Structure–activity relationship studies on tetralin carboxamide growth hormone secretagogue receptor antagonists

Induction of intrinsic apoptosis pathway in colon cancer HCT-116 cells by novel 2-substituted-5,6,7,8-tetrahydronaphthalene derivatives

2-Acetyl tetralin (1) reacted with N,N-dimethylformamide dimethylacetal (DMF-DMA) to afford the enaminone 3. The reaction of 3 with piperidine and morpholine afforded the trans enaminone 5a,b, respectively. Compound 3 was treated with primary aromatic amines to give secondary enaminones 6a-e. The enaminone 3 reacted with acetylglycine and hippuric acid to yield pyranones 10a, b, respectively. The reaction of enaminone 3 with 1,4-benzoquinone and 1,4-naphthoquinone gave benzofuranyl tetralin derivatives 14a,b, respectively. Also, when 3 reacted with 5-amino-3-phenyl-1H-pyrazole 15a and 5-amino-1,2,3-triazole 15b, it afforded the new pyrazolo[1,5-a]pyrimidine 17a and 1,2,3-triazolo[1,5-a]pyrimidine 17b, respectively. While the reaction of 3 with pyrimidines 18a, b resulted in the formation of pyrido[2,3-d]pyrimidine derivatives 20a, b, respectively. Investigations of the cytotoxic effect of those compounds against different human cell lines indicated that some compounds showed high selective cytotoxicity against colon cancer HCT-116 cells. Some of these compounds led to DNA damaging and fragmentation that was associated with the induction of apoptosis via mitochondrial pathway. This pathway is initiated by the impairment of mitochondrial transmembrane potential (Δψm) and in response to that the mitochondria released cytochrome c increased, that in turn activated caspase-9 and caspase-3 and induced apoptosis. Compounds 17b and 20b were promising anti-cancer agents that induced intrinsic apoptosis pathway in colon cancer cells.

Ghrelin receptor in energy homeostasis and obesity pathogenesis

The ghrelin receptor, also known as growth hormone secretagogue receptor (GHS-R), was identified in porcine and rat anterior pituitary membranes, where the synthetic secretagogue MK-0677 causes amplified pulsatile growth hormone (GH) release. In addition to its function in the stimulation of GH secretion, ghrelin, the natural ligand of ghrelin receptor is now recognized as a peptide hormone with fundamental influence on energy homeostasis. Despite the potential existence of multiple subtypes of ghrelin receptor, the effects of ghrelin on energy metabolism, obesity, and diabetes are mediated by its classical receptor GHS-R1a, whose activation requires the n-octanoylation of ghrelin. Here we review the current understanding of the role of the ghrelin receptor in the regulation of energy homeostasis. An overview of the ghrelin receptor is presented first, followed by the discussion on its effects on food intake, glucose homeostasis, and lipid metabolism. Finally, potential strategies for treating obesity and diabetes via manipulation of the ghrelin/ghrelin receptor axis are explored.

MC4R dimerization in the paraventricular nucleus and GHSR/MC3R heterodimerization in the arcuate nucleus: is there relevance for body weight regulation?

The worldwide obesity epidemic is increasing, yet at this time, no long-acting and specific pharmaceutical therapies are available. Peripheral hormonal signals communicate metabolic status to the hypothalamus by activating their corresponding receptors in the arcuate nucleus (ARC). In this brain region, a variety of G protein-coupled receptors (GPCRs) are expressed that are potentially involved in weight regulation, but so far, the detailed function of most hypothalamic GPCRs is only partially understood. An important and underappreciated feature of GPCRs is the capacity for regulation via di- and heterodimerization. Increasing evidence implicates that heterodimerization of GPCRs results in profound functional consequences. Recently, we could demonstrate that interaction of the melanocortin 3 receptor (MC3R) and the growth hormone secretagogue receptor (GHSR)-1a results in a modulation of function in both receptors. Although the physiological role of GPCR-GPCR interaction in the hypothalamus is yet to be elucidated, this concept promises new avenues for investigation and understanding of hypothalamic functions dependent on GPCR signaling. Since GPCRs are important targets for drugs to combat many diseases, identification of heterodimers may be a prerequisite for highly specific drugs. Therefore, a detailed understanding of the mechanisms and their involvement in weight regulation is necessary. Fundamental to this understanding is the interplay of GPCR-GPCR in the hypothalamic nuclei in energy metabolism. In this review, we summarize the current knowledge on melanocortin receptors and GHSR-1a in hypothalamic weight regulation, especially as they pertain to possible drug targets. Furthermore, we include available evidence for the participation and significance of GPCR dimerization.

Modeling of the growth hormone secretagogue receptor antagonistic activity using chemometric tools

A quantitative structure-activity relationship (QSAR) study has been carried out on growth hormone secretagogue receptor antagonistic activity of the derivatives of 2,4-diaminopyrimidine. To obtain significant QSARs, the approaches involving the non-parametric such as Fujita-Ban, and the parametric based on physicochemical and DRAGON descriptors in Hansch type of analysis have been employed. The Fujita-Ban approach, however, was constrained to 18 compounds only due to a limited number of substituents appeared at varying positions. The derived contributions of different substituents and the parent moiety were used to identify the potential congeners. The physicochemical model of Hansch was subsequently used to interpret the type of interactions involved between the receptor sites and varying positions of these compounds. The study, employing DRAGON descriptors in Hansch approach was also carried out on this data set to discuss the prevailing interactions in terms of topological descriptors. The derived highest significant model was discussed to delineate the type of interactions involved and suggestions have been made for different alterations to lead to further potential compounds of the series.

Constitutive activation of G protein-coupled receptors and diseases: insights into mechanisms of activation and therapeutics

The existence of constitutive activity for G protein-coupled receptors (GPCRs) was first described in 1980s. In 1991, the first naturally occurring constitutively active mutations in GPCRs that cause diseases were reported in rhodopsin. Since then, numerous constitutively active mutations that cause human diseases were reported in several additional receptors. More recently, loss of constitutive activity was postulated to also cause diseases. Animal models expressing some of these mutants confirmed the roles of these mutations in the pathogenesis of the diseases. Detailed functional studies of these naturally occurring mutations, combined with homology modeling using rhodopsin crystal structure as the template, lead to important insights into the mechanism of activation in the absence of crystal structure of GPCRs in active state. Search for inverse agonists on these receptors will be critical for correcting the diseases cause by activating mutations in GPCRs. Theoretically, these inverse agonists are better therapeutics than neutral antagonists in treating genetic diseases caused by constitutively activating mutations in GPCRs.

Recent Developments in Ghrelin Receptor Ligands

The 28-amino acid peptide ghrelin is a neuroendocrine hormone synthesized primarily in the stomach. It stimulates growth hormone secretion and appetite, thus promoting food intake and body-weight gain. The pharmacological properties of this peptide are mediated by the growth hormone secretagogue receptor type 1a (GHS-R1a). Given its wide spectrum of biological activities, it is evident that the discovery of ghrelin and its receptor has opened up many perspectives in the fields of neuroendocrine and metabolic research and has had an influence on such fields of internal medicine as gastroenterology, oncology, and cardiology. It is therefore increasingly likely that synthetic, peptidyl, and nonpeptidyl GHS-R1a ligands, acting as agonists, partial agonists, antagonists, or inverse agonists, could have both clinical and therapeutic potential. This review summarizes the various types of GHS-R1a ligands that have been described in the literature and discusses the recent progress made in this research area.

Optimization of 2,4-diaminopyrimidines as GHS-R antagonists: Side chain exploration

The synthesis and structure-activity relationships of the 4- and 6-substituents of 2,4-diaminopyrimidine-based growth hormone secretagogue receptor (GHS-R) antagonists are described. Diaminopyrimidines with 6-norbornenyl (4n) and 6-tetrahydrofuranyl (4p) substitutents were found to exhibit potent GHS-R antagonism and good selectivity (approximately 1000-fold) against dihydrofolate reductase.