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Biomedical subjects

D I Edwards

Publications and source records attributed to D I Edwards.

At least 19 recordsLinked to original sources
PubMed1992

The interaction of reduced metronidazole with DNA bases and nucleosides.

The electrochemical behavior of the 1-electron couple for the bioreductive drug metronidazole has been examined in the presence and absence of the biological target molecules, DNA bases, and nucleosides, including uracil and uridine. Using cyclic voltammetry as the investigation technique, the change in return-to-forward peak current ratio, ipr/ipf, from the control, recorded in the absence of target, was measured as a function of scan rate and biological target concentration. All target molecules, except adenosine and guanine, resulted in interaction with RNO2.-, as measured by the decrease in the ipr/ipf ratio in the following order of increasing reactivity: adenine, guanosine, thymine, uracil, uridine, and thymidine (at a metronidazole:target ratio of 1:1). No decrease in ipr/ipf was observed with cytosine or cytidine until ratios of 1:20 and 1:30, respectively, were attained. An approximately linear relationship was found between the percentage change in the CV response and log[target] allowing us to determine the sensitivity of RNO2.- to the concentration of the target species. The implication for the biological action of metronidazole and other nitro-heterocyclic drugs is discussed.

DNA↗
PubMed1992

Repair of damage induced by SR 4233.

The benzotriazine di-N-oxide, SR 4233, was electrolytically reduced at constant potential at pH 4.0 at a reduction rate of 5%/hr under N2 in the presence of phi X174 DNA. During the reduction process, the biological infectivity of the bacterial phage was measured by a double transfection technique, either into the wild-type Escherichia coli strain, or into a series of seven mutants with specific, known defects in their capacity to repair DNA. The survival of phi X174 was evaluated as an index of drug damage and from this we conclude that SR 4233 induces pH-dependent DNA damage in E coli, which is recognized and repaired primarily by the uvrC gene product and by the exonuclease III and endonuclease III gene products. These gene products act primarily upon and are responsible for the recognition of strand breaks and repair of oxidized and fragmented pyrimidine products, indicating that SR 4233 induces strand breaks in DNA resulting from oxidative damage to pyrimidines. As damage is maximized at acid pH, we further propose that the damage mechanism is a process of electron transfer from pyrimidine nucleotides in DNA (i.e., oxidation) to the protonated benzotriazine di-N-oxide one-electron radical anion.

Antineoplastic Agents↗
PubMed1992

Electrochemical characteristics of nitroheterocyclic compounds of biological interest. VIII. Stability of nitro radical anions from cyclic voltammetric studies.

The stability of the one electron addition product of four biologically important nitroheterocyclic compounds has been examined electrochemically. Using cyclic voltammetry the tendency of the nitro radical anion to undergo disproportionation was studied by two methods of analysis. The first was based on determining the voltammetric time-constant required for half of the reduction product, RNO2-., to react further. The second concerned the minimum volume of dimethylformamide which had to be added to the aqueous electrolytic medium to give a specific cyclic voltammetric response. Both methods were found to compare well with the results obtained for RNO2-. stabilities using a theoretically derived procedure for a second order reaction following a charge-transfer step. The use of these alternative approaches for quantifying the reactivity of reduction products is discussed. The time-constant method in particular may be useful in studying complex reaction pathways.

Chloramphenicol↗
PubMed1992

Studies on DNA damage and induction of SOS repair by novel multifunctional bioreducible compounds. II. A metronidazole adduct of a ruthenium-arene compound.

A new transition metal complex of the 5-nitroimidazole, metronidazole (1-beta-hydroxyethyl-2-methyl-5-nitroimidazole), has been prepared and its potential use as a hypoxic cell cytotoxic agent examined. The preparation of the complex [(eta6-C6H6)RuCl2(metronidazole)] is described together with its characterization using standard spectroscopic techniques. Electrochemical investigations showed that coordination to the metal centre had not altered the electron affinity of the metronidazole, but kinetic studies using the cyclic voltametric mode demonstrated that the one-electron addition product, the nitro radical anion, had a decreased lifetime, with a half-life of 7.75 and 11.9 s for the coordinated and free metronidazole ligand respectively. Biological studies employed viscosity measurements, DNA SOS repair capacity and a transfection assay to examine the effect on DNA. Conductance studies were also employed to determine the influence on intact Escherichia coli growth rates. The ruthenium-metronidazole complex showed greater activity than metronidazole aerobically, but a higher differential activity under hypoxic reduction conditions, due to activation of the NO2 group. Results with intact cells suggested a greater selective cytotoxicity with metronidazole coordinated to ruthenium than attained with the free ligand.

Antineoplastic Agents↗
PubMed1991

A comparison of the relative activities of 8 radiosensitizers in the SOS chromotest.

Misonidazole, and RSU 1069 and 6 of its analogues are all reported to show increased cytotoxicity towards hypoxic cells compared to oxic cells. DNA is considered to be the target through which these drugs exert their cytotoxic activity. Therefore we monitored induction of the SOS response in uvrABC excinuclease proficient and deficient strains of E. coli, under oxic and hypoxic conditions, as an indirect method of assessing the activity of these drugs towards DNA in a biological system. This was done using the SOS chromotest which utilizes E. coli strains which possess a sfiA::lacZ fusion allowing induction of the SOS response to be monitored by assaying beta-galactosidase activity. All of the drugs tested here show some induction of the SOS response in both uvrABC excinuclease proficient and deficient strains. Data shown here suggests that the uvrABC excinuclease is important in the production of a SOS induction signal from RSU 1069-induced DNA lesions and that RSU 1069 may act as a crosslinking agent. The data also shows that SOS induction activity and toxicity do not necessarily correlate and that production of a SOS induction signal may occur via a different pathway for RSU 1069 than for its analogues.

Cell Hypoxia↗
PubMed1991

Electrochemical studies of nitroheterocyclic compounds of biological interest. VII. Effect of electrode material.

The electrochemical behaviour of three nitrofuran compounds, nitrofurazone, nitrofurantoin and furazolidone, has been studied in three solvent types; aprotic, aqueous and mixed, and at four working electrodes. Particular attention has focused on the 1-electron RNO2/RNO2.- couple as measured by the cyclic voltammetric mode. Using Hg in aqueous buffer, reduction of the NO2 group proceeds directly to the hydroxylamine with no intermediate stages being identified. Addition of an aprotic solvent gave a 2-stage reduction, initially forming the RNO2.- species. At all solid electrodes, however, the RNO2/RNO2.- couple was identified under simple aqueous conditions. The switch to a mixed aqueous/aprotic solvent medium produced only minor changes in the response compared with the situation on Hg. This presents the opportunity of using nitrofuran complexes as model systems for the redox behaviour of nitro aromatic compounds in general at solid electrode surfaces where the latters' more negative reduction potentials makes direct study difficult. The conditions have been defined whereby we can examine pH effects and RNO2.- biological target interactions in simple aqueous media to allow the further refinement of the electrolytic model system for studying bio-reducible drug action.

Carbon↗
PubMed1990

DNA damaging effects and voltammetric studies on the hypoxic cell toxin 3-amino-1,2,4-benzotriazine-1,4-dioxide, SR4233, as a function of pH.

The compound 3-amino-1,2,4-benzotriazine-1,4-dioxide, SR4233, has recently attracted considerable attention as a possible hypoxic cell radiation sensitizer and cytotoxic agent. The present study examines the influence of pH on the DNA damaging ability of SR4233 upon electrolytic reductive activation, and the corresponding changes in electrochemistry. A phi X174 double transfection assay has been employed to assess the DNA damaging ability of SR4233 between pH 4 to 7. Upon electrolytic reduction the drug was found to be more effective in damaging DNA at acidic pH than at neutral conditions. This indicated that the damaging species was probably protonated. The DNA damaging ability of SR4233, as measured by a viral transfection assay, was linearly related to pH between the values of 4 and 7, and this feature has implications for its potential efficacy in the treatment of hypoxic tumors. The electrochemistry of SR4233 has been examined as a function of pH between the ranges 2 and 10.5. Three investigation techniques have been employed, cyclic voltammetry and differential pulse and dc polarographies. A general shift towards less negative potentials with increasing acidity was found between pH 2 and 8.5 giving a linear relationship. The behaviour was found to be relatively invariant at alkaline pH.

Antineoplastic Agents↗
PubMed1990

Electrochemical studies and DNA damaging effects of the benzotriazine-N-oxides.

The electrochemical behaviour of eight benzotriazine 1,4 di-N-oxides has been examined and compared with the mono- and zero-N-oxides. The di-N-oxides all show two reduction steps, an irreversible followed by a quasi-reversible response assigned to the 4 electron reduction of both N-oxide groups, followed by the 2 electron reduction of the benzotriazine ring. Mono- and zero-N-oxides show only a single, quasi-reversible reduction step, similar in character to the second reduction of the di-N-oxides. This has been assigned to reduction of the benzotriazine ring, with the available, redox-active, N-oxide group of the mono-N-oxide complex being reduced at less negative potentials, but only after ring reduction, hence only a single electrode response. The importance of reductive activation of the N-oxide group has been examined using a phi X174 double transfection technique which assays biologically relevant DNA damage. For the di-N-oxides, no effect on DNA was recorded under oxic conditions, however, DNA damage was marked under anoxic reduction conditions. The extent of DNA damage was found to increase with the acidity of the medium, suggesting the protonated form of the reduction product as being responsible for the cytotoxic action. The mono-N-oxide was shown to be biologically inactive under all conditions.

Antineoplastic Agents↗
PubMed1990

Electrochemical characteristics of nitroheterocyclic compounds of biological interest. V. Measurement and comparison of nitro radical lifetimes.

Using mixed aqueous/dimethylformamide solvents we have generated nitro radical anions by electrochemical reduction of nitroaromatic compounds. Six drugs have been examined: metronidazole, nitrofurazone, nifuroxime, chloramphenicol, M&B 4998 and 4(5)-nitroimidazole, chosen to represent a variety of ring structures and a range of reduction potentials. Analysis of the cyclic voltammetric response as a function of scan rate and dimethylformamide content yields information on the reactivity of RNO2.-. A kinetic analysis of the return-to-forward peak current ratio based on a theoretical treatment was employed. Second-order kinetics for the decay of RNO2.- for all six drugs examined was established. By extrapolation, first half-lives in purely aqueous media were found to increase in the order: nitrofurazone, nifuroxime, chloramphenicol, metronidazole and M&B 4998 (from 8.9 x 10(-2) seconds for nitrofurazone to 98s for M&B 4998 at a radical anion concentration of 1 x 10(-6) mol/dm3). Comparison with reduction potentials showed that as the lifetime of RNO2.- increased, the drug became progressively less electron-affinic (reduced at more negative potentials). The reactivity of RNO2.- was also examined in relation to the DNA damaging capability following electrochemical reduction of these nitroaromatic drugs.

Chloramphenicol↗
PubMed1990

Electrochemical characteristics of five quinolone drugs and their effect on DNA damage and repair in Escherichia coli.

The object of this study was to determine whether 4-quinolone antimicrobials were reduced under biologically attainable redox conditions and whether they had any effect on DNA in the absence of the DNA gyrase enzyme. Electrochemical characteristics of the drugs were investigated using d c polarography, differential pulse polarography and cyclic voltammetry. The ability of the drugs to interact with, and cause damage to, naked DNA was investigated by a phi X174 DNA double transfection assay. Induction of DNA SOS repair was assessed using a stain of Escherichia coli in which the synthesis of beta-galactosidase was under the control of the su1A gene. Growth studies were performed using a conductimetric method in a Malthus system. All five 4-quinolones examined had redox potentials lower (more negative) than -1.2 V and thus were incapable of being reduced in biological systems, even under strict anaerobiosis. Exposure of all drugs to single-stranded phi X174 DNA for up to 50 h engendered no detectable damage. However, all the drugs induced DNA SOS repair, in the order ciprofloxacin greater than fleroxacin = pefloxacin greater than norfloxacin greater than nalidixic acid. This rank order corresponds approximately with antibacterial efficiency. The growth studies indicated that redoxyendonuclease III and excision repair enzymes may be involved in the fixation of quinolone-induced damage.

4-Quinolones↗
PubMed1990

Electrochemical characteristics of nitroheterocyclic compounds of biological interest. VI. The misonidazole radical anion.

The addition of four aprotic solvents to misonidazole in an aqueous buffer system has been examined electrochemically. Qualitatively they all result in separation of the initial irreversible 4 electron reduction step into two stages, the RNO2/RNO2- and RNO2-/RNHOH couples respectively. Despite some difficulties in achieving measurements for the discrete RNO2/RNO2- without interference from the following reduction step, it was clear that the various aprotic solvents influenced the lifetime of the RNO2- species to different degrees. Resolution of the two processes was best achieved using a water-acetone system and this has been employed to study the lifetimes of the misonidazole radical anion as a function of acetone content and drug concentration. Analysis of the cyclic voltammetric response showed a second order decay pathway, in line with the metronidazole system studied under similar conditions. This has been compared with results from pulse radiolysis work, which suggested a first order reaction of unknown pathway for 2-nitroimidazole radical anions.

Anions↗
PubMed1990

Electrochemical properties as a function of pH for the benzotriazine di-N-oxides.

The electrochemistry of five benzotriazine di-N-oxides has been examined by cyclic voltammetry and differential pulse and dc polarographies as a function of pH. Between the pH range 8.5 and 2 the trend to less negative potentials with lowering of pH can be described by an equation of the type Ep = -apH + b. Comparison has been made with the mono- and zero-N-oxides which were found to show virtually identical trends in electron affinity with pH. The general electrochemical characteristics for the di- and mono-N-oxides under acidic conditions were found to be comparable with the zero-N-oxide. This was particularly the case on repeat scanning in the cyclic voltammetric mode. The redox mechanism involved reduction by a 4-electron addition step and subsequent loss of the N-oxide group(s) yielding the intact benzotriazine heterocycle. The heterocycle was also redox active, involving a reversible 2-electron reduction. For the di-N-oxides these two stages could be identified as separate processes at alkaline pH, but only a single step at acidic values. The mono-N-oxide in which the electrochemical behaviour was dominated by the triazine, showed only a single reduction step, although the single N-oxide group was redox active.

Antineoplastic Agents↗
PubMed1989

Comparative DNA damage and repair induced by misonidazole, CB 1954 and RSU 1069.

We have studied the ability of CB 1954, misonidazole, and RSU 1069 to induce biologically relevant DNA damage in single- and double-stranded phi X174 DNA under oxic, anoxic, and anoxic reductive conditions using a double transfection technique. In addition, the ability of the three drugs to induce the SOS repair response in E. coli under the same conditions was measured. Whereas the relative order of DNA damage was RSU 1069 greater than CB 1954 greater than misonidazole the order in inducing SOS repair was RSU 1069 greater than misonidazole greater than CB 1954. Drug-induced damage by RSU 1069 involves enhanced damage by endonuclease III suggesting drug-induced pyrimidine damage. There appears to be no correlation between drug-induced damage and the degree of SOS repair induction. Thus it appears that enzymes other than, or in addition to, those of the SOS repair system are involved in the repair of DNA damage induced by these drugs.

Antineoplastic Agents↗
PubMed1989

Electrochemical characteristics of nitro-heterocyclic compounds of biological interest. II. Nitrosochloramphenicol.

The electrochemical characteristics of nitrosochloramphenicol have been studied in aqueous buffer systems (pH 7.1) using direct current (d.c.) and differential pulse polarography, cyclic voltammetry and coulometric techniques. Up to 4 charge-transfer steps can be identified. The first reduction step is reversible both chemically and electrochemically, the charge-transfer product showing no tendency to undergo further reaction on the electrochemical time-scale. In contrast, the second reduction step is irreversible, with the product undergoing a fast following reaction to yield a redox-active species which was detected by cyclic voltammetry. From the data and by comparison with related systems, two reduction mechanisms are possible and are discussed.

Chloramphenicol↗
PubMed1989

Electrochemical characteristics of nitro-heterocyclic compounds of biological interest. III. Nitroso derivative formation.

Upon electrolytic reduction of a range of nitro-aromatic complexes (including imidazoles, benzenoids, furans and pyrazoles) an associated oxidation-reduction process is observed at more positive potentials with respect to nitro group reduction when using repeat scan cyclic voltammetry. This new couple has been identified as the reversible first reduction of the nitroso derivative for chloramphenicol, by the addition of a genuine sample of nitrosochloramphenicol to the electrochemical cell. We have failed to observe formation of the new redox-active species for five 5-nitroimidazoles examined. Possible reaction schemes for nitroso formation under electrolytic reduction conditions and the importance of the nitroso redox couple with respect to the cytotoxic action of the parent drug are discussed. The applicability of nitrosochloramphenicol as a model for the behaviour of nitro-heterocycles in general is shown.

Chloramphenicol↗
PubMed1989

Electrochemical characteristics of nitro-heterocyclic compounds of biological interest. IV. Lifetime of the metronidazole radical anion.

Electrochemical studies on metronidazole using mixed aqueous/dimethylformamide (DMF) solvents have allowed us to generate the one-electron addition product, the nitro radical anion, RNO2(-.). Cyclic volt-ammetric techniques have been employed to study the tendency of RNO2-.to undergo further chemical reaction. The return-to-forward peak current ratio, ipr/ipf, was found to increase towards unity with increasing DMF content of the medium, indicating the extended lifetime of RNO2(-.). Second order kinetics for the decay of RNO2-were established at all DMF concentrations examined. Extrapolation has allowed the rate constant and a first half-life of 8.4 x 10(4) dm3/mol-sec and 0.059 seconds respectively, to be determined for the decay of RNO2-in a purely aqueous media. This is impossible by direct electrochemical measurement in water, due to a different reduction mechanism, giving the hydroxylamine derivative in a single 4-electron step. The application of the technique to other nitro-aromatic compounds is discussed.

Drug Stability↗
PubMed1989

Studies on DNA damage and induction of SOS repair by novel multifunctional bioreducible compounds. I. A metronidazole adduct of dirhodium (II) tetraacetate.

A novel bifunctional hypoxia-selective compound [Rh2(O2CCH3)4.2C6N3O3H9] has been synthesized and its genotoxic and potential mutagenic effects studied with reference to those of dirhodium tetraacetate (RAc) and metronidazole. The properties of the two functional components have been examined by comparing its oxic genotoxicity, a measure of the DNA damage induced by RAc, with its anoxic genotoxicity by electrochemical reduction, a measure of DNA damage resulting from the combined activity of reduced nitro group intermediates and RAc. The induction of DNA SOS repair has also been studied as well as the strand-breaking ability of the compound using viscometry. The genotoxic effects observed are proportional to the drug concentration over the range tested and the compound exhibits a high selective toxicity differential to hypoxic bacteria. The strand-breaking and mutagenic properties are governed by the metronidazole component and other effects, such as inhibition of DNA synthesis, are governed by the RAc component.

DNA↗