Linaclotide: promising IBS-C efficacy in an era of provisional study endpoints

Recent disappointing developments in the pharmacotherapy of irritable bowel syndrome (IBS) have not dampened the enthusiasm surrounding linaclotide, a novel guanylate cyclase-C agonist for the management of constipation-predominant IBS (IBS-C). Two recent phase 3 studies reporting on a single, daily dose of linaclotide are presented in this issue of the American Journal of Gastroenterology. Importantly, these studies are the first to examine a provisional Food and Drug Administration (FDA) combined response endpoint for IBS-C, which mandates improvements of both abdominal pain and defecatory symptoms. Potential limitations of this FDA endpoint relate to a lack of inclusion of other potentially important IBS symptoms and an inability to directly compare findings with other recent IBS-C trials. Both studies successfully reached this endpoint in approximately one-third of study subjects, resulting in numbers needed to treat (NNT) of five to eight, to achieve an FDA responder. Individual symptom responses to linaclotide were seen in nearly 50% of participants, and potential explanations for these discrepancies when compared with the FDA endpoint are offered. Adequate relief measures also were assessed and, with NNTs of 3.4-6.8, compared favorably with other contemporary IBS-C studies. Overall, both linaclotide trials found the medication to be safe in terms of serious adverse events, though the secretagogue mechanism of action led to diarrhea in approximately one in five subjects. Together, these studies inspire several other important questions regarding linaclotide, including its role in the management of IBS-C relative to existing treatment options, such as lubiprostone. Greater clinical use of linaclotide will reveal whether the observed responses measured with the FDA provisional endpoint will translate into real-world experiences of improvement in IBS patients.