JAK Inhibitors: How They Work and Where They're Used

Mechanism of Action

Janus kinase (JAK) inhibitors block JAK enzymes (JAK1, JAK2, JAK3, TYK2), which are critical for cytokine and growth factor signaling. When a cytokine binds its receptor, associated JAKs phosphorylate each other, then phosphorylate STAT proteins. These phosphorylated STATs dimerize, enter the nucleus, and regulate gene expression driving inflammation and immune cell proliferation. By inhibiting this JAK-STAT pathway, these drugs reduce inflammatory mediators and calm aberrant immune responses.

Therapeutic Applications

JAK inhibitors are approved for multiple immune-mediated conditions:

• Rheumatoid arthritis: tofacitinib, baricitinib, upadacitinib
• Psoriatic arthritis: upadacitinib, tofacitinib
• Ulcerative colitis: tofacitinib, upadacitinib
• Atopic dermatitis: upadacitinib, abrocitinib
• Ankylosing spondylitis: upadacitinib
• Myelofibrosis: ruxolitinib (JAK1/2 inhibitor)
• Alopecia areata: baricitinib, ritlecitinib

Administration & Safety Considerations

Most are oral tablets taken once or twice daily, usable as monotherapy or combined with conventional DMARDs. Key safety considerations include infection risk (especially herpes zoster), cytopenias, lipid elevations, thrombosis risk (higher at increased doses), and malignancy surveillance. Regular monitoring of blood counts, liver enzymes, and lipids is recommended.