Current Drug Targets - Volume 4, Issue 2, 2003

Alzheimer's disease (AD), a progressive, degenerative disorder of the brain, is believed to be the most common cause of dementia amongst the elderly. AD is characterized by the presence of amyloid deposits and neurofibrillary tangles in the brain of afflicted individuals. AD is associated with a loss of the presynaptic markers of the cholinergic system in the brain areas related to memory and learning. AD appears to have a heterogeneous etiology with a large percentage termed sporadic AD arising from unknown causes and a smaller fraction of early onset familial AD (FAD) caused by mutations in one of several genes, such as the β-amyloid precursor protein (APP) and presenilins (PS1, PS2). These proteins along with tau, secretases, such as β-amyloid cleaving enzyme (BACE), and apolipoprotein E play important roles in the pathology of AD. On therapeutic fronts, there is significant research underway in the development of new inhibitors for BACE, PS-1 and γ-secretase as targets for treatment of AD. There is also a remarkable advancement in understanding the function of cholinesterase (ChE) in the brain and the use of ChE-inhibitors in AD. A new generation of acetyl- and butyryl cholinesterase inhibitors is being studied and tested in human clinical trials for AD. The development of vaccination strategies, anti-inflammatory agents, cholesterol-lowering agents, anti-oxidants and hormone therapy are examples of new approaches for treating or slowing the progression of AD. In addition, nutritional, genetic and environmental factors highlight more effective preventive strategies for AD. Developments of early diagnostic tools and of quantitative markers are critical to better follow the course of the disease and to evaluate different therapeutic strategies. In this review, we attempt to critically examine recent trends in AD research from molecular, genetic to clinical areas. We discuss various neurobiological mechanisms that provide the basis of new targets for AD drug development. All these current research efforts should lead to a deeper understanding of the pathobiochemical processes that occur in the AD brain in order to effectively diagnose and prevent their occurrence.

Because conventional chemotherapy is not specific for cancer cells leading to toxic side effects there is a need for novel agents with high grade antitumor specificity. The major prerequisite to develop such drugs is to understand the targets that these agents should attack. In recent years a number of promising new anticancer drugs have been developed which target intracellular pathways or extracellular cell molecules. The clinically most effective compounds function as tyrosine kinase inhibitors. In the past, various tyrosine kinase receptors have been identified as regulators of tumor or tumor vessel growth. Having shown their expression characteristics in different tumor entities, specific inhibitors of the ATP binding sites of these receptors or antibodies were developed and entered clinical trials. The pathognomonic role of the tyrosine kinase defines the way of action of the inhibiting drug, whereas the amount of expression in tumor tissue defines the rationale to use the inhibitor to treat a specific protein. The future will define indications for such drugs by tumor kinase profiles instead of tumor entities. Gleevec, inhibiting the BCR-ABL tyrosine kinase; Iressa, inhibiting the EGF-receptor tyrosine kinase, Herceptin, inhibiting the Her2 / neu tyrosine kinase and PTK787 / ZK222584, inhibiting the VEGF-receptor tyrosine kinase will be discussed as representatives of selective tyrosine kinase inhibitors whereas ZD6474 and SU6668 will be discussed as representatives of multitarget tyrosine kinase inhibitors.

The integrin receptor αvβ3 has been shown to play a critical role in several distinct processes, such as angiogenesis, osteoclast-mediated bone resorption and tumor metastasis. Its expression is upregulated in newly synthesized blood vessels produced in response to a variety of tumors and purified angiogenic factors. Studies show that αvβ3 is a critical target downstream from perhaps all angiogenic factors. Proof-of-principle that αvβ3 antagonists such as monoclonal antibodies and small molecules block angiogenesis and tumor growth has been obtained in several animal models. Many endogenous inhibitors of angiogenesis such as angiostatin, endostatin and tumstatin seem to work through the αvβ3 receptor further emphasizing the critical role of this receptor in angiogenesis. In addition, the αvβ3 receptor has been clearly implicated in several pathological processes such as rheumatoid arthritis, osteoporosis, and metastasis of prostate cancer to bone. Thus αvβ3 may prove to be an important target for pharmacological intervention in more than one clinical setting.

The light-induced reactions of 3,7-bis(dialkylamino)phenothiazin-5-ium compounds with biological substrates are briefly discussed. Their biomedical applications, in particular those related with biological staining, interaction with proteins and antiviral, antibacterial and antitumour activity are reviewed.

Receptors of vascular cells and coagulation proteins form a tightly integrated and balanced system, providing regulation to coagulation and mediating a response to coagulation by the vascular cells. Endothelial and smooth muscle cells express a variety of proteins directly participating in hemostasis. Engagement of activated coagulation proteins by their specific receptors on the vascular cell surface, in turn, activates these cells and leads to expression of genes involved in coagulation, angiogenesis, leukocyte adhesion, regulation of the vascular wall tone, etc. The signals inducing the expression of target genes are mediated by protease-activated receptors, which are shared among coagulation proteases. However, differences in mechanisms of activation of these receptors, as well as the presence of specific receptors for each coagulation protein and structures of promoters of target genes, may provide specificity in the responses of vascular cell types to different coagulation factors. Activation of gene expression in vascular cells by coagulation proteases accounts for the long-term consequences of coagulation in disorders such as atherosclerotic lesion development, cancer growth, and inflammation. Multiple intracellular pathways and specific trancsriptional mechanisms activated by coagulation proteins represent an attractive target for drug design, providing the possibility of controlling the adverse effects of coagulation activation without interfering with the hemostatic requirements of coagulation.This review discusses regulation of gene expression in vascular cells by thrombin, tissue factor, factor VIIa, factor Xa and protein C. Differences and similarities in mechanisms of receptor activation, the pathological profiles of genes activated by these coagulation factors, and recently described transcriptional mechanisms that they induce are discussed.

The call for the discovery of less toxic, more selective, and more effective agents to treat cancer has become more urgent. Inhibition of angiogenesis continues to be one of the main streams in the current cancer drug discovery activity. Insights into tumor angiogenesis biology have led to the identification of a number of molecules, which are important for the progression of these processes. Of particular interest is a group of growth factors including fibroblast growth factor, platelet-derived growth factor, and vascular endothelial growth factor. These growth factors and their corresponding receptor tyrosine kinases have become important targets for inhibition of the proliferation of endothelial cells, the main component of blood vessels. The validated targets for inhibition of angiogenesis also include a family of matrix metalloproteinases and cell adhesion molecules. In the closely related area, protein kinases have emerged as one of the most important targets for drug discovery. Besides growth factor receptor tyrosine kinases, numerous other protein kinases implicated in malignancies have been identified including non-receptor kinases such as Bcl-Abl and Src kinases. In addition, the cell cycle regulators (cyclin-dependent kinases, p21 gene) and apoptosis modulators (Bcl-2 oncoprotein, p53 tumor suppressor gene, survivin protein, etc) have also attracted renewed interest as potential targets for anticancer drug discovery. Other molecular targets include protein farnesyltransferase (FTase), histone deacetylase (HDAC), and telomerase, which have essential roles in cellular signal transduction pathways (FTase, HDAC) and cell life-span (telomerase). This review presents a comprehensive summary and discussion on the most important targets currently attracting a great deal of interest in contemporary anticancer drug design and discovery. Recent advances complementing these targets are also highlighted.

The quinolones are a potent group of drugs that target the essential bacterial enzymes DNA gyrase and topoisomerase IV. DNA gyrase is the primary target of Gram negative organisms however, it is topoisomerase IV that is the primary target of Gram positive organisms. Within these enzymes is a highly conserved region centered round the active site where resistance mutations occur. These mutations are almost always identical, irrespective of organism. In spite of the homology of this region, amino acid sequence analysis shows that there are defined differences between the Gram groups, particularly in topoisomerase IV, and it is speculated that herein lies the origin of target preference.Since the first quinolone nalidixic acid was developed, the quinolones have undergone structural modifications, in particular the addition of a fluorine at position 6, to produce the fluoroquinolones. This has seen their potency and pharmakokinetic profile greatly increase. In vitro selection of resistance mutations has allowed the observation of how resistance is acquired and some of the modifications in newer fluoroquinolones have resulted in the shift of primary target from topoisomerase IV to gyrase with Gram positives. Curiously, purified topoisomerase IV is still more sensitive even if gyrase is the primary target. Gyrase remains the primary target for Gram negatives.