Study to Evaluate the Efficacy and Safety of Filgotinib in Participants With Active Ankylosing Spondylitis Who Are Naive to Biologic Disease-Modifying Antirheumatic Drug Therapy

Key Inclusion Criteria:

• Ambulatory male or female, ≥ 18 years of age (≥ 20 years of age in Japan) on the dayof signing initial informed consent
• Have an established diagnosis of radiographic axial spondyloarthritis (SpA)/ankylosing spondylitis (AS) by a rheumatologist (or other specialist with expertisediagnosing AS)
• Meet Assessment of SpondyloArthritis international Society (ASAS) classificationcriteria for axial SpA with radiographic sacroiliitis on imaging at screening asfollows:
• History of back pain ≥ 3 months and age at onset of back pain < 45 years, AND
• Radiographic sacroiliitis Grade ≥ 2 bilaterally or Grade 3-4 unilaterally, AND
• ≥ 1 SpA feature (refer to protocol; inflammatory back pain, arthritis, heelenthesitis, uveitis anterior, dactylitis, psoriasis, inflammatory bowel disease,good response to nonsteroidal anti-inflammatory drugs (NSAIDs), family history ofSpA, historically positive human leukocyte antigen B27 (HLA-B27), elevatedC-reactive protein (CRP))
• Have active AS at screening and Day 1 defined by:
• Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) ≥ 4, AND
• Spinal pain score ≥ 4 (based on BASDAI question 2)
• Have had an inadequate response to ≥ 2 NSAIDs at a therapeutic dose range for ≥ 2weeks each (a total duration of NSAID trial ≥ 4 weeks) or intolerance to ≥ 2 NSAIDsfor the treatment of AS
• If using allowed conventional synthetic disease-modifying antirheumatic drug (csDMARD), NSAID, or corticosteroid therapy, must have been on stable doses (asoutlined in protocol) prior to Day 1

Key Exclusion Criteria:

• Contraindication to magnetic resonance imaging (MRI)
• Prior exposure to a biologic disease-modifying antirheumatic drug (bioDMARD) (including investigational agents)
• Prior exposure to a Janus kinase (JAK) inhibitor > 2 doses
• Total ankylosis of the spine
• Any active/recent infection, as specified in the protocol
• Diagnosis of fibromyalgia
• Any musculoskeletal disorder other than AS that would interfere with assessment ofstudy parameters, as per judgement of investigator
• Note: Prior history of reactive or other types of inflammatory arthritis ispermitted if there is documentation of change in diagnosis to AS or additionaldiagnosis of AS
• Any history of an inflammatory arthritis with onset age before 16 years old Note: Other protocol defined Inclusion/Exclusion criteria may apply.