1. Drugs for the treatment of irritable bowel syndrome (IBS)
Domina Petric, MD

2. Introduction
IBS is an idiopathic, chronic, relapsing disorder characterized by:
abdominal discomfort (pain, bloating, distention, cramps)
in association with alterations in bowel habits (diarrhea,constipation)

3. Introduction Pharmacotherapy goals:
relieving abdominal pain and discomfort
improving bowel function
For patients with predominant diarrhea, antidiarrheal agents (loperamide especially) are helpful in reducing stool frequency and fecal urgency.
For patients with predominant constipation, fiber supplements may lead to softening of stools and reduced straining.
Increased gas production may exacerbate bloating and abdominal discomfort.

4. Introduction
Osmotic laxatives, especially milk of magnesia, are used to soften stools and promote increased stool frequency.
For chronic abdominal pain, low doses of tricyclic antidepressants (amitriptyline or desipramine, mg/d) appear to be helpful.

5. Introduction
Low doses of tricyclic antidperessants have no effect on mood, but may alter central processing of visceral afferent information.
The anticholinergic properties of these agents may have effects on gastrointestinal motility and secretion, reducing stool frequency and liquidity.

6. Introduction
Tricyclic antidepressants may alter receptors for enteric neurotransmitters such as serotonin, affecting visceral afferent sensation.

7. Antispasmodics (anticholinergics)
Antispasmodics work primarily through anticholinergic activities.
Dicyclomine, hyoscyamine: inhibit muscarinic cholinergic receptors in the enteric plexus and on smooth muscle.
At low doses, they have minimal autonomic effects.

8. Antispasmodics (anticholinergics)
At higher doses these agents exibit significant additional anticholinergic effects: dry mouth, visual disturbances, urinary retention and constipation.

9. Serotonin 5-HT3 receptor antagonists
5-HT3 receptors in the gastrointestinal tract activate visceral afferent pain sensation via extrinsic sensory neurons from the gut to the spinal cord and CNS.
Inhibition of afferent gastrointestinal 5-HT3 receptors may reduce unpleasant visceral afferent sensation: nausea, bloating and pain.

10. Serotonin 5-HT3 receptor antagonists
Blockade of central 5-HT3 receptors also reduces the central response to visceral afferent stimulation.
5-HT3 receptor blockade on the terminals of enteric cholinergic neurons inhibits colonic motility, especially in the left colon, increasing total colonic transit time.

11. Alosetron
5-HT3 receptor antagonist for the treatment of patients with severe IBS with diarrhea.
Alosetron is highly potent and selective antagonist.
It is rapidly absorbed from the GI tract.
Bioavailability is 50-60%.
Plasma half-life is 1,5 hours.

12. Alosetron
It undergoes extensive hepatic cytochrome P450 metabolism with renal excretion of most metabolites.
Binds with higher affinity and dissociates more slowly from 5-HT3 receptors than other 5-HT3 antagonists.

13. Clinical use
Alosetron is approved for the treatment of women with severe IBS with predominant diarrhea.
Dosage is 1 mg 1-2 daily.
It reduces IBS-related lower abdominal pain, cramps, urgency and diarrhea.
Reduces the mean number of bowel movements per day and improves stool consistency.

14. Adverse events
Alosetron is associated with rare, but serious gastrointestinal toxicity.
Constipation occurs in up to 30% of patients with diarrhea-predominant IBS.
Serious complications of constipation requiring hospitalization (with or without surgery) and episodes of ischemic colitis have been reported.

15. Adverse events
Alosetron is restricted to women with severe diarrhea predominant IBS who have not responded to conventional therapies.
It is important to educate patients about relative risks and benefits.

16. Chloride channel activator
Lubiprostone is a prostanoic acid derivative that stimulates the type 2 chloride channel (CIC-2) in the small intestine.
It is used in the treatment of chronic constipation.
It is approved for the treatment of women with IBS with predominant constipation.

17. Chloride channel activator
Dose for IBS is 8 mcg twice daily.
It should be avoided in pregnant women and women of child-bearing age.

18. Literature
Katzung, Masters, Trevor. Basic and clinical pharmacology.