Efficacy and Safety of Namilumab for Moderate-to-severe Axial Spondyloarthritis

Inclusion Criteria: * Age ≥ 18 and ≤ 75 years of age. * Diagnosis of axSpA by an appropriately qualified physician and classified using ASAS criteria ≥ 3 months prior to Baseline. * Bath Ankylosing Spondylitis Disease Activity Index score ≥ 4 and spinal pain score ≥ 40, at screening and Baseline. * MRI evidence of active axSpA ≤ 6 (ideally ≤ 3) months prior to randomisation using ASAS criteria. * Stable NSAID use prior to study entry. * Stable use of MTX, sulfasalazine or leflunomide prior to study entry. * Stable oral corticosteroid dose prior to study entry. * Capable of giving signed informed consent. * Inadequately responded to or experienced intolerance to previous treatment with an anti-TNF agent (some subjects). Exclusion Criteria: * Current diagnosis of axSpA with a BASDAI \> 4 but no evidence of inflammation on MRI. * Discontinued biologic therapy \< 8 weeks prior to Baseline. * Previous or current use of oral corticosteroid as defined in protocol. * Received intra-articular or i.v. corticosteroids prior to or during Screening. * Received anti-IL-17A or anti-IL-12/23 therapy. * Received cyclosporine, tacrolimus or mycophenolate mofetil prior to Baseline. * Previously received stem cell transplantation. * Infection(s) requiring treatment with i.v. anti-infectives or oral anti-infectives prior to Baseline. * Abnormal screening laboratory and other analyses. * Receipt of any live vaccine within 2 weeks prior to randomisation, or will require live vaccination during study participation. * Evidence of current or prior dysplasia or history of malignancy. * Has had any uncontrolled and/or clinically significant illness, hospitalisation, or any surgical procedure requiring general anaesthesia prior to Screening, or any planned surgical procedure within 6 months after randomisation. * Known current or previous interstitial lung disease. * Positive pregnancy test at Screening (serum) or Baseline (urine). * Female subjects who are breastfeeding or considering becoming pregnant during the study. * Considered by the Investigator to be an unsuitable candidate for the study. * Received any investigational agent or procedure within 30 days or 5 half-lives prior to Baseline. * Related to or a dependent of the site staff, or a member of the site staff.