A potent, orally bioavailable benzazepinone growth hormone secretagogue

A competitive and highly selective 7-, 6- and 5-annulation with 1,3-migration through C-H and N-H - alkyne coupling

We demonstrated a highly competitive and selective C-C and N-C cross-coupled 7-, 6- and 5-annulation utilizing 2-ethynylanilides to afford functionalized 1H-benzo[b]azepin-2(5H)-ones, 2-quinolinones, and 3-acylindoles in high yield. ZnCl2 was found to be the smart catalyst for 7- and 5-annulation with 1,3-migration through C-H and N-H functionalization, respectively, whereas molecular iodine performed the C-H functionalized 6-annulation with a nonconventional 1,3 H-shift. The mechanism was investigated by intermediate trapping, control, and labeling experiments.

Synthesis of 5-Amino-2,5-dihydro-1H-benzo[b]azepines Using a One-Pot Multibond Forming Process

Rapid access to allylic trichloroacetimidates bearing a 2-allylaminoaryl group from readily available 2-iodoanilines combined with a one-pot multibond forming process has allowed the efficient synthesis of a series of 5-amino-2,5-dihydro-1H-benzo[b]azepines. The potential of these compounds as synthetic building blocks was demonstrated by the preparation of a late-stage intermediate of the hyponatremia agent, mozavaptan.

Macrocyclic Inhibitors of GPCR's, Integrins and Protein–Protein Interactions

This chapter summarizes some highlights of macrocyclic drug discovery in the area of GPCRs, integrins, and protein–protein interactions spanning roughly the last 30 years. Several examples demonstrate that incorporation of pharmacophores derived from natural peptide ligands into the context of a constrained macrocycle (“lock of the bioactive conformation”) has proven a powerful approach for the discovery of potent and selective macrocyclic drugs. In addition, it will be shown that macrocycles, due to their semi-rigid nature, can exhibit unique properties that can be beneficially exploited by medicinal chemists. Macrocycles can adapt their conformation during binding to a flexible protein target surface (“induced fit”), and due to their size, can interact with larger protein interfaces (“hot spots”). Also, macrocycles can display favorable ADME properties well beyond the rule of 5 in particular exhibiting favorable cell penetrating properties and oral bioavailability.

Synthesis of novel polysubstituted (2SR,4RS)-2-heteroaryltetrahydro-1,4-epoxy-1-benzazepines and cis-2-heteroaryl-4-hydroxytetrahydro-1H-1-benzazepines as antiparasitic agents

New series of polysubstituted (2SR,4RS)-2-heteroaryltetrahydro-1,4-epoxy-1-benzazepines and cis-2-heteroaryl-4-hydroxytetrahydro-1H-1-benzazepines were designed and synthesized in moderate to high yields using a three-step procedure from ortho-allylanilines. Their antiparasitic activity was evaluated against the extracellular and intracellular forms of Trypanosoma cruzi and Leishmania (Leishmania) infantum parasites. Their cytotoxicity was also determined on Vero and THP-1 mammalian cells. Many of the tested compounds inhibited significantly the growth of extracellular forms of T. cruzi and L. (L.) infantum without showing cytotoxicity on Vero and HTP-1 cells. Only compounds 10h and 14f demonstrated good activity against amastigotes of T. cruzi, but none was able to inhibit the growth of L. (L.) infantum amastigotes.

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.

Quinic Acid Derivatives as Sialyl Lewisx-Mimicking Selectin Inhibitors: Design, Synthesis, and Crystal Structure in Complex with E-Selectin

A search for noncarbohydrate sLe(x) mimics led to the development of quinic acid derivatives as selectin inhibitors. At Wyeth we solved the first cocrystal structure of a small molecule, quinic acid, with E-selectin. In the cocomplex two hydroxyls of quinic acid mimic the calcium-bound fucose of the tetrasaccharide sLe(x). The X-ray structure, together with structure based computational methods, was used to design quinic acid based libraries that were synthesized and evaluated for their ability to block the interaction of sLex with P-selectin. A large number of analogues were prepared using solution-phase parallel synthesis. Selected compounds showed decrease in leukocyte rolling in the IVM mouse model. Compound 2 inhibited neutrophil influx in the murine TIP model and demonstrated good plasma exposure.

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.

A Versatile Synthetic Approach to Peptidyl Privileged Structures Using a “Safety-Catch” Linker

Peptidyl privileged structures have been widely used by many groups to discover biologically active molecules. In this context, privileged substructures are used as "hydrophobic anchors", to which peptide functionality is appended to gain specificity. Utilization of this concept has led to the discovery of many different active compounds at a wide range of biological receptors. A synthetic approach to these compounds has been developed on a "safety-catch" linker that allows rapid preparation of large libraries of these molecules. Importantly, amide bond formation/cleavage through treatment with amines is the final step; it is a linker strategy that allows significant diversification to be easily incorporated, and it only requires the inclusion of an amide bond. In addition, chemistry has been developed that permits the urea moiety to be inserted at the N-terminus of the peptide, allowing the same set of amines (either privileged substructures or amino acid analogues) to be used at both the N- and C-termini of the molecule. To show the robustness of this approach, a small library of peptidyl privileged structures were synthesized, illustrating that large combinatorial libraries can be synthesized using these technologies.

Biological, physiological, pathophysiological, and pharmacological aspects of ghrelin

Ghrelin is a peptide predominantly produced by the stomach. Ghrelin displays strong GH-releasing activity. This activity is mediated by the activation of the so-called GH secretagogue receptor type 1a. This receptor had been shown to be specific for a family of synthetic, peptidyl and nonpeptidyl GH secretagogues. Apart from a potent GH-releasing action, ghrelin has other activities including stimulation of lactotroph and corticotroph function, influence on the pituitary gonadal axis, stimulation of appetite, control of energy balance, influence on sleep and behavior, control of gastric motility and acid secretion, and influence on pancreatic exocrine and endocrine function as well as on glucose metabolism. Cardiovascular actions and modulation of proliferation of neoplastic cells, as well as of the immune system, are other actions of ghrelin. Therefore, we consider ghrelin a gastrointestinal peptide contributing to the regulation of diverse functions of the gut-brain axis. So, there is indeed a possibility that ghrelin analogs, acting as either agonists or antagonists, might have clinical impact.

Discovery and Evaluation of Potent P1 Aryl Heterocycle-Based Thrombin Inhibitors

In an effort to discover potent, clinically useful thrombin inhibitors, a rapid analogue synthetic approach was used to explore the P(1) region. Various benzylamines were coupled to a pyridine/pyrazinone P(2)-P(3) template. One compound with an o-thiadiazole benzylic substitution was found to have a thrombin K(i) of 0.84 nM. A study of ortho-substituted five-membered-ring heterocycles was undertaken and subsequently demonstrated that the o-triazole and tetrazole rings were optimal. Combination of these potent P(1) aryl heterocycles with a variety of P(2)-P(3) groups produced a compound with an extraordinary thrombin inhibitory activity of 1.4 pM. It is hoped that this potency enhancement in P(1) will allow for more diversification in the P(2)-P(3) region to ultimately address additional pharmacological concerns.

Recognition of Privileged Structures by G-Protein Coupled Receptors

Privileged structures are ligand substructures that are widely used to generate high-affinity ligands for more than one type of receptor. To explain this, we surmised that there must be some common feature in the target proteins. For a set of class A GPCRs, we found a good correlation between conservation patterns of residues in the ligand binding pocket and the privileged structure fragments in class A GPCR ligands. A major part of interior surface of the common ligand binding pocket of class A receptors, identified in many GPCRs, is lined with variable residues that are responsible for selectivity in ligand recognition, while other regions, typically located deeper into the binding pocket, are more conserved and retain a predominantly hydrophobic and aromatic character. The latter is reflected in the chemical nature of most GPCR privileged structures and is proposed to be the common feature that is recognized by the privileged structures. Further, we find that this subpocket is conserved even in distant orthologs within the class A family. Three pairs of ligands recognizing widely different receptor types were docked into receptor models of their target receptors utilizing available structure- activity relationships and mutagenesis data. For each pair of ligands, the ligand-receptor complexes reveal that the nature of the privileged structure binding pocket is conserved between the two complexes, in support of our hypothesis. Only part of the privileged structures can be accommodated within the conserved subpocket. Some contacts are established between the privileged structure and the nonconserved parts of the binding pocket. This implies that any one particular privileged structure can target only a subset of receptors, those complementary to the full privileged structure. Our hypothesis leads to a valuable novelty in that ligand libraries can be designed without any foreknowledge of the structure of the endogenous ligand, which in turn means that even orphan receptors can in principle now be addressed as potential drug targets.

Substituted bridged phenyl piperidines: orally active growth hormone secretagogues

A new series of growth hormone secretagogues have been discovered. The best compound, 26j, shows excellent ability to release growth hormone both in vitro and in vivo. The synthesis and biological activity of these compounds are discussed.

Identification, synthesis, and activity of novel blockers of the voltage-gated potassium channel Kv1.5

The voltage-gated potassium channel Kv1.5 is regarded as a promising target for the development of new atrial selective drugs with fewer side effects. In the present study the discovery of ortho,ortho-disubstituted bisaryl compounds as blockers of the Kv1.5 channel is presented. Several compounds of this new class were synthesized and screened for their ability to block Kv1.5 channels expressed in Xenopus oocytes. The observed structure-activity relationship (SAR) is described by a pharmacophore model that consists of three hydrophobic centers in a triangular arrangement. The hydrophobic centers are matched by a phenyl or pyridyl ring of the bisaryl core and both ends of the side chains. The most potent compounds (e.g., 17c and 17o) inhibited the Kv1.5 channel with sub-micromolar half-blocking concentrations and displayed 3-fold selectivity over Kv1.3 and no significant effect on the HERG channel and sodium currents. In addition, compounds 17c and 17m have already shown antiarrhythmic effects in a pig model.

5-Substituted-1H-tetrazoles as carboxylic acid isosteres: medicinal chemistry and synthetic methods

5-Substituted-1H-tetrazoles (RCN4H) are often used as metabolism-resistant isosteric replacements for carboxylic acids (RCO2H) in SAR-driven medicinal chemistry analogue syntheses. This review provides a brief summary of the medicinal chemistry of tetrazolic acids and highlights some examples of tetrazole-containing drug substances in the current literature. A survey of representative literature procedures for the preparation of 5-substituted-1H-tetrazoles, focusing on preparations from aryl and alkyl nitriles, is presented in sections by generalized synthetic methods.

Permeability of the peptidic GH secretagogues hexarelin and EP 51389, across rat jejunum

The intestinal permeability of hexarelin and EP 51389, two growth hormone releasing hexa- and tri- peptide analogues, was assessed in vitro with side-by-side diffusion chambers in the apical-to-basolateral (AP-to-BL) and in the basolateral-to-apical (BL-to-AP) direction using excised rat jejunal segments. The effect of EP 51389 on P-glycoprotein (P-gp) was evaluated by rhodamine 123 accumulation on monolayers of CH(R)C5 cells with increasing concentrations of EP 51389. Hexarelin and EP 51389 permeability were found to be < 1%. Permeability coefficients (P(app)) were 18.87 +/- 2.86 (x10(-7) cm/s) and 5.87 +/- 0.45 (x10(-7) cm/s) for hexarelin and EP 51389, respectively. Bidirectional studies revealed that hexarelin transport was similar in both directions. EDTA did not influence hexarelin permeability. Permeability was predominantly secretory for EP 51389 as P(app) in the BL-to-AP direction [32.56 +/- 6.11 (x10(-7) cm/s)] was greater than AP-to-BL. Confirming involvement of a secretory transport system, chlorpromazine inhibited EP 51389 transport across the jejunum. EP 51389 inhibited P-gp in a dose dependent manner resulting in the intracellular accumulation of rhodamine in CH(R)C5 cells. These results suggest that: 1) the intestinal permeability of hexarelin and EP 51389 is poor; 2) the passage of hexarelin is mainly via a transcellular passive pathway since the contribution of paracellular permeability to the overall permeability is rather low; 3) P-gp may act as a potential barrier for the intestinal absorption of EP 51389.

Proceedings of the 10th hGH Symposium. Seville, April 23-24, 1999

A series of structurally diverse growth hormone (GH) releasing substances have been synthesized that are distinct from the naturally occurring GH releasing hormone (GHRH). These synthetic molecules range from the family of GH releasing peptides and mimetics such as MK-0677. The physiological importance of these molecules and their receptor is exemplified by studies in the elderly. For example, when MK-0677 was administered chronically to 70- to 90-year-old subjects, once daily, the age-related reduced amplitude of GH pulses was reversed to that of the physiological profile typical of young adults. In 1996, the synthesis of ³⁵S-MK-0677 was reported and used as a ligand to characterize a common receptor (GH secretagogue receptor [GHS-R]) for the GH releasing substances. The GHS-R is distinct from the GHRH receptor. Subsequently, the GHS-R gene was cloned and shown to encode a unique G-protein coupled receptor with a deduced protein sequence that was 96% identical in human and rat. Because of the physiological importance of the GHS-R, a search for family members (FMs) was initiated and its molecular evolution investigated. Three FMs GPR38, GPR39 and FM3 were isolated from human genomic libraries. To accelerate the identification of other FMs, a vertebrate organism with a compact genome distant in evolutionary terms from humans was exploited. The pufferfish (Spheroides nephelus) genome provides an ideal model for the discovery of human genes. Three distinct full-length clones encoding proteins of significant sequence identity to the human GHS-R were cloned from the pufferfish. Remarkably, the pufferfish gene with highest sequence homology to the human receptor was activated by the hexapeptide and non-peptide ligands. These intriguing results show that the structure and function of the ligand binding pocket of the human GHS-R has been highly conserved in evolution (<sim>400 million years) and strongly suggests that an endogenous natural ligand has been conserved. This new information is consistent with a natural ligand for the GHS-R playing a fundamentally important and conserved role in physiology.

Development of analytical and preparative chromatographic separations of novel growth hormone secretagogue compounds

Chromatographic separations of new growth hormone secretagogue compounds were developed to support structure-activity relationship (SAR) studies in conjunction with lead optimization. These new compounds differed from Merck's MK-677 by having two chiral centers and thus diastereomeric mixtures were generated. Separation of initial compounds in the SAR was achieved on a Kromasil C18 column using an ammonium acetate buffer and acetonitrile. However, additional candidates were not separable on C18 columns and a chiral Kromasil CHI-DMB column was used to resolve the diastereomeric compounds. The Kromasil CHI-DMB packing was also used in a preparative chromatographic system to resolve multigram quantities of secretagogue candidates for testing. Chiral separations of different intermediates were also developed in support of evolution of an asymmetric synthetic route. This report summarizes development of the preparative chromatographic system used to purify diastereomeric mixtures and chiral separations of intermediates in the synthesis.

Growth hormone secretagogues: recent advances and applications

The discovery of a new class of compounds that stimulate the release of growth hormone (GH) in a manner distinctly different from growth hormone-releasing hormone (GHRH) is advancing the understanding of the mechanisms that control GH secretion. These compounds, the GH secretagogues, act at both pituitary and hypothalamic levels, and might even elicit effects in the CNS and peripheral systems. A receptor with high affinity for the GH secretagogues has been identified and several observations suggest the presence of additional receptors. The existence of these specific endogenous receptors could indicate that the mechanism of GH release is not yet fully understood. Several potential indications have been explored clinically and, as some of these compounds are orally active, they could offer attractive alternatives to recombinant human growth hormone (hGH) in treating GH disorders such as growth hormone deficiency (GHD), age-related conditions, obesity and catabolic conditions.

Nonpeptidic ligands for peptide and protein receptors

The first potent nonpeptidic ligands for somatostatin, luteinizing hormone-releasing hormone, glucagon and bradykinin receptors have been reported. Nonpeptidic clinical candidates have been identified or are currently under study for substance P, bradykinin, endothelin, growth hormone secretagogue, angiotensin, vasopressin, motilin and cholecystokinin. Design, screening, combinatorial chemistry and classical medicinal chemistry all played important roles in these advances.

Thiazole-derived potent, highly bioavailable short duration growth hormone secretagogues

Replacement of the phenyl in 3 with a 2-pyridyl or 4-thiazolyl group resulted in increased potency in the rat pituitary cell GH release assay and in beagles.

Synthesis and biological activities of spiroheterocyclic growth hormone secretagogues

The synthesis and biological activities of a series of spiroheterocyclic growth hormone secretagogues are reported. Modification of the spiroindane part-structure of the prototypal secretagogue L-162,752 revealed that the spiroindane could be replaced with spirobenzodihydrothiophen derivatives to enhance not only in vitro potency but also oral activity. In this study non-aromatic D-2-amino-4-cyclohexylbutanoic analogs (8a-8d) were also identified to be active secretagogues.

A New Orphan Receptor Involved in Pulsatile Growth Hormone Release

In all species studied to date, growth hormone (GH) is released episodically. Traditionally, the regulation of this process was considered to be mediated by two hypothalamic hormones, growth hormone-releasing hormone (GHRH) and somatostatin (sst). More recently, we identified a new orphan G-protein-coupled receptor that causes episodic GH release upon activation by synthetic ligands. These ligands include the GH-releasing peptides (GHRPs) first described by Bowers and their small molecule mimetics such as L-692,429 and MK-0677. Site-directed mutagenesis of this GH secretagogue receptor (GHS-R) has defined key amino acid residues essential for binding and activation by the synthetic ligands. The GHS-R is not activated by GHRH or sst. It is expressed exclusively in the anterior pituitary lobe and central nervous system and although this new receptor does not belong to any of the known families of G-protein-coupled receptors, the GHS-R is highly conserved across species. The Puffer fish homolog, in common with the human GHS-R, is activated by the structurally distinct ligands GHRP-6, MK-0677 and L-163,540. Thus, the GHS-R ligand-binding pocket has apparently been conserved for at least 400 million years. Studies in humans suggest that production of an endogenous ligand declines during aging. For example, chronic treatment with the synthetic ligand MK-0677 reverses the age-related physiological changes in the GH/IGF-I (insulin-like growth factor I) axis of 70-94 year old subjects. Based on the localization of expression of GHS-R in the brain, reduced production of the natural ligand might also be involved in age-associated changes in cognition, memory, mood and behavior.