(Reuters) - RedHill Biopharma Ltd said on Tuesday its drug to treat diarrhea-predominant irritable bowel syndrome met the main goal in a mid-stage study, but the positive data failed to impress investors, sending its U.S.-listed shares lower.

The company said the drug, Bekinda, improved stool consistency in patients when compared with a placebo. The drug, however, did not show statistical significance in improving abdominal pain and overall response in patients - secondary goals in the study.

U.S.-listed shares of the Tel Aviv-based company are down about 11.2 percent at $9.60 in late morning trade.

“The study was not designed or meant to show significance in the secondary endpoints ... the results are actually good according to the small sample size,” RedHill’s Chief Executive Dror Ben-Asher told Reuters.

The trial was testing a 12-mg dose - a once-daily oral pill formulation of a 5-HT3 receptor antagonist called ondansetron - in 126 patients over 18 years of age.

Receptor antagonists such as 5-HT3 are a class of drugs known to slow intestinal movement in patients.

The U.S Food and Drug Administration has already approved a different kind of 5-HT3 antagonist called Lotronex in the United States to treat women with IBS-D.

Other drugs to treat IBS-D in both men and women include Valeant Pharmaceuticals International Inc’s Xifaxan and Allergan Plc’s Viberzi.

RedHill told Reuters it expects to start a late-stage trial on Bekinda by late 2018.

Irritable bowel syndrome is a chronic condition that affects up to 30 million people in the United States, out of which over 50 percent cases belong to IBS-D, the company said.

The disease has two other forms: constipation (IBS-C) and mixed (IBS-M).

RedHill is also testing Bekinda to treat acute gastroenteritis and gastritis, an inflammation of the stomach and intestines that causes vomiting and diarrhea.

In June, RedHill reported positive results from a late-stage trial testing the drug in patients with gastroenteritis.