- Email: [email protected]
p38 MAP kinase inhibitors: A future therapy for inflammatory diseases
Drug Discovery Today: Therapeutic Strategies
Vol. 3, No. 1 2006
Editors-in-Chief Raymond Baker – formerly University of Southampton, UK and Merck Sharp & Dohme, UK Eliot Ohlstein – GlaxoSmithKline, USA DRUG DISCOVERY
TODAY THERAPEUTIC
STRATEGIES
Immunological disorders and autoimmunity
p38 MAP kinase inhibitors: A future therapy for inflammatory diseases Ruth J. Mayer*, James F. Callahan GlaxoSmithKline Pharmaceuticals, Respiratory and Inflammation CEDD, P.O. Box 1539, King of Prussia, PA 19406, USA
p38 mitogen-activated kinase is a target for therapeutic intervention in inflammatory diseases. The pharmacology of p38 inhibitors is reviewed briefly, and
Section Editors: Claudine Bruck – GlaxoSmithKline, King of Prussia, USA Michel Goldman – University of Brussels, Brussels, Belgium
potential other therapeutic targets in this important signaling pathway are outlined. The evaluation of p38 inhibitors in early clinical studies is reviewed with emphasis on pharmacodymanic assessment. Ex vivo and in vivo lipopolysaccharide (LPS) challenges have
better kinase selectivity and potency, clinical evaluation has been more promising. The past few years have seen positive Phase II studies in inflammatory diseases such as rheumatoid arthritis and psoriasis.
been successful in Phase I studies and will be of value in the early assessment of future p38 inhibitors.
Introduction Inhibitors of p38 mitogen-activated protein kinase (MAPK) were first identified in the late 1980s, although the target was not yet identified. Fig. 1 shows the current understanding of the p38 pathway and identifies the principal components of the signaling cascade. P38 MAPK is a central point of activation in response to a wide variety of stimuli that subject cells to stress, as detailed below. With identification of p38a as the target for a class of compounds with broad, potent anti-inflammatory activity [1], the efforts to identify an effective therapeutic have increased dramatically. At least a dozen p38 inhibitors have been evaluated in clinical trials, with very few progressing beyond Phase I. Early compounds have been associated with elevations in liver function enzymes (LFTs, transaminases), as well as CNS effects characterized as dysphoria [2–5]. As compounds were identified with improved absorption–distribution–metabolism–excretion (ADME) properties, as well as *Corresponding author: R.J. Mayer ([email protected]) 1740-6773/$ ß 2006 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddstr.2006.03.003
Potential disease indications for p38 inhibitors A large number of diseases have been considered as indications for a p38 inhibitor, all having an inflammatory component. The list includes rheumatoid arthritis (RA), inflammatory bowel disease (IBD) or Crohn’s disease, atherosclerosis, chronic obstructive pulmonary disease (COPD), severe asthma and psoriasis. To date, Phase II studies have been completed in rheumatoid arthritis and psoriasis, with encouraging results as detailed below.
p38 MAP kinase The tremendous interest in p38 MAPK as a therapeutic target for inflammatory diseases stems from an ever-growing body of data demonstrating the importance of the p38 pathway in the cellular response to inflammatory stimuli and the very broad efficacy of p38 inhibitors in preclinical animal models of disease. The relevant target is considered to be p38a, one of the four isoforms of the enzyme (MAPK14, MAPK11, MAPK13, MAPK12 or p38a, b, d, g, respectively). P38a is found as the predominant isoform in leukocytes, epithelial cells, smooth muscle cells, whereas p38d is more highly expressed in macrophages and p38g in skeletal muscle [6]. At this time, all inhibitors under investigation are dual p38a/b 49
Drug Discovery Today: Therapeutic Strategies | Immunological disorders and autoimmunity
Vol. 3, No. 1 2006
Figure 1. The p38 MAPK signaling cascade. Upstream activators include apoptosis signal-regulated kinase (ASK1) and the mitogen-activated protein kinase (MKKs or MEKs); downstream effectors include mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK2) and other MAPKAPs. The c-jun N-terminal kinase (JNK) and extracellular signal-regulated (ERK) MAPK pathways are related functionally to the p38 pathway. Additional details on the pathways can be found at http://www.biocarta.com.
inhibitors. A p38b / mouse has recently been described [7]. The lipopolysaccharide (LPS)-induced cytokine response in these mice is normal, suggesting again that p38a is the major target for the known inhibitors. The p38 pathway (Fig. 1) is activated in response to cytokines including tumor necrosis factor-a (TNFa) and transforming growth factor-b (TGFb), UV light, osmotic stress, oxidative stress and microbial pattern recognition (Toll receptor engagement) [6]. Activation of the pathway through surface receptors or cellular sensing mechanisms results in phosphorylation of MEKKs, MEKs and p38 MAPK in sequence. In turn, p38 phosphorylates downstream kinases such as PRAK (MAPKAPK5) or MAPKAPK2, which directly phosphorylate several transcription factors and heat shock proteins (see Related articles). A list of some of the relevant functions or genes modulated by the p38 pathway is given in Table 1.
The kinase selectivity of p38 inhibitors has been generally addressed by cross-screening against several available kinases, and more recently by using phage-display approach to evaluating binding affinity to over 100 kinases [8]. BIRB-796 and VX-745 were part of the panel evaluated in this recent profiling exercise, and showed remarkable selectivity, with over tenfold tighter binding to p38 than any other kinase evaluated. However, several kinases, both in the Tyr kinase and Ser/Thr kinase families were inhibited by all of the p38 inhibitors evaluated, with the c-jun N-terminal kinase (JNK) family members most often inhibited. No common pattern emerged, suggesting that each p38 inhibitor would have a distinct profile. One of the tool compounds evaluated in this profile was SB203580, which has been used as a prototypic p38 inhibitor in a large number of studies to characterize the role of p38 in cellular processes. SB-203580 has been reported to inhibit
Table 1. Some key functions of p38a or the p38 kinase pathway Cellular function
Mechanism
Refs
Cytokine and chemokine expression (e.g. TNFa, IL-1b, IL-6)
Modulation of mRNA levels containing an AU-rich region
[6,35]
Cell survival (e.g. UV stress)
Phosphorylation of p53
[28,36]
TGFb signaling
Activated downstream of receptor
[19]
Neutrophil function (e.g. superoxide burst)
Regulation of surface receptor expression Assembly of NAPDH oxidase
[37]
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Drug Discovery Today: Therapeutic Strategies | Immunological disorders and autoimmunity
JNKs, as well as p38, and some additional targets in the epidermal growth factor receptor pathway were identified. Mechanistic studies with SB-203580, therefore, must be interpreted carefully and in some cases repeated with more selective compounds or genetic knockdown methods.
Preclinical evaluation of p38 inhibitors P38 inhibitors have been evaluated preclinically in a wide range of disease models, with good efficacy demonstrated in many models. In the collagen-induced arthritis or rat adjuvant arthritis models of RA, p38 inhibitors have been shown to reduce the arthritis score, to reduce the expression of cytokines and chemokines such as IL-1b, TNFa and KC (murine analog of IL-8), and to block the loss of bone density and the expression of bone degradation markers such as RANK-L [9,10]. In a rat model of cardiovascular disease, p38 inhibitors protect against cardiac hypertrophy, and end-organ damage in spontaneously hypertensive rats fed a high-salt, high-fat diet [11]. In a second model of cardiovascular disease, a p38 inhibitor offered protection against cardiac remodeling 12 weeks following induction of myocardial infarction in mice [12]. In models of COPD, p38 inhibitors block neutrophil influx into the lung, as well as reduce cytokine levels in the bronchoalveolar lavage [13,14]. p38 inhibitors have also been reported to be efficacious in models of pain, IBD and asthma [15–17].
What will a p38 inhibitor deliver? Although there is extensive preclinical data on p38 inhibitors, successful clinical proof-of-concept studies are still very few in number. Based on the preclinical data, a typical p38 inhibitor would be expected to reduce levels of proinflammatory cytokines and chemokines and reduce cellular infiltration to sites of inflammation, thereby reducing local damage. In diseases such as RA and IBD, TNFa blockade through either anti-TNFa antibodies or use of soluble TNFa receptors has been shown to be an effective therapy [18]. Inhibition of p38a offers the potential of significant inhibition of TNFa, as well as inhibition of cytokines such as IL-1b and IL-6, which would be expected to offer additional therapeutic efficacy. In diseases such as cardiovascular disease or COPD, similar data on the role of cytokines is lacking. In addition to these well-studied roles in the inflammatory response, p38 inhibition might be expected to modulate the remodeling process that occurs with resolution of inflammation, through the role of p38 in TGFb signaling [19].
available. Inhibition of kinases has been considered to be of high risk owing to the concerns related to selectivity and the crucial regulatory role of many kinases in cell-signaling pathways. The profiling of kinase inhibitors including p38 inhibitors has interestingly not identified the potent inhibition of any unexpected off-target activity of known p38 inhibitors. Recent compounds are likely to be even more selective, suggesting that safety might not be significantly affected by lack of selectivity. Nonetheless, inhibition of p38 still has the potential for mechanism-based toxicity, especially at high levels of inhibition. Full reports on toxicology of p38 inhibitors have yet not appeared, but given the number and range of cellular events under the regulatory control of p38 MAPK, it is very probable that the therapeutic index, or ratio between desirable and undesirable effects, will be relatively small [20].
Compounds in development Several compounds of interest that have been in recent clinical development are listed in Table 2, with the primary indications and phase of development achieved. Structures, where known, are provided in Fig. 2. The current status of any of these compounds is uncertain, and several might have been already discontinued. In addition to these compounds, several compounds have been studied previously with instructive results, although various concerns prevented further development. Results with VX-745 in a study of 44 patients with RA were reported [4] to significantly improve ACR20 response (43%, p = 0.04), following 12 weeks of dosing. Treatment-related elevation in LFTs was reported in six patients on VX-745 versus none on placebo, following 4 weeks of dosing. This first positive proof-of-concept study with a p38 inhibitor has lent confidence to the further development of p38 inhibitors for RA as well as other diseases.
Phase I studies and pharmacodynamic assessment SB-242235 was shown to dose-dependently suppress ex vivo production of cytokines in response to LPS stimulaTable 2. Compounds reported recently in clinical development Compound
Key indication
Phase achieved
Refs
VX-702
RAa, angina
II
[26]
BIRB-796
Psoriasis, RA, Crohn’s
IIb/III
[38]
b
SCIO-469
RA, IBD
Safety considerations
RWJ 67657
Discontinued
There are both theoretical and known safety concerns with p38 inhibition. Several reports of transient elevations of LFTs following repeat dosing have raised concerns over long-term hepatotoxicity, although information on the clinical safety profile of more recent compounds in development is not
TAK-715
RA
SB-681323
II
[38] [38]
c
RA, COPD , athero
II
[39]
I/II
[38]
a
RA: rheumatoid arthritis. IBD: irritable bowel disease. c COPD: chronic obstructive pulmonary disease. b
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Figure 2. Structures of p38 inhibitors of interest in clinical development. See Table 2 for additional information.
tion of whole blood, providing one approach to pharmacodynamic evaluation of p38 inhibition [21]. A similar study was reported [22] for another compound, RWJ 67657, with similar conclusions, suggesting that ex vivo TNF inhibition is an adequate method for an early assessment of efficacy. Early studies with BIRB-796 utilized several approaches to pharmacodynamic evaluation, including ex vivo LPS-stimulated cytokine production, ex vivo stimulation of neutrophil surface markers and in vivo LPS challenge [3,23]. The biochemical endpoints evaluated for the in vivo LPS challenge included phosphorylation of p38, ex vivo p38 activity assay, several cytokines, CD11b surface expression and plasma Creactive protein (CRP) [23]. All of these measures were sensitive to doses of 50 mg BIRB-796. Again, a similar study design was used in the evaluation of RWJ-67657, showing both inhibition of cytokines including TNFa, IL-6 and IL-8 up to 90%, as well as surface expression of ICAM-1 and L-selectin, confirming these endpoints as potential markers for p38 inhibitory activity [24,25]. 52
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Phase II studies Following the successful Phase IIa study with VX-745, several other compounds have been reported to have entered Phase II studies of significant size (Table 2). None of these studies have reported data as yet. A study with BIRB-796 in psoriasis at doses up to 30 mg b.i.d. was reported to be positive in a press release, but no data have appeared. A very limited Phase IIa study with VX-702 in acute coronary syndrome reported inhibition of the increase in C-reactive protein following percutaneous coronary intervention (PCI). VX-702 was dosed 2 days prior and 3 days post-PCI, and decreased CRP significantly across all dose groups [26]. Additional Phase II studies are in progress.
Alternate approaches to p38 pathway inhibition Inhibition of p38 MAP kinase is not the only mechanism under consideration for modulation of the p38 pathway (Table 3). Mitogen-activated protein kinase (MAPK)-activated protein kinase (MAPKAP-K2 or MK2), which is a downstream kinase implicated in many of the effector functions of p38 kinase [27,28], has also been evaluated as a target and has
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Drug Discovery Today: Therapeutic Strategies | Immunological disorders and autoimmunity
Table 3. Approaches to inhibition of p38 MAP kinase pathway Target
Pros
Cons
Latest developments
Who is working on this approach
p38
Chemistry well established, biology well documented
Safety and toxicity concerns
Several compounds have reached Phase II
Many pharma and biotech
MAPKAPK2a
None known, possibly better safety
Possibly reduced efficacy
Preclinical
BMSb, Pfizer Amphora, Teijin
ASK1c
Upstream inhibition of both p38 and JNKd pathways, activated by oxidative stress
Safety considerations unknown for inhibition of both p38 and JNK; importance of oxidative stress unclear
Emerging target
Early stages
Kissei a
MAPKAPK2: mitogen-activated protein kinase-activated protein kinase. BMS: Bristol Myers Squibb. ASK-1: apoptosis signal regulating kinase. d JNK: c-jun N-terminal kinase. b c
generated some patent activity [29]. A MAPKAP-K2 knockout mouse (which is viable, in contrast to a p38a knockout) has been studied in disease models. MK2 / mice subjected to focal ischemia demonstrated reduced infarct size compared to wild-type mice [30]. However, reduced TNFa expression was not observed in these mice, which is somewhat surprising. Another study with MK2 / mice demonstrated protection from ischemia-reperfusion injury [31]. These results might not be solely owing to the role of MAPKAPK2 because levels of p38a are significantly reduced in MK2-deficient tissues as these two kinases form a stabilizing complex [32]. A relatively small degree of inhibition of MK2 might be efficacious, however, owing to destabilization of p38a. Another member of the MAP kinase family that is of emerging interest is apoptosis-signal-regulating kinase, or ASK-1 (MAP3K5) [33]. This kinase is upstream of both the p38 and JNK MAPKs [34], and is regulated in response to changes in oxidation state of thioredoxin. Interest lies in the dual pathway role and perhaps, more focused role in activation of MAPKs by sensing oxidative stress. There is recent patent activity in this area as well, although this target is by no means as well characterized as either p38a or MAPKAPK2.
several diseases, but are not desirable for long-term oral therapy owing to the side effect profile. The potential for anti-inflammatory therapy with statins might not yet have been realized, and they have been shown to be safe for longterm therapy. So, the medical need for additional mechanisms to treat inflammatory diseases remains. Since the mid-1990s, several p38 inhibitors have entered clinical studies, and the clinical development of almost all of these compounds has been discontinued. The full reasons for the attrition rate are not known. Sufficient positive data have been generated in Phase I or IIa studies to maintain industry interest in development of p38 inhibitors despite the challenges. Preclinical data continue to support important roles for p38a in several diseases. Alternative mechanisms within the p38 pathway, such as ASK1, pose similar problems and will also be slow to reach clinical efficacy studies. As our understanding of the importance of inflammation in chronic disease continues to be refined, other alternatives for effective anti-inflammatory treatments might emerge. The near future will demonstrate whether compounds currently in development have the appropriate properties for long-term development for chronic disease.
Conclusions The potential value and therapeutic importance of potent, safe and effective anti-inflammatory drugs are very high. Current anti-inflammatory therapy includes NSAIDs, corticosteroids and statins. Nonsteroidal anti-inflammatories or NSAIDS, although highly effective for acute treatment, have not been shown to be disease modifying for long-term treatment of, for example, RA. Corticosteroids are effective in
Links http://stke.sciencemag.org/cgi/cm/stkecm;CMC_826 (Kinase knowledge site maintained by Science) http://www.biocarta.com/pathfiles/h_mapkPathway.asp (Complete pathway information for the MAPK pathways)
References 1 Lee, J.C. et al. (1994) A protein kinase involved in the regulation of inflammatory cytokine biosynthesis. Nature 372, 739–746 2 Polmar, S.H. et al. (2002) Safety and pharmacokinetics of an oral p38 MAP kinase inhibitor (BIRB-796 BS) administered twice daily for 14 days to healthy volunteers. AAAAI 58th Annual Meeting (Abstract #157) 3 Gupta, A. et al. (2002) Safety, pharmacokinetics and pharmacodynamics of single doses or an oral p38 MAP kinase inhibitor (BIRB-796 BS) in healthy human males a placebo-controlled, randomized study, double-blinded at each dose level. AAAAI 58th Annual Meeting (Abstract #158) 4 Weisman, M. et al. (2002) A double-blind, placebo controlled trial of VX745, an oral p38 mitogen-activated protein kinase (MAPK) inhibitor in patients with rheumatoid arthritis (RA). 2002 Annual European Congress of Rheumatology, Stockholm, Sweden (Abstract FRI0018) 5 Lee, M.R. and Dominguez, C. (2005) MAP kinase p38 inhibitors: clinical results and an intimate look at their interactions with p38 alpha protein. Curr. Med. Chem. 12, 2979–2994 6 Shieven, G.L. (2005) The biology of p38 kinase: a central role in inflammation. Curr. Top. Med. Chem. 5, 921–928 www.drugdiscoverytoday.com
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7 Beadmore, V.A. et al. (2005) Generation and characterization of p38b (MAPK11) gene-targeted mice. Mol. Cell Biol. 25, 10454–10464 8 Fabian, M.A. et al. (2005) A small molecule–kinase interaction map for clinical kinase inhibitors. Nat. Biotechnol. 23, 329–336 9 Badger, A.M. et al. (2001) Disease-modifying activity of SB-242235, a selective inhibitor of p38 mitogen-activated protein kinase, in rat adjuvant arthritis. Arthritis Rheum. 43, 175–183 10 Nishikawa, M. et al. (2003) Prevention of the onset and progression of collagen-induced arthritis in rats by the potent p38 mitogen-activated protein kinase inhibitor FR167653. Arthritis Rheum. 48, 2670–2681 11 Behr, T.M. et al. (2001) Hypertensive end-organ damage and premature mortality are p38 mitogen-activated protein kinase-dependent in a rat model of cardiac hypertrophy and disfunction. Circulation 104, 1292–1298 12 Liu, Y.H. et al. (2005) Inhibition of p38 mitogen-activated protein kinase protects the heart against cardiac remodeling in mice with heart failure resulting from myocardial infarction. J. Card. Fail. 11, 74–81 13 Underwood, D.C. et al. (2000) SB 239063, a p38 MAPK inhibitor, reduces neutrophilia, inflammatory cytokines, MMP-9, and fibrosis in lung. Am. J. Physiol. Lung Cell Mol. Physiol. 279, L895–L902 14 Barnes, P.J. and Stockley, R.A. (2005) COPD: current therapeutic interventions and future approaches. Eur. Respir. J. 25, 1084–1106 15 Sweiter, S.M. et al. (2004) Antinociceptive action of a p38alpha MAPK inhibitor, SD-282, in a diabetic neuropathy model. Pain 109, 409–419 16 Hollenback, E. (2004) Inhibition of p38 MAP kinase- and RICK/NFkappaB-signaling suppresses inflammatory bowel disease. FASEB J. 18, 1550–1552 17 Salmon, M. et al. (2002) Inhibition of antigen-induced airway inflammation and remodeling in sensitised Brown-Norway rats by SB 239063 a potent and selective p38 MAP kinase inhibitor. Am. J. Respir. Crit. Care Med. 165, A538 18 Aggarwal, B.B. et al. (2006) TNF blockade: and inflammatory issue. Ernst Shering Res. Found. Workshop 56, 161–186 19 Choi, M.E. (2000) Mechanism of transforming growth factor-beta1 signaling. Kid. Intl. Suppl. 77, S53–S58 20 Dambach, D. (2005) Potential adverse effects associated with inhibition of p38a/b MAP kinases. Curr. Top. Med. Chem. 5, 929–939 21 Fullerton, T. et al. (2000) Suppression of ex vivo cytokine production by SB-242235, a selective inhibitor of p38 MAP kinase. Clin. Pharm. Ther. 67, 81 22 Parasrampuria, D.A. et al. (2003) Single-dose pharmacokinetics and pharmacodymanics of RWJ-67657, a specific p38 mitogen-activated protein kinase inhibitor: a first-in-human study. J. Clin. Pharmacol. 43, 406–413 23 Branger, J. et al. (2002) Anti-inflammatory effects of a p38 mitogenactivated protein kinase inhibitor during human endotoxemia. J. Immunol. 168, 4070–4077
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25
26
27 28
29
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31
32 33 34 35
36
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38
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Fijen, J.W. et al. (2002) Inhibition of p38 mitogen-activated protein kinase: a dose-dependent suppression of leukocyte and endothelial response after endotoxin challenge in humans. Crit. Care Med. 30, 841–845 Fijen, J.W. et al. (2001) Suppression of the clinical and cytokine response to endotoxin by RWJ-67657, a p38 mitogen-activated protein kinase inhibitor, in healthy human volunteers. Clin. Exp. Immunol. 124, 16–20 Vertex Pharmaceuticals (2004) Preliminary Phase IIa data for VX-702 demonstrate tolerability and reduction in C-reactive protein in cardiovascular patients. European Society of Cardiology’s Acute Cardiac Care Symposium, Rome, Italy Gaestel, M. (2006) MAPKAP kinases: MKs two’s company, three’s a crowd. Nat. Rev. Mol. Cell Biol. 7, 120–130 Manke, I.A. et al. (2005) MAPKAP kinase-2 is a cell cycle checkpoint kinase that regulates the G2/M transition and S phase progression in response to UV irradiation. Mol. Cell 17, 37–48 Anderson, D.R. et al. (2005) Aminocyanopyridine inhibitors of mitogenactivated protein kinase-activated protein kinase 2 (MK2). Bioorg. Med. Chem. Lett. 15, 1587–1590 Wang, X. et al. (2002) Mitogen-activated protein kinase-activated protein (MAPKAP) kinase 2 deficiency protects brain from ischemic injury in mice. J. Biol. Chem. 15, 43968–43972 Shiroto, K. et al. (2005) MK2 / gene knockout mouse hearts carry antiapoptotic signal and are resistant to ischemia reperfusion injury. J. Mol. Cell. Cardiol. 38, 93–97 Kotlyarov, A. et al. (2002) Distinct cellular functions of MK2. Mol. Cell. Biol. 22, 4827–4835 Matsukawa, J. (2004) The ASK-1-MAP kinase cascades in mammalian stress response. J. Biochem. 136, 261–265 Tobiume, K. et al. (2001) ASK1 is required for sustained activations of JNK/ p38 MAP kinases and apoptosis. EMBO J. 2, 222–228 Winzen, R. et al. (1999) The p38 MAP kinase pathway signals for cytokineinduced mRNA stabilization via MAP kinase-activated protein kinase 2 and an AU-rich region-targeted mechanism. EMBO J. 18, 4969–4980 Bulavin, D.V. (1999) Phosphorylation of human p53 by p38 kinase coordinates N-terminal phosphorylation and apoptosis in response to UV radiation. EMBO J. 18, 6845–6854 Dang, P.M. et al. (2003) Phosphorylation of the NADPH oxidase component p67(PHOX) by ERK2 and P38MAPK: selectivity of phosphorylated sites and existence of an intramolecular regulatory domain in the tetratricopeptide-rich region. Biochemistry 42, 4520–4526 Behr, T.M. et al. (2003) p38 Mitogen-activated protein kinase inhibitors for the treatment of chronic cardiovascular disease. Curr. Opin. Invest. Drugs 4, 1059–1064 Miwatashi, S. et al. (2005) Novel inhibitor of p38 MAP kinase as an antiTNF-a drug: discovery of N-[4-[2-ethyl-4-(3-methylphenyl)-1,3-thiazol-5yl]-2-pyridyl]benzamide (TAK-715) as a potent and orally active antirheumatoid arthritis agent. J. Med. Chem. 48, 5966–5979
Vol. 3, No. 1 2006
Editors-in-Chief Raymond Baker – formerly University of Southampton, UK and Merck Sharp & Dohme, UK Eliot Ohlstein – GlaxoSmithKline, USA DRUG DISCOVERY
TODAY THERAPEUTIC
STRATEGIES
Immunological disorders and autoimmunity
p38 MAP kinase inhibitors: A future therapy for inflammatory diseases Ruth J. Mayer*, James F. Callahan GlaxoSmithKline Pharmaceuticals, Respiratory and Inflammation CEDD, P.O. Box 1539, King of Prussia, PA 19406, USA
p38 mitogen-activated kinase is a target for therapeutic intervention in inflammatory diseases. The pharmacology of p38 inhibitors is reviewed briefly, and
Section Editors: Claudine Bruck – GlaxoSmithKline, King of Prussia, USA Michel Goldman – University of Brussels, Brussels, Belgium
potential other therapeutic targets in this important signaling pathway are outlined. The evaluation of p38 inhibitors in early clinical studies is reviewed with emphasis on pharmacodymanic assessment. Ex vivo and in vivo lipopolysaccharide (LPS) challenges have
better kinase selectivity and potency, clinical evaluation has been more promising. The past few years have seen positive Phase II studies in inflammatory diseases such as rheumatoid arthritis and psoriasis.
been successful in Phase I studies and will be of value in the early assessment of future p38 inhibitors.
Introduction Inhibitors of p38 mitogen-activated protein kinase (MAPK) were first identified in the late 1980s, although the target was not yet identified. Fig. 1 shows the current understanding of the p38 pathway and identifies the principal components of the signaling cascade. P38 MAPK is a central point of activation in response to a wide variety of stimuli that subject cells to stress, as detailed below. With identification of p38a as the target for a class of compounds with broad, potent anti-inflammatory activity [1], the efforts to identify an effective therapeutic have increased dramatically. At least a dozen p38 inhibitors have been evaluated in clinical trials, with very few progressing beyond Phase I. Early compounds have been associated with elevations in liver function enzymes (LFTs, transaminases), as well as CNS effects characterized as dysphoria [2–5]. As compounds were identified with improved absorption–distribution–metabolism–excretion (ADME) properties, as well as *Corresponding author: R.J. Mayer ([email protected]) 1740-6773/$ ß 2006 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddstr.2006.03.003
Potential disease indications for p38 inhibitors A large number of diseases have been considered as indications for a p38 inhibitor, all having an inflammatory component. The list includes rheumatoid arthritis (RA), inflammatory bowel disease (IBD) or Crohn’s disease, atherosclerosis, chronic obstructive pulmonary disease (COPD), severe asthma and psoriasis. To date, Phase II studies have been completed in rheumatoid arthritis and psoriasis, with encouraging results as detailed below.
p38 MAP kinase The tremendous interest in p38 MAPK as a therapeutic target for inflammatory diseases stems from an ever-growing body of data demonstrating the importance of the p38 pathway in the cellular response to inflammatory stimuli and the very broad efficacy of p38 inhibitors in preclinical animal models of disease. The relevant target is considered to be p38a, one of the four isoforms of the enzyme (MAPK14, MAPK11, MAPK13, MAPK12 or p38a, b, d, g, respectively). P38a is found as the predominant isoform in leukocytes, epithelial cells, smooth muscle cells, whereas p38d is more highly expressed in macrophages and p38g in skeletal muscle [6]. At this time, all inhibitors under investigation are dual p38a/b 49
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Vol. 3, No. 1 2006
Figure 1. The p38 MAPK signaling cascade. Upstream activators include apoptosis signal-regulated kinase (ASK1) and the mitogen-activated protein kinase (MKKs or MEKs); downstream effectors include mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK2) and other MAPKAPs. The c-jun N-terminal kinase (JNK) and extracellular signal-regulated (ERK) MAPK pathways are related functionally to the p38 pathway. Additional details on the pathways can be found at http://www.biocarta.com.
inhibitors. A p38b / mouse has recently been described [7]. The lipopolysaccharide (LPS)-induced cytokine response in these mice is normal, suggesting again that p38a is the major target for the known inhibitors. The p38 pathway (Fig. 1) is activated in response to cytokines including tumor necrosis factor-a (TNFa) and transforming growth factor-b (TGFb), UV light, osmotic stress, oxidative stress and microbial pattern recognition (Toll receptor engagement) [6]. Activation of the pathway through surface receptors or cellular sensing mechanisms results in phosphorylation of MEKKs, MEKs and p38 MAPK in sequence. In turn, p38 phosphorylates downstream kinases such as PRAK (MAPKAPK5) or MAPKAPK2, which directly phosphorylate several transcription factors and heat shock proteins (see Related articles). A list of some of the relevant functions or genes modulated by the p38 pathway is given in Table 1.
The kinase selectivity of p38 inhibitors has been generally addressed by cross-screening against several available kinases, and more recently by using phage-display approach to evaluating binding affinity to over 100 kinases [8]. BIRB-796 and VX-745 were part of the panel evaluated in this recent profiling exercise, and showed remarkable selectivity, with over tenfold tighter binding to p38 than any other kinase evaluated. However, several kinases, both in the Tyr kinase and Ser/Thr kinase families were inhibited by all of the p38 inhibitors evaluated, with the c-jun N-terminal kinase (JNK) family members most often inhibited. No common pattern emerged, suggesting that each p38 inhibitor would have a distinct profile. One of the tool compounds evaluated in this profile was SB203580, which has been used as a prototypic p38 inhibitor in a large number of studies to characterize the role of p38 in cellular processes. SB-203580 has been reported to inhibit
Table 1. Some key functions of p38a or the p38 kinase pathway Cellular function
Mechanism
Refs
Cytokine and chemokine expression (e.g. TNFa, IL-1b, IL-6)
Modulation of mRNA levels containing an AU-rich region
[6,35]
Cell survival (e.g. UV stress)
Phosphorylation of p53
[28,36]
TGFb signaling
Activated downstream of receptor
[19]
Neutrophil function (e.g. superoxide burst)
Regulation of surface receptor expression Assembly of NAPDH oxidase
[37]
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Drug Discovery Today: Therapeutic Strategies | Immunological disorders and autoimmunity
JNKs, as well as p38, and some additional targets in the epidermal growth factor receptor pathway were identified. Mechanistic studies with SB-203580, therefore, must be interpreted carefully and in some cases repeated with more selective compounds or genetic knockdown methods.
Preclinical evaluation of p38 inhibitors P38 inhibitors have been evaluated preclinically in a wide range of disease models, with good efficacy demonstrated in many models. In the collagen-induced arthritis or rat adjuvant arthritis models of RA, p38 inhibitors have been shown to reduce the arthritis score, to reduce the expression of cytokines and chemokines such as IL-1b, TNFa and KC (murine analog of IL-8), and to block the loss of bone density and the expression of bone degradation markers such as RANK-L [9,10]. In a rat model of cardiovascular disease, p38 inhibitors protect against cardiac hypertrophy, and end-organ damage in spontaneously hypertensive rats fed a high-salt, high-fat diet [11]. In a second model of cardiovascular disease, a p38 inhibitor offered protection against cardiac remodeling 12 weeks following induction of myocardial infarction in mice [12]. In models of COPD, p38 inhibitors block neutrophil influx into the lung, as well as reduce cytokine levels in the bronchoalveolar lavage [13,14]. p38 inhibitors have also been reported to be efficacious in models of pain, IBD and asthma [15–17].
What will a p38 inhibitor deliver? Although there is extensive preclinical data on p38 inhibitors, successful clinical proof-of-concept studies are still very few in number. Based on the preclinical data, a typical p38 inhibitor would be expected to reduce levels of proinflammatory cytokines and chemokines and reduce cellular infiltration to sites of inflammation, thereby reducing local damage. In diseases such as RA and IBD, TNFa blockade through either anti-TNFa antibodies or use of soluble TNFa receptors has been shown to be an effective therapy [18]. Inhibition of p38a offers the potential of significant inhibition of TNFa, as well as inhibition of cytokines such as IL-1b and IL-6, which would be expected to offer additional therapeutic efficacy. In diseases such as cardiovascular disease or COPD, similar data on the role of cytokines is lacking. In addition to these well-studied roles in the inflammatory response, p38 inhibition might be expected to modulate the remodeling process that occurs with resolution of inflammation, through the role of p38 in TGFb signaling [19].
available. Inhibition of kinases has been considered to be of high risk owing to the concerns related to selectivity and the crucial regulatory role of many kinases in cell-signaling pathways. The profiling of kinase inhibitors including p38 inhibitors has interestingly not identified the potent inhibition of any unexpected off-target activity of known p38 inhibitors. Recent compounds are likely to be even more selective, suggesting that safety might not be significantly affected by lack of selectivity. Nonetheless, inhibition of p38 still has the potential for mechanism-based toxicity, especially at high levels of inhibition. Full reports on toxicology of p38 inhibitors have yet not appeared, but given the number and range of cellular events under the regulatory control of p38 MAPK, it is very probable that the therapeutic index, or ratio between desirable and undesirable effects, will be relatively small [20].
Compounds in development Several compounds of interest that have been in recent clinical development are listed in Table 2, with the primary indications and phase of development achieved. Structures, where known, are provided in Fig. 2. The current status of any of these compounds is uncertain, and several might have been already discontinued. In addition to these compounds, several compounds have been studied previously with instructive results, although various concerns prevented further development. Results with VX-745 in a study of 44 patients with RA were reported [4] to significantly improve ACR20 response (43%, p = 0.04), following 12 weeks of dosing. Treatment-related elevation in LFTs was reported in six patients on VX-745 versus none on placebo, following 4 weeks of dosing. This first positive proof-of-concept study with a p38 inhibitor has lent confidence to the further development of p38 inhibitors for RA as well as other diseases.
Phase I studies and pharmacodynamic assessment SB-242235 was shown to dose-dependently suppress ex vivo production of cytokines in response to LPS stimulaTable 2. Compounds reported recently in clinical development Compound
Key indication
Phase achieved
Refs
VX-702
RAa, angina
II
[26]
BIRB-796
Psoriasis, RA, Crohn’s
IIb/III
[38]
b
SCIO-469
RA, IBD
Safety considerations
RWJ 67657
Discontinued
There are both theoretical and known safety concerns with p38 inhibition. Several reports of transient elevations of LFTs following repeat dosing have raised concerns over long-term hepatotoxicity, although information on the clinical safety profile of more recent compounds in development is not
TAK-715
RA
SB-681323
II
[38] [38]
c
RA, COPD , athero
II
[39]
I/II
[38]
a
RA: rheumatoid arthritis. IBD: irritable bowel disease. c COPD: chronic obstructive pulmonary disease. b
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Figure 2. Structures of p38 inhibitors of interest in clinical development. See Table 2 for additional information.
tion of whole blood, providing one approach to pharmacodynamic evaluation of p38 inhibition [21]. A similar study was reported [22] for another compound, RWJ 67657, with similar conclusions, suggesting that ex vivo TNF inhibition is an adequate method for an early assessment of efficacy. Early studies with BIRB-796 utilized several approaches to pharmacodynamic evaluation, including ex vivo LPS-stimulated cytokine production, ex vivo stimulation of neutrophil surface markers and in vivo LPS challenge [3,23]. The biochemical endpoints evaluated for the in vivo LPS challenge included phosphorylation of p38, ex vivo p38 activity assay, several cytokines, CD11b surface expression and plasma Creactive protein (CRP) [23]. All of these measures were sensitive to doses of 50 mg BIRB-796. Again, a similar study design was used in the evaluation of RWJ-67657, showing both inhibition of cytokines including TNFa, IL-6 and IL-8 up to 90%, as well as surface expression of ICAM-1 and L-selectin, confirming these endpoints as potential markers for p38 inhibitory activity [24,25]. 52
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Phase II studies Following the successful Phase IIa study with VX-745, several other compounds have been reported to have entered Phase II studies of significant size (Table 2). None of these studies have reported data as yet. A study with BIRB-796 in psoriasis at doses up to 30 mg b.i.d. was reported to be positive in a press release, but no data have appeared. A very limited Phase IIa study with VX-702 in acute coronary syndrome reported inhibition of the increase in C-reactive protein following percutaneous coronary intervention (PCI). VX-702 was dosed 2 days prior and 3 days post-PCI, and decreased CRP significantly across all dose groups [26]. Additional Phase II studies are in progress.
Alternate approaches to p38 pathway inhibition Inhibition of p38 MAP kinase is not the only mechanism under consideration for modulation of the p38 pathway (Table 3). Mitogen-activated protein kinase (MAPK)-activated protein kinase (MAPKAP-K2 or MK2), which is a downstream kinase implicated in many of the effector functions of p38 kinase [27,28], has also been evaluated as a target and has
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Drug Discovery Today: Therapeutic Strategies | Immunological disorders and autoimmunity
Table 3. Approaches to inhibition of p38 MAP kinase pathway Target
Pros
Cons
Latest developments
Who is working on this approach
p38
Chemistry well established, biology well documented
Safety and toxicity concerns
Several compounds have reached Phase II
Many pharma and biotech
MAPKAPK2a
None known, possibly better safety
Possibly reduced efficacy
Preclinical
BMSb, Pfizer Amphora, Teijin
ASK1c
Upstream inhibition of both p38 and JNKd pathways, activated by oxidative stress
Safety considerations unknown for inhibition of both p38 and JNK; importance of oxidative stress unclear
Emerging target
Early stages
Kissei a
MAPKAPK2: mitogen-activated protein kinase-activated protein kinase. BMS: Bristol Myers Squibb. ASK-1: apoptosis signal regulating kinase. d JNK: c-jun N-terminal kinase. b c
generated some patent activity [29]. A MAPKAP-K2 knockout mouse (which is viable, in contrast to a p38a knockout) has been studied in disease models. MK2 / mice subjected to focal ischemia demonstrated reduced infarct size compared to wild-type mice [30]. However, reduced TNFa expression was not observed in these mice, which is somewhat surprising. Another study with MK2 / mice demonstrated protection from ischemia-reperfusion injury [31]. These results might not be solely owing to the role of MAPKAPK2 because levels of p38a are significantly reduced in MK2-deficient tissues as these two kinases form a stabilizing complex [32]. A relatively small degree of inhibition of MK2 might be efficacious, however, owing to destabilization of p38a. Another member of the MAP kinase family that is of emerging interest is apoptosis-signal-regulating kinase, or ASK-1 (MAP3K5) [33]. This kinase is upstream of both the p38 and JNK MAPKs [34], and is regulated in response to changes in oxidation state of thioredoxin. Interest lies in the dual pathway role and perhaps, more focused role in activation of MAPKs by sensing oxidative stress. There is recent patent activity in this area as well, although this target is by no means as well characterized as either p38a or MAPKAPK2.
several diseases, but are not desirable for long-term oral therapy owing to the side effect profile. The potential for anti-inflammatory therapy with statins might not yet have been realized, and they have been shown to be safe for longterm therapy. So, the medical need for additional mechanisms to treat inflammatory diseases remains. Since the mid-1990s, several p38 inhibitors have entered clinical studies, and the clinical development of almost all of these compounds has been discontinued. The full reasons for the attrition rate are not known. Sufficient positive data have been generated in Phase I or IIa studies to maintain industry interest in development of p38 inhibitors despite the challenges. Preclinical data continue to support important roles for p38a in several diseases. Alternative mechanisms within the p38 pathway, such as ASK1, pose similar problems and will also be slow to reach clinical efficacy studies. As our understanding of the importance of inflammation in chronic disease continues to be refined, other alternatives for effective anti-inflammatory treatments might emerge. The near future will demonstrate whether compounds currently in development have the appropriate properties for long-term development for chronic disease.
Conclusions The potential value and therapeutic importance of potent, safe and effective anti-inflammatory drugs are very high. Current anti-inflammatory therapy includes NSAIDs, corticosteroids and statins. Nonsteroidal anti-inflammatories or NSAIDS, although highly effective for acute treatment, have not been shown to be disease modifying for long-term treatment of, for example, RA. Corticosteroids are effective in
Links http://stke.sciencemag.org/cgi/cm/stkecm;CMC_826 (Kinase knowledge site maintained by Science) http://www.biocarta.com/pathfiles/h_mapkPathway.asp (Complete pathway information for the MAPK pathways)
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