Current Medicinal Chemistry - Volume 15, Issue 26, 2008

Expression levels of intact tumor suppressor proteins and molecular targets of anti-neoplastic agents are critical in defining cancer cell drug sensitivity; however, the intracellular location of a specific protein may be as important. Many tumor suppressor proteins must be present in the cell nucleus to perform their policing activities or for the cell to respond to chemotherapeutic agents. Nuclear proteins needed to prevent cancer initiation or progression or to optimize chemotherapeutic response include the tumor suppressor proteins p53, APC/β-catenin, and FOXO family genes; negative regulators of cell cycle progression and survival such as p21CIP1and p27KIP1; and chemotherapeutic targets such as DNA topoisomerases I and IIα. Mislocalization of a nuclear protein into the cytoplasm can render it ineffective as a tumor suppressor or as a target for chemotherapy. Blocking nuclear export of any or all of these proteins may restore tumor suppression or apoptosis or, for topoisomerases I and IIα, reverse drug resistance to inhibitors of these enzymes. During disease progression or in response to the tumor environment, cancer cells appear to acquire intracellular mechanisms to export anti-cancer nuclear proteins. These mechanisms generally involve modification of nuclear proteins, causing the proteins to reveal leucine-rich nuclear export signal protein sequences. Subsequent export is mediated by CRM1. This review defines the general processes involved in nuclear export mediated by CRM1/RanGTP (exportin/XPO1), examines the functions of individual tumor suppressor nuclear proteins and nuclear targets of chemotherapy, and explores potential mechanisms of cancer cells to induce export of these proteins. Novel drugs that could potentially counteract nuclear export of specific proteins are also discussed.

Most current anticancer therapies induce tumor cell death through apoptosis where its specific involved pathways are poorly understood. For example, for many DNA-damaging agents, the specific biochemical lesions (DNA adducts) are associated with the induction of apoptosis via the mitochondria death pathway. However, several of these DNA-damaging agents like cisplatin induce apoptosis through plasma membrane disruption, triggering the Fas death receptor pathway. In this review, we focus on the role of early plasma membrane events in cisplatin-induced apoptosis. Special attention is given to changes in plasma membrane fluidity, inhibition of NHE1 exchanger, activation of acid sphingomyelinase and their consequences on the Fas death pathway in response to cisplatin.

The coumarin (benzopyran-2-one, or chromen-2-one) ring system, present in natural products (such as the anticoagulant warfarin) that display interesting pharmacological properties, has intrigued chemists and medicinal chemists for decades to explore the natural coumarins or synthetic analogs for their applicability as drugs. Many molecules based on the coumarin ring system have been synthesized utilizing innovative synthetic techniques. The diversity oriented synthetic routes have led to interesting derivatives including the furanocoumarins, pyranocoumarins, and coumarin sulfamates (COUMATES), which have been found to be useful in photochemotherapy, antitumor and anti-HIV therapy, and as stimulants for central nervous system, antibacterials, anti-inflammatory, anti-coagulants, and dyes. Of particular interest in breast cancer chemotherapy, some coumarins and their active metabolite 7-hydroxycoumarin analogs have shown sulfatase and aromatase inhibitory activities. Coumarin based selective estrogen receptor modulators (SERMs) and coumarinestrogen conjugates have also been described as potential antibreast cancer agents. Since breast cancer is the second leading cause of death in American women behind lung cancer, there is a strong impetus to identify potential new drug treatments for breast cancer. Therefore, the objective of this review is to focus on important coumarin analogs with antibreast cancer activities, highlight their mechanisms of action and structure-activity relationships on selected receptors in breast tissues, and the different methods that have been applied in the construction of these pharmacologically important coumarin analogs.

In modern industrialized societies, people are exposed to thousands of naturally occurring and synthetic chemicals throughout their lifetime. Although certain occupational chemicals are known to be carcinogenic in humans, it has been difficult to definitively determine the adverse health effects of many environmental pollutants due to their tremendous chemical diversity and absence of a consistent structural motif. Many environmental chemicals are metabolized in the body to reactive intermediates that readily react with DNA to form modified bases known as adducts, while other compounds mimic the biological function of estrogen. Because environmental chemicals tend to accumulate in human tissues and have carcinogenic and/or estrogenic properties, there is heightened interest in determining whether environmental chemicals increase risk for endocrine-related cancers, including breast cancer. Breast cancer is the most common cancer in women worldwide, but established risk factors account for a relatively small proportion of cases and causative factors remain ambiguous and poorly defined. In this review, we outline the structural chemistry of environmental contaminants, describe mechanisms of carcinogenesis and molecular pathways through which these chemicals may exert detrimental health effects, review current knowledge of relationships between chemicals and breast cancer risk, and highlight future directions for research on environmental contributions to breast cancer. Improved understanding of the relationship between environmental chemicals and breast cancer will help to educate the general public about real and perceived dangers of these pollutants in our environment and has the potential to reduce individual risk by changing corporate practices and improving public health policies.

The 90 kDa heat shock proteins (Hsp90), which are integrally involved in cell signaling, proliferation, and survival, are ubiquitously expressed in cells. Many proteins in tumor cells are dependent upon the Hsp90 protein folding machinery for their stability, refolding, and maturation. Inhibition of Hsp90 uniquely targets client proteins associated with all six hallmarks of cancer. Thus, Hsp90 has emerged as a promising target for the treatment of cancer. Hsp90 exists as a homodimer, which contains three domains. The N-terminal domain contains an ATP-binding site that binds the natural products geldanamycin and radicicol. The middle domain is highly charged and has high affinity for cochaperones and client proteins. Initial studies by Csermely and co-workers suggested a second ATP-binding site in the Cterminus of Hsp90. This C-terminal nucleotide binding pocket has been shown to not only bind ATP, but cisplatin, novobiocin, epilgallocatechin-3-gallate (EGCG) and taxol. The coumarin antibiotics novobiocin, clorobiocin, and coumermycin A1 were isolated from several streptomyces strains and exhibit potent activity against Gram-positive bacteria. These compounds bind type II topoisomerases, including DNA gyrase, and inhibit the enzyme-catalyzed hydrolysis of ATP. As a result, novobiocin analogues have garnered the attention of numerous researchers as an attractive agent for the treatment of bacterial infection. Novobiocin was reported to bind weakly to the newly discovered Hsp90 C-terminal ATP binding site (∼700 M in SkBr3 cells) and induce degradation of Hsp90 client proteins. Structural modification of this compound has led to an increase of 1000-fold in activity in antiproliferative assays. Recent studies of structure-activity relationship (SAR) by Renoir and co-workers highlighted the crucial role of the C-4 and/or C-7 positions of the coumarin and removal of the noviose moiety, which appeared to be essential for degradation of Hsp90 client proteins. Unlike the N-terminal ATP binding site, there is no reported co-crystal structure of Hsp90 C-terminus bound to any inhibitor. The Hsp90 C-terminal domain, however, is known to contain a conserved pentapeptide sequence (MEEVD) which is recognized by co-chaperones. Cisplatin is a platinum-containing chemotherapeutic used to treat various types of cancers, including testicular, ovarian, bladder, and small cell lung cancer. Most notably, cisplatin coordinates to DNA bases, resulting in cross-linked DNA, which prohibits rapidly dividing cells from duplicating DNA for mitosis. Itoh and co-workers reported that cisplatin decreases the chaperone activity of Hsp90. This group applied bovine brain cytosol to a cisplatin affinity column, eluted with cisplatin and detected Hsp90 in the eluent. Subsequent experiments indicated that cisplatin exhibits high affinity for Hsp90. Moreover Csermely and co-workers determined that the cisplatin binding site is located proximal to the Cterminal ATP binding site. EGCG is one of the active ingredients found in green tea. EGCG is known to inhibit the activity of many Hsp90- dependent client proteins, including telomerase, several kinases, and the aryl hydrocarbon receptor (AhR). Recently Gasiewicz and co-workers reported that EGCG manifests its antagonistic activity against AhR through binding Hsp90. Similar to novobiocin, EGCG was shown to bind the C-terminus of Hsp90. Unlike previously identified N-terminal Hsp90 inhibitors, EGCG does not appear to prevent Hsp90 from forming multiprotein complexes. Studies are currently underway to determine whether EGCG competes with novobiocin or cisplatin binding. Taxol, a well-known drug for the treatment of cancer, is responsible for the stabilization of microtubules and the inhibition of mitosis. Previous studies have shown that taxol induces the activation of kinases and transcription factors, and mimics the effect of bacterial lipopolysaccharide (LPS), an attribute unrelated to its tubulin-binding properties. Rosen and coworkers prepared a biotinylated taxol derivative and performed affinity chromatography experiments with lysates from both mouse brain and macrophage cell lines. These studies led to identification of two chaperones, Hsp70 and Hsp90, by mass spectrometry. In contrast to typical Hsp90-binding drugs, taxol exhibits a stimulatory response. Recently it was reported that the geldanamycin derivative 17-AAG behaves synergistically with taxol-induced apoptosis. This review describes the different C-terminal inhibitors of Hsp90, with specific emphasis on structure-activity relationship studies of novobiocin and their effects on anti-proliferative activity.

Thyrotropin-releasing hormone (TRH), a hypothalamic orally active neuropeptide, has been manifested in a wide range of biological responses. Besides its central role in regulating the pituitary-thyroid axis by simulating the release of thyrotropin, TRH has considerable influence on the activity of a number of neurobiological systems. Due to the therapeutic potential of TRH to treat several CNS maladies, the development of CNS-selective and metabolically stable TRH analogs is an area of interest. TRH is known to elicit its biological response through two G-protein coupled receptors for TRH (namely, TRH-R1 and TRH-R2). The distinct distribution of TRH receptors in tissues has provided opportunity to discover receptor subtype-specific analogs, which would demonstrate high CNS activities, and are completely free of hormonal activities. In this review, an in-depth analysis of the chemistry and biology of TRH and its analogs is provided. Recent discoveries of TRH-R2 selective analogs, TRH super agonists, metabolically stable TRH analogs, and targeted delivery of TRH analogs have been also discussed.

Cushing's syndrome is a rare disease with significant morbidity and mortality. Surgical intervention represents the most effective treatment option in both adrenocorticotropin-dependent and -independent forms of hypercortisolism. It is not uncommon, however, that surgery fails to cure or control the disease. Pharmacotherapy with drugs inhibiting steroid biosynthesis can be effectively used in these cases in order to alleviate symptoms or even to induce chemical adrenalectomy. A few drugs inhibiting single or multiple steps in adrenal steroid biosynthesis can be used in clinical practice. Drugs predominantly inhibiting single enzymatic steps include the 11β-hydroxylase inhibitor metyrapone and the 3β- hydroxysteroid dehydrogenase inhibitor trilostane, whereas mitotane, aminoglutethimide, ketoconazole and etomidate block multiple enzymatic reactions. Etomidate is the only agent available for parenteral administration that renders it as a treatment of choice in critically ill patients requiring a rapid control of hypercortisolemia. Ketoconazole, metyrapone and aminoglutethimide can be used alone or in combination for the treatment of hypercortisolism caused by benign adrenocorticotropin- or cortisol-secreting tumors. The clinical utility of trilostane is variable. Besides blocking multiple steps in adrenal steroid biosynthesis, the DDT (insecticide) analogue mitotane also has adrenolytic properties by inducing mitochondrial degeneration that renders it superior to other drugs in the treatment of adrenocortical cancer. Severe side effects may develop during therapy with each aforementioned drug that include hepatic, endocrine and neurological toxicity. After summarizing the chemical and biological properties of steroid biosynthetic inhibitors, the authors describe their possible clinical applications and limitations.

Dementia with Lewy bodies (DLB), the second most frequent cause of dementia after Alzheimer disease (AD), is characterized by the widespread distribution of Lewy bodies in virtually every brain area. Clinically, DLB is distinguished from AD by fluctuating cognition, prominent visual hallucinations and parkinsonism, and from Parkinson disease, by the appearance of parkinsonism within one year of cognitive or behavioral decline. The main component of Lewy bodies is alpha-synuclein. Accumulating evidence suggests that its aggregation constitutes one of the first steps preceeding Lewy body formation, so that antiaggregation strategies would be very useful to prevent alpha-synuclein fibril formation. Main therapies nevertheless applied up to the present remain symptomatological. In this context, cholinesterase inhibitors such as rivastigmine, galantamine and donepezil, are used for the treatment of delusions and other psychotic symptoms. This review focuses on the recent discovery of possible alpha-synuclein anti-aggregation factors, where four main classes can be defined. First, beta-synuclein as well as alpha-synuclein derived peptides in addition to antibodies present a group of proteins and peptides that directly interact with alpha-synuclein and so inhibit its aggregation. Second, small molecules interfere with alpha-synuclein aggregation by their covalent binding, although not all of them are suitable for an appropriate inhibition of alpha-synuclein aggregation. Third, to inhibit the expression of alpha-synuclein and its isoforms at the RNA level, the use of interference RNA represents a future challenge. The fourth strategy is based on the enhancement of inclusion body formation to accelerate the elimination of soluble alpha-synuclein oligomers. Each chapter section includes the discussion of possible strategies for the development of drugs and therapies.

Tuberculosis causes nearly two million deaths per year world-wide. In addition multidrug-resistant mycobacterial strains rapidly emerge so novel therapeutic approaches are needed. Recently, several promising mycobacterial target molecules were identified, which are involved in bacterial or host cell signalling e.g. the serine/threonine protein kinases, PknB and PknG, NAD kinase and the NAD synthetase. Here we describe some early efforts in the development of novel signal transduction inhibitory anti-mycobacterial drugs using a multiple target approach, with special emphasis on the kinase inhibitory field. Initially, we are using the Nested Chemical Library™ (NCL) technology and pharmacophore modelling. A hit-finding library, consisting of ∼19000 small molecules with a bias for prototypic kinase inhibitors from our NCL library and commercial sources was virtually screened against these validated target molecules. Protein structures for the virtual screening were taken from the published three dimensional crystal structures of the enzymes. The hits from the virtual screening were subsequently tested in enzymatic assay systems. Potent hits were then tested for biological activity in macrophages, infected with mycobacteria. The final goal of this exercise is not only to identify potent antimycobacterial substances, but also a common pharmacophore for the mycobacterial target PknG in combination with PknB, NAD kinase and/or NAD synthetase. This common pharmacophore still needs to be a unique pharmacophore for the mycobacterial target proteins over human off-targets. Such a pharmacophore might then drive the optimization of a completely new profile of an antibiotic agent with activity against latent mycobacteria and resistance mycobacterial strains.