Selective interaction of tirapazamine with DNA bases and DNA. A comparison of cyclic voltammetry and electrolysis techniques

An electrochemical model has been used to study the reductive activation of the hypoxic cell cytotoxin tirapazamine (TPZ, 3-amino-1,2,4-benzotriazine-1,4-dioxide). Cyclic voltammetry and controlled potential electrolysis have been used to generate and study the 1-electron reduction product, the assumed biologically active species. Cyclic voltammetry of tirapazamine in dimethylformamide shows a quasi-reversible 1-electron reduction with the product showing a tendency to participate in a following chemical reaction. Controlled potential electrolysis to generate the 1-electron reduction product was unsuccessful due to the formation of a new redox-active species at less negative reduction potentials. However, the cyclic voltammetry of tirapazamine in the presence of E. coli DNA shows a decrease in the lifetime of the radical anion, signifying direct interaction with the DNA. The radical lifetime also decreased in the presence of adenine, thymine and guanine, but increased upon addition of cytosine and ribose. The study shows that cyclic voltammetry is an extremely useful tool for investigating the interaction between bio-reductive drugs and biological target molecules.

Reductive activation of nitroheterocyclic compounds

1. Nitroaromatic compounds are important chemotherapy agents. 2. Their selective toxicity is determined by reduction to the biologically active form in the absence of oxygen. 3. Nitroaromatics are extensively used in the treatment of anaerobic infections and to target hypoxic tumor cells in cancer therapy. 4. Possible mutagenic action is related to the relative ease of nitro group reduction. 5. The mode of action and clinical application of nitroaromatic compounds is summarized.

The interaction of nitroaromatic drugs with aminothiols

The effect of cysteamine and glutathione addition on the redox behaviour of metronidazole, chloramphenicol, M&B 4998, nitrofurazone, and nifuroxime has been studied by electrochemical techniques. The presence of thiol influences the redox behaviour of the nitro compound in a number of ways. In aqueous media, the single-step nitro/hydroxylamine reduction shows a decrease in current and a shift to more positive potentials, which is assigned to the thiol acting as the reducing agent, but only after the formation of the nitro radical anion. In addition, the reversible RNO/RNHOH couple is greatly diminished or removed. In a dimethylformamide/H2O solvent, the nitro radical anion can be selectively generated. The effect of thiol addition on the stability of the radical anion is strongly dependent on the drug, the identity of the thiol, and the concentration of the supporting electrolyte. The presence of thiol can result in an increase or a decrease in the lifetime of the radical with no apparent correlation with the redox couple of the nitro compound, or can act as an oxidizing agent and regenerate the original nitro compound. These disparate routes by which thiol can modify the redox characteristics of nitro compounds suggest that the traditional role of thiol as a radical scavenger needs to be extended.

Electrochemical studies of tirapazamine: generation of the one-electron reduction product

The electrochemical properties of the benzotriazine di-N-oxide, tirapazamine (SR4233), and the mono- and zero-N-oxides, SR4317 and SR4330 respectively, have been investigated in dimethylformamide and acetonitrile. The voltammetry of tirapazamine is complicated, with up to 6 reduction steps being identified, depending on the solvent. Both SR4317 and SR4330 show two reduction steps. The first reduction of all three compounds is a reversible or quasi-reversible step, which is assigned to a 1-electron addition. Cyclic voltammetric studies show that the anion radical product is stable, although the tirapazamine 1-electron addition product shows a tendency to participate in a chemical following reaction. Subsequent reduction steps are all highly irreversible in nature. The 2nd electron transfer of SR4317 results in the formation of the free base, SR4330, which is identified voltammetrically. Comparison is made with the voltammetric behaviour of quinoline and quinoline-oxide.

Evidence for the direct interaction of reduced metronidazole derivatives with DNA bases

The electrochemical behaviour of the bioreductive redox active nitroimidazole drug metronidazole has been examined in the presence and absence of the DNA bases using three electrochemical techniques, all of which indicate the capacity for interaction between reduced products and DNA bases. The 4-electron metronidazole (RNO2) metronidazole-hydroxylamine (RNHOH) couple in an aqueous medium shows a positive shift in reduction potential upon addition of thymine, adenine and guanine, but a negative shift for cytosine. Interpretation of these results for an irreversible process is, however, inconclusive. In dimethylformamide/H2O the presence of DNA base on the one-electron addition product, the nitro radical anion, was examined by cyclic voltammetry. All except guanine resulted in interaction with the metronidazole nitro radical anion (RNO2-), as measured by the decrease in the return-to-forward peak current ratio, in the following order of increasing reactivity: cytosine, adenine and thymine (at a metronidazole: base ratio of 1:1). The increase in the stability of the radical anion by increasing the pH of the dimethylformamide/H2O medium resulted in a decreased reaction with thymine.

The reactivity of chloramphenicol reduction products with DNA bases

PURPOSE

The interaction between the constituent bases of deoxyribonucleic acid and the reduction products of the nitro-aromatic compound chloramphenicol and its nitroso derivative have been studied using an electrochemical system.

METHODS AND MATERIALS

The changes to the voltammetry of chloramphenicol and nitrosochloramphenicol upon addition of adenine, cytosine, guanine, and thymine at various concentrations have been measured. The biological implications of reductive activation of both chloramphenicol and nitrosochloramphenicol were examined using a phi X174 double transfection technique which measures biologically relevant deoxyribonucleic acid damage.

RESULTS

Measurement of the voltammetric response of chloramphenicol shows that the most noticeable change upon base addition is a decrease in the lifetime of the nitro radical anion in the following order of decreasing activity: adenine, thymine, and cytosine. No effect was observed with guanine. The reversible 2-electron nitrosochloramphenicol-hydroxychloramphenicol couple showed no interaction on the voltammetric timescale, although binding of the hydroxylamine to guanine was observed. Interaction of the azo derivative, formed as a consequence of further reduction plus chemical reaction of nitrosochloramphenicol was observed. Biological studies showed that no significant effect on deoxyribonucleic acid by chloramphenicol or nitrosochloramphenicol was observed under oxic conditions. Controlled reduction of nitrosochloramphenicol to the hydroxylamine gave considerably less damage than when nitrosochloramphenicol or chloramphenicol was completely reduced.

CONCLUSION

The chloramphenicol nitro radical anion reacts selectively with the bases of deoxyribonucleic acid. Reduction products of nitrosochloramphenicol beyond the 2-electron hydroxylamine are highly reactive to deoxyribonucleic acid.

Heterocyclic mono-N-oxides with potential applications as bioreductive anti-tumour drugs: Part 1. 8-Alkylamino-substituted phenylimidazo [1,2-a] quinoxalines

A series of imidazo [1,2-a] quinoxaline mono-N-oxides and their 6- and 9-aza analogues have been substituted in the 8-position with a variety of secondary and tertiary amines, and the compounds evaluated as bioreductively activated cytotoxins. Cytotoxic action against hypoxic cells in vitro was critically dependent upon the structural nature of the 8-substituent and its basicity, with little dependence upon reduction potential. 1,2-Dihydro-8-(4-methylpiperazin-1-yl)-4-phenylimidazo [1,2-a] pyrido [3,2-e] pyrazine 5-oxide (11) had differential hypoxic:oxic toxicity of 15.3 and some novel analogues had differential hypoxic:oxic toxicities of 7.5-17. Other related compounds with either substituted or unsubstituted 8-piperazinyl substituents, or certain straight-chain aminoalkyl substituents, show comparable activity in vitro. Less basic 8-substituents abolished activity, although the 8-morpholinyl derivatives (7 and 8) had differential hypoxic:oxic toxicities of 3-4. Substitution of the 4-phenyl ring with an electron-withdrawing group (F) improved hypoxic potency, but only with a small effect on hypoxic:oxic toxicity, whereas an electron-donating substituent (MeO) reduced hypoxic potency. Perhaps significantly, the 8-unsubstituted analogue 3 was 6-fold less potent, but had comparable differential cytotoxicity in vitro. The most effective novel hypoxia-selective cytotoxins synthesized were the bifunctional 2-nitro-imidazole derivative 1,2-dihydro-8-((4-(3-(2-nitro-1-imidazoyl)-1-hydroxypropyl)- piperazin-1-yl))-4-phenylimidazo [1,2-a] quinoxaline 5-oxide bishydrochloride (37) and its 9-aza analogue 38. These compounds also exhibited the lowest aerobic toxicities in vitro of the new compounds.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Comparative DNA damage induced by nitroimidazole-aziridine drugs: 1. Effects of methyl substitution on drug action

RSU-1069 (1-(-3-aziridinyl-2-hydroxypropyl)-2-nitroimidazole) is a bifunctional chemo- and radiosensitizing agent. The properties of these functional groups may be examined by comparison of drug-induced DNA damage oxically, a measure of aziridine-induced damage and, during anoxic electrochemical reduction of the nitro-group, a measure of DNA damage resulting from the combined activity of reduced nitro group intermediates and alkylation by the aziridine moiety. In this study, a series of nitroimidazole aziridines have been studied and compared. The compounds used were RSU-1069, five methyl substituted derivatives: RSU-1131, RSU-1150, RSU-1164, RSU-1172, RB-7040; a 4-nitroimidazole derivative, RSU-1170, and RSU-1137, the non-alkylating hydrolysis product of RSU-1069. DNA damage, occurring oxically or as a consequence of nitro reduction, decreases with increasing substitution of the aziridine ring. Most DNA damage occurring oxically is produced by RSU-1069 and RSU-1170, both compounds having unsubstituted aziridine rings; least DNA damage is produced by RSU-1137. In general, the extent of DNA damage during electrochemical reduction is greater than that occurring oxically, this being due to an assumed combination of alkylation and reduced nitro-group intermediates. There is a direct correlation between the half-lives of the compounds and the extent of DNA damage occurring under oxic conditions. A direct correlation of the aerobic toxicities of the compounds tested, relative to RSU-1069, and the number of unsubstituted sites available for nucleophilic attack on the aziridine moiety has also been shown.

Electrochemical characteristics of nitro-heterocyclic compounds of biological interest. I. The influence of solvent

The electrochemical properties of three nitroimidazoles, a nitropyrazole, a nitrofuran and three nitrobenzenoid compounds have been extensively investigated in a range of solvents. The reduction pathway for the nitro group is independent of the cyclic function to which it is attached, but is strongly influenced by the nature of the solvent. In aqueous media, generally, a single, irreversible 4-electron reduction occurs to give the hydroxylamine. In aprotic media (dimethylformamide, methylene chloride or dimethylsulphoxide), a reversible one-electron reduction takes place to form a stable nitro radical anion. At more negative values, a further 3-electron reduction occurs, irreversibly to give the hydroxylamine. In mixed aqueous-organic systems, intermediate behaviour is found, with the reversibility of the RNO2/RNO.-2 couple increasing with addition of organic medium. The control of the reduction pathway, by changing the electrolytic medium is discussed in relation to the biological activities of the drugs and identification of the short-lived reduction intermediate responsible for DNA damage.