Current Drug Targets - Volume 5, Issue 4, 2004

This issue of CURRENT DRUG TARGETS is focused on cancer therapy, one of the most challenging problems for humanity. Indeed, this problem represents the nexus of life science with many other disciplines. Outburst of modern biology uncovers the fundamental mechanisms of single cell behavior (e.g., gene expression regulation, macromolecular biosynthesis and transport, cell cycle progression, division, death) which, in turn, sets stage for understanding tumor cell-organism interactions such as control of differentiation, clonal proliferation, evasion from surveillance, to mention a few major paradigms of cancer biology. This basic knowledge needs to be translated into therapeutically applicable strategies, a task for unified efforts of biologists and medical doctors with pharmacologists, physico-chemists, medicinal chemists... The contributors to the present volume, although their areas of expertise differ, follow the key principle of mechanism based anticancer therapy, namely, identification of a particular structure critical for cancer cell and search for means of targeting the process(-es) in which this structure is involved. For the deliberate therapeutic intervention the authors analyze signal transduction machinery, drug metabolism systems, and cell death cascades. The importance of preferential accumulation of the killer in the tumor over normal tissues is emphasized in novel approaches for delivery of small molecules and genetic constructs. We sought to combine laboratory investigations with clinically relevant topics; a trend for 'translational approach' is demonstrated in the studies of individual mechanisms as predictive factors of response to therapy. Still the major question remains to be resolved: would the intervention into one single mechanism be sufficient for successful treatment? It is the redundant regulation of key cellular functions, the notorious biological plasticity that allows tumor cells to escape treatment. If cancer is the 'unfolding' of this plasticity, the 'grown-to-perfection' ability to avoid control, then little is left for a simplified hope. This by no means underscores the necessity for studying cancer related mechanisms: rather, these investigations build the foundation for manipulation with multiple targets. The more processes in tumor cells become available for therapeutic management the better should be the outcome. One should believe that enormous intellectual effort and unprecedented material investment aimed at cancer treatment will be justified by any advancement in this everlasting enigma, as complex as life itself...

Some types of cancer respond far less favorably to treatment than do others. A quantitative estimate of this intuition can be obtained from the SEER (Surveillance, Epidemiology and End-Results) Cancer Statistics Review. Of particular interest, from a drug resistance perspective, are the five-year survival data for patients presenting with tumors that were diagnosed as “distant”. A good correlation can be found between those numbers and an estimate of treatment successes obtained from a survey of current literature on chemotherapy applied to cancers originating from these various tissues. These two measures, considered together, define “the axis of intractability”, a parameter that characterizes the (possibly) inherent, physiological basis of the tissue-by-tissue intractability of cancers. Exploring the basis of this intractability, it appears that factors other than the classical ABC transporter-based, multidrug resistance systems probably play a major role. An ineffective DNA repair system, coupled to reduced apoptosis, is the basis for the inherent tractability of testicular cancer. For other tissues, important contributions to resistance arise from cell adhesion-mediated drug resistance, which is overcome when cells are released from tissues during anoikis. Making a direct comparison between gene expression in solid tumors and their corresponding cell lines, genes controlling the extracellular matrix and cell-cell communication appear among the genes that are over-expressed in the solid tumors, while genes coding for the protein biosynthesis system are over-expressed in the cell lines. The more tractable cancers are closer to the cell lines in their expression profiles of these sets of genes.

Since late 1950s the main strategies to treat cancer, besides surgery, have been radiotherapy or chemotherapy. These approaches work primarily by damaging proliferating cells at the level of DNA replication or cell division, and inducing apoptotic cell suicide as a secondary response to the damage. In recent years, efforts to improve cancer therapy have focused on the development of more selective, biological mechanism based approaches that can help to overcome tumor resistance as well as minimize toxic side effects. In the present review new strategies and new targets for biological cancer therapy will be discussed. In particular, new angiogenic pathways discovered in melanoma will be discussed in relationship to a more efficient anticancer strategy. In summary, this review tries to identify the most logical targets and the most useful mechanisms of tumor inhibition in light of new knowledge from the basic research including human genome project.

Poly(ADP-ribose) polymerization is a unique post-translation protein modification that utilizes an ADP-ribose moiety from NAD+ to form long and branched polymers attached via glutamic acid residues to nuclear acceptor proteins. The corresponding enzyme, poly(ADPribose) polymerase (PARP-1), is a zinc finger-containing protein, which allows PARP-1 binding to either double- or single-strand DNA breaks. The catalytic activity of PARP-1 is strictly dependent on the presence of strand breaks in DNA, and is modulated by the level of automodification. PARP-1 is regarded as an intracellular sensor for DNA strand breaks, and its function has been implicated in cellular processes that require DNA cleavage and rejoining reactions, such as DNA replication, recombination and repair. Recent studies have also implicated PARP-1 in the regulation of gene expression through modification of transcription factors by poly(ADP-ribosyl)ation or its direct binding to gene-regulating DNA sequences. The latter is attributable to PARP's ability to recognize and bind to various structural discontinuities in the DNA duplex in the absence of DNA strand breaks, such as three- or four-way junctions, bent DNA, and base unpaired regions. Cumulatively, these findings indicate that PARP-1 plays a pivotal role in the maintenance of the genome integrity during the normal functioning of eukaryotic cells as well as in the cellular responses to DNA damage, and that PARP-DNA interactions are indispensable for PARP function. This review summarizes the data on DNA-binding properties of PARP- 1 and relates them to the development of strategies for sensitizing tumor cells to genotoxic treatments.

Proteins containing a caspase-associated recruitment domain (CARD) have been established as key regulators of cell death and, more recently, cytokine production. During the last several years, the number of proteins identified within this family has grown immensely and many aspects of their function point to their potential utility as novel drug targets in the treatment of cancer. Several CARD family proteins are critical components of the conserved cell death machinery which, when dysregulated, promotes oncogenesis and contributes prominently to tumor resistance to chemotherapy. The pro-apoptotic protein Apaf1, which is inactivated in some cancers, is a CARD protein that is indispensable for mitochondria-induced apoptosis. Other antiapoptotic CARD proteins, such as TUCAN / CARDINAL / CARD8, have been shown to protect tumors from cell death stimuli and to be over-expressed in certain forms of cancer. Therapeutics that activate or inhibit CARD proteins may therefore be potentially utilized as novel chemo-sensitizing agents when used in conjunction with conventional chemotherapy. Other CARD proteins influence cellular processes through the regulation of NF-kB or caspase-1, which governs the levels of interleukin-1β (IL-1β). In addition to its pro-inflammatory properties, this cytokine also contributes to neoplastic progression by promoting angiogenesis, proliferation, and the metastasis of many tumors. Many of the IL- 1β-regulating CARD proteins also contain a nucleotide binding / oligomerization domain known as a NACHT and may therefore be amenable to targeting by small molecule compounds. This review examines the role of CARD proteins in cytoprotection and cytokine processing in the context of neoplasia and presents strategies for using this information in devising potential novel anticancer agents.

Treatment with anti-cancer agents in most cases ultimately results in apoptotic cell death of the target tumor cells. Unfortunately, tumor cells can develop multidrug resistance, e.g., by a reduced propensity to engage in apoptosis by which they become insensitive to multiple chemotherapeutics. Ceramide, the central molecule in cellular sphingolipid metabolism, has been recognized as an important mediator of apoptosis. Moreover, an increased cellular capacity for ceramide glycosylation has been identified as a novel multidrug resistance mechanism. Indeed, virtually all multidrug resistant cell types exhibit a deviating sphingolipid composition, most typically an increased level of glucosylceramide. Thus, the enzyme glucosylceramide synthase, which converts ceramide into glucosylceramide, has emerged as a potential target to increase apoptosis and decrease drug resistance of tumor cells. In addition, several other steps in the pathways of sphingolipid metabolism are altered in multidrug resistant cells, opening a perspective on additional sphingolipid metabolism enzymes as targets for anti-cancer therapy. In this article, we present an overview of the current understanding concerning drug resistance-related changes in sphingolipid metabolism and how interference with this metabolism can be exploited to over come multidrug resistance.

The efficacy of cancer therapy is compromised by the fact that there are currently no good ways to predict which patients will benefit from treatment. This long standing goal is closer to becoming a reality as more is learned about the molecules that affect the activities of various therapeutic agents. The fluoropyrimidine antimetabolites drugs have been in clinical use for over 4 decades and the cellular proteins important for their activities have been studied in detail. The most important are the major target enzyme, thymidylate synthase (TS) and the rate limiting enzyme in the degradation pathway, dihydropyrimidine dehydrogenase (DPD), equally important for the analogue capecitabine is thymidine phosphorylase (TP), which is rate limiting for activation of this prodrug. A number of assays are available for these enzymes, including enzyme activity measurements, quantitative PCR for RNA expression and immunological methods for protein expression. With each of these methods, more clinical studies are required to validate their clinical usefulness.

This review presents molecular targeting approaches in anticancer drug delivery systems (DDS) and identifies new developments in these systems. Targeting approaches include passive targeting (enhanced permeability and retention effect), targeting specific tumor conditions, topical delivery and active targeting, namely, targeting organs, cells, intracellular organelles and molecules, sandwich targeting, promoter targeting, indirect targeting and targeting by external stimuli. A novel advanced proapoptotic anticancer DDS that utilizes several molecular targets will be considered. Experimental data suggest that this DDS can simultaneously: (1) induce cell death; (2) prevent adverse effects on healthy tissues; (3) suppress and prevent multidrug resistance; and (4) inhibit cellular antiapoptotic defense.