Current Drug Targets - Volume 8, Issue 2, 2007

In recent years, the dramatic progress in basic cancer research has finally reached clinical practice. Five years after the introduction of imatinib for the treatment of Bcr-Abl driven diseases, several novel targeted treatment modalities have been introduced into clinical medicine. For various reasons, hematological malignancies remain amongst the most promising candidates to be successfully treated by such modalities. This Theme Issue, entitled “Highlights on Important Signaling Pathways in Hematological Malignancies” describes some of the reasons, why this hope for our patients is realistic, especially for those suffering from Ph-negative myeloproliferative disorders. Here, major breakthroughs have been the discovery of various activating tyrosine kinases, including the most recent finding of a single, recurrent point mutation in the Janus kinase 2 (JAK2) in the large majority of patients with polycythemia vera, but also in about 50 % of patients with essential thrombocythemia and idiopathic myelofibrosis. Among other questions, Reiter and co-workers address this surprising result of a single mutation causing a variety of related diseases in their description of tyrosine kinases as therapeutic targets in Bcr-Abl negative chronic myeloproliferative disorders. However, the future will not only be bright. We will also have to face that the astounding success of imatinib in the treatment of chronic myeloid leukemia will not be easily copied to the other, often more complex, fully malignant and multigenic hematological malignancies. Here, it will be important to identify the pathways into the disease, to define their contribution to the initiation, progression and maintenance, to define molecular targets, and finally to combine specific therapeutic modalities with our cytotoxic armamentarium. It Is the second purpose of this Theme Issue to describe recent progress along these lines. In a comprehensive review, Morgan et al. thoroughly describe the Ras-signaling pathway and ways of therapeutic interference with this pathway by inhibition of the mevalonate pathway in malignant hematological diseases, including farnesyl transferase inhibitor (FTI) treatment. The mevalonate pathway as a novel therapeutic target in polycythemia vera and related diseases is separately described by Hasselbalch & Riley who focus on the potential beneficial effects of statins and zoledronic acid in these disorders. This novel concept is based upon in vitro and in vivo studies showing that statins - in addition to the well-known cholesterol lowering effect - display anticancer potential as evidenced by their antiproliferative, proapoptotic, and antiangiogeneic properties. Furthermore, statins also have antithrombotic effects. All these drug effects may be beneficial in clonal myeloid diseases that feature myeloproliferation, myeloaccumulation (decreased apoptosis), increased angiogenesis and thrombotic complications.........

Acquired constitutive activation of protein tyrosine kinases is a central feature in the pathogenesis of chronic myeloproliferative disorders (CMPDs). The most commonly involved genes are the receptor tyrosine kinases PDGFRA, PDGFRB, FGFR1 or c-KIT and the non-receptor tyrosine kinases JAK2 and ABL. Activation occurs as a consequence of specific point mutations or fusion genes generated by chromosomal translocations, insertions or deletions. Mutant kinases are constitutively active in the absence of the natural ligands resulting in deregulation of haemopoiesis in a manner analogous to BCR-ABL in chronic myeloid leukaemia. With the advent of targeted signal transduction therapy with tyrosine kinase inhibitors, an accurate diagnosis of CMPDs by morphology, karyotyping and molecular genetics has become increasingly important. Imatinib induces high response rates in patients associated with constitutive activation of ABL, PDGFRα , PDGFRβ and some KIT mutants. Other inhibitors under development are promising candidates for effective treatment of patients with constitutive activation of JAK2, FGFR1 and imatinib-resistant KIT mutants.

Molecularly targeting signaling pathways that are involved in the pathogenesis of hematopoietic malignancies may lead to more specific and efficacious therapies. Activation of the RAS signal transduction cascade is a common feature in the molecular pathogenesis of hematologic malignancies. A number of novel agents targeting RAS signaling have been developed over the past decade. This review will focus on these agents, which include inhibitors of RAS post-translational modification (farnesyl transferase (FTase)-, geranylgeranyl transferase-I (GGTase-I)-, isoprenylcysteine carboxylmethyltransferase (ICMTase)-inhibitors, statins, bisphosphonates), and inhibitors of RAF and MEK activity. Although some of these inhibitors (e.g. FTase, RAF and MEK inhibitors) were developed to specifically inhibit RAS signaling, it has become clear that RAS may not be the only critical target of these compounds. This review provides a background on RAS signaling in hematologic malignancies and discusses opportunities to exploit aberrant cancer cell signaling in order to develop better treatment options for patients suffering from these diseases.

Acute myeloid leukemia (AML) is a malignant disease of the bone marrow. Despite intensive treatment only one third of AML patients are cured. Numerous genetic events have been identified in the last years that have shed light into the mechanisms dictating increased self-renewal, proliferation, survival and block in differentiation. It is increasingly recognized that AML represents a hierarchical disease, originating from a leukemia stem cell population. Sophisticated animal models have helped to elucidate rate-limiting steps in initiation, development and maintenance of AML. This review discusses the fundamental genetic events identified to date that determine the pathogenesis of AML, with particular emphasis on the oncogenic signaling events that result from translocation of myeloid transcription factors and mutations in receptor tyrosine kinases. Our current understanding of the biology of AML has fueled the development of promising anti-leukemic agents, which may improve the treatment of the disease in the future.

The Ph-negative chronic myeloproliferative disorders (CMPDs) polycythaemia vera, essential thrombocytosis and idiopathic myelofibrosis are acquired stem cell disorders, which pathophysiologically are featured by clonal myeloproliferation and accumulation of myeloid cells, the latter being consequent to decreased apoptosis. Myelofibrosis and neoangiogenesis in the bone marrow and spleen are the histopathological hallmarks of idiopathic myelofibrosis but may develop in the other diseases as well. In patients with myelofibrosis elevated levels of circulating CD34+ cells are highly characteristic being partly explained by a proteolytic bone marrow mileu owing to excessive release of various proteases with ensuing extracellular matrix degradation and constitutive mobilisation of CD34+ cells into the peripheral blood. Thrombohaemorrhagic complications are major clinical problems contributing significantly to morbidity and mortality. Based upon in vitro and in vivo studies of the effects of statins (antithrombotic, antiproliferative, antiangiogenic, antiproteolytic) and zoledronic acid (antiproliferative antiproliferative, antiangiogenic, antiproteolytic) this review focusses on the translation of these effects into potential clinical benefits of combinational therapy with these agents in patients with CMPDs.

Angiogenesis is defined as formation of new blood vessels from the preexisting vasculature, a process which is essential for malignant tumor growth. While this has been accepted for solid forms of cancer there is now emerging evidence that progression of hematological malignancies also requires the induction of new blood vessels. Vascular endothelial growth factor (VEGF) is known to be an essential regulator of physiological and pathological angiogenesis. Numerous preclinical and clinical studies have validated VEGF as target for antiangiogenesis and anticancer therapy. With regard to hematological malignancies a stimulating effect of VEGF for proliferation, survival and migration of leukemia cells could be demonstrated. Bone marrow of leukemia patients shows an increased microvessel density as well as VEGF expression. Complete remissions in acute myeloid leukemia (AML) have been reported by targeting the receptor tyrosine kinase system of VEGF. While the pathophysiology behind the contribution of VEGF to leukemia progression is not yet completely understood, VEGF and its receptors may provide promising targets not only in solid tumors but also hematological malignancies such as AML.

The more the baby-boomer generation ages, the more patients will be suffering from chronic, degenerative diseases, like osteoarthritis. This means they will experience pain, swelling, weakness and loss of functional ability in their diseased joints. These symptoms will progress to significantly impact the quality of life and the productivity of the affected people. In the US Centers for Disease Control report from 2002 “Prevalence of self-reported Arthritis of chronic joint symptoms among adults ” United States, 2001” the prevalance of arthritis/chronic joint symptoms were given as 33%, representing 69.9 million adults. This translates directly into a significant economic burden, which comprises both the direct costs (diagnosis, hospitalization, therapies) and the indirect costs including premature mortality, chronic and short-term disability and not to forget losses of productivity. Not surprisingly, indirect costs far exceed direct costs. Despite tremendous efforts to understand pathomechanisms of osteoarthritic joint changes both at the level of basic and at the level of applied research, the major unmet medical need for osteoarthritis remain disease-modifying agents and better diagnostic tools. With the COX-2 inhibitors we thought to have alternatives to NSAIDs with their gastrointestinal side effects to treat at least symptoms of osteoarthritis, but the withdrawal of rofecoxib (Vioxx; Merck) in September 2004 and the subsequent discussion on a class effect of coxibs in respect to cardiovascular risks, depredated us of our illusions. But did not biology advance tremendously in recent years with all its genomics, proteomics, metabolomics and other ‘omics’ to show hundreds of new molecules to explain the academic researchers the intricacies of disease pathways and at the same time to quench the thirst of the industry researcher for novel drug targets? Reading through the collected reviews, a tremendous amount of knowledge on individual targets has been gained. But quite notably, an other notion emanates from most of the reviews: Understanding individual targets does not seem to be sufficient to develop effective therapeutic strategies. Roach et al. set the stage by describing OA as a disease of an organ system, implying that a true understanding of the disease must eventually be based on ‘molecular portraits’ of the affected tissues including articular cartilage, bone, synovium, capsule, ligaments and muscles. Burrage and Brinckerhoff summarise our understanding of apparently the most obvious drug targets for OA, the proteases that are directly responsible for the degradation of the cartilage matrix components. Although some very attractive targets (MMP-13, ADAMTS aggrecanases and TACE) are known, they speculate, that we need to follow the concept of “molecular polypharmacy” i.e. to target several proteases at the same time to achieve clinical effects. Salminen-Mankonen et al. report on Cystein cathepsins, most notably on Cathepsin K, whose role as a protease responsible for degradative events in bone and in cartilage earns more attention than in the past, although here the development of tissue specific inhibitors poses problems. Turning to the level of communication between cells, Bloom et al. point towards cytokines with IL-1 inhibition shown to ameliorate OA-like pathology and TGFb providing an approach to promote cartilage integrity and repair, but also in the context of a combination therapy. Our hopes to find therapeutic approaches at the level of regulation of the MAPK and NFkB signaling pathways are dampened by Saklatvala with the arguments that it is not clear which pathway would have to be blocked for most effective MMP suppression and what effects it may have on normal cartilage turnover to block these central pathways.......

Osteoarthritis, a degenerative joint disease, is the most disabling condition of the Western world. It affects first and foremost the articular cartilages and leads to a molecular and supramolecular destruction of the extracellular cartilage matrix. In addition, the cells, the chondrocytes, show severe alterations of their phenotype: they get anabolically and catabolically activated, change accordingly their gene expression pattern, lose their differentiated phenotype, and undergo focally cell death and cell degeneration. All these processes represent potential targets for therapeutic intervention and drug development. Apart from the cartilage itself, however, other joint tissues are also involved in the disease: thus, the synovial capsule and membrane as well as the subchondral bone account not only for most of the symptoms of the disease, but are also presumably involved in the progression of the degenerative process. Both, inflammation and stiffening within the joint capsule accelerate joint destruction. Stiffening of the subchondral bone increases the mechanical stress over the overlying cartilage during physiological movement. Altogether, there is a plethora of tissues, disease processes and targets for treating osteoarthritic joint degeneration, which will need to be followed up systematically in the future.

Cytokines are involved in osteoarthritis (OA) at several levels. They are involved in primary cartilage damage, but also in synovial activation that is observed in osteoarthritic joints. From in vitro studies and animal models for OA, several cytokines have been identified that are potential targets for OA therapy. Two promising targets are the destructive cytokine Interleukin-1 (IL-1) and the anabolic growth factor transforming growth factor (TGF)β and these will be discussed in more detail. Inhibition of IL-1 has been proven to result in amelioration of osteoarthritis-like pathology in animal models and the role of IL-1 is substantiated in studies in IL-1 deficient mice. In contrast, application of the anabolic growth factor TGFβ may provide an alternative approach to promote cartilage integrity and repair. TGFβ is a potent stimulator of chondrocyte matrix production, and therefore has a potency to repair already damaged cartilage. However, TGFβ induces tissue fibrosis and osteophytes at the joint margins and can only be applied to promote cartilage repair when these side effects can be blocked. This appears possible with concomitant, compartmentalized application of selective inhibitors of TGFβ in soft tissues, using local gene therapy with inhibitory Smad 6 and 7. Since OA is often limited to a few joints, local gene therapy may provide a suitable way to treat OA patients. Depending on the phenotype of a particular OA patient, e.g. with or without marked synovial activation, treatment may be focused mainly on suppression of catabolism or stimulation of anabolism, but combination therapy seems most warranted.

The debilitating destruction of joint tissues seen in osteoarthritis (OA) is due, in large part, to the degradative activity of metalloproteinase (MP) enzymes that target extracellular matrix (ECM) components within articular cartilage. Although successful in suppressing the pain and inflammation associated with this disease, conventional OA therapeutics do not inhibit the underlying tissue catabolism, allowing the disease to progress into irreversible ECM loss and chronic disability. Therapeutic inhibition of metalloproteinase activity is not a new concept, however, its transfer into clinical use has been frustrating. Disappointing results from clinical trials with small molecule inhibitors of metalloproteinases have highlighted the critical importance of inhibitor specificity, and the need to identify the individual metalloproteinases responsible for joint destruction. We discuss strategies of inhibition using small molecule inhibitors and tissue inhibitors of metalloproteinases (TIMPs) engineered to increase inhibitory specificity, and present new data using of new reagents such as ribozymes and inhibitory RNAs that repress expression of specific enzymes. Recent data has implicated the disease stagedependent involvement of matrix metalloproteinase-1, -2, -3, -9, -13, ADAM-17/TACE (tumor-necrosis factor-α converting enzyme), and ADAMTS-5 (a disintegrin and metalloproteinase with thrombospondin 1 motifs) as major in vivo mediators of the ECM degradation seen in OA, and as such, they represent promising therapeutic targets. We conclude that the concept of molecular polypharmacy, in which the relevant enzymes are selectively targeted with multiple directed therapies, may offer a new therapeutic strategy that prevents joint destruction and minimizes toxicities.

Osteoarthritis is characterised by degeneration of articular cartilage. It is thought to be primarily a disease of cartilage. Inflammatory response genes, such as proteinases, cyclooxygenase, and cytokines are implicated in its pathogenesis. The evidence for expression of these genes in articular cartilage in osteoarthritis is reviewed. The expression of inflammatory response genes is controlled by four major intracellular signalling pathways. These lead to activation of the three mitogen-activated protein kinases (MAPK) and the transcriptional regulator nuclear factor kappa (NFκ )-B. The current state of knowledge of the structure of these pathways is summarized. Pharmacological inhibitors of the protein kinases of the pathways in current use are described, and insights into chondrocyte gene expression obtained with them are discussed. Very limited use of these inhibitors has yet been made in animal models of osteoarthritis. The main use of the inhibitors in the near future will be in investigation of pathogenetic mechanisms in osteoarthritis, both in experimental animals and in vitro, with a view to identifying therapeutic targets. Prospects for using signalling pathway inhibitors for therapy in osteoarthritis are distant.

Cysteine cathepsins are a large family of proteolytic enzymes active at acidic pH as found in lysosomes. Since its discovery in 1990's, cathepsin K has been shown to be a key enzyme in osteoclastic bone resorption through its activity in the resorption lacuna. Although characteristic to osteoclasts, the expression of cathepsin K has also been observed at other sites in skeleton. Several recent observations have demonstrated up-regulation of cathepsin K in osteoarthritic cartilage and inflamed synovial tissue. As cathepsin K is one of the few extracellular proteolytic enzymes capable of degrading native fibrillar collagen, it may play an important role in the progressive destruction of articular cartilage both in osteoarthritis and in inflammatory arthritides. Also transgenic mouse models have provided evidence supporting the important role of cathepsin K in both groups of arthritides. The aim of this chapter is to review the accumulating evidence for the role of cathepsin K in degradation of articular cartilage regardless of its pathogenic background, and to discuss the potential efficacy of cathepsin K inhibitors to slow down or prevent articular cartilage degradation.

It has been known for a very long time that aging is the most prominent risk factor for the initiation and progression of osteoarthritis. This might be related to continuous mechanical wear and tear and/or result from time/age-related modifications of cartilage matrix components. Also a mere loss of viable cells over time, due to apoptosis or any other mechanism, might contribute. More recent evidence, however, supports that stressful conditions for the cells might promote chondrocyte senescence and might be in particular important for the progression of the osteoarthritic disease process. One of the most important implications of this hypothesis is that it points to issues of cellular degeneration as the basis for understanding of the initiation and the progression of osteoarthritis. Equally important, it emphasizes that whatever treatment we envisage for osteoarthritis, we must take into account that we are dealing with aged/(pre) senescent cells which no longer have the abilities of their juvenile counterparts to respond to the many mechanical, inflammatory, and traumatic assaults to the tissue. Thirdly, this directs treatment options to deal with the senescence of cells, which are only conceptually available today. Clearly, if accumulation of wear and tear over time is the major scenario of osteoarthritis, any therapy will largely be hopeless as moving and loading the joints is unavoidable as implication of their use. However, this review intends to open up the idea that age-related changes are less a fate, but rather a challenge for therapeutic intervention which can be taken.

Osteoarthritis (OA) is the most common chronic joint disease in the elderly population, causing significant pain and disability. Because the cardinal feature of OA is a progressive loss of articular cartilage, a great portion of the research endeavour into the pathogenesis of OA has been focused on the regulation of matrix synthesis and degradation. The phenotypic stability and survival of the chondrocytes are essential for the maintenance of a proper cartilage matrix. This has lead to the long-standing assumption that cell death is a central feature in OA cartilage degeneration. The important role of apoptosis in OA has been demonstrated in in vitro and in vivo models. However, it should be noted that the relative contribution of apoptotic cell death in the pathogenesis of OA is still difficult to assess because of the chronic nature of the disease process. Therefore, the apoptosis of chondrocytes seems to be a potential target for therapeutic interventions in OA. The death receptor, mitochondrial and endoplasmic reticulum pathways are the major cellular pathways of apoptosis. Of all these elements involved in the apoptosis of chondrocytes, caspase inhibition has been studied with the most detail. Other molecules with the capacity to modulate mitochondria function, phosphatase (PP-1A/B) activity and pro-apoptosis stimuli (NO, prostaglandins, cytokines, ROS) could be excellent targets to block apoptosis of chondrocytes. Finally, the regulation of the natural inhibitors of apoptosis (c-FLIP, BAR, ARC and HC-gp39) could complement the other strategies to reduce cartilage degradation.

Osteoarthritis is one of the most common chronic diseases that causes pain and physical disability in patient. Although OA is considered as a global disease affecting all joint tissues, cartilage degradation is the end point. The degradation of cartilage results of the combination of mechanical stress and biochemical factors, mainly metalloproteinases and reactive oxygen species (ROS). The activity of reactive oxygen species is balanced by enzymatic and non-enzymatic antioxidants, that act by inhibiting oxidative enzymes, scavenging free radicals or chelating ion metals. Until now, few information is available on the antioxidative status of chondrocytes. Further, the modification of the antioxidative system in osteoarthritis remains unknown. Some antioxidant supplements or drugs with antioxidant properties have been developed to reinforce the cellular antioxidant status. However, until now, there is no consistent evidence that additional antioxidant supply is efficient to relieve OA symptoms or to prevent structural changes in OA cartilage.

While a great deal of information is available on the cellular and molecular biology of cartilage degradation, less is known about anabolism in normal cartilage and degenerating cartilage. A consistent amount of evidence is now available on the neo-synthesis of matrix molecules and enzymes in OA: the entire cell metabolism appears to be increased leading to the hypothesis that chondrocytes in OA are actually “activated”. This chapter will focus on anabolic events that are now known to occur in articular cartilage. We will begin to view articular cartilage as a complex three-dimensional tissue in which local events may be different. We will also be interested in viewing the development of degenerative arthritis as a continuum from functionally normal tissue to degeneration.

Osteoarthritis (OA) is a progressive disease of cartilage degradation that significantly impacts quality of life. There are currently no effective treatments and, while a large number of potential therapeutic targets exist, most have not been validated in vivo. The range of OA models in the mouse has dramatically expanded in the last decade, beyond spontaneous models, to include genetically modified transgenic, knockout (KO) and knock-in (KI) mice that can develop premature cartilage degeneration reminiscent of OA. In addition, instability models of OA, either induced by intra-articular (IA) collagenase or surgery, are providing a set of tools to assist in the identification of disease-modifying OA drug (DMOAD) targets. These models are now vital tools to dissect the pathways essential to the pathogenesis of OA. Two targets, ADAMTS (a disintegrin and metalloproteinase with thrombospondin-like motifs)-5 and IL-1 β(interleukin-1 beta), have been validated in the surgical destabilization of the medial meniscus model (DMM) in KO mice. Other potential targets evaluated in instability models, either showed no disease modification or a worsening of disease, suggesting that those targets have no role, a protective role or that other, more destructive enzymes etc., can overcompensate. Development of small molecule or protein antagonist inhibitors of therapeutic targets require many years to bring to clinical trials and often confront potency and safety issues which impede successful progress. Validation, or confirmation of therapeutic targets in vivo is most clearly and efficiently obtained by using KO studies, than by creating potent and selective DMOADs to multiple potential targets. While the results in the mouse will not always transpose to the human condition, the track record of mouse knockouts corresponding to the human phenotype have been excellent. These results indicate that the evaluation of genetically modified mice will become increasingly important as we unravel the genes contributing to OA. 8,9].

Gene expression analysis including large scale gene expression profiling has become a very basic tool for investigating the pathogenesis of degenerative joint diseases as well as for the search of new drug targets. However, gene expression analysis so far revealed very complex expression patterns rather than a clear picture of molecular changes occurring during the initiation and progression of the disease. To elucidate the molecular changes in osteoarthritis the analysis of the fetal growth plate as a developmental model for phenotypic changes in chondrocytes occurring in osteoarthritis can help in three ways: it allows to interpret gene expression patterns in the context of disease-relevant processes also occurring in developing cartilage (e.g. cell differentiation, proliferation, matrix synthesis, catabolism and calcification), it offers the chance to investigate gene function in these functional contexts by knocking out or overexpressing genes in animals, and it provides a suitable model for testing the effect of therapeutic compounds on these processes within the growing cartilage.