Anti-Cancer Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Anti-Cancer Agents) - Volume 8, Issue 7, 2008

Calcium is a ubiquitous second messenger and is involved in virtually all cellular functions. Cellular events being regulated by calcium include gene transcription, metabolism, proliferation and apoptosis. Cancer growth is based on increased proliferation, decreased differentiation and decreased apoptosis. Therefore, the intracellular Ca2+-homeostasis has become one of the focuses in current cancer research. Elevation of the cytoplasmic Ca2+-concentration can result from Ca2+-influx from the extracellular space or from Ca2+-release from intracellular stores. The main intracellular Ca2+-store is the endoplasmic reticulum (ER). The Ca2+-content of the ER is maintained by trans-membrane proteins involving the sarco/endoplasmic reticulum Ca2+-ATPase and the inositol-1,4,5-phosphat receptor. In this review, we summarize the current knowledge of the ER and its trans-membrane proteins as regulating structures of the intracellular Ca2+- homeostasis, what changes occur in malignant cells and how this promotes cancer. We further review possible pharmacological intervention and show future perspectives of the intracellular Ca2+-homeostasis as an anti-cancer target.

Ribosomal S6 kinase (RSK) is a family of serine/threonine kinases that has been identified as a promising anti-cancer target. While a number of protein kinase inhibitors that have potent activity against other serine/threonine kinases were shown to also inactivate RSK, there is keen interest in the three different inhibitor chemotypes that were shown to be RSK specific, since these compounds have tremendous utility as chemical probes in elucidating the biochemistry of the RSK signaling cascade and unraveling the molecular basis of cancer. Because each compound may have therapeutic potential, the nonspecific kinase inhibitors as well as the RSK specific inhibitors will be discussed.

Proteins with tyrosine kinase activity are recognized as key regulators of cellular processes including growth and differentiation. Tyrosine kinase receptors e.g. EGFR and soluble tyrosine kinase proteins e.g. JAK-2, have emerged as essentials in cell survival for cervical carcinoma and acute myeloblastic leukemia, respectively. These receptors and soluble cytoplasm networks have been studied in detail and finally pharmacological agents, targeted at key molecules, could be produced. Tyrphostins are kinases inhibitors synthesized on the basic structure of erbstatin a natural kinase inhibitor. The JAK-2 specific inhibitor, Tyrphostin AG490 is used to inhibit phosphorylation of EGFR and signal transducer and activator of transcription 3 [STAT-3], and subsequently reduce invasion and adhesion potential of malignant cells. This review summarizes experiments providing a detailed picture of how hematopoietic cancer c-Kit+, Jak-2+ and non hematopoietic tumors c-Kit+, HER-2+, JAK-2+ can be inhibited by the chemosensitizing agent AG490 causing programmed cell death. Furthermore, studies presented herein analyzed several cellular targets that can be modified by the same death effector. The highly conserved JAK-2/STAT-3, c-Kit, and HER-2 signaling pathways play pleiotropic roles during embryonic development and are important for the regulation of self-renewing tissues. The physiological functions of these signaling cascades range from stem cell maintenance and influencing cell fate decisions of progenitor cells, to the induction of terminal differentiation processes, all of which have been found to be recapitulated in different forms of cancers. Inhibiting their action by AG490 represents a therapeutic approach for the treatment of individual types of cancer and several broad-spectrum.

Cyclins and CDKs play critical roles in DNA synthesis and cell division. Alterations in their function may lead to the disruption of normal cell growth and apoptosis, and subsequently, result in carcinogenesis. Elevated levels of cyclins and CDKs are frequently observed in a wide range of different types of human cancers. Understanding of molecular mechanisms underlying the cell cycle effects in response to the chemotherapeutic agents is of great importance for improving the efficacy of targeted therapeutics and overcoming resistance to chemotherapeutic agents. Despite the clinical applications of cell cycle specific chemotherapeutic agents, there is still an urgent need to develop novel drugs that can target multiple sites and pathways of the cell cycle while avoiding drug induced cytotoxicity. In this review article, we will summarize the development of novel agents that specifically target cell cycle pathways in human cancer. We will discuss drugs that can directly interfere with the mitotic process of tumor cells. Moreover, we tend to address the significance of using small molecule CDK inhibitors that are derived from natural products.

The steroid sulfatase (STS) enzyme plays a pivotal role in the formation of biologically active steroid hormones. Its involvement in the hydrolysis of estrone sulfate and dehydroepiandrosterone sulfate to estrone and dehydroepiandrosterone, respectively, is an important step in the formation of estradiol and androstenediol, both of which are estrogenic steroids that can stimulate tumor growth. Consequently, as STS is widely distributed throughout the entire body, it has a substantial influence on hormone-dependent cancer mitogenesis. It is a useful prognostic marker of disease as a significant majority of breast tumors over-express the enzyme and there are indications of STS having a role in prostate cancer. This knowledge has led to the development of potent STS inhibitors for use as anti-cancer agents. There are now several steroidal and non-steroidal STS inhibitors available. New in vivo models, using ovariectomized female nude mice, have been developed to pre-clinically test these inhibitors. These studies have demonstrated the excellent efficacy and effect of STS inhibitors on breast carcinoma development. Recently, 667 COUMATE, an irreversible type of inhibitor which utilizes a phenol sulfamate ester as its active pharmacophore, has completed a Phase I clinical trial in postmenopausal women with breast cancer. These studies have indicated the potential clinical benefit for the use of STS inhibitors. Most pre-clinical and clinical studies have focused on breast cancer as the target for STS inhibition. However, there are other hormone-dependent malignancies, such as endometrial and prostate cancer, that could in the future be treated with these new potent STS inhibitors.

The sulfonamides constitute an important class of drugs, which display a variety of activities including antibacterial, anticarbonic anhydrase, diuretic, hypoglycemic and antithyroid effectiveness. A number of sulfonamides have been reported to be potent anticancer agents, which interact with a wide range of different cellular targets. Among these interesting sulfonamides, the diaryl or heterocyclic sulfonamides have recently emerged as an important class of antimitotic agents against different types of cancer, including multidrug resistant tumors. This review summarizes the recent advances of the diaryl or heterocyclic sulfonamides as novel antimitotic agents.

Lamellarins are a large family of marine alkaloids with potential anticancer activity that have been isolated from diverse marine organisms, mainly ascidians and sponges. All lamellarins feature a 3,4-diarylpyrrole system. Pentacyclic lamellarins, whose polyheterocyclic system has a pyrrole core, are the most active compounds. Some of these alkaloids are potently cytotoxic to various tumor cell lines. To date, Lam-D and Lam-H have been identified as lead compounds for the inhibition of topoisomerase I and HIV-1 integrase, respectively— nuclear enzymes which are over-expressed in deregulation disorders. Moreover, these compounds have been reported for their efficacy in treatment of multi-drug resistant (MDR) tumors cells without mediated drug efflux, as well as their immunomodulatory activity and selectivity towards melanoma cell lines. This article is an overview of recent literature on lamellarins, encompassing their isolation, recent synthetic strategies for their total synthesis, the preparation of their analogs, studies on their mechanisms of action, and their structure-activity relationships (SAR).

One promising therapeutic strategy for treating cancer is to specifically target signal transduction pathways that have a key role in oncogenic transformation and malignant progression. Hsp90 is an emerging therapeutic target of interest for the treatment of cancer. It is responsible for modulating cellular response to stress by maintaining the function of numerous signalling proteins - known as ‘client proteins’ - that are associated with cancer cell survival and proliferation. Many cancers result from specific mutations in, or aberrant expression of, these client proteins. Small molecule Hsp90 inhibitors bind to the ATP binding pocket, inhibit chaperone function and could potentially result in cytostasis or cell death. Consequently, many client proteins are targeted for degradation via the ubiquitin-proteasome pathway including receptor and non receptor kinases (Erb-B2, epidermal growth factor receptor, and Src family kinases), serine/threonine kinases (c-Raf-1 and Cdk4), steroid hormone receptors (androgen and estrogen), and apoptosis regulators such as mutant p53. Inhibition of Hsp90 function has also proven effective in killing cancer cells that have developed resistance to targeted therapies such as kinase inhibitors. This review is intended to update recent developments in new Hsp90 inhibitors as antitumors agents, the design, biological evaluation and their clinical trials studies.

Cladribine, i.e.2-deoxy-Chloroadenosine is currently in use as chemotherapeutic agent in chronic lymphoid malignancies and pediatric acute myelogenous leukemia whereas the structurally related counterpart, 2-Chloroadenosine, has been less studied. Nevertheless, 2-Chloroadenosine has been shown to be capable of inducing apoptosis in several cell lines by acting either via adenosine receptors or via uptake that is followed by metabolic transformations leading to nucleotide analogues, i.e. antimetabolites effective in the treatment of a variety of malignancies. Triphosphate nucleoside analogues show specificity for cell in S-phase, inhibit DNA synthesis and kill the cells by mechanisms still largely unknown. 2-Chloroadenosine, at low micromolar concentration, acts as a metabolic precursor of an Sphase specific nucleoside analogue in human prostate cancer PC3 cells and inhibits DNA synthesis thereby leading to accumulation of cells in the S-phase. However, although responsible for the acquisition of resistance, the adenosine derivative is capable of sensitising the cells to the action of other antineoplastic agents and the ability of nucleoside analogues to trigger cell cycle arrest can be exploited to maximize cytotoxicity in combination with cell cycle checkpoint disregulators. 2-Chloroadenosine, in combination with Docetaxel, known to improve the survival of hormone-refractory prostate cancer patients, further decreases in vitro PC3 cell proliferation and invasiveness. Moreover, 2-Chloroadenosine is capable of modulating PAR-1 and IL-23 gene expression suggesting a modulation of cancer metastasis and immune system activity. The present review summarizes research performed in our laboratory to propose a novel role for 2-Chloroadenosine as an anticancer agent.

A solid tumour forms an organ-like structure that is comprised of cancer cells as well as stroma cells (fibroblasts, inflammatory cells) that are embedded in an extracellular matrix and are nourished by vascular network. However, tumoral microenvironment is heterogeneous due to the abnormal vasculature network and high proliferation rate of cancer cells. Because of these features, some regions are starved from oxygen, a phenomenon called hypoxia. Transient hypoxia is associated with inadequate blood flow while chronic hypoxia is the consequence of the increased oxygen diffusion distance due to tumour expansion. Both types of hypoxia are correlated with poor outcome for patients. Moreover, hypoxia also enhances chemoresistance of cancer cells. Firstly, the delivery of drugs in hypoxic area and cellular uptake of it are affected by hypoxia or associated acidity. Secondly, some chemotherapeutic drugs require oxygen to generate free radicals that contribute to cytotoxicity. Last, hypoxia induces cellular adaptations that compromise the effectiveness of chemotherapy. In response to nutrient deprivation due to hypoxia, the rate of proliferation of cancer cells decreases but chemotherapeutic drugs are more effective against proliferating cells. On the other hand, hypoxia induces adaptation by post-translational and transcriptional changes that promote cell survival and resistance to chemotherapy. Through these changes, hypoxia promotes angiogenesis, shift to glycolytic metabolism, expression of ABC transporters, cell survival by inducing the expression of genes encoding growth factors and the modulation of apoptotic process. The aim of this review is to provide a description of known hypoxia-induced mechanisms of chemoresistance at a cellular level.

Protein kinase CK2 is a ubiquitous, essential, and highly pleiotropic protein kinase whose abnormally high constitutive activity is suspected to underlie its pathogenic potential in neoplasia and other diseases. A number of structurally unrelated CK2 inhibitors, tested on a variety of cells derived from tumours, including lymphomas, leukaemias, multiple myeloma and prostate carcinoma, display a proapoptotic effect which is roughly proportional to their in vitro inhibitory potency. In the present review we summarize the most recent discovery of potent and selective CK2 inhibitors and their prospective as future anticancer agents.

Numerous proteins responsible for cell proliferation and differentiation exist either as hetero or homodimers or become activated through dimerization as a key step in their respective signaling cascade. Many of these proteins have been identified as major components in oncogenic signaling pathways and have become popular targets for the development of anti-tumor agents. For this reason, bivalent anti-cancer drugs that could potentially interact with each monomer of a dimeric protein target have been developed. This review provides a brief background on prevalent dimeric drug targets within the anti-cancer field and focuses mainly on dimeric natural product and synthetic cancer chemotherapeutics.