Emerging and Investigational JAK Inhibitors for Rheumatoid Arthritis: Efficacy and Safety

Janus kinase (JAK) inhibition is an established therapeutic strategy in rheumatoid arthritis, but heterogeneous treatment responses, persistent safety concerns, and the need for more precise therapeutic positioning continue to stimulate the development of additional JAK-directed compounds. This structured narrative review evaluates the pharmacological rationale, clinical efficacy, structural and imaging outcomes, safety evidence, and developmental maturity of emerging, regionally approved, and investigational agents. Peficitinib has the most mature evidence base, including Phase 3 efficacy, radiographic protection, long-term extension data, and pooled safety analyses. Ivarmacitinib has phase 3 and exploratory MRI data, whereas the phase 3 trial of TLL-018 has reached primary completion, with sponsor-reported topline findings available but complete results not yet published. CPL409116/CPL'116 remains supported by limited Phase 2 evidence. Decernotinib represents an informative historical proof-of-concept programme. Overall, no emerging agent has yet demonstrated a reproducible advantage over established JAK inhibitors in sustained remission, structural preservation, difficult-to-treat disease, or overall benefit-risk balance. Future development should prioritise active comparators, prespecified structural outcomes, prolonged safety assessment, and prospectively validated treatment-specific biomarkers.