Rheumatol Ther . 2019 Dec;6(4):503-520. doi: 10.1007/s40744-019-00167-6. Epub 2019 Aug 13.Peficitinib (PEF) 100 mg QD demonstrated a stable safety profile and sustained effectiveness in patients with moderate-to-severe RA in a two-year extension of two global phase IIb studies. Patients enrolled in the LTE had completed one of two 12-week phase IIb PEF trials, one with MTX and one without. The primary objective of the LTE was to assess treatment-emergent adverse events and clinical laboratory evaluations for 105 weeks. As a secondary objective, an additional 2-years of effectiveness was assessed by ACR 20/50/70 scores. AEs caused 48% of patients to discontinue treatment. The most common AEs were upper respiratory tract infections (9.9/100 PYs) and UTIs (7.2/100 PYs). Serious AEs were reported in 13% of patients with incidences per 100 PYs of serious infections 2.7, herpes zoster 1.5 and malignancies 0.6. The most frequently reported grade ≥3 AEs were RA, appendicitis, UTI, anaemia and headache. AEs of special interest showed UTI to have the highest incidence (0.4 events/100 PYs), followed by appendicitis (0.3 events/100 PYs). The most common AEs leading to discontinuation were blood creatine phosphokinase increase, anaemia and headache. At week 105, the ACR20/50/70 responses were 44%, 32% and 21% respectively. This LTE study of PEF showed that treatment had a favourable benefit-risk ratio however, there were no clinically meaningful differences in the frequency of any AEs between treatment groups. Patients that reached completion had ACR20 responses that were sustained from week 12 to 105, along with stable safety profile. The main limitation of the study is that half of the population discontinued treatment.