Background: Guselkumab, a human monoclonal antibody that binds to the p19 subunit of interleukin 23, has been approved for the treatment of moderate-to-severe psoriasis. Psoriatic arthritis is a common comorbidity of psoriasis with an umet need for novel treatments. We assessed the efficacy and safety of guselkumab in patients with active psoriatic arthritis. Methods: We did a randomised, double-blind, placebo-controlled, phase 2a trial at 34 rheumatology and dermatology practices in Canada, Germany, Poland, Romania, Russia, Spain, and the USA. Eligible participants were aged 18 years or older with active psoriatic arthritis and plaque psoriasis affecting at least 3% of their body surface area, with three or more of 66 tender joints and three or more of 68 swollen joints, who had an inadequate response or intolerance to standard treatments. We randomly assigned patients (2:1) via a central interactive web-response system using computer-generated permuted blocks with a block size of six, stratified by previous anti-tumour necrosis factor-α use, to receive subcutaneous guselkumab 100 mg or placebo at week 0, week 4, and every 8 weeks thereafter for 24 weeks. Patients, investigators, and site staff were masked to treatment assignment until final database lock at week 56. At week 16, patients with less than 5% improvement in swollen and tender joint counts were eligible for early escape to ustekinumab. At week 24, the remaining placebo-treated patients crossed over to receive guselkumab 100 mg at weeks 24, 28, 36, and 44 and guselkumab-treated patients received a placebo injection at week 24, followed by guselkumab injections at weeks 28, 36, and 44. The primary endpoint was the proportion of patients with at least 20% improvement at week 24 in signs and symptoms of psoriatic arthritis according to American College of Rheumatology criteria (ACR20) in the modified intention-to-treat population (ie, all randomly assigned patients who received at least one dose of study treatment). Safety analyses included patients according to the study drug received. This study is registered with ClinicalTrials.gov, number NCT02319759. Findings: Between March 27, 2015, and Jan 17, 2017, we randomly assigned 149 patients to treatment: 100 to guselkumab and 49 to placebo. 17 (35%) of 49 patients in the placebo group and ten (10%) of 100 patients in the guselkumab group were eligible for early escape to ustekinumab at week 16. 29 (59%) of 49 patients in the placebo group crossed over and received guselkumab at week 24. Three (6%) of 49 patients in the placebo group, one (3%) of 29 patients who crossed over from placebo to guselkumab, and six (6%) of 100 patients in the guselkumab group discontinued study treatment before week 44. 58 (58%) of 100 patients in the guselkumab group and nine (18%) of 49 patients in the placebo group achieved an ACR20 response at week 24 (percentage difference 39·7% [95% CI 25·3–54·1]; p<0·0001). Between week 0 and week 24, 36 (36%) of 100 guselkumab-treated patients and 16 (33%) of 49 placebo-treated patients had at least one adverse event. The most frequent adverse event was infection in both groups (16 [16%] of 100 patients in the guselkumab group vs ten [20%] of 49 patients in the placebo group). The prevalence of adverse events between week 0 and week 56 in guselkumab-treated patients (51 [40%] of 129) indicated no disproportional increase with longer guselkumab exposure. No deaths occurred. Interpretation: Guselkumab, a novel anti-interleukin 23p19 antibody, significantly improved signs and symptoms of active psoriatic arthritis and was well tolerated during 44 weeks of treatment. The results of this study support further development of guselkumab as a novel and comprehensive treatment in psoriatic arthritis. Funding: Janssen Research & Development.
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