Temporal Trends in Janus Kinase Inhibitor ( JAKi ) Use Across Five European Countries: A DARWIN EU Network Study

ABSTRACT Introduction Janus kinase inhibitors (JAKi) are increasingly used for autoimmune conditions despite existing safety concerns. This study aims to estimate the incidence of new JAKi use over time and characterise users in Europe to support future safety studies. Methods The study included individuals from five European data sources within the DARWIN EU network: FinOMOP‐HILMO (Finland; primary and secondary care), IPCI (the Netherlands; primary), IQVIA DA Germany (Germany; primary), NLHR (Norway; primary and secondary), VID (Spain; primary and secondary) from 1 January 2017. Annual incidence of JAKi (abrocitinib, baricitinib, filgotinib, tofacitinib, upadacitinib) was up to 31 December 2023. Patient characteristics and treatment duration were described. Results Incidence of JAKi use increased over time, with transient declines during the COVID‐19 pandemic. Initiators, except for abrocitinib, were predominantly aged 41–60 years. Rheumatoid arthritis was the most common indicated condition, except for abrocitinib, for which atopic dermatitis predominated. Median treatment duration was generally less than 1 year. Tofacitinib and baricitinib had the lowest proportions of prior JAKi use, whereas up to 17% and 16% of upadacitinib and filgotinib initiators had received prior treatment with another JAKi. Findings were consistent across data sources, although incidence rates varied according to the healthcare sector captured. Conclusions Incidence of JAKi use generally increased, with tofacitinib being the most initiated JAKi. Treatment durations were relatively short and switching between JAKi was common. The current study provides an overview of indication distribution and characterises JAKi treatment, while offering a framework for future research.