RA diagnosis prior to 16 years of age (Juvenile RA)

Lack of response to \>3 disease modifying anti-rheumatic drugs (DMARDs) or exposure to \>1 biologic DMARD

Use of DMARDs other than MTX within 12 weeks prior to Day 1

Intra-articular and/or parenteral corticosteroids within 4 weeks prior to study Day 1

Blood donation or receipt of live vaccine within 4 weeks prior to Day 1

Major surgical procedure within 8 weeks prior to Day 1

Any systemic inflammatory condition, recurrent infection, or current infection other than onychomycosis

History of cancer within 5 years prior to Day 1

Presence of hepatic or biliary disease

History of tuberculosis

History of human immunodeficiency virus (HIV)

Study of LX3305 in Subjects With Active Rheumatoid Arthritis on Stable Methotrexate