Endogenous and induced oxidative stress in multi-cellular tumour spheroids: implications for improving tumour therapy.

The endogenous oxidative stress in tumours is determined by the status of mitochondrial, metabolic, oxygen (hypoxia) and inherent enzymatic as well as non-enzymatic antioxidant defense systems, which influence tumour growth and respond to anticancer therapeutics. Induced oxidative stress is one of the important determinants of the outcome of treatment with certain chemotherapeutic drugs and ionizing radiation. The mild to moderate levels of reactive oxygen species (ROS) have often been found to trigger prosurvival responses, thereby contributing to the resistance against therapy. The higher levels of ROS stimulate multiple death pathways viz. typical and atypical apoptosis, necrosis etc, thereby enhancing the therapeutic efficiency. Therefore, approaches employing therapeutic agents that generate ROS efficiently in the tumour cells and enhance the antioxidant defense system in the normal cells could significantly enhance the therapeutic gain. Multi-cellular tumour spheroids (MCTS) offer an excellent in vitro system that mimics endogenous oxidative stress often observed in tumours, arising due to a number of factors (gradients of oxygen and nutrients, altered intercellular interaction and tumour necrosis factor), besides antioxidant defense systems similar to tumours in vivo. More importantly, MCTS resemble tumours in vivo with reference to the induced oxidative stress related responses, particularly following combinations of certain chemotherapeutic drugs and metabolic inhibitors and differs significantly from the responses in monolayer cultures. Therefore, MCTS appear to be excellent in vitro models, ideally suited for developing novel therapies that are based on the generation of oxidative stress in tumours. The present review provides a modest account on the utility of MCTS in understanding the role of oxidative stress in treatment-induced responses of tumours for designing therapies and therapeutics.

Heterogeneity in the radiosensitizing effects of the DNA ligand hoechst-33342 in human tumor cell lines.

The AT specific minor grove DNA binding ligands bisbenzimidazole derivatives like hoechst-33342 and hoechst-33258 which scavenge free radicals and stabilize macromolecular structure have been shown to afford radioprotection by reducing the induction of DNA damage. However, their ability to inhibit topoisomerases I & II, which play important roles in damage response pathways including DNA repair can enhance radiation damage under certain conditions. Since pool sizes of the topoisomerases differ not only between normal and tumor cells, but also among different tumors, it is anticipated that radiosensitization by hoechst-33342 can vary among tumors. The present studies were, therefore, undertaken to verify this proposition in human glioma (BMG-1 &U-87) and squamous carcinoma (4197 &4451) cell lines which differ in their biological behavior (ploidy, p53, cyclins, bcl, bax etc). Isotoxic concentrations of hoechst-33342 (IC50 i.e producing 50% cell kill) administered immediately following irradiation resulted in the radiosensitization of all cell lines, with a 4&7 fold increase in the cell death (loss of clonogenic cell survival) in U-87&BMG-1 and a 3 fold increase in 4197 &4451 cells. Growth inhibition and increase in cytogenetic damage (micronuclei formation) as well as delayed apoptosis observed under these conditions corroborated well with the enhanced cell death. The ligand induced a significant cell cycle delay, particularly in the late S and G2 phases of BMG-1, U-87 and 4197 cells, while no significant changes could be observed in 4451 cells. Higher endogenous levels of cyclin B1 found in both the glioma cell lines, was enhanced further by the ligand as compared to the squamous carcinoma cells. These results clearly demonstrate that the radiosensitizing effects of the ligand are indeed heterogeneous among different human tumor cell lines. The radiaosensitization is p53 independent and accompanied by enhanced mitotic death (linked to cytogenetic damage) as well as induction of cyclin B1 mediated apoptosis.

Inhibitors of topoisomerases as anticancer drugs: problems and prospects.

DNA topoisomerases, which solve topological problems associated with various DNA transactions, are the targets of many therapeutic agents. Various topoisomerase inhibitors especially, topo-poisons, camptothecin (topo-I) and etoposide (topo-II) are some of the drugs that are used in the current treatment protocols, particularly for the treatment of leukemia (AML, ALL etc). However, tumor resistance, normal and non-specific tissue cytotoxicity are the limitations for successful development of these drugs as one of the primary therapeutic agents for the treatment of tumors in vitro. This brief review presents the current understanding about cytotoxicity development and outlines various approaches to overcome the limitations for enhancing the efficacy of topo-poison based anticancer drugs.