Dediazoniation of p-hydroxybenzenediazonium ion in a neutral aqueous medium

Spin Trapping Hydroxyl and Aryl Radicals of One-Electron Reduced Anticancer Benzotriazine 1,4-Dioxides

Hypoxia in tumors results in resistance to both chemotherapy and radiotherapy treatments but affords an environment in which hypoxia-activated prodrugs (HAP) are activated upon bioreduction to release targeted cytotoxins. The benzotriazine 1,4-di-N-oxide (BTO) HAP, tirapazamine (TPZ, 1), has undergone extensive clinical evaluation in combination with radiotherapy to assist in the killing of hypoxic tumor cells. Although compound 1 did not gain approval for clinical use, it has spurred on the development of other BTOs, such as the 3-alkyl analogue, SN30000, 2. There is general agreement that the cytotoxin(s) from BTOs arise from the one-electron reduced form of the compounds. Identifying the cytotoxic radicals, and whether they play a role in the selective killing of hypoxic tumor cells, is important for continued development of the BTO class of anticancer prodrugs. In this study, nitrone spin-traps, combined with electron spin resonance, give evidence for the formation of aryl radicals from compounds 1, 2 and 3-phenyl analogues, compounds 3 and 4, which form carbon C-centered radicals. In addition, high concentrations of DEPMPO (5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide) spin-trap the •OH radical. The combination of spin-traps with high concentrations of DMSO and methanol also give evidence for the involvement of strongly oxidizing radicals. The failure to spin-trap methyl radicals with PBN (N-tert-butylphenylnitrone) on the bioreduction of compound 2, in the presence of DMSO, implies that free •OH radicals are not released from the protonated radical anions of compound 2. The spin-trapping of •OH radicals by high concentrations of DEPMPO, and the radical species arising from DMSO and methanol give both direct and indirect evidence for the scavenging of •OH radicals that are involved in an intramolecular process. Hypoxia-selective cytotoxicity is not related to the formation of aryl radicals from the BTO compounds as they are associated with high aerobic cytotoxicity.

Tumor-promoting activity of p-hydroxybenzenediazonium is accelerated in Mg-deficient rats

Tumor-promoting activity caused by the short-term administration of p-hydroxybenzenediazonium (PDQ) has been assayed in rats fed on a Mg-deficient diet as a reference model versus rats fed on a standard diet as controls. For 5 weeks groups of 20 rats, fed either on the Mg-deficient or standard diet, were treated simultaneously with PDQ. A group of 10 Mg-deficient rats remained untreated. Topical application of PDQ was followed by the appearance of macroscopic alterations in the skin, which were more evident in the Mg-deficient rats. No deaths occurred during the treatment. After 5 weeks' PDQ treatment the rats were killed and histological analyses were made. Tissues from the skin, liver, heart, kidney, lung and thymus were screened by conventional staining methods. Both the PDQ-treated Mg-deficient and PDQ-treated control rats presented tissue lesions, although to a different extent. The untreated Mg-deficient rats showed no such lesions. Mg-deficient rats treated with PDQ developed significant incipient fibrosarcomas (p<0.05) and extended hyperplasia (p<0.001), particularly in the skin, accompanied by a significant increase in the thickness of collagen (mean value: 445.4+/-47.2microm, p<0.05) compared to the control PDQ-treated group (mean values: 258.7+/-36.4microm). The overall results provide objective proof of tumor-promoting activity after 5 weeks' treatment with PDQ. Such a fast response is interpreted as being linked to the increased vulnerability of the membrane caused by Mg deficiency, which would more readily facilitate the toxic activity of p-hydroxybenzenediazonium ions.

The Structural Influence in the Stability of Polymer-Bound Diazonium Salts

Various new diazonium ions on a polymeric support that have a variety of counterions and complexation with crown ethers have been prepared, and the thermal stability of these resins over a larger temperature interval has been investigated. Nonisothermal kinetics applied to DSC data have been used predicting the lifetime of the resins.

Oxidative effects induced by dediazoniation of the p-hydroxybenzenediazonium ion in a neutral aqueous medium

The toxicity of arenediazonium ions is believed to result from the appearance of very reactive compounds during the dediazoniation process. In the case of the p-hydroxybenzenediazonium ion (PDQ), radical species generated during dediazoniation could potentially initiate lipid peroxidation. The data obtained in spectrophotometric experiments suggest that an interaction between PDQ and linoleic acid (LA) gives rise to the characteristic absorption of oxidized products deriving from LA, both in the presence and absence of a mixed micellar medium containing the surfactant Tween 20 (Tw20). Spectroscopic evidence also clearly points to the interference of these processes in the dediazoniation of PDQ. Analysis by reverse-phase, high-pressure liquid chromatography (HPLC) confirms that the decomposition of PDQ in a mixed micellar medium induces the peroxidation of both LA and methyl linoleate (MEL), thus causing the appearance of peaks characteristic of dienic conjugated hydroperoxides. The same products are observed after interaction between LA and the water-soluble 2,2'-azobis (2-amidinopropane), a frequently used initiator of lipid peroxidation. The proportion of isomers produced during the peroxidation process agrees well with that reported for reactions mediated by free radicals. A further chromatographic analysis of the decomposition of PDQ in the presence of 2-methylcyclohexa-2,5-diene-1-carboxylic acid (CHD) shows that phenol and quinone are the main products of the reaction. These results are discussed on the understanding that aryl and peroxyl radicals abstract a hydrogen atom from CHD, in accordance with our general scheme for PDQ dediazoniation described in a previous publication.

Dediazoniation of p-hydroxy and p-nitrobenzenediazonium ions in an aqueous medium: Interference by the chelating agent diethylenetriaminepentaacetic acid

We have made a comparative study of the dediazoniation of p-hydroxy and p-nitrobenzenediazonium ions. The electron-withdrawing and donating properties of the -NO2 and -OH groups strongly determine the reactivity of both compounds, thus exerting different influences upon the dediazoniation reaction. We describe here how the decomposition of p-hydroxy and p-nitrobenzenediazonium ions in a neutral aqueous medium follows a different pattern in the presence of the metal-chelator diethylenetriaminepentaacetic acid (DTPA). The decomposition rate of p-hydroxybenzene diazonium decreases whilst the decomposition of the p-nitrobenzenediazonium ion is enhanced. The experimental data are discussed with reference to a common scheme of interference for both benzenediazonium ions in the light of the radical-scavenging capacity of DTPA.Key words: p-hydroxybenzenediazonium ion, p-nitrobenzenediazonium ion, di-ethylenetriaminepentaacetic acid, dediazoniation, radical scavenging, artifacts.

Monitoring dediazoniation product formation by high-performance liquid chromatography after derivatization

A derivatization protocol that exploits the rapid reaction between arenediazonium ions and a suitable coupling agent followed by high-performance liquid chromatography analyses of the reaction mixture was employed to determine the product distribution, the rate constants for product formation and the association constant of 4-nitrobenzenediazonium, PNBD, ion with beta-cyclodextrin, beta-CD. The derivatization of PNBD with the coupling agent leads to the formation of a stable azo dye that prevents by-side reactions of PNBD with the solvents of the mobile phase, including water, or the metallic parts of the chromatographic system that would eventually lead to erroneous identification and quantification of dediazoniation products. The results show that in the presence of beta-CD, nitrobenzene is formed at the expense of 4-nitrophenol, which is the major product in its absence. The observed rate constants for the interaction between PNBD and beta-CD increase upon increasing [beta-CD] showing a saturation profile indicative of the formation of an inclusion complex between PNBD and beta-CD. By fitting the experimental data to a simplified Lineaweaver-Burk equation, the corresponding association constant and the maximum acceleration rate of beta-CD towards PNBD were estimated. The protocol is applicable under a variety of experimental conditions provided that the rate of the coupling reaction is much faster than that of dediazoniation.