Study of Asimadoline to Treat Diarrhea-Predominant Irritable Bowel Syndrome (D-IBS)

The primary objective of the study is to compare the efficacy of the two treatments with respect to improvement in IBS-related abdominal pain severity and reduction in stool frequency. During the 12-week treatment period, daily IBS-related abdominal pain severity score and daily frequency of bowel movements will be averaged over each week to determine average values for each endpoint. For each subject, weekly response to treatment will be based on the following parameters: * Decrease from baseline of at least 30% in the average IBS-related daily abdominal pain severity score * Decrease from baseline of at least 25% in the average number of daily bowel movements A subject must meet both criteria to be considered a weekly responder. The primary efficacy endpoint is based on an "overall study responder," defined as a subject having 6 or more weeks of weekly response to treatment out of the 12 weeks in the treatment period. Overall study responder will be stratified by timing of bowel preparation and endoscopy.