Efficacy and Safety of Ivarmacitinib, a Selective JAK1 Inhibitor, in Patients With Active Ankylosing Spondylitis: Results From an Adaptive Seamless Phase 2/3 Trial

Objective This randomized, double‐blind, adaptive phase 2/3 trial aimed to evaluate the efficacy and safety of ivarmacitinib, a novel selective JAK1 inhibitor, in patients with active ankylosing spondylitis (AS). Methods In phase 2, patients were randomized (1:1:1:1) to receive ivarmacitinib at a dose of 2, 4, or 8 mg or placebo once daily for 12 weeks. Based on interim analysis, ivarmacitinib 4 mg was selected as the recommended dose. In phase 3, patients were randomized (1:1) to receive ivarmacitinib 4 mg or placebo for 12 weeks; from week 12 onward, all patients received ivarmacitinib 4 mg once daily through week 24. The primary end point was the proportion of patients achieving a ≥20% improvement in the Assessment of SpondyloArthritis international Society (ASAS20) response criteria at week 12. Results During the entire study, 504 patients were randomized (ivarmacitinib 4 mg, n = 187; placebo, n = 186). At week 12, the ASAS20 response rate was significantly higher in the ivarmacitinib 4 mg group compared to the placebo group (48.7% vs 29.0%; one‐sided P = 0.0001). The ASAS40 response rate (32.1% vs 18.3%) and all other secondary end points also favored ivarmacitinib 4 mg. Efficacy was sustained through 24 weeks. During the placebo‐controlled period, treatment‐emergent adverse events occurred in 79.7% of the ivarmacitinib group and 65.6% of the placebo group. Conclusion Ivarmacitinib 4 mg significantly improved the signs and symptoms of AS at week 12 compared to placebo, with sustained efficacy through 24 weeks and a tolerable safety profile. These findings support ivarmacitinib 4 mg as a new treatment option for active AS.