GSK962040: a small molecule motilin receptor agonist which increases gastrointestinal motility in conscious dogs

BACKGROUND

GSK962040, a small molecule motilin receptor agonist, was identified to address the need for a safe, efficacious gastric prokinetic agent. However, as laboratory rodents lack a functional motilin system, studies in vivo have been limited to a single dose, which increased defecation in rabbits. Motilin agonists do not usually increase human colonic motility, so gastric prokinetic activity needs to be demonstrated.

METHODS

The effect of intravenous GSK962040 on gastro-duodenal motility was assessed in fasted dogs implanted with strain gauges. Activity was correlated with blood plasma concentrations of GSK962040 (measured by HPLC-MS/MS) and potency of GSK962040 at the dog recombinant receptor [using a Fluorometric Imaging Plate Reader (Molecular Devices, Wokingham, UK) after expression in HEK293 cells].

KEY RESULTS

GSK962040 activated the dog motilin receptor (pEC(50) 5.79; intrinsic activity 0.72, compared with [Nle(13) ]-motilin). In vivo, GSK962040 induced phasic contractions, the duration of which was dose-related (48 and 173 min for 3 and 6 mg kg(-1) ), driven by mean plasma concentrations >1.14 μmol L(-1) . After the effects of GSK962040 faded, migrating motor complex (MMC) activity returned. Migrating motor complex restoration was unaffected by 3 mg kg(-1) GSK962040 but at 6 mg kg(-1) , MMCs returned 253 min after dosing, compared with 101 min after saline (n=5 each).

CONCLUSIONS & INFERENCES

The results are consistent with lower potency for agonists at the dog motilin receptor, compared with humans. They also define the doses of GSK962040 which stimulate gastric motility. Correlation of in vivo and in vitro data in the same species, together with plasma concentrations, guides further studies and translation to other species.

5-HT(2B) receptors modulate visceral hypersensitivity in a stress-sensitive animal model of brain-gut axis dysfunction

BACKGROUND

Irritable bowel syndrome (IBS) is associated with an enhanced perception to visceral stimuli and exaggerated stress response. The serotonergic neurotransmitter system has been strongly implicated as a key player in the manifestation of IBS symptomatology including visceral hypersensitivity. However the role of 5-HT(2B) receptors in visceral pain, although speculated, is currently unclear. Thus we assessed the impact of a selective 5-HT(2B) receptor antagonist, RS-127445, on visceral hypersensitivity in a model of brain gut axis dysfunction the Wistar Kyoto (WKY) rat.

METHODS

Colorectal distension (CRD) was used to assess the visceral sensitivity of the WKY rat compared to normosensitive Sprague Dawley (SD) rats. Once we verified the visceral sensitivity of the WKY rat we assessed the efficacy of RS-127445 in pain signalling from the colorectum. We administered the compound peripherally (i.p.) and centrally (i.c.v.) in order to ascertain the site of action of RS 127445. Behavioural responses to colorectal distention were then monitored.

KEY RESULTS

The WKY rats were more viscerally hypersensitive than the SD as previously shown. RS-127445 (5 mg kg(-1), i.p.) significantly reversed visceral hypersensitivity in WKY animals. Moreover, when administered intracerebroventricularly RS-127445 (100 nM) also decreased the number of pain behaviours during noxious CRD in the WKY animals.

CONCLUSIONS & INFERENCES

Taken together, blockade of 5-HT(2B) receptors offers an exciting novel therapeutic target for pain relief in stress-related gastrointestinal disorders such as IBS.

Influence of the pattern of jejunal distension on mesenteric afferent sensitivity in the anaesthetized rat

Vagal, spinal and intestino-fugal fibres all potentially transmit mechanosensory afferent information from the gastrointestinal tract. We aimed to characterize the relative mechanosensitivity of these three different afferent populations supplying the rat jejunum. Afferent nerve discharge was recorded from pentobarbitone-anaesthetized rats during different distension protocols. Saline ramp distension (1 mL min(-1)) and barostat ramp distension (2 mmHg 4 s(-1)) each evoked biphasic responses but with the latter significantly attenuated especially at low distending pressures. Barostat controlled phasic distensions (10-50 mmHg, 25 s) evoked an afferent response with a peak at the onset of distension adapting to a plateau level that was maintained and comparable to the barostat ramp responses at the corresponding pressures. Chronic subdiaphragmatic vagotomy significantly attenuated the low pressure component of the response to balloon ramp distension and both peak and plateau responses to phasic distension. Single unit analysis showed an absence of low threshold afferent activity after vagotomy while the response to fibres with wide-dynamic range and high threshold sensitivity were preserved hexamethonium had no effect on the responses to either ramp or phasic distension. These findings suggest that the nature of the distension stimulus is critical in determining the pattern of response observed from the various subpopulations of afferents supplying the bowel wall.

Peripheral opiate action on afferent fibres supplying the rat intestine

The aim of the present study was to examine the sensitivity of mesenteric afferents supplying the rat small intestine to mu-opioid receptor ligands. Mesenteric afferent discharge was recorded electrophysiologically in response to [D-ALA2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO; 100 mug kg(-1) i.v.), before and after treatment with the mu-receptor antagonist alvimopan (1 mg kg(-1) i.v.). DAMGO markedly stimulated whole nerve mesenteric afferent discharge (P < 0.05), an effect completely blocked by alvimopan. The response of mesenteric afferents to 2-methyl-5-hydroxytryptamine (30 microg kg(-1) i.v.), bradykinin (0.1-1 microg kg(-1) i.a.) and both low- and high-threshold distension (0-60 mmHg) was unaffected by alvimopan. In chronically vagotomized animals, the low-threshold response to distension was attenuated while the remaining high-threshold response was unaffected by alvimopan. In conclusion, mesenteric afferent fibres are markedly stimulated by mu-opioid receptor agonists, an effect blocked by alvimopan, which may contribute to the gastrointestinal reflex and behavioural responses to opiate treatment or abuse. However, alvimopan did not influence the normal sensitivity of intestinal afferents to chemical and mechanical stimuli that activate different subpopulations of vagal and spinal afferents. Thus, alvimopan may be useful for the treatment of gastrointestinal sequelae following opiate treatment for postoperative or chronic pain.