BACKGROUND: The Janus kinase 1 (JAK1) pathway has been implicated in the 
pathogenesis of psoriatic arthritis. We aimed to investigate the efficacy and 
safety of filgotinib, a selective JAK1 inhibitor, for the treatment of psoriatic 
arthritis.
METHODS: The EQUATOR trial was a randomised, double-blind, placebo-controlled 
phase 2 trial that enrolled adults from 25 sites in seven countries (Belgium, 
Bulgaria, Czech Republic, Estonia, Poland, Spain, and Ukraine). Patients (aged 
≥18 years) had active moderate-to-severe psoriatic arthritis (defined as at 
least five swollen joints and at least five tender joints) fulfilling 
Classification for psoriatic arthritis (CASPAR) criteria, active or a documented 
history of plaque psoriasis, and an insufficient response or intolerance to at 
least one conventional synthetic disease-modifying anti-rheumatic drug 
(csDMARD). Patients continued to take csDMARDs during the study if they had 
received this treatment for at least 12 weeks before screening and were on a 
stable dose for at least 4 weeks before baseline. Using an interactive web-based 
system, we randomly allocated patients (1:1) to filgotinib 200 mg or placebo 
orally once daily for 16 weeks (stratified by current use of csDMARDs and 
previous use of anti-tumour necrosis factor). Patients, study team, and sponsor 
were masked to treatment assignment. The primary endpoint was proportion of 
patients achieving 20% improvement in American College of Rheumatology response 
criteria (ACR20) at week 16 in the full analysis set (patients who received at 
least one dose of study drug), which was compared between groups with the 
Cochran-Mantel-Haenszel test and non-responder imputation method. This trial is 
registered with ClincalTrials.gov, number NCT03101670.
FINDINGS: Between March 9, and Sept 27, 2017, 191 patients were screened and 131 
were randomly allocated to treatment (65 to filgotinib and 66 to placebo). 60 
(92%) patients in the filgotinib group and 64 (97%) patients in the placebo 
group completed the study; five patients (8%) in the filgotinib group and two 
patients (3%) in the placebo group discontinued treatment. 52 (80%) of 65 
patients in the filgotinib group and 22 (33%) of 66 in the placebo group 
achieved ACR20 at week 16 (treatment difference 47% [95% CI 30·2-59·6], 
p<0·0001). 37 (57%) patients who received filgotinib and 39 (59%) patients who 
received placebo had at least one treatment-emergent adverse event. Six 
participants had an event that was grade 3 or worse. The most common events were 
nasopharyngitis and headache, occurring at similar proportions in each group. 
One serious treatment-emergent adverse event was reported in each group 
(pneumonia and hip fracture after a fall), one of which (pneumonia) was fatal in 
the filgotinib group.
INTERPRETATION: Filgotinib is efficacious for the treatment of active psoriatic 
arthritis, and no new safety signals were identified.
FUNDING: Galapagos and Gilead Sciences.