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Biomedical subjects

P Wardman

Publications and source records attributed to P Wardman.

At least 55 records · Page 3Linked to original sources
PubMed1995

Enhancement of lipid peroxidation by indole-3-acetic acid and derivatives: substituent effects.

The peroxidation of liposomes by a haem peroxidase and hydrogen peroxide in the presence of indole-3-acetic acid and derivatives was investigated. It was found that these compounds can accelerate the lipid peroxidation up to 65 fold and this is attributed to the formation of peroxyl radicals that may react with the lipids, possibly by hydrogen abstraction. The peroxyl radicals are formed by peroxidase-catalyzed oxidation of the enhancers to radical cations which undergo cleavage of the carbon-carbon bond on the side-chain to yield CO2 and carbon-centred radicals that rapidly add oxygen. In competition with decarboxylation, the radical cations deprotonate reversibly from the N1 position. Rates of decarboxylation, pka values and rate of reaction with the peroxidase compound I indicate consistent substituent effects which, however, can not be quantitatively related to the usual Hammett or Brown parameters. Assuming that the rate of decarboxylation of the radical cations taken is a measure of the electron density of the molecule (or radical), it is found that the efficiency of these compounds as enhancers of lipid peroxidation increases with increasing electron density, suggesting that, at least in the model system, the oxidation of the substrates is the limiting step in causing lipid peroxidation.

Cholesterol↗
PubMed1994

Radiation chemistry applied to drug design.

Radiation chemistry can contribute to drug design by quantifying redox properties of drugs (useful parameters in quantitative structure-activity relationships), and where free radicals are suspected intermediates in drug action, radiation can be used to generate these putative species and help characterize relevant reactions. Steady radiolysis produces radicals at a readily-varied but quantified rate; pulse radiolysis with fast spectrophotometric and/or conductimetric detection enables the kinetic properties of radicals to be monitored directly. Using these methods, radical intermediates from drugs with specific cytotoxicity towards hypoxic cells have been shown to react rapidly with oxygen, a reaction probably responsible for the therapeutic differential. Radical oxidants from activated neutrophils include superoxide and hydroxyl radicals, and radiation-chemical methods have an important role to play in rational drug design to exploit such oxidative chemistry. Antioxidants can also be evaluated quantitatively by radiolysis methods; the conjugation reactions of thiyl radicals with thiolate and oxygen are now recognised to be major contributions of pulse radiolysis to thiol biochemistry.

Drug Design↗
PubMed1994

Free-radical repair by a novel perthiol: reversible hydrogen transfer and perthiyl radical formation.

2-(3-Aminopropyl-amino) ethaneperthiol (RSSH, the perthiol analogue of the thiol radioprotector, WR-1065) reacts with the alpha-hydroxy alkyl radical (CH3)2C.OH by donating a hydrogen atom as indicated by the characterization of perthiyl radicals (RSS.; lambda max approximately 374 nm, epsilon 374 approximately 1680 +/- 20 dm3 mol-1 cm-1) by pulse radiolysis. The perthiyl radical abstracts a hydrogen from the alcohol to establish a reversible hydrogen-transfer equilibrium. This equilibrium lies predominantly on the side of radical repair since the rate constants for the forward and reverse reactions at pH 4 are: kappa(RSSH+(CH3)2C.OH) = (2.4 +/- 0.1) x 10(9) dm3 mol-1 s-1 and kappa(RSS.+(CH3)2CHOH) = (3.8 +/- 0.3) x 10(3) dm3 mol-1 s-1 respectively. The pKa (RSSH<-->RSS(-)+H+) = 6.2 +/- 0.1 was determined from the pH dependence of the rate of perthiol repair. Identical experiments have been performed with WR-1065 allowing a direct comparison of free-radical repair reactivity to be made with the parthiol analogue. At pH approximately 7.4 the reactivities of the thiol and perthiol were similar, both repairing the alcohol radical with a rate constant of approximately (2.4 +/- 0.1) x 10(8) dm3 mol-1 s-1. However, at pH 5 whilst the hydrogen-donation rate of the thiol was 15-20% higher than at pH 7.4, the perthiol reactivity was over an order of magnitude higher. The thermodynamic driving force for the observed enhanced free-radical repair reactivity of RSSH compared to RSH is attributed to the resonance stabilization energy of 8.8 kJ mol-1 within the RSS. radical. These results indicate a possible application of RSSH/RSS- as DNA-targeted antioxidants or chemoprotectors.

Free Radicals↗
PubMed1994

Formation of hydroxyl radicals on reaction of hypochlorous acid with ferrocyanide, a model iron(II) complex.

Hypochlorous acid reacts with the model iron(II) complex, ferrocyanide (Fe(CN)6(4-)) in aqueous solution with the rate constant 220 +/- 15 dm3 mol-1 s-1. Free hydroxyl radicals are formed in this reaction in 27% yield as shown by the hydroxylation of benzoate to give a product distribution identical to that of free (radiolytically generated) hydroxyl radicals. This reaction is three orders of magnitude faster than the analogous reaction involving hydrogen peroxide (the Fenton reaction), suggesting that the hypochlorous acid generated by activated neutrophils may be a source of hydroxyl radicals.

Benzoates↗
PubMed1994

Radiosensitizing and cytotoxic properties of DNA targeted phenanthridine-linked nitroheterocycles of varying electron affinities.

2-Nitroimidazoles targeted to DNA via intercalation have previously been shown to be as much as 10-100 times more efficient on a molar basis than the untargeted nitroimidazole, misonidazole, in vitro as hypoxic cell selective radiosensitizers and cytotoxins based on extracellular concentrations. In this work the effect of varying the nitroaromatic group has been examined through the preparation of a DNA-targeted 4-nitroimidazole (4-MeNLP-3), a 5-nitroimidazole (5-NLP-3) and a 5-nitrofuran (FEP-2) linked to phenanthridinium ions. With the previously synthesized 2-nitroimidazoles, this provides a series of DNA targeted compounds of varying electron affinity as well as structure at the nitroaromatic position. The present series of compounds was tested for partition coefficient, DNA binding ability, reduction potentials and in vitro radiosensitizing and cytotoxic abilities. The 4-nitro compound displays good radiosensitizing ability, with an SER of approximately 2, despite its low reduction potential, -551 mV. However, hypoxia selective toxicity is lost at the lower reduction potentials presumably due to redox-independent mechanisms. The results obtained indicate that targeting such compounds to DNA diminishes the dependency of radiosensitizing and cytotoxic properties on reduction potential and may allow significant uncoupling of toxicity from radiosensitizing ability.

Animals↗
PubMed1993

Free radical intermediates in the oxidation of flavone-8-acetic acid: possible involvement in its antitumour activity.

The sulphate radical (SO4.-), a model one-electron oxidant, reacts with the antitumour drug flavone-8-acetic acid (FFA) with the rate constant 9.1 x 10(8) dm3mol-1s-1 to yield an uncharged radical that reacts with oxygen (k approximately 1 x 10(9) dm3mol-1s-1). The oxidation of FAA by SO4.- in a steady-state system was found to release carbon dioxide with a yield of 96% relative to that of the SO.4-. The results are interpreted by fast (t1/2 < or = 1 microsecond) and efficient decarboxylation of the FAA radical cation, resulting in a carbon-centred radical. The reaction of the latter with oxygen is a possible source of radical-driven cytotoxic pathways, such as singlet oxygen formation via the Russell mechanism or H-abstraction from lipids. On the basis of the observations in the model system, a possible free radical mechanism for the antitumour action of the drug is suggested.

Antineoplastic Agents↗
PubMed1992

Bioreductive fluorescent markers for hypoxic cells: a study of 2-nitroimidazoles with 1-substituents containing fluorescent, bridgehead-nitrogen, bicyclic systems.

The oxygen-sensitive bioreductive binding of 2-nitroimidazoles labeled with fluorescent side chains has been used to stain hypoxic mammalian cells selectively. Several novel compounds were synthesized with a 1-substituent containing a fluorescent, bicyclic system having a bridgehead-nitrogen atom. Additional amine and secondary alcohol substituents were also included in the link between the fluorophor and the nitroimidazole to improve water solubility. Their ability to discriminate between hypoxic and oxic cells was compared by flow cytometric analysis. A wide range of cellular fluorescence and hypoxic-oxic differentials in fluorescence was observed when compounds with indolizine fluorophors were incubated with cells, and one such compound was considered suitable for further evaluation in vivo. Two compounds with bimane fluorophors gave very little cellular fluorescence when incubated with hypoxic cells.

Animals↗
PubMed1992

Uptake and cytotoxicity of novel nitroimidazole-polyamine conjugates in Ehrlich ascites tumour cells.

A number of tumour cells, including Ehrlich ascites tumour cells (EATC), possess a polyamine uptake system which selectively accumulates endogenous polyamines and structurally related compounds by an active energy dependent system(s). We suggest that it may be possible to utilize this uptake system to target certain cytotoxic agents to those tumour cells possessing this system. In an initial attempt to determine the feasibility of this suggestion, we have synthesized a series of 2- and 5-nitroimidazoles linked to polyamines and determined their ability to utilize the polyamine uptake system. Within the limited series of compounds synthesized, 2-nitroimidazole-polyamine conjugates were more potent inhibitors of spermidine uptake into EATC than the 5-nitroimidazole conjugates. It has been assumed partly based on the competitive nature of this inhibition, that the ability of these compounds to inhibit spermidine uptake is also a measure of their ability to be accumulated by EATC. A greater than 700-fold variation was observed in the ability of different analogues to inhibit spermidine uptake. The most potent inhibitors retained certain structural characteristics similar to those of spermidine. Those compounds linked to polyamines were much more potent inhibitors of polyamine uptake than the parent nitroimidazoles i.e. metronidazole and misonidazole. The toxicity of the parent compounds and their polyamine conjugates in control and polyamine-depleted EATC was assessed by measuring inhibition of tritiated thymidine incorporation. Polyamine depletion, by prior exposure to difluoromethylornithine, results in a compensatory increase in the uptake of polyamines and related structures which may result in an increase in toxicity. Whilst many of the novel conjugates showed only little or moderate toxicity to control cells, the toxicity of several of the conjugates but not the parent nitroimidazoles increased in the polyamine-depleted cells. A clear distinction was also observed between the ability to inhibit spermidine uptake (and hence affinity for the uptake system) and toxicity, e.g. compound 430, a dinitroimidazole-polyamine conjugate, was the best inhibitor of spermidine uptake studied but showed no toxicity. These results support the hypothesis that linking polyamines to nitroimidazoles facilitates the entry of the latter into cells, such as EATC, which possess the polyamine uptake system and may therefore have therapeutic application in the delivery of polyamine-linked cytotoxics to certain tumours.

Animals↗
PubMed1992

Extracellular: intracellular and subcellular concentration gradients of thiols.

Chinese hamster V79 cells in Eagle's minimum essential medium in vitro at room temperature were incubated with the aminothiol, WR-1065, or glutathione (GSH) at extracellular concentrations of approximately 1 mmol dm-3. Average intracellular concentrations of GSH, cysteine, and WR-1065 were measured by high performance liquid chromatography, and the effective reducing environment near DNA probed by staining the cells with acridine orange (AO) and measuring the delayed fluorescence. Exposure to either thiol resulted in a rapid, 10-fold increase in average intracellular cysteine concentrations (to about 1 mmol dm-3). Adding extracellular GSH after prior depletion of GSH by treatment with L-buthionine sulfoximine (BSO) did not restore intracellular GSH, but intracellular cysteine was elevated 10-fold. These results are ascribed to thiol/disulfide exchange with cystine in the medium. WR-1065 slowly concentrated intracellularly to approximately 160% of the extracellular concentration. Chemical conjugation of GSH in cells decreased the reducing environment near DNA, but BSO treatment altered the uptake of AO. The electrostatic attraction of WR-1065 toward isolated DNA was markedly affected by ionic strength.

Animals↗
PubMed1992

Correlation of the radiosensitization potency afforded by nitroacridine intercalators with their electron scavenging efficiency in DNA.

Nitracrine (1-NC) and its nitro-positional analogues are electron-affinic DNA intercalating compounds that act as hypoxic cell radiosensitizers in cell culture, but are too rapidly metabolized to provide radiosensitization in vivo. We have explored the electron-trapping efficiencies of nitroacridines to see if such compounds can scavenge radicals formed in irradiated DNA and if the efficiency of such trapping is related to their radiosensitization properties. We have shown that a correlation does indeed exist as 1-NC, the most potent radiosensitizer, scavenges approximately 45% of the migrating electrons (formed by attachment of eaq- to the DNA) compared with approximately 4% for 4-NC, the least efficient radiosensitizer. The quenching of the delayed fluorescence from acridine orange bound to DNA was also used to probe intracellular uptake and association with DNA. The results are in accord with earlier measurements of average uptake and a suggestion that intercalators which form DNA complexes with short residence times will be preferable to highly bound agents as radiosensitizers. Since 1-NC possesses the smallest association constant with DNA of the four compounds, it is suggested that its high radiosensitization potency may well be related to it being able to interact with more radical sites on the DNA than the other analogues, in addition to its capture of migrating electrons in DNA.

Animals↗
PubMed1991

Fluorescent markers for hypoxic cells. A study of novel heterocyclic compounds that undergo bio-reductive binding.

The bioreductive metabolism and binding of nitroaromatic compounds has been suggested as a method for the identification of hypoxic tumour cells. Bound metabolites of suitable nitroaryl compounds (and some other reducible aromatic compounds) may fluoresce, offering an alternative to radiolabelling or NMR etc. as a diagnostic method. In this paper, the synthesis of some heteroaromatic nitro-compounds is given together with the results obtained from testing of these and other mainly nitroaromatic compounds in vitro as potential bioreductive fluorescent probes for hypoxic cells in tumours. Compounds were incubated with oxygenated or hypoxic mammalian cell suspensions for various times before evaluation of the cellular fluorescence from bioreductive metabolites by fluorescence microscopy and flow cytometry. Among those compounds yielding fluorescent metabolites in cells, considerable variation in hypoxic:oxic differential fluorescence was observed. The in vitro mammalian cell test system showed several of the compounds to be sufficiently promising to merit further investigation in vivo.

Animals↗
PubMed1991

"Radiation chemistry" literature compilations: their wider value in free radical research.

The data obtained from radiation chemistry experiments, and, particularly, pulse radiolysis studies that allow absolute rate constants to be obtained, can be of considerable value to investigations in free radical biology. In this article, we review leading sources of rate, spectral, and thermodynamic data (including critical review articles) where readers can obtain this information.

Chemical Phenomena↗