Until a little over 10years ago, the therapeutic options for the treatment of diseases such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), spondyloarthropathies (SA), psoriasis (PsO) and inflammatory bowel disease (IBD)—primarily Crohn's disease (CD) and ulcerative colitis (UC)—were very limited. At the present time, there is a wide spectrum of therapeutic options, including monoclonal antibodies to interleukins (IL), IL receptors, antigen recognition receptors and intercellular communication.1 Recently, protein kinases (PK) have emerged as therapeutic targets for the treatment of these diseases, which has led to the development of inhibitors that block the activity of these proteins.2 All this has contributed to deepen our knowledge of the immune mechanisms underlying the disease and the therapeutic response.

Protein kinases are enzymes that modify other proteins and/or enzymes biochemically, activating or deactivating them, depending on the objective of the intracellular communication from the membrane toward the nucleus. The sequencing of the human genome and advances in proteomics have enabled the identification of more than 500 PK, which have been cataloged in a system referred to as the “human kinome”.3 Depending on their cellular location and function, PK have been classified into 7 major groups, which include the tyrosine kinases (TK). Tyrosine kinases are enzymes that serve as mediators between the reception of an extracellular signal and the occurrence of an effector response. Their activation is produced by the phosphorylation or transfer of phosphate groups to the hydroxyl group of the tyrosine residues of the enzyme.4

The catalytic activity of PK is exhibited with: (a) redundancy: an action or response can be catalyzed by the simultaneous activation of separate enzymes; (b) pleiotropy: the activation of a single enzyme can promote different cell responses; and (c) synergy: the activation of several signaling pathways is necessary to achieve a specific effect and, thus, proper cell communication and function.

Yet, where within the framework of the immune system do PK fit in? The complex function of the cells of the immune system depends in part on the PK because of their decisive role in the cell signaling processes of growth, maturation, differentiation, migration, inflammation, aging and apoptosis. Thus, it is essential that PK activation be efficient, specific and appropriate, since defects in the activation and regulation of these mechanisms can affect the endocrine system and/or promote the development of malignant and/or anomalous cell phenotypes, as in cancer and autoimmunity.2,5,6

As we mentioned above, the objective of intracellular signaling is to produce a nuclear response to the reception of a stimulus that is recognized by the cell membrane receptors. Protein kinases can form part of these receptors or be associated with the cell membrane, as they comprise 2 types: receptors with intrinsic kinase activity and receptors that have no intrinsic kinase activity but are associated with the kinases in their cytoplasmic domain.7

This signaling process can be summarized as follows:

• (a)

  Recognition of different cytokines—chemokines, interleukins, growth factors or antigens—on the part of the specific receptors found on the cytoplasmic membrane.

• (b)

  Binding of this receptor to the stimulus that induces conformational changes in the receptor and adjacent proteins.

• (c)

  These conformational changes promote the activation of the kinases and, in the case of TK, the phosphorylation of its tyrosine residues.

• (d)

  The activation of the kinases triggers that of other proteins to create a cascade directed toward the cell nucleus

• (e)

  The start of gene transcription for protein synthesis, or the initiation of cellular processes such as cell differentiation, maturation, proliferation and apoptosis. This can be accompanied by the synthesis/activation of regulatory and/or inhibitory proteins, such as the phosphorylases which, in the case of TK, interrupts the intracellular signaling process by dephosphorylation (Fig. 1).

  
  

  Fig. 1.

  Diagram of the intracellular signaling pathways MAPK, Syk–Btk and JAK–STAT.

  (0.69MB).

The cell communication system is made up of different “signaling pathways”, in which PK participate according to the objective of the signaling and the cell subpopulation they are in. To understand and describe the signaling process or cascade, the terms “upstream” and “downstream” are used. The literal translation of these 2 terms in Spanish would be “corriente arriba” and corriente abajo”, analogy of the flow of a current of water that is utilized to indicate the direction and position of the proteins within the intracellular signaling cascade. Thus, when we say that an action takes place upstream it means that it occurs in the direction of the cell membrane, like cell receptor recognition and binding, On the other hand, when we say that this action takes place downstream, it means that it occurs in the direction of the cell nucleus, like gene transcription. Thus, if we talk about a TK like Janus kinase 3 (JAK-3), any component that it activates and/or stimulates, such as the cytokines that bind to the common γ chain, is upstream (above it in the signaling cascade) and any component that the enzyme itself activates or regulates, as, for example, the activation and development of T lymphocytes and natural killer (NK) cells and their homeostasis is downstream (beneath it in the signaling cascade). Table 1 provides a detailed list of the meanings of these terms and the abbreviations or acronyms of each of the proteins, genes and transcription factors described in this review.

Autoimmune and inflammatory diseases are a group of chronic diseases in which, despite the availability of a number of therapeutic options, a percentage of patients do not respond adequately, probably due to the complexity of the immune system, as well as to the characteristics of each disease, of the drug and of the patient him- or herself. For all these reasons, it is still a challenge to select the appropriate therapeutic targets for the development of new drugs that increase the therapeutic options when the conventional and biologic therapies presently approved have failed. In recent decades, we have seen the development of a number of monoclonal antibodies to cytokines and cytokine receptors characteristic of the extracellular matrix or located in the cell membrane. For this reason, and due to the fact that JAK–STAT are pleiotropic enzymes that participate in the intracellular signaling of multiple cytokines, TK inhibitors have been developed as a new, more specific and effective therapeutic option in the inhibition of each disease process.26

The main advantage of these small molecular weight kinase inhibitors (SMKI) is their oral administration versus the subcutaneous or intravenous administration of the monoclonal antibodies. This administration route facilitates adherence and increases the willingness of patients to receive the treatment, and could reduce costs with respect to the existing drugs. This group of drugs is absorbed in the stomach and small intestine. In the liver, the cytochrome P450 system takes charge of its metabolism, in which the presence of genetic polymorphisms like the single nucleotide polymorphism (SNP), can alter the pharmacokinetics, bioavailability and efficacy of the active ingredient, as has been reported with another type of drugs metabolized by this system. The TK inhibitors block the site for the binding of TK to adenosine triphosphate (ATP). This point is critical in the development of these new drugs because it is indispensable that the block be selective; for the inhibition to be effective and specific, and to prevent the inhibition of multiple TK and an increase in the risk of toxicity and/or of secondary effects.

This group of drugs has been included in the category of antineoplastic agents and has been assigned the suffix -itinib (International Nonproprietary Name [INN]).32 The nomenclature utilized for assigning generic names follows the recommendations of an advisory committee of the American Medical Association and the American Pharmacist's Association (USAN, AM, USP, APhA).33

The clinical success of selective tyrosine kinase inhibitors in the treatment of tumors and leukemias (colorectal, gastric and breast cancer, hepatocellular carcinoma, sarcoma, metastatic melanoma, acute lymphoblastic leukemia and chronic myeloid leukemia) approved by the United States Food and Drug Administration (FDA) and the European Medicines Agency (EMA) has led to an increase in preclinical and clinical trials and studies of therapeutic options for the treatment of autoimmune and inflammatory diseases (www.clinicaltrials.gov). It has also deepened our understanding of the role of TK in intracellular signaling and inflammation.34

In murine models of RA treated with p38 (SB203580 and PH-797804), JNK (SP600125) and ERK (PD98059), a decrease in proinflammatory factors, such as prostaglandins, and in IL-10 and joint inflammation has been observed.35,36 A recent study reports that SP600125 inhibits MMP expression, reduces cartilage loss and increases the number of regulatory T lymphocytes, also in a murine model of RA.37 Due to the limited efficacy observed with the use of JNK and ERK inhibitors, the existing studies focus on its use in autoimmune diseases. In IBD, the utilization of p38 inhibitors such as semapimod (Ferring Pharmaceuticals) reduces inflammatory activity in CD, although the response is less than satisfactory in severe forms.38 Tests with the drug doramapimod (Boehringer Ingelheim) in RA, AS and CD resulted in a decrease in the production of proinflammatory cytokines by certain cell lines.39,40 Treatment with pamapimod (Hoffman-La Roche) was found to have little efficacy in RA, and was accompanied by a high rate of infections and skin conditions.41,42

With respect to the inhibition of Syk, studies in vitro and in animal models of RA showed that, PRT062070 (cerdulatinib), an inhibitor of Syk and JAK-1/3, reduced inflammatory activity and synovitis resulting from the suppression of autoantibody production caused by the blocking of signaling and B lymphocyte activation.43 A study involving 2 models of RA in mice treated with P505-15, a selective Syk inhibitor, corroborated its efficacy in blocking BCA signaling and activation and in inhibiting basophil activation via Fcγ.44 In clinical trials, fostamatinib disodium (AstraZeneca-Rigel), a Syk inhibitor, is being evaluated in patients with SLE, Pso and autoimmune thrombocytopenia purpura. In animal models of RA, the prodrug fostamatinib (R-788) was shown to reduce proinflammatory IL and MMP production and delay joint destruction, probably due to its acting on T lymphocytes and osteoclasts.45 On the other hand, it was seen to prevent renal damage and the development of skin lesions in a murine model of lupus.46 However, despite these findings, and in a preliminary phase III study, fostamatinib was not superior to placebo in patients with RA who had had a poor response to biologic therapies.47

Different Btk inhibitors, like dasatinib (Bristol-Myers Squibb), PCI-32765 (ibrutinib, Janssen Research & Development, LLC), CC-292 (azacitidine, Celgene Corporation) and GDC-0834, have been approved for the treatment of multiple myeloma, lymphoma and B-cell leukemia and are being evaluated in preclinical and clinical studies of RA and SLE. In vitro studies have demonstrated that inhibition of Btk reduces bone loss in murine models of receptor activator of NF-kB ligand (RANKL)-induced osteoporosis, as well as autoantibody production, and delays the development of kidney disease in a murine model of lupus.48–50 In a model of collagen-induced arthritis in rats, GDC-0834 promoted a reduction of joint inflammation.24,51 Moreover, in a murine model of lupus, a decrease in IgG deposits and in cell infiltrate and activation was observed when the mice were treated with the inhibitor RN-486.52

PI3K inhibitors are being widely studied in hematopoietic disorders and solid tumors. In preclinical studies, molecules such as IC-87114 and CZC24832 reduced IL-17 production, blocking Th17 lymphocyte differentiation and osteocyte generation in murine models of RA. Another inhibitor of PI3K, AS-605240, reduced inflammation, neutrophil infiltrate and erosions. This same molecule increased survival and reduced autoantibody production and attenuated glomerulonephritis in a murine model of lupus.19

The results obtained with different molecules directed against the JAK enzymes have shown promise in different clinical trials for the treatment of Pso and RA, and the second generation of these molecules is defined as being more specific and effective as inhibitors. In models of RA treated with decernotinib (Vertex Pharmaceuticals Inc.), a molecule directed against JAK-3, there was a reduction in joint inflammation,53 although preliminary clinical results revealed an increase in the risk of infections and in the transaminase levels.54 ASP015K (Astellas Pharma Inc.), a JAK-3 inhibitor that, to a lesser extent, inhibits JAK-1 and JAK-2 as well, has promoted the reduction of the epidermis thickness and of cell proliferation in patients with Pso.55 Baricitinib (Incyte and Eli Lilly Company), a JAK-1/JAK-2 inhibitor, has been found to reduce inflammatory activity and favor the preservation of cartilage and bone in a murine model.56 Ruxolitinib (Incyte Corporation and Novartis) is an oral drug that inhibits JAK-1/JAK-2; approved for the treatment of proliferative disorders, its topical form is being evaluated for use in the treatment of Pso. The study of this molecule in murine models showed that it promoted a decrease in STAT-3 phosphorylation and, with that, the reduction of edema, lymphocyte infiltration and psoriatic plaques.57 Results in clinical trials indicated that this drug is well tolerated and is effective in the treatment of Pso.58 On the other hand, tofacitinib (Pfizer) is the only TK inhibitor that is commercially available in the United States and other countries for the treatment of RA. It is a functional inhibitor specific for JAK-3 that also inhibits JAK-1 and, to a lesser extent, JAK-2.59,60 In preclinical studies, this inhibitor was found to reduce the levels of chemokines, interleukins, acute phase reactants and RANKL, promoting the reduction of bone reabsorption mediated by osteoclasts and inflammation.61 It has also been seen to inhibit the proinflammatory response of Th1 and Th1762 and prevent cartilage damage in animal models.63

In more recent studies, it has been observed that, in RA patients treated with tofacitinib, there is an inhibition of the proliferation of CD4+ T lymphocytes without affecting their absolute number on the periphery. Moreover, it has been found that, before treatment, a small number of CD8+ T lymphocytes are correlated with the development of secondary effects during the treatment in patients with RA.64 Given the efficacy of tofacitinib in the treatment of RA, phase III clinical trials are now underway in Pso, as well as UC, CD and AS.

A summary of the major clinical trials carried out in autoimmune and inflammatory diseases is provided in Table 2. Despite the challenge posed by the development of these inhibitors when compared with monoclonal antibodies—that is, intracellular proteins versus extracellular proteins, the secondary effects reported do not seem to differ from those observed up to now with biologic therapies: neutropenia, high rates of infection, hepatotoxicity and elevated liver enzymes, altered thyroid function, fatigue, hypertension, skin rashes, delayed wound closure, myelosuppression, diarrhea, elevated creatinine levels and hyperlipidemia, among others. For this reason, work is currently underway to develop inhibitory molecules with greater specificity and an effect better aimed at each disease.

In November 2012, the FDA granted approval to tofacitinib (Xeljanz®) for the treatment of moderate to severe RA in adults who have had an inadequate response or intolerance to methotrexate (MTX). According to its technical specifications, tofacitinib is administered orally, at doses of 5mg twice daily, as monotherapy or in combination with MTX or other synthetic drugs. It should not be used in combination with biologic therapies or immunosuppressive agents such as azathioprine and cyclosporine. At the present time, in addition to the United States, tofacitinib has been approved for the treatment of RA in several countries, including Japan, Russia, Canada and Switzerland.

As for the European Community, in April 2013, the Committee for Medicinal Products for Human Use (CHMP) of the EMA considered that, despite the evidence of a reduction in the signs and symptoms of RA and an evident improvement in the physical function of these patients, the benefit was not sufficient to outweigh the associated secondary effects, such as serious infections, gastrointestinal perforations and cancer, despite the fact that the dossier presented to the FDA was the same as that presented to the EMA. Therefore, due to the uncertainty caused by the magnitude of the risks and their management in clinical practice, it was considered that making it commercially available was not worthwhile in terms of the benefits obtained with the treatment (EMA/CHMP/425279/2013, Procedure No. EMA/H/C/002542/0000). In view of this, the company requested a revaluation of the risks/benefits of the drug, and the group of experts, in accordance with article 12 of Regulation (EC) no. 726/2004, ruled that the risk/benefit ratio of tofacitinib was inadequate or had not been convincingly demonstrated and, thus, refused once again to authorize that it be marketed (EMA/CHMP/425279/2013). However, since this drug has been commercially available in the United States for 2 years, we hope that we will soon have evidence from clinical practice to corroborate the results obtained in the clinical trials. On the other hand, tofacitinib continues to be evaluated in different clinical trials for its use in other conditions involving autoimmunity and inflammation, to take position as a new therapeutic option in the treatment of this group of diseases.

The use of drugs that inhibit PK, like TK, could change the therapeutic approach in diseases involving failures in the regulation of the immune system, due to the ubiquity of these molecules in the process of intracellular signaling and, thus, in their effect on autoimmune and inflammatory processes. This could benefit those patients in whom multiple treatments have failed because of a loss of efficacy and secondary effects. Likewise, the introduction of these molecules that are administered orally would change patient adherence and reduce the cost associated with the treatment compared to the administration of monoclonal antibodies. However, the anything but promising results with some of these PK inhibitors have shown us that not all the intracellular signaling pathways are susceptible to being blocked to provide beneficial clinical effects. As new results in clinical practice concerning the use of these therapeutic targets are obtained, it will be possible to identify those signaling pathways that are most appropriate for the goal of obtaining a proper balance between the desired clinical efficacy and the adverse effects. In this respect, and in light of the evidence that is being obtained with the use of inhibitors of the JAK kinases, like tofacitinib, the JAK–STAT pathway is seen as one of the main therapeutic targets for the development of new drugs and the treatment of diseases of the immune system.

Lara Valor has received professional fees from Abbvie, Bristol-Myers Squibb, Pfizer, Roche Farma, UCB Pharma and Merck Sharp & Dohme Limited (MSD).Diana Hernández-Flórez declares that she has no conflicts of interest.