This Month in Gastroenterology

Article Outline

• Expression of Serotonin-Related Proteins in Irritable Bowel Syndrome
• High-Dose Mesalamine Preparation for the Treatment of Active Ulcerative Colitis
• A Less Gastrotoxic NSAID by Linkage to a Hydrogen Sulfide-Releasing Molecule
• Another Tool in Treating Acetaminophen Liver Injury
• Copyright

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Expression of Serotonin-Related Proteins in Irritable Bowel Syndrome 

The pathophysiology of irritable bowel syndrome (IBS) remains elusive, but increasing evidence for disordered serotonin signaling in the gastrointestinal tract was presented over the last few years. Potentially relevant alterations in IBS patients include changes in postprandial release of serotonin into the circulation from the gastrointestinal tract, and decreased expression of tryptophan hydroxylase and of the serotonin reuptake transporter protein, proteins that respectively synthesize serotonin, and terminate its action at the synapse.

In this issue, Camilleri et al and researchers from the Mayo Clinic present the results of a study of the mRNA expression for 2 serotonin-related proteins in sigmoid and rectal biopsies from patients with IBS and controls. Forty IBS patients (16 constipation predominant and 24 diarrhea predominant) were selected from a database of IBS patients and recruited via mailing. Twenty-five healthy controls were recruited through advertising. Reproducibility of the molecular assays was demonstrated in 10 subjects (5 patients and 5 controls) studied 3 months apart.

For one of these proteins, the serotonin reuptake transporter (SERT) and the mRNA level in sigmoid and rectal biopsies was not found to be different in patients with diarrhea-predominant or constipation-predominant IBS when compared with controls (Figure 1). Hence, this study did not confirm the hypothesis of decreases SERT transcription in IBS compared with controls. It is clear that further studies are needed to address the pathophysiologic role of altered SERT expression in (subgroups of) IBS patients.

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• Figure 1. 

  (A) SERT mRNA expression in mucosal biopsies from sigmoid colon in healthy controls, diarrhea-predominant IBS, and patients. (B) p11 mRNA expression in mucosal biopsies from sigmoid colon in healthy controls and diarrhea-predominant IBS and constipation-predominant patients.

The authors also studied mRNA expression for p11, a protein that until now had not received a lot of attention with regard to gastrointestinal function. The protein p11 is a member of the S100 protein family, and it forms a heterotetrameric complex with annexin 2. It has been shown to interact with a number of proteins including several ion channels involved in membrane excitability; and the serotonin 5-HT(1B) receptor. In mice models, decreased expression of p11 has been linked to depression-like states and to decreased nociception.

In the present study, a significant increase in mRNA expression of p11 was found in the sigmoid and rectal mucosa of the whole group of IBS patients. When comparing subgroups of IBS, statistical significance was found on sigmoid biopsies for the constipation-predominant IBS subgroup compared with controls, but not for the diarrhea-predominant IBS group. In the rectum, significantly higher levels were found for the diarrhea-predominant IBS subgroup compared with controls, but not for constipation-predominant IBS.

At present, it is unknown whether increased p11 mRNA in the mucosa of IBS patients is related to altered bowel function. In the central nervous system, p11 is colocalized with 5-HT1B receptors and increased expression of p11 enhances 5-HT1B receptor function. The role of the 5-HT1B receptor in the physiology or pathophysiology of the gastrointestinal tract remains to be elucidated. p11 has also been implicated in the control of trafficking of a number of ion channels involved in neural excitability. Through such pathway, p11 expression might contribute to altered visceroperception or nociception, but this again requires additional studies.

Although many questions remain, these observations maintain 5-HT signaling in the focus for research on functional bowel disorders. Further research should, hopefully, lead to better understanding of disease mechanisms and the development of better therapeutic approaches.

See page 17.

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High-Dose Mesalamine Preparation for the Treatment of Active Ulcerative Colitis 

5-Aminosalicylic acid (5-ASA) is considered the first-line therapy of choice in the treatment of mild-to-moderate left-sided or extensive ulcerative colitis. There is increasing evidence that the therapeutic effect of 5-ASA is dose dependent, with doses >4 g/d leading to faster and higher remission rates. Currently available treatment regimens with 5-ASA are based on 3–4 times daily administrations of tablets containing variable amounts of 5-ASA. It is generally accepted that more than once-a-day dosing is associated with decreased treatment compliance, and the same is likely to be true for ulcerative colitis. Hence, once-daily dosing has the potential to provide a significant therapeutic advance in ulcerative colitis.

In this issue, Kamm et al report the results of a randomized, placebo-controlled trial to assess the efficacy of Multi Matrix System (MMX) mesalamine in the treatment of ulcerative colitis. MMX mesalamine is a novel that uses MMX technology to release 5-ASA throughout the colon. This delivery system uses lipophilic and hydrophilic matrices enclosed within a gastroresistant, pH-dependent coating to facilitate prolonged exposure of the colonic mucosa to 5-ASA. A total of 343 patients with active mild-to-moderate ulcerative colitis were randomized to receive MMX mesalamine 2.4 g/d given once daily, MMX mesalamine 4.8 g/d given once daily, ASACOL 2.4 g/d given in 3 divided doses (0.8 g 3 times daily), or placebo for 8 weeks, using a double-dummy treatment design. The endpoint was the number of patients in clinical and endoscopic remission after 8 weeks.

The treatment groups were demographically and clinically well matched. After 8 weeks, clinical and endoscopic remission was obtained in a significantly higher proportion of patients receiving MMX mesalamine 2.4 or 4.8 g once daily, compared with placebo-treated patients (Figure 2). The proportion of patients receiving ASACOL 0.8 g 3 times a day who reached clinical and endoscopic remission did not differ significantly from placebo-treated patients (Figure 2). Treatments were well tolerated and no significant differences were found between the treatment groups with respect to the frequency of treatment emergent adverse events.

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• Figure 2. 

  Percentage of patients in clinical and endoscopic remission at week 8 (intent-to-treat population, n = 341). ^**P < .01; ^***P < .001. QD, once daily.

The study demonstrated that MMX mesalamine 2.4 g/d or 4.8 g/d given once per day is well tolerated and effective for the induction of clinical and endoscopic remission in patients with mild-to-moderate ulcerative colitis. The observation that a 5-ASA preparation is active in ulcerative colitis is not novel, and confirms previous studies. MMX mesalamine is attractive because it is being studied and found active as a once-per-day drug. In theory, the single dose may constitute a therapeutic advance over current thrice daily formulations and treatment regimens, but this needs to be addressed in appropriately designed large clinical trials.

See page 66.

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A Less Gastrotoxic NSAID by Linkage to a Hydrogen Sulfide-Releasing Molecule 

A major limitation of the use of nonsteroidal anti-inflammatory drugs (NSAIDs) is its toxicity toward the gastrointestinal mucosa, resulting in erosions, ulceration, and significant bleeding. There have been different approaches toward reducing the toxicity of NSAIDs while maintaining their anti-inflammatory function, such as the development of selective cyclooxygenase 2 (COX-2) inhibitors that relatively spare the constitutive COX-1 activity and leaving largely intact the protective barrier in the stomach, and the development of nitric oxide (NO)-releasing NSAIDs that allow the release of NO to enhance gastric mucosal defense. Another molecule, hydrogen sulfide, like NO, has a favorable profile for gastric mucosal defense, and the key enzymes that generate endogenous hydrogen sulfide are inhibited by many NSAIDs. Moreover, hydrogen sulfide exhibits anti-inflammatory effects, is a potent vasodilator, and down-regulates leukocyte adherence to vascular endothelium, a notable early event in inflammation and might prove counter to the cardiovascular side effects of selective COX-2 inhibitors.

In the study by Wallace et al, ATB-337 (diclofenac linked to a hydrogen sulfide-releasing molecule) was evaluated for its ability to cause gastrointestinal mucosal injury, its effect on tumor necrosis factor (TNF)-α, myeloperoxidase, and ICAM-1 expression, its effect on leukocyte adherence, as well as its ability to inhibit COX-2, COX-1, thromboxane, and edema formation. Using male Wistar rats, ATB-337 markedly inhibited prostaglandin production but demonstrated no gastric hemorrhagic erosions and >90% reduction in small intestinal damage with no intestinal hemorrhage when compared with diclofenac-treated rats. Unlinking the hydrogen sulfide moiety, ADT-OH, from diclofenac and treating rats with a separate combination of the 2 molecules showed no difference than diclofenac alone (Figure 3), indicating that the intact compound molecule was most efficacious in mucosal protection. ATB-337 failed to show the diclofenac-induced increases in TNF-α, myeloperoxidase, and ICAM-1 expression, did not show significant effects on leukocyte adherence, but did generate an increase in plasma levels of hydrogen sulfide. Both diclofenac and ATB-337 suppressed cystathione γ-lyase and cystathone β-synthase, key enzymes in the endogenous mucosal generation of hydrogen sulfide, suppressed COX-1 and COX-2 in a rat airpouch model, produced inhibition of thromboxane synthesis, and reduced edema in carrageenan-injected rat paw model.

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• Figure 3. 

  ATB-337, diclofenac linked to a hydrogen-sulfide releasing molecule, does not induce the formation of hemorrhagic erosions in the stomach (upper panel) despite profoundly suppressing gastric prostaglandin synthesis (lower panel). Co-administration of ADT-OH, the separated hydrogen sulfide-releasing moiety of ATB-337 and diclofenac, did not affect the severity of gastric damage produced as compared with diclofenac alone. Equimolar doses (27 μmol/kg) of diclofenac, ATB-337, and ADT-OH were administered orally, and the stomach examined 3 hours later. Each bar represents the mean ± SEM, and each group consisted of 5–6 rats. *P < .05 versus the vehicle-treated group.

The study indicates that modification of diclofenac by linking with a hydrogen sulfide releasing molecule results in a substantial reduction in diclofenac’s ability to induce gastrointestinal damage and bleeding, while preserving its ability to inhibit prostaglandin synthesis or reduce edema formation.

See page 261.

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Another Tool in Treating Acetaminophen Liver Injury 

Acetaminophen can be detrimental to a patient with a stressed liver. The normal glucuronidation and sulfation detoxification mechanisms in the liver are easily saturable, and this threshold is lowered with a prior compromised liver. Once saturated, acetaminophen metabolism is shunted through cytochrome P-450 causing the formation of highly reactive compounds such as N-acetyl-p-benzoquinone imine, which freely and covalently bind cellular molecules, damaging them. Acetaminophen toxicity results in hepatic glutathione depletion, massive hepatocyte damage with marked elevation of AST and ALT in the serum, and subsequent patient consequences of hepatic failure such as bleeding, hypoproteinemia, encephalopathy, coma, intracranial hypertension, and possible death. Current treatment involves the use of N-acetylcysteine, a replenisher of hepatic glutathione stores, with the timing of administration critical for effect, and supportive care during which the hope for hepatic recuperation occurs spontaneously after the acetaminophen insult. Ambient body temperature drops with acetaminophen toxicity, in part because of its antipyretic function, but also in part because of the acute liver failure induced by the drug. Mild hypothermia has been beneficial in treating intracranial hypertension that develops with acute liver failure, but it is unknown if this benefits or is detrimental toward hepatic recovery after acetaminophen toxicity.

In the study by Vaquero et al, the effect of mild hypothermia (32.0°C–35.0°C) on the liver was compared with normothermia (35.5°C–37.5°C) in male C57B6 mice treated with acetaminophen. Hypothermia spontaneously developed in the mice, requiring active warming to maintain normothermia in the control mice. Mild hypothermia significantly increased 24-hour survival after acetaminophen administration over normothermic mice (100% vs 40% survival; Figure 4). Mice kept in mild hypothermic conditions had less histologic hepatic necrosis, lower serum ALT levels, recovered depleted hepatic glycogen levels quicker, had milder hepatic hemoglobin congestion, and demonstrated attenuation of hepatocyte apoptosis. However, mild hypothermia did not significantly reduce hepatic concentration of the acetaminophen–protein adducts, suggesting that the protective effect of mild hypothermia lies downstream of the initial bioactivation of acetaminophen.

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• Figure 4. 

  Survival rates of normothermic (NT) and mild hypothermic (mild HT) mice (n = 9–10 per group) in the first 24 hours after acetaminophen (APAP) challenge. Log-rank test was used for statistical analysis.

It appears that in this mouse model of acetaminophen toxicity, mild hypothermia affords protection against acetaminophen-induced mortality, with reductions in hepatocellular necrosis, hepatic congestion, and apoptosis, coupled with improved recovery of hepatic glycogen stores. Thus, mild hypothermia could benefit hepatic function recovery in addition to improving acute liver failure-induced intracranial hypertension.

See page 372.