Current Pharmaceutical Design - Volume 8, Issue 22, 2002

Activation of signaling pathways following DNA damage induced by topoisomerase (topo) poisons can lead to cell death by apoptosis. NF-κB, a major regulator of the stress response and a negative regulator of apoptosis is often activated following treatment with topoisomerase poisons. Since activation of NF-κB is generally considered to relay an anti-apoptotic signal, inactivation of this signaling molecule is considered to represent an important strategy to improve therapeutic efficacy. Although this strategy seems to be effective in some model systems, our results in human nonsmall cell lung cancers differed. In this review we will discuss the role of NF-κB in mediating topoisomerase poison-induced DNA damage and apoptosis and the consequence of inhibiting its activity. Newer insights about the importance of proteasome inhibitors and anti-apoptotic genes in topoisomerase poison-induced signaling mechanisms leading to apoptosis will also be reviewed. The knowledge obtained from these studies may be useful for translation to a clinical setting for development of more effective therapeutic strategies.

Receptor tyrosine kinases (RTKs) are cell surface transmembrane proteins responsible for intracellular signal transduction. They are expressed in several cell types and, after activation by growth factor binding, trigger a series of intracellular pathways, leading to a wide variety of cell responses (e.g., differentiation, proliferation, migration and invasion, angiogenesis, survival). Over-expression and / or structural alteration of RTKs family members are often associated to human cancers and tumor cells are known to use RTK transduction pathways to achieve tumor growth, angiogenesis and metastasis. Therefore, RTKs represent pivotal target in approaches of cancer therapy. A number of small molecules acting as RTK inhibitors have been synthesized by pharmaceutical companies and are under clinical trials, are being analyzed in animal models or have been successfully marketed. Liganddependent downregulation of RTKs is a critical step for modulating their activity and the adaptor Cbl has been indicated as the key protein involved in negative regulation of RTKs, such as EGF and HGF receptors. These data suggest novel potential pharmacological targets for the treatment of human malignancies associated to oncogenic activation of RTKs.

Cancer-homing toxins are a group of man-made cytotoxic molecules targeting cancer cells. In the past decade they have demonstrated potential as cancer therapeutics. These molecules contain a toxin, natural or usually derivatized, connected to a cancerhoming module, such as a monoclonal antibody or growth factor or their derivatives. Various cancer-homing toxins have been designed and tested in cell-lines, animalmodels and clinical trials. We review some of these data and discuss ways to better design cancer-homing toxins in the light of advances in cancer genomics, antibodyengineering techniques and computational algorithms.

Blood-borne drug molecules are transported through as well as around cells in tissue. For small molecule drugs with a molar weight <1000, the wall of the capillary blood vessels in tumors usually is not a barrier. Just after a rise in the drug concentration in the blood, the cells closest to the microvessels are exposed to the highest drug concentrations. Short or long lasting concentration gradients away from the capillary vessels will develop. Since in a tumor the distance to the nearest blood vessel can be relatively large, inefficient transport of drugs to some cancer cells may limit drug efficacy. Studies on in vitro drug gradients have given insight into the factors determining this transport. Small intercellular distances, high cellular drug influx and low drug efflux rates, and high intracellular and extracellular drug binding favor the development of drug gradients. In the absence of drug metabolism, gradients “level out” over time and may reverse as the blood concentration drops. Understanding the drug transport process from the microvessels to every cancer cell will be important for optimizing cancer chemotherapy. Cancer cells that can “hide” for the drug may lead to regrowth of the tumor.

DNA topoisomerases are ubiquitous group of enzymes altering the topology of DNA by concerted breakage and rejoining of the phosphodiester backbone of DNA. The enzymes are classified based on the pattern of DNA cleavage. Type IA enzymes found in all bacteria nick the DNA and attach themselves covalently to the 5' side of the nick during the first transesterification reaction. Most of the information on this group of enzymes comes from studies with E. coli topoisomerase I and III. Members of type IA group are single subunit Zn++ metalloenzymes recognizing single stranded DNA without high degree of sequence specificity during relaxation reaction of negatively super coiled DNA. So far no inhibitors are known for this group of enzymes inspite of their important role in maintaining homeostasis of DNA topology. Molecular characterization of DNA topoisomerase I from mycobacteria has revealed some of the important features of type IA enzymes hitherto unknown and provide scope for identifying novel inhibitors. The present review describes the recent developments in the area summarizing the distinctive features of mycobacterial topoisomerase I. The enzyme has several properties not shared by either type IA or IB enzymes with respect to DNA binding, recognition, sequence specificity and interaction pattern. The physiological basis of the unusual features is discussed. The unique properties described would aid in developing the enzyme as a target molecule in pharmaceutical design. In addition, the findings lead to address some fundamental questions on the intracellular role of topoisomerase I in the biology of mycobacteria which are one of the most formidable group of pathogenic organisms.