Opposite effects of kappa-opioid agonists on gastric emptying of liquids and solids in dogs

Methylnaltrexone mechanisms of action and effects on opioid bowel dysfunction and other opioid adverse effects

OBJECTIVE

To review the mechanisms of action of methylnaltrexone and its effects on opioid bowel dysfunction, as well as its effects on other opioid-induced adverse effects (ADEs), and its potential roles in clinical practice.

DATA SOURCES

A literature search using the MEDLINE and Cochrane Collaboration databases for articles published between 1966 and March 2007 was performed. Additional data sources were obtained from manual searches of recent journal articles, book chapters, and monographs. An updated literature search showed no additional publications.

STUDY SELECTION AND DATA EXTRACTION

Abstracts and original preclinical and clinical research reports published in the English language were identified for review. Review articles, commentaries, and news reports of this compound were excluded. Literature related to opioids, opioid receptors, opioid antagonists, methylnaltrexone, opioid-induced bowel dysfunction, constipation, nausea, and vomiting was evaluated and selected based on consideration of the support shown for the proof of concept, mechanistic findings, and timeliness. Fifty-eight original articles from preclinical studies and clinical trials using methylnaltrexone were identified. Pharmacologic action, benefits, and ADEs of methylnaltrexone were reviewed, with a focus on its effects on bowel dysfunction after opioids. Emphases were placed on its receptor binding activities and therapeutically relevant sites of action (peripheral vs central), in which peripheral opioid receptors in the body contribute to physiological and drug-induced effects.

DATA SYNTHESIS

Morphine and related opioids are associated with a number of limiting ADEs, including opioid-induced bowel dysfunction. Methylnaltrexone, a quaternary derivative of naltrexone, blocks peripheral effects of opioids while sparing central analgesic effects. It is currently under late-stage clinical investigation for the treatment of opioid-induced constipation in patients with advanced illness. Reported results showed the drug to be generally well-tolerated. The rapid reversal of constipation is very encouraging. Hastening postoperative discharge may also be possible.

CONCLUSIONS

Methylnaltrexone has the potential to prevent or treat opioid-induced peripherally mediated ADEs on bowel dysfunction without interfering with central analgesia. The study of methylnaltrexone leads to a greater understanding of the mechanisms of action of opioid pharmacology.

Opioid-induced gastrointestinal dysfunction

Use of opioid analgesics is associated with a number of side effects, especially opioid-induced gastrointestinal dysfunction. The extensive use of these compounds and the significant negative impact of the resulting gastrointestinal dysfunction on patients' quality of life make it an important clinical issue. In recent years our understanding of the mechanisms of opioid-induced gastrointestinal dysfunction has advanced greatly. This article reviews the underlying pathophysiological mechanisms of specific gastrointestinal adverse effects of opioids. The role of endogenous opioid peptides in certain gastrointestinal diseases is also discussed. A better understanding of the pathophysiological mechanisms of opioid-induced bowel dysfunction should lead to the development of newer opioid analgesics and improved regimens resulting in reduced gastrointestinal adverse effects.

The effect of the GABAB receptor agonist baclofen on liquid and solid gastric emptying in mice

The effect of the GABA(B) receptor agonist baclofen, a potential treatment for gastroesophageal reflux, on gastric emptying has not been determined. The effect of 1-4 mg/kg baclofen on liquid and solid gastric emptying in mice was evaluated by noninvasive [13C] breath tests. Baclofen accelerated gastric emptying of solids but delayed emptying of liquid, suggesting that it may have differential effects on proximal and distal stomach emptying.

Dilutions of low- vs. high-fat diets on intakes and gastric volumes in rats

Rats adapt to changes in dietary energy to maintain nearly constant energy intakes. This regulation indicates that animals sense and respond to nutrient content. We sought to determine whether this response was affected by the fat content of the diet. Our second goal was to determine how energy dilution affected intragastric volumes. Rats were randomized to high (18% w/w) and low fat (4.5% w/w) as the energy density of the diet was altered from 2.0 to 3.5 kcal/g. Average energy intake during 7-h feeds rose steeply (P<.01) when density was increased from 2.0 to 3.0 kcal/g, but modestly as density increased from 3.0 to 3.5 kcal/g. In other rats on 18% vs. 32% fat diets, energy intakes increased significantly (P<.01) as density of the diet was raised from 3.5 to 4.5 kcal/g. During diets at 2.0 and 2.5 kcal/g, animals on 18% fat ate fewer kilocalories than those on 4.5% fat; but over 3.0-4.5 kcal/g, energy intake was similar regardless of fat concentration (4.5-32%). Gastric contents after 7-h feeds increased with grams of food ingested similarly for high- and low-fat diets. We concluded that in rats: (a) compensation to energy dilution or concentration was inexact but (b) was about equal for high- vs. low-fat diets; thus, high fat was as well sensed as high carbohydrate; (c) compensations for energy densities were made despite varied gastric volumes; thus, rats learned to override the stimulus of gastric stretch and to sense energy via extra gastric mechanisms.

Pharmacology and clinical experience with fedotozine

Fedotozine [(1R)-1-phenyl-1-[(3,4,5-trimethoxy)benzyloxymethyl]-N,N- dimethyl-n-propylamine, (2S,3S-tartrate] is derived from the arylacetamide series. As with other compounds of this series, fedotozine is more or less selective of kappa(1)-opioid receptors and particularly for the kappa(1a)-receptor subtype, where it acts as an agonist. Pharmacological studies have shown that fedotozine exerts a peripheral antinociceptive action, comparable with that of other kappa-agonists. Its main effects have been demonstrated at the level of the afferent nerve pathways originating from the gut. Fedotozine alters the processing of visceral sensations along these pathways and hence, the perception of gut stimuli at the brain level. It modifies reflexes induced in various pathological conditions, like experimental inflammation of the gut, chemically-induced peritonitis or post-operative ileus. Fedotozine also decreases the nociceptive reflexes triggered by noxious gut distension in animals. In humans, fedotozine decreases the perception of gut distension, both in physiological and pathological conditions. Clinical trials undertaken in patients with functional digestive disorders, non-ulcer dyspepsia and irritable bowel syndrome, have shown that fedotozine relieves abdominal pain in these patients in 6-week treatments. kappa-Opioid receptors remain an interesting area for future development of new treatments for abdominal pain in patients with functional digestive disorders.

The effect of trimebutine maleate on gastric emptying in patients with non-ulcer dyspepsia

The study was designed to investigate the effect of trimebutine maleate, a drug used in both hyperkinetic and hypokinetic motility disorders, on gastric emptying in patients with non-ulcer dyspepsia having prolonged gastric emptying rates and to compare the parameters used for the determination of the lag period observed during the emptying of solid foods from the stomach. Gastric emptying was measured by the radionuclide technique. Twenty normal volunteers and 43 patients with non-ulcer dyspepsia participated in the study. Radionuclide imaging was performed by using a solid meal labeled with 99mTc-tin colloid. Of the patients with non-ulcer dyspepsia, 20 had prolonged gastric emptying. They were given three weeks of oral treatment with trimebutine maleate and had their radionuclide gastric emptying study repeated. Treatment with trimebutine maleate resulted in reduction in duration of the lag period and less retention of food at 100 minutes (p < 0.0005). After treatment with trimebutine maleate, no significant difference has been observed in the mean symptom score of patients with prolonged gastric emptying. Among the parameters used for the determination of the lag period, lag period determined by a mathematical equation (TLAG) has been found to be longer than the lag period determined by visual inspection of the images (VLAG) and there was correlation between the two parameters when the lag time was short.

The kappa agonist fedotozine modulates colonic distention-induced inhibition of gastric motility and emptying in dogs

BACKGROUND/AIMS

Gastric motor disturbances, associated with a delay in gastric emptying, occur in patients with the irritable bowel syndrome. The influence of fedotozine and kappa agonists on the cologastric reflex produced by nonpainful colonic distention was evaluated in conscious dogs.

METHODS

Colonic distention was applied in dogs fitted with either strain gauges or gastric cannula to assess its influence on gastrointestinal motility and gastric emptying, respectively.

RESULTS

Colonic distention delayed the occurrence of gastric migrating motor complex by 141%, an effect blocked by intravenous fedotozine, U 50,488 (25 and 50 micrograms/kg), and hexamethonium (0.5 mg/kg) but not by D-Ala2, N-methyl, Phe4, Gly5-ol enkephalin (1, 5, and 10 micrograms/kg), granisetron (50 and 100 micrograms/kg), or bretylium tosylate (5 mg/kg). Nor-binaltorphimine hydrochloride (1 mg/kg intravenously) eliminated the suppressive action of fedotozine. Colonic distention reduced the 1-hour gastric emptying of solids by 40.1%, an effect blocked by fedotozine and U 50,488 (50 and 100 micrograms/kg); nor-binaltorphimine hydrochloride (1 mg/kg) antagonized the blocking effect of fedotozine.

CONCLUSIONS

Fedotozine acts through kappa receptors to block the colonic distention-induced delay on gastric motility and emptying. The cologastric reflex involves nicotinic ganglionic receptors but not adrenergic pathway and 5-hydroxytryptamine 3 receptors.

Double-blind dose-response multicenter comparison of fedotozine and placebo in treatment of nonulcer dyspepsia

The efficacy and safety of the peripheral kappa-receptor agonist fedotozine was investigated in a double-blind, placebo-controlled, dose-ranging study involving 146 patients with nonulcer dyspepsia (NUD). After a two-week washout, patients were assigned to one of four groups to receive either placebo or fedotozine three times a day at doses of 10, 30, or 70 mg for six weeks. Analysis of mean symptom intensity scores showed that the 30-and 70-mg doses of fedotozine were superior to placebo in relieving postprandial fullness, bloating, abdominal pain, and nausea. Eructation and early satiety were marginally affected. The 30-mg dose was significantly more effective than placebo in reducing the total symptom score. Eight-two mostly minor adverse effects were recorded, but no significant differences in distribution emerged between placebo and treatment groups. The number of withdrawals declined significantly as a function of increasing dose. These results indicate that 30 mg three times a day is the minimal effective dose of fedotozine in the treatment of NUD symptoms and that this treatment is safe.

A new system for gastric emptying analysis using impedance measurement

A tetra-polar electrical impedance device has been constructed to permit study of gastric emptying using a microcomputer. An alternative current of 100 kHz, 4 mM peak-to-peak, is injected through a pair of surface electrodes on the upper abdomen. The voltage variations picked up across the second pair of electrodes correspond to the variations of epigastric impedance in response to the applied current. The low frequency voltage corresponding to gastric emptying and recording noise is then conditioned and digitised at a sampling frequency of 1 Hz. A parallel 8-bit signal is finally converted to standard serial form and sent, in real time, to a microcomputer via a serial port. In a process of off-line analysis, the emptying trace was extracted from measured traces by taking successive Fast Fourier Transform (FFT) of length 64 points and was then fit by three mathematical models: linear, exponential and Weibull. The rate of emptying was calculated in terms of the time needed to achieve 50% emptying (T 1/2) from the best fit model. A clinical experiment was done in 20 healthy volunteers to investigate the reproducibility of the method and compared to a scintigraphic method. Successive measurements on the same subject gave statistically similar results and were statistically independent. No correlation has been observed between impedance and scintigraphic methods.

Oral administration of Tyr-MIF-1 stimulates gastric emptying and gastrointestinal motility in rodents

The effects of orally administered Tyr-MIF-1, an agonist of an endogenous antiopiate system, were examined on gastric emptying in mice and gastrointestinal myoelectric activity in rats. Tyr-MIF-1 (5 mg/kg in mice, 20 mg/kg in rats) accelerated gastric emptying of a methylcellulose test meal, increased the frequency of antral spike bursts, and disrupted intestinal migrating myoelectric complexes. These effects were reproduced by a subcutaneous administration of Tyr-MIF-1 at the same dosage. They were blocked by naloxone (1 mg/kg) but not by the kappa receptor subtype antagonist MR 2266 (1 mg/kg). The GABAA antagonist bicuculline (0.5 mg/kg), but not the GABAB antagonist 2-hydroxysaclofen (4 mg/kg), also antagonized the effects of Tyr-MIF-1. These data demonstrate that oral Tyr-MIF-1 stimulates gastric emptying and gastrointestinal motility through a systemic or central action that involves opioid and GABA systems.

Gastric emptying of solid food is most potently inhibited by carbohydrate in the canine distal ileum

Although glucose sensors regulating the gastric emptying of liquid meals are uniformly distributed throughout the canine small intestine, some data suggest that the distal small bowel more potently inhibits gastric emptying of solid foods. The aims of this study were to compare (a) the inhibition of gastric emptying by glucose sensors in the proximal intestine with the feedback from the distal intestine, (b) these effects on the gastric emptying of solids vs. liquids, and (c) the inhibitory effect of unhydrolyzed starch with glucose. In 7 dogs with chronic duodenal fistulas, the second, third, and fourth quarters of small bowel were perfused via chronically implanted transmural catheters. Gastric emptying of either solids or liquids was tracked by gamma camera while gastric output was diverted out the duodenal fistula and the small bowel perfused with test solutions of glucose (0.06-2.0 mol/L), 0.15 mol/L NaCl, or 8.5% soluble starch. It was found that (a) gastric emptying of solids but not liquids was approximately 3 times more potently inhibited by glucose in the fourth quarter vs. the first or second quarter of small bowel, and (b) only hydrolyzed starch inhibited gastric emptying of solids.

Stimulation of kappa opiate receptors in intestinal wall affects stress-induced increase of plasma cortisol in dogs

In dogs, an acoustic stress (A.S.) produced by hearing of intense music (less than or equal to 90 dB) through earpieces for 1 h induced a 520% maximal rise in plasma cortisol 15-30 min after the beginning of stress. Oral administration of the specific kappa agonists, U-50488 (0.1 mg/kg) and PD 117302 (0.05 mg/kg), 30 min before the A.S. session reduced significantly (P less than 0.01) by 71.2% and 80.9% the maximal increase of plasma cortisol but did not affect the increase observed after intracerebroventricular administration of ovineCRF (100 ng/kg). These effects which are not reproduced by intravenous administration of the drugs at similar doses, were blocked by previous treatment with MR 2266 (0.1 mg/kg) or local anesthesia and vagotomy, suggesting that kappa opioid agonists inhibit the stress-induced activation of the hypothalamo-pituitary-adrenocortical (HPA) system by acting selectively on specific receptors located in the wall of the proximal gut.

Effect of butorphanol on equine antroduodenal motility

Six healthy six to eight-month-old horses were surgically prepared with Ag bipolar electrodes sutured to the gastric antrum and duodenum. Leads from the electrodes were exteriorised through a stab incision in the flank. During experimental sessions the horses were lightly restrained in stocks and electrode leads were connected to a physiograph to record antroduodenal myoelectrical activity. Intravenous (i.v.) injection of 0.05 mg/kg bodyweight (bwt) of the opioid agonist/antagonist, butorphanol was followed within 2 to 3 mins by a normal appearing period of repetitive spike activity, or phase III, of the migrating motor complex (MMC) on the duodenum. This was followed by a period of no spike activity, or phase I, of the MMC and then resumption of intermittent spike activity, or phase II, of normal duration. Pre-treatment with 15 micrograms/kg bwt of the non-selective opioid antagonist, naloxone, or with 1 mg/kg bwt of the alpha 2-adrenergic antagonist, tolazoline, did not block the myoelectrical response to butorphanol. It was concluded that a dose of butorphanol that has effective analgesic effects in a colicky horse resets the antroduodenal MMC without causing undesirable effects on antroduodenal motility. This particular effect of butorphanol might not be mediated by either a2-adrenergic or opioid receptors, although the latter question needs further investigation.