Potent achiral agonists of the growth hormone secretagogue (ghrelin) receptor. Part 2: Lead optimisation

Identification and Pharmacological Profile of an Indane Based Series of Ghrelin Receptor Full Agonists

Cachexia and muscle wasting are very common among patients suffering from cancer, chronic obstructive pulmonary disease, and other chronic diseases. Ghrelin stimulates growth hormone secretion via the ghrelin receptor, which subsequently leads to increase of IGF-1 plasma levels. The activation of the GH/IGF-1 axis leads to an increase of muscle mass and functional capacity. Ghrelin further acts on inflammation, appetite, and adipogenesis and for this reason was considered an important target to address catabolic conditions. We report the synthesis and properties of an indane based series of ghrelin receptor full agonists; they have been shown to generate a sustained increase of IGF-1 levels in dog and have been thoroughly investigated with respect to their functional activity.

Ligand-Promoted meta-C-H Amination and Alkynylation

Using a modified norbornene (methyl bicyclo[2.2.1]hept-2-ene-2-carboxylate) as a transient mediator, meta-C-H amination and meta-C-H alkynylation of aniline and phenol substrates have been developed for the first time. Both the identification of a monoprotected 3-amino-2-hydroxypyridine/pyridone-type ligand and the use of a modified norbornene as a mediator are crucial for the realization of these two unprecedented meta-C-H transformations. A variety of substrates are compatible with both meta-C-H amination and meta-C-H alkynylation. Amination and alkynylation of heterocyclic substrates including indole, indoline, and indazole afford the desired products in moderate to high yields.

Hypotensive effects of ghrelin receptor agonists mediated through a novel receptor

BACKGROUND AND PURPOSE

Some agonists of ghrelin receptors cause rapid decreases in BP. The mechanisms by which they cause hypotension and the pharmacology of the receptors are unknown.

EXPERIMENTAL APPROACH

The effects of ligands of ghrelin receptors were investigated in rats in vivo, on isolated blood vessels and on cells transfected with the only molecularly defined ghrelin receptor, growth hormone secretagogue receptor 1a (GHSR1a).

KEY RESULTS

Three agonists of GHSR1a receptors, ulimorelin, capromorelin and CP464709, caused a rapid decrease in BP in the anaesthetized rat. The effect was not reduced by either of two GHSR1a antagonists, JMV2959 or YIL781, at doses that blocked effects on colorectal motility, in vivo. The rapid hypotension was not mimicked by ghrelin, unacylated ghrelin or the unacylated ghrelin receptor agonist, AZP531. The early hypotension preceded a decrease in sympathetic nerve activity. Early hypotension was not reduced by hexamethonium or by baroreceptor (sino-aortic) denervation. Ulimorelin also relaxed isolated segments of rat mesenteric artery, and, less potently, relaxed aorta segments. The vascular relaxation was not reduced by JMV2959 or YIL781. Ulimorelin, capromorelin and CP464709 activated GHSR1a in transfected HEK293 cells at nanomolar concentrations. JMV2959 and YIL781 both antagonized effects in these cells, with their pA2 values at the GHSR1a receptor being 6.55 and 7.84.

CONCLUSIONS AND IMPLICATIONS

Our results indicate a novel vascular receptor or receptors whose activation by ulimorelin, capromorelin and CP464709 lowered BP. This receptor is activated by low MW GHSR1a agonists, but is not activated by ghrelin.

Cognitive enhancing effects of ghrelin receptor agonists

RATIONALE

Ghrelin, the endogenous ligand for the growth hormone secretagogue receptor, has been shown to play a role in multiple physiological processes including appetite regulation, metabolism and, more recently, dendritic spine architecture, long-term potentiation and cognition.

OBJECTIVE

The objective of this study was to determine the effects of two structurally non-peptide ghrelin receptor agonists (GSK894490A and CP-464709-18) on rodent cognition.

METHODS

All experiments were performed in male Lister hooded rats. Effects of the test compounds on rat cognitive performance was determined using the novel object recognition test, a modified water maze paradigm and a scopolamine-induced deficit in cued fear conditioning. These tests were chosen as they each probe a relatively independent cognitive domain and therefore potentially have differing underlying neural substrates.

RESULTS

Both compounds significantly improved performance in the novel object recognition and modified water maze tests but were unable to attenuate a scopolamine deficit in cued fear conditioning.

CONCLUSIONS

These results demonstrate that the small-molecule ghrelin receptor agonists profiled here readily cross the blood/brain barrier and elicit pro-cognitive effects in recognition and spatial learning and memory tests. Based on these observations, the central ghrelin receptor would appear to be a chemically tractable receptor and perhaps should be considered as a new drug target for therapeutic approaches to treat diseases affecting cognition.

Ghrelin receptor modulators and their therapeutic potential

Background: Ghrelin is a peptide produced predominantly in the stomach and intestines, and is a natural growth hormone (GH) secretagogue-receptor ligand. It is able to stimulate GH release, but it also exhibits an important role in conditions related to processes regulating nutrition, body composition and growth, and heart, liver, thyroid or kidney dysfunction. Drug discovery efforts initially focused on ghrelin-receptor agonists, known as GH secretagogues, to be used as anabolic agents, but none of them reached the market. Discussion: The latest developments in this field are constituted by the discovery of new nonpeptidic compounds endowed with interesting properties: oxindole agonists are able to exert an increase in the fat-free mass, while ghrelin was reported to increase the fat mass gain, and triazole- and 2,4-diaminopyrimidine-based antagonists were shown to be able to reduce food intake, without inhibition of GH secretion stimulated by an agonist to the ghrelin receptor. Other antagonist compounds (quinazolinones) were discovered as antiobesity/antidiabetic agents. Moreover, inverse agonists have been discovered that are able to reduce weight gain. Conclusions: Taking into account the great number of pathological conditions related to ghrelin, and the discovery of several compounds able to modulate the ghrelin receptor, its importance in the field of medicinal chemistry research is set to increase significantly.

Oral administration of a centrally acting ghrelin receptor agonist to conscious rats triggers defecation

Agonists of ghrelin receptors that cross the blood-brain barrier, but not ghrelin itself, administered peripherally (intravenous or subcutaneous), cause defecation by acting on centres in the lumbo-sacral spinal cord. It is not established whether orally administered ghrelin receptor agonists can have this action. We tested GSK894281 for its effectiveness at the ghrelin receptor and its ability to cross the blood-brain barrier. GSK894281 was effective at the human and rat ghrelin receptors at 1-10 nmol L(-1), but was >1000-fold less potent at the motilin receptor. It achieved a similar blood concentration by oral or intravenous administration. Oral bioavailability was 74% and brain : blood ratio at steady state was 0.7 : 1. GSK894281 administered orally (1-100 mg kg(-1)) caused a prompt, dose-related production of faecal pellets; at 10 mg kg(-1) faecal output was four times greater than after carrier. The output was the greatest in the first half hour and subsided over the next 90 min. At an oral dose of 10 mg kg(-1), the compound was effective on eight successive days. Faecal output was, on average, increased threefold over control in the 2 h after administration on each of the 8 days. This dose also significantly increased food consumption. Rats showed no adverse behavioural effects to the drug on a single application, but at the end of a week of administration they avoided the gavaging pipette. Oral administration of ghrelin receptor agonists that enter the central nervous system could possibly be used to relieve acute cases of constipation or to clear the bowel for colonoscopy.