Current Cancer Drug Targets - Volume 4, Issue 5, 2004

p53 is a key tumor suppressor that plays a critical role in coordinating the response of cells to a diverse range of stress conditions, e.g. oncogenic activation, hypoxia or DNA damage. Induction of cell death by apoptosis in response to stress by p53 is crucial for the prevention of tumor development as well as for the response to anticancer therapy. p53 triggers apoptosis through multiple mechanisms, including mitochondrial and death receptor pathways, cytoskeleton changes, suppression of survival signalling, and induction of hypoxia. Lesions in the p53 pathway occur so frequently in cancer, regardless of patient age or tumor type, that they appear to be part of the life history of a majority of cancer cells. Given an extremely high potency of apoptosis induction by functional p53, it appears that anti-cancer strategies based on p53 reactivation should be efficient and applicable in a wide range of human tumors. Tumor cells are prone to p53-induced apoptosis due to oncogene activation. Therefore it is conceivable that p53-based therapeutic strategies will not require selective targeting of tumor cells.

Tumor development, growth, and progression depend on some combination of altered cell cycle regulation, excessive growth factor pathway activation, and decreased apoptosis. Understanding the complex molecular mechanisms that underlie these processes should therefore lead to the identification of potential targets for therapeutic intervention. The estrogen receptor and HER-2 / neu were among the earliest targets investigated, ultimately leading to the widespread use of tamoxifen and trastuzumab, respectively, in the treatment of breast cancer. Major research advances have since led to other classes of targeted therapies, including cyclin-dependent kinase inhibitors, histone deactylase inhibitors, and receptor tyrosine kinase inhibitors. The following review provides a discussion of the molecular biology associated with each of these types of therapies as well as a detailed summary of the preclinical and clinical data published on selected compounds from each of these subgroups.

The majority of proteins are modified in post-translational events and one of the most common of these is glycosylation. Many reports describe alterations to the normal cellular glycosylation in cancer but detailed knowledge of the underlying structures and mechanisms that result in the altered glycosylation of cancer glycoproteins have been hindered by the inherent complexity of glycans themselves. Improved analytical tools for the study of glycosylation and application of molecular techniques for the characterisation of the genes encoding glycosyltransferases have, however, enabled the structural identification of some of the cancer-associated changes in glycosylation. The observed alterations in protein glycosylation in cancer have led to clinical trials in which glycans on cancer cell-surface proteins are targeted. These new approaches to cancer treatment include immunotherapy and carbohydrate-processing inhibitor-based strategies. Compounds that mimic glycans involved in the metastatic dissemination of cancer are also actively sought. The results that have been obtained and the long-term potential of these new approaches are discussed in this review article.

Identification of relevant targets for cancer therapy is a major goal in cancer research. In this field, the identification of tumor antigens has opened the possibility of inducing specific anti-tumor immune responses. Among these antigens, carcinoembryonic antigen (CEA) is especially relevant because CEA is expressed in a wide variety of adenocarcinomas such as colon, rectum, pancreas, gastric, breast, etc. The present review focuses on different strategies to induce anti-CEA immune responses. In a first group of strategies, the antigen is administered using viral and bacterial vectors expressing CEA, dendritic cells loaded with CEA protein, or dendritic cells transfected with DNA or RNA expressing CEA. A second group of strategies is based on immunizations with antigenic peptide determinants from CEA, rather than with immunogens containing the whole protein. This has been possible due to the identification of different peptide determinants from CEA, which when presented by MHC class I molecules, are recognized by T cytotoxic lymphocytes. More recently, due to the importance of CD4+ T cells in the induction of immune responses, T helper peptides presented by MHC class II molecules have also been identified. To overcome the poor immunogenicity of CEA-derived peptide determinants, a common feature of self-antigens, their sequence has been modified to improve binding to MHC molecules or recognition by T cell receptors. Finally, in order to enhance immunization efficacy, some of these strategies have combined the administration of immunogens and cytokines or co-stimulatory molecules. Some of the immunization protocols developed are being tested in clinical trials with promising results. Thus, CEA may prove to be a valuable target antigen for the therapy of a high number of malignancies.

Prostate cancer is the most frequently diagnosed tumor in industrialized countries. Endocrine therapy, which is based on interference with androgen signaling is only palliative. Drugs used in prostate cancer therapy are luteinizing hormone releasing hormone (LHRH) agonists and antiandrogens. Application of LHRH agonists leads to suppression of the levels of circulating androgens, and antiandrogens block the function of the androgen receptor (AR). The steroidal antiandrogen cyproterone acetate and nonsteroidal compounds hydroxyflutamide and bicalutamide are used most frequently. They prevent acquisition of a transcriptionally active conformation of the AR. It became clear that tumors progress to therapy resistance in the presence of the AR which might be structurally altered. These mutations generate receptors that respond to other steroids and antiandrogens by increased activation. In addition, AR expression increases during endocrine treatment. AR is also activated by nonsteroidal compounds such as growth factors, interleukin-6, and neuropeptides. Therefore, new experimental approaches are needed to antagonize AR expression and function more efficiently. The AR associates with a number of proteins, coactivators and corepressors. There are indications that expression of some of these proteins is altered in prostate cancer, a fact which might be important for improvement of endocrine therapy.