Janus kinases (JAKs) belong to the non-receptor tyrosine kinase family, and their JAK-STAT signaling pathway with signal transducers and activators of transcription (STAT) can affect cell proliferation, differentiation, apoptosis, and immune regulation. Since the approval of the first JAK inhibitor, small molecule JAK inhibitors have become a hot topic in global drug development.
1. First-generation JAK inhibitors
The first-generation JAK inhibitors developed in the early stage are non-selective small molecule JAK inhibitors that can block two or more different JAKs, including ruxolitinib (1), tofacitinib (2), baricitinib (3), peficitinib (4), and momelotinib (5). Their clinical applications in the treatment of immune diseases such as RA, MF, and atopic dermatitis are relatively mature, and their indications are continuously expanding.
1.1 Ruxolitinib
Ruxolitinib, developed by Incyte Corporation, is a JAK inhibitor with a pyrrolo[2,3-d]pyrimidine structure. It is the first JAK inhibitor approved for marketing. It targets JAK1/JAK2, with IC50 values of 3.3 and 2.8 nmol·L-1 for JAK1 and JAK2. This drug was approved by the FDA and the European Medicines Agency (EMA) in 2011 and 2012 for the treatment of MF, making it the first approved drug for MF treatment.
1.2 Tofacitinib
Tofacitinib, developed by Pfizer, is a pyrrolo[2,3-d]pyrimidine-based pan-JAK kinase inhibitor. Its IC50 values for JAK1, JAK2, and JAK3 are 112, 20, and 1 nmol·L-1, respectively. The drug was approved by the FDA in 2012 for the treatment of moderate-to-severe RA patients who had an inadequate response to methotrexate (MTX), making it the first JAK inhibitor approved for the treatment of RA. Tofacitinib was also approved by the FDA in 2018 for the treatment of UC, in 2021 for polyarticular juvenile idiopathic arthritis (pcJIA) and ankylosing spondylitis (AS), and by the European Commission (EC) in 2020 for the treatment of psoriatic arthritis (PsA) in combination with MTX. Studies have also shown that Tofacitinib has certain efficacy in treating NK/T-cell lymphoma (NKTCL) caused by JAK3 gene mutations. The most common adverse reaction of this drug in clinical treatment is the induction of infections, mainly herpes zoster.
1.3 Baricitinib
Baricitinib, developed jointly by Incyte Corporation and Eli Lilly, is a pyrrolo[2,3-d]pyrimidine-based JAK1/JAK2 inhibitor. Its IC50 values for JAK1 and JAK2 are 5.9 and 5.7 nmol·L-1, respectively. It was approved by the EC and the FDA in 2017 and 2018, respectively, for the treatment of RA. In 2020, it was approved by the EC for the treatment of adult atopic dermatitis, becoming the first JAK inhibitor approved for the treatment of atopic dermatitis. It is worth mentioning that Baricitinib was granted emergency use authorization by the FDA in November 2020 for the treatment of COVID-19 in combination with Remdesivir. The results showed that this treatment regimen can reduce the mortality rate of COVID-19 patients with a lower risk of severe adverse reactions. Common adverse reactions during the treatment of Baricitinib include upper respiratory tract infections, nausea, and herpes simplex infections.
1.4 Peficitinib
Peficitinib, developed by Astellas, is a pyrrolo[2,3-b]pyridine-based pan-JAK inhibitor. Its IC50 values for JAK1, JAK2, JAK3, and TYK2 are 3.9, 5.0, 0.71, and 4.8 nmol·L-1, respectively.
1.5 Momelotinib
In addition to the four JAK inhibitors mentioned above that are already on the market, momelotinib, which has multiple targets and inhibitory effects on JAK1 and JAK2, is also worth noting. This drug, developed by Gilead, is a JAK1/JAK2/activin receptor type I (ACVR1)/activin receptor-like kinase 2 (ALK2) inhibitor. Its IC50 values for JAK1 and JAK2 are 11 and 78 nmol·L-1, respectively. Compared to Ruxolitinib, Momelotinib can inhibit ACVR1, resulting in a decrease in hepcidin and restoration of iron homeostasis and red blood cell production. It can significantly improve anemia, making it a major candidate drug for the treatment of moderate-to-severe anemia in MF patients. In addition, clinical trials of Momelotinib for the treatment of PV are also underway.
2. Second-generation JAK inhibitors
Different subtypes of JAK mediate the signaling of various cytokines, resulting in different physiological functions. The selectivity of 1st generation JAK inhibitors is poor, as they inhibit different subtypes within the JAK family. In order to regulate the signaling of cytokines in the JAK-STAT pathway more precisely and improve drug efficacy while reducing adverse reactions, 2nd generation JAK inhibitors with subtype selectivity have attracted increasing attention from researchers.
2.1 Selective JAK1 inhibitors
JAK1 is involved in the signaling of γc cytokines (including IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21), gp130 receptor cytokines (including IL-6, IL-12, and IL-23), as well as IFN (IFN-α, IFN-β, and IFN-γ). IL-6 primarily signals through JAK1, and its signaling imbalance is associated with the occurrence of autoimmune diseases such as RA, UC, and CD. Clinical studies have shown that inhibiting the expression of IFN-γ may have therapeutic effects on RA. In addition, JAK1 can also regulate the signaling of multiple cytokines associated with the pathophysiology of atopic dermatitis. Therefore, selective JAK1 inhibitors may play a key therapeutic role in autoimmune diseases.
2.2 Selective JAK2 inhibitors
JAK2 is involved in the signaling of βc cytokines (including IL-3, IL-5, GM-CSF, EPO, GH), gp130 receptor cytokines (including IL-6, IL-12, and IL-23), as well as IFN-γ. JAK2 gene mutations are the main driving factors in blood system diseases such as PV, MF, and essential thrombocythemia (ET), making JAK2 inhibitors promising in the treatment of these diseases.
2.3 Selective JAK3 inhibitors
JAK3 is only involved in the signaling of γc cytokines (including IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21). Alopecia areata (AA) is an autoimmune disease characterized by circular patches of hair loss on the scalp, and its etiology is not yet clear. However, the pathogenesis is related to overexpression of IFN-γ and γc cytokines, which promote inflammation through activation of the JAK-STAT signaling pathway, disrupt the immune function of hair follicles, and promote the proliferation and differentiation of cytotoxic T cells in the skin. Therefore, JAK3 is an ideal therapeutic target for AA. Existing studies have shown that selective JAK3 inhibitors can effectively promote hair regeneration and reduce AA-related inflammation in C3H/HeJ mouse models.
There are two approaches to the development of selective JAK3 inhibitors. One is the development of ATP-competitive inhibitors that can reversibly bind to specific regions of JAK3. The other is irreversible covalent inhibitors that bind covalently to the unique cysteine residue Cys909 in the catalytic domain of JAK3. Currently, the development of selective JAK3 inhibitors mainly focuses on covalent inhibitors.
2.4 Selective TYK2 inhibitors
TYK2 and JAK1 jointly participate in the signaling of gp130 receptor cytokines IL-6, IL-12, IL-23, as well as IFN-α and IFN-β. The signaling of IL-6 is mainly associated with the occurrence of autoimmune diseases such as RA, UC, and CD. In addition, IL-23/IL-17 induces the activation of nuclear factor kappa B, leading to the production of granulocyte colony-stimulating factor (G-CSF), GM-CSF, and various chemokines, which in turn promotes the recruitment of neutrophils and is one of the mechanisms of PsA. Therefore, targeting TYK2 has potential therapeutic effects on autoimmune diseases such as RA, UC, CD, and PsA.
3. Conclusion and Prospects
The emergence of 1st and 2nd generation JAK inhibitors has brought great benefits to many patients. With further research on the regulatory mechanisms of the JAK-STAT signaling pathway in immune and related diseases, as well as the ongoing drug design work targeting the structural characteristics of different JAK subtypes, there will be more selective JAK inhibitors demonstrating their development advantages and therapeutic value in clinical practice. In addition, multi-target inhibitors with JAK inhibitory activity can simultaneously act on multiple aspects of the same disease, producing synergistic effects and potentially regulating the disease in various ways to reduce the occurrence of adverse reactions. It deserves the attention of researchers.