This study will evaluate the effectiveness and safety of the investigational drug 683699 in treating subjects with moderately to severely active CD (Crohn's Disease).

Inclusion criteria:

• Diagnosed with CD and reconfirmed in previous 2 years by a radiologic or endoscopic assessment including histology.
• CDAI (Crohn's Disease Activity Index) score of 220-450 after 1 week of screening.
• CRP (C-Reactive Protein) level of >4mg at screening.

Exclusion criteria:

• Significantly abnormal laboratory tests or ECG (electrocardiogram) results.
• Current use of an elemental diet or parenteral nutrition.
• Clinically significant positive stool culture.
• Ongoing neoplastic disease of the bowel.
• Bowel perforation other than fistulae.
• Has an ileostomy or colostomy.
• Fixed symptomatic gastrointestinal (GI) stricture within 6 months or obstructive symptoms within 3 months of screening.
• Any bowel resection within 12 months of screening or bowel resection without subsequent demonstration of recurrence of active CD.
• More than 100cm of bowel resected.
• Non-curative bowel surgery with 2 months of screening.
• Symptoms attributed to short bowel syndrome.
• Uncontrolled bacterial, viral, or fungal infection or congenital or acquired immunodeficiency.
• Women who are pregnant, breast feeding, or planning to become pregnant during the study.

Permitted medications:

• Less than or equal to 20mg/day oral corticosteroids for at least 4 weeks prior to screening and on stable doses for 2 weeks prior to screening.
• Azathioprine and 6-MP (6-Mercaptopurine) stable dose for 3 months prior to screening.
• 5-ASA (5-Aminosalicylic acid) stable dose for 1 month prior to screening.

Prohibited medications:

• Greater than 20mg/day oral corticosteroids, or systemic intravenous corticosteroids, or antibiotics as a treatment for CD within 4 weeks prior to screening.
• Cyclosporine or methotrexate during the 2 months prior to screening.
• Infliximab or other biological treatments within 3 months prior to screening.