SMT19969-C006 study was terminated in alignment with corporate decision to pursue further development of drug candidate with a partner.

Study to evaluate the safety of ridinilazole in adolescent subjects and how ridinilazole is metabolized.

Inclusion Criteria:

• Is aged 12 to <18 years.
• Has signs and symptoms of CDI including diarrhea such that in the Investigator's opinion CDI antimicrobial therapy is required, and the subject has tested positive for toxin A and/or B of C. difficile in the stool. Diarrhea is defined as ≥ 3 unformed bowel movements (UBMs) based on types 5, 6, 7 on the Bristol Stool Chart and diarrhea information is within 24 hours prior to randomization.

Exclusion Criteria:

• Has had more than the equivalent of 48 hours of dosing of antimicrobial treatment active against the current episode of CDI prior to randomization.
• Has received ridinilazole or an investigational vaccine against C. difficile any time in the past, anti-toxic antibodies including bezlotoxumab within the past 6 months, or any other investigational medicinal product for treatment of CDI or fecal microbiota replacement therapy within the past 3 months.
• Has a clinically relevant positive stool test for pathogens other than C. difficile, within 48 hours of randomization.
• Has life-threatening or fulminant CDI with evidence of hypotension, septic shock, peritoneal signs or absence of bowel sounds, toxic megacolon, or ileus.
• Has had major GI surgery (e.g. significant bowel resection or pancreatectomy but not including appendectomy or cholecystectomy) within past 3 months or has the presence of a colostomy or ileostomy or has the likely requirement of an ostomy during the study.
• Is receiving treatment that generally is associated with severe diarrhea, intractable vomiting, severe nausea, or inability to swallow that cannot be managed with antiemetics or antidiarrheals and that limits the ability to take oral medications. Cancer treatment that does not comprise ability to take study medication or cause severe diarrhea is allowed.