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JAK2, a member of the Janus kinase (JAK) family of protein tyrosine kinases (PTKs), is an important intracellular mediator of cytokine signaling. Mutations of the JAK2 gene are associated with hematologic cancers, and aberrant JAK activity is also associated with a number of immune diseases, including rheumatoid arthritis. Accordingly, the development of JAK2-specific inhibitors has tremendous clinical relevance. Critical to the function of JAK2 is its PTK domain. We report the 2.0 A crystal structure of the active conformation of the JAK2 PTK domain in complex with a high-affinity, pan-JAK inhibitor that appears to bind via an induced fit mechanism. This inhibitor, the tetracyclic pyridone 2-tert-butyl-9-fluoro-3,6-dihydro-7H-benz[h]-imidaz[4,5-f]isoquinoline-7-1, was buried deep within a constricted ATP-binding site, in which extensive interactions, including residues that are unique to JAK2 and the JAK family, are made with the inhibitor. We present a structural basis of high-affinity JAK-specific inhibition that will undoubtedly provide an invaluable tool for the further design of novel, potent, and specific therapeutics against the JAK family.
BACKGROUND: Janus kinase (JAK) inhibitors are effective oral therapies for inflammatory bowel disease (IBD). While acne is a known adverse event in dermatological cohorts, its incidence and risk factors in the IBD population are not well-defined. We aimed to determine the pooled incidence of acne in IBD patients treated with JAK inhibitors and to explore this risk across key clinical subgroups. METHODS: We conducted a systematic review and meta-analysis following PRISMA guidelines. MEDLINE, EMBASE, and CENTRAL were searched from inception to September 2025 for randomized controlled trials (RCTs) and observational studies reporting acne incidence in IBD patients on JAK inhibitors. Data were pooled using a random-effects generalized linear mixed-effects model. Pre-specified subgroup analyses were performed. RESULTS: A total of 50 studies (5 RCTs, 45 observational) involving 9902 IBD patients were included. The overall pooled incidence of acne was 8.6% (95% CI: 6.4%-11.6%). Acne rates were significantly higher (P < .0001) with the upadacitinib (12.2%), compared to tofacitinib (2.6%) and filgotinib (2.3%). A numerically higher incidence was observed during induction (8.6%) versus maintenance (4.2%) therapy, though this difference was not statistically significant (P = .07). The incidence was significantly higher in the pediatric population (12.2%) compared to adults (7.4%) (P = .03). In RCTs, JAK inhibitors were associated with significantly increased odds of acne compared to placebo (OR 2.43, 95% CI: 1.33-4.43, P = .019). No statistically significant difference was observed by IBD subtype. CONCLUSION: Acne is a common adverse event in IBD patients treated with JAK inhibitors. The reported incidence of acne was significantly higher with upadacitinib, in the pediatric population, and numerically higher during the induction phase of treatment.
Systemic Janus kinase inhibitors (JAKIs) have markedly advanced the therapeutic landscape for alopecia areata (AA). Although baricitinib and ritlecitinib are approved in the United States (US) and Europe, and deuruxolitinib in the US for severe AA, the lack of head-to-head randomized controlled trials (RCTs) limits evidence-based prescribing decisions. Moreover, prior meta-analyses excluded data on certain oral JAKIs or incorporated findings from agents and dosing regimens that were abandoned, investigational, clinically ineffective, or associated with unacceptable safety profiles. To compare the efficacy of oral JAKIs, limited to FDA, EMA, or MHRA approved drugs and doses-baricitinib (2 and 4 mg QD), ritlecitinib (50 mg QD), and deuruxolitinib (8 mg BID)-for severe AA, using advanced indirect comparison methodologies. A systematic review was performed following PRISMA 2020 guidelines (CRD420251116775). Bayesian network meta-analysis (NMA) synthesized data from RCTs reporting Week 24 outcomes on Severity of Alopecia Tool (SALT) ≤ 10 and SALT ≤ 20 thresholds. Multilevel network meta-regression (ML-NMR) evaluated heterogeneity and adjusted for baseline imbalances. Additionally, unanchored matching-adjusted indirect comparisons (MAIC) were conducted using individual patient-level data from THRIVE trials. Surface under the cumulative ranking (SUCRA) values were calculated to rank treatments. Seven RCTs (n = 4560 participants) were included. Deuruxolitinib 8 mg significantly outperformed baricitinib 2 and 4 mg on both SALT endpoints. Differences with ritlecitinib 50 mg were directionally favorable for deuruxolitinib but not statistically significant in NMA and ML-NMR models. MAICs confirmed superior odds for deuruxolitinib versus baricitinib 2 mg (OR = 71.55) and ritlecitinib (OR = 18.27) for SALT ≤ 20. SUCRA rankings also consistently favored deuruxolitinib. Among approved oral JAKIs, deuruxolitinib 8 mg shows the highest short-term efficacy for severe AA. These findings provide preliminary evidence to guide treatment decisions but should be interpreted as exploratory pending confirmation.
BACKGROUND & AIMS: We conducted a retrospective cohort study comparing the safety of Janus kinase (JAK) inhibitors vs tumor necrosis factor-a (TNF) antagonists in patients with inflammatory bowel diseases (IBDs). METHODS: Using an administrative claims database, we identified patients with IBD who were new users of either JAK inhibitors or TNF antagonists between 2016 and 2023 and had insurance coverage for at least 1 year before and after treatment initiation. We compared the risk of infections (overall and serious infections requiring hospitalization), venous thromboembolism (VTE), and major adverse cardiovascular events (MACE) through stabilized inverse probability of treatment weighted Cox proportional hazards model accounting for disease characteristics, health care utilization, comorbidities, prior and concomitant medications, and competing risk of mortality. RESULTS: We included 856 patients treated with JAK inhibitors (age, 47 ± 17 years; 82% with ulcerative colitis [UC]) and 9422 patients treated with TNF antagonists (age, 45 ± 18 years; 44% with UC). JAK inhibitors were associated with higher risk of overall infections (incidence rate, 62.4 per 100 person-years [PY] vs 37.4 per 100 PY; hazard ratio [HR], 1.60; 95% confidence interval [CI], 1.33-1.93), but not serious infections (4.9 vs 5.4; HR, 0.97; 95% CI, 0.66-1.44) compared with TNF antagonists. There was no difference in the risk of VTE (1.3 vs 1.2; HR, 0.66; 95% CI, 0.28-1.57) and MACE (0.4 vs 0.7; HR, 0.50; 95% CI, 0.19-1.30). Findings were largely stable on subgroup analyses based on type of IBD, type of JAK inhibitors, age, prior biologic exposure, concomitant use of corticosteroids, and baseline risk of adverse events. CONCLUSIONS: In an observational study of patients with IBD, JAK inhibitors were not associated with an increased risk of serious infections, VTE, or MACE compared with TNF antagonists, although the overall risk of infections was higher.
INTRODUCTION: Behçet's disease e (BD) is a chronic, relapsing, multisystem inflammatory disorder that if not successfully treated can lead to severe, organ or life-threatening complications. Despite treatment with glucocorticoids, immunosuppressants, and tumor necrosis factor (TNF) inhibitors, some patients still have refractory disease that mandates additional therapeutic options. The pathogenesis of BD involves dysregulated innate and adaptive immune responses with multiple cytokines signaling through the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway. By targeting multiple inflammatory pathways, JAK inhibitors have emerged as a promising therapeutic option. However, current evidence remains limited and heterogeneous. Therefore, we conducted this systematic review to evaluate their efficacy and safety in BD. METHODS: We conducted a systematic literature review in accordance with PRISMA 2020 guidelines (PROSPERO registration: CRD420261381955). PubMed/MEDLINE, Embase, Scopus, and Web of Science were searched from inception to March 2026. Original clinical studies evaluating Janus kinase (JAK) inhibitors in BD were included. Two reviewers independently performed study selection, data extraction, and quality assessment using Joanna Briggs Institute tools. Due to heterogeneity, results were synthesized narratively, focusing on efficacy and safety outcomes. RESULTS: Seventeen studies (99 patients) were included, predominantly case reports and small observational cohorts with overall high methodological quality. All evaluated tofacitinib, baricitinib, or upadacitinib, with no data on other JAK inhibitors. Patients were highly treatment-refractory, with prior failure of conventional and biologic therapies. Upadacitinib was the most frequently studied agent and demonstrated an overall response rate of 85.2% and complete remission in 59.3% in a multi-center study. Efficacy was observed across multiple domains, with the most consistent responses in intestinal disease, including clinical and endoscopic remission, alongside frequent glucocorticoid-sparing effects. Safety findings were consistent with known JAK inhibitor safety profiles, with mainly mild to moderate infections and manageable laboratory abnormalities, and no clear signal for increased thrombotic events, although follow-up was limited. CONCLUSION: JAK inhibitors demonstrate promising efficacy in BD, particularly in refractory and multisystem disease. The most consistent evidence of efficacy was observed in gastrointestinal involvement, whereas data for other disease domains remain limited. Their safety profile appears consistent with existing data, although further follow up and validation is required. High-quality randomized controlled studies are an imminent need to study the potential role of JAK inhibitors in BD.
The Janus family of protein tyrosine kinases (JAKs) and STAT transcription factors regulate cellular processes involved in cell growth, differentiation, and transformation through their association with cytokine receptors. The CIS family of proteins (also referred as the SOCS or SSI family) has been implicated in the regulation of signal transduction by a variety of cytokines. Among them, we have shown that JAB/SOCS-1 is strongly induced by interferon-gamma and forced expression of JAB/SOCS-1I conferred cells interferon resistance. This resistance was caused by inhibition of JAK1 and JAK2 activation in response to IFNgamma. Moreover, recent detailed analysis of JAB/SOCS-1 knockout mice revealed that JAB/SOCS-1 is indeed a "negative feedback regulator" that determine the sensitivity of cells to IFNgamma. Using in vitro mutagensis, we defined a functional structure of JAB/SOCS-1 and proposed a mechanism for how JAB inhibits JAK kinase activity.
BACKGROUND: Vitiligo is a chronic autoimmune disease causing skin depigmentation and psychosocial issues. Non-segmental vitiligo often shows limited response to standard therapies. The IFN-γ-JAK-STAT pathway plays a key role, and combining JAK inhibitors with NB-UVB may improve repigmentation. AIM: To evaluate the efficacy of JAK inhibitors (baricitinib or tofacitinib) plus NB-UVB versus NB-UVB without JAK inhibitors in adult NSV. METHODS: PubMed, Cochrane, and ClinicalTrials.gov were searched till August 2024 for randomized controlled trials. Four studies, comprising 217 adults (121 in the combination group and 96 in the control group), were analyzed using RevMan 5.4. Bias was assessed via Newcastle-Ottawa, Cochrane ROB-2, and ROBINS-I tools. RESULTS: Combination therapy significantly reduced total VASI compared to controls (MD = -4.96, 95% CI [-9.29, -0.63], p = 0.02), with a greater effect on sensitivity analysis (MD = -6.84, p = 0.0007). Significant reductions were seen in face/neck (MD = -0.17, p = 0.002), trunk (MD = -3.62, p = 0.0001); acral (MD = -0.85, p = 0.0002) and extremity (MD = -4.61, p < 0.00001) regions after sensitivity analysis. Patients were more likely to achieve ≥50% (RR = 6.87, p < 0.00001) and ≥75% (RR = 15.13, p = 0.006) repigmentation. CONCLUSIONS: JAK inhibitor plus NB-UVB markedly improves the repigmentation in adult NSV compared to NB-UVB alone.
BACKGROUND: Tyrphostin AG490 has recently been shown to block interleukin (IL)-2 receptor gamma-chain-associated Janus kinase 3. Here, we analyzed the effect of AG490 on T-cell alloresponses in vitro. METHODS: For the evaluation of T-cell activation, DNA synthesis, surface marker expression, cytokine secretion, intracellular calcium mobilization, early protein tyrosine phosphorylation, and apoptosis were measured. RESULTS: AG490 effectively inhibited T-cell proliferation in human mixed lymphocyte culture (MLC) even when added 4 days after culture initiation. Inhibition of IL-2-dependent proliferation in T-cell blasts and the incapability of IL-2 or IL-15 to restore proliferation in AG490-treated MLC suggests interference with cytokine receptor signaling. T-cell receptor-triggered early protein tyrosine phosphorylation, calcium mobilization, up-regulation of CD69, and initial CD25 expression were not affected. Interestingly, AG490 substantially inhibited production of IL-2 and interferon-gamma in T cells stimulated with alloantigen or via CD3 and CD28. In CD28-independent activation models (e.g., stimulation with phorbol myristate acetate plus ionomycin), however, cytokine secretion was not inhibited. Pretreatment of primary MLC with AG490 resulted in substantial down-regulation of secondary responses to cells from the original donor as opposed to third-party cells or phytohemagglutinin. Unresponsiveness was induced also in T cells stimulated with CD3 monoclonal antibody. Induction of apoptosis in polyclonally activated T cells and the incapability of IL-2 to reverse specific hyporesponsiveness, suggest programmed cell death as an important mechanism underlying antigen-specific down-regulation of alloresponses. CONCLUSIONS: We demonstrate that AG490 blocks different manifestations of T-cell activation. This and its ability to induce alloantigen-specific hyporesponsiveness point to a potential use for interfering with alloreactivities in vivo.
It has been shown that interferons (IFNs) exert their signals through receptor-associated Janus kinases (JAKs) and signal transducers and activators of transcription (STATs). However, molecular mechanism of regulation of IFN signaling has not been fully understood. We have reported novel cytokine-inducible SH2 protein (CIS) and JAK binding protein (JAB) family genes that can potentially modulate cytokine signaling. Here we report that JAB is strongly induced by IFN-gamma but not by IFN-beta in mouse myeloid leukemia M1 cells and NIH-3T3 fibroblasts. NIH-3T3 cells ectopically expressing JAB but not CIS3 lost responsiveness to the antiviral effect of IFN-beta and IFN-gamma. M1 leukemic cells stably expressing JAB were also resistant to IFN-gamma and IFN-beta-induced growth arrest. In both NIH-3T3 and M1 transformants expressing JAB, IFN-gamma did not induce tyrosine phosphorylation and DNA binding activity of STAT1. Moreover, IFN-gamma-induced activation of JAK1 and JAK2 and IFN-beta-induced JAK1 and Tyk2 activation were inhibited in NIH-3T3 JAB transformants. These results suggest that JAB inhibits IFN signaling by blocking JAK activity. We also found that IFN-resistant clones derived from LoVo cells and Daudi cells expressed high levels of JAB without stimulation. In IFN-resistant Daudi cells, IFN-induced STAT1 and JAK phosphorylation was partially reduced. Therefore, overexpression of JAB could be, at least in part, a mechanism of IFN resistance.
The Janus kinase (JAK) family of protein tyrosine kinases are activated in response to a wide variety of external stimuli. Here we have investigated whether the janus kinase 2 (JAK2) is involved in the induction of nitric oxide synthase type II (iNOS) expression in a mouse fetal skin dendritic cell line (FSDC). In FSDC the expression of iNOS protein and nitric oxide production, in response to the lipopolysaccharide (LPS) stimulus (5 microg/ml), is inhibited by the specific inhibitor of the JAK2, tyrphostin B42 with an half maximal inhibitory concentration (IC(50)) of 9.65 microM. The antioxidant compound pyrrolidinedithiocarbamate (PDTC) inhibits both the nitrite production with an IC(50) of 16.6 microM and the iNOS protein expression in FSDC. In addition, LPS induces the activation of NF-kappa B, and tyrphostin B42 prevents the degradation of the cytosolic factor I kappa B-alpha and blocks the translocation of the NF-kappa B p65 protein subunit into the nucleus. These results indicate that the JAK family of protein kinases and the transcription factor NF-kappa B are involved in the induction of iNOS protein expression in FSDC stimulated with LPS.
Potent inhibition of Janus kinase 3 was found for a series of naphthyl(beta-aminoethyl)ketones (e.g. 7, pIC50 = 7.1+/-0.3). Further studies indicated that these compounds fragment in less than 1 h by retro-Michael reaction in the Jak3 in vitro ELISA assay procedure. The breakdown product of 7, 2-naphthylvinyl ketone (22, pIC50 = 6.8+/-0.3) showed very similar inhibitory activity to 7. Compounds 7 (in neutral buffer) and 22 will be useful pharmacological tools for the investigation of the Janus tyrosine kinase Jak3.
4-(4'Hydroxyphenyl)-amino-6,7-dimethoxyquinazoline (WHI-P131) is a potent and selective inhibitor of the Janus kinase 3, which triggers apoptosis in human acute lymphoblastic leukemia (ALL) cells. In this preclinical study, we evaluated the pharmacokinetics and toxicity of WHI-P131 in rats, mice, and cynomolgus monkeys. Following i.v. administration, the terminal elimination half-life of WHI-P131 was 73.2 min in rats, 103.4 min in mice, and 45.0 min in monkeys. The i.v. administered WHI-P131 showed a very wide tissue distribution in mice. Following i.p. administration, WHI-P131 was rapidly absorbed in both rats and mice, and the time to reach the maximum plasma concentration (tmax) was 24.8 min in rats and 10.0 min in mice. Subsequently, WHI-P131 was eliminated with a terminal elimination half-life of 51.8 min in rats and 123.6 min in mice. The estimated i.p. bioavailability was 95% for rats, as well as for mice. WHI-P131 was quickly absorbed after oral administration in mice with a tmax of 5.8 min, but its oral bioavailability was relatively low (29.6%). The elimination half-life of WHI-P131 after oral administration was 297.6 min. WHI-P131 was not acutely toxic to mice at single i.p. bolus doses ranging from 0.5-250 mg/kg. Two cynomolgus monkeys treated with 20 mg/kg WHI-P131 and one cynomolgus monkey treated with 100 mg/kg WHI-P131 experienced no side effects. Plasma samples from WHI-P131-treated monkeys exhibited potent antileukemic activity against human ALL cells in vitro. To our knowledge, this is the first preclinical toxicity and pharmacokinetic study of a Janus kinase 3 inhibitor. Further development of WHI-P131 may provide the basis for new and effective treatment programs for relapsed ALL in clinical settings.
We analyzed the effects of the Janus kinase 3 (Jak3)-specific inhibitor WHI-P131 (4-(4'-hydroxyphenyl)-amino-6,7-dimethoxyquinazoline) and the Jak3/Syk inhibitor WHI-P154 (4-(3'-bromo-4'-hydroxyphenyl)-amino-6,7-dimethoxyquinazoline) on the antigen-induced activation of mast cells. In the rat mast cell line RBL-2H3, both WHI-P131 and WHI-P154 inhibited the antigen-induced degranulation and phosphorylation of p44/42 mitogen-activated protein kinase (MAPK), p38 MAPK and c-Jun N-terminal kinase (JNK). The phosphorylation of Gab2, Akt and Vav was also inhibited by WHI-P131 and WHI-P154, indicating that these inhibitors suppress the activation of phosphatidylinositol 3-kinase (PI3K). In bone marrow-derived mast cells (BMMCs) from Jak3-deficient (Jak3-/-) mice, degranulation and activation of MAPKs were induced by the antigen in almost the same extent as in BMMCs from wild-type mice. In addition, the antigen-induced degranulation and activation of MAPKs were inhibited by WHI-P131 and WHI-P154 in both groups of BMMCs, indicating that these compounds inhibit a certain step except for Jak3. The antigen-induced increase in the activity of Fyn, a probable tyrosine kinase of Gab2, was also inhibited by WHI-P131 and WHI-P154 in RBL-2H3 cells. In BMMCs from Jak3-/- mice, the antigen stimulation induced tyrosine phosphorylation of Fyn, which was inhibited by WHI-P131, as well as in BMMCs from wild-type mice and in RBL-2H3 cells. These findings suggest that Jak3 does not play a significant role in the antigen-induced degranulation and phosphorylation of MAPKs, and that WHI-P131 and WHI-P154 inhibit the PI3K pathway by preventing the antigen-induced activation of Fyn, thus inhibiting the antigen-induced degranulation and phosphorylation of MAPKs in mast cells.
BACKGROUND & AIMS: We compared the efficacy of different advanced therapies by disease location in patients with Crohn's disease (CD) through a systematic review and meta-analysis. METHODS: Through a systematic review, we identified 14 randomized controlled trials in 3139 patients with moderate-to-severe CD who were treated with different advanced therapies vs placebo, and reported efficacy in inducing clinical remission, stratified by disease location (isolated colonic vs ileal disease, excluding ileocolonic disease). We grouped advanced therapies based on the primary mechanism of action: anti-interleukins, Janus kinase inhibitors (JAK inhibitors), anti-integrins, and tumor necrosis factor (TNF) antagonists. We calculated treatment efficacy (drug vs placebo), overall and by drug class, for colonic vs ileal disease. RESULTS: Overall treatment efficacy of advanced therapies vs placebo was higher in patients with colonic (odds ratio [OR], 4.09; 95% confidence interval [CI], 3.02-5.54) vs ileal CD (OR, 1.80; 95% CI, 1.23-2.63; P < .001). By drug class, anti-interleukins demonstrated a higher efficacy in colonic disease (OR, 4.29; 95% CI, 2.77-6.64) vs ileal disease (OR, 2.31; 95% CI, 1.44-3.70; P = .059), whereas no difference in efficacy was observed with anti-integrins (colonic vs ileal: OR, 1.79; 95% CI, 0.55-5.87 vs 2.10; 95% CI, 0.80-5.53; P = .84). For JAK inhibitors, efficacy was observed only in patients with isolated colonic disease (OR, 4.37; 95% CI, 2.67-7.15), but not in ileal disease (OR, 1.01; 95% CI, 0.54-1.89; P < .001). All analyses had minimal to moderate heterogeneity. CONCLUSIONS: The magnitude of efficacy of advanced therapies for ileal CD is generally lower compared with isolated colonic CD, with JAK inhibitors showing particularly limited efficacy for ileal disease. These results may help inform treatment selection.
BACKGROUND: Therapeutic targeting of Janus kinase 3 (JAK3) has received particular attention, because it is associated with the common gamma signaling of cytokine receptors and thus vitally influences T-cell growth and survival. Recent evidence, however, indicates a critical role for JAK3 in signaling linked to the T-cell antigen receptor. METHODS: In this study we investigated whether targeting JAK3 with a rationally designed inhibitor affects early T-cell activation events. T cells were stimulated by CD3 and CD28 cross-linking, and interleukin (IL)-2 production, activation marker expression, increase of free intracellular Ca2+ concentration, activation of the extracellular-related kinase, and nuclear translocation of transcription factors were evaluated. RESULTS: We found that JAK3 inhibitor treatment dramatically impaired T-cell-receptor (TCR)-induced IL-2 production, surface activation marker expression (CD69, CD154), and homotypic T-cell aggregation. Accordingly, mRNA production of IL-2, interferon-gamma, and IL-10 was profoundly inhibited. Molecular analysis revealed that TCR-triggered phosphorylation of phospholipase C-gamma1, increase in cytoplasmic Ca2+ concentration, and activation of extracellular-related kinase were markedly reduced by the JAK3 inhibitor, resulting in substantially decreased DNA binding of nuclear factor of activated T cells and alkaline phosphatase-1 and subsequent IL-2 promoter activation. Remarkably, on TCR-independent stimulation, IL-2 production, CD69 expression, and blast formation were completely insensitive to JAK3 inhibitor treatment. CONCLUSION: These data indicate that pharmacologic targeting of JAK3 uncouples early TCR-triggered signaling from essential downstream events, which may have important implications for the use of such compounds in T-cell-mediated disorders such as allograft rejection or graft-versus-host disease.
Because of its requirement for signaling by multiple cytokines, Janus kinase 3 (JAK3) is an excellent target for clinical immunosuppression. We report the development of a specific, orally active inhibitor of JAK3, CP-690,550, that significantly prolonged survival in a murine model of heart transplantation and in cynomolgus monkeys receiving kidney transplants. CP-690,550 treatment was not associated with hypertension, hyperlipidemia, or lymphoproliferative disease. On the basis of these preclinical results, we believe JAK3 blockade by CP-690,550 has potential for therapeutically desirable immunosuppression in human organ transplantation and in other clinical settings.
A new class of bicyclic pyrrolopyrimidine-based Janus kinase 3 (JAK-3) inhibitors are described. Many of these inhibitors showed low nanomolar activity against JAK-3.