The primary objective of this study is to investigate the safety and efficacy of OPT-80 versus vancomycin in subjects with Clostridium difficile-associated diarrhea (CDAD).

This is a multicenter, double-blind, randomized, parallel group study. The subjects who meet all of the inclusion criteria and none of the exclusion criteria will be randomized, and will orally receive either OPT-80 twice daily or vancomycin powder four times daily for 10 days. A follow-up investigation will be performed 28 (±3) days after the completion of study drug administration.

Inclusion Criteria:

• Inpatients who have symptoms of CDAD as defined by;

  • (1)Diarrhea: with ≥4 unformed bowel movements (or ≥200 mL unformed stool for subjects having rectal collection devices) and
  • (2)Presence of either toxin A and/or B of C. difficile in the stool

• Subjects who have not received antibacterials (vancomycin, metronidazole, et.al.) aiming at CDAD treatment before the study

Exclusion Criteria:

• Life-threatening or fulminant CDAD
• Ileus paralytic or toxic megacolon
• Likelihood of death before the completion of study from any cause
• Concurrent use of oral vancomycin, metronidazole, et.al. aiming at the treatment of CDAD
• The anticipated need to continue other antibacterials for a period exceeding seven days from providing the informed consent
• Subjects who in the opinion of the investigator require other drugs to control diarrhea
• Need of change in dosage regimen of opiates during the study period
• Need of change in dosage regimen of probiotic products during the study period
• History/complications of ulcerative colitis or Crohn's disease
• Multiple occurrences of CDAD within the past three months
• Hypersensitivity to vancomycin
• Previous exposure to OPT-80 (fidaxomicin)
• Female patients who are pregnant, breastfeeding or possibly pregnant, or wishing to become pregnant during the course of study
• Participation in other clinical research studies or Post Marketing Clinical Trials utilizing an investigational agent within one month prior to providing the informed consent or within five half-lives of the investigational agent, whichever is longer

Access to anonymized individual participant level data will not be provided for this trial. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.