Anti-obesity effects of small molecule melanin-concentrating hormone receptor 1 (MCHR1) antagonists

Over the past ten years, tremendous advances in our understanding of the role of the hypothalamic neurohormone, melanin-concentrating hormone (MCH), and its involvement in the regulation of food intake and body weight have been achieved. The MCHR1 receptor has been actively targeted as a much-needed, novel treatment for obesity, a disease of epidemic proportion in the United States. Numerous companies have joined the competition to be the first to produce a small molecule antagonist targeting MCHR1 receptors in the race for therapeutics for this disease. This review details the rising need for new treatments for obesity; the rationale and target validation of MCHR1 receptor antagonists as potential treatments for this disease; and the current status of the numerous small molecule MCHR1 antagonists in development by different companies. MCHR1 antagonists might find an additional usage in the treatment of anxiety and depression disorders. The rationale and current status of this effort by several companies is also reviewed.     

Discovery of a novel series of melanin-concentrating hormone receptor 1 antagonists for the treatment of obesity

A new class of MCHR1 antagonists was discovered via a high-throughput screen. Optimization of the lead structure resulted in the identification of indole 10e. This compound possesses good pharmacokinetic properties across preclinical species and is efficacious in reducing food consumption in an MCH cannulated rat model and a cynomolgus monkey food consumption model.     

Characterization of a neuronal cell line expressing native human melanin-concentrating hormone receptor 1 (MCHR1)

Melanin-concentrating hormone (MCH), an orexigenic neuropeptide in mammals, activates a G-protein coupled receptor, MCHR1. It is expected that antagonists of MCHR1 function will prove therapeutically useful as anti-obesity agents. Intracellular signaling by MCHR1 has been investigated primarily using non-neural cell lines expressing the recombinant receptor, in which MCHR1 has been shown to couple to G alpha(i/o) and G alpha(q) G-proteins. While these cell lines have been widely utilized to discover and optimize small molecule antagonists, it is unknown whether the intracellular signaling pathways in these cells accurately reflect those in neurons. Thus, we sought to develop a neurally derived cell line endogenously expressing MCHR1. IMR32, a human neuroblastoma cell line, has been shown to express MCHR1 mRNA; however, we were unable to detect either MCH-binding or MCH-stimulated Ca++-mobilization in these cells. Following transfection of IMR32 cells with a plasmid encoding human G alpha(16) G-protein, we isolated a cell line, I3.4.2, which responded to MCH in Ca++-mobilization assays. We found that the expression level of MCHR1 mRNA in I3.4.2 cells was 2000-fold higher than in the parent cell line. Using [125I]MCH saturation-binding to I3.4.2 cell membranes, we estimated the Bmax as 0.72 pmol/mg protein and the Kd as 0.35 nM. We report that Ca++-mobilization in I3.4.2 cells was insensitive to pertussis toxin (Ptx) treatment, indicating that signaling was via G alpha(q) G-proteins. Furthermore, negative results in cAMP accumulation assays confirmed the lack of signaling via the G alpha(i/o) G-proteins. Our results suggest that the I3.4.2 cell line may be useful for characterization of MCHR1 activity in a neural-derived cell line.     

Novel pyrrolidine melanin-concentrating hormone receptor 1 antagonists with reduced hERG inhibition

We discovered novel pyrrolidine MCHR1 antagonist 1 possessing moderate potency. Profiling of pyrrolidine 1 demonstrated that it was an inhibitor of the hERG channel. Investigation of the structure-activity relationship of this class of pyrrolidines allowed us to optimize the MCHR1 potency and decrease the hERG inhibition. Increasing the acidity of the amide proton by converting the benzamide in lead 1 to an anilide provided single digit nanomolar MCHR1 antagonists while replacing the dimethoxyphenyl ring of 1 with alkyl groups possessing increased polarity dramatically reduced the hERG inhibition.     

Discovery and characterization of a potent and selective antagonist of melanin-concentrating hormone receptor 2

A series of spiropiperidine carbazoles were synthesized and evaluated as MCHR2 antagonists using a FLIPR assay. The pharmacokinetic properties of selected compounds have also been studied. This effort led to the discovery of potent and specific MCHR2 antagonists. Compound 38 demonstrated good pharmacokinetic properties across rat, beagle dog and rhesus monkey and had a favorable selectivity profile against a number of other receptors. These MCHR2 antagonists are considered appropriate tool compounds for study of the function of MCHR2 in vivo.     

Discovery of novel,orally available benzimidazoles as melanin concentrating hormone receptor 1 (MCHR1) antagonists

Melanin concentrating hormone (MCH) is an important mediator of energy homeostasis and plays role in several disorders such as obesity, stress, depression and anxiety. The synthesis and biological evaluation of novel benzimidazole derivatives as MCHR1 antagonists are described. The in vivo proof of principle for weight loss with a lead compound from this series is exemplified.     

Amine-free melanin-concentrating hormone receptor 1 antagonists: Novel 1-(1H-benzimidazol-6-yl)pyridin-2(1H)-one derivatives and design to avoid CYP3A4 time-dependent inhibition

Melanin-concentrating hormone (MCH) is an attractive target for antiobesity agents, and numerous drug discovery programs are dedicated to finding small-molecule MCH receptor 1 (MCHR1) antagonists. We recently reported novel pyridine-2(1H)-ones as aliphatic amine-free MCHR1 antagonists that structurally featured an imidazo[1,2-a]pyridine-based bicyclic motif. To investigate imidazopyridine variants with lower basicity and less potential to inhibit cytochrome P450 3A4 (CYP3A4), we designed pyridine-2(1H)-ones bearing various less basic bicyclic motifs. Among these, a lead compound 6a bearing a 1H-benzimidazole motif showed comparable binding affinity to MCHR1 to the corresponding imidazopyridine derivative 1. Optimization of 6a afforded a series of potent thiophene derivatives (6q–u); however, most of these were found to cause time-dependent inhibition (TDI) of CYP3A4. As bioactivation of thiophenes to form sulfoxide or epoxide species was considered to be a major cause of CYP3A4 TDI, we introduced electron withdrawing groups on the thiophene and found that a CF₃ group on the ring or a Cl adjacent to the sulfur atom helped prevent CYP3A4 TDI. Consequently, 4-[(5-chlorothiophen-2-yl)methoxy]-1-(2-cyclopropyl-1-methyl-1H-benzimidazol-6-yl)pyridin-2(1H)-one (6s) was identified as a potent MCHR1 antagonist without the risk of CYP3A4 TDI, which exhibited a promising safety profile including low CYP3A4 inhibition and exerted significant antiobesity effects in diet-induced obese F344 rats.     

Building a MCHR1 homology model provides insight into the receptor-antagonist contacts that are important for the development of new anti-obesity agents

Melanin-concentrating hormone (MCH) regulates feeding and energy homeostasis through interaction with its receptor, the melanin-concentrating receptor 1 (MCHR1), making it a target in the treatment of obesity. Molecular modeling and docking studies were performed in order to find a binding model for the docking of two new series of MCHR1 antagonists to the receptor. Results suggested interactions between the ligands and two glutamines (Gln5.42 and Gln6.55) not conserved in many of the GPCRs family members. Histamine 3 receptor (HRH3) presents two apolar residues in the aforementioned positions and the available biological data against this receptor supported the role of the two glutamines in the binding of antagonists to the MCHR1. This knowledge could be useful in the development of new, more active and more selective MCHR1 antagonists.     

Design and optimization of quinazoline derivatives as melanin concentrating hormone receptor 1 (MCHR1) antagonists

Melanin concentrating hormone (MCH) is an important mediator of energy homeostasis and plays a role in metabolic and CNS disorders. The modeling-supported design, synthesis and multi-parameter optimization (biological activity, solubility, metabolic stability, hERG) of novel quinazoline derivatives as MCHR1 antagonists are described. The in vivo proof of principle for weight loss with a lead compound from this series is exemplified. Clusters of refined hMCHR1 homology models derived from the X-ray structure of the β2-adrenergic receptor, including extracellular loops, were developed and used to guide the design.     

Potent MCH-1 receptor antagonists from cis-1,4-diaminocyclohexane-derived indane analogs

Benzimidazole and indane are the two key fragments in our potent and selective MCH-1 receptor (MCHR1) antagonists. To identify novel linkers connecting the two fragments, we investigated diamino-cycloalkane-derived analogs and discovered highly potent antagonists with cis-1,4-diaminocyclohexane as a unique spacer in this chemical class. Structural overlay suggested that cis-1-substituted-4-aminocyclohexane functions as a bioisostere of 4-substituted-piperidine and that the active conformation adopts a U-shaped orientation.     

Osteoporosis in MCHR1-deficient mice

It is well recognized that the hypothalamus is of central importance in the regulation of food intake and fat mass. Recent studies indicate that it also plays an important role in the regulation of bone mass. Melanin concentrating hormone (MCH) is highly expressed in the hypothalamus and has been implicated in regulation of energy homeostasis. We developed MCHR1 inactivated mice to evaluate the physiological role of this receptor. Interestingly, the MCHR1(-/-) mice have osteoporosis, caused by a reduction in the cortical bone mass, while the amount of trabecular bone is unaffected. The reduction in cortical bone mass is due to decreased cortical thickness. Serum levels of c-telopeptide, a marker of bone resorption, are increased in MCHR1(-/-) mice, indicating that the MCHR1(-/-) mice have a high bone turnover osteoporosis. In conclusion, the MCHR1(-/-) mice have osteoporosis, indicating that MCHR1-signalling is involved in a tonic stimulation of bone mass.     

Signalling pathway of goldfish melanin-concentrating hormone receptors 1 and 2

Melanin-concentrating hormone (MCH) is the natural ligand for the MCH-1 receptor (MCHR1) and MCH-2 receptor (MCHR2). The MCH-MCHR1 system plays a central role in energy metabolism in rodents. Recently, we identified MCHR1 and MCHR2 orthologues in goldfish, designated gfMCHR1 and gfMCHR2. In a mammalian cell-based assay, calcium mobilization was evoked by gfMCHR2 via both Gαi/o and Gαq, while the gfMCHR1-mediated response was exclusively dependent on Gαq. This coupling capacity to G proteins is in contrast to human MCHR1 and MCHR2. Here, we extended our previous characterization of the two gfMCHRs by examining their different signalling pathway. We found that MCH caused activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) via both gfMCHR1 and gfMCHR2 in dose-dependent manners. Unlike the case for gfMCHR2, gfMCHR1 signalling was not sensitive to pertussis toxin, suggesting Gαq coupling of gfMCHR1 in the ERK1/2 pathway as well as a calcium mobilization system. Cyclic AMP assays revealed that gfMCHR2 was efficiently coupled to Gαi/o, while gfMCHR1 was weakly coupled to Gαs. Finally, we investigated the transduction features stimulated by two mammalian MCH analogues. As expected, Compound 15, which is a full agonist of human MCHR1, was a potent gfMCHR1 agonist in multiple signalling pathways. On the other hand, Compound 30, which is a human MCHR1-selective antagonist with negligible agonist potency, unexpectedly acted as a selective agonist of gfMCHR1. These results are the first to demonstrate that gfMCHR1 and gfMCHR2 have quite different signalling properties from human MCHRs.     

MCH receptor peptide agonists and antagonists

Melanin-concentrating hormone (MCH) is an important neuropeptide hormone involved in multiple physiological processes. Peptide derivatives of MCH have been developed as tools to aid research including potent radioligands, receptor selective agonists, and potent antagonists. These tools have been used to further understand the role of MCH in physiology, primarily in rodents. However, the tools could also help elucidate the role for MCHR1 and MCHR2 in mediating MCH signaling in higher species.     

Single nucleotide polymorphisms of the MCHR1 gene do not affect metabolism in humans

Melanin concentrating hormone receptor-1 (MCHR1) is a centrally and peripherally expressed receptor that regulates energy expenditure and appetite. Single nucleotide polymorphisms (SNPs) of the MCHR1 gene have been previously associated with obesity, but the results are inconsistent among different populations. This study was performed to determine whether SNPs of MCHR1 affect glucose and energy metabolism. We screened six SNPs of MCHR1 in a cross-sectional study of 217 middle-age, non-diabetic Finnish subjects who were offspring of type 2 diabetic patients. Insulin secretion was evaluated by an intravenous glucose tolerance test and insulin sensitivity and energy metabolism by the hyperinsulinemic euglycemic clamp and indirect calorimetry. SNPs of MCHR1 were not associated with BMI, waist circumference, subcutaneous or intra-abdominal fat area, glucose tolerance, first-phase insulin release, insulin sensitivity, or energy metabolism. One SNP, which was in >0.50 linkage disequilibrium with the other five SNPs, was also screened in 1455 unrelated Finnish middle-age subjects in a population-based study. No differences in BMI, waist circumference, or glucose or insulin levels in an oral glucose tolerance test among the genotypes were found. In conclusion, SNPs of MCHR1 did not have effects on metabolic variables in humans.     

Discovery of a novel melanin concentrating hormone receptor 1 (MCHR1) antagonist with reduced hERG inhibition

An initial SAR study resulted in the identification of the novel, potent MCHR1 antagonist 2. After further profiling, compound 2 was discovered to be a potent inhibitor of the hERG potassium channel, which prevented its further development. Additional optimization of this structure resulted in the discovery of the potent MCHR1 antagonist 11 with a dramatically reduced hERG liability. The decrease in hERG activity was confirmed by several in vivo preclinical cardiovascular studies examining QT prolongation. This compound demonstrated good selectivity for MCHR1 and possessed good pharmacokinetic properties across preclinical species. Compound 11 was also efficacious in reducing body weight in two in vivo mouse models. This compound was selected for clinical evaluation and was given the code AMG 076.     

Synthesis of novel bicyclo[4.1.0]heptane and bicyclo[3.1.0]hexane derivatives as melanin-concentrating hormone receptor R1 antagonists

To address the hERG liability of MCHR1 antagonists such as 1 and 2, new analogs such as 4 and 5 that incorporated a polar heteroaryl group were designed and synthesized. Biological evaluation confirmed that these new analogs retained MCH R1 activity with greatly attenuated hERG liabilities as indicated in the Rb efflux assay.     

Beyond skin color: emerging roles of melanin-concentrating hormone in energy homeostasis and other physiological functions

Melanin-concentrating hormone (MCH) is a cyclic peptide that mediates its effects by the activation of two G-protein-coupled seven transmembrane receptors (MCHR1 and MCHR2) in humans. In contrast to its primary role in regulating skin color in fish, MCH has evolved in mammals to regulate dynamic physiological functions, from food intake and energy expenditure to behavior and emotion. Chronic infusion or transgenic expression of MCH stimulates feeding and increases adipocity, whereas targeted deletion of MCH or its receptor (MCHR1) leads to resistance to diet-induced obesity with increased energy expenditure and thermogenesis. The involvement of MCH in energy homeostasis and in brain activity has also been validated in mice treated with non-peptide antagonists, suggesting that blockade of MCHR1 could provide a viable approach for treatment of obesity and certain neurological disorders. This review focuses on emerging roles of MCH in regulating central and peripheral mechanisms.     

Melanin-concentrating hormone receptor 1 antagonists: synthesis, structure-activity relationship, docking studies, and biological evaluation of 2,3,4,5-tetrahydro-1H-3-benzazepine derivatives

Melanin-concentrating hormone receptor 1 (MCHR1) antagonists have been studied as potential agents for the treatment of obesity. Initial structure-activity relationship studies of in-house hit compound 1a and subsequent optimization studies resulted in the identification of tetrahydroisoquinoline derivative 23, 1-(2-acetyl-1,2,3,4-tetrahydroisoquinolin-7-yl)-4-[4-(4-chlorophenyl)piperidin-1-yl]butan-1-one, as a potent hMCHR1 antagonist. A homology model of hMCHR1 suggests that these compounds interact with Asn 294 and Asp 123 in the binding site of hMCHR1 to enhance binding affinity. Oral administration of compound 23 dose-dependently reduced food intake in diet-induced obesity (DIO)-F344 rats.     

Mice lacking melanin-concentrating hormone receptor 1 demonstrate increased heart rate associated with altered autonomic activity

Melanin-concentrating hormone (MCH) plays an important role in energy balance. The current studies were carried out on a new line of mice lacking the rodent MCH receptor (MCHR1(-/-) mice). These mice confirmed the previously reported lean phenotype characterized by increased energy expenditure and modestly increased caloric intake. Because MCH is expressed in the lateral hypothalamic area, which also has an important role in the regulation of the autonomic nervous system, heart rate and blood pressure were measured by a telemetric method to investigate whether the increased energy expenditure in these mice might be due to altered autonomic nervous system activity. Male MCHR1(-/-) mice demonstrated a significantly increased heart rate [24-h period: wild type 495 +/- 4 vs. MCHR1(-/-) 561 +/- 8 beats/min (P < 0.001); dark phase: wild type 506 +/- 8 vs. MCHR1(-/-) 582 +/- 9 beats/min (P < 0.001); light phase: wild type 484 +/- 13 vs. MCHR1(-/-) 539 +/- 9 beats/min (P < 0.005)] with no significant difference in mean arterial pressure [wild type 110 +/- 0.3 vs. MCHR1(-/-) 113 +/- 0.4 mmHg (P > 0.05)]. Locomotor activity and core body temperature were higher in the MCHR1(-/-) mice during the dark phase only and thus temporally dissociated from heart rate differences. On fasting, wild-type animals rapidly downregulated body temperature and heart rate. MCHR1(-/-) mice displayed a distinct delay in the onset of this downregulation. To investigate the mechanism underlying these differences, autonomic blockade experiments were carried out. Administration of the adrenergic antagonist metoprolol completely reversed the tachycardia seen in MCHR1(-/-) mice, suggesting an increased sympathetic tone.