Use Patterns of Novel Janus Kinase Inhibitors After the First Safety Alerts: A Nationwide Register‐Based Time Series Analysis

ABSTRACT Purpose Janus kinase inhibitors (JAKi) are medicines used to treat inflammatory rheumatic musculoskeletal diseases (IRMDs) and inflammatory bowel diseases (IBDs). In Europe, JAKis received marketing authorisation in 2017 for rheumatoid arthritis. Subsequently, the European Medicines Agency (EMA) issued multiple safety alerts concerning this drug class. We aimed to examine how the use of JAKis developed in Finland. Methods We conducted a retrospective time series analysis on the initiations and use of JAKis in the treatment of IRMD and IBD, utilising the national medicines reimbursement registry 2018–2023, including nearly all JAKi prescriptions. We assessed the initiators' age, line of treatment and atherosclerotic risk factors. Results Between 2018 and 2023, the prevalence of JAKi use increased from 0.14 to 0.43/1000 inhabitants, from 756 to 1740 among IRMD patients and from 11 to 672 among IBD patients. In IRMD , JAKis were rapidly adopted with 677 initiations in 2018, declining to 328 initiations in 2023. The corresponding numbers in IBD were 11 in 2018 and 358 in 2023. The mean age of IRMD patients initiating JAKis decreased from 54 years in 2021 to 41 years in 2023, while IBD patients' mean age changed from 42 to 39 years. In 2020, 52% of IRMD Patients Initiated JAKi in second‐line of treatment but by 2023, this decreased to 31%. Prevalence of atherosclerotic risk factors declined. Conclusions The uptake of JAKis among IRMD patients were initially fast. After safety alerts, initiation rates declined and shifted towards younger patients with fewer atherosclerotic risk factors. In contrast, the uptake of JAKis among IBD patients increased steadily.