The radiosensitizing effect of ornidazole in hypoxic mammalian tissue: an in vivo study

Radio-Sensitizing Effects of CuII and ZnII Complexes of Ornidazole: Role of Nitro Radical Anion

The treatment of malignant cells that are deficient in oxygen due to the insufficient flow of blood is often seen as a major hindrance in radiotherapy. Such cells become radio-resistant because molecular oxygen, the natural and best radio-sensitizer, is depleted. Hence, to compensate this deficiency in oxygen, there is a need for agents that enhance radiation-induced damage of cells (radio-sensitizers) in a manner that normal cells are least affected. Simultaneously, agents capable of showing activity under hypoxic conditions are known as hypoxic cytotoxins that selectively and preferably destroy cells under hypoxic environments. 5-Nitroimidazoles fit both definitions. Their efficiency is based on their ability to generate the nitro radical anion that interacts with the strands of DNA within cells, either damaging or modifying them, leading to cell death. 5-Nitroimidazoles are important radio-pharmaceuticals (radio-sensitizers) in cancer-related treatments where the nitro radical anion has an important role. Since its generation leads to neurotoxic side effects that may be controlled through metal complex formation, this study looks at the possibility of two monomeric complexes of Ornidazole [1-chloro-3-(2-methyl-5-nitro-1H-imidazole-1-yl)propan-2-ol] with Cu^II and Zn^II to be better radio-sensitizers and/or hypoxic cytotoxins than Ornidazole. The study reveals that although there is a decrease in nitro radical anion formation by complexes, such a decrease does not hamper their radio-sensitizing ability. Nucleic acid bases (thymine, cytosine, and adenine) or calf thymus DNA used as targets were irradiated with ⁶⁰Co γ rays either in the absence or presence of Ornidazole and its monomeric complexes. Radiation-induced damage of nucleic acid bases was followed by high-performance liquid chromatography (HPLC), and modification of calf thymus DNA was followed by ethidium bromide fluorescence. Studies indicate that the complexes were better in performance than Ornidazole. Cu^II-ornidazole was significantly better than either Ornidazole or Zn^II-ornidazole, which is attributed to certain special features of the Cu^II complex; aspects like having a stable lower oxidation state enable it to participate in Fenton reactions that actively influence radio-sensitization and the ability of the complex to bind effectively to DNA.

Use of ornidazole in fractionated radiotherapy: dose tolerance, serum and tumour tissue concentration

Sensitizing and neurotoxic effect of ornidazole, was tested in a double-blind randomized study in patients with carcinoma of the cervix and larynx. Ornidazole or placebo were given orally, two times weekly, for 3 weeks. Dose was 2.5 g/m2 for each administration. Total dose given was 15 g/m2. Radiation therapy was given 3 h after the drug administration. Ornidazole was well tolerated in the majority of the patients. No neurotoxic side effects, such as peripheral neuropathy or convulsion, were observed with a total dose of up to 30 g. Dizziness, somnolence and nausea were the prominent acute side effects, seen mostly (70%) in women. In the placebo group this rate was 17% (p less than 0.01). No important side effect was observed in men receiving ornidazole. Serum concentration of ornidazole reached the maximum level in 2-4 h after oral administration and ranged (23 patients) from 65.1 to 139.8 micrograms/ml. Mean half-life was 15.6 +/- 2.8 h. Peak concentration in tumour tissue was achieved 1-3 h after the administration, ranging from 13.0 to 78.0 micrograms/g. Tumour concentration of ornidazole ranged from 14 to 93% of the serum concentration at the time of irradiation.