Lucas & TanSmall-molecule inhibitors of spleen tyrosine
kinase as therapeutic agents for immune disor-ders
6
16
2014
Following on the heels of the US FDA approval of tofacitinib (Xeljanz, Pfizer, USA), an inhibitor of the JAK family members, and ibrutinib (Imbruvica, Janssen, Belgium), an inhibitor of BTK, for the treatment of rheumatoid arthritis and chronic lymphocytic leukemia, respectively, there is now renewed interest in the biopharmaceutical industry in the development of orally active small-molecule agents targeting key protein kinases implicated in immune regulation. One such ‘immunokinase’ target is SYK, a non-receptor tyrosine protein kinase critical for transducing intracellular signaling cascades for various immune recognition receptors, such as the B-cell receptor and the Fc receptor. Here, we review and discuss the progress and challenges in the development of small-molecule inhibitors of SYK and their potential as a new class of disease-modifying immunosuppressive agents for certain inflammatory and autoimmune disorders.
The clinical utility of biologics targeting TNF-α, the IL-6 receptor, CD20 (B-cell depletion) and CD80/86 (inhibition of T-cell costimulatory signal) in the man-agement of rheumatoid arthritis (RA) sup-ports the notion that several major and redundant pathways are capable of promot-ing and/or sustaining inflammation and autoimmunity. However, there is still a tremendous unmet clinical need for more efficacious therapeutic agents, as a signifi-cant proportion of RA patients either do not respond to these marketed biologics or are unable to achieve a sustained response when treatment is ceased. This is not surprising as many human autoimmune disorders, includ-ing RA, are polygenic; therefore, therapeu-tic perturbation of multiple redundant and distinct mechanisms is required in order to achieve greater disease activity control, if not to ultimately modify the disease to induce a state of disease remission.
Therapeutic modulation predicated on targeting key intracellular enzymes essen-tial for integrating signal transduction path-ways represents a reasonable, if not attrac-tive, strategy to deliver the much-needed
breakthrough efficacy. Much of the initial enthusiasm for such an approach, however, has been hampered by the disappointing clinical results of various small-molecule inhibitors targeting the p38 pathway [1–3]. A change of tides may be happening following the recent US FDA approval of tofacitinib (Xeljanz, Pfizer, USA) and ibrutinib (Imbru-vica, Janssen, Belgium) for RA and chronic lymphocytic leukemia therapy, respectively. Tofacitinib is an inhibitor of the JAK fam-ily members, whereas ibrutinib targets BTK. Interestingly, the potential therapeutic utility of another kinase inhibitor, namely imatinib (Gleevec, Novartis, Switzerland), is being explored for RA therapy. Originally devel-oped to inactivate the BCR–ABL tyrosine kinase for the treatment of certain leukemic cancers, imatinib was found to be efficacious in preclinical models of RA and may exert its effects by selectively inhibiting a variety of signaling pathways central to the develop-ment of RA [4]. Perhaps one lesson gleaned from the clinical success of BCR–ABL, JAK and BTK inhibitors is that these enzymes and their functions were first elucidated by the identification of disease-associated
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human mutations in the corresponding genes. In the case of JAK3, a member of the JAK kinase family, and BTK, human mutations have been linked to severe immunodeficiencies, confirming their important role in the immune response and suggesting that pharma-cological inhibition of these kinases is likely tolerable in human.
In addition, as we have learned in the field of anti-cancer therapy, exquisite kinase selectivity may be desirable but not the rule of thumb in the development of successful therapeutic kinase inhibitors. Predomi-nantly expressed in hematopoietic cells, JAK3 medi-ates the signal transduction of the common gamma chain (γc) of the type I cytokine receptor family (e.g., IL-2R, -4R, -7R, -9R, -15R and -21R). Blunting these proinflammatory cytokine signals is expected to halt disease progression of inflammatory and autoimmune disease. While the development of highly selective JAK3 inhibitors is conceptually preferred, it is clear that in the JAK signaling pathway for cytokines, JAK members function as dimers by pairing with each other (e.g., JAK1 and JAK3). Furthermore, JAK1 is a primary signaling pathway for IL-6, so a JAK1 inhibi-tor could function like an IL-6 inhibitor. Inhibition of JAK2 may also benefit RA patients as this JAK member is important for GM-CSF-dependent signal-ing and an anti-GM-CSF antibody (mavrilimumab, CAM-3001, MedImmune, USA) has met its clinical endpoint in a recent RA trial with an acceptable safety profile [5]. Thus, targeting different combinations of JAK members may still have specificity while offer-ing greater efficacy, provided an adequate therapeutic window can be attained.
Notwithstanding these questions, the biopharma-ceutical industry has continued to invest in the devel-opment of protein kinase inhibitors as therapeutic agents for immune-related disorders [6]. The biophar-maceutical deals centering on less than a handful of ‘immunokinase’ targets, specifically JAK, BTK, SYK and PI3Kδ, have generated an estimated US$12 billion in value creation for the industry. Among these, SYK was at one point considered to be a ‘hot’
immunokinase target but has rapidly fallen from its pedestal, with AstraZeneca and Pfizer both return-ing rights to Rigel’s oral and inhaled SYK inhibitors for RA and asthma therapy, respectively. Here, we review and discuss the challenges and progress in the development of SYK inhibitors for the treatment of inflammatory and autoimmune disorders.
SYK as a therapeutic target in immunological disordersSYK is a cytoplasmic tyrosine kinase of 72 kDa that contains a tandem pair of SH2 domains and a kinase domain. Expressed at higher levels in the hematopoi-etic lineage of the immune system, SYK plays a critical role in mediating signal transduction triggered by the activation of a number of immune recognition recep-tors, including but not limited to the B-cell receptor (BCR), Fc receptors, CD74 and integrins [7,8]. These immune recognition receptors or ‘immunorecep-tors’ activate SYK through a common mechanism involving SYK interaction with the immunoreceptor tyrosine-based activation motifs (ITAMs) present in the cytoplasmic tail of the immunoreceptor. Upon immunoreceptor engagement, the tyrosine residues on ITAMs are rapidly phosphorylated by membrane-associated Src family non-receptor tyrosine kinases, such as LYN and FYN. The SH2 domains of SYK recognize and bind the phosphotyrosines on ITAMs. Thus, phophorylated ITAMs act as docking sites for SYK, where SYK becomes activated as a consequence of protein conformational changes. The functional role of SYK is best delineated in B cells, where upon antigen engagement of the BCR, activated SYK orches-trates the formation of a plasma membrane-associated signaling complex known as the BCR signal osome. The signalosome, which includes SYK itself, PLCγ2, PI3Kδ, BTK and BLNK, in turn catalyzes the phos-phorylation of a number of protein substrates and, thus, facilitates the propagation of downstream signal-ing events. Depending on the cellular context, includ-ing the provision of other extracellular signals, the signal transduction cascades are integrated to generate the variable cellular responses, such as proliferation, survival, differentiation, phagocytosis, migration and secretion of cytokines.
Aberrant activation of immunoreceptor signaling can contribute to the initiation and/or maintenance of chronic inflammation and autoimmunity. For exam-ple, inappropriate activation of B cells as a result of chronic engagement of the BCR by extracellular auto-antigen can lead to B-cell differentiation and secretion of pathogenic autoantibodies and proinflammatory cytokines. Similarly, autoantigen–immuno globulin complexes can activate monocytes/macrophages
Key terms
Autoimmunity: A condition in which the body’s normal immune response is misdirected towards its own tissues, which can lead to hypersensitivity and various disease states.
Immunokinase: A kinase that is part of the immune signaling cascade, for example BTK , IRAK4, JAK1, JAK2, JAK3, NIK, NIK beta subunit, PI3K delta, ZAP70, MYD88 and TYK2.
Signalosome: A group of proteins that form a complex that mediates signal transduction.
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and mast cells through FcγR and FceR engagement, respectively. Although no human mutations linking SYK function and immunological defects have been described yet, the importance of B cells in the patho-genesis of autoimmune disease is unequivocally dem-onstrated by the clinical benefits of B-cell-directed therapies, such as rituximab and belimumab [9]. Autoantibodies, which are a hallmark of many auto-immune diseases, can also contribute to disease by engaging the FcR expressed on many immune effector cells, including macrophages, mast cells and neutro-phils. Furthermore, FcR-deficient mice are resistant to the development of autoimmunity in preclinical models [10]. Given the critical role of SYK in BCR and FcR signaling, as well as in other signaling pathways (discussed below), therapeutic approaches targeting SYK are expected to deliver better and broader effi-cacy for treatment of autoimmune diseases. The rela-tive higher expression of SYK in the hematopoietic tissues also suggests pharmacological inhibition of SYK may have a wider therapeutic window compared with proteins that are more ubiquitously expressed. As expected, inhibition of SYK, either by pharmacologi-cal or genetic means, resulted in attenuation of various immunoreceptor-mediated cellular responses in vitro and encouraging therapeutic effects in various pre-clinical disease models [11]. In the following section, we review progress towards positioning SYK inhibi-tors in different immune disease indications, as well as discuss some of the shortcomings of these studies.
Rheumatoid arthritisInitial preclinical studies in several murine arthri-tis models showed administration of SYK inhibitor R406 resulted in a dose-dependent improvement in disease severity, suggesting that SYK inhibition might be beneficial in RA [11]. Subsequent clinical evalua-tion of fostamatinib (R788; the methylene phosphate pro-drug of R406, Rigel, USA) in RA patients who did not adequately respond to the standard-of-care treatment with methotrexate showed fostamatinib was significantly superior to the placebo group with respect to American College of Rheumatology criteria 50 and 70 at doses of 100 mg twice daily and 150 mg once daily [12]. However, fostamatinib failed to show significant efficacy in another clinical trial with RA patients who displayed inadequate response to the anti-TNF-α biologic, and although post-hoc analyses of the data suggested that a further trial in this popu-lation is warranted, this appears unlikely at this time [13]. Adverse events reported in these trials included hypertension, diarrhea and neutropenia.
Because fostamatinib displays a relatively poor kinase selectivity profile [14], it is difficult to ascertain
if SYK is solely responsible for clinical benefit and/or the side effects. The blood pressure increase associ-ated with fostamatinib use is believed to be an off-tar-get effect, owing to inhibition of other kinases – the VEGFR2 being a prime suspect [12]. Approximately 25% of the treated patients needed antihypertensive therapy or needed to increase their dose of antihy-pertensive therapy, with the subjects’ blood pressure returning to baseline at the end of the study. Thus, the risk for hypertension associated with fostamatinib use appears to be manageable, although the long-term effects of fostamatinib in RA patients, who are prone to cardiovascular risk, remain unknown.
Asthma & allergic rhinitisGiven the role of SYK in mediating FceRI signaling in mast cells and basophils, SYK inhibitors should have a therapeutic niche in allergic diseases, such as asthma and allergic rhinitis (reviewed in [11]). However, there have been some setbacks in the clinical evaluation of SYK inhibitors in these indications. One of the earli-est SYK inhibitors to progress into the clinic, R112, was able to improve symptoms of seasonal allergic rhinitis [15]. A follow-up compound, R343, completed several Phase I clinical trials for asthma and although it was safe and well tolerated, it fell short of its primary or secondary endpoints in a Phase II clinical trial and development was subsequently halted. However, both these compounds were derived from a poorly selective scaffold, so it is difficult to determine if the lack of clinical efficacy is due to inadequate coverage of SYK inhibition.
More recent studies have further substantiated a role for SYK in mediating allergic airway responses in rat and sheep allergen-induced airway constriction models and ex vivo in non-human primate lungs [16]. Furthermore, a SYK inhibitor, NVP-QAB-205, was effective in reducing airway hyper-responsiveness in a chronic mouse model of asthma [17]. Interestingly, depletion of SYK by topical administration of siRNA via nasal instillation seemed to inhibit recruitment of inflammatory cells to the bronchoalveolar lavage fluid of allergen-sensitized mice [18].
Immune thrombocytopenic purpuraImmune thrombocytopenic purpura (ITP) is a dis-ease associated with autoimmune-driven destruc-tion of platelets, which leads to thrombocytopenia, bruising and bleeding. The mechanism involves for-mation of anti-platelet antibodies and accelerated phagocytosis of platelets carried out by macrophages expressing FcγR. In a murine model of ITP, mice pre-treated with R788 were protected from thrombocy-topenia. Following these encouraging results, R788
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was administered to 16 adult human patients with chronic refractory ITP in a small Phase II trial. Half of the ITP patients achieved a sustained response at a median R788 dose of 125 mg twice daily [19]. A Phase III trial was recently announced and results are expected in 2015 [20].
IgA nephropathyImmunoglobulin A nephropathy (IgAN) is the most common cause of glomerulonephritis and is a sig-nificant cause of end-stage renal disease. Current treatment options for IgAN are inadequate, consist-ing of supportive care to control blood pressure and proteinuria [21]. The disease pathogenesis involves the synthesis and release into the circulation an aberrant form of IgA1 antibodies with characteristics that favor mesangial cell deposition. SYK inhibition may arrest or slow destruction of the glomeruli by reducing the production of the IgA1 antibodies and thus mesangial deposition. Moreover, Kim et al. showed fostamatinib can reduce proinflammatory cytokine production and proliferation of human mesangial cells stimulated with IgA1 immune complex from IgAN patients [22]. Thus, SYK inhibitor may also halt or slow the destruc-tive inflammatory pathways leading to downstream renal tissue damage by blocking the signaling of IgA1 immune complex receptors on mesangial cells. These proposed mechanisms might contribute the therapeu-tic effects seen with fostamatinib (18) in its ability to reverse the inflammation in the glomeruli and improve kidney function in a mouse model with established glomerulonephritis [23]. Predicated on these encourag-ing preclinical results and a high unmet medical need, a Phase II study evaluating fostamatinib in IgAN has been announced [20].
Systemic lupus erythematosusAnother immune disorder that might benefit from SYK inhibitor therapy is systemic lupus erythema-tosus (SLE). A chronic autoimmune disease, SLE is characterized by the presence of autoantibodies, the formation of immune complexes and inflamma-tion in multiple organs. B cells in SLE have multiple abnormalities and are believed to play a central role in these aspects [24]. Preclinical studies evaluating R788 in various lupus models support the development of SYK inhibitors for SLE therapy [25,26]. However, as
discussed above, these results are tempered by the fact that R788 is not a SYK-selective inhibitor.
The best evidence supporting a critical role for B cells in SLE pathogenesis comes from clinical practice of therapeutic agents targeting B cells [9]. Despite not meeting its clinical endpoints in SLE clinical trials, B-cell-depletion therapy using rituximab is being used off-label for refractory cases. Belimumab, a monoclo-nal antibody that neutralizes the B-cell survival factor BLyS fared better in SLE clinical trials and received FDA approval for SLE therapy. In this regard, it is worth pointing out that optimal BLyS responsiveness in B cells requires SYK, suggesting a potential cross-talk mechanism between the BLyS and BCR pathways in transmitting its survival signal [27].
Abnormal T-cell activation and production of cyto-kines are also well-known characteristics of SLE and these may also drive the initiation and maintenance of the autoimmune reaction [28]. Intriguingly, higher SYK expression and activity were detected in T cells of SLE patients compared with controls [29]. How-ever, the hyporesponsiveness of the T cells in these SLE patients did not correlate with SYK expression. The mechanism underlying this heightened T-cell receptor (TCR) response is apparently due to the swapping out of one of the TCR subunits, CD3ζ, with the Fcγ receptor common gamma chain (FcRγ), which recruits and signals through SYK [30]. If confirmed in a larger SLE patient cohort, these results might sug-gest a potential strategy for selecting a subset of SLE patients based on dysregulated SYK-dependent T-cell activation who may benefit from treatment with a SYK inhibitor.
Post-operative ileusPost-operative ileus is a gastrointestinal motility dis-order caused by physical disturbances to the bowel during abdominal surgery. Inflammation within the intestinal muscularis, characterized by infiltration of neutrophils, mast cells and macrophages, is thought to contribute to the development of post-operative ileus. van Bree et al. evaluated a potent and selec-tive aminopyrimidine SYK inhibitor 25 in a mouse model of post-operative ileus [31]. The investigators found 25 dampened manipulation-induced intestinal muscular inflammation, restored intestinal transit and prevented recruitment of immune effector cells to the muscularis, probably by inhibiting mast cell degranulation and the activation of macrophages in response to intestinal manipulation. Thus, SYK inhi-bition may exert additional effects through inhibition of migration and recruitment of immune cells to the site of tissue inflammation, either via FcR or integrin signaling.
Key term
T-cell receptor: Found on the surface of T cells, it is responsible for recognizing antigens and, thus, triggering a cascade that leads to downstream events, such as cellular proliferation, differentiation and cytokine production. Aberrant signaling can lead to disease.
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SYK inhibitors: recent progressPatent literature review: 2010–2014A steady stream of patents and patent applications have been published over the last few years and sev-eral reviews of this literature have been prepared [32–34]. A review of the patent literature from 2010–2012 is therefore redundant and is not included here. Rather, selected patent applications published in 2013 and 2014 are discussed either if they complement these reviews or if they are important to highlight and aug-ment the discussion of some of the common scaffolds that have emerged in the peer-reviewed literature discussed below.
Merck Sharp & Dohme (USA) have been the source of several new patent applications in 2013 and 2014. Based on some of their most potent compounds, one can speculate that they are seeking selective inhibi-tors of SYK and that they may be following a strat-egy to optimize binding interactions with key resi-dues Pro455 and Asn458 in the SYK binding pocket similar to that described by Lucas et al. and Currie et al. (vide infra) [35,36]. Interestingly, the Merck group appears to have found a way to achieve this using a different vector and an aminopyrimidine scaffold [37]. Given the high-molecular weights and the pres-ence of the carboxylic acid in many of their potent
examples (e.g., 1 & 2, Figure 1), one can speculate that they will have encountered similar challenges to the Gilead (USA) and Roche (Switzerland) investigators (vide infra). The Merck group appears to also have found alternatives (e.g., 3, Figure 1) [38], which have allowed them to replace the carboxylic acid, although they seem to have returned to the acid functionality in subsequent applications (e.g., 4, Figure 1) [39].
In 2013, a patent application from Boehringer Ingelheim (Germany) disclosed another variation of the aminopyrimidine scaffold that is favored by Merck [40]. Amongst the compounds claimed was compound 5, an exceptionally potent (SYK IC
50 = 0.5 nM) inhibi-
tor with only a small shift in the presence of 1% human serum albumin. This suggests that cellular potency might also be reasonable. Although no information on selectivity is provided in the patent, again, one could speculate that the indole ring might interact with Pro455 to achieve some level of selectivity of binding.
A series of Merck GMBH patent applications have appeared in the recent literature covering wide structural diversity. Most recently, Burgdorf et al. disclosed a series of pyridopyrimidine derivatives [41]. The medicinal chemistry behind this scaffold is discussed below. This scaffold is clearly distinguished from the set of other scaffolds published by the group,
Figure 1. Aminopyrimidine SYK inhibitors. SYK IC50 binding potency is shown for comparison, but it should be noted that these values may have been obtained from different assay types and conditions.
HN
NN
N
N
NH
ON
N
5, 0.5 nM
NN
HN
1, 1.7 nM
OH
CO2H
CF3
HN
NN
NN
4, 0.9 nM
CO2H
CF3
NN
NH
N
N
3, 1 nM
OH
OH
CHF2
HN
NN
SO
ONH
2, 2.2 nMCO2H
CF3
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such as 6 [42] and 7 [43], both of which are reported to exhibit potency <100 nM, and the macrocyclic 8 [44], reported to be less than 300 nM in potency (Figure 2). The macrocycle appears to have been designed as a constrained version of the Rigel series of aminopy-rimidines, perhaps to improve selectivity; however, the scaffolds that gave rise to 6 & 7 appear completely unrelated.
In 2013, Gilead published a patent [45] that describes a series of imidazopyrazine compounds that are closely related to those that delivered the clinical candidate GS-9973 (38). In the patent application, the authors include solubility data rather than potency; this sug-gests that these compounds may have partly solved the solubility issues that were encountered and described in the discovery effort that led to 38 [36]. For example,
compound 9 (Figure 3) is reported to have a solubility of 100 μM across a wide pH range of 2–7.4.
Roche, in addition to publishing details around sev-eral scaffolds (vide infra), also published in the pat-ent literature a novel thienopyrimidine SYK inhibi-tor scaffold [46]. This scaffold delivered some highly potent SYK inhibitors. For example, compound 10 is a potent inhibitor in both the SYK biochemical assay as well as a human whole-blood assays (IC
50 <1 nM
and 190 nM, respectively). This scaffold likely binds in a similar fashion to the anilinopyrimidine-5-carbox-amide scaffolds that are described below, based upon the similar use of the diaminocyclohexyl functionality. Also from the Roche group is a pyrrolo[2,3-B]pyrazine scaffold that is closely related to one already published and discussed at length below (e.g., 11) [47].
Figure 2. Merck GMBH SYK inhibitors. SYK IC50 binding potency is shown for comparison, but it should be noted that these values may have been obtained from different assay types and conditions to those used in other figures.
N
N
F
OO
O
O
O
N
8, <300 nM
NH
NH NH
HN
N
NN
N
O
N N
O
7, <100 nM
NH
NHN
O
NO
N
O
O
O
6, <100 nM
NH2
Figure 3. Miscellaneous SYK inhibitor scaffolds appearing in the patent literature 2013–2014. SYK IC50 binding potency is shown for comparison only; it should be noted that these values may have been obtained from different assay types and conditions.
N
N NH
S
O
N
NH2NH
10, <1 nM
NN
NH
O
NN
HN
HN
11, 14 nM (76 nM HWB)
N
NN
N
O
NH
O
9, 7 nM
HN
OMe
N
NN
NH
NN
NH2
HN
12, <10 nM
H2NOC
N
N
NH
N
NH2
HN
13, 0.5 nM
H2NOC
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Portola Pharmaceuticals (USA) also remains quite active in the field. In 2013, the company published a series of triazine carboxamides, closely related to their other scaffolds, which delivered clinical candidate molecule 21. Little detail as to the potential advantage of this new core can be gleaned from the application, however, it can deliver highly potent compounds, with exemplars such as 12 showing SYK IC
50 of <10 nM [48].
Patent applications originating from the Novartis group have also been published [49], the most recent of which was closely followed by a manuscript in the peer-reviewed literature (vide infra). An earlier scaffold dis-closes a series of potent compounds (SYK IC
50 = 8 nM)
that were apparently inspired by the Astellas and Portola chemistry, and likely acted as the bridge in the design of their most recent scaffolds (e.g., 13, Figure 3) [50]. Inter-estingly, compounds in this application overlap closely with those in the recent Portola patent described above (compare 12 with 13), a testament to the close space that all these groups are working within and high-lighting the competitive nature of the chemical space around SYK inhibition. The main difference between the scaffolds seems to be the specific focus on bicyclic heteroaryl groups, such as the 4-aminoindole 13. As detailed below, the 7-aminoindole functionality report-edly imparts good selectivity and lowers hERG chan-nel-binding potential on the derivatives. Based on the literature, hERG channel binding is a recurring safety issue that appears in these carboxamide scaffolds.
For all of the scaffolds disclosed in the patent lit-erature, little information regarding the biophysical properties and kinase selectivity is available. We can, therefore, only make inferences based on those scaf-folds for which more detail has become available in the peer-reviewed scientific literature, described in the next section.
Scientific literature review: 2010–2013In contrast to the large quantity of patent literature that has emerged in the last few years, relatively few peer-
reviewed manuscripts have emerged, although the pace might be accelerating. Fortunately, the manuscripts that have been published encompass many of the scaf-fold types that have been the subject of patent claims. These serve to provide insight and clarity around some of the medicinal chemistry obstacles encountered, and the solutions found, in the various campaigns seeking potent and selective SYK inhibitors.
A thorough perspective of the history of the devel-opment of SYK inhibitors, providing a chronological treatise on the development of SYK inhibitors is avail-able [51]. This review will focus on key structural types, as well as providing a contemporary and complemen-tary update with respect to the state of the art in SYK inhibitor research.
The first SYK inhibitors that enjoyed success-ful clinical trials were designed and synthesized at Rigel Pharmaceuticals. These early successes created momentum in the industry and pushed SYK research from the periphery into the mainstream. Excitement over apparent safety and later efficacy in the first patients established a great promise for SYK as a target for the treatment of autoimmune and allergic disease and prompted many other companies to enter the field. Unfortunately, on closer examination, these pioneer-ing first-in-class SYK inhibitors exhibited a number of weaknesses, which included poor solubility, low cellular potency and poor kinome selectivity. These flaws opened the door to differentiating strategies and mapped a path to providing best-in-class compounds. Second-generation scaffolds that purported to address one or more of these deficiencies rapidly emerged. More recently, improving the selectivity, likely to be the cause of some of the toxicities observed in Phase II trials of the Rigel inhibitor R-788 (including, in particular, hypertension) [52], has become a frequent theme amongst groups seeking to develop improved SYK inhibitors as drug candidates.
Rigel obtained their 2-aminopyrimidine scaffold 14 (Figure 4) from a high-throughput screening campaign.
Figure 4. Evolution of Rigel Pharmaceuticals’ 2-aminopyrimidine scaffold.
N
N
NH
F
NH
N O
O
NO
OO
R
R = H, R406, 17, Ki = 30 nMR = CH2OPO3Na2, R788, 18
N
N
NH
F
NH
O O
14
N
N
NH
F
NH
O
HN
O
O
NH
O
FF
R343, 16, Ki = 2 nM
N
N
NH
F
NH
R112, 15, Ki = 96 nM
OHHO
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It is noteworthy that they used a cell-based screen and that it was only after several rounds of modification that lead compound 15 was characterized in biochemi-cal assays and found to potently inhibit the kinase SYK (as well as Src). Further optimization ultimately led to the development of R343 (16), which was positioned as an inhaled asthma therapy, and later R406 (17), and its pro-drug R788 (18), as oral formulations for the treatment of RA. Ensuing studies examined 18 for its potential as a therapy for a variety of autoimmune diseases; indeed, even as recently as 2013 the primary indication pursued by Rigel switched to IgAN. While at first the expanding number of clinical trials and ever-increasing quantity of disease indications paved the way for more SYK inhibitors to enter the clinic, more recently the tremendous excitement around the target has waned as one by one these trials stalled and failed to meet the high expectations. Despite 16 and 18 attracting financial support of some large collabo-rators, including Pfizer and AstraZeneca, respectively, Rigel resumed control of 16 from Pfizer in 2011, then later stopped its development in 2013 after it failed to meet the primary or secondary endpoints in a Phase II clinical study aimed at treating allergic asthma. Simi-larly, after licensing 18 to AstraZeneca in 2010 follow-ing initial exciting data in patients with RA, the rights were returned after Phase III data disappointed. Ulti-mately, the development of 18 for RA halted. While 16 has been officially abandoned by Rigel, 18 is still under investigation for the treatment of immune thrombocy-topenic purpura, with Phase III trial data expected in 2015, and IgAN.
Although initially claimed to be selective, Rigel has acknowledged that at clinical doses of 18 used in RA trials, inhibition of other targets might contribute to the observed pharmacology. This is in alignment with recent publications that show 17 is a multikinase inhibitor. This off-target activity may be the cause of some undesired toxicities observed for this scaffold. Whether or not this is true remains unproven, but it has influenced the strategy of several groups following behind with alternative scaffolds.
Figure 5 shows the evolution of several structur-ally similar SYK inhibitor scaffolds that have evolved from the 4-anilinopyrimidine-5-carboxamide scaffold first published by the Astellas group [53]. The 4-ani-linopyrimidine-5-carboxamides were obtained when the hit 19 emerged from a high-throughput screening campaign. Many of the features that were found to be important attributes by Hisamichi et al. have persisted in subsequent, related scaffolds. The hit compound 19 was developed into compound 20, which was some-what selective for SYK, active in a cell-based assay, and efficacious in the passive cutaneous anaphylaxis
assay in rodent. This promising set of properties pre-sumably inspired others to embark on further devel-opment of this scaffold. Several features, including a 2’-ethylene diamine moiety, a 4-anilino group and a 5-carboxamide, generally recur in follow-on scaffolds. Structure–activity relationships (SAR) developed by Astellas determined that both the terminal amine as well as the secondary amine of the ethylene diamine moiety are important for potency; crystal structures of 20 bound to the SYK kinase domain provide clear explanations why [54]. Replacing the terminal amine with a hydroxyl group led to over 10-fold loss in potency. Substitution on the 4-anilino group showed small lipophilic 3’-substitution to be superior to either 2’- or 4’-substitution (although subsequent scaffolds have demonstrated that there is a lot of opportunity for further diversity). The requirement for a flat aro-matic 4-substituent on the pyrimidine ring has been established; crystallography data shows that SYK has a narrow channel that can only seem to accommodate planar (usually aromatic) functionality at this position. The 5-carboxamide is also crucial to potency; either mono-methylation or di-methylation leads to signifi-cantly decremented potency. Although the primary amide is most commonly maintained in other scaf-folds in this structural class, if the amide is locked in a cis-conformation (e.g., through cyclization, vide supra) then further variety is possible. The NH of the carbox-amide makes an essential H-bond interaction at the hinge of the kinase domain.
The early examples of 4-anilinopyrimidine-5-car-boxamide SYK inhibitors were quite selective when measured against a small panel of kinases. On the other hand, they showed less robust cellular activities, which were attributed to poor membrane permeability. Despite the low cellular activity, however, 20 did show efficacy in an in vivo model of anaphylaxis in mice after subcutaneous dosing and this surely encouraged other research groups to develop this scaffold further.
Portola Pharmaceuticals, partnered with Biogen Idec (USA), is among the few companies following closely behind Rigel in the clinic. Building from the pioneering work of the Astellas team, Portola opti-mized the pyrimidine carboxamide scaffold to deliver PRT2607 (BIIB-057, 21), a SYK-specific inhibitor with improved selectivity compared with 18. Portola collaborated with Biogen Idec to develop compound 21 in RA and lupus, although it was withdrawn from a planned clinical trial for RA, perhaps because of the weakened rationale for SYK in this disease after 18 failed its Phase III clinical trial for RA [55]. It is not present on the Biogen pipeline [56], which could imply progress in other indications has also stalled. The medicinal chemistry discovery story has not been
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disclosed; however, recent publications from other research groups with closely related scaffolds do pro-vide some insight into the advantages and challenges around this scaffold.
Liddle et al. took on this scaffold and, after iden-tifying additional areas of improvement, were able to deliver highly potent and selective SYK inhibitors with oral efficacy [57]. Using an information-based approach beginning from 20, Liddle and his team focused on reducing hERG activity, improving selectivity (they
noted in particular that activity at Aurora B kinase was a primary concern) and developing cellular potency. They confirmed much of the SAR discussed above, finding, for example, that either replacing the 2′-NH with a 2′-O to generate an ether or alkylating the 2′-NH to generate a tertiary amine was detrimental to SYK potency. Following a broader exploration of the SAR around the 2′-diamine moiety they established that secondary amines, such as 24, offered improved selectivity and potency in human whole blood, but
Figure 5. Evolution of arylcarboxamide SYK scaffolds.
N
N
NH
NN
N
N
N
NH
O
NN
NH
N
N
N
NH N
N
NH
N
N
NH
HN
NN
NH
N
O
N NHH
N
N
O
N
N
NH
N
N
N
NH
N
NC
N N
N NH
O
HN
O
28, 44 nM(HWB IC50 = 540 nM)
29, 78 nM(HWB IC50 = 590 nM)
20, 30 nM19, 4 nM
21, 1–13 nM 24, 126 nM (HWB IC50 = 398 nM)
26, 6 nM(HWB IC50 = 87 nM)
27, 40 nM(HWB IC50 = 126 nM)
25, 32 nM(HWB IC50 = 251 nM)
22, 4 nM (cellular blood IC50 = 307 nM)
23, 22 nM(cellular blood IC50 = 50 nM)
NH2
NH2
NH2
NH2 NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH
HNHN
HN
HN
HN
HN
HN
HN HN
HNHN
CHF2
H2NOC H2NOC
H2NOC
H2NOCH2NOC
H2NOCH2NOC
CF3
1820 Future Med. Chem. (2014) 6(16) future science group
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that insufficient progress was made with respect to hERG channel activity. They also found that con-straining the ethylene diamine into a ring, such as cyclohexyl, and maintaining a distal primary amine, were beneficial for binding. Unfortunately, potency in human whole blood was poor. The shift from binding potency to whole-blood potency could not be attrib-uted to any single dominating property (a problem that has also confounded other groups, vide infra); however, by substituting the cyclohexyl for a pyran, an improved therapeutic window over the hERG chan-nel was achievable. Curiously, this improvement was driven not by reduced hERG-binding potency, but through a smaller shift from binding to human whole-blood potency. Interestingly, to widen the difference between human whole-blood potency and hERG, the team found that 4′-substituents on the aniline ring were superior to 3′, and this led to the identification of GSK-143 (25). Compound 25 maintained good physi-cochemical properties including solubility of almost 0.42 mg/ml in simulated intestinal fluid (remark-ably good for kinase inhibitors, which are frequently insoluble), and a high plasma free fraction. This com-pound demonstrated oral efficacy in the reverse passive Arthus reaction, a rat model of inflammation, at doses as low as 10 mg/kg. Unfortunately, mutagenicity in an Ames bacterial assay halted progression. Efforts to overcome this mutagenicity were not discussed. Muta-genicity is not unique to this scaffold; a related scaffold with a pyridazine core rather than the pyrimidine suf-fered from a similar challenge, although in this case a method was established to design this property out of the molecules (vide infra).
Lucas et al. designed a 4-anilinopyridazine-5-car-boxamide scaffold based upon compounds such as 20 and 21 [58]. Their approach was focused on improving the kinase selectivity profile, following a hypothesis in which they sought to optimize hydrophobic interac-tions with Pro455, an amino acid residue that is rarely found in the same position in other kinase catalytic domains [35]. To enable this approach they needed to replace the pyrimidine core with a pyridazine core, and the 4-aniline with a 4-pyridine moiety. This allowed them to optimize the pyridine substitution to interact with Pro455. As well as improving selectivity, install-ing a pyridine was considered a better alternative to an aniline, because anilines can lead to mutagenicity and other toxicities via the formation of reactive metabo-lites. These pyridazine compounds do show superior selectivity in a binding selectivity panel as well as a cell-based selectivity assay, supporting the hypothesis driving their design process. Lucas’ team then followed a similar strategy to that described by Liddle et al. [57], hoping to capitalize on the favorable ADME properties
displayed by this scaffold. 5′,6′-disubstituted pyridines were ultimately favored for their balanced profile and RO9021 (26) demonstrated efficacy in a mouse col-lagen-induced arthritis model of inflammation when dosed orally [59]. Similar to the GSK pyrimidines, such as 25, members of this pyridazine scaffold also showed mutagenicity in the Ames test. Replacing the 4′-amine with a 4′-ether, or removing the distal primary amine, gave non-mutagenic compounds, but these changes were incompatible with SYK binding. Based on the premise that the flat scaffold could intercalate into DNA double helices, a hypothesis to help identify Ames-negative compounds that retained SYK-binding potency was devised. Increasing the ovality of the mol-ecules by adding larger, lipophilic substituents on the pyridine group gave several compounds that were no longer mutagenic. The first derivatives had reduced potency (in human blood), inferior kinase selectiv-ity and a diminished therapeutic window between hERG binding and primary SYK potency. By combin-ing sufficient ovality with lower lipophilicity in later derivatives, they were able to generate compounds with improved therapeutic windows as well as negative (that is, desirable) Ames test results. Ultimately, as found by Liddle et al., substituents para to the amine, such as 27, were found that offered sufficient hERG windows. Compound 27 showed good oral pharmacokinetic (PK) properties and was negative in an MNT assay.
Thoma et al. have described two series of selective SYK inhibitors with low hERG channel inhibition [60]. The selectivity that the group achieved is ultimately better than Rigel’s compound 17 and equivalent to Portola’s compound 21. The group measured selectiv-ity using a combination of a binding assay panel with a cell-based functional assay; it appears that the group was using a cell-based assay to try to better estimate the true kinase selectivity. Thoma’s team differentiated their series from 21 by reducing the hERG-binding potency and improving cellular activity in the presence of 90% human blood. They first developed the pyrido[4,3-d]pyrimidin-4(3H)-one scaffold (e.g., 22, Figure 5) in which they cyclized the primary carboxamide into a pyrimidinone. This change created important and much needed novelty in the crowded chemical space. However, while the primary carboxamides seem to show good pharmacokinetic properties, this new scaf-fold showed poor oral bioavailability when investigated in rats. They attributed the high clearance that they observed to aldehyde oxidase-mediated metabolism of the newly formed pyrimidinone core. Ensuing efforts to block this route of metabolism then led them to a new naphthyridone-based scaffold (e.g., 23, Figure 5). Unfor-tunately, despite successfully reducing the aldehyde oxidase-mediated metabolism, the new compounds still
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Small-molecule inhibitors of spleen tyrosine kinase as therapeutic agents for immune disorders Review
showed high clearance and low exposure, preventing their examination in in vivo models.
Burgdorf recently described the development of a novel pyridopyrimidine based SYK scaffold, inspired by the pyrimidine carboxamide Astellas compound 20 [61]. Like the Thoma group, they also found novel chemical space by essentially incorporating the key features of the primary carboxamide into a ring. However, rather than cyclizing back around onto the core pyrimidine, they replaced one of the arylamine nitrogens with a carbon and created an aminopyridopyrimidine. In doing so, they also designed a new hinge-binding motif. Burg-dorf et al. were quickly able to generate compounds with good potency in human whole blood, and with an improved hERG therapeutic window, by replacing a phenyl front group with a pyrazole moiety. However, this pyridopyridine scaffold suffered from very poor bioavailability, and they found that the heterobicyclic core was a metabolic soft spot. Replacing the amine with a metabolically stable difluoromethyl resulted in improved PK, and installing an indole nitrile provided a compound with sufficiently low protein binding to recover good human whole-blood potency (e.g., 29,
IC50
= 590 nM in a human whole-blood assay). Another noteworthy feature of this scaffold is the exceptional selectivity that was reported for this compound and attributed to the non-classical difluoromethyl H-bond donor as a hinge binder.
So, despite the pyrimidine carboxamide scaffold prototype 19 being discovered over two decades ago, it continues to be a rich source of related compounds. The scaffold continues to evolve, and might yet offer promising new clinical candidates.
A new set of similar scaffolds that are discrete from both the aminopyrimidines and the pyrimidine car-boxamides (Figures 4 & 5, respectively) is highlighted in Figure 6. Two separate research groups appear to have independently identified these scaffolds. Roche group researchers first detailed the structure-guided devel-opment of some highly selective SYK inhibitors [35]. They describe a selectivity hypothesis that drove their design process. Their hypothesis was based on optimiz-ing inhibitor interactions with two specific amino acid residues (Pro455 and Asn458) in the SYK ATP-binding pocket. Studying kinase sequence alignments, these two residues were found less frequently. Thus, starting
Figure 6. Selected Roche SYK inhibitors 30–34, with primary SYK binding IC50 shown. Also shown in parentheses is the cellular activity.
N
N
NN
OH
OH
HN
Roche HTS hit (30)
N
N
O
SN
O O NH
HN
CO2H31, 7 nM
(Ramos B cell IC50 = 156 nM)
N
O
N
NN
O O NH
HN
CO2H
32, 3 nM(Ramos B cell IC50 = 444 nM)
N
N
O
NN
O O NH
HN
CO2H
33, 1 nM(Ramos B cell IC50 = 482 nM)
N
N
NN
N OH
HN
34, 25 nM(Ramos B cell IC50 = 359 nM)
1822 Future Med. Chem. (2014) 6(16) future science group
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from the high-throughput screening hit 30, they grew along vectors that sought to optimize close interactions with Pro455 and Asn458. In this way, the Roche group was able to derive highly SYK inhibitors with excellent binding potencies and cellular potencies commensurate with 17 and 21. The group identified several alternative hinge binding core replacements (e.g., 31–33), which all showed excellent selectivity against a large panel of kinases, supporting their selectivity hypothesis. The interaction with both Pro455 and Asp458 was con-firmed by x-ray crystallography. Thiazolopyrimidine 31 strongly (greater than 90%) inhibited only 4% of kinases in a kinase-selectivity panel, in contrast to 50 and 30% for 17 and 21, respectively. However, permeability and solubility were very poor, which they attributed to the benzoic acid moiety, and potency was approximately 11.9 μM when measured in a human whole-blood assay. Replacing the carboxylate with bioisosteres was poorly tolerated, and ultimately the molecules were trun-cated to eliminate the carboxylic acid altogether and to improve physicochemical properties. Unfortunately, this necessary change resulted in the loss of the interaction with the Asn458, as well as some hydrophobic interac-tions with Pro455, and a loss of selectivity. A focused effort to reoptimize interactions with just the Pro455 led to 34, which recovered good, albeit slightly dimin-ished, selectivity. This suggests that optimizing interac-tions with only the Pro455 residue of SYK is an excellent way to derive selective compounds. Unfortunately, the
biopharmaceutical properties of the scaffold remained suboptimal and no in vivo characterization was possible.
In 2014 the Gilead group (formerly CGI Pharmaceu-ticals) shared the discovery of GS-9973 (38), a highly selective spleen tyrosine kinase inhibitor that is currently in Phase II clinical trials (Figure 7) [36]. Currie et al. identified a scaffold similar to the Roche high-through-put screening hit 30 from their own high-throughput screening effort. Starting from 35, which was already a very potent SYK inhibitor (SYK IC
50 = 65 nM),
they replaced the lipophilic tert-butyl group with a dimethoxyphenyl (a typical tyrosine kinase inhibitor motif) and then reversed the central amide to obtain 36, a highly potent and selective SYK inhibitor (bind-ing potency 0.8 nM, 8/317 kinases inhibited >90% at 10 μM). In what is emerging as a common challenge in the effort to find SYK inhibitor scaffolds, compound 36 showed poor cellular activity and was inactive when measured in a human whole-blood assay. Like the Roche group, the Gilead team predicted that the carboxylic acid was responsible for the large shift in potency, and after extensive bioisosteric replacement attempts, presumably reached similar conclusions regarding likely poor devel-opability of the carboxylate or of its bioisosteres. Follow-ing a similar strategy to that described by Lucas et al., the Gilead group also chose to truncate the molecule, and found that even with some loss of potency, the improved molecular properties compensated. The benzamide 37 demonstrated a reduced shift in the cell assay, it retained good selectivity and even showed some oral bioavail-ability in rat (10 mg/kg, F = 22%). Screening hetero-cyclic replacements of the benzamide that has improved physicochemical properties led the group to an inda-zole. Finally, introducing a morpholine substituent to replace the dimethoxyphenyl finally gave GS-9973 (38). Compound 38 maintained good selectivity, a balanced biopharmaceutical profile and showed in vivo efficacy in a rat collagen-induced arthritis model. In 2014, this compound entered Phase II clinical trials to study the safety and efficacy in participants with hematological tumors [62]. It is also being studied in combination with Idelalisib, an oral inhibitor of PI3K delta, in patients with relapsed or refractory hematologic tumors [63,64]. It is notable that these clinical trials are both for oncology indications and that one these studies is a combination approach. SYK has been considered an autoimmune disease target primarily; however, it would appear that this molecule is not being pursued in inflammation. It could be that, based on the reported challenging physi-cochemical properties for this scaffold, the attributes were insufficient to support a monotherapy. Alterna-tively, together with the failure of other SYK inhibitors to progress through the clinic in autoimmune disease, this might indicate that selective SYK inhibitors are not Figure 7. Selected Gilead SYK inhibitors 35–38.
N
NN
O
HN
HN
CO2H
Gilead HTS hit (35)
N
NN
OO O NH
HN
CO2H36, 1 nM
(pBLNK EC50 = 126 nM)
N
NN
OO
HN
CONH2
37, 27 nM(pBLNK EC50 = 214 nM)
N
NN
N
N
O
HN
HN
GS-9973 (38), 8 nM(pBLNK EC50 = 26 nM)
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Small-molecule inhibitors of spleen tyrosine kinase as therapeutic agents for immune disorders Review
able to live up to their original expectations. In order for SYK inhibitors to meet their full potential, it might be necessary to pivot toward other biological pathways, as Gilead appears to have done.
In 2013, the medicinal chemistry around the devel-opment of the pyrrolopyrazine class of SYK inhibitors was presented (Figure 8). Starting from a non-selective SYK/JAK scaffold 39 [65–68], which possessed improved pharmacological properties compared with compounds 31–34, Padilla et al. applied the same strategy to enhance selectivity that they had disclosed previously [69]. Improving solubility through the introduction of a diamine group, and improving selectivity by selecting an indazole that was conformationally restricted to force it to interact with the Pro455 moiety, the group generated 40. This compound had improved PK properties, but solubility was still poor. The scaffold was terminated due to acute in vivo toxicity observed during rodent effi-cacy models. The scaffold has been developed further, as evidenced by a recent patent (vide supra), in which the bicyclic indazole ring was replaced by an amine-linked heterocycles. Although the properties of these com-pounds are unknown, they appear to have maintained good human whole-blood activity (IC
50 = 76 nM) [47].
Castillo et al. reported a set of aminopyridines (Figure 9, 41–44), using a knowledge-based design to generate a low micromolar SYK inhibitor 41 [70]. The Almirall team found that by installing a piperazine moiety they were able to access the SYK Asp512 resi-due to form a salt bridge – the same residue that proved so critical for potency in the heteroaryl carboxamides (vide supra). Indeed, they observed a dramatic improve-ment in potency of approximately two orders of magni-tude, to generate low nanomolar potency compounds. Replacing the picoline methyl with a trifluoromethyl group to form compound 42 led to a further 10-fold potency enhancement. The authors propose that this is due to hydrophobic interactions with the gatekeeper res-idue Met448. However, the high lipophilicity was a con-cern and this may have contributed to the compounds suffering from low micromolar cytotoxicity. Reducing the lipophilicity by replacing the picoline with an ami-nopyrazine, (e.g., compound 43) dramatically improved the cytotoxicity profile, albeit with a 10-fold loss of potency. Finally, installing an amine on the central pyri-dine ring recovered this loss (e.g., 44, SYK IC
50 = 4 nM).
This compound also showed good activity in a human mast cell degranulation assay (IC
50 = 118 nM). It is not
clear how selective for SYK this series is; when tested in a 32-member panel at 1 μM concentration, compound 44 inhibited 12% of the kinases tested. This suggests that it could be more selective than some of the earliest scaffolds such as 17 and 21, but that it may still be more promiscuous than some of the other scaffolds discussed
above. Nevertheless, one could envisage that by applying some of the recent learnings and hypotheses that have resulted in highly selective scaffolds to this one, that this could offer a route to highly selective and potent compounds.
Conclusion & future perspectiveLargely because of problems for compounds such as 16 and 18 in the clinic being attributed to poor kinome selectivity, a subsequent focus on more selective SYK inhibitors ensued. While selectivity might have offered improved safety profiles, it could have come at the cost of some efficacy. Perhaps the exquisite selectivity that some groups strived for, and in some cases have achieved, has actually contributed to their failure to treat immune diseases. While most early studies focused on autoim-munity, SYK inhibition in oncology indications also has a firm rationale. The recent advancement of 38 through Phase II for an oncology indication, as well as the interest in combining SYK inhibition with other kinase inhibi-tors, might suggest that the industry has found it neces-sary to shift the focus back to the biology of SYK. A shifting paradigm not back towards indiscriminate, mul-tikinase inhibition, but rather carefully targeting mul-tiple, defined kinases combinations might be occurring.
Figure 8. Pyrrolopyrazine SYK inhibitors.
NN
NH
O
O
OO
HN
39
NN
NH
O
NNH
F
NH2
HN
40
Figure 9. Almirall SYK inhibitors 41–44.
N
N N
41
HN
43
N N
NN
N
NH
HN
N
NN
N
42
NH
HN
CF3
N N
NN
N
44
NH
HN
H2N
1824 Future Med. Chem. (2014) 6(16) future science group
Review Lucas & Tan
Recent publications from Rigel Pharmaceuticals indicate that they are pursuing dual SYK/JAK inhibitors in Phase II for dry eye, and Portola Pharmaceuticals is pursuing dual SYK/JAK inhibitors (PRT2070). Of course, it is possible that this is simply opportunistic owing to com-mon features shared by these two kinases, rather than a requirement for inhibition of additional targets. How-ever, based on some high-profile failures and stumbles following a mono-targeted approach, perhaps the most exciting development for selective SYK inhibitors will be finding the right combination with other biological tar-gets that are able to bring additional pathways to bear in order to strengthen the desired biological outcome. For example, it has been reported that as a single agent 21 shows promising activity in chronic lymphocytic leuke-mia, but that it is synergistic with Fludarabine, a purine analog that interferes with ribonucleotide reductase and DNA polymerase [71]. While some research groups have made it a priority to avoid certain off-targets, such as
Zap70 [60], the emergence of routes to prepare dual ZAP70/SYK inhibitor compounds should allow more investigation and potentially uncover new opportunities for SYK inhibition in the treatment of allergy, autoim-munity or organ transplant therapies [72]. Ultimately, however, incorporation of such oral therapeutic agents in the clinical management of chronic inflammatory and autoimmune disorders will be dictated by the risks and benefits of treatment, regulatory and pricing pressures of treatment and the patient’s preference. It is fair to say that the jury is still out.
AcknowledgmentsThe authors thank Kristopher Hahn and Ron Dolle for proof-
reading the manuscript and providing helpful comments.
Financial & competing interests disclosureThe authors have no relevant affiliations or financial involve-
ment with any organization or entity with a financial inter-
est in or financial conflict with the subject matter or mate-
rials discussed in the manuscript. This includes employment,
consultancies, honoraria, stock ownership or options, expert
testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this
manuscript.
Executive summary
• The clinical utility of biologics in the management of rheumatoid arthritis (RA) is unquestionable, however, a significant proportion of RA patients do not respond to these marketed biologics or are unable to achieve a sustained response when treatment is ceased.
• Much of the initial enthusiasm for an alternative small-molecule approach has been hampered by the disappointing clinical results of various small-molecule inhibitors targeting the p38 pathway.
• The recent US FDA approval of tofacitinib (a JAK inhibitor) and ibrutinib (a Btk inhibitor) for RA and chronic lymphocytic leukemia therapy, respectively, has renewed interest in the development of orally active small-molecule agents targeting key protein kinases implicated in immune regulation.
• SYK was at one point considered to be a ‘hot’ immunokinase target but fell from grace when Rigel’s oral and inhaled SYK inhibitors for RA and asthma therapy, respectively, failed to live up to initial expectations.
• Based upon recent literature, a revival in the development of SYK inhibitors for the treatment of inflammatory and autoimmune disorders may be imminent.
• SYK is a cytoplasmic tyrosine kinase that plays a critical role in mediating signal transduction triggered by the activation of a number of immune recognition receptors. This catalyzes the phosphorylation of a number of protein substrates and thus facilitates the propagation of downstream signaling events that lead to cell proliferation, cell survival, cell differentiation, phagocytosis, cell migration and secretion of cytokines.
• Aberrant activation of immunoreceptor signaling can contribute to the initiation and/or maintenance of chronic inflammation and autoimmunity. Given the critical role of SYK in BCR and FcR signaling as well as in other signaling pathways, therapeutic approaches targeting SYK are expected to deliver better and broader efficacy for treatment of autoimmune diseases.
• Diseases that might benefit from treatment with SYK inhibitors include RA, asthma and allergic rhinitis, immune thrombocytopenic purpura, IgA nephropathy, systemic lupus erythematosus and post-operative ileus.
• Failure of the early SYK inhibitors that entered the clinic was attributed to the poor kinome selectivity.• More selective SYK inhibitor scaffolds were pursued, which were expected to provide safer compounds, but so
far no SYK inhibitors have been able to treat immune diseases.• The recent advancement of a SYK inhibitor through Phase II for an oncology indication, as well as the
interest in combining SYK inhibition with other defined kinases to bring additional pathways to bear, might potentially uncover new opportunities for SYK inhibition.
Key term
ZAP70: The second member of the SYK family of kinases. Zeta-chain-associated protein kinase 70 plays a role in T cells that is the equivalent of the role that SYK plays primarily in B cells.
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Small-molecule inhibitors of spleen tyrosine kinase as therapeutic agents for immune disorders Review
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