Current Cancer Drug Targets - Volume 5, Issue 3, 2005

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 cellcell 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.

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.

Receptor and non-receptor tyrosine kinases (TKs) have emerged as clinically useful drug target molecules for treating gastrointestinal cancer. Imatinib mesilate (STI-571, Gleevec™), 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™) has been on trial for esophageal and colorectal cancer (CRC) and erlotinib (OSI-774, Tarceva™) 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™), 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™), 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™), matrix metalloproteinase inhibitors, proteasome inhibitor bortezomib (Velcade™), 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 hypoxiainducible factor (HIF) inhibitors, aurora kinase inhibitors, hedgehog inhibitors, and TGF-β signalling inhibitors.

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 (NFKB), 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.

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.

Prevention of cancer through the activation of the immune system has been explored in recent years in preclinical systems thanks to the availability of several new transgenic mouse models that closely mimic the natural history of human tumors. The most thoroughly investigated model of cancer immunoprevention is the mammary carcinoma of HER-2/neu transgenic mouse. In this system it has clearly been shown that the activation of immune defences in healthy individuals can effectively prevent the subsequent onset of highly aggressive mammary carcinomas. A complete prevention was obtained using a combination of three signals (the so called “triplex” vaccine) that included the specific antigen (p185, the product of HER-2/neu) and nonspecific signals like allogeneic histocompatibility antigens and interleukin 12. The analysis of protective immune responses in models of cancer immunoprevention revealed some unexpected features, in particular the central role of antibodies in immunoprevention, at variance with conventional immunotherapy which is firmly based on cytotoxic T cells. In the HER-2/neu system anti-p185 antibodies, in addition to immunological functions leading to tumor cell lysis, inhibit p185 dimerization and induce its internalization, resulting in the inhibition of mitogenic signaling. Most current tumor antigens appear to be unsuitable targets for cancer immunoprevention. An ideal antigen should have a crucial pathogenetic role in tumor growth to avoid the selection of antigen loss variants. Downregulation of major histocompatibility complex (MHC) expression during tumor progression frequently limits antigen recognition by MHC-restricted T cells. Thus an ideal antigen for cancer immunoprevention should be recognized both by T cells and by antibodies. Antibody binding to cell surface oncogenic determinants, in addition to complement- and cell-mediated tumor cell lysis, can block mitogenic signaling and induce internalization, resulting in tumor growth arrest. A search for new tumor antigens should be conducted among molecules that are directly involved in neoplastic transformation and are recognizable by the immune response also in MHC loss variants. Novel tumor antigens fulfilling both conditions will be crucial for the development of cancer immunoprevention and will provide new targets also for cancer immunotherapy.