The combination of multiple doses of etanidazole and pimonidazole in 48 patients: a toxicity and pharmacokinetic study

An overview of imidazole and its analogues as potent anticancer agents

The quest for novel, physiologically active imidazoles remains an exciting topic of research among medicinal chemists. The imidazole ring is a five-membered aromatic heterocycle that is found in both natural and synthesized compounds. Multiple anticancer drug classes are currently available on the market, but concerns including toxicity, limited efficacy and solubility have lowered the overall therapeutic index. Therefore, the hunt for new potential chemotherapeutic agents persists. The development of imidazole as a reliable and safer alternative to anticancer treatment is generating much attention among experts. Tubulin or microtubule polymerization inhibition and changes in the structure and function of DNA, VEGF, topoisomerase, kinases, histone deacetylases and certain other proteins that affect gene expression are among the putative targets.

Nitro-Group-Containing Drugs

The nitro group is considered to be a versatile and unique functional group in medicinal chemistry. Despite a long history of use in therapeutics, the nitro group has toxicity issues and is often categorized as a structural alert or a toxicophore, and evidence related to drugs containing nitro groups is rather contradictory. In general, drugs containing nitro groups have been extensively associated with mutagenicity and genotoxicity. In this context, efforts toward the structure-mutagenicity or structure-genotoxicity relationships have been undertaken. The current Perspective covers various aspects of agents that contain nitro groups, their bioreductive activation mechanisms, their toxicities, and approaches to combat their toxicity issues. In addition, recent advances in the field of anticancer, antitubercular and antiparasitic agents containing nitro groups, along with a patent survey on hypoxia-activated prodrugs containing nitro groups, are also covered.

Bioreductively-activated prodrugs for targeting hypoxic tissues: elimination of aspirin from 2-nitroimidazole derivatives

2-Nitroimidazoles were synthesised substituted with aspirin or salicylic acid, as leaving groups linked through the (imidazol-5-yl)methyl position. Activation of aqueous solutions by CO2*- (a model one-electron reductant) resulted in release of aspirin or salicylate, probably via the 2-hydroxyaminoimidazole. The analogous 2-nitroimidazole with bromide as leaving group eliminated bromide in < 1 ms via the radical-anion.

Apoptosis and 1-methyl-2-nitroimidazole toxicity in CHO cells

The time course and characteristics of the selective hypoxic cytotoxicity of the 2-nitroimidazole model compound 1-methyl-2-nitroimidazole (INO2) were analysed during prolonged time periods (up to 5 days post treatment). When control populations were seeded at the same cell density as drug-treated cells, they entered confluency at day 3 and underwent apoptosis at day 5, which appeared to be mediated by an autocrine mechanism. In subsequent studies of drug-treated cells, the seeding density of treated cells was adjusted to avoid this cell confluency effect. Treatment with a low INO2 concentration (2.5 mM) resulted in apoptotic DNA fragmentation (ladders), which was observed 4-5 days after an acute 6-h hypoxic drug exposure. In contrast, at a high INO2 concentration (40 mM) for 2 h, which was equitoxic to the low concentration, no characteristic DNA ladders were observed. Fluorescence microscopy revealed apoptotic bodies and pyknotic nuclei 5 days following hypoxic 2.5 mM INO2 exposure, whereas 40 mM INO2 hypoxic treatment produced cellular ghosts devoid of DNA 5 days after exposure, consistent with the DNA ladder results. However, characteristic apoptotic morphology was previously observed immediately after the acute hypoxic exposure of 40 mM INO2. Cell cycle analysis and DNA fragmentation as measured by the TdT assay suggested that dose-dependent differences in the apoptotic response occur post exposure after an equitoxic acute hypoxic exposure to either the low or the high INO2 concentration. This dose-dependent differential in response may be attributed to the degree of initial DNA damage as measured by the comet assay.

Mechanism of the selective hypoxic cytotoxicity of 1-methyl-2-nitroimidazole

2-Nitroimidazoles were introduced into radiation therapy to test their ability to radiosensitize hypoxic cells in solid human tumours. In addition, they are selectively reduced in hypoxic cells to form reactive metabolites that may be effective cytotoxins. 1-Methyl-2-nitroimidazole (INO2) was investigated as a model compound to study the mechanism of selective bioreduction in hypoxic cells. Results demonstrated that INO2 was toxic under hypoxic conditions (tested via colony-forming assay) at concentrations where no toxicity was observed for aerobic cells. This selective hypoxic toxicity was a function of both concentration and time. The depletion of both glutathione and protein thiols occurred under hypoxic conditions and preceded a rise in intracellular calcium levels. Previous work with INO, the nitroso intermediate of INO2 reduction, also showed concentration-dependent cytotoxicity, and glutathione and protein thiol depletion, which was followed by an increase in intracellular calcium levels. The kinetics of cytotoxicity and cellular reactions were slower for the parent compound, INO2, as compared with the 2e- reductive metabolite, INO, reflecting the limited enzymatic production of the reactive intermediate in the INO2 experiments. Zeiosis (membrane blebbing) and chromatin condensation occurred shortly after treatment of cells with equitoxic concentrations of both INO2 (under hypoxic conditions) and INO (under aerobic conditions), suggesting that an apoptotic-like death mechanism may be involved. However, analysis of DNA isolated from both INO2- and INO-treated cells, up to 2 hr after treatment, did not reveal any nucleosomal fragmentation, another characteristic feature of cells undergoing apoptosis. The toxicity of high INO2 concentrations toward CHO cells is consistent with the production of an INO intermediate and has several features characteristic of an apoptotic mechanism of cell death.

Tumor hypoxia, reoxygenation and oxygenation strategies: possible role in photodynamic therapy

The concept of hypoxia and its role in tumor therapy are currently under re-evaluation. Poor oxygenation is no longer visualized as an independent feature promoting necrosis and resistance to treatments, but rather as one of the several interdependent microenvironmental parameters associated with impaired blood perfusion. Tumor cells display several survival strategies and remain clonogenic for long periods in nutrient-deprived situations. Reoxygenation may cause lethal damage, improve the response to therapy, or else allow the cell variants adapted to hypoxia to resume proliferation with enhanced aggressiveness and resistance to treatment. The blood supply parameters, oxygenation status and metabolism of malignant cells are discussed here from the standpoint of tumor photodynamic therapy. The role of the tumor interstitial fluid as oxygen- and sensitizer-carrier is discussed. Techniques for assessing tumor oxygenation and for mapping hypoxic territories are described. Strategies for locally improving the oxygenation levels or for selectively destroying the hypoxic populations are outlined.