Acetorphan, an enkephalinase inhibitor, decreases gastric secretion in cats

A comprehensive review of the pharmacodynamics, pharmacokinetics, and clinical effects of the neutral endopeptidase inhibitor racecadotril

Racecadotril, via its active metabolite thiorphan, is an inhibitor of the enzyme neutral endopeptidase (NEP, EC 3.4.24.11), thereby increasing exposure to NEP substrates including enkephalins and atrial natriuretic peptide (ANP). Upon oral administration racecadotril is rapidly and effectively converted into the active metabolite thiorphan, which does not cross the blood–brain-barrier. Racecadotril has mainly been tested in animal models and patients of three therapeutic areas. As an analgesic the effects of racecadotril across animal models were inconsistent. In cardiovascular diseases such as hypertension or congestive heart failure results from animal studies were promising, probably related to increased exposure to ANP, but clinical results have not shown substantial therapeutic benefit over existing treatment options in cardiovascular disease. In contrast, racecadotril was consistently effective in animal models and patients with various forms of acute diarrhea by inhibiting pathologic (but not basal) secretion from the gut without changing gastro-intestinal transit time or motility. This included studies in both adults and children. In direct comparative studies with loperamide in adults and children, racecadotril was at least as effective but exhibited fewer adverse events in most studies, particularly less rebound constipation. Several guidelines recommend the use of racecadotril as addition to oral rehydration treatment in children with acute diarrhea.

The role of peripheral opioid receptor subtypes in the modulation of gastric acid secretion and plasma gastrin in dogs

The peripheral opioid receptor subtypes involved in the regulation of gastric acid secretion were studied in dogs with both a gastric fistula and a Heidenhain pouch, by using the putative mu-opioid receptor agonist dermorphin, the delta-opioid receptor agonist [D-Ala2,D-Leu5]enkephalin (DADLE) and the kappa-opioid receptor agonist dynorphin-(1-13). Dermorphin caused a significant increase in basal acid secretion from both the gastric fistula and the Heidenhain pouch, while DADLE and dynorphin-(1-13) did not. Acid secretion stimulated by 2-deoxy-D-glucose from the gastric fistula was not modified by dermorphin and dynorphin-(1-13), while DADLE significantly inhibited it; at the same time gastric secretion from the Heidenhain pouch was significantly increased by dermorphin and unmodified by DADLE and dynorphin-(1-13). Dermorphin, DADLE or dynorphin-(1-13) did not modify plasma gastrin during basal or 2-deoxy-D-glucose-stimulated conditions. Submaximal bethanechol-stimulated secretion was increased by dermorphin and DADLE but unaffected by dynorphin-(1-13). Acid secretion from the gastric fistula stimulated by pentagastrin was enhanced by dermorphin, inhibited by DADLE and unaffected by dynorphin-(1-13). Dermorphin and DADLE significantly increased acid secretion from the Heidenhain pouch stimulated by pentagastrin, while dynorphin-(1-13) was ineffective. Naloxone prevented the stimulatory effects of dermorphin and DADLE on the Heidenhain pouch, but it reduced acid secretion from the gastric fistula further when given with DADLE. The inhibitory effects of DADLE on secretion from the gastric fistula were prevented by naltrindole, a selective antagonist of delta-opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracisternal neutral endopeptidase-24.11 inhibitors produce inhibition in gastric acid output: independence from opiate, bombesin, or neurotensin-mediated mechanisms

Intracisternal (ic) injection of the neutral endopeptidase-24.11 inhibitor phosphoramidon (1-100 nmol) produced a dose-dependent inhibition of gastric acid secretion in 2-h pylorus-ligated rats. The response resulted from a reduction in acid concentration and volume. Likewise, ic injection of another neutral endopeptidase-24.11 inhibitor Zincov (200 nmol) produced a 63% inhibition in gastric acid output. In contrast, neither intravenous injection of phosphoramidon (100 nmol) nor ic injection of the aminopeptidase inhibitor amastatin (100 nmol) produced any change in gastric acid secretion. The inhibitory effect of ic phosphoramidon (10 nmol) was not reversed by a dose of naloxone sufficient to antagonize the acid inhibitory effects of ic [D-Ala2-D-met5]enkephalinamide (8.5 nmol). Moreover, phosphoramidon-induced inhibition of acid was not reduced by the centrally effective bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 or by reserpine pretreatment at a dose effective to antagonize ic neurotensin-induced inhibition in acid secretion. These results suggest that an endogenous neutral endopeptidase-24.11 sensitive substrate may act in the brain to inhibit gastric acid output by mechanisms independent of CNS opiate, bombesin or neurotensin activity.

Action of enkephalinase (EC 3.4.24.11) inhibition on the pre- and post-prandial electromyographic patterns of colon in rats

Endogenous opioids are an important regulatory factor for the digestive tract and specially for its motility pattern. Enkephalin degradation includes an enkephalinase (EC 3.4.24. 11) and the effects on the colic electromyographic profile of its inhibition by acetorphan has been investigated in the unrestrained rat. Electromyogram consisted of Long Spike Bursts (LSB). In fasted state, they propagated indifferently in both aboral or oral directions from any point of the colon. Feeding privileges LSB which start near the cecal junction and propagated aborally to the distal colon. The acetorphan treatment (A) increases the percentage of LSB propagating aborally on the entire colon in fasted state and (B) reinforces the increased percentage of LSB which propagated down on the entire colon induced by feeding. All the actions of acetorphan on colic motility pattern disappear after inhibition of opioid receptors by naloxone. That may account for involvement of enkephalins in acetorphan properties on the pattern of the colic electrical activity.

Immunoautoradiographic localisation of enkephalinase (EC 3.4.24.11) in rat gastrointestinal tract

Enkephalinase (EC 3.4.24.11, membrane metalloendopeptidase) is a zinc peptidase expressed by neurons and a variety of epithelial cells, and responsible for the inactivation of enkephalins in brain. Its functions in the gastrointestinal (GI) tract are less well understood although enkephalinase inhibitors were reported to induce a constellation of antisecretory and motor responses. Its localisation in various segments of the rat GI tract was established autoradiographically using a 125I-labelled monoclonal antibody. All along the GI tract, the highest immunoreactivity was found in mucosal layers e.g., in intestinal villi, basal epithelial layers of the oesophagus or gastric cardia, muscularis mucosae of the stomach and large intestine. The immunoreactivity was also high in the stomach submucosae and moderate in the muscularis propria of the caecum. A faint patchy immunoreactivity was also observed in several other layers. This distribution suggests that the membrane peptidase is expressed by enterocytes and a variety of other cells. Its high expression in mucosal layers is consistent with its participation in protein digestion and also in the inactivation of endogenous peptides, particularly the enkephalins, acting at this level to control secretory mechanisms and hydroelectrolytic fluxes. Its presence in submucosal layers may account for some naloxone-reversible motor responses elicited by enkephalinase inhibitors.

Endogenous opioid peptides in the control of food intake in cats

In this report, we investigated the role of exogenous and endogenous enkephalins on food intake in the cat, using, respectively, exogenous [D-Ala2-Met5]-enkephalin (DAME) and acetorphan (Ac) in order to inhibit the degradation of endogenous enkephalins. In addition, the selective peripheral antagonist naltrexone methylbromide (NTxMB) and the nonselective antagonist naloxone (Nx) were used in an attempt to discriminate central and peripheral opioid receptors. In 18-hours food-deprived animals, Ac (5 mg/kg IV) increased milk intake during sham feeding (+18%, p less than 0.05), but did not modify it in feeding conditions. Nx (1 mg/kg SC) reduced milk intake in sham-feeding experiments (-67%, p less than 0.01) more than in milk-feeding conditions (-30%, p less than 0.01). NTxMB (1 mg/kg SC) did not modify milk intake in sham-feeding but decreased it in feeding experiments. In nonfasted animals, Ac did not modify food intake. IV infusion of DAME (50 micrograms/kg) resulted in a reduction of daily food intake (-32%, p less than 0.01). Nx (1 mg/kg SC) decreased the earlier 30 min intake followed by reduction of daily intake (-30%, p less than 0.01). NTxMB (1 and 4 mg/kg SC) increased the 30-min intake dose dependently, without significant change in daily intake. In conclusion, Ac increases food intake in sham-feeding conditions, suggesting that endogenous enkephalins are likely to be involved in the stimulation of food intake. The effects of Nx and NTxMB furthermore suggest both a central activation, and a peripheral inhibition of food intake by opiates when food is allowed to proceed normally through the digestive tract.

Thiorphan and acetorphan inhibit gastric secretion by a central, non-opioid mechanism in the rat

Thiorphan and its prodrug, acetorphan, are two inhibitors of enkephalinase (EC 3.4.24.11), a membrane-bound peptidase which plays an important role in the metabolic degradation of enkephalins. Since exogenous opioids have been reported to both stimulate and inhibit gastric secretion, the effects of thiorphan and acetorphan were studied in conscious rats equipped with chronic gastric fistulas. While i.v. thiorphan had no effect, both i.c.v. thiorphan and i.v. acetorphan potently inhibited the basal gastric acid output and the acid output stimulated by pentagastrin. Conversely, neither drug affected the histamine- or methacholine-induced stimulation of acid secretion. Neither thiorphan or acetorphan had any effect on the acid secretion stimulated by a combination of pentagastrin and acetylcholine in vagotomized rats. The results strongly suggest that both drugs inhibit gastric secretion through an effect at the level of the central nervous system, which decreases the vagal drive to the stomach. However, the effects of thiorphan and acetorphan were not prevented by naloxone. This is at variance with most of the effects of these drugs reported to date, including the inhibition of gastric secretion in cats. Furthermore, these effects were observed at doses which could inhibit other enzymes apart from enkephalinase. This suggests that the antisecretory action in rats is related to the protection of some non-opioid peptide(s) from degradation. In conclusion, both peptidase inhibitors inhibit gastric secretion of the rat through a central mechanism involving unknown, non-opioid peptide(s).