Abnormal radiosensitizing and cytotoxic properties of ortho-substituted nitroimidazoles

Nitroimidazoles as hypoxic cell radiosensitizers and hypoxia probes: misonidazole, myths and mistakes

Nitroimidazoles have been extensively explored as hypoxic cell radiosensitizers but have had limited clinical success, with efficacy restricted by toxicity. However, they have proven clinically useful as probes for tumour hypoxia. Both applications, and probably much of the dose-limiting toxicities, reflect the dominant chemical property of electron affinity or ease of reduction, associated with the nitro substituent in an aromatic structure. This single dominant property affords unusual, indeed extraordinary flexibility in drug or probe design, suggesting further development is possible in spite of earlier limitations, in particular building on the benefit of hindsight and an appreciation of errors made in earlier studies. The most notable errors were: the delay in viewing cellular thiol depletion as a likely common artefact in testing in vitro; slow recognition of pH-driven concentration gradients when compounds were weak acids and bases; and a failure to explore the possible involvement of pH and ascorbate in influencing hypoxia probe binding. The experience points to the need to involve a wider range of expertise than that historically involved in many laboratories when studying the effects of chemicals on radiation response or using diagnostic probes.

Synthesis, characterisation and antiamoebic activity of new thiophene-2-carboxaldehyde thiosemicarbazone derivatives and their cyclooctadiene Ru(II) complexes

Reaction of new thiosemicarbazones (1-4) derived from thiophene-2-carboxaldehyde and cycloalkylaminothiocarbonylhydrazine with [Ru(eta(4)-C8H12)(CH3CN)2Cl2] leads to form complexes (1a-4a) of the type [Ru(eta(4)-C8H12)(TSC)Cl2] (where TSC=thiosemicarbazone). All the compounds have been characterised by elemental analysis, IR, 1H NMR, electronic spectra and thermogravimetric analysis. It is concluded that the thionic sulphur and the azomethine nitrogen atom of the ligands are bonded to the metal ion. In vitro antiamoebic screening against (HK-9) strain of Entamoeba histolytica indicated that the Ru(II) complexes of thiophene-2-carboxaldehyde thiosemicarbazones were found more active than the thiosemicarbazones.

Electron-affinic radiosensitizers possessing NPSH-reactive side chains: cytotoxicity and radiosensitizing activity towards hypoxic EMT6 cells in vitro

A new class of dual-function nitroazole derivatives that are composed of electron-affinic nitroazole rings and a thiol-reactive alpha, beta-unsaturated carbonyl side chain were synthesized to evaluate their physico-chemical properties, reactivity with glutathione (GSH), and cytotoxicity and radiosensitizing activity towards EMT6/KU cells in vitro. Among this class of nitroazole compounds, 2-nitroimidazole-derivative (1), 3-nitro-1,2,4-triazole derivative (2), and 2-methyl-4-nitroimidazole derivative (3) with a common side-chain structure of trans CH2CH = CHCO2CH3 readily reacted with GSH in phosphate-buffer solution (pH 7.2, 310 K). These compounds showed higher cytotoxicities to both aerobic and hypoxic EMT6/KU cells than the corresponding alpha, beta-saturated counterparts (6-8) with a side-chain structure of CH2CH2CH2CO2CH2CH3. The hypoxic cytotoxicity of 1 and 2 with similar electron affinities to that of misonidazole (9) was potentiated by the combined effects of depletion of non-protein thiols (NPSH) by the alpha, beta-unsaturated carbonyl side chains and bioreduction of the nitroazole rings. The sensitizer enhancement ratios in vitro (SERvitro) of 1 (2.80 +/- 0.20) and 2 (2.63 +/- 0.27) at a dose of 1.0 mmol dm-3 are comparable with the oxygen enhancement ratio (OER = 2.90 +/- 0.10) and are significantly larger than those of their respective counterparts 6 (1.28 +/- 0.06) and 7 (1.22 +/- 0.09). A less electron-affinic compound, 3, also gave a large SERvitro = 1.80 +/- 0.18, whereas the counterpart 8 was not effective (1.10) in radiosensitizing hypoxic cells. Compounds 1-3 not only altered the slope, but also reduced the shoulder of the dose-survival curve. These 'dual-function' nitroazole radiosensitizers show much lower levels of in vitro radiosensitization, as measured by the C1-6, than the previously reported 'anomalous' radiosensitizers such as 5-substituted 4-nitroimidazole radiosensitizers.

A new ruthenium radiosensitizer: RuCl2(DMSO)2(4-nitroimidazole)2

Earlier studies on Ru complexes, such as, cis-RuCl2(DMSO)4, as antineoplastic agents, have suggested a DNA binding mechanism similar to that of the clinically successful platinum complex, cis-diammine(dichloro)platinum(II), (cis-DDP). As part of our study on metal-radiosensitizer complexes, cis-RuCl2(DMSO)4 is used as a precursor for synthesis of Ru(II)-nitroimidazole complexes. These complexes were identified and characterized and their toxicity and radiosensitizing abilities were examined in vitro. In the series of Ru(II)-nitroimidazole complexes synthesized, RuCl2(DMSO)2(4-nitroimidazole)2, "Ru-(4-nitro)," was the most effective radiosensitizer. At 200 microM, Ru-(4-nitro) produced a sensitizer enhancement ratio (SER) of 1.6 in hypoxic Chinese hamster ovary (CHO) cells, but did not sensitize oxic CHO cells. Other Ru-nitroimidazole complexes gave SER values of 1.2-1.4 at 200 microM. These Ru(II)-nitroimidazole complexes also showed lower toxicity than the free nitroimidazoles alone at equimolar concentration. The enhanced radiosensitizing effect of Ru-(4-nitro) may be due to the metal's ability to target the sensitizer to DNA, or may be related to changes in reduction potential: from -685 mV for the free ligand to -355 mV and -615 mV for the complex. This complex did not deplete non-protein sulfhydryls (NPSH) at the time intervals and concentrations used. DNA binding was studied using inhibition of restriction enzyme activity on plasmid DNA.