To evaluate the efficacy of Brodalumab in axSpA (ankylosing spondylitis [AS] and non-radiographic axial spondyloarthritis [nr-axSpA]) subjects compared to placebo, as measured by the proportion of axSpA subjects achieving an Assessment of SpondyloArthritis international Society (ASAS) 40 response at week 16.

Inclusion Criteria:

• Subject with age at onset <45 years and continuous chronic back pain for ≥ 3 months fulfills the ASAS classification criteria of axial spondyloarthritis (with the exception of the Crohn's disease)
• Subject has Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score ≥ 4 at screening and enrollment
• Subject has spinal pain score (BASDAI question #2) ≥ 4 at screening and enrollment
• Subject has had adequate therapy with non-steroidal anti-inflammatory drugs (oral NSAIDs) for back pain for at least 3 months with inadequate treatment response before enrollment

Exclusion Criteria:

• Complete ankylosis (fusion) of the spine
• Subject with active ongoing inflammatory diseases other than axSpA that might confound the evaluation of Brodalumab therapy, including reactive arthritis, spondyloarthritis associated with inflammatory bowel disease, SAPHO syndrome (pustulotic arthro-osteitis), fibromyalgia, ankylosing spinal hyperostosis, osteitis condensans ilii, spondylosis deformans, or osteoarthritis sacroiliac joint disease
• Subject has a prior history of >1 anti-tumor necrosis factor (TNF) therapy
• Subject has a history or evidence of suicidal ideation (severity of 4 or 5) or any suicidal behavior based on an assessment with the Columbia-Suicide Severity Rating Scale (C-SSRS) at enrollment
• Subject has a history or evidence of a psychiatric disorder, alcohol and/or substance abuse
• Subject has severe depression based on a total score of ≥ 15 on the Patient Health Questionnaire-8 (PHQ-8) at enrollment (note: subjects with a total score of 10 to 14 on the PHQ-8 should be referred to a mental health care professional)