From Screening to Targeted Degradation: Strategies for the Discovery and Optimization of Small Molecule Ligands for PCSK9.

Proprotein convertase substilisin-like/kexin type 9 (PCSK9) is a serine protease involved in a protein-protein interaction with the low-density lipoprotein (LDL) receptor that has both human genetic and clinical validation. Blocking this protein-protein interaction prevents LDL receptor degradation and thereby decreases LDL cholesterol levels. Our pursuit of small-molecule direct binders for this difficult to drug PPI target utilized affinity selection/mass spectrometry, which identified one confirmed hit compound. An X-ray crystal structure revealed that this compound was binding in an unprecedented allosteric pocket located between the catalytic and C-terminal domain. Optimization of this initial hit, using two distinct strategies, led to compounds with high binding affinity to PCSK9. Direct target engagement was demonstrated in the cell lysate with a cellular thermal shift assay. Finally, ligand-induced protein degradation was shown with a proteasome recruiting tag attached to the high-affinity allosteric ligand for PCSK9.

Restoration of insulin receptor improves diabetic phenotype in T2DM mice.

Type 2 diabetes mellitus (T2DM), also known as adult-onset diabetes, is characterized by ineffective insulin action due to insulin resistance in key metabolic tissues. Insulin receptor (IR) plays an important role in insulin signal transduction, defect of which has been considered the fundamental cause of T2DM. IR content reduction in diabetes is one key contributor to the defective insulin signaling and diabetes progression. Rescuing IR levels by transgenic complementation has not been considered as a treatment option because it is limited by uncontrollable expression level, tissue selectivity, or developmental defects. In the current study, we demonstrated that single-dose adeno-associated virus (AAV) vector delivered expression of human IR (hIR) in the liver of inducible IR-knockout mice and significantly improved the diabetic phenotype caused by IR deletion during adulthood. Such an approach was also applied, for the first time to our knowledge, to treating ob/ob mice, a model of severe T2DM attributed to superfluous calorie intake and insulin resistance. Interestingly, similar treatment with AAV-hIR had no obvious effect in healthy animals, indicative of low hypoglycemic risk as a consequence of potential excessive insulin action. The results described here support restoration of IR expression as a safe and effective T2DM therapeutic with a long-lasting profile.

Identification of DGAT2 Inhibitors Using Mass Spectrometry.

Mass spectrometry offers significant advantages over other detection technologies in the areas of hit finding, hit validation, and medicinal chemistry compound optimization. The foremost obvious advantage is the ability to directly measure enzymatic product formation. In addition, the inherent sensitivity of the liquid chromatography/mass spectrometry (LC/MS) approach allows the execution of enzymatic assays at substrate concentrations typically at or below substrate Km. Another advantage of the LC/MS approach is the ability to assay impure enzyme systems that would otherwise be difficult to prosecute with traditional labeled methods. This approach was used to identify inhibitors of diacylglycerol O-acyltransferase-2 (DGAT2), a transmembrane enzyme involved in the triglyceride (TG) production pathway. The LC/MS approach was employed because of its increased assay window (compared with control membranes) of more than sevenfold compared with less than twofold with a conventional fluorogenic assay. The ability to generate thousands of dose-dependent IC50 data was made possible by the use of a staggered parallel LC/MS system with fast elution gradients. From the hit-deconvolution efforts, several structural scaffold series were identified that inhibit DGAT2 activity. Additional profiling of one chemotype in particular identified two promising reversible and selective compounds (compound 15 and compound 16) that effectively inhibit TG production in mouse primary hepatocytes.

Discovery and Pharmacology of a Novel Class of Diacylglycerol Acyltransferase 2 Inhibitors.

DGAT2 plays a critical role in hepatic triglyceride production, and data suggests that inhibition of DGAT2 could prove to be beneficial in treating a number of disease states. This article documents the discovery and optimization of a selective small molecule inhibitor of DGAT2 as well as pharmacological proof of biology in a mouse model of triglyceride production.

Glucagon receptor antagonism induces increased cholesterol absorption.

Glucagon and insulin have opposing action in governing glucose homeostasis. In type 2 diabetes mellitus (T2DM), plasma glucagon is characteristically elevated, contributing to increased gluconeogenesis and hyperglycemia. Therefore, glucagon receptor (GCGR) antagonism has been proposed as a pharmacologic approach to treat T2DM. In support of this concept, a potent small-molecule GCGR antagonist (GRA), MK-0893, demonstrated dose-dependent efficacy to reduce hyperglycemia, with an HbA1c reduction of 1.5% at the 80 mg dose for 12 weeks in T2DM. However, GRA treatment was associated with dose-dependent elevation of plasma LDL-cholesterol (LDL-c). The current studies investigated the cause for increased LDL-c. We report findings that link MK-0893 with increased glucagon-like peptide 2 and cholesterol absorption. There was not, however, a GRA-related modulation of cholesterol synthesis. These findings were replicated using structurally diverse GRAs. To examine potential pharmacologic mitigation, coadministration of ezetimibe (a potent inhibitor of cholesterol absorption) in mice abrogated the GRA-associated increase of LDL-c. Although the molecular mechanism is unknown, our results provide a novel finding by which glucagon and, hence, GCGR antagonism govern cholesterol metabolism.

Utilizing HaloTag Technology to Track the Fate of PCSK9 from Intracellular vs. Extracellular Sources.

The function of a particular protein is dependent upon its localization and milieu. The ability to track the "fate" of a protein is a valuable tool to elucidate its function. We present the use of HaloTag technology to study the localization and fate of human Proprotein Convertase Subtilisin-like Kexin type 9 (PCSK9).The role of PCSK9 in the regulation of circulating low density lipoprotein-cholesterol (LDL-c) levels is ascribed to binding of circulating PCSK9 to the LDL receptor (LDLR) and subsequent lysosomal degradation of LDLR. However, hints in the literature indicate that intracellular PCSK9 may act on the LDLR, possibly during processing of newly synthesized protein. To address this question, the source and fate of intracellular PCSK9 requires further investigation.We applied HaloTag technology to distinguish the source of intracellular PCSK9 and showed that newly synthesized intracellular PCSK9 has unique localization from the PCSK9 after re-uptake. This suggests different functions of PCSK9 while interacting with the LDLR.

Discovery of a new role of human resistin in hepatocyte low-density lipoprotein receptor suppression mediated in part by proprotein convertase subtilisin/kexin type 9.

OBJECTIVES: In this study, our goal was to determine if human resistin plays a role in regulating the uptake of atherogenic low-density lipoproteins in human hepatocytes. BACKGROUND: Serum levels of resistin, an adipose tissue-derived adipokine, are increased in human obesity and are positively correlated with atherosclerotic cardiovascular diseases. However, the function of resistin in humans is enigmatic. METHODS: Human hepatocytes (HepG2 and primary) were treated (24 h) with the following: 1) purified human resistin at various concentrations, with and without lovastatin; and 2) obese human serum with elevated resistin levels or serum from which resistin was removed via antibody-immunoprecipitation. The effect of the treatments on cellular low-density lipoprotein receptor (LDLR) and proprotein convertase subtilisin/kexin type 9 (PCSK9) messenger ribonucleic acid and protein levels were determined by using real-time polymerase chain reaction and Western blotting, respectively. RESULTS: Resistin, at physiological levels observed in human obesity, down-regulated hepatocyte LDLR expression substantially (by 40%). A key mechanism by which human resistin inhibited LDLR levels was by increased cellular expression of the recently identified protease, PCSK9, which enhances intracellular LDLR lysosomal degradation. The quantitatively important role of human resistin in LDLR expression was demonstrated by antibody-immunoprecipitation removal of resistin in human serum, which decreased serum stimulation of hepatocyte LDLRs markedly (by 80%). Furthermore, resistin diminished statin-mediated up-regulation of the LDLR by 60%, implicating resistin in the relative ineffectiveness of statins in selective target populations. CONCLUSIONS: These results reveal for the first time that resistin is a highly attractive therapeutic target in ameliorating elevated serum low-density lipoprotein and, thereby, atherosclerotic cardiovascular diseases in obese humans.

The design and synthesis of potent, selective benzodiazepine sulfonamide bombesin receptor subtype 3 (BRS-3) agonists with an increased barrier of atropisomerization.

Bombesin receptor subtype 3 (BRS-3) is an orphan G-protein coupled receptor expressed primarily in the hypothalamus which plays a role in the onset of both diabetes and obesity. We report herein our progress made towards identifying a potent, selective bombesin receptor subtype-3 (BRS-3) agonist related to the previously described MK-7725(1) Chobanian et al. (2012) that would prevent atropisomerization through the increase of steric bulk at the C-2 position. This would thereby make clinical development of this class of compounds more cost effective by inhibiting racemization which can occur over long periods of time at room/elevated temperature.

Discovery of MK-7725, A Potent, Selective Bombesin Receptor Subtype-3 Agonist for the Treatment of Obesity.

Extensive structure-activity relationship studies of a series derived from atropisomer 1, a previously described chiral benzodiazepine sulfonamide series, led to a potent, brain penetrant and selective compound with excellent preclinical pharmacokinetic across species. We also describe the utilization of a high throughput mouse pharmacodynamic assay which allowed for expedient assessment of pharmacokinetic and brain distribution.

Pyridinesulfonylureas and pyridinesulfonamides as selective bombesin receptor subtype-3 (BRS-3) agonists.

Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein coupled receptor belonging to the subfamily of bombesin-like receptors. BRS-3 is implicated in the development of obesity and diabetes. We report here small-molecule agonists that are based on a 4-(alkylamino)pyridine-3-sulfonamide core. We describe the discovery of 2a, which has mid-nanomolar potency, selectivity for human BRS-3 versus the other bombesin-like receptors, and good bioavailability.

Discovery of MK-5046, a Potent, Selective Bombesin Receptor Subtype-3 Agonist for the Treatment of Obesity.

We report the development and characterization of compound 22 (MK-5046), a potent, selective small molecule agonist of BRS-3 (bombesin receptor subtype-3). In pharmacological testing using diet-induced obese mice, compound 22 caused mechanism-based, dose-dependent reductions in food intake and body weight.

Discovery of benzodiazepine sulfonamide-based bombesin receptor subtype 3 agonists and their unusual chirality.

We report herein the discovery of benzodiazepine sulfonamide-based bombesin receptor subtype 3 (BRS-3) agonists and their unusual chirality. Starting from a high-throughput screening lead, we prepared a series of BRS-3 agonists with improved potency and pharmacokinetic properties, of which compound 8a caused mechanism-based, dose-dependent food intake reduction and body weight loss after oral dosing in diet-induced obese mice. This effort also led to the discovery of a novel family of chiral molecules originated from the conformationally constrained seven-membered diazepine ring.

Antiobesity effect of MK-5046, a novel bombesin receptor subtype-3 agonist.

Bombesin receptor subtype-3 (BRS-3) is an orphan G protein-coupled receptor implicated in the regulation of energy homeostasis. Here, we report the biologic effects of a highly optimized BRS-3 agonist, (2S)-1,1,1-trifluoro-2-[4-(1H-pyrazol-1-yl)phenyl]-3-(4-{[1-(trifluoromethyl)cyclopropyl]methyl}-1H-imidazol-2-yl)propan-2-ol (MK-5046). Single oral doses of MK-5046 inhibited 2-h and overnight food intake and increased fasting metabolic rate in wild-type but not Brs3 knockout mice. Upon dosing for 14 days, MK-5046 at 25 mg · kg(-1) · day(-1) reduced body weight of diet-induced obese mouse by 9% compared with vehicle-dosed controls. In mice, 50% brain receptor occupancy was achieved at a plasma concentration of 0.34 ± 0.23 µM. With chronic dosing, effects on metabolic rate, rather than food intake, seem to be the predominant mechanism for weight reduction by MK-5046. The compound also effectively reduced body weight in rats and caused modest increases in body temperature, heart rate, and blood pressure. These latter effects on temperature, heart rate, and blood pressure were transient in nature and desensitized with continued dosing. MK-5046 is the first BRS-3 agonist with properties suitable for use in larger mammals. In dogs, MK-5046 treatment produced statistically significant and persistent weight loss, which was initially accompanied by increases in body temperature and heart rate that abated with continued dosing. Our results demonstrate antiobesity efficacy for MK-5046 in rodents and dogs and further support BRS-3 agonism as a new approach to the treatment of obesity.

Synthesis of 7-benzyl-5-(piperidin-1-yl)-6,7,8,9-tetrahydro-3H-pyrazolo[3,4-c][2,7]naphthyridin-1-ylamine and its analogs as bombesin receptor subtype-3 agonists.

The original structure of a high-throughput screening hit obtained from an external vendor was revised based on multiple NMR studies. The active compound was re-synthesized via a novel route and its structure and biological activity as a BRS-3 agonist were unambiguously confirmed. Multi-gram quantities of the hit were prepared for pharmacokinetic and efficacy studies. The synthetic strategy allowed for the preparation of multiple analogs for SAR exploration.

Synthesis and SAR of heterocyclic carboxylic acid isosteres based on 2-biarylethylimidazole as bombesin receptor subtype-3 (BRS-3) agonists for the treatment of obesity.

SAR around non-peptidic potent bombesin receptor subtype-3 (BRS-3) agonist lead 2 is presented. Attempts to replace the carboxylic acid with heterocyclic isosteres to improve oral bioavailability and brain penetration are described.

Synthesis and SAR of derivatives based on 2-biarylethylimidazole as bombesin receptor subtype-3 (BRS-3) agonists for the treatment of obesity.

This Letter describes a series of potent and selective BRS-3 agonists containing a biarylethylimidazole pharmacophore. Extensive SAR studies were carried out with different aryl substitutions. This work led to the identification of a compound 2-{2-[4-(pyridin-2-yl)phenyl]ethyl}-5-(2,2-dimethylbutyl)-1H-imidazole 9 with excellent binding affinity (IC(50)=18 nM, hBRS-3) and functional agonist activity (EC(50)=47 nM, 99% activation). After oral administration, compound 9 had sufficient exposure in diet induced obese mice to demonstrate efficacy in lowering food intake and body weight via BRS-3 activation.

Discovery of substituted biphenyl imidazoles as potent, bioavailable bombesin receptor subtype-3 agonists.

We report SAR studies on a novel non-peptidic bombesin receptor subtype-3 (BRS-3) agonist lead series derived from high-throughput screening hit RY-337. This effort led to the discovery of compound 22e with significantly improved potency at both rodent and human BRS-3.

Regulation of energy homeostasis by bombesin receptor subtype-3: selective receptor agonists for the treatment of obesity.

Bombesin receptor subtype 3 (BRS-3) is a G protein coupled receptor whose natural ligand is unknown. We developed potent, selective agonist (Bag-1, Bag-2) and antagonist (Bantag-1) ligands to explore BRS-3 function. BRS-3-binding sites were identified in the hypothalamus, caudal brainstem, and several midbrain nuclei that harbor monoaminergic cell bodies. Antagonist administration increased food intake and body weight, whereas agonists increased metabolic rate and reduced food intake and body weight. Prolonged high levels of receptor occupancy increased weight loss, suggesting a lack of tachyphylaxis. BRS-3 agonist effectiveness was absent in Brs3(-/Y) (BRS-3 null) mice but was maintained in Npy(-/-)Agrp(-/-), Mc4r(-/-), Cnr1(-/-), and Lepr(db/db) mice. In addition, Brs3(-/Y) mice lost weight upon treatment with either a MC4R agonist or a CB1R inverse agonist. These results demonstrate that BRS-3 has a role in energy homeostasis that complements several well-known pathways and that BRS-3 agonists represent a potential approach to the treatment of obesity.

4-Aminoquinoline melanin-concentrating hormone 1-receptor (MCH1R) antagonists.

Structure-activity relationships of a 4-aminoquinoline MCH1R antagonist lead series were explored by synthesis of analogs with modifications at the 2-, 4-, and 6-positions of the original HTS hit. Improvements to the original screening lead included lipophilic groups at the 2-position and biphenyl, cyclohexyl phenyl, and hydrocinnamyl carboxamides at the 6-position. Modifications of the 4-amino group were not well tolerated.