metabolite-target pairs such as nicotinic acid-GPR109a or linoleoyl ethanolamide-GPR119 and inferred interactions of interest including oleanolic acid-GABRG2 and alpha-CEHC-THRB. Eleven metabolites were tested for bioactivity in vitro using human primary cell-types. By expanding the universe of possible microbial metabolite-host protein interactions, we provide multiple drug targets for potential immune-therapies.

Identification of potent, nonabsorbable agonists of the calcium-sensing receptor for GI-specific administration

Modulation of gastrointestinal nutrient sensing pathways provides a promising a new approach for the treatment of metabolic diseases including diabetes and obesity. The calcium-sensing receptor has been identified as a key receptor involved in mineral and amino acid nutrient sensing and thus is an attractive target for modulation in the intestine. Herein we describe the optimization of gastrointestinally restricted calcium-sensing receptor agonists starting from a 3-aminopyrrolidine-containing template leading to the identification of GI-restricted agonist 19 (GSK3004774).

Exploration of phenylpropanoic acids as agonists of the free fatty acid receptor 4 (FFA4): Identification of an orally efficacious FFA4 agonist

The long chain free fatty acid receptor 4 (FFA4/GPR120) has recently been recognized as lipid sensor playing important roles in nutrient sensing and inflammation and thus holds potential as a therapeutic target for type 2 diabetes and metabolic syndrome. To explore the effects of stimulating this receptor in animal models of metabolic disease, we initiated work to identify agonists with appropriate pharmacokinetic properties to support progression into in vivo studies. Extensive SAR studies of a series of phenylpropanoic acids led to the identification of compound 29, a FFA4 agonist which lowers plasma glucose in two preclinical models of type 2 diabetes.

Identification of nonabsorbable inhibitors of the scavenger receptor-BI (SR-BI) for tissue-specific administration

The identification of a low-permeability scavenger receptor BI (SR-BI) inhibitor starting from the ITX-5061 template is described. Structure-activity and structure-permeability relationships were assessed for analogs leading to the identification of compound 8 as a potent and nonabsorbable SR-BI inhibitor.

Identification of diarylsulfonamides as agonists of the free fatty acid receptor 4 (FFA4/GPR120)

The exploration of a diarylsulfonamide series of free fatty acid receptor 4 (FFA4/GPR120) agonists is described. This work led to the identification of selective FFA4 agonist 8 (GSK137647A) and selective FFA4 antagonist 39. The in vitro profile of compounds 8 and 39 is presented herein.

Discovery of 3-aryl-4-isoxazolecarboxamides as TGR5 receptor agonists

A series of 3-aryl-4-isoxazolecarboxamides identified from a high-throughput screening campaign as novel, potent small molecule agonists of the human TGR5 G-protein coupled receptor is described. Subsequent optimization resulted in the rapid identification of potent exemplars 6 and 7 which demonstrated improved GLP-1 secretion in vivo via an intracolonic dose coadministered with glucose challenge in a canine model. These novel TGR5 receptor agonists are potentially useful therapeutics for metabolic disorders such as type II diabetes and its associated complications.

Constitutive receptor systems for drug discovery

This paper discusses the use of constitutively active G-protein-coupled receptor systems for drug discovery. Specifically, the ternary complex model is used to define the two major theoretical advantages of constitutive receptor screening-namely, the ability to detect antagonists as well as agonists directly and the fact that constitutive systems are more sensitive to agonists. In experimental studies, transient transfection of Chinese hamster ovary cyclic AMP response element (CRE) luciferase reporter cells with cDNA for human parathyroid hormone receptor, glucagon receptor, and glucagon-like peptide (GLP-1) receptor showed cDNA concentration-dependent constitutive activity with parathyroid hormone (PTH-1) and glucagon. In contrast, no constitutive activity was observed for GLP-1 receptor, yet responses to GLP-1 indicated that receptor expression had taken place. In another functional system, Xenopus laevi melanophores transfected with cDNA for human calcitonin receptor showed constitutive activity. Nine ligands for the calcitonin receptor either increased or decreased constitutive activity in this assay. The sensitivity of the system to human calcitonin increased with increasing constitutive activity. These data indicate that, for those receptors which naturally produce constitutive activity, screening in this mode could be advantageous over other methods.

Recombinant human CXC-chemokine receptor-4 in melanophores are linked to Gi protein: seven transmembrane coreceptors for human immunodeficiency virus entry into cells

This article describes the transient expression of the CXC chemokine receptor-4 in Xenopus laevis melanophores and the resulting functional assay for the endogenous ligand for this receptor stromal cell-derived factor (SDF)-1alpha. Specifically, it will be shown that SDF-1alpha produces increased light transmittance in transfected cells that is consistent with the activation of Gi protein. This stimulus pathway is further implicated by the abolition of this response after pretreatment of the cells with pertussis toxin, a known method for the inactivation of Gi protein. The fact that SDF-1alpha does not produce responses in nontransfected cells and that treatment of the cells with 12G5, an antibody specific for the CXC chemokine receptor-4, eliminates this response indicates that this ligand produces responses by activation of this receptor in these cells. The possible relevance to human immunodeficiency virus (HIV) entry into cells was explored by observing the effects of SDF-1alpha on HIV-mediated cell fusion. It was found that SDF-1alpha blocked cell-to-cell fusion (as has been previously reported) at concentrations 1200-fold greater than those required to produce Gi protein mediated responses. The implications of the functional assay to screening for new drugs to block HIV-mediated fusion is discussed.

Regulation of calcium levels in brain tissue from adult and aged rats

The possibility that regulation of Ca2+ levels in brain nerve terminals is altered as the brain ages was examined in synaptosomes from adult and aged Fischer 344 rats. Free intrasynaptosomal [Ca2+]i was monitored with fura-2 as synaptosomes were depolarized with KCl, veratridine and ibotenic acid. With all three depolarizing agents, synaptosomes from aged animals reached higher free Ca2+ levels, and the maximal Ca2+ increases (delta Ca2+) estimated from computer assisted-fitting of the curves, ranged from 35% to 80% greater in synaptosomes from aged animals. The total Ca2+ content of the brain and of synaptosomes was also found to be considerably higher in aged than in adult animals. These results suggest that the aging process in brain is accompanied by alterations in both dynamic aspects of Ca2+ handling in nerve endings and the overall content of Ca2+ in the brain and synaptic terminals.

Purification of a synaptic membrane Na+/Ca2+ antiporter and immunoextraction with antibodies to a 36-kDa protein

The conditions for optimal solubilization and reconstitution of bovine brain synaptic plasma membrane Na+/Ca2+ exchange activity were examined and a series of chromatographic procedures were used for the isolation of a protein involved in this transport activity. The zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate in the presence of 20% (vol/vol) glycerol led to optimal solubilization, and soybean phospholipids in low-pH medium were found to produce optimal reconstitution of activity after dialysis to remove the detergent. Sequential chromatography steps involving the use of gel filtration on Sephacryl S-400 HR, ion exchange on diethylaminoethyl-Sephacel, and metal chelate chromatography on tris-(carboxymethyl)ethylenediamine loaded with LaCl3 led to the isolation of a fraction highly enriched in both Na+/Ca2+ exchange activity and two protein bands identified by denaturing electrophoresis. The estimated molecular masses of the two proteins were 50 and 36 kDa. Development of polyclonal antibodies to the 36-kDa protein permitted immunoextraction of greater than 95% of the antiporter activity from solubilized synaptic plasma membranes. These antibodies cross-reacted with the electroeluted 50-kDa protein on enzyme-linked immunosorbent assays, suggesting a close relationship between the two proteins. These results indicate that the 36-kDa protein is at least a component of the brain membrane Na+/Ca2+ antiporter.