Showing papers in "Current Cancer Drug Targets in 2005"

MDM2 is a Central Node in the p53 Pathway: 12 Years and Counting

Abstract: Twelve years ago, the Mdm2 oncogene was shown to bind to and inhibit the tumor suppressor protein, p53. During the past 12 years, both genetic and biochemical studies have demonstrated that Mdm2 is a key negative regulator of the tumor suppressor p53. Mdm2 and p53 form an oscillating auto-regulatory feedback loop, which is tightly controlled to allow the appropriate response to environmental stresses in order to suppress tumor formation. When Mdm2 activity is inappropriately heightened, as it is in many human tumors, p53 activity is attenuated and tumor susceptibility arises. The p53 gene is mutated in 50% of all human tumors, but in those tumors that retain wild type p53, inhibiting Mdm2 activity could activate p53 tumor suppression and therefore provide a therapeutic strategy for the treatment of cancer.

Matrix metalloproteinases as therapeutic targets in cancer.

Abstract: Degradation of extracellular matrix is crucial for malignant tumor growth, invasion, metastasis and angiogenesis. Matrix metalloproteinases (MMPs) are a family of zinc-dependent neutral endopeptidases collectively capable of degrading essentially all matrix components. Elevated levels of distinct MMPs can be detected in tumor tissue or serum of patients with advanced cancer, and their role as prognostic indicators in cancer has been widely examined. In addition, therapeutic intervention in tumor growth and invasion based on inhibition of MMP activity is under intensive investigation and several MMP inhibitors (MMPIs) are in clinical cancer trials. Even though results of the first clinical trials in advanced cancer have been mostly disappointing, there are also positive results. Recent observations show, that certain MMPs limit tumor growth. Therefore, identification of proper MMPs for therapeutic intervention with array-based molecular classifications of tumors and targeting these with more specific MMPIs in combination with conventional chemotherapy is expected to provide a feasible approach for cancer therapy. MMPIs represent a totally different therapeutic modality from proven anti-cancer drugs and thus traditional approaches to evaluate drug efficiency cannot be used without modification. In this review, we discuss the current view on the feasibility of MMPs as targets for therapeutic intervention in cancer.

MDM2 and Human Malignancies: Expression, Clinical Pathology, Prognostic Markers, and Implications for Chemotherapy

Abstract: The human homologue of the mouse double minute 2 (MDM2) oncogene is overexpressed in more than forty different types of malignancies, including solid tumors, sarcomas and leukemias. Because of its prevalent expression and its interactions with p53 and other signaling molecules, MDM2 plays a central role in cancer development and progression. The expression of this oncoprotein is being studied by researchers world-wide, and the amount of data published about it is increasing exponentially. Although there are some conflicting data about the effects of MDM2 expression in individual cancers, the overall evidence is convincing, indicating that increased MDM2 expression is related to a worse clinical prognosis. There is an increased likelihood of distant metastases, as well as a decreased response to therapeutic intervention in MDM2-positive cancers. MDM2 may also serve as a diagnostic marker, not only for cancer stage, but to differentiate between similar cancers. MDM2 may also be associated with drug resistance in cancer chemotherapy. These findings make studying the oncoprotein necessary to aid in our understanding of cancer development, to identify novel cancer drug targets, and to increase the efficacy of cancer therapy.

Induction of Apoptosis by Curcumin and Its Implications for Cancer Therapy

Abstract: Curcumin (diferuloyl methane), the yellow pigment in turmeric (Curcuma longa), is a potent chemopreventive agent that inhibits proliferation of cancer cells by arresting them at various phases of the cell cycle depending upon the cell type. Curcumin-induced apoptosis mainly involves the mitochondria-mediated pathway in various cancer cells of different tissues of origin. In some cell types like thymocytes, curcumin induces apoptosis-like changes whereas in many other normal and primary cells curcumin is either inactive or inhibits proliferation, but does not appear to induce apoptosis. These together with reports that curcumin protects cells against apoptosis induced by other agents, underscore the need for further understanding of the multiple mechanisms of cell death unleashed by curcumin. Tumor cells often evade apoptosis by expressing several antiapoptotic proteins, down-regulation and mutation of proapoptotic genes and alterations in signaling pathways that give them survival advantage and thereby allow them to resist therapy-induced apoptosis. Many researchers including ourselves, have demonstrated the involvement of several pro and antiapoptotic molecules in curcumin-induced apoptosis, and ways to sensitize chemoresistant cancer cells to curcumin treatment. This review describes the mechanisms of curcumin-induced apoptosis currently known, and suggests several potential strategies that include down-regulation of antiapoptotic proteins by antisense oligonucleotides, use of proapoptotic peptides and combination therapy, and other novel approaches against chemoresistant tumors. Several factors including pharmacological safety, scope for improvement of structure and function of curcumin and its ability to attack multiple targets are in favor of curcumin being developed as a drug for prevention and therapy of various cancers.

Anti-Angiogenic and Anti-Inflammatory Effects of Statins: Relevance to Anti-Cancer Therapy

Abstract: Angiogenesis is indispensable for the growth of solid tumors and angiogenic factors are also involved in the progression of hematological malignancies. Targeting the formation of blood vessels is therefore regarded as a promising strategy in cancer therapy. Interestingly, besides demonstration of some beneficial effects of novel anti-angiogenic compounds, recent data on the activity of already available drugs point to their potential application in anti-angiogenic therapy. Among these are the statins, the inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Statins are very efficient in the treatment of hypercholesterolemia in cardiovascular disorders; however, their effects are pleiotropic and some are not directly related to the inhibition of cholesterol synthesis. Some reports particularly highlight the pro-angiogenic effects of statins, which are caused by low, nanomolar concentrations and are regarded as beneficial for the treatment of cardiovascular diseases. On the other hand, the anti-angiogenic activities, observed at micromolar concentrations of statins, may be of special significance for cancer therapy. Those effects are caused by the inhibition of both proliferation and migration and induction of apoptosis in endothelial cells. Moreover, the statin-mediated inhibition of vascular endothelial growth factor synthesis, the major angiogenic mediator, may contribute to the attenuation of angiogenesis. It has been suggested that the anti-cancer effect of statins can be potentially exploited for the cancer therapy. However, several clinical trials aimed at the inhibition of tumor growth by treatment with very high doses of statins did not provide conclusive data. Herein, the reasons for those outcomes are discussed and the rationale for further studies is presented.

The MCM Complex: Its Role in DNA Replication and Implications for Cancer Therapy

Abstract: The MCM complex controls the once per cell cycle DNA replication in eukaryotic cells. In a process known as DNA replication licensing, it primes chromatin for DNA replication by binding origins of DNA replication during the late M to early G1 phase of the cell cycle. Activated by S phase promoting protein kinases, the origin-bound MCM complexes unwind the double stranded DNA at the origins, recruit DNA polymerases and initiate DNA synthesis. Coupled with the initiation of DNA replication in the S phase, the MCM complexes move away from replication origins as a component of the DNA replication fork, likely serving as DNA helicases. Their departure deprives replication origins the ability to re-initiate DNA replication for the reminder of the cell cycle. Because of its vital role in genome duplication in proliferating cells, deregulation of the MCM function results in chromosomal defects that may contribute to tumorigenesis. The MCM proteins are highly expressed in malignant human cancers cells and pre-cancerous cells undergoing malignant transformation. They are not expressed in differentiated somatic cells that have been withdrawn from the cell cycle. Therefore, these proteins are ideal diagnostic markers for cancer and promising targets for anti-cancer drug development. In this article, I will overview the structures and functions of the MCM complex with an effort to integrate insights from recent biochemical and structural studies. Discussions will also cover activities and structures of the complex that may be useful for the development of drug screens.

EphA2 receptor tyrosine kinase as a promising target for cancer therapeutics.

Abstract: Eph receptors are a unique family of receptor tyrosine kinases (RTK) that play critical roles in embryonic patterning, neuronal targeting, and vascular development during normal embryogenesis. Eph RTKs and their ligands, the ephrins, are also frequently overexpressed in a variety of cancers and tumor cell lines. In particular, one family member, EphA2, is overexpressed in breast, prostate, lung, and colon cancers. Unlike traditional oncogenes that often function only in tumor cells, recent data show that Eph receptors mediate cell-cell interactions both in tumor cells and in the tumor microenvironment, namely the tumor stroma and tumor vasculature. Thus, EphA2 receptors are attractive targets for drug design, as targeting these molecules could simultaneously inhibit several aspects of tumor progression. This review focuses on the multiple roles of EphA2 in cancer progression, the mechanisms by which EphA2 inhibition may halt this progression, and the pre-clinical results of EphA2 inhibition in various cancer model systems.

Matrix metalloproteinase inhibitors as anticancer therapeutics.

Abstract: Matrix metalloproteinases (MMPs), also designated as matrixins, play a central role in many biological processes and are involved both in physiologic cellular processes and in pathologic situations such as tumor growth, invasion and metastasis. For more than 30 years MMPs have been considered as promising targets for cancer therapy and a number of different synthetic and natural MMP inhibitors have been identified as cytostatic and anti-angiogenic agents and have begun clinical testing in view of their specific implication in malignant tissues. Although preclinical studies were so compelling to encourage several clinical trials, the past years have seen a consistent number of disappointments and limited success. The critical examination of previous studies shed light on new information about the cellular source, substrates and mode of action of MMPs, focusing the attention of future research on the identification of specific MMP targets in tumors at different stage of tumor progression, both in order to improve efficacy and to reduce the side effect profile. In this review we discuss the current view on the feasibility of MMPs as target for therapeutic intervention in cancer, taking into account that the perspective may be of great value for molecular medicine for the twenty-first century, providing intriguing information about the MMPs as mediators in biology and pathology, and as targets for disease therapies.

Mechanisms of focal adhesion kinase regulation.

Abstract: Focal adhesion kinase (FAK) is a tyrosine kinase whose phosphorylation state and activity is tightly linked to cell adhesion to the extracellular matrix through integrin receptors. FAK's regulation by adhesion places it in a key position to be able to influence cellular events that are either dependent on cell adhesion like cell proliferation and survival, or that require modulation of cell adhesion like cell migration. FAK's involvement in cellular pathways that regulate cell growth and cell movement suggests that it may contribute to the development of cancer or other diseases. FAK's possible involvement in these pathways makes it a potential drug target. In this review we will focus on the developing view how FAK's activity and phosphorylation are regulated within the cell. Specifically, we will address the contribution of integrins and growth factor dependent pathways to FAK's activation. The role of the tyrosine kinase Src in FAK's regulation will be discussed. The contribution of various negative regulators of FAK's phosphorylation on its regulation including phosphatases and proteases will be discussed. Lastly, the emerging role of FAK's amino terminal FERM like domain in FAK's regulation will be explored. FAK's function within a cell are tightly linked to its phosphorylation state, thus understanding its normal regulation in the cell will provide important insight into drug development by highlighting novel regulatory mechanisms within FAK that potentially may be exploited.

P-glycoprotein--implications of metabolism of neoplastic cells and cancer therapy.

Abstract: Multidrug resistance (MDR) of neoplastic tissues is a major obstacle in cancer chemotherapy The predominant cause of MDR is the overexpression and drug transport activity of P-glycoprotein (P-gp, a product of the MDR gene) P-gp is a member of the ATP binding cassette (ABC) transporters family, with broad substrate specificity for several substances including anticancer drugs, linear and cyclic peptides, inhibitors of HIV protease, and several other substances The development of P-gp-mediated MDR is often associated with several changes in cell structure and metabolism of resistant cells In the present review are discussed the relations between glucosylceramide synthase activity, Pregnane X receptor and development of P-gp mediated MDR phenotype Attention is also focused on the changes in protein kinase systems (mitogen-activated protein kinases, protein kinase C, Akt kinase) that are associated with the development of MDR phenotype and to the possible role of these kinase cascades in modulation of P-gp expression and function The overexpression of P-gp may be associated with changes in metabolism of sugars as well as energy production Structural and ultrastructural characteristics of multidrug resistant cells expressing P-gp are typical for cells engaged in a metabolically demanding process of protein synthesis and transport P-gp mediated MDR phenotype is often also associated with alterations in cytoskeletal elements, microtubule and mitochondria distribution, Golgi apparatus, chromatin texture, vacuoles and caveolae formation The current review also aims at bringing some state-of-the-art information on interactions of P-glycoprotein with various substances To capture and transport the numerous unrelated substances, P-gp should contain site(s) able to bind compounds with a molecular weight of several hundreds and comprising hydrophobic and/or base regions that are protonated under physiological conditions Drug binding sites that are able to recognize substances with different chemical structures may have a complex architecture in which different parts are responsible for binding of different drugs For P-gp substrates and inhibitors, a pharmacophore-based model has been described The pharmacophores have to contain parts with hydrophobic and aromatic characteristics and functional groups that can act as hydrogen-bond donors and/or acceptors Several drugs are known to be P-glycoprotein antagonizing agents They represent a large group of structurally unrelated substances that can act via direct interaction with P-gp and inhibition of its transport activity, or via possible modulation of processes (such as phosphorylation) regulating P-gp transport activity Effects of MDR reversal agents on the P-gp expression have also been reported Function and expression of P-gp can be affected indirectly as well, eg through cyclooxygenase-2 or carbonic anhydrase-IX expression and effects

The chick embryo chorioallantoic membrane as a model system for the study of tumor angiogenesis, invasion and development of anti-angiogenic agents.

Abstract: Angiogenesis, the formation of new blood vessels, is essential for tumor growth, progression and metastasis. The development of agents that target tumor vasculature is ultimately dependent on the availability of appropriate preclinical screening assays. The chorioallantoic membrane (CAM) assay is well established and widely used as a model to examine angiogenesis, and anti-angiogenesis. This review 1) summarizes the currently used angiogenesis assays and the importance of CAM model among them; 2) summarizes the current knowledge about the development and structure of the CAM's capillary bed; 3) reports findings regarding the role played by molecular signaling pathways in angiogenesis process; 4) discusses the use, advantages and limitations of the CAM as a model for studying tumor angiogenesis and invasiveness, as well as development of angiogenic and/or anti-angiogenic agents; 5) discusses the importance of standardization of the major methodologies for all aspects of the use of the CAM in angiogenesis-related studies; 6) and finally, summarizes major findings regarding the agents developed by the use of CAM model in the study of tumor angiogenesis, invasion and development of anti-angiogenic agents.

p53-independent activities of MDM2 and their relevance to cancer therapy.

Abstract: The feed-back auto-regulatory loop between p53 and MDM2 has been extensively investigated. MDM2 is under the transcriptional control of p53, and MDM2 acts as a negative regulator of p53. There is increasing evidence, however, supporting the notion that MDM2 has activities independent of p53. In the absence of p53, MDM2 may retain its role in cell cycle control, differentiation, cell fate determination, DNA repair, transcription regulation, signal transduction of steroid receptors, cellular response to hypoxia, internalization of surface receptors, and other processes. MDM2 also has oncogenic transformational activities independent of p53. Moreover, anti-MDM2 antisense oligonucleotides have in vitro and in vivo antitumor activity and chemosensitizing and radiosensitizing effects in several human cancer models, regardless of their p53 status. In this article, the p53 independent activities of MDM2 and its interactions with various cellular proteins are considered. The studies reviewed provide a basis for developing novel MDM2 inhibitors as a therapy against human malignancies.

Inflammation and cancer: when NF-kappaB amalgamates the perilous partnership.

Abstract: Chronic inflammation has long been suggested to constitute a risk factor for a variety of epithelial cancers such as malignancies of prostate, cervix, esophagus, stomach, liver, colon, pancreas, and bladder. An inflammatory response is typically accompanied by generation of free radicals, stimulation of cytokines, chemokines, growth and angiogenic factors. Free radicals, capable of both directly damaging DNA and affecting the DNA repair machinery, enhance genetic instability of affected cells, thus contributing to the first stage of neoplastic transformation also known as "initiation". Cytokines and growth factors can further promote tumor growth by stimulating cell proliferation, adhesion, vascularization, and metastatic potential of later stage tumors. Nuclear factor kappa B (NF-kappaB) is a family of ubiquitously expressed transcription factors that are widely believed to trigger both the onset and the resolution of inflammation. NF-kappaB also governs the expression of genes encoding proteins essential in control of stress response, maintenance of intercellular communications, and regulation of cellular proliferation and apoptosis. Recent data have expanded the concept of inflammation as a critical component in carcinogenesis suggesting new anti-inflammatory therapies for a complementary approach in treating a variety of tumor types. These observations highlighted the NF-kappaB pathway as an attractive avenue for drug discovery and development. The present review will outline recent advances in our understanding of NF-kappaB function in the inflammatory processes and its input in tumor initiation/promotion, as well as summarize the development of animal and cell culture models for validating drug candidates with NF-kappaB-modulating activities, and applications of the latter in cancer therapy.

Molecularly targeted therapy for gastrointestinal cancer.

Abstract: Receptor and non-receptor tyrosine kinases (TKs) have emerged as clinically useful drug target molecules for treating gastrointestinal cancer. Imatinib mesilate (STI-571, Gleevec(TM)), an inhibitior of bcr-abl TK, which was primarily designed to treat chronic myeloid leukemia is also an inhibitor of c-kit receptor TK, and is currently the drug of choice for the therapy of metastatic gastrointestinal stromal tumors (GISTs), which frequently express constitutively activated forms of the c-kit-receptor. The epidermal growth factor receptor (EGFR), which is involved in cell proliferation, metastasis and angiogenesis, is another important target. The two main classes of EGFR inhibitors are the TK inhibitors and monoclonal antibodies. Gefitinib (ZD1839, Iressa(TM)) has been on trial for esophageal and colorectal cancer (CRC) and erlotinib (OSI-774, Tarceva(TM)) on trial for esophageal, colorectal, hepatocellular, and biliary carcinoma. In addition, erlotinib has been evaluated in a Phase III study for the treatment of pancreatic cancer. Cetuximab (IMC-C225, Erbitux(TM)), a monoclonal EGFR antibody, has been FDA approved for the therapy of irinotecan resistant colorectal cancer and has been tested for pancreatic cancer. Vascular endothelial growth factor (VEGF) and its receptor (VEGFR) are critical regulators of tumor angiogenesis. Bevacizumab (Avastin(TM)), a monoclonal antibody against VEGF, was efficient in two randomized clinical trials investigating the treatment of metastatic colorectal cancer. It is also currently investigated for the therapy of pancreatic cancer in combination with gemcitabine. Other promising new drugs currently under preclinical and clinical evaluation, are VEGFR2 inhibitor PTK787/ZK 222584, thalidomide, farnesyl transferase inhibitor R115777 (tipifarnib, Zarnestra(TM)), matrix metalloproteinase inhibitors, proteasome inhibitor bortezomib (Velcade(TM)), mammalian target of rapamycin (mTOR) inhibitors, cyclooxygenase-2 (COX-2) inhibitors, platelet derived growth factor receptor (PDGF-R) inhibitors, protein kinase C (PKC) inhibitors, mitogen-activated protein kinase kinase (MEK) 1/2 inhibitors, Rous sarcoma virus transforming oncogene (SRC) kinase inhibitors, histondeacetylase (HDAC) inhibitors, small hypoxia-inducible factor (HIF) inhibitors, aurora kinase inhibitors, hedgehog inhibitors, and TGF-beta signalling inhibitors.

Purine nucleoside analogues for the treatment of hematological malignancies: pharmacology and clinical applications.

Abstract: The purine nucleoside analogues (PNAs), fludarabine (FA), 2-CdA (2-chlorodeoxyadenosine, 2-CdA) and pentostatin (2'-deoxycoformycin, DCF) represent a group of cytotoxic agents with high activity in lymphoid and myeloid malignancies PNAs share similar chemical structure and mechanism of action Several mechanisms could be responsible for their cytotoxicity both in proliferating and quiescent cells, such as inhibition of DNA synthesis, inhibition of DNA repair and accumulation of DNA strand breaks Induction of apoptosis through the mitochondrial pathway, direct binding to apoptosome or modulation of p53 expression all lead to apoptosis, which is the main end-point of PNA action However, individual PNAs exhibit significant differences, especially in their interaction with enzymes involved in adenosine and deoxyadenosine metabolism Synergistic interactions between PNAs and other cytotoxic agents (alkylating agents, anthracycline antitumor antibiotics, cytarabine, monoclonal antibodies) have been demonstrated in both preclinical and clinical studies PNAs are highly effective in chronic lymphoid leukemias and low grade B- and T-cell non-Hodgkin's lymphomas, including Waldenstrom's macroglobulinemia DCF and 2-CdA are currently the drugs of choice in hairy cell leukemia Moreover, clinical studies have confirmed the efficacy of PNAs alone or in combination protocols in the treatment of acute myeloid leukemia and myelodysplastic syndromes Finally, PNAs, especially FA, play an important role in non-myeloablative conditioning regimens for allogenic stem cell transplantation in high-risk patients The toxicity profiles of PNAs are similar for all agents and consist mainly of dose-limiting myelotoxicity and prolonged immunosuppression Three other compounds: clofarabine, nelarabine and immucillin-H are currently being evaluated clinically

NO News is not Necessarily Good News in Cancer

Abstract: The diatomic radical nitric oxide has been the focus of numerous studies involved with every facet of cancer. It has been implicated in carcinogenesis, progression, invasion, metastasis, angiogenesis, escape from immune surveillance, and modulation of therapeutic response. In recent years, an increasing number of studies have suggested the possible involvement of nitric oxide in multiple cancer types, including melanoma. It is perhaps not surprising that conflicting viewpoints have arisen as to whether nitric oxide is beneficial or deleterious in cancer. However, it has become clear that nitric oxide possesses modulatory properties in a number of signal transduction pathways that depend on concentration and context. Our laboratory has shown that tumor expression of inducible nitric oxide synthase in melanoma patients results in poor survival. Furthermore, we demonstrated that the removal of endogenous nitric oxide in melanoma cell lines led to increased sensitivity to cisplatin-induced apoptosis in a p53-dependent manner. Others have shown antiapoptotic properties of NO in melanoma cells. However, several studies also suggest that NO can inhibit metastasis and diminish resistance. Despite the apparently conflicting observations, it is evident that NO is involved in melanoma pathology. The purpose of this review is to summarize the current literature relating to the role of NO in cancer with particular emphasis on its relevance to therapeutic resistance in melanoma. Recent evidence suggests the involvement of an intricate and complex interplay between reactive nitrogen species and reactive oxygen species. The importance of nitric oxide and its balance with other oxidative agents in the regulation of cancer cell response to therapies will be discussed. This balance may serve as an important focal point in determining patient response to therapy. The ability to control this balance could significantly influence outcome.

Small molecule antagonists of the MDM2 oncoprotein as anticancer agents.

Abstract: In this early phase of the new era of molecularly targeted patient friendly cancer chemotherapy, there is a need for novel viable anticancer molecular targets. The MDM2 oncoprotein has been validated as a potential target for cancer drug development. MDM2 amplification and/or overexpression occur in a wide variety of human cancers, several of which can be treated experimentally with MDM2 antagonists. MDM2 interacts primarily with the p53 tumor suppressor protein in an autoregulatory negative feedback loop to attenuate p53's cell cycle arrest and apoptosis functions. Inhibition of the p53-MDM2 interaction has been shown to cause selective cancer cell death, as well as sensitize cancer cells to chemotherapy or radiation effects. Consequently, this interaction has been the main focus of anticancer drug discovery targeted to MDM2. The promotion of the proteasomal degradation of the p53 protein by MDM2 is central to its repression of the tumor suppressor functions of p53, and many proteins impinge upon this activity, either enhancing or inhibiting it. MDM2 also has oncogenic activity independent of its interaction with p53, but this has so far not been explored for drug discovery. Among the approaches for targeting MDM2 for cancer therapy, small molecule antagonists have recently featured as effective anticancer agents in experimental models, although the repertoire is currently limited and none has yet entered human clinical trials. Small molecules that have been reported to disrupt the p53-MDM2 binding, thereby enhancing p53 activity to elicit anticancer effects include the following: synthetic chalcones, norbornane derivatives, cis-imidazoline derivatives (Nutlins), a pyrazolidinedione sulfonamide and 1,4-benzodiazepine-2,5-diones, as well as tryptophan derivatives. In addition to compounds disrupting p53pMDM2 binding, three compounds have been discovered that are effective in inhibiting the E3 ligase activity of MDM2 towards p53, and should serve as leads for drug discovery targeting this aspect of the p53-MDM2 interaction as well. These compounds were discovered from library screening and/or structure-based rational drug design strategies.

The MYCN Oncogene as a Specific and Selective Drug Target for Peripheral and Central Nervous System Tumors

Abstract: MYCN belongs to the MYC family of proto-oncogenes, which encode for transcription factors of the basic-helix-loop-helix-zipper (bHLHZ) class and is fundamental in the development of the peripheral and central nervous systems (PNS and CNS). While Myc is ubiquitous, MYCN has a very restricted expression pattern: it is mainly expressed during embryonic development, but then becomes downregulated, while in adults it is usually detected in B-cell development. Identification of selective inhibitors of MYCN and its mRNA and protein could be important for the development of more specific, effective and less toxic therapeutic agents for tumors of the PNS and CNS. In children, the most common tumors of the PNS and CNS are neuroblastomas and medulloblastomas, respectively. About 30% of neuroblastoma (NB) tumors present MYCN amplification/over-expression, which is associated with rapid progression and poor prognosis. N-Myc is essential during neurogenesis for the rapid expansion of progenitor cells in the brain. MYCN amplification and over-expression has been reported in medulloblastoma, and especially in the desmoplastic type. Other tumors associated with MYCN overexpression include retinoblastoma, small cell lung carcinoma, glioblastoma and certain embryonal tumors. A cell-based, N-Myc-dependent luciferase reporter gene assay to identify specific N-Myc small-molecule inhibitors has allowed identification of five compounds showing significant activity. Antisense oligodeoxynucleotides have been shown to inhibit N-Myc production and anti-tumoral activity in vitro and in vivo for NB. Peptide nucleic acids (PNA), which belong to the most recent (third) generation of nucleic acid therapeutics, form highly stable duplexes with DNA and RNA, and are resistant to degradation by nucleases and proteases. Encouraging results have been reported utilizing a PNA-based antisense strategy for inhibition of N-Myc expression in neuroblastoma.

Copper lowering therapy with tetrathiomolybdate as an antiangiogenic strategy in cancer.

Abstract: Tetrathiomolybdate (TM) is a novel anticopper agent under development for use in Wilson's disease. It acts by forming a stable tripartite complex with serum albumin and copper, rendering the complexed copper unavailable for cellular uptake. TM is a very potent anticopper agent and has an excellent safety profile. It has been shown that normal copper levels are required for optimal angiogenesis. Based on this background, we decided to evaluate TM as an anticancer agent. TM treatment of Her/2neu mice, genetically programmed to develop breast cancer, completely prevented the development of visible mammary cancers, although avascular microscopic clusters of cancer cells were present in the breasts of TM treated animals. Controls developed grossly visible tumors. TM was able to strongly inhibit tumor growth in six other rodent models. In a phase 1/2 clinical trial of advanced and metastatic cancers, freedom from progression averaged 11 months, and some individual results were quite dramatic. Eight phase 2 studies of specific cancers have been launched. TM's hypothesized mechanism of action is inhibition of angiogenic cytokines. Unlike other current approaches to antiangiogenic therapy which target single agents, we hypothesize that TM inhibits multiple angiogenic cytokines. Part of this effect appears to stem from inhibition of nuclear factor kappa B (NF(K)B), which in turn controls transcription of many angiogenic and other cytokines. However, there are probably multiple mechanisms, in that some angiogenic cytokines appear to have separate mechanisms of copper dependence. The inhibition of multiple angiogenic cytokines gives TM the potential to be a more global inhibitor of angiogenesis.

Modulation of pRB/E2F functions in the regulation of cell cycle and in cancer.

Abstract: Cell proliferation is regulated by the cell cycle, and in order to divide the cell must enter a mitotic state. Prior to mitosis the cell is required to pass through a number of checkpoints, including the critical G1/S restriction point governed by the successive phosphorylation of the retinoblastoma protein, pRb. The various proteins and regulatory factors governing pRb phosphorylation have been a major focus of study in recent years, given the central importance of G1/S transition deregulation in cancer development. This review summarises the molecular biology around the G1/S transition, focussing on the critical roles of the transcription factor family E2F and the cyclin-dependent kinase (CDK) and cyclin families involved in E2F release from pRb. Interestingly, E2F release from pRb is associated with cell proliferation; however, above a certain threshold E2F has the potential to trigger apoptosis. The review focuses on the following topics: (i) how E2F and other substrates bind to pRb at the molecular level; (ii) mechanisms by which pRb function is modulated within the cell; (iii) mechanisms that inhibit or enhance cell proliferation via the pRb/E2F pathway; (iv) how E2F can potentiate apoptotic pathways; and (v) what controls whether E2F mediates cell proliferation or apoptosis. The case for the development of agents that perturb pRb:E2F interactions will be made, as a strategy to further inform the molecular biology around this important target and as a therapeutic strategy against cancer.

Novel antisense anti-MDM2 mixed-backbone oligonucleotides: proof of principle, in vitro and in vivo activities, and mechanisms.

Abstract: The MDM2 oncogene has been suggested as a novel target for cancer therapy, based on the following observations: 1) MDM2 is overexpressed in many human cancers, including breast, colon, and prostate cancer; 2) high MDM2 levels are associated with poor prognosis in patients with cancer; 3) MDM2 overexpression is associated with advanced cancer phenotypes such as metastatic tumors and hormone-independent tumors; 4) MDM2 overexpression is associated with tumor resistance to chemotherapy and radiation therapy; and 5) inhibiting MDM2 expression or function results in tumor growth inhibition and regression. There are many options for inhibiting MDM2 function, including the use of gene silencing technologies, antibodies, peptides and small molecules. Considering the complexity of MDM2 functions, we have chosen to use gene silencing technologies including antisense oligonucleotides and RNA interference. In this article, we summarize the investigation of the antisense technology for inhibiting MDM2 expression. Antisense mixed-backbone oligonucleotides (MBO) specifically inhibit MDM2 expression in a dose- and time-dependent manner, resulting in significant anti-tumor activity in vitro and in vivo. The MBO also potentiates the therapeutic effects of chemotherapeutic agents and radiation therapy in various tumors, through both p53-dependent and p53-independent mechanisms, indicating that MDM2 inhibitors have a broad spectrum of anti-tumor activity in human cancers, regardless of p53 status. These results provide a basis for clinical evaluation of antisense anti- MDM2 oligonucleotides as chemosensitizers and radiosensitizers. In addition, the MBO has been successfully used to identify novel functions of MDM2.

Antiangiogenic therapy in acute myelogenous leukemia: targeting of vascular endothelial growth factor and interleukin 8 as possible antileukemic strategies.

Abstract: Acute myelogenous leukemia (AML) is an aggressive disorder with an overall disease-free survival of 40-50% even for the younger patients under 60 years of age who can receive the most intensive treatment. The median age at the time of diagnosis is 60-65 years, and the large majority of elderly patients usually receive less intensive chemotherapy or only supportive therapy due to the high treatment-related mortality when using intensive therapy for elderly individuals. Thus, there is a need for new therapeutic approaches to improve the treatment in younger patients and to make AML-directed therapy with acceptable toxicity possible in elderly individuals. Angiogenesis seems to be important both for leukemogenesis and susceptibility to intensive chemotherapy, and antiangiogenic strategies are therefore considered for the treatment of AML. The two proangiogenic mediators vascular endothelial growth factor (VEGF) and interleukin 8, (IL-8, also referred to as CXCL8) seem to be important in human AML: VEGF is released at increased levels due to interactions between AML cells and neighboring nonleukemic cells, whereas IL-8 is released at high levels by native human AML cells. Thus, VEGF as a therapeutic target in AML is suggested both by experimental and clinical observations, whereas IL-8 as a target is mainly suggested by experimental evidence. In the present review we describe and discuss (i) the angioregulatory network of soluble mediators in AML, including both the systemic levels and local release by native human AML cells; and (ii) various therapeutic approaches to target VEGF and IL-8. Although single angioregulatory mediators can be targeted, it should be emphasized that the final effect of soluble mediators on angioregulation is determined by a complex angioregulatory network that varies between AML patients, and the final effect of targeting single mediators may therefore differ between patient subsets.

New Indications for Established Drugs: Combined Tumor-Stroma-Targeted Cancer Therapy with PPARγ Agonists, COX-2 Inhibitors, mTOR Antagonists and Metronomic Chemotherapy

Abstract: In search of new strategies for the treatment of cancer, the interaction between tumor and stroma attracts more and more attention. Disruption of stroma functions, e.g. angiogenesis, has evolved into a promising target for cancer therapies. Since stromal cells are genetically stable, stroma-targeted therapies seem to be less susceptible to the development of drug resistance. Several well-established drugs, which had initially been developed for other indications, also exhibit antitumor activity. Among those, PPARgamma agonists, COX-2 inhibitors, and mTOR antagonists are the most remarkable examples. Current research data and clinical experience suggest that these drugs target stroma functions in cancer, in particular angiogenesis, but immunological mechanisms and direct antitumor effects seem to participate as well. In addition to these drugs, frequent administration of low-dose chemotherapeutics, referred to as metronomic chemotherapy, reveals profound anti-angiogenic effects. In the meantime, a multitude of preclinical and clinical studies have been undertaken, which demonstrate the efficacy of these drugs in cancer therapy. Combinatorial use of these agents has been suggested to be superior in terms of antitumor efficacy and prevention of drug resistance. The toxicity of these therapies is surprisingly low compared with conventional high-dose chemotherapy regimens. Patients with advanced disease, often heavily pretreated and presenting multiple drug resistance, could particularly profit from such tumor-stroma-targeted therapies. However, larger randomized clinical trials are required for further evaluation and optimization of this concept.

Control of copper status for cancer therapy.

Abstract: Copper is a trace element which is tightly regulated in mammals and lower animals. Disruptions of copper homeostasis in humans are rare and they cause serious disorders such as Wilsons disease and Menkes disease. Copper plays an important role in promoting physiological and malignant angiogenesis. Formation of new blood vessels by a tumor enables tumor growth, invasion, and metastasis are copper requiring processes. The copper chelator tetrathiomolybdate (TM), which quickly and effectively depletes copper stores, is under investigation as an anti-angiogenic agent. Promising results from in vitro experiments, in pre-clinical animal models, and in a phase I clinical trial have led to several phase II trials of TM in patients with advanced cancers.

Treatment modalities for hepatocellular carcinoma.

Abstract: Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide, accounting for approximately 5 to 10% of all cancers. It is estimated to cause approximately 1 million deaths annually. Currently, no adjuvant or palliative treatment modalities have been conclusively shown to prolong survival in HCC. Despite the high mortality and frequency of this cancer, surgical resection is an option for only a small proportion of patients, less than 18%. Liver cirrhosis is the most common cause of HCC and necessitates the preservation as much liver as possible, resulting in local ablation, intra-arterial and systemic treatments being major therapeutic modalities. Through better understanding of the molecular basis of hepatocarcinogenesis, new preventative and treatment modalities have recently emerged. This article reviews the current treatment options and new therapeutic advances for HCC including antiangiogenesis therapy, targeted therapy and antisense gene targeting. Future clinical trials and research will help to evaluate and improve both systemic and targeted molecular therapies for this complex disease.

Antiproliferative Efficacy of Angiotensin II Receptor Blockers in Prostate Cancer

Abstract: An apparent low prevalence of cancer in hypertensive patients receiving angiotensin converting enzyme inhibitors is reported; however, the molecular mechanisms have not been elucidated. Angiotensin-II (Ang-II) is well known to be associated with hypertension, as a main peptide of the renin-angiotensin system, and its detailed molecular mechanisms have recently been elucidated. For instance, Ang-II directly activates the mitogenic signal transduction pathway through the angiotensin-II type-1 (AT1) receptor in smooth muscle cells and cardiac myocytes. Ang-II receptor blockers (ARBs), a class of antihypertensive agent, suppress signal transduction pathways mediated by growth factors such as epidermal growth factor (EGF), through the AT1 receptor. Our studies demonstrated that an ARB had the potential for antiproliferative effects and inhibition of angiogenesis in prostate cancer cells. The AT1 receptor is categorized in the guanosine phosphate binding protein-coupled receptors (GPCRs), which are viewed as critical regulators of the interactions between epithelial and stromal cells. Hence, we consider that in overcoming prostate cancer, it is very important to inhibit GPCR signaling in cancer cells by ARBs. It is unclear how prostate cancer growth changes from being hormone dependent to independent, and no effective therapy has therefore been developed. Our clinical data revealed that ARB administration decreased prostate specific antigen (PSA) and improved performance status in patients with hormone-refractory prostate cancer. This review provides an insight into the key role of Ang-II and the possibility of ARBs for molecular targeting of mitogenesis and angiogenesis in prostate cancer.

Hypomethylation of Urokinase (uPA) Promoter in Breast and Prostate Cancer: Prognostic and Therapeutic Implications

Abstract: Methylation of CpG islands of tumor suppressor genes, growth factors, and hormone receptors among other genes causes epigenetic changes in chromatin structure without altering DNA sequence to regulate transcription of these genes. This epigenetic regulation of gene expression plays an important role in the process of tumor invasion, growth and metastasis in malignancies. In hormone dependent malignancies such as breast and prostate cancer, sex steroids play an important role in the process of tumor initiation and progression. These malignancies are often initiated as a less aggressive hormone-responsive type that gradually progresses to become highly invasive and hormone-insensitive. At the early stages, cells lose a functional hormone receptor due to mutations, blockage of signaling pathway or hormone receptor gene silencing. This transition of cancer cells causes them to become refractory to the standard hormone therapies. In later stages, important factors like growth factors, cytokines and proteases promote tumor growth, invasion and metastases. The most commonly implicated protease in these processes is urokinase type plasminogen activator (uPA), which is known to be expressed in a number of malignancies including breast and prostate cancer and is directly associated with the higher invasive and metastatic potential of malignancies. In this chapter, we will review DNA methylation as the underlying molecular mechanism regulating uPA gene expression and its potential diagnostic, prognostic and therapeutic implication.

Cellular and molecular surrogate markers to monitor targeted and non-targeted antiangiogenic drug activity and determine optimal biologic dose.

Abstract: Perhaps the most significant recent advance in oncology therapeutics has been the approval of various "molecularly targeted" anti-cancer drugs. Currently, there are a large number of similar drugs in early or late stage development, including antiangiogenic agents. Clinical development of such drugs suffers from several handicaps including determining whether a patient's cancer expresses the target and is functionally contributing to cancer growth, monitoring biologic activity, and determining optimal biologic dose. The last problem is related to the low frequency of objective tumor responses (tumor shrinkage) caused by such drugs, or the lack of dose limiting toxicities necessary to define a maximum tolerated dose (MTD), or expression of optimal therapeutic activity at doses below the MTD, when one can be defined. These problems necessitate the development of alternative pharmacodynamic surrogate markers. Here we summarize several such promising markers for monitoring targeted antiangiogenic activity, and establishing optimal therapeutic/biologic dosing. The first is molecular--plasma VEGF--levels of which are rapidly and significantly increased in a dose dependent manner after injection of normal or tumor bearing mice with anti-VEGFR-2 antibodies. The second is a cellular marker, and more generic in nature--circulating VEGF receptor-2 positive cells found in peripheral blood, some of which may be circulating endothelial progenitor cells. Levels of such cells are suppressed in a dose dependent manner which correlate with previously determined optimal biologic/therapeutic anti-tumor activity of various antiangiogenic drugs or treatments. Finally, another promising marker we discuss is soluble VEGFR-2.

Chemosensitization by antisense oligonucleotides targeting MDM2.

Abstract: The MDM2 oncogene is overexpressed in many human cancers, including sarcomas, certain hematologic malignancies, and breast, colon and prostate cancers. The p53-MDM2 interaction pathway has been suggested as a novel target for cancer therapy. To that end, several strategies have been explored, including the use of small polypeptides targeted to the MDM2-p53 binding domain, anti-MDM2 antisense oligonucleotides, and natural agents. Different generations of anti-human-MDM2 oligonucleotides have been tested in in vitro and in vivo human cancer models, revealing specific inhibition of MDM2 expression and significant antitumor activity. Use of antisense oligos potentiated the effects of growth inhibition, p53 activation and p21 induction by several chemotherapeutic agents. Increased therapeutic effectiveness of chemotherapeutic drugs in human cancer cell lines carrying p53 mutations or deletions have shown the ability of MDM2 inhibitors to act as chemosensitizers in various types of tumors through both p53-dependent and p53-independent mechanisms. Inhibiting MDM2 appears to also have a role in radiation therapy for human cancer, regardless of p53 status, providing a rationale for the development of a new class of radiosensitizers. Moreover, MDM2 antisense oligonucleotides potentiate the effect of epidermal growth factor receptor (EGFR) inhibitors by affecting in vitro and in vivo proliferation, apoptosis and protein expression in hormonerefractory and hormone-dependent human prostate cancer cells. These data support the development, among other MDM2 inhibitors, of anti-MDM2 antisense oligonucleotides as a novel class of anticancer agents, and suggest a potentially relevant role for the oligonucleotides when integrated with conventional treatments and / or other signaling inhibitors in novel therapeutic strategies.

Squalamine: A Polyvalent Drug of the Future?

Abstract: The purpose of this mini-review is to summarize and highlight the different advances in our understanding of the antimicrobial and antiangiogenic activity of squalamine, a cationic steroid isolated in 1993 from the dogfish shark Squalus Acanthias. Indeed, squalamine has shown to be useful for the treatment of important diseases such as cancers (lung, ovarian, brain and others), age-related macular degeneration (AMD) and the control of body weight in man. All these results led to a question: could we consider squalamine as a polyvalent drug of the future?