Nitric oxide and some nitric oxide donor compounds enhance the cytotoxicity of cisplatin

A major emphasis in cancer therapy research is finding mechanisms to enhance the effectiveness of clinically used chemotherapeutic agents. In this report, we show the effects of direct NO exposure or NO delivery agents such as NONOate NO donors, DEA/NO ((C2H5)2N[N(O)NO]-Na+) and PAPA/ NO (NH2(C3H6)(N[N(O)NO]C3H7)), or S-nitrosothiol NO donors (GSNO, S-nitrosoglutathione, and SNAP, S-nitroso-N-acetylpenicillamine) on the cytotoxicity of cisplatin with Chinese hamster V79 lung fibroblast cells. Cells pretreated with bolus NO or NO delivered from NONOate NO donors were markedly sensitized to subsequent cisplatin treatment, whereas S-nitrosothiol NO donors exerted little effect. The enhancement in cisplatin cytotoxicity from pretreatment with DEA/NO and PAPA/ NO persisted for approximately 180 and 240 min, respectively; thereafter cytotoxicity returned to a level consistent with cisplatin treatment alone. Pretreatment of cells with GSNO or SNAP did not enhance cisplatin cytotoxity. To discern why there were differential effects among the different NO donors, formation of NO over the time course of the experiment was assessed by the nitrosation of 2,3-diaminonaphthylene. Bolus NO, DEA/NO, and PAPA/NO produced more reactive nitrogen oxide species (RNOS) than did treatment with GSNO or SNAP. Previously reported electrochemical studies revealed that temporal NO concentrations measured from DEA/NO and PAPA/NO (1 mM) were greater than 5 microM. It appears that the flux of NO, as well as the amount of RNOS, is important in the NO-mediated enhancement of cisplatin cytotoxicity. Our results demonstrate the importance of NO delivery systems in the enhancement of cisplatin cytotoxicity and may provide insights into strategies for participation of NO donors and nitric oxide synthase with cisplatin therapy.

Direct evidence for in vivo nitroxide free radical production from a new antiarrhythmic drug by EPR spectroscopy

The new Class I anti-arrhythmic agent 2,2,5,5-tetramethyl-3-pyrroline-1-carboxamide derivative, is currently being evaluated in clinical trials in patients with a high risk for cardiac arrhythmias. In this study we show that this antiarrhythmic drug can be chemically converted to the nitroxide free radical analog. Further, using an in vivo Electron Paramagnetic Resonance (EPR) spectroscopy model by detecting free radicals in the distal portion of the tail of an anesthetized mouse, we demonstrate that the drug is oxidized to the corresponding nitroxide. In vitro studies using Chinese hamster V79 cells suggest that the oxidation products of the drug, namely, the hydroxylamine and the nitroxide protect against oxidative damage induced by hydrogen peroxide (H2O2). Taken together, our results suggest that, in addition to the antiarrhythmic effects of the parent drug, sufficient levels of nitroxides may accumulate from the parent drug in vivo to provide antioxidant defense to cardiac tissue that may be subject to ischemia and oxidation-driven injury.

Nitric oxide enhancement of melphalan-induced cytotoxicity

The effects of the diatomic radical, nitric oxide (NO), on melphalan-induced cytotoxicity in Chinese hamster V79 and human MCF-7 breast cancer cells were studied using clonogenic assays. NO delivered by the NO-releasing agent (C2H5)2N[N(O)NO]- Na+ (DEA/NO; 1 mM) resulted in enhancement of melphalan-mediated toxicity in Chinese hamster V79 lung fibroblasts and human breast cancer (MCF-7) cells by 3.6- and 4.3-fold, respectively, at the IC50 level. Nitrite/nitrate and diethylamine, the ultimate end products of DEA/NO decomposition, had little effect on melphalan cytotoxicity, which suggests that NO was responsible for the sensitization. Whereas maximal sensitization of melphalan cytotoxicity by DEA/NO was observed for simultaneous exposure of DEA/NO and melphalan, cells pretreated with DEA/NO were sensitized to melphalan for several hours after NO exposure. Reversing the order of treatment also resulted in a time-dependent enhancement in melphalan cytotoxicity. To explore possible mechanisms of NO enhancement of melphalan cytotoxicity, the effects of DEA/NO on three factors that might influence melphalan toxicity were examined, namely NO-mediated cell cycle perturbations, intracellular glutathione (GSH) levels and melphalan uptake. NO pretreatment resulted in a delayed entry into S phase and a G2/M block for both V79 and MCF-7 cells; however, cell cycle redistribution for V79 cells occurred after the cells returned to a level of cell survival, consistent with treatment with melphalan alone. After 15 min exposure of V79 cells to DEA/NO (1 mM), GSH levels were reduced to 40% of control values; however, GSH levels recovered fully after 1 h and were elevated 2 h after DEA/NO incubation. In contrast, DEA/NO (1 mM) incubation did not reduce GSH levels significantly in MCF-7 cells (approximately 10%). Melphalan uptake was increased by 33% after DEA/NO exposure in V79 cells. From these results enhancement of melphalan cytotoxicity mediated by NO appears to be complex and may involve several pathways, including possibly alteration of the repair of melphalan-induced lesions. Our observations may give insights for improving tumour kill with melphalan using either exogenous or possibly endogenous sources of NO.

Do nitroxide antioxidants act as scavengers of O2-. or as SOD mimics?

Stable nitroxide radicals were reported to act as SOD mimics and catalyze the dismutation of O2-. through two different catalytic pathways including reductive and oxidative reaction mechanisms (Samuni, A., Krishna, C. M., Riesz, P., Finkelstein, E. & Russo, A. (1988) J. Biol Chem. 263, 17921-17924). Recent studies directly monitoring O2-. and employing kinetics analysis did not reveal SOD activity of nitroxides (Weiss, R. H., Flickinger, A. G., Rivers, W. J., Hardy, M. M., Aston, K. W., Ryan, U. S. & Riley, D. P. (1993) J. Biol. Chem. 268, 23049-23054). Such discrepancy may result in cases where distinction of stoichiometric scavengers from catalytic detoxifiers of O2-. is not readily feasible. Nitroxides are effective antioxidants that protect against oxidative injury in various pathological processes. The distinction of their SOD mimic activity from O2-. scavenging was established by examining the validity of direct and indirect methods employed to assay SOD-like catalytic activity. Kinetics analysis along with direct EPR monitoring were used to study the mechanism underlying nitroxide reactions with O2-.. The nitroxide EPR signal decayed in the presence of NADH but otherwise did not decrease with time, thus substantiating its catalytic role in O2-. dismutation. The catalytic rate constants for O2-., dismutation, determined for the nitroxides tested, were found to increase with [H+], indicating that .OOH rather than O2-. is oxidizing the nitroxide. The results demonstrate the limitations associated with direct kinetics analysis in evaluating SOD mimic activity, underscoring the need for independent assays for valid discrimination of SOD mimics from stoichiometric scavengers of O2-..

Stimulation by nitroxides of catalase-like activity of hemeproteins. Kinetics and mechanism

The ability of stable nitroxide radicals to detoxify hypervalent heme proteins such as ferrylmyoglobin (MbFeIV) produced in the reaction of metmyoglobin (MbFeIII) and H2O2 was evaluated by monitoring O2 evolution, H2O2 depletion, and redox changes of the heme prosthetic group. The rate of H2O2 depletion and O2 evolution catalyzed by MbFeIII was enhanced by stable nitroxides such as 4-OH-2,2,6,6-tetramethyl-piperidinoxyl (TPL) in a catalytic fashion. The reduction of MbFeIV to MbFeIII was the rate-limiting step. Excess TPL over MbFeIII enhanced catalase-like activity more than 4-fold. During dismutation of H2O2, [TPL] and [MbFeIV] remained constant. NADH caused: (a) inhibition of H2O2 decay; (b) progressive reduction of TPL to its respective hydroxylamine TPL-H; and (c) arrest/inhibition of oxygen evolution or elicit consumption of O2. Following depletion of NADH the evolution of O2 resumed, and the initial concentration of TPL was restored. Kinetic analysis showed that two distinct forms of MbFeIV might be involved in the process. In summary, by shuttling between two oxidation states, namely nitroxide and oxoammonium cation, stable nitroxides enhance the catalase mimic activity of MbFeIII, thus facilitating H2O2 dismutation accompanied by O2 evolution and providing protection against hypervalent heme proteins.

Protection from radiation-induced alopecia with topical application of nitroxides: fractionated studies

PURPOSE

Hair loss resulting from irradiation of the head and neck or from whole brain irradiation often leads to cosmetic, social, and psychological problems for the radiotherapy patient. Few successful clinical interventions are available. We have shown that nitroxides (stable free radicals) afford radiation protection against single-dose radiation-induced alopecia in a guinea pig model. Here we determine if topical nitroxide application provides protection from fractionated radiation treatment.

MATERIALS AND METHODS

Two symmetrical and contralateral areas (3 x 5 cm) of skin on the dorsal trunk of guinea pigs were shaved to a hair length of 0.25 cm. A 2 mL solution containing 70 mg/mL nitroxide (Tempo or Tempol) in 70% ethanol was topically applied to the skin surface of one side; 70% ethanol was applied to the contralateral (control) side 10 minutes before irradiation. Animals were placed in a special jig that held skin without decreasing blood flow to the treatment area and fractionated external beam radiation (7 Gy) was delivered daily for eight fractions over 10 days via a 4 MeV linear accelerator. Alopecia (hair density) was scored weekly for 13 to 14 weeks after radiotherapy, using a standardized reference with respect to hair loss and regrowth in the treatment field.

RESULTS

After radiation treatment, dry desquamation and gradual hair loss were observed for both control and nitroxide-treated skin; however, over weeks 4 to 11 postirradiation hair loss was much more pronounced in control animals when compared with nitroxide-treated animals. Hair density measurements for Tempol treatment over weeks 9 to 13 were approximately 75% compared with measurements in controls of approximately 25%. Tempo-treated animals exhibited hair density values of approximately 90% compared with 12% in controls over weeks 11 to 14. Tempol and Tempo treatments resulted in significant radioprotection. Histologic evaluation showed that radiation treatment alone in ethanol controls resulted in a marked decrease in the number of hair follicles and poor development of remaining follicles; however, nitroxide pretreatment resulted in no appreciable decrease in hair follicles and hair follicles appeared mature. This was also observed in unirradiated ethanol controls. Electron paramagnetic resonance studies revealed that topical nitroxide application did not result in measurable systemic concentrations of either drug.

CONCLUSIONS

The results of this study suggest that topical application of nitroxides may be useful in a clinical setting to reduce the undesirable toxicity of radiation-induced alopecia.

The effect of various nitric oxide-donor agents on hydrogen peroxide-mediated toxicity: a direct correlation between nitric oxide formation and protection

The role that nitric oxide (NO) plays in various degenerative and disease states has remained a mystery since its discovery as a biological messenger, prompting the question, "NO, friend or foe?" Some reports have suggested that NO is cytotoxic, and yet others have shown that it possesses protective properties against reactive oxygen species (ROS). Many studies have used various NO donor complexes arriving at seemingly different conclusions. This report will address the effects of various NO donor compounds on ROS-mediated toxicity. Consistent with our previous study, the NO donor compound, DEA/NO ((C2H5)2N[N(O)NO]-Na+), afforded protection against hydrogen peroxide-mediated cytotoxicity in V79 Chinese hamster lung fibroblasts at concentrations as low as 10 microM DEA/NO. Furthermore, a survey of other NO donor complexes revealed that some either protected or potentiated hydrogen peroxide-mediated cytotoxicity. 3-Morpholinosynodiomine.HCl (SIN-1) and sodium nitroprusside (SNP) enhanced hydrogen peroxide-mediated cytotoxicity, while S-nitrosoglutathione (GSNO), and S-nitroso-N-acetylpenicillamine (SNAP) afforded protection. Electrochemical detection of NO in cell culture medium revealed that neither 1000 microM SIN-1 nor SNP yielded appreciable NO concentrations (<0.3 microM). In contrast, DEA/NO, SNAP, and GSNO yielded fluxes of NO >1.0 microM. Thus, a direct correlation between inhibition of hydrogen peroxide cytotoxicity and NO production was observed: agents that release NO during hydrogen peroxide treatment afford significant protection, whereas agents that do not release NO do not protect. Similar results were observed for NO donors studied when hypoxanthinesolidusxanthine oxidase was used as the source for ROS, although the S-nitrosothiol agents were much less protective. These results demonstrate that NO possesses properties which protect against ROS toxicity and demonstrate how the use of different NO donor compounds can lead to different conclusions about the role that NO can play in the cytotoxicity of ROS.

Radiation sensitisation by nitric oxide releasing agents

Previous studies have shown that nitric oxide (NO) sensitises hypoxic cells to ionising radiation. In the present study, four different nitric oxide (NO) donor agents were evaluated for both NO release and hypoxic radiosensitisation. The S-nitrosothiol NO donor agents, S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP), were shown to release sustained NO concentrations (microM) and significantly radiosensitise hypoxic cells. The extent of hypoxic radiosensitisation by both of these agents at 1.0 mM concentration was similar to that obtained with molecular oxygen. In contrast, neither 3-morpholinosydnonimine (SIN-1) nor sodium nitroprusside (SNP) released detectable NO concentrations and neither agent enhanced the hypoxic radiation response to the extent of that observed for GSNO or SNAP. NO-mediated hypoxic cell radiosensitisation by NO donor drugs may offer a new approach for clinical consideration, particularly if such agents can be selectively delivered to hypoxic cells.

Convenient colorimetric and fluorometric assays for S-nitrosothiols

S-nitrosothiols have been shown to affect a number of physiological functions. Several techniques have been used to detect these species in biological systems, primarily by methods utilizing chemiluminescence. Since the apparatus required for measurement of chemiluminescence are not readily available in most laboratories, methods employing more conventional techniques such as uv-vis and fluorescence spectroscopy may be of greater use. Herein, we report the development of colorimetric and fluorometric methods for the reliable quantitation of S-nitrosothiols. Solutions containing sulfanilamide/N-(1-naphthyl)- ethylenediamine dihydrochloride or 2,2'-azinobis (3-ethylbenzthiazoline-6-sulfonic acid), when exposed to S-nitrosoglutathione (GSNO), S-nitrosocysteine, or S-nitrosoacteylpenicillamine, resulted in no absorbance changes in the range of 400-800 nm. Exposure to HgCl2 or Cu(acetate)2 resulted in release of nitric oxide (NO) from the S-nitrosothiols. The liberated NO reacted subsequently with oxygen and formed a chemical species which reacted with either analysis solution, resulting in an increase in absorption between 400 and 800 nm. A plot of RSNO versus absorbance was linear for both mercury(II) and copper(II) ions where the slope in the presence of mercury ion was significantly greater than that for copper ion. The sensitivity was as low as 5 microM RSNO using HgCl2. The fluorometric method using 2, 3-diaminonaphthalene as the scavenger of the NOsolidusO2 products gave a sensitivity of 50 nM for GSNO. In addition, S-nitrosylated proteins were quantitated using the fluorometric technique. These methods provide accurate determination of low concentrations of S-nitrosothiols, utilizing conventional spectroscopic techniques available in most laboratories.

Electron paramagnetic resonance imaging of rat heart with nitroxide and polynitroxyl-albumin

Electron paramagnetic resonance (EPR) imaging utilizing stable nitroxyl radicals is a promising technique for measuring free radical distribution, metabolism, and tissue oxygenation in organs and tissues [Kuppusamy, P., Chzhan, M., Vij, K., Shteynbuk, M., Lefer, D. J., Giannella, E., & Zweier, J. L. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 3388-3392]. However, the technique has been limited by the rapid reduction of nitroxide in vivo to its hydroxylamine derivative, a diamagnetic, EPR-inactive species. In this report a novel, polynitroxylated derivative of human serum albumin is shown to be capable of reoxidizing the hydroxylamine back to nitroxide in vivo. Polynitroxyl-albumin (PNA) is shown to be effective in maintaining the signal intensity of the nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL or TPL) in the ischemic isolated rat heart, allowing the acquisition of high-resolution three-dimensional (3D) EPR images of the heart throughout a prolonged 2.5 h period of global cardiac ischemia. In serial transverse sections of the 3D image, TPL intensity maps of the heart showed cardiac structure with submillimeter resolution. TPL intensities in coronary arteries and myocardium showed that nitroxide concentration decreases with increasing distance from large blood vessels. These results demonstrate that EPR imaging in vivo is possible using nitroxides in conjunction with PNA. In addition to its utility in the emerging technology of EPR imaging, the greatly prolonged half-life of TPL observed in the presence of PNA may facilitate the therapeutic application of nitroxides in a variety of disease processes.

Role of antioxidants in the nitric oxide-elicited inhibition of dopamine uptake in cultured mesencephalic neurons. Insights into potential mechanisms of nitric oxide-mediated neurotoxicity

Under aerobic conditions the addition of (C2N5)2N(N[O]NO)-.Na+(DEA/NO), S-nitroso-N-acetyl penicillamine and nitric oxide (NO)-saturated buffer, but not S-nitroso-L-glutathione, to dopamine solutions resulted in dopamine o-semiquinone formation that was dependent on the formation of a NO/oxygen intermediate. High pressure liquid chromatography (HPLC) electrochemical analysis of dopamine demonstrated that the DEA/NO-induced oxidation of dopamine was abrogated in the presence of the antioxidants, ascorbate and glutathione. NO spontaneously released from DEA/NO decreased [3H]dopamine accumulation in primary cultures of mesencephalic neurons in a dose-dependent fashion. In contrast, [3H] gamma-aminobutyric acid uptake by mesencephalic neurons tested under the same conditions was unchanged. When DEA/NO was added to incubation buffer that contained [3H]dopamine and the antioxidant, ascorbate or glutathione, [3H]dopamine uptake was also inhibited. These data excluded that oxidation of extracellular [3H]dopamine by the intermediates of the NO/O2 reaction could have caused this decrease. Instead, NO may have acted directly on a not yet identified target operative in the regulation of dopamine storage and release. Analysis of the rate constants for the NO reaction with ascorbate, glutathione and dopamine revealed that dopamine quinone formation was delayed by the presence of antioxidants. Since the formation of NO as well as neurotransmitter release are activated during ischemia reperfusion injury, it is possible that prolonged NO exposure could deplete antioxidants and facilitate the oxidation of dopamine and thereby cause neurotoxicity.

Nitric oxide (NO) protects against cellular damage by reactive oxygen species

Since the discovery of nitric oxide (NO) as an endogenously formed radical, its effect on numerous physiological processes has been intensively investigated. Some studies have suggested NO to be cytotoxic while others have demonstrated it protective under various biological conditions. Though NO shows minimal cytotoxicity to a variety mammalian cell cultures, it does modulate the toxicity of some agents such as reactive oxygen species. Often, NO is generated in the presence of these reactive oxygen species in response to foreign pathogens or under various pathophysiological conditions. We will show that NO can play a protective role under oxidative stress resulting from superoxide, hydrogen peroxide and alkyl peroxides. It was found by measuring the time-concentration profiles of NO released from various NO donor compounds that only microM levels of NO were required for protection against the toxicity of these reactive species. It was found that there are several chemical reactions which may account for these protective effects such as NO preventing heme oxidation, inhibition of Fenton-type oxidation of DNA, and abatement of lipid peroxidation. Taken together, NO at low concentrations clearly protects against peroxide-mediated toxicity.

Nitric oxide potentiates hydrogen peroxide-induced killing of Escherichia coli

Previously, we reported that nitric oxide (NO) provides significant protection to mammalian cells from the cytotoxic effects of hydrogen peroxide (H2O2). Murine neutrophils and activated macrophages, however, produce NO, H2O2, and other reactive oxygen species to kill microorganisms, which suggests a paradox. In this study, we treated bacteria (Escherichia coli) with NO and H2O2 for 30 min and found that exposure to NO resulted in minimal toxicity, but greatly potentiated (up to 1,000-fold) H2O2-mediated killing, as evaluated by a clonogenic assay. The combination of NO/H2O2 induced DNA double strand breaks in the bacterial genome, as shown by field-inverted gel electrophoresis, and this increased DNA damage may correlate with cell killing. NO was also shown to alter cellular respiration and decrease the concentration of the antioxidant glutathione to a residual level of 15-20% in bacterial cells. The iron chelator desferrioxamine did not stop the action of NO on respiration and glutathione decrease, yet it prevented the NO/H2O2 synergistic cytotoxicity, implicating metal ions as critical participants in the NO/H2O2 cytocidal mechanism. Our results suggest a possible mechanism of modulation of H2O2-mediated toxicity, and we propose a new key role in the antimicrobial macrophagic response for NO.

Nitric oxide protects against alkyl peroxide-mediated cytotoxicity: further insights into the role nitric oxide plays in oxidative stress

Endogenously formed nitric oxide (NO) possesses diverse properties such as regulating physiological functions, exerting specific toxic effects, and protecting against various toxic substances. Recent studies suggest that in the presence of reactive oxygen species, NO can serve as an antioxidant. We show here that NO delivered from the NO donor compound, PAPA/NO (NH2(C3H6)(N[N(O)NO](C3H7)), protects Chinese hamster V79 lung fibroblasts from the cytotoxicity of t-butyl hydroperoxide and cumene hydroperoxide. In contrast, the other end products of PAPA/NO degradation in aqueous solution, NH2(C3H6)NH(C3H7) and nitrite, did not protect. The NONOate DEA/NO releases NO six times faster than PAPA/NO, yet did not afford protection, which implies that NO must be present throughout the alkyl hydroperoxide exposure. Measurements of NO concentrations released from PAPA/NO suggest that micromolar levels protect against cytotoxicity induced by alkyl hydroperoxides. These findings demonstrate that the flux of NO sustained over the duration of the peroxide exposure determines protection and not the total of NO delivered. These results suggest that concentrations of NO produced in the microenvironment of endothelial cells are high enough to protect cells from Fenton-type-mediated toxicity and support the premise that NO may exert a salutary effect in certain diseases associated with membrane damage.

Protection against SR 4233 (Tirapazamine) aerobic cytotoxicity by the metal chelators desferrioxamine and tiron

PURPOSE

Metal chelating agents and antioxidants were evaluated as potential protectors against aerobic SR 4233 cytotoxicity in Chinese hamster V79 cells. The differential protection of aerobic and hypoxic cells by two metal chelators, desferrioxamine and Tiron, is discussed in the context of their potential use in the on-going clinical trials with SR 4233.

METHODS AND MATERIALS

Cytotoxicity was evaluated using clonogenic assay. SR 4233 exposure was done in glass flasks as a function of time either alone or in the presence of the following agents: superoxide dismutase, catalase, 5,5-dimethyl-1-pyrroline, Trolox, ICRF-187, desferrioxamine, Tiron (1,2-dihydroxybenzene-3,5-disulfonate), and ascorbic acid. Experiments done under hypoxic conditions were carried out in specially designed glass flasks that were gassed with humidified nitrogen/carbon dioxide mixture and with a side-arm reservoir from which SR 4233 was added to cell media after hypoxia was obtained. Electron paramagnetic resonance studies were also performed.

RESULTS

Electron paramagnetic resonance and spectrophotometry experiments suggest that under aerobic conditions SR 4233 undergoes futile redox cycling to produce superoxide. Treatment of cells during aerobic exposure to SR 4233 with the enzymes superoxide dismutase and catalase, the spin trapping agent DMPO, the water-soluble vitamin E analog Trolox, and the metal chelator ICRF-187 provided little or no protection against aerobic SR 4233 cytotoxicity. However, two other metal chelators, desferrioxamine and Tiron, afforded significant protection against aerobic SR 4233 cytotoxicity (protection factors at 50% survival were 3.8 and 3.1, respectively), while exhibiting minimal protection to hypoxic cells treated with SR 4233.

CONCLUSIONS

One potential mechanism of aerobic cytotoxicity is redox cycling of SR 4233 with molecular oxygen resulting in several potentially toxic oxidative species that overburden the intrinsic intracellular detoxification systems such as superoxide dismutase, catalase, and glutathione peroxidase. This study identifies two metal chelating agents, desferrioxamine and Tiron, that were able to protect against aerobic but not hypoxic SR 4233 cytotoxicity.

Modulation of streptonigrin cytotoxicity by nitroxide SOD mimics

Nitroxides are cell-permeable, stable radicals that react readily with paramagnetic species such as transition metals or short-lived free radicals, though not generally with diamagnetic molecules. Nitroxides can undergo one-electron selective redox reactions and thereby potentially modify the activity of cytotoxic drugs. Streptonigrin (SN) toxicity requires bioreduction to yield the semiquinone radical, and the toxicity is reportedly mediated by transition metals and oxygen-derived reactive species via redox-cycling of the semiquinone intermediate. The present study shows that (1) nitroxides protected isolated DNA and also aerated or hypoxic bacterial cells from SN toxicity; (2) H2O2 potentiated the hypoxic cytotoxicity of the drug but inhibited the damage to aerated cells; (3) pretreatment of cells with H2O2 conferred some protection, but not when the drug alone was preexposed to H2O2; and (4) desferrioxamine and 2,2-dipyridyl, though neither diethylenetriamino pentaacetate, exogenous catalase, or superoxide dismutase, decreased SN-induced cell killing. The mechanisms by which nitroxides protect from SN toxicity involve both a selective radical-radical reaction with SN semiquinone and the reoxidation of reduced cellular transition metal ions. On the other hand, H2O2 appears to exert two opposing effects: (1) facilitation of cell killing by the Fenton reaction and (2) lowering the cellular level of reducing equivalents, thus inhibiting the bioreductive activation of SN.

Free radical modes of cytotoxicity of adriamycin and streptonigrin

Free radical modes of cytotoxicity of streptonigrin (STN) and Adriamycin (ADR) in Chinese hamster V79 cells under aerobic conditions were evaluated using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TP), a low molecular weight stable nitroxide free radical with antioxidant properties and desferrioxamine (DF), a transition metal chelator. In addition, exogenous superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6), were tested for cytoprotective effects. EPR studies showed that TP reacts with the semiquinones of both ADR and STN and also with O2- radicals generated during aerobic redox cycling of the respective semiquinone radicals. Pulsed field gel electrophoresis studies confirmed that DNA double-strand breaks (dsb) induced by STN in V79 cells were inhibited completely by TP, whereas ADR-induced DNA dsb were not affected by TP. Clonogenic cell survival studies showed that STN-induced cytotoxicity could be inhibited completely by DF or TP. Both agents were ineffective in inhibiting ADR-induced cytotoxicity. SOD and CAT were ineffective in protecting against both STN and ADR cytotoxicity. Our results are consistent with a mechanism requiring the semiquinone radical intermediate of STN for cytotoxicity and minimal free radical involvement in ADR-induced V79 cell cytotoxicity.

Reaction kinetics for nitrosation of cysteine and glutathione in aerobic nitric oxide solutions at neutral pH. Insights into the fate and physiological effects of intermediates generated in the NO/O2 reaction

The critical regulatory function of nitric oxide (NO) in many physiologic processes is well established. However, in an aerobic aqueous environment NO is known to generate one or more reactive and potentially toxic nitrogen oxide (NOx) metabolites. This has led to the speculation that mechanisms must exist in vivo by which these reactive intermediates are detoxified, although the nature of these mechanisms has yet to be elucidated. This report demonstrates that among the primary bioorganic products of the reaction of cellular constituents with the intermediates of the NO/O2 reaction are S-nitrosothiol (S-NO) adducts. Anaerobic solutions of NO are not capable of nitrosating cysteine or glutathione, while S-NO adducts of these amino acids are readily formed in the presence of O2 and NO. Investigation of the kinetics for the formation of these S-NO adducts has revealed a rate equation of d[RSNO]/dt = kSNO[NO]2[O2], where kSNO = (6 +/- 2) x 10(6) M-2S-1, a value identical to that for the formation of reactive intermediates in the autoxidation of NO. Competition studies performed with a variety of amino acids, glutathione, and azide have shown that cysteine residues have an affinity for the NOx species that is 3 orders of magnitude greater than that of the nonsulfhydryl amino acids, and > 10(6) times greater than that of the exocyclic amino groups of DNA bases. The dipeptide alanyltyrosine reacts with the intermediates of the NO/O2 reaction with an affinity 150 times less than that of the sulfhydryl-containing compounds. Furthermore, Chinese hamster V79 lung fibroblasts depleted of glutathione display enhanced cytotoxicity on exposure to NO.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperthermic sensitization by the radical initiator 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH). I. In vitro studies

AAPH (2,2'-azobis-(2-amidinopropane dihydrochloride)) is a water-soluble, heat-labile azo compound which undergoes thermal decomposition to produce carbon-centred free radicals. These carbon-centred radicals might be directly cytotoxic or may react with oxygen to produce potentially cytotoxic alkoxyl and peroxyl radicals. The rate of free radical production as a result of AAPH thermal decomposition increases with increasing temperature. We have evaluated the efficacy of AAPH as a heat sensitizer for Chinese hamster V79 cells by the clonogenic assay. AAPH (50 mM) was not cytotoxic to V79 cells at 37 degrees C for exposures up to 3 h. In contrast, AAPH (50 mM) was found to markedly sensitize cells exposed to 42, 43 and 45 degrees C. For a 75 min exposure to 42 degrees C alone, cell survival was reduced to 9 x 10(-1); however, a 75 min exposure at 42 degrees C+AAPH resulted in survival of 5.5 x 10(-4). For 43 and 45.5 degrees C heating, cell survival was potentiated by AAPH at the 1% survival level by 4.1 and 1.4-fold, respectively. AAPH was also found to sensitize both hypoxic cells and thermotolerant cells. These findings would encourage in vivo evaluation of AAPH (or analogues) as a temperature-dependent heat sensitizer. AAPH represents a new class of heat sensitizers which may have use in unravelling the mechanism(s) of heat killing and may have utility in local hyperthermia treatment.