The signalling cascade from initial stimulus to downstream effector — and where therapeutic intervention is possible at each node.
The Janus kinase–signal transducer and activator of transcription (JAK-STAT) pathway is a central intracellular signalling cascade through which over 50 cytokines, growth factors, and interferons transmit their effects. Upon cytokine binding to its receptor, receptor-associated JAK kinases (JAK1, JAK2, JAK3, TYK2) undergo transphosphorylation, creating docking sites for STAT proteins. Activated STATs dimerize and translocate to the nucleus, directly driving transcription of inflammatory genes — including cytokines, chemokines, and survival factors — making the pathway a convergence point for diverse inflammatory signals.
In rheumatoid arthritis and psoriatic arthritis, JAK-STAT activation downstream of IL-6, IL-17, and GM-CSF drives synovial inflammation and joint destruction. In ulcerative colitis, JAK1-mediated signalling from multiple mucosal cytokines sustains epithelial barrier dysfunction and immune cell recruitment. Atopic dermatitis involves JAK1/3-dependent IL-4, IL-13, and IL-31 signalling, while myelofibrosis is characterised by constitutive JAK2 activation — frequently via the JAK2 V617F somatic mutation — driving dysregulated myeloproliferation.
JAK inhibitors (jakinibs) are small molecules that penetrate cell membranes and compete with ATP at the kinase active site, offering oral bioavailability unlike biologic therapies. Selectivity profiles differ across approved agents: JAK1-selective inhibitors aim to preserve haematopoiesis and innate immunity mediated by JAK2/TYK2, while pan-JAK inhibitors provide broader cytokine suppression. TYK2 inhibition, as exemplified by deucravacitinib, targets the regulatory pseudokinase domain rather than the active site, offering a differentiated selectivity mechanism relevant to psoriasis management.
Upstream blockade vs downstream blockade — understanding the distinction is critical for treatment selection and sequencing.
The downstream disease manifestations of dysregulated signalling in this pathway.
Clinically actionable insights for treatment selection and sequencing
All JAK inhibitors carry a class-level requirement for assessment of cardiovascular risk, venous thromboembolism risk, and malignancy history before initiation, as reflected in updated MHRA guidance (2023).
JAK inhibitors are small molecules offering oral administration, avoiding the need for injections required with biologic therapies — an important patient preference consideration.
Deucravacitinib targets the TYK2 regulatory pseudokinase domain (allosteric mechanism) rather than the ATP-binding active site, conferring selectivity for TYK2-dependent signalling pathways relevant to psoriasis.
Dose adjustment is required for renal or hepatic impairment with most JAK inhibitors; consult individual SmPCs for specific thresholds and recommendations.
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