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Background: Ixekizumab, a high-affinity interleukin-17A (IL-17A) monoclonal antibody, has previously shown efficacy in radiographic axial spondyloarthritis (also known as ankylosing spondylitis). We aimed to evaluate the efficacy and safety of ixekizumab, an IL-17 inhibitor, in non-radiographic axial spondyloarthritis. Here, we report the primary results of COAST-X.Methods: COAST-X was a 52-week, randomised, double-blind, placebo-controlled, parallel-group study done at 107 sites in 15 countries in Europe, Asia, North America, and South America. Eligible participants were adults (aged ≥18 years) with active axial spondyloarthritis without definite radiographic sacroiliitis (non-radiographic axial spondyloarthritis), objective signs of inflammation (via MRI or C-reactive protein), and an inadequate response or intolerance to non-steroidal anti-inflammatory drugs (NSAIDs). Patients were randomly assigned (1:1:1) to receive subcutaneous 80 mg ixekizumab every 4 weeks (Q4W) or every 2 weeks (Q2W), or placebo. Changing background medications or switching to open-label ixekizumab Q2W, or both, was allowed after week 16 at investigator discretion. Primary endpoints were Assessment of SpondyloArthritis international Society-40 (ASAS40) response (defined as an improvement of 40% or more and an absolute improvement from baseline of 2 units or more [range 0-10] in at least three of the four domains [patient global, spinal pain, function, and inflammation] without any worsening in the remaining one domain) at weeks 16 and 52. Patients who switched to open-label ixekizumab were imputed as non-responders in logistic regression analysis. This trial is registered with ClinicalTrials.gov, number NCT02757352.Findings: Between Aug 2, 2016, and Jan 29, 2018, 303 patients were enrolled (105 to placebo, 96 to ixekizumab Q4W, and 102 to ixekizumab Q2W). Both primary endpoints were met: ASAS40 at week 16 (ixekizumab Q4W: 34 [35%] of 96, p=0·0094 vs placebo; ixekizumab Q2W: 41 [40%] of 102, p=0·0016; placebo: 20 [19%] of 105) and ASAS40 at week 52 (ixekizumab Q4W: 29 [30%] of 96, p=0·0045; ixekizumab Q2W: 32 [31%] of 102, p=0·0037; placebo: 14 [13%] of 105). 60 (57%) of 104 patients in the placebo group, 63 (66%) of 96 in the ixekizumab Q4W group, and 79 (77%) of 102 in the ixekizumab Q2W group had at least one treatment-emergent adverse event. The most common treatment-emergent adverse events in the ixekizumab groups were nasopharyngitis and injection site reaction. Of the treatment-emergent adverse events of special interest, there was one case of serious infection in the ixekizumab Q4W group. The frequency of serious adverse events was low (four [1%] of 302) and similar across the three groups. There were no malignancies or deaths. No new safety signals were identified.Interpretation: Ixekizumab was superior to placebo for improving signs and symptoms in patients with non-radiographic axial spondyloarthritis at weeks 16 and 52. Reports of adverse events were similar to those of previous ixekizumab studies. Ixekizumab offers a potential therapeutic option for patients with non-radiographic axial spondyloarthritis who had an inadequate response or were intolerant to NSAID therapy.Funding: Eli Lilly and Company. [ABSTRACT FROM AUTHOR]