Current Drug Targets - Volume 7, Issue 10, 2006

Myocardial infarction caused by thrombotic occlusion of a coronary artery represents the most dreaded complication of atherosclerotic disease. Conjoined with other forms of appearance of atherosclerotic vascular disease it represents one of the most important causes of mortality in the western civilization and is thus a major socio-economic problem. The immediate thrombotic occlusion of a diseased vessel represents a complicated scenario spanning from rupture of an atherosclerotic plaque to the ultimate necrosis of the depending myocardial tissue. A sequelae of pathophysiological events take place within seconds of the development of vascular thrombosis, which include numerous triggering elicitors and mediators that support, augment or confine the several steps of primary and secondary hemostasis. Past years' scientific advances have further unraveled and helped to classify the roles of various agents that support arterial thrombosis and specified cellular signaling mechanisms, which put these actions into effect. In addition, it has become clear that the balance of vascular homeostatic small molecule mediators and the activation state of the cellular structure that is the crucial mediator of primary thrombotic occlusion - the anucleate platelet - may be the decisive determinants of fatal arterial occlusion. This thematic issue of Current Drug Targets embraces fine review articles by scientific leaders in the field of platelet pathophysiology, vascular biology, and antithrombotic pharmacology. Their contributions comprise insights into new concepts derived from medicinal chemistry and pharmacology to recent experimental perceptions with potential clinical relevance. The composition of this issue is designed to provide the scientifically interested clinicians with important new information about evolving outstanding research concepts, and also provide the basic researcher with up-to-date reviews on emerging topics in vascular pathophysiology. The first article of this issue by Essex and Li is a brilliant example of such an emerging scientific topic. Although platelet glycoprotein receptors like the fibrinogen receptor (GPIIb/IIIa) have long been known to be the most important mechanical mediators of platelet-platelet interaction, the molecular regulation of their redox-sensitive disulfite sites has not been understood so far. However, Essex and colleague nicely demonstrate, how redox state may form the basis for functionality of such glycoprotein receptors. In close relation to redox state, free reactive oxidant or nitrogen species are increasingly recognized to dominate vascular homeostasis not only by regulating endothelial function, but also by modulating platelet signaling. The article by Tziros and Freedman explains why this has recently led to the development of new NO donors that attempt to enhance the antithrombotic actions of NO as a means to manipulate arterial thrombosis. The articles by Shankar and colleagues and the review by Wee and Jackson further underline the integral role of platelets and their activatory signaling that form the basis for the development of more specific and highly effective antiplatelet agents. Based upon the finding that the P2Y12 receptor, the target of thienopyridine antiplatelet drugs like Clopidogrel, mediates its actions through G-protein dependent signaling, Shankar and colleagues have assembled an impressing overview of how several crucially important agonists like adenosine diphosphate (ADP), thrombin and thromboxane A2 (TXA2) activate platelets by acting via G-proteins and demonstrate how many of anti-thrombotic drugs mediate their beneficial effects by interfering with or preventing the initiation of the G-protein signaling pathway. The crucial importance of specific signaling in platelet-dependent thrombosis is further being highlighted in the article by Wee and Jackson. For the first time, their contribution reviews in a most fascinating way, how tyrosine phosphorylation events regulate key signaling molecules that either stimulate signaling pathways in platelets, such as the collagen-dependent activation of glycoprotein GPVI, or are associated with regulatory pathways that limit the extent of platelet activation. Lastly, the articles by Sohn and colleagues and the article by Klauss and Spannagl span a bridge from preclinical physiological knowledge to either clinical pathophysiology leading to myocardial infarction or to the development and first clinical experiences with a promising new antithrombotic class of drugs, which target at inhibiting the activated factor X or thrombin directly.

Platelets contain several glycoprotein receptors including the adhesion receptor glycoprotein Ib and the fibrinogen receptor glycoprotein IIbIIIa, also know as the αIIbβIIIa integrin. Both of these receptors contain thiol groups and vicinal thiols representing redox sensitive sites are present in αIIbβIIIa. Disulfide isomerases such as protein disulfide isomerase (PDI) that are on or recruited to the platelet surface have a role in platelet aggregation. Dynamic rearrangement of disulfide bonds in receptor signaling and platelet activation is a developing concept that requires an attacking thiol. Biochemically, a role for disulfide isomerization is suggested as the αIIbβIIIa integrin undergoes major structural changes upon activation centered around a disulfide knot in the integrin. Additionally, the P2Y12 ADP receptor is involved in platelet activation by most platelet agonists and contains extracellular thiols, making it a possible site for redox modification of platelet aggregation. Various forms of redox modulation of thiols or disulfides in platelet glycoproteins exist. These include modification by low molecular weight thiols such as reduced glutathione or homocysteine, oxidized glutathione or by nitric oxide (NO) derived from s-nitrosothiols. Levels of these redox compounds change in various disease states and in some cases physiologic concentrations of these compounds have been shown to modify platelet responsiveness. Additionally, platelets themselves contain a transplasma membrane redox system capable of reducing extracellular disulfide bonds. It is likely that a redox homeostasis exists in blood with the redox environment being controlled in a way analogous to the control of ionized calcium levels or the pH of blood. Changes in this homeostasis induced by disease states or pharmacologic agents that modify the platelet redox environment will modify platelet function.

Vessel occlusion within a coronary artery is the precipitating event in unstable coronary syndromes and is primarily due to rupture of atheromatous plaque and subsequent thrombus formation. In the nondiseased vessel, the intact endothelium releases the vasodilator and antithrombotic agent nitric oxide (NO) preventing platelet adherence and activation. In the diseased vessel and during unstable coronary syndromes, release of both endothelial and platelet NO is impaired contributing to thrombus formation. Nitric oxide availability in the vascular system has been associated with various disease states, genetic variants, and medication use. Recently, through the development of new NO donors and by targeting specific signaling pathways, there has been an attempt to enhance the antithrombotic actions of NO as a means to manipulate arterial thrombosis.

Platelet activation and aggregation is an integral component of the pathophysiology that leads to thrombotic and ischemic diseases such as cerebral stroke, peripheral vascular disease and myocardial infarction. Anti-platelet agents (such as aspirin, ADP receptor antagonists, and GPIIb/IIIa antagonists), phosphodiesterase inhibitors and anti-coagulants are major part of the current treatment towards treating ischemic diseases. However, their limited efficacy in the setting of arterial thrombosis, unfavorable side effect profile and costto- benefit issues substantiate the need for the development of newer and more efficacious antithrombotic drugs. Various platelet agonists like adenosine diphosphate (ADP), thrombin and thromboxane A2 (TXA2) activate platelets by acting via their respective surface receptors, which couple to one or more distinct G-proteins belonging to either the Gi, Gq, G12/13 or Gs families. Upon activation, each of these G-proteins trigger a series of intracellular signaling cascades, causing the platelets to undergo shape change, secrete their granular contents, generate positive feedback mediators and form stable platelet aggregates. In addition, various G-protein-mediated signaling cascades act in synergy with one another to amplify the magnitude of the platelet responses. The significance of G-proteins as key mediators of the platelet function and normal hemostasis is further corroborated by extensive gene knockout studies. In this review we will limit our discussion to understanding the role of G-proteins in the process of platelet activation and discuss some of the anti-thrombotic drugs that mediate their beneficial effects by interfering with or preventing the initiation of the G-protein signaling pathway.

Platelet activation is crucial for normal hemostasis to arrest bleeding following vascular injury. However, excessive platelet activation in narrowed atherosclerotic blood vessels that are subject to high shear forces may initiate the onset of arterial thrombosis. When platelets come into contact with, and adhere to collagen exposed by damaged endothelium, they undergo morphological and functional changes necessary to generate a platelet-rich thrombus. This process is complex and involves precise co-ordination of various signaling pathways which lead to firm platelet adhesion to sites of tissue damage, release of granule contents from activated platelets, platelet shape change, platelet aggregation and subsequent thrombus formation and consolidation. Induction of tyrosine phosphorylation of key signaling molecules has emerged as a critical event central to stimulatory signaling pathways that generate platelet activation, but is an essential component associated with regulatory pathways that limit the extent of platelet activation. Understanding mechanisms that regulate platelet activation may contribute to the development of novel therapeutics that control common vascular diseases such as myocardial infarction and ischaemic stroke.

Cyclooxygenases represent a major target of pharmaceutical therapy. Cyclooxygenase inhibitors are applied in order to reduce inflammation, to relieve from pain, or to prevent atherothrombotic complications in cardiovascular disease. Inhibition of platelet aggregation by aspirin, which is due to inhibition of platelet cyclooxygenase-dependent formation of thromboxane A2, is a cheap, safe and effective strategy to prevent myocardial infarction or stroke and is thus the most established strategy of secondary prevention of atherothrombotic disease. However, the existence of several isoforms of the cyclooxygenase enzyme, their tissue-specific expression patterns, their spatial and functional association with enzymes that further degrade the major cyclooxygenase products and the specific pharmacological properties of substances that have the potential of inhibiting cyclooxygenase make the ultimate physiological effects of a specific cyclooxygenase inhibitor a highly sophisticated pharmacological question. Specific inhibitors of the cyclooxygenase-2 isoform (COX-2) have at first promised to represent a major improvement of pharmaceutical therapy, but later have been suggested to enhance the risk of atherothrombotic events in vivo. This has led to the withdrawal of some of these substances from global markets and also initiated a discussion as to whether some non-selective cyclooxygenase inhibitors (non-steroidal anti-inflammatory drugs, NSAID), such as naproxen would also have an antithrombotic effect in vivo. This review summarizes current pathophysiological and clinical knowledge about the effects on atherothrombosis of drugs that target cyclooxygenases either as specific inhibitors of a cyclooxygenase isoform, or as non-specific cyclooxygenase inhibitors. It specifically discusses the question, whether all selective COX-2 inhibitors may have an intrinsic risk of enhancing the risk of atherothrombosis.

Cardiovascular disease is the leading cause for mortality and morbidity in the western world. Arterial thrombosis has multiple origins and may present with different clinical presentations such as acute coronary syndromes, stroke, and peripheral embolization. Furthermore, thrombotic complications may occur during percutaneous interventions. The underlying causes range from atherosclerosis with plaque rupture or erosion, embolization, stasis and hypercoagulable states. Thrombotic complications lead to activation of the intrinsic coagulation system and to platelet aggregation. Despite the development of effective platelet inhibitors, there is still the need for an optimal anticoagulation regimen. While unfractionated heparin is the most commonly used antithrombotic agent, which has major inherent limitations. Direct thrombin inhibitors and anti factor Xa agents are agents which may overcome the limitation of unfractionated heparin. The potential advantages of these new compounds are discussed on the basis of available clinical data in patients with coronary artery disease.

These are exciting days for physicians treating patients with Lymphoma. The introduction of chemotherapy has resulted in dramatic improvement of outcome in the early 1970s, when the diagnosis of leukaemia or lymphoma meant a death warrant in the vast majority of cases. However, variation of chemotherapeutic components and increasing the dose during the following decades of research has only resulted in small additional benefits - a notable exception is Hodgkin 's disease which can be cured by aggressive polychemotherapy regimens in most cases. Therefore, chemotherapy regimens of the early days like CHOP still remained the gold standard for lymphoma therapy at the beginning of this century. The same was true for many other haematological diseases with very view improvements, if any, in terms of survival for most patients. Beginning with the success story of the tyrosine kinase inhibitor Imatinib in CML, strong efforts have been made to treat haematological malignancies with more specific drugs which are directed against cell surface molecules, signal transduction pathways or the cell cycle. This special issue of current drug targets, entitled “Beyond chemotherapy: future directions in lymphoma treatment” contains reviews dealing with very exciting fields of clinical and preclinical research in the field of targeted therapy in malignant lymphoma. The issue starts with a review in which the current status of radioimmunotherapy is presented by Bruce Cheson. Another approach to enhance cytotoxic activity of monoclonal antibodies is to add a plant or bacteria toxin - available data are summarized in the following paper by Kreitman and Pastan. The next review by Winkler et al, nicely illustrates the interplay of genetics, gene expression and targeted therapy using B-CLL as a prototype disease. Like BCR-ABL in CML, NPM-ALK is an oncogenic tyrosine kinase which is involved in lymphomagenesis. The review of Miething and colleagues describes the efforts to better understand and finally target this oncogenic event. The BCL-10/MALT- 1 pathway is another signal transduction pathway which is involved in lymphomagenesis and might be a promising drug target as reviewed by Jost and coworkers. A real success story of targeted therapy is described by Mitsiades et al. Proteasome inhibition has already been shown to be a very effective treatment approach in multiple myeloma and is just being tested in lymphoma treatment as well. In contrast to cytotoxic chemotherapy, drugs targeting the cell cycle might be able to stop disease progression or maintain remissions for prolonged periods of time. Ringshausen and coworkers present preclinical and clinical data currently available. Another promising approach is to modulate pro- and antiapoptotic mechanisms in cancer and lymphoma as reviewed by Meiler and Schuler. Last but not least, considerable progress has been made in bringing immnotherapeutic approaches from bench to bedside. Different strategies including active and passive immunotherapy are reviewed by Kofler and colleagues at the end of this issue. Taken together, these nine reviews present current research dealing with new drug targets in lymphoma treatment. Some of the therapeutic approaches presented in this issue will likely alter the way we treat lymphoma in the future.

The availability of active monoclonal antibodies has altered the treatment paradigms for patients with non-Hodgkin's lymphomas (NHL). Nevertheless, some patients do not respond, while almost all of the others eventually relapse and require additional treatment. Thus, more effective alternatives are needed. Radioimmunotherapy (RIT) is an attractive option because of the inherent radiosensitivity of most NHL. Yttrium-90 ibritumomab tiuxetan and iodine-131 tositumomab are the first two radioimmunoconjugates currently available for clinical use. These agents appear comparably active in patients with follicular and low-grade NHL after failure following chemotherapy and/or rituximab. Activity has also been demonstrated against other histologies, including diffuse large B-cell NHL, mantle cell NHL, and transformed NHL. Toxicities primarily include myelosuppression, with a potential risk of treatmentassociated myelodysplastic syndrome and acute myelogenous leukemia. Current clinical trials are attempting to optimize the use of these agents by evaluating them earlier in the course of the disease, and sequenced with a variety of chemotherapy regimens. Hopefully, the rational development of RIT will lead to a prolongation of survival for patients with NHL.

Immunotoxins, composed of protein toxins connected to cell binding ligands including monoclonal antibodies and growth factors, have been developed for several decades to target hematologic malignancies. Protein toxins from either plants or bacteria are extremely potent based on their enzymatic inhibition of protein synthesis and induction of apoptosis. Plant toxins, particularly ricin, are useful for chemically conjugating to monoclonal antibodies, and have shown clinical activity in several types of lymphoma and leukemia. Their dose is generally limited by vascular leak syndrome. Bacterial toxins have been used to produce single chain fusions with either growth factors or recombinant antibody fragments. These agents are smaller in size (55-65 kDa) and exit the bloodstream much more rapidly than the chemical conjugates, and generally do not cause severe vascular leak syndrome. The only approved drug containing a protein toxin is denileukin diftitox, a fusion of human interleukin 2 with truncated diphtheria toxin. Denileukin diftitox has shown efficacy in cutaneous T-cell lymphoma, chronic lymphocytic leukemia, and non-Hodgkin's lymphoma. Recombinant immunotoxin BL22 is an anti-CD22 Fv fragment fused to truncated Pseudomonas exotoxin; it induces complete remissions in a high percentage of patients with chemoresistant hairy cell leukemia. The anti-CD25 recombinant immunotoxin LMB-2 is active in several CD25+ hematologic malignancies. Several other recombinant immunotoxins are undergoing preclinical development for other target antigens expressed on hematologic malignancies.

Chronic lymphocytic leukemia (CLL) represents the most common leukemia among adults in the Western countries. CLL is a remarkably diverse disorder following an extremely variable clinical course. Some patients have an indolent disease that may never require treatment. In others a progressive clinical course is rapidly fatal. CLL affects mainly elderly individuals, but about a third of patients are less than 60 years of age at diagnosis. Traditionally, the therapeutic procedures were aimed at palliation, but over the recent years highly effective and potentially curative approaches such as combined antibody-chemotherapy and autologous or allogeneic stem cell transplantation have been developed. In parallel there has been progress in the understanding of pathogenesis and outcome prediction. The cornerstones to estimate prognosis are the clinical staging systems of Rai and Binet. To refine outcome prediction for individual patients there has been intensive work on biological factors of potential prognostic relevance. Among these, the genetic characteristics of the CLL cells that can be divided into genomic aberrations and the mutation status of the variable segments of immunoglobulin- heavy chain genes (VH) have attained considerable importance. In addition, data on gene expression of CLL cells are accumulating which further characterize the CLL subgroups. In this context, the expression of ZAP-70 has been recognized a useful surrogate marker to predict the VH mutation status and outcome of CLL patients. At present, targeted therapies are focused on humanized antibodies that bind proteins expressed on the surface of CLL cells. The most prominent agents of these are the anti-CD52 antibody alemtuzumab and the anti-CD20 antibody rituximab, which are currently being tested in clinical trials. To identify CLL-specific gene expression products as candidates for targeted therapies will be an important part of CLL research in the next years.

Oncogenic tyrosine kinases play a ever growing role in the pathogenesis of human malignancies. In human non-hodgkin lymphomas, the NPM-ALK oncogene arising from the t(2;5) chromosomal translocation represents the most important oncogenic tyrosine kinase identified so far. The ALK-kinase is constitutively activated by NPM-induced dimerization and signals through a multitude of growth promoting and antiapoptotic pathways. Murine models have made a significant impact on the elucidation of the molecular pathogenesis and new treatment options of malignant diseases. Here, the latest developments in the analysis of NPM-ALK induced lymphomagenesis by murine models is reviewed.

The development of lymphomas and leukemias is frequently caused by chromosomal translocations that deregulate cellular pathways of differentiation, proliferation or survival. The molecules that are involved in these aberrations provide rational targets for selective drug therapies. Recently, several disease specific translocations have been identified in human MALT lymphoma. These aberrations either upregulate the expression of BCL10 or MALT1 or induce the formation of API2-MALT1 fusion proteins. Genetic and biochemical experiments identified BCL10 and MALT1 as central components of an oligomerization - ubiquitinylation - phosphorylation cascade that activates the transcription factor NF-κB in response to antigen receptor ligation. Deregulation of the signaling cascade is directly associated with antigen independent MALT lymphoma growth. Here we provide an overview of the physiological and pathological functions of BCL10 / MALT1 signal transduction and discuss the potential of this pathway as a drug target.

The intracellular concentration of proteins in both normal and tumor cells are regulated by the balance between the rates of protein synthesis vs. degradation. The ubiquitin-proteasome pathway is the main intracellular cascade for controlled degradation of proteins and has attracted in recent years major interest not only because of its biochemical complexity and the intricate regulation of its function, but also because diverse cell cycle regulators and modulators of apoptosis are subject to regulation by proteasome function, and can therefore be significantly affected by small molecule inhibitors of the proteolytic activity of the proteasome. In fact, bortezomib, the prototypic member of this class of agents, was recently approved by the U.S. Food and Drug Administration for the treatment of advanced multiple myeloma patients. This review article focuses on the exciting recent progress in the use of proteasome inhibitors, with emphasis on the bench-to-bedside research effort which provided the foundation for clinical development of bortezomib for the treatment of multiple myeloma, as well as other hematologic malignancies, such as mantle cell lymphoma.

Disruption of a proper regulation of cell proliferation can ultimately cause cancer. Most human B cell malignancies are driven by chromosomal translocations or other genetic alterations which directly affect the function of critical cell cycle proteins, such as cyclins and cyclin-dependent kinases. In addition, the transformation of indolent lymphomas into aggressive malignancies is often accompanied by a loss of tumor suppressors controlling important cell cycle checkpoints. A better understanding of cell cycle deregulations in human tumors has promoted the introduction of a new class of antiproliferative drugs into cancer therapies. These drugs exert their function by specifically blocking important cell cycle proteins. In the present review we discuss how alterations in the cell cycle control contribute to the malignant transformation of B cells. Furthermore, we provide an overview of novel direct and indirect cell cycle inhibitors and their impact on the treatment of patients with B cell lymphomas.

Programmed cell death (apoptosis) is a key tumor suppressor mechanism. Consequently, most if not all cancers develop mechanisms to abolish or circumvent this genetic program. Besides enabling malignant transformation and tumor progression, defects in apoptosis can result in resistance to cytotoxic cancer therapies. Much progress has been made in the delineation of the molecular pathways leading to apoptosis. This allows the identification of target molecules and lead compounds to develop novel therapies, which make use of this intrinsic death program for the treatment of cancer. Here, we review the current understanding of apoptotic signal transduction pathways, and strategies of their therapeutic modulation in relation to lymphoma and other cancers.

Conventional treatment of hematologic malignancies mainly consists of chemotherapeutic agents or a combination of both, chemotherapy and monoclonal antibodies. Despite recent advances, chemotherapeutic treatments often remain unsatisfying due to severe side effects and incomplete long-term remission. Therefore the evaluation of novel therapeutic options is of great interest. B cell malignancies, in particularly follicular lymphomas, chronic lymphocytic leukemia and multiple myeloma, represent the most immuneresponsive types of all human cancer. Several immunotherapeutic strategies are presently employed to combat these B-cell malignancies. Active immunotherapies include vaccination strategies with dendritic cells (DCs) and genetically-modified tumor cell preparations as well as DNA and protein vaccination. Most of these vaccines target the tumor-specific immunoglobulin idiotype and have already demonstrated some anti-lymphoma activity in early phase clinical trials while their definitive impact is evaluated in ongoing phase III randomized trials. In contrast to these active immunizations, T cells transduced with chimeric antigen receptors and donor leukocyte infusions (DLI) represent adoptive (passive) immunotherapies. Recent advances of gene transduction technologies enabled improvement of immunotherapeutic strategies based on genetic modification of malignant cells or adoptive T cells. Current early phase clinical trials are investigating the potential of these innovative approaches. At the moment it remains unclear if the novel immunotherapeutic strategies will be able to play a similar role in the treatment of B cell malignancies than the already established antibody-based immunotherapy.