Experimental studies: Alverine citrate is an antispasmodic drug that inhibits calcium uptake and modulates smooth muscle activity. An experimental study on anesthetized cats showed that alverine acts on vagal sensory endings of the GI tract, where it decreases the responses of mechanoreceptors to mechanical and chemical stimuli[43]. Because chemically induced responses and smooth muscle contraction are both calcium dependent, decreased chemical sensitivity and smooth muscle relaxation can be explained by reduced calcium influx. In addition, a calcium-independent mechanism of action, such as selective 5-HT_1A receptor antagonism, might also be present, which has been demonstrated in rats using a 5-HT agonist-induced hypersensitivity model[44]. However, the picture is more complex than first thought - in guinea pig urinary bladder preparations, the frequency of spontaneous contractions in endogenously active smooth muscle was surprisingly increased by low doses of alverine citrate, whereas contraction amplitude was decreased. Higher doses of the drug could suppress both the frequency and amplitude of contractions; nevertheless, the authors speculated that the doses used in clinical practice would not reach this concentration in humans. They concluded that alverine citrate should be considered a true spasmolytic, because it suppresses the duration of spontaneous contractions of the gut, preventing local ischemia and reflectoric pain in the colonic wall evoked by “spasms”. In addition, it has also been noted that alverine can increase calcium influx during action potentials by inhibiting the inactivation of calcium channels, but it reduces the sensitivity of contractile proteins to calcium, consequently suppressing the evoked muscular activity. Stress-induced colonic motility changes are an important factor in the pathogenesis of IBS; therefore, the clinical effects of alverine and the antifoaming agent, simethicone were tested in a rat model of colonic hypersensitivity induced by acute restraint stress[45]. Treatment with simethicone (200 mg/kg po) or alverine citrate (10 mg/kg po) reduced stress-induced increases in colonic permeability and hypersensitivity to distension, but lower doses were ineffective[45]. However, the combination of inactive doses of simethicone (100 mg/kg) with low doses of alverine (7 mg/kg) completely abolished the effects of stress, suggesting a synergistic action.

Clinical trials: Clinical studies on alverine citrate in IBS have been scarce (Table 1). In a randomized, placebo-controlled, double-blind clinical trial conducted in three British centers, after a 2-wk screening period, IBS patients selected according to the modified Rome criteria received 12 wk of treatment with 120 mg alverine citrate three times daily[46]. The patients completed diary cards about abdominal pain or discomfort, bloating, bowel movements, stool consistency, and general well-being; furthermore, the severity and frequency of abdominal pain, bloating, nausea and early satiety were assessed at study visits four times during treatment. Although abdominal pain, bloating and general well-being were all slightly more improved in the alverine-treated group than in the placebo group, when comparing the first diary card to the third, the difference was not statistically significant. This finding might be explained by the placebo effect being exceptionally high, sometimes reaching almost 70%, which would require a study with a much great number of participants to detect a possible positive effect of alverine. Regarding safety, no serious adverse events were reported in the study, and more patients experienced adverse events in the placebo group (48.1%) than in the alverine-treated group (39.6%). Using the well-known spasmolytic properties of alverine, a randomized, placebo-controlled trial showed that pretreatment of patients over 5 d with 60 mg alverine citrate plus 300 mg simethicone three times daily reduced intubation time during colonoscopy by 19%; nonetheless, it did not affect scores for pain, spasm, difficulty or cleanliness[47]. The combination of alverine with simethicone was also tested in a double-blind, placebo-controlled, randomized trial conducted in 17 sites in Hungary and Poland[48]. A total of 412 IBS patients meeting the Rome III criteria received a combination of 60 mg alverine citrate and 300 mg simethicone or placebo three times per day for 4 wk. Combined alverine citrate and simethicone treatment achieved a higher reduction in abdominal pain and discomfort, as measured using visual analog scale (VAS) scores, and significantly more patients responded to therapy than to placebo, regardless of stool pattern. A visible, but not statistically significant, trend was also observed, showing greater improvement in IBS life impact scores with combination therapy than with placebo. No severe drug-related adverse events were noted in the study, and the numbers of adverse events were similar in both groups.

Experimental studies: Mebeverine is a beta-phenyletylamine derivative of reserpine, which has relatively specific effects on smooth muscle cells without atropine-like side effects in humans[49]. It directly blocks voltage-operated sodium channels and inhibits intracellular calcium accumulation[49,50]. It is three times more potent than papaverine in inhibiting the peristaltic reflex of the guinea-pig ileum[51], but further animal studies on its pharmacological effect have been lacking.

Clinical trials: Mebeverine became treatment of interest for IBS in the 1960s. In an early study by Connell[52], iv mebeverine decreased all sigmoid colonic motility, especially in hyperactive subjects, and it had less or no effect in hypoactive subjects. In a subsequent part of the study, mebeverine was superior to placebo at each time point over 12 wk of treatment in IBS patients in terms of symptom improvement and general well-being. Using prolonged ambulant manometry in 12 IBS patients and 6 healthy controls, compared to a placebo period, mebeverine had no significant effects on interdigestive small bowel motor parameters in controls; in contrast, a higher phase 2 motility index was observed in both IBS-D and IBS-C patients, and phase 3 motility was also affected[53]. These alterations in small bowel motile activity by mebeverine suggest possible spasmolytic and prokinetic effects in IBS patients.

Regarding symptom control in IBS, non-placebo-controlled studies have shown positive results. Significant improvement was observed after 6 wk of treatment with both the plain and sustained-release forms of mebeverine, with a minimal number of adverse events[54]. When comparing pinaverium bromide to mebeverine in 91 IBS-D patients, the improvements in global well-being were similar in the two groups, the daily defecation frequencies were markedly decreased, and stool consistencies became well formed in both groups, while no significant side effects were observed[55]. In a clinical trial comparing the effects of ramosteron, a 5-HT₃ receptor antagonist, to those of mebeverine in patients with IBS-D, both treatments were equally effective in reducing abdominal pain/discomfort and urgency and improving the stool form score and stool frequency compared to baselines[56].

However, when the effects of mebeverine have been compared to placebo and not compared to another drug or measured by self-control, the results have been controversial. A recent systematic review, including eight randomized trials, revealed that clinical improvement and relief of abdominal pain by mebeverine treatment were not statistically significant compared to placebo[57-59]. No differences were found in the effectiveness of 200 and 135 mg mebeverine doses. Tolerability was excellent, without significant adverse effects. Similarly, no positive effects of mebeverine over placebo were seen in an exploratory study performed in 135 IBS patients fulfilling the Rome III criteria who were recruited from general practice, when mebeverine, methylcellulose and placebo were compared, with or without the combination of a cognitive behavioral therapy-based self-management web site (with or without additional telephone and e-mail support)[60,61]. Disappointingly, the use of the web site also did not improve IBS symptom severity scores or quality of life scores significantly over the “no web site” group; nevertheless, there was a visible trend toward continued improvement in the self-management group (particularly those with telephone support) throughout the study, while the “no web site” group and the medication groups seemed to lose their therapeutic gains from weeks 6 to 12. However, in a study performed in London, personal sessions of cognitive behavioral therapy were beneficial in addition to mebeverine, and the effects persisted for up to six months after therapy, both in terms of symptom relief and improvement in social and work disability[62]. Depression and anxiety predict poor outcomes in mebeverine-treated IBS patients, and in cases of patients with unhelpful coping behaviors (e.g., avoidance), the combination of mebeverine with cognitive-behavioral therapy could be useful[63].

Experimental studies: Otilonium bromide is weakly absorbable from the GI tract due to its quaternal ammonium structure; thus, it is almost completely excreted in the feces[64]. In experimental studies, it accumulated in the walls of the GI tract after oral administration, with minimal systemic absorption[65]. Its effects are rather complex, consisting mainly of L-type calcium-channel blockade, but binding to muscarinic M1, M2, M4 and M5 receptors has also been observed[66]. Antagonism of M3-coupled calcium signals in human colonic crypt cells suggested an anti-secretory action in IBS-D patients[67]. Additionally, by antagonism of tachykinin NK-2 receptors, otilonium not only causes spasmolysis but also reduces peripheral sensory afferent transmission to the central nervous system[64]. These effects suggest that otilonium could be effective in reducing both of the main symptoms of IBS: spasms and abdominal pain.

Clinical trials: In a small study of 15 IBS patients, one week of treatment with otilonium bromide significantly increased the pain threshold of IBS patients to anorectal distension, while thresholds for first sensation and stool remained unchanged[68]. In a multicenter, double-blind, placebo-controlled trial with 72 IBS patients in Italy, treatment with 40 mg otilonium bromide three times daily significantly decreased abdominal pain and bloating, improved well-being and global assessment, while it markedly increased the pain threshold during sigmoid distension. Nevertheless, these results did not differ from those of the placebo group[69]. However, otilonium significantly reduced sigmoid motility during distension, whereas placebo did not, suggesting the need for larger studies, a different setup or more accurate patient selection. Seven years later, the results of a larger trial were published in Italy, including 375 IBS patients selected by the Drossman criteria, when a 2-wk placebo run-in period was included to exclude patients with low compliance or with quickly resolving symptoms[70]. After randomization, the patients received 40 mg otilonium bromide or placebo three times daily for 15 wk, and their symptoms were assessed at weeks 5, 10 and 15. Abdominal pain frequency was reduced in both of the groups, with a statistically significant difference in favor of otilonium after 10 and 15 wk of treatment. The global score improvements in abdominal pain and discomfort were significantly greater in the otilonium group throughout the whole study. Therapy was successful in significantly more patients treated with otilonium than in those treated with placebo. Defecation disturbances improved similarly in both groups. Tenderness of the sigmoid colon, general well-being and global judgements by the investigators all differentially improved in the otilonium-treated group. Extended analysis of the data from this study with different analysis forms 3 years later revealed that otilonium had therapeutic gains over placebo not only in terms of pain intensity, pain frequency and meteorism but also regarding the severity of diarrhea/constipation, the number of evacuations and the presence of mucus in stool[71]. When sorting patients according to stool habits, otilonium was more effective than placebo in treating diarrhea but was only as effective as placebo in managing constipation.

Otilonium bromide in irritable bowel syndrome (OBIS) was a recent international clinical trial in which patients diagnosed according to the Rome II criteria received 40 mg otilonium bromide three times a day or placebo over 15 wk after randomization[72,73]. Otilonium bromide effectively reduced abdominal pain frequency and bloating, while improving stool frequency and patients’ global assessments, compared to placebo. The prominent outcome of this study was the efficacy of otilonium in dramatically reducing abdominal pain frequency from more than half of the days to less than one day per week, compared to the persistent 1-3 episodes in the placebo group. Otilonium had no significant effects on pain severity, stool consistency or mucus in the stool. During the 10-wk follow-up period after finishing treatment, the likelihood of symptom recurrence was significantly higher in the placebo group than in the otilonium group. This finding might be explained by the elongated persistence of otilonium in the colonic wall due to its lipophilic properties. No serious adverse events occurred in the study, and only three adverse events, consisting of dry mouth or nausea, were judged by the investigator to be related to otilonium’s side effects.

When compared to other spasmolytics in a meta-analysis, otilonium bromide performed outstandingly among 12 different antispasmodics in terms of IBS symptom control[74]. In a double-blind, randomized, active-controlled trial conducted in China of IBS patients selected according to the Rome II criteria, the results confirmed the similar, but not superior, efficacy of otilonium to that of mebeverine in the management of the frequency and intensity of abdominal pain, and abdominal bloating, flatulence and satisfactory stool frequency were all improved similarly by both therapies[75]. The most common side effects of dry mouth and nausea/dizziness - recorded in previous studies as well-might have been caused by peripheral and central muscarinic antagonism, respectively, and could be explained by the known ability of otilonium to bind to muscarinic receptors[66].

Experimental studies: Pinaverium bromide is also a quaternary ammonium derivate that is poorly absorbed, with pronounced pharmacological effects in the gastrointestinal tract instead of the cardiovascular system[76]. It has a low absorption rate from the GI tract, corroborated by hepato-biliary excretion[64]. It has been shown that its effects are very similar to those of the established L-type calcium-channel blockers (nitrendipine, diltiazem, D600); that is, it reduces the plateau phase of slow waves, thereby inhibiting calcium influx and preventing consequent contractions[77]. Pinaverium has been shown to inhibit the contractile response in dog and rat colonic smooth muscle preparations to acetylcholine, the neurotransmitter of cholinergic intrinsic nerves[77,78]. Similarly, in colonic smooth muscle cells isolated from normal or inflamed human colons, pinaverium bromide inhibits contraction induced by different agonists (CCK 8, carbachol or KCl)[79]. In inflamed colonic cells, pinaverium exerts more pronounced inhibition than the non-GI-selective L-type calcium channel blockers nicardipine and diltiazem. This effect of pinaverium on colonic smooth muscle cells is mediated mainly by the inhibition of calcium influx through L-type calcium channels, thereby inhibiting contractions induced by acetylcholine or KCl in rat preparations[80]. Stress plays an important role in the pathogenesis of IBS; therefore, colonic smooth muscle preparations from cold restraint-stressed rats have also been examined, revealing that the hypermotility observed after stress is mostly related to increased calcium influx into the cells[80]. This observation supports the use of pinaverium in IBS, in which pathological colonic hypermotility must be suppressed. Furthermore, in rats chronically fitted with intraparietal electrodes in the proximal colon, pinaverium bromide has been found to have no effect on colonic long spike bursts in the fasting state, but it inhibits increases in colonic spike burst frequency induced by a meal or by CCK-8[81]. However, it loses its effects in capsaicin-pretreated animals, showing the participation of sensory afferent neurons in the mechanism of action, which might also explain the efficacy of pinaverium bromide in treating the two main features of IBS: motility disorders and gut hypersensitivity.

Clinical trials: Pinaverium bromide has been used for managing functional bowel disorders for decades, with double-blind studies performed as early as 1977[82]. Its effects on colonic smooth muscle have been well established by animal experiments; however, intensive research has only started to characterize its mechanism of modifying GI motility in humans. In an early pilot study performed in 12 IBS patients, colonic motility was detected by surface electromyography over a 2-h fasting period and a 2-h postprandial period following a standard meal, before and after 10 d of treatment with 50 mg pinaverium bromide three times daily[83]. The leading symptoms, such as abdominal pain, bloating and altered bowel habits, started to ameliorate on day 4 of treatment. Abnormal colonic motility patterns (viz., increased frequency and amplitude of contraction, arrhythmia in motoric activity), which were particularly pronounced post-prandially, diminished after 10 d of treatment. In a continuation of this pilot study, the authors studied 22 IBS patients and 7 healthy controls[84]. The healthy controls received no treatment but served as controls for electromyographic measurements. The study protocol was as previously described, except for the length of pinaverium bromide therapy, which was extended to 14 d. The results showed increased fasting and postprandial colonic motility parameters in IBS patients compared to controls, which was effectively reduced by 14 d of pinaverium bromide therapy. Abdominal pain and bloating were also significantly improved by treatment. Stool frequency was normalized by pinaverium bromide therapy in both diarrheic and constipated IBS patients. The effects of pinaverium bromide on intestinal motility were affirmed by a further randomized, double-blind, placebo-controlled trial on IBS patients[85]. Pinaverium bromide was administered in a 50 mg dose (po, tid), and myoelectrical and mechanical activities of the rectum and the internal anal sphincter were recorded before treatment, in the fasting state and at 2 h post-prandially. Post-prandial rectal spike amplitude and frequency, as well as the frequency of the spontaneous recto-anal inhibitory reflex, were significantly decreased after treatment with pinaverium bromide. Pinaverium bromide was also able to change colonic transit and colonic responses to food in IBS patients, as demonstrated by a technique using orally ingested radiopaque markers visible on plain abdominal X-rays[86]. Beneficial effects of pinaverium bromide treatment were also demonstrated by an open trial, in which 61 treated IBS patients experienced significantly reduced abdominal pain, improved stool consistency, reduced defecation straining and urgency, and decreased mucus in stool, with good drug tolerance and few side effects[87]. The clinical efficacy of pinaverium bromide was also evaluated using a statistical technique new to the field-namely, by employing polar vectors on data from a phase IV clinical trial with 1677 Rome III IBS patients receiving pinaverium bromide combined with simethicone[88]. The results showed amelioration of stool frequency and consistency in IBS-C, IBS-D and IBS-M patients; furthermore, the intensity of abdominal pain and bloating was also significantly reduced.

When comparing pinaverium to otilonium bromide in IBS, both treatments were similarly useful in reducing the intensity of pain and in regulating bowel movements, but otilonium was superior to pinaverium in terms of decreasing pain frequency[89]. The side effects were similar in the two groups. The use of pinaverium has generally been considered safe; however, the drug is not licensed for use in pregnant women. In a letter reporting ten involuntary cases of pregnant women taking pinaverium bromide due to dispensing errors, nine individuals delivered healthy babies, while the tenth experienced a spontaneous abortion 1 week after the ingestion of pinaverium[90]. Several women complained of abdominal pain and constipation in parallel with pinaverium use.

Experimental studies: Phloroglucinol is a phenol derivative with non-specific antispasmodic properties, together with its methylated form trimethylphloroglucinol. The mechanism of action is most likely based on the direct inhibition of the voltage-dependent calcium channels of smooth muscle; however, the modulation of prostaglandin or nitric oxide release has also been suggested[91]. Although it has long been used in clinical practice as an antispasmodic for painful urogenital and gastrointestinal conditions, in an early study on anesthetized rats, phloroglucinol was found to be inactive toward the contraction of the duodenum, ileum and colon[92]. Similarly, in anesthetized dogs, phloroglucinol plus trimethyl-phloroglucinol failed to antagonize acetylcholine-induced contraction of the colon[93].

Clinical trials: In parallel with animal studies, phloroglucinol plus trimethyl-phloroglucinol had no clear effects in humans on ascending and sigmoid colon hypermotility evoked by neostigmine[94]. However, in 20 IBS patients, iv phloroglucinol effectively reduced postprandial rectosigmoid motility increases after a test meal, compared to placebo[95]. In another study of IBS patients, phloroglucinol inhibited phasic contractions provoked by intrarectally injected glycerol, but it did not modify colonic tone[96]. In an open-label study of 100 IBS patients selected according to the Rome II criteria, po 50 mg phloroglucinol was administered three times daily for two months[97]. The 68 patients who completed the study reported significant improvement in abdominal pain, frequency of stools per day, urgency, passage of mucus per the rectum, sense of incomplete defecation and bloating. Nevertheless, straining was unchanged. Further, a multicenter, randomized, double-blind, placebo-controlled trial examined the effects of phloroglucinol/trimethylphloroglucinol (62.2 mg P plus 80 mg TMP three times daily) or placebo for 7 d in 307 IBS patients diagnosed using the Rome II criteria[98]. The relative decrease in pain intensity and the responder rate were significantly higher in the P/TMP-treated group, compared to the placebo-treated group. Further, the treatment effect persisted up to the 7^th day in a higher percentage of patients treated with P/TMP than in those treated with placebo. The frequency and severity of adverse events did not differ between the two treatment groups, and no adverse events were considered sufficiently serious to stop treatment. Finally, according to a preliminary report, 72 patients with D-IBS, based on the ROME III criteria, were involved in a double-blind, placebo-controlled trial and were treated with placebo or phloroglucinol (80 mg) three times daily for 14 d after a 1-wk run-in period[99]. Significantly more patients reported “moderate or more improvement” in Subjects Global Assessment in the phloroglucinol group than in the placebo group over the 2-wk period of treatment and the 1-wk post-treatment period. Stool frequency decreased significantly in the phloroglucinol group, compared to the placebo group. Individual symptom scores and stool consistency also improved significantly, but they did not differ from those of the placebo group. Regarding its safety, a French epidemiologic study of phloroglucinol in pregnancy did not find evidence of a teratogenic risk in humans[100].