New highly potent dipeptidic growth hormone secretagogues with low molecular weight

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.

Modeling of human ghrelin receptor (hGHS-R1a) in its close state and validation by molecular docking

The objective of this study was to generate a reliable model of human ghrelin receptor (hGHS-R1a) in its close state by means of a hybrid fragmental approach in which the transmembrane bundle was modeled using the rhodopsin as the template to assure a marked closeness among the transmembrane helices, while the remaining segments (i.e., loops plus terminal domains) were modeled searching different templates to favor the local homologies. The reliability of this model was assessed docking both a tetrapeptide, which represents the ghrelin's active core, and a set of 50 peptidomimetic secretagogues taken from the literature. The analysis of obtained complexes unveils a set of stabilizing interactions with crucial hGHS-R1a residues in remarkable agreement with both mutational analyses and pharmacophore hypotheses. Also the significant correlation between docking scores and biological activities affords an encouraging validation for such hGHS-R1a model, suggesting that also the receptor in its close state (similarly to the hGHS-R1a in its open state which was modeled in our previous study, Pedretti A, Villa M, Pallavicini M, Valoti E, Vistoli G. J. Med. Chem.2006, 49, p 3077.) may be involved in ligand binding and could find fertile applications in ligand design.

Construction of human ghrelin receptor (hGHS-R1a) model using a fragmental prediction approach and validation through docking analysis

The objective of this study was to investigate the reliability of a fragmental approach to build a full-length model of the human ghrelin receptor (hGHS-R1a) in its open state. The soundness of the model was verified by docking the tetrapeptide Gly-Ser-Ser(n-octanoyl)-Phe-NH2, which represents the ghrelin active core, and a dataset of 35 peptidomimetic GH secretagogues taken from literature. Docking results confirm the relevance of two distinct subpockets: a polar cavity bearing the key residues involved in receptor activation and an aromatic/apolar subpocket, which plays a crucial role in determining the high constitutive activity of hGHS-R1a. The docking scores of both subpockets are in remarkable agreement with biological data, emphasizing that the model can be used to predict the activity of novel ligands. Moreover, the subpocket selectivity of peptidomimetic GHSs suggests a cooperative role of the aromatic/apolar subpocket. Taken globally, the results highlight the potential of the fragmental approach to build improved models for any GPCR.

Development of growth hormone secretagogues

The GH secretagogues (GHS) were developed by reverse pharmacology. The objective was to develop small molecules with pharmacokinetics suitable for once-daily oral administration that would rejuvenate the GH/IGF-I axis. Neither the receptor nor the ligand that controlled pulse amplitude of hormone release was known; therefore, identification of lead structures was based on function. I reasoned that GH pulse amplitude could be increased by four possible mechanisms: 1) increasing GHRH release; 2) amplifying GHRH signaling in somatotrophs of the anterior pituitary gland; 3) reducing somatostatin release; and 4) antagonizing somatostatin receptor signaling. Remarkably, the GHS act through all four mechanisms to reproduce a young adult physiological GH profile in elderly subjects that was accompanied by increased bone mineral density and lean mass, modest improvements in strength, and improved recovery from hip fracture. Furthermore, restoration of thymic function was induced in old mice. The GHS receptor (GHS-R) was subsequently identified by expression cloning and found to be a previously unknown G protein-coupled receptor expressed predominantly in brain, pituitary gland, and pancreas. Reverse pharmacology was completed when the cloned GHS-R was exploited to identify an endogenous agonist (ghrelin) and a partial agonist (adenosine); ghsr-knockout mice studies confirmed that GHS are ghrelin mimetics.

Identification of Structurally Diverse Growth Hormone Secretagogue Agonists by Virtual Screening and Structure−Activity Relationship Analysis of 2-Formylaminoacetamide Derivatives

Two molecules with known growth hormone secretagogue (GHS) agonist activity were used as templates to computationally screen approximately 80000 compounds. A total of 108 candidate compounds were selected, and five of them were found to be active in the low-micromolar range in both cell-based and direct binding assays. These compounds were structurally diverse and significantly differed from known GHS agonists. The most active compound was subjected to SAR evaluation, which slightly increased its potency and identified molecular regions important for specific GHS agonist activity.

The influence of conformational restriction in the C-terminus of growth hormone secretagogues on their potency

In order to obtain more potent growth hormone secretagogues, a comparison of ipamorelin and NN703 suggested the addition of a polar group at the C-terminus of NN703. A study was conducted using constrained amines for this purpose. Here, substituted 4-piperidinylamino- and 4-dimethylaminopiperidino-substituents were found to give the most active compounds. A replacement of the 4-dimethylaminopiperidino-substituent with 4-hydroxypiperidino resulted in a series of compounds, which showed in vitro activity with EC(50) values in the low nanomolar range, and favourable kinetic properties, such as 40% oral bioavailability. The most promising compound was also tested in a swine in vivo model, resulting in a growth hormone level with a C(max) of over 40 ng mL(-1).