Introduction
Psoriatic arthritis (PsA) is one of several spondyloarthritides (SpA), a grouping of diseases with shared common immunological and inflammatory components, but unique clinical manifestations.1 Despite having distinct presentations, consistencies in genetic susceptibility markers and associated aberrations in immune response (including activation of the interleukin (IL)−23/IL-17 axis),2 can result in overlapping clinical phenotypes of SpA. Patients with PsA and ankylosing spondylitis (AS), the archetype for axial SpA, can both present with axial arthritis, peripheral arthritis and enthesitis.3 4 One of the most notable genetic susceptibility markers is expression of the human-leucocyte-antigen B27 allele (HLA-B27) of major histocompatibility class I region.5
PsA patients with axial involvement will typically also have peripheral disease. The prevalence of axial PsA increases with duration of disease, that is, 5%–28% among patients with early-onset versus 25%–70% with long-standing PsA.4 Further, PsA patients with axial disease are more likely to be HLA-B27+ than are those with only peripheral arthritis,3 and HLA-B27+ status predicts earlier development of axial disease.4 Although HLA-B27+ is highly associated with the pathogenesis of AS and is present in the vast majority of these patients,4 6 only 20% of patients with PsA are HLA-B27+.4
While it is known that axial disease in PsA may develop later in life than in AS, yields less severe symptoms, and has radiographic features distinct from AS, a universally accepted definition of axial disease in PsA is lacking and currently being researched.4 7 In contrast, the Assessment of SpondyloArthritis International Society’s (ASAS) classification criteria for axial SpA encompass patients both with and without definite radiographic sacroiliitis. Specifically, patients with chronic back pain for >3 months and age at onset <45 years can be classified with axial SpA in the presence of imaging-confirmed sacroiliitis plus ≥1 typical SpA feature, or in the presence of HLA-B27 plus ≥2 other SpA features.8
Ustekinumab is a fully human monoclonal antibody with high affinity for the p40-subunit shared by IL-12 and IL-23. Ustekinumab demonstrated efficacy in treating multiple domains of PsA, including peripheral arthritis, enthesitis and dactylitis, and significantly inhibited radiographic progression of joint damage in the PSUMMIT-1&2 phase 3 studies.9–11 In these studies, approximately 30% of tumour necrosis factor-inhibitor (TNFi)-naïve and experienced patients in PSUMMIT-1&2 had peripheral arthritis with physician-reported spondylitis (PA-PRS); ustekinumab demonstrated significant improvements in axial signs and symptoms through Week 24 in these patients, regardless of prior TNFi use.12 In contrast, ustekinumab was not effective when evaluated in phase 3 placebo-controlled trials of AS patients,13 which prompted additional post-hoc analyses of the PSUMMIT 1&2 trial data focused on evaluating the efficacy of ustekinumab in treating spondylitis-related signs and symptoms among PA-PRS patients who were naïve to TNFi treatment. Response to ustekinumab was also assessed in patients with or without HLA-B27 expression.