Current Cancer Drug Targets - Volume 1, Issue 2, 2001

The most prevalent problem in cancer therapy is the regrowth and metastasis of malignant cells after standard treatment with surgery, radiation, and / or chemotherapy. Gene therapy approaches have suffered from the inadequate transduction efficiencies of replication-defective vectors that have been used thus far. Replication-competent vectors, particularly adenoviruses that cause cytolysis as part of their natural life cycle, represent an emerging technology that shows considerable promise as a novel treatment option, particularly for locally advanced or recurrent cancer. A number of oncolytic adenoviruses that are designed to replicate selectively in tumor cells by targeting molecular lesions inherent in cancer, or by incorporation of tissue-specific promoters driving the early genes that initiate viral replication, are currently being tested in clinical trials. The results of these clinical trials indicate that, in its current form, oncolytic adenovirus therapy shows the best results and achieves an enhanced tumoricidal effect when used in combination with chemotherapeutic agents such as cisplatin, leucovorin and 5-fluorouracil. Nevertheless, each of the oncolytic adenoviruses in current use exhibits characteristic shortcomings, and there is still considerable room for improvement. Current strategies for improving the selectivity and efficacy of oncolytic adenoviruses include molecular engineering of tumor cell-specific binding tropism, selective modifications of viral early genes and incorporation of cellular promoters to achieve tumor-specific replication, augmentation of anti-tumor activity by incorporation of suicide genes, and manipulation of the immune response.

A key problem in the effective treatment of patients with cancer (both leukemia and solid tumors) is to distinguish between tumor and normal cells. This problem is the main reason why current treatments for cancer are often ineffective. There have been remarkable advances in our understanding of the molecular biology of cancer that provides new selective tumor destruction mechanisms. The molecular characterization of the tumor-specific chromosomal abnormalities has revealed that fusion proteins are the consequence in the majority of cancers. These fusion proteins result from chimeric genes created by the translocations, which form chimeric mRNA species that contain exons from the genes involved in the translocation. Obviously, these chimeric molecules are attractive therapeutic targets since they are unique to the disease (they only exist in the tumor cells but not in the normal cells of the patient), allowing the design of specific anti-tumor drugs. Inhibition of chimeric gene expression by anti-tumor agents specifically kills leukemic cells without affecting normal cells. As therapeutic agents targeting chimeric genes, zinc-finger proteins, antisense RNAs or hammerhead-based ribozymes have been used. All of these agents have some limitations, indicating that new therapeutic tools are required as gene inactivating agents that should be able to inhibit any chimeric fusion gene product. Recently, we have used the catalytic RNA subunit of RNase P from Escherichia coli, which can be specifically directed to cut any mRNA sequence, to specifically destroy tumor-specific fusion genes created as a result of chromosomal translocations. In this chapter, we will review the advances made to selectively destroy tumor cells through specific inhibition of products resulting from chromosomal translocations.

The catalogue of gene alterations in human cancer is growing rapidly. Alterations in specific genes that play important roles in diverse cellular functions such as cell adhesion, signal transduction, differentiation, development or DNA-repair have been identified. Cancer-associated mutant cell surface molecules are very attractive candidates to target tumor cells because they offer the possibility of minimizing toxic effects to non-tumor cells. The cell adhesion molecule E-cadherin has been shown to play a major role in determining which of the two subtypes of gastric cancer, diffuse or intestinal type, develops. E-cadherin gene mutations typically affect the extracellular portion of the homophilic receptor and are frequently found in patients with diffuse-type tumors. Cancer-specific monoclonal antibodies against the E-cadherin mutational hot spot region are now available. In cell culture and in animal studies we have shown that mutation-specific antibodies exclusively target cells expressing abnormal E-cadherin. Those cells expressing the normal protein were not affected, demonstrating the specificity of our approach. After linking to toxins, drugs or radiolabeled mutation-specific antibodies could serve as very specific agents to treat small tumor deposits. Patients for this novel individualized cancer therapy can be identified within a day using routine immunohistochemistry of biopsies.

Increasing knowledge of the structure and function of the Epidermal Growth Factor Receptor (EGFR) subfamily of tyrosine kinases, and of their role in the initiation and progression of various cancers has led to the search for inhibitors of signaling molecules that may prove to be important in cancer therapy. The complex nature of EGFR biology allows for potential opportunities for EGFR inhibitors in a number of areas of cancer therapy, including proliferative, angiogenic, invasive, and metastatic aspects. Different approaches have been used to target either the extracellular ligand-binding domain of the EGFR or the intracellular tyrosine kinase region that results in interference with its signaling pathways that modulate cancer-promoting responses. Examples of these include a number of monoclonal antibodies, immunotoxins and ligand-binding cytotoxic agents that target the extracellular ligand binding region of EGFR, and small molecule inhibitors that target the intracellular kinase domain and act by interfering with ATP binding to the receptor. During the past 3 years, significant progress has been made towards the identification of new structural classes of small molecule inhibitors that show high potency and specificity towards EGFR. The search for new small molecules that inhibit kinases has included traditional approaches like the testing of natural products, random screening of chemical libraries, the use of classical structure-activity-relationship studies, and the incorporation of structure-based drug design and combinatorial chemistry techniques. There has been a significant improvement in the development of selective EGFR inhibitors with the use of a structure-based design approach employing a homology model of the EGFR kinase domain. Molecular modeling procedures have been used to generate novel molecules that are complementary in shape and electrostatics to the EGFR kinase domain topography. This review focuses on some examples of the successful use of this method.

Several cytokines and growth factors modulate angiogenesis through a fine tuned paracrine or autocrine mode of action. Among them is plateled-derived endothelial cell growth factor (PD-ECGF), which is highly is expressed in tumors, and is angiogenic by stimulation of endothelial cell migration. Studies have shown that PD-ECGF is identical to the well known enzyme thymidine phosphorylase (TP), which is involved in thymidine metabolism and homeostasis. Interestingly, PD-ECGF plays an angiogenic role as a result of its TP enzyme activity. In light of these findings, PD-ECGF / TP should not be considered a true growth factor, and its PD-ECGF name is now actually a misnomer. Recently, TP activity was thought of as an interesting potential two-face target for controling tumor-dependent angiogenesis. In fact, on one hand, its high levels of expression in tumors compared to non-neoplastic regions, and its broad substrate specificity suggested that TP could be used as an enzymatic tool to locally activate anticancer nucleoside bases or base analogs. On the other hand, its enzyme-dependent angiogenic activity engendered the search for specific inhibitors to reduce TP-dependent angiogenesis. This review will describe TP, its activity, its possible mechanisms of action and its role in angiogenesis. Particular attention will be focused on the design and biological characterization of novel TP inhibitors which recently showed promising anticancer activity.

The introduction of microarray technology has dramatically changed the way that researchers address many biomedical questions. DNA microarrays can measure expression of thousands of genes simultaneously, providing extensive information on gene interaction and function. Microarray technology is a powerful tool for identifying novel molecular drug targets and for elucidating mechanisms of drug action. Furthermore, microarrays can monitor the global profile of gene expression in response to specific pharmacologic agents, providing information on drug efficacy and toxicity. Over the last several years, dramatic advancements have occurred in array technology. In this review we describe basic aspects of microarray instrumentation and experimentation. Each of the major array formats including oligonucleotides arrays, spotted arrays, and macroarrays are examined, and advantages and options for using each format are presented. Important factors in the design and analysis of microarray experiments are also discussed. Most importantly, we explore recent developments in microarray technology that are relevant to pharmacogenomics and the discovery of gene function.