Current Cancer Drug Targets - Volume 4, Issue 2, 2004

Cancer drugs have traditionally been identified in screens designed to produce broad biological end points such as cell death. A serious undesired outcome of drugs discovered in these screens is that the mechanism of drug action is unknown and such drugs often have adverse side effects. Designing cancer drugs that act on specific targets offer the advantage that the mechanism of drug action can be understood and accurately monitored in clinical trials leading to development of better drugs. The pharmacological industry has recently shifted to a target directed drug discovery model. However, until recently potential cancer drug targets comprised of only a small fraction of the human genome. The human genome project and high-throughput structural and functional genomics have dramatically increased the number of cancer drug targets. Deciphering cancer drug targets requires the understanding of biochemical pathways that are affected in the cancer genome. It has been suggested that utilization of Single-nucleotide polymorphisms (SNPs) will aid in identifying individuals at high risk of developing certain cancers, and will also help in development of tailored medication or identify genetic profiles of specific drug action and toxicity. Achieving successful new cancer drug development schemes will require a merger of research disciplines that include pharmacology, genomics, comparative genomics, functional genomics, proteomics and bioinformatics. In this review the significance and challenges of these rapidly evolving technologies in cancer drug target discovery are discussed.

Protein kinase C (PKC) comprises a family of isozymes (α, βI, βII,γ δ, ε, θ,& eegr;, λ/ι [mouse/human], and ζ) which are involved in signal transduction from membrane receptors to the nucleus. Activation of PKC by phorbol esters promotes tumor formation, and from that it was concluded that inhibitors of PKC might prevent carcinogenesis or inhibit tumor proliferation. However, the situation is more complicated because the exact function of the different PKC isozymes is not known at present. They have been shown to be involved in synaptic transmissions, the activation of ion fluxes, secretion, cell cycle control, differentiation, proliferation, tumorigenesis, metastasis and apoptosis. Modulators such as bryostatin-1, phospholipid analogues, PKC-activating adriamycin derivatives, CGP41251, UCN-01, and antisense oligonucleotides directed against PKCα, have shown antitumor activity in cancer patients. PKC inhibitors are not specific to PKC, but also interact with other signaling molecules, which may contribute to the antitumor effects. Modulators of PKC have also been shown to influence non-MDR1-mediated and MDR1- mediated antitumor drug resistance. This review is focussed on the role of PKC isozymes in human cell proliferation, apoptosis and antitumor drug resistance, and on the use of PKC modulators as antitumor agents.

In normal healthy tissues, an equilibrium is established between cell death and survival. This equilibrium ensures that cells survive in the right milieu, but undergo programmed cell death (apoptosis) when damaged, or when the environment is no longer supportive. Diseases may occur with alterations in this homeostasis. For example, cancer cells may survive in an environment in which they would not normally exist. This is accomplished by alterations in the expressions or functions of genes controlling both survival and apoptotic signaling pathways. Survival signaling pathways involve the activation of cell surface receptors, serine threonine kinases, transcription factors as well as other molecules. In breast and ovarian cancers, the ErbB2 growth factor receptor is overexpressed and this contributes to the progression of these cancers, in part by constitutively activating survival signaling pathways. In contrast, apoptotic signal transduction pathways are often inhibited in cancer. For example, overexpression of Bcl-2 blocks apoptosis and this contributes to the accumulation of cells in follicular lymphomas and chronic lymphocytic leukemia. Furthermore, alterations in these signaling pathways in cancer cells may lead to drug resistance. Recent advances in molecular targeted therapies have taken advantage of alterations in survival and apoptotic signaling pathways in cancer to specifically target aberrantly regulated molecules. For example, Herceptin inhibits ErbB2 function and antisense oligonucleotides against Bcl-2 reduce Bcl-2 expression. These agents can thus induce apoptosis in the specific cancer cell against which they have been targeted. In this review, we will discuss alteration in survival and apoptotic signal transduction pathways in cancer and the development of novel chemotherapeutic drugs to target these pathways.

The Transforming Growth Factor-β (TGFβ) superfamily of cytokines is comprised of a number of structurally-related, secreted polypeptides that regulate a multitude of cellular processes including proliferation, differentiation and neoplastic transformation. These growth regulatory molecules induce ligandmediated hetero-oligomerization of distinct type II and type I serine / threonine kinase receptors that transmit signals predominantly through receptor-activated Smad proteins but also induce Smad-independent pathways. Ligands, receptors and intracellular mediators of signaling initiated by members of the TGFβ family are expressed in the mammary gland and disruption of these pathways may contribute to the development and progression of human breast cancer. Since many facets of TGFβ and breast cancer have been recently reviewed in several articles, except for discussion of recent developments on some aspects of TGFβ, the major focus of this review will be on the role of activins, inhibins, BMPs, nodal and MIS-signaling in breast cancer with emphasis on their utility as potential diagnostic, prognostic and therapeutic targets.

Inhibitors of estrogen-related pathways have been used with some success in the treatment of breast cancer. These include the antiestrogens tamoxifen, more recently faslodex, and the aromatase inhibitor anastrazole. However, failure and recurrence rates are substantial with drugs countering the effects of estrogens. Progestins, unlike estrogens, have generally been considered to oppose breast cancer and have been used with reasonable efficacy after antiestrogen failure. However, a building body of evidence, from cell culture, animal studies, and, most recently, several major clinical studies involving hormone replacement therapy, strongly supports the notion that progestins generally stimulate breast cancer. Our studies and those of others suggest that progestins increase the numbers of breast cancer cells by both stimulating the rate of proliferation and inhibiting cell death. These data indicate that progestin-related pathways might provide effective targets for breast cancer therapy. This review addresses the rationale for using inhibitors of progestin-related pathways to treat breast cancer and comments on some possible points of attack.

The fluoropyrimidines were first synthesised nearly 50 years ago as rationally designed anti-cancer agents. Their target was pyrimidine and hence DNA synthesis. 5-Fluorouracil has been the most extensively used in a wide variety of malignancies. In more recent years a fuller understanding of the pharmacokinetics of these agents has lead to their utilisation as more effective and versatile anti-cancer drugs than might have been initially envisaged. This in part has occurred due to recognition of the schedule dependency of efficacy of 5-FU and modulation of its activity by leucovorin. However the development of novel fluropyrimidines such as capcetabine, UFT, and eniluracil which can be administered orally, has offered equal if not superior efficacy with improved tolerability and patient acceptance. It is now recognised that enzyme polymorphism's and heterogeneity of expression of key molecules are important determinants of the pharmacokinetic handling and pharmacodynamic effects of these drugs in individual patients. Further characterisation of such interindividual and inter-tumoral variability, for example in enzymes such as DPD and thymidine phosphorylase is ongoing. This work offers the promise of improvements in efficacy and tolerability by the process of individualisation of chemotherapy (for both patient and tumour). In contrast to the advances made in the understanding of the pharmacokinetics, less progress has been made in Fluropyrimidine pharmacodynamics. The inhibition of thymidylate synthetase by dFUMP and thereby dTMP and DNA synthesis is thought to be the critical mechanism. The incorporation of FUTP and dFUTP into RNA and DNA are also postulated to be of importance. While these events have been well defined, exactly how they lead to cell death is less clearly understood. Similarly, the mechanism of selective cancer cell cytotoxicity is not well understood. Pharmacokinetics and cell cycle kinetics provide a partial explanation. There is some evidence to suggest that the most important factor in determining cytotoxicity is the cellular response to fluoropyrimidine induced biochemical abnormalities rather than the lesions themselves. In this hypothesis the difference in response between normal and cancer cells is of critical importance. Further improvements in efficacy and tolerability could be made by elucidation of the molecular mechanisms behind this process. This knowledge in combination with the advances already made (and ongoing) in pharmacokinetics may allow the full potential of fluoropyrimidines as anti-cancer agents to be realised in the future.

The histone deacetylase inhibitors are a new class of cytostatic agents that inhibit the proliferation of tumor cells in culture and in vivo by inducing cell cycle arrest, differentiation and / or apoptosis. Histone acetylation and deacetylation play important roles in the modulation of chromatin topology and the regulation of gene transcription. Histone deacetylase inhibition induces the accumulation of hyperacetylated nucleosome core histones in most regions of chromatin but affects the expression of only a small subset of genes, leading to transcriptional activation of some genes, but repression of an equal or larger number of other genes. Non-histone proteins such as transcription factors are also targets for acetylation with varying functional effects. Acetylation enhances the activity of some transcription factors such as the tumor suppressor p53 and the erythroid differentiation factor GATA-1 but may repress transcriptional activity of others including T cell factor and the co-activator ACTR. Recent studies in our laboratory and others have shown that the estrogen receptor α (ERα) can be hyperacetylated in response to histone deacetylase inhibition, suppressing ligand sensitivity and regulating transcriptional activation by histone deacetylase inhibitors. Conservation of the acetylated ERα motif in other nuclear receptors suggests that acetylation may play an important regulatory role in diverse nuclear receptor signaling functions. A number of structurally diverse histone deacetylase inhibitors have shown potent antitumor efficacy with little toxicity in vivo in animal models. Several compounds are currently in early phase clinical development as potential treatments for solid and hematological cancers both as monotherapy and in combination with cytotoxics and differentiation agents. This report reviews the biology and clinical development of histone deacetylase inhibitors for cancer therapy.

The human genome contains a unique class of domains, referred to as AT islands, which consist typically of 200-1000 bp long tracts of up to 100% A / T DNA. The significance of AT islands as potential targets for chemotherapeutic intervention stems from two main aspects. First, AT islands are inherently unstable (expandable) minisatellites that are found in various known loci of genomic instability, such as ATrich fragile sites. Second, AT islands are involved in the organization of the genomic DNA on the nuclear matrix by acting as scaffold / matrix attachment regions, S / MARs. DNA duplexes of AT islands are unusually flexible and prone to base unpairing, which are crucial MAR attributes. Various AT islands show high binding affinity for isolated nuclear matrices and associate with the nuclear matrix in the cell. The cellular MAR function of AT islands may differ in cancer and normal cells. The abnormally expanded AT islands in the FRA16B fragile site in leukemic CEM cells act as strong, permanent MARs, while their unexpanded counterparts in normal cells are loop localized. Given their instability and involvement in the remodeling of the nuclear architecture, AT islands may be a factor in cancerous phenotypes. AT islands are preferentially targeted by the extremely potent DNA-alkylating antitumor drugs, bizelesin and U78779. High lethality of lesions in AT islands is consistent with the critical role of MAR domains in DNA replication. The abnormal structure / function of AT islands, such as their expansion and acquired strong MAR properties, may sensitize cancer cells to AT island targeting drugs.