TYK2 and JAK1 are intracellular enzymes that help immune cells translate cytokine signals into changes in gene expression. They sit inside the JAK-STAT signaling network, a pathway used by interferons and multiple interleukins to coordinate inflammation. That makes the pathway an attractive drug target in autoimmune disease, but selectivity matters because different JAK-family enzymes carry different biological functions and safety trade-offs.
JAK-STAT signaling turns an outside cytokine message into a nuclear response
Cytokines are signaling proteins released by immune and other cells. Many cytokine receptors do not have their own intrinsic kinase activity, so they rely on intracellular Janus kinases, or JAKs, attached to the receptor complex.
When a cytokine binds its receptor, the associated JAK enzymes become activated and phosphorylate downstream signal transducers and activators of transcription, known as STAT proteins. Activated STATs dimerize, move into the nucleus, and change expression of target genes.
The JAK family contains JAK1, JAK2, JAK3, and tyrosine kinase 2, or TYK2. Different cytokine receptors use different combinations of these enzymes, which is one reason inhibitors with different selectivity profiles can have different clinical effects.
TYK2 carries signals from several immune pathways linked to autoimmunity
TYK2 is especially important in signaling downstream of type I interferons, interleukin-12, and interleukin-23. These pathways influence antiviral responses and the behavior of T-helper-cell populations involved in inflammatory and autoimmune disease.
IL-12 signaling promotes T-helper 1 responses and is strongly linked to STAT4 activation. IL-23 helps sustain inflammatory T-helper 17 biology and preferentially activates STAT3. TYK2 participates in both pathways, typically partnering with another JAK-family member at the receptor complex.
This is why TYK2 has become a therapeutic target in disorders such as psoriasis, psoriatic arthritis, lupus, inflammatory bowel disease, and dermatomyositis. The exact role of the pathway differs by disease, so blocking TYK2 does not produce the same clinical effect in every inflammatory condition.
Selective TYK2 inhibition is different from broad JAK inhibition
One major development has been the ability to target TYK2 more selectively. Deucravacitinib, marketed as Sotyktu, binds the regulatory domain of TYK2 rather than competing at the conserved catalytic ATP-binding site used by traditional JAK inhibitors.
According to the FDA label, this allosteric binding stabilizes an inhibitory interaction within TYK2 and blocks receptor-mediated TYK2 activation and downstream STAT signaling in cell-based assays. The drug is approved for moderate-to-severe plaque psoriasis in adults who are candidates for systemic therapy or phototherapy, and its label now also includes active psoriatic arthritis.
The appeal of this strategy is that more selective inhibition may avoid some of the biological consequences of simultaneously blocking JAK1, JAK2, and JAK3. That does not mean selective TYK2 inhibition is risk-free, only that its target profile is different.
Brepocitinib deliberately targets both TYK2 and JAK1
Brepocitinib takes a broader approach. It is an oral selective TYK2-JAK1 inhibitor designed to suppress cytokine signaling implicated in autoimmune disease. JAK1 participates in signaling from several cytokine families, including interferons, while TYK2 contributes to interferon, IL-12, and IL-23 pathways.
That dual profile is relevant to dermatomyositis, a disease with prominent interferon-associated immune activity. In the phase 3 VALOR trial, adults with active dermatomyositis were randomized to brepocitinib 30 mg, brepocitinib 15 mg, or placebo for 52 weeks while background therapy continued and glucocorticoids were tapered.
The approved 30 mg dose significantly improved the trial’s composite Total Improvement Score compared with placebo. That trial formed the evidence base for the 2026 FDA approval of Lisraya, which we covered in our Lisraya approval explainer.
Blocking a signaling pathway can suppress disease without identifying one single “bad cytokine”
A biologic drug often targets one extracellular cytokine or receptor. A kinase inhibitor works farther downstream, where several receptor pathways converge. That can make a small-molecule inhibitor useful when multiple inflammatory signals contribute to disease.
The trade-off is that broader intracellular inhibition can affect more pathways at once. That may increase therapeutic reach, but it can also widen the safety profile. This is one reason it is inaccurate to treat every JAK or TYK2 inhibitor as interchangeable.
The boxed-warning issue depends on the specific drug and its regulatory class
Traditional JAK inhibitors used for inflammatory diseases carry prominent safety warnings that include serious infections, malignancy, major adverse cardiovascular events, thrombosis, and mortality in selected populations. Those warnings arose from clinical safety data with the class and are reflected in individual prescribing information.
Brepocitinib’s U.S. label carries a boxed warning covering serious infections, mortality, malignancy, major adverse cardiovascular events, and thrombosis. By contrast, the current U.S. label for the selective TYK2 inhibitor deucravacitinib does not carry the same JAK-inhibitor boxed warning.
That regulatory difference should not be interpreted as proof that one mechanism is universally safer. The drugs inhibit different enzymes, are used in different populations, and have different evidence bases. Long-term comparative safety data remain limited.
Genetics helped establish TYK2 as a biologically credible target
Human genetics also supports a role for TYK2 in immune regulation. Variants that alter TYK2 function have been associated with susceptibility to several autoimmune diseases. Experimental work has shown that TYK2 contributes to signaling through IL-12 and IL-23 and helps shape T-helper-cell responses.
This convergence of genetics, immunology, and clinical pharmacology is one reason TYK2 has attracted so much drug-development interest. It is not simply a convenient enzyme to inhibit; it sits at a signaling junction repeatedly implicated in immune-mediated disease.
Selectivity is a spectrum, not a binary label
Terms such as “selective JAK1 inhibitor” or “TYK2 inhibitor” can sound more absolute than they are. Drug selectivity depends on concentration, assay conditions, binding mode, and the relative potency against other kinases. A compound can be much more potent against one kinase without being completely inactive against the others.
That is especially important when comparing an allosteric TYK2 inhibitor such as deucravacitinib with a catalytic-site inhibitor such as brepocitinib. The drugs reach the pathway through different molecular strategies and should not be assumed to have identical downstream effects.
What the evidence cannot yet answer
The long-term clinical consequences of targeting different combinations of JAK-family enzymes are still being defined. Many autoimmune diseases are chronic, so safety questions require years of exposure and large patient populations.
Mechanistic studies also do not guarantee clinical benefit. A disease can show strong activation of a signaling pathway without becoming fully dependent on that pathway. That is why randomized trials remain necessary even when the biology appears compelling.
For dermatomyositis specifically, the VALOR trial provides phase 3 evidence for brepocitinib, but it does not establish that TYK2-JAK1 inhibition is the best strategy for every patient or that the same approach will work equally well in other autoimmune disorders.
TYK2 and JAK1 are best understood as signaling hubs, not disease-specific switches
The JAK-STAT pathway is powerful because it converts extracellular immune messages into gene-expression programs. Drugs that block TYK2 or JAK1 can interrupt those programs, but the clinical outcome depends on which cytokines are driving the disease, which kinase combinations the receptors use, and how selectively the drug inhibits them.
That is why recent approvals such as Lisraya for dermatomyositis are scientifically interesting beyond a single drug. They show how increasingly precise manipulation of intracellular immune signaling is becoming part of modern autoimmune therapy, alongside biologics and other targeted treatments. Our drug repurposing explainer covers a different route by which existing pharmacology can be redirected toward new disease biology.
References
- U.S. Food and Drug Administration. Sotyktu (deucravacitinib) Prescribing Information. 2026. FDA label.
- Vleugels RA, Paik JJ, Bauer Ventura I, et al. A Phase 3 Trial of Brepocitinib in Dermatomyositis. N Engl J Med. 2026;394:1883-1893. DOI: 10.1056/NEJMoa2503531.
- Ishizaki M, Akimoto T, Muromoto R, et al. Involvement of tyrosine kinase-2 in both the IL-12/Th1 and IL-23/Th17 axes in vivo. J Immunol. 2011;187:181-189. DOI: 10.4049/jimmunol.1003244.