The purpose of this study is to verify the safety and efficacy of OPC-6535 and determine the optimal dose by once-daily oral administration of OPC-6535 at 25 or 50 mg or placebo for 8 weeks in combination with base treatment (either a fixed oral dose of 5-aminosalicylic acid [5-ASA] or a fixed oral dose of 5-ASA plus enteral nutrition) in 180 patients with active Crohn's disease.

Inclusion Criteria:

• Primary lesion in either small intestine or large intestine
• C-reactive protein (CRP) level above the upper limit of the normal range
• Patients who have been receiving a 5-ASA formulation (oral mesalazine) at a fixed dose of 2.25 g/day or higher (not exceeding the approved dose) and at a fixed dosing regimen
• Patients who have not received enteral nutrition or who have been receiving enteral nutrition at a fixed intake of 1200 kcal/day or less

Exclusion Criteria:

• Patients with an uncontrolled external fistula (including anal fistula)
• Patients with a history of total proctocolectomy or subtotal colectomy
• Patients with short bowel syndrome
• Patients with an artificial anus
• Patients with serious infectious disease (intra-abdominal abscess, etc)
• Patients with malignant tumor
• Female patients who are pregnant, lactating, or possibly pregnant, or who wish to become pregnant during the trial period

Full analysis set comprised all subjects who received at least one dose of investigational medicinal product (IMP) and for whom postdosing efficacy data were obtained.

From the start of IMP administration up to 3 weeks after the end of the trial

Adverse events (AEs) occurring after IMP administration were included in assessment of AEs.

The safety analysis population comprised subjects who received at least one dose of IMP.