JAK

Class

Janus kinase (JAK) inhibitor

Pathway

JAK-STAT signalling

Notes

Selectivity varies by drug (see each drug's novelty_note) — pan-JAK vs. JAK1-selective.

Janus kinases (JAK1, JAK2, JAK3, and TYK2) are enzymes that sit just inside the cell membrane, attached to receptors for cytokines—signaling proteins that coordinate immune and blood-forming activity. When a cytokine binds its receptor, paired JAKs activate each other and then phosphorylate STAT proteins, which travel to the nucleus and switch on genes involved in inflammation, immune cell proliferation, and blood cell production. In autoimmune and inflammatory diseases, this JAK-STAT relay is chronically overactive, driving tissue damage. Small-molecule JAK inhibitors block the enzymes' catalytic activity, dampening downstream gene transcription and reducing inflammatory signaling broadly across multiple cytokine pathways at once—an advantage over single-cytokine antibodies when disease is driven by several overlapping pathways. Because the four JAK family members are unevenly distributed across tissues and cytokine systems, selectivity matters: some inhibitors block all JAK isoforms (pan-JAK), while others are engineered to preferentially inhibit JAK1 or JAK3, aiming to preserve efficacy against inflammatory cytokines while reducing effects on blood-cell-regulating pathways. This class is used broadly across rheumatologic, dermatologic, and gastrointestinal inflammatory conditions.