The study consists of 2 parts: Main Study (screening through Week 16) and Long-Term Extension (LTE) (up to 1.5 years after Week 16). Participants who permanently discontinue treatment in the Main Study will not be eligible for the Long-Term extension.
The primary objective of this study is to evaluate the effect of filgotinib versus placebo on signs and symptoms of active ankylosing spondylitis (AS), evaluated by Assessment of SpondyloArthritis international Society 40% improvement (ASAS40) response at Week 16 in participants with active AS who have an inadequate response to biologic disease-modifying antirheumatic drug therapy.