Irritable Bowel Syndrome The Possible Benefits of Oral Antibiotic Therapy

From AccessMedicine from McGraw-Hill

Kurt J. Isselbacher

Irritable bowel syndrome (IBS) is one of the most common conditions in clinical practice, yet the number of effective treatments is limited. Perhaps this is not surprising because we lack a complete understanding of the underlying pathogenesis of this disorder and undoubtedly the disease is heterogeneous. Because gut flora may play an important role in the pathophysiology of IBS, Pimentel and colleagues (2011) have evaluated the role of rifaximin, a minimally absorbed antibiotic, as a potential treatment of IBS patients without constipation.

They carried out two identically designed, large, double-blind, placebo-controlled trials. A total of 1260 patients were randomly assigned to receive rifaximin at a dose of 550 mg three times daily or placebo for 2 weeks. There was then a follow-up period of 10 weeks after treatment. The primary endpoint was the proportion of patients who reported adequate relief of symptoms assessed weekly during the first 4 weeks after treatment. The second endpoint was the proportion of patients reporting adequate relief of bloating during the same period.

In the rifaximin groups, compared with the placebo groups, there was a significantly higher proportion of patients reporting (1) adequate relief of IBS symptoms in the two trials combined—41% vs 32% (p < .001)—and (2) relief of bloating—40% vs 30%— (p < .001) for at least the first 4 weeks.

In an accompanying editorial, Dr. Jan Tack (2011) comments positively on these data. He notes that (1) the sustained benefit over at least 10 weeks after the short pretreatment course was a positive sign; (2) the beneficial effects of rifaximin, including its effects on bloating, were impressive; and (3) the similarity of the results in the two studies confirms the reproducibility of the therapeutic effect of rifaximin. Commenting on the possible mode of action of rifaximin, Dr. Tack notes that "the most likely mode of action of rifaximin is a reduction in overall bacterial load, especially in the large bowel." He suggests that this obviously would lead to a decrease in bacterial fermentation and therefore less bloating.

It should be noted that there may be a subpopulation of IBS patients who have a better result with oral nonsystemic antibiotics than others. And finally, Dr. Tack noted that "it seems prudent to restrict the use of nonabsorbable antibiotics to patients in whom small-intestine bacterial over-growth has been confirmed, or to single-treatment cycles in patients who have IBS without constipation and who have not had a response to currently available symptom-directed therapies."