Imuran (Azathioprine) Dose-Ranging Study in Crohn's Disease

Inclusion Criteria:

• Males and females ≥ 14 years old, including women of childbearing potential who arenot pregnant or nursing at the time of enrollment.

• Body weight between 40 and 100 kg (88-220 lbs), inclusive.

• Subjects diagnosed with Crohn's disease, based upon the criteria of Lennard-Jones, forat least a 3-month period. The date of diagnosis will be the date of the firstdiagnostic test that confirms the diagnosis of Crohn's disease. Subjects with adiagnosis of less than 3 months may be considered after review of primary diagnosticdata by the study safety monitor.

• Need for treatment with oral prednisone, based upon the treating physician's clinicaljudgment, for active Crohn's disease as indicated by a (Crohn's Disease ActivityIndex) CDAI between 200 and 450, inclusive; OR Currently being treated with prednisonefor at least 4 weeks with a stable dose of 40mg/day or less for at least 2 weeks, orbudesonide (Entocort EC) 9 mg/day for at least 4 weeks with a stable dose for at least 4 weeks, and active Crohn's disease as indicated by a CDAI between 200 and 450,inclusive.

• Able to swallow tablets.

• Able to provide written informed consent (subjects ≥ 18 years old) or in the case of aminor provide parental consent along with child assent (subjects 14-17 years old).

• If sexually active, willing to comply with effective contraception during the study;or is abstinent.

Exclusion Criteria:

• Diagnosis of indeterminate, microscopic, lymphocytic, collagenous, or ulcerativecolitis.

• Previous or current therapy with 6-mercaptopurine, azathioprine, thioguanine,methotrexate, cyclosporine, tacrolimus, thalidomide or mycophenolate mofetil.

• Previous or current treatment with infliximab.

• Treatment with narcotic pain medications. (Anti-diarrheal agents such as loperamideand diphenoxylate are permitted, providing that the dose is not increased while onprotocol.)

• Subjects with short gut syndrome (defined as requiring oral or parenteral supplementalor total nutrition in order to maintain stable body weight, or more than 100 cm ofsmall bowel resected).

• Subjects with obstructive symptoms or demonstrated stenosis and prestenotic dilatationon barium study.

• Subjects with active infection.

• Subjects with a stoma.

• Subjects with heterozygous or recessive homozygous genotype for TPMT.

• Poor access for peripheral venous phlebotomy.

• History of pancreatitis, except for self-limited episodes from a known cause, such asgallstone pancreatitis.

• White blood cell count (WBC) <4.5 x 10^9/L, hemoglobin <8 gm/dL, Platelets (PLT) <100,000/mm3 at screening (or within the previous 6 months, if known).

• History of abnormal liver function tests, including aspartate aminotransferase (AST)or alanine aminotransferase (ALT) >1.5 times upper limit of normal, alkalinephosphatase >2 times upper limit of normal, total bilirubin >2.5 mg/dL at screening (or within the previous 6 months, if known).

• Subjects needing treatment with orally administered corticosteroids for the treatmentof other medical conditions. Inhaled or dermatologic preparations are acceptable.

• History of HIV infection (if known) or opportunistic infection.

• History of cancer, with the exception of basal cell carcinoma of the skin.

• Concurrent treatment, or need for treatment, with allopurinol.

• Women who are pregnant or nursing at the time of eligibility screening, or who intendto be during the study period.

• Inability to comply with planned schedule of study visits.

• Participation in a clinical trial within the past 6 months.