Current Drug Targets - Volume 6, Issue 6, 2005

Although the hematological malignancies constitute a minor fraction of the total number of newly diagnosed cancers per year, they pose a substantial burden to the individual patient. In general neoplasias affecting the lymphohematopoietic system carry a poor prognosis with median 5- year survival rates usually below 50-60%, and some are still considered incurable, like chronic lymphoid leukemia and multiple myeloma. Today the best treatment modalities are often composed of a combination of intensive chemotherapy and stem cell transplantation. These therapeutic regimens are usually offered to the younger population of patients (< 60 years), the treatmentrelated side effects might be severe, and importantly, they are costly in terms of financial and personnel resources. Hence there is a continuous need to expand our knowledge of normal and malignant hematopoiesis in order to improve our treatment repertoire of these diseases. This issue of Current Drug Targets contains an update of recent advances in the development of molecular-based strategies for treating chronic diseases with special emphasis on cancers, in particular the hematological malignancies. In the growing era of proteomics, Sjoholt and colleagues describe how these new and fascinating techniques can be adapted to study leukemogenesis, how putative therapeutic targets can be identified, and they also add to our understanding of how current cytostatic drugs affect the leukemic cell. The possibility of selective silencing of genes involved in the development and dissemination of malignant cells, is the focus of the paper by Sioud and Iversen. The advent of small-molecular oligonucleotides to effectively impair gene expression is pawing their way into modern cancer treatment. Skalhegg and co-workers have a long standing interest in the biology of protein A kinases, and here they pinpoint potential targets for treating diseases characterized by dysfunctional T cells by interfering with the protein kinase A signaling pathways. The transition from a single malignant cell clone to spread of these cells to more distant sites, is still far from being adequately understood. In their review Fjelstad and Kolset focus on the extracellular compartment and how the inherent components operate and can be inhibited during the metastatic process. Angiogenesis is increasingly being accepted as an important mechanism for how blood cancers disseminate, and putative angiogenic targets for cancer therapy are outlined by Negaard et al. In addition to the invariable impairment of bone marrow hematopoiesis, patients with multiple myeloma frequently experience bone pains due to osteolytic destructions of their skeletons. Hjertner and colleagues describe the molecular mechanisms for such osteolytic destructions and define possible targets for therapy. The myelodysplastic syndromes have remained a major obstacle to clinicians, both in terms of ill-defined diagnostic criteria and the lack of adequate treatment. Hellstrøm-Lindberg presents an overview of how this group of hematolological malignancies can be categorized and how new molecular-based knowledge about the onset and development of these diseases can be exploited for design of more efficient treatment. Collectively, it is my hope that these reviews will aid in our understanding how molecular-based strategies can be implemented to identify putative targets for therapy and thus for translating basic science into applied clinical practice.

Enduring efforts into determination of the molecular biological status of acute myelogenous leukaemia (AML), a stem cell disease characterised by distinct blastic differentiation blocks and their extensive growth, continue to provide us with prognostically important information for more than half of all patients. In subsets of AML, molecular diagnostics rigorously guide the clinician toward the choice of optimal therapy. The in-depth characterization of leukemogenesis associated genetic alterations, such as the combined presence of activating mutations of tyrosine kinases together with altered transcription factors, and the documented impact of these mutations upon prognosis of AML, suggests AML as a primary candidate for pioneering proof-of-principle studies with new high throughput protein analysis techniques. This review aims to introduce the reader to proteomic methodology, e.g. two-dimensional polyacrylamide gel electrophoresis, mass spectrometry, SELDI and protein arrays. Examples of its use, including single cell phosphoprotein profiling in risk stratification, the probing of cellular effects of conventional chemotherapeutics and novel target determination are presented. Based on original proteomic analysis of AML, molecular characteristics of AML, in addition to knowledge of conventional therapeutics and novel drugs, we attempt to forecast the influence of proteomics in therapy development for AML.

Selective gene silencing by nucleic acid enzymes has provided researchers with a new strategy to block gene expression and drug target validation. Ribozymes, DNAzymes and small interfering RNAs (siRNAs) are being explored as genetic inhibitors of gene expression as well as potential therapeutics against viral infections, inflammatory disorders, haematological diseases and cancers. We review the mode of action of these molecules, with special emphasis on their construction and the possibility to enhance their serum half-lives via specific chemical modifications. Their potential use in cell cultures and in animal models for disease is also highlighted.

In several cases of immunodeficiency and autoimmunity, the dysfunctional immune system is associated with either hypo- or hyperactive T and B cells. In autoimmune conditions such as systemic lupus erythematosus (SLE) and immunodeficiencies such as acquired immunodeficiency syndrome (AIDS), it has been demonstrated that the regulatory effect of the signaling pathway of cyclic 3', 5' adenosine monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) is abrogated. PKA is well-known as a key regulator of immune responses in that it inhibits both early and late phases of antigen induced T and B cell activation. Here we will discuss a potential useful strategy for therapeutic interventions of dysfunctional T cells associated with SLE and HIV by modulation of the cAMP-PKA pathway. Therefore, we will describe the components and architecture of the cAMP-PKA signaling pathway in T cells in order to point out one or several steps which potentially may serve as targets for therapeutic intervention.

The heterogeneity of proteoglycans (PG)s contributes to their functional diversity. Many functions depend on their ability to bind and modulate the activity of components of the extracellular matrix (ECM). The ability of PGs to interact with other molecules, such as growth factors, is largely determined by the fine structure of the glycosaminoglycan (GAG) chains. Tumorigenesis is associated with changes in the PG synthesis. Heparan sulfate (HS) PGs are involved in several aspects of cancer biology including tumor progression, angiogenesis, and metastasis. PGs can have both tumor promoting and tumor suppressing activities depending on the protein core, the GAG attached, molecules they associate with, localization, the tumor subtype, stages, and degree of tumor differentiation. Perlecan is an angiogenic factor involved in tumor invasiveness. The C-terminal domain V of perlecan, named endorepellin, has however been shown to inhibit angiogenesis. Another angiogenic factor is endostatin, the COOH-terminal domain of the part-time PG collagen XVIII. Glypicans and syndecans may promote local cancer cell growth in some cancer tissues, but inhibit tissue invasion and metastasis in others. The GAG hyaluronan (HA) promotes cancer growth by providing a loose matrix for migrating tumor cells and mediates adhesion of cancer cells. HSPG degrading enzymes like heparanase, heparitinase, and other enzymes such as hyaluronidase and MMP are also important in tumor metastasis. Several different treatment strategies that target PGs have been developed. They have the potential to be effective in reducing tumor growth and inhibit the formation of metastases. PGs are also valuable tumor markers in several cancers.

Angiogenesis is essential for tumor growth and metastasis. This is firmly established in solid tumors, but accumulating evidence suggests that this is also an important event in hematological neoplasias. Angiogenesis is therefore a putative target for therapy. The potential application of different angiogenesis inhibitors is currently under intense clinical investigation, and we will here review a number of these trials. The association between cancer and thromboembolic disease is even better documented, and again, this is not limited to solid tumors. It appears that many patients with hematological malignancies have a dysfunctional hemostatic system, with increased risk of thromboembolism. Furthermore, effective antithrombotic therapy seems to reduce the risk of cancer progression and even prolongs overall survival. In this review we will thus discuss the mechanisms involved in the regulation of angiogenesis and hemostasis and present evidence for a shared biology. A number of factors regulating the hemostatic system also have pro- or anti-angiogenic properties. Tissue factor (TF) and TF pathway inhibitor (TFPI) seem to play a central role, and there are several lines of evidence suggesting a close cooperation between TF/TFPI and pro-angiogenic factors like members of the vascular endothelial growth factor family. A better understanding of this shared biology may reveal new targets, and will probably increase the safety of targeting the blood supply.

One of the most characteristic features of multiple myeloma is the development of osteolytic bone lesions. Myeloma-associated bone disease is caused by an increase in osteoclastic bone resorption and a decrease in osteoblastic new bone formation. Insight into the molecular mechanisms of osteoclastogenesis has been provided by the detection of receptor activator of NF-kB ligand (RANKL), its specific receptor (RANK) and its decoy receptor antagonist osteoprotegerin (OPG). The RANK signaling system is abnormally regulated in multiple myeloma and targeting this system may ameliorate myeloma bone disease. Less is known about the development of osteoblastic dysfunction, and further knowledge about the interaction between myeloma cells and osteoblasts is required. The aim of this review is to focus on the principles of bone biology for a better understanding of the development of myeloma bone disease and to identify possible therapeutic targets.

The myelodysplastic syndromes (MDS) constitute a group of clonal stem cell disorders characterized by cytopenia, ineffective hematopoiesis, bone marrow dysplasia, and a risk of progression to acute myeloid leukemia (AML). Disease mechanisms can be divided into two main groups; those underlying the increased apoptosis of bone marrow progenitors, and those associated with progressive blast proliferation, and transformation to acute myeloid leukemia. The recently published WHO classification includes one subtype with a specific cytogenetic lesion, the 5q- syndrome, but otherwise classification of MDS is based solely on clinical and morphological criteria. Subsequently, few therapeutic options have been directed towards specific biological or molecular mechanisms in MDS. Progenitor apoptosis in MDS may be initiated by extrinsic and intrinsic mechanisms. The extrinsic pathway includes T-cell mediated bone marrow failure, for which antithymocyte globulin treatment may be an effective, as well as negative effects caused by the marrow microenvironment. New therapeutic options targeting the microenvironment include thalidomide and its analogue, lenalidomide, which has proven extremely effective for patients with 5q- syndrome. The erythroid apoptosis of in particular sideroblastic anemia is mediated by mitochondrial release of cytochrome c, which may be inhibited by treatment with erythropoietin and granulocyte-colony-stimulating-factor. Important mechanisms for disease progression are DNA hypermethylation, histone deacetylation, and possibly RAS mutations. Two new DNA hypomethylating agents, azacytidine and decitabine, have shown efficacy in patients with high-risk MDS, and may prolong time to progression. In conclusion, recent advances in the pathogenetic understanding of MDS have led to significant therapeutic progress.