The study will assess the effect of namilumab, a GM-CSF inhibitor, on the clinical response in subjects with axial spondyloarthritis. Subjects will receive treatment with either namilumab or placebo.

A phase 2a proof-of-concept, randomised, double-blind, placebo-controlled study to evaluate the safety/tolerability and efficacy of 4 subcutaneous injections of namilumab (150 mg) given over 10 weeks in subjects with moderate-to-severely active axial spondyloarthritis including those previously exposed to anti- TNF therapy (NAMASTE study).

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.

28 weeks