Mammalian cell studies with diamide

Oxidative modification of rat sulfotransferase 1A1 activity in hepatic tissue slices correlates with effects on the purified enzyme

Mammalian cytosolic sulfotransferases (SULTs) catalyze the sulfation of xenobiotics as well as numerous endogenous molecules. The major aryl (phenol) SULT in rat liver, rSULT1A1, has been used extensively as a model enzyme for understanding the catalytic function of SULTs. Previous studies showed that purified rSULT1A1 displays significant catalytic changes in the presence of GSSG and other oxidants. In the present study, the effects of diamide [1,1'-azobis(N,N-dimethylformamide)] and tert-butyl hydroperoxide (TBHP) on the activity of rSULT1A1 in rat hepatic slices were compared with the effects of these oxidants on a homogeneous preparation of the enzyme. Precision-cut hepatic slices were incubated with 10 μM 7-hydroxycoumarin (7-HC) in the presence of varied concentrations of either diamide or TBHP. Analysis of the 7-hydroxycoumarin sulfate released into the incubation medium indicated that both oxidants significantly increased the sulfation of 7-HC, and this occurred at optimal concentrations of 5 and 10 μM, respectively. Cellular GSH and GSSG levels in the hepatic slices were not significantly altered from control values at these concentrations of diamide and TBHP. Exposure of homogeneous rSULT1A1 to diamide or TBHP also increased the rate of sulfation of 7-HC, although the optimal concentrations of diamide and TBHP were lower (50- and 100-fold, respectively) than those required for effects with the hepatic slices. These results indicate that both diamide and TBHP may modify the rSULT1A1 in intact cells in a manner similar to that observed with the homogeneous purified enzyme.

The thiol-oxidizing agent diamide reduces isoproterenol-stimulated amylase release in rat parotid acinar cells

In parotid acinar cells, activation of beta-adrenergic receptors provokes exocytotic amylase release via the accumulation of intracellular cAMP. Cellular redox status plays a pivotal role in the regulation of various cellular functions. Cellular redox imbalance caused by the oxidation of cellular antioxidants, as a result of oxidative stress, induces significant biological damages. In this study, we examined effect of diamide, a thiol-oxidizing reagent, on amylase release in rat parotid acinar cells. In the presence of diamide, isoproterenol (IPR)-induced cAMP formation and amylase release were partially reduced. Diamide had no effect on amylase release induced by forskolin and mastoparan, an adenylate cyclase activator and heterotrimeric GTP binding protein activator, respectively. In the cells pretreated with diamide, the binding affinity of [(3)H]dihydroalprenolol to beta-receptors was reduced. These results suggest that oxidative stress results in reduction of binding affinity of ligand on beta-receptor and consequently reduces protein secretory function in rat parotid acinar cells.

The Role of Non-protein Sulphydryls in Determining the Chemical Repair Rates of Free Radical Precursors of DNA Damage and Cell Killing in Chinese Hamster V79 Cells

Chinese hamster V79 fibroblasts were irradiated in the gas explosion apparatus and the chemical repair rates of the oxygen-dependent free radical precursors of DNA double-strand breaks (dsb) and lethal lesions measured using filter elution (pH 9.6) and a clonogenic assay. Depletion of cellular GSH levels, from 4.16 fmol/cell to 0.05 fmol/cell, by treatment with buthionine sulphoximine (50 mumol dm-3; 18 h), led to sensitization as regards DNA dsb induction and cell killing. This was evident at all time settings but was particularly pronounced when the oxygen shot was given 1 ms after the irradiation pulse. A detailed analysis of the chemical repair kinetics showed that depletion of GSH led to a reduction in the first-order rate constant for dsb precursors from 385 s-1 to 144 s-1, and for lethal lesion precursors from 533 s-1 to 165 s-1. This is generally consistent with the role of GSH in the repair-fixation model of radiation damage at the critical DNA lesions. However, the reduction in chemical repair rate was not proportional to the severe thiol depletion (down to approximately 1% for GSH) and a residual repair capacity remained (approximately 30%). This was found not to be due to compartmentalization of residual GSH in the nucleus, as the repair rate for dsb precursors in isolated nuclei, washed virtually free of GSH, was identical to that found in GSH-depleted cells (144 s-1), also the OER remained substantially above unity. This suggests that other reducing agents may have a role to play in the chemical repair of oxygen-dependent damage. One possible candidate is the significant level of protein sulphydryls present in isolated nuclei.

Chemical radiosensitizers for use in radiotherapy

Radiosensitizers are intended to enhance tumour cell killing while having much less effect on normal tissues. Some drugs target different physiological characteristics of the tumour, particularly hypoxia associated with radioresistance. Oxygen is the definitive hypoxic cell radiosensitizer, the large differential radiosensitivity of oxic vs hypoxic cells being an attractive factor. The combination of nicotinamide to reduce acute hypoxia with normobaric carbogen breathing is showing clinical promise. 'Electron-affinic' chemicals that react with DNA free radicals have the potential for universal activity to combat hypoxia-associated radioresistance; a nitroimidazole, nimorazole, is clinically effective at tolerable doses. Hypoxia-specific cytotoxins, such as tirapazamine, are valuable adjuncts to radiotherapy. Nitric oxide is a potent hypoxic cell radiosensitizer; variations in endogenous levels might have prognostic significance, and routes to deliver nitric oxide specifically to tumours are being developed. In principle, many drugs can be delivered selectively to hypoxic tumours using either reductase enzymes or radiation-produced free radicals to activate drug release from electron-affinic prodrugs. A redox-active agent based on a gadolinium chelate is being evaluated clinically. Pyrimidines substituted with bromine or iodine are incorporated into DNA and enhance free radical damage; fluoropyrimidines act by different mechanisms. A wide variety of drugs that influence the nature or repair of DNA damage are being evaluated in conjunction with radiation; it is often difficult to define the mechanisms underlying chemoradiation regimens. Drugs being evaluated include topoisomerase inhibitors (e.g. camptothecin, topotecan), and the hypoxia-activated anthraquinone AQ4N; alkylating agents include temozolomide. Drugs involved in DNA repair pathways being investigated include the potent poly(ADP ribose)polymerase inhibitor, AG14,361. Proteins involved in cell signalling, such as the Ras family, are attractive targets linked to radioresistance, as are epidermal growth factor receptors and linked kinases (drugs including vandetanib [ZD6,474], cetuximab and gefitinib), and cyclooxygenase-2 (celecoxib). The suppression of radioprotective thiols seems to offer more potential with alkylating agents than with radiotherapy, although it remains a strategy worthy of exploration.

Redox cycling by motexafin gadolinium enhances cellular response to ionizing radiation by forming reactive oxygen species

PURPOSE

To examine the mechanism of radiation enhancement by motexafin gadolinium (Gd-Tex) in vitro.

METHODS AND MATERIALS

Oxidation of ascorbate and NADPH by Gd-Tex was evaluated in a neutral buffer. Growth inhibition of human uterine cancer cell line MES-SA was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) dye. Clonogenic assays were used to measure radiation response in MES-SA, A549 human lung carcinoma, E89, a CHO cell line variant deficient in glucose-6-phosphate dehydrogenase activity, and murine lymphoma cell lines LYAR and LYAS.

RESULTS

Gd-Tex catalyzed the oxidation of NADPH and ascorbate under aerobic conditions, forming hydrogen peroxide. Decreased viability was observed in MES-SA cells incubated with Gd-Tex in media containing NADPH or ascorbate. Gd-Tex and ascorbate increased fluorescence in dichlorofluorescin acetate-treated cultures. Synergistic effects on the aerobic radiation response in MES-SA and A549 were seen using Gd-Tex in combination with L-buthionine-(S,R)-sulfoximine (BSO). Incubation with Gd-Tex in the presence of ascorbate increased the aerobic radiation response of E89 and the apoptosis-sensitive B-cell line (LYAS).

CONCLUSIONS

Gd-Tex sensitizes cells to ionizing radiation by increasing oxidative stress as a consequence of futile redox cycling. Optimization of the concentration of ascorbate (or other reducing species) may be required when evaluating Gd-Tex activity in vitro.

Differential cytotoxicity of buthionine sulfoximine to "normal" and transformed human lung fibroblast cells

Glutathione (GSH) depletion has been studied extensively as a possible means to sensitive tumor cells to radiation treatment and chemotherapy. The present study was undertaken to compare the cytotoxicity of GSH depletion in normal and transformed cells. The results showed that specific inhibition of GSH synthesis by L-buthionine sulfoximine (BSO) caused significantly higher cytotoxicity in "normal" human-lung fibroblast cells than in their transformed counterparts. This finding suggests a possibility that depletion of GSH could be more harmful to normal cells than to transformed and/or tumor cells and that the selective cytotoxicity of BSO to normal cells could limit its potential as an effective sensitizer for cancer treatment.

Different characteristics of ferrochelatase in cultured fibroblasts of erythropoietic protoporphyria patients and normal controls

Ferrochelatase activity was measured in crude extracts of fibroblasts, obtained from erythropoietic protoporphyria patients and healthy controls. The enzyme activity in erythropoietic protoporphyria fibroblasts was about 50% lower, compared to the controls. The sulfhydryl-oxidising reagent diamide inhibited the normal enzyme by about 50%, whereas ferrochelatase from erythropoietic protoporphyria fibroblasts was completely insensitive to the reagent. Pb2+ inhibits ferrochelatase activity by reacting with essential sulfhydryl groups. Low concentrations of Pb2+ inhibited the normal enzyme by 56%, but the mutant enzyme by only 8%. The photodynamic activity of bound mesoporphyrin substrate caused a biphasic inactivation of the normal enzyme. During the first 5 min of illumination a fast decrease of enzyme activity occurred to about 60% of the initial value. Experimental evidence indicates that this first phase of inactivation is caused by photooxidation of sulfhydryl groups. During further illumination inactivation continued at a much slower rate. With ferrochelatase from erythropoietic protoporphyria fibroblasts only the second, slow phase of photodynamic inactivation was observed. These observations suggest a mutation of ferrochelatase in erythropoietic protoporphyria, affecting the reactivity of sulfhydryl groups, involved in the catalytic activity of the enzyme.

Diamide induced shift in protein and glutathione thiol: disulfide status delays DNA rejoining after X-irradiation of human cancer cells

By treating a human tumor cell line with various concentrations of diamide, we explored the relationship between extent and duration of protein and nonprotein thiol oxidation, initiation of DNA double-strand break rejoining after X-rays, and the degree of radiosensitization. We also examined the relationship between protein thiol status and the non-protein thiol, glutathione (GSH). A549 cells were irradiated and incubated postirradiation with 0, 100, 300 or 500 microM diamide for 1 h. The dose of radiation required to give 10% survival decreased from 4.8 Gy to 3.2 Gy with 300 microM and to 2.7 Gy with 500 microM diamide (enhancement ratios of 1.5 and 1.8, respectively) but was not significantly affected by 100 microM diamide. The time of initiation of double-stranded DNA rejoining after X-irradiation (DNA repair) was delayed by 300 and 500 microM diamide. Furthermore, DNA rejoining began only after total cellular protein thiol content recovered to 55% of pretreatment levels for both concentrations. Intracellular GSH/GSSG ratios decreased immediately after diamide addition to less than 1. Large decreases in GSH/GSSG ratio preceded significant loss of protein thiols, but protein-glutathione mixed disulfides accounted for a minor percentage of the total protein thiol oxidized (up to 20%). We believe that diamide-induced protein thiol loss, and not GSH oxidation, is related to the cessation of DNA strand rejoining after X-irradiation, thereby affecting survival.

Role of glutathione in the radiation response of mammalian cells in vitro and in vivo

Radiation interacts with biological systems to produce many types of molecular lesions. Much of the molecular damage is of little consequence with regard to cell killing. The lesions that are most likely to contribute to cell killing are DNA lesions produced by clusters of radicals. The formation of clusters of radicals is characteristic of ionizing radiation and accounts for its high efficiency as a cytotoxic agent. The mechanism by which these lesions kill cells is probably the formation of DNA double-strand breaks, ultimately resulting in chromosomal breaks. There is a possibility that some of the other types of molecular lesions produced by radiation may participate in more subtle mechanisms of cell damage. For instance, radiation induces a self-destructive process (apoptosis) in certain cell types, and the molecular lesions that initiate this process have not been identified. Glutathione (GSH) is a versatile protector. Several distinct mechanisms of radioprotection by GSH can be identified. These include radical scavenging, restoration of damaged molecules by hydrogen donation, reduction of peroxides and maintenance of protein thiols in the reduced state. Of these mechanisms, hydrogen donation to DNA radicals is probably the most important. Since competing reactions are very rapid, this mechanism requires a high concentration of GSH. Radioprotection by hydrogen donation to DNA radicals is not effective in oxygenated cells because the normal intracellular GSH concentration is not sufficient for effective competition with oxygen. Consequently, moderate depletion of GSH has no effect on the radiosensitivity of oxygenated cells. Under hypoxic conditions GSH becomes more competitive, and GSH depletion can markedly affect radiosensitivity. The radiosensitivity of hypoxic cells is most affected by GSH depletion in the presence of low concentrations of radiosensitizers. Since hypoxic cells are a characteristic feature of tumors, moderate depletion of GSH in combination with treatment with hypoxic cell radiosensitizers appears to be a promising strategy for selective tumor sensitization in radiation therapy. Oxidation of GSH can result in radiosensitization of both hypoxic and oxygenated cells. The mechanism of this effect appears to involve oxidation of protein thiols which are important for DNA repair. In principle, modification of DNA repair could have a greater impact on radiation therapy than modification of the number of lesions produced by radiation. However, a strategy for modification of GSH or protein thiol redox state in vivo has not yet been devised.

Application of non-hypoxic cell sensitizers in radiobiology and radiotherapy: rationale and future prospects

The effects of commonly used non-hypoxic cell radiosensitizers are briefly reviewed. Emphasis is placed on the effects and the mechanism of action of halogenated pyrimidines, since recent clinical trials indicated the potential importance of these compounds in the treatment of certain types of human tumors. Evidence is presented suggesting that halogenated pyrimidines sensitize cells to radiation by increasing induction of DNA and chromosome damage per cell per unit absorbed dose, as well as by increasing the susceptibility to fixation of radiation induced PLD. The former mode of action correlates with an increase in survival curve slope, whereas the latter probably causes the reduction observed in shoulder width. The effects of repair inhibitors such as the nucleoside analogs are briefly reviewed and their possible clinical importance discussed. Results are presented indicating that combined treatment with halogenated pyrimidines and nucleoside analogs may enhance the radiosensitizing effect of the former and the specificity on tumor cells of the latter. Finally, the effects of other radiation sensitizers such as 3-aminobenzamide and diamide are briefly summarized.

Pre- and post-irradiation radiosensitization by SR 4233

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is a bioreductive agent which exhibits highly selective killing of hypoxic cells in a variety of mammalian cell lines in vitro and in murine tumors in vivo. The selective toxicity of the drug results from its one-electron reduction under hypoxic conditions to form a free radical intermediate capable of damaging DNA, through the formation of strand breaks. Using the neutral filter elution assay, SR 4233 was found to be more efficient at producing DNA double strand breaks in Chinese hamster ovary (CHO) cells than an equitoxic dose of gamma-rays. Drug and radiation sequencing experiments were also performed, with both cell survival and DNA strand break rejoining used as endpoints. As a result of these studies, we now describe two additional properties of SR 4233: (a) radiosensitization of aerobic cells in culture produced by hypoxic incubation with drug either before or after irradiation, and (b) the inhibition of subsequent rejoining of radiation-induced DNA double strand breaks after hypoxic pretreatment with drug. The magnitude of the radiosensitization produced did not vary for drug treatments which, when given alone, reduced cell survival over a range from 30% to 2%. The extent of DNA repair inhibition increased with increasing severity of the SR 4233 pretreatment, but was quite small for non-lethal drug exposures.

Keynote address: hypoxic cell radiosensitizers: where next?

The second generation hypoxic cell radiosensitizers SR 2508 (etanidazole) and Ro 03-8799 (pimonidazole) are now undergoing Phase III clinical trials and are predicted from in vitro, animal, and clinical studies to be equivalent to an increase in the tolerable dose of Miso of 5-6 fold. Nonetheless, this will only produce an SER of the hypoxic cells of approximately 1.5 when given with each dose of a 24 fraction course of radiotherapy. There is ample opportunity, therefore, for a sufficiently improved third generation radiosensitizer to warrant the considerable time, effort, and money that would be involved for its clinical testing. Many groups are now involved in the synthesis and testing of drugs with the goal of producing this third generation sensitizer. The purpose of the present review is to summarize what we have learned from the many studies of new radiosensitizers and attempt to draw conclusions about the most fruitful directions for further work. One of the conclusions is that in vitro testing of radiosensitizers, essential though it is as a first screen, can lead to many false predictions. Fortunately, these in vitro systems tend to "overpredict" (i.e., produce false positives rather than false negatives) for in vivo activity. There is only one class of drugs, so far, (those that increase tumor oxygenation) which radiosensitize tumors in vivo, but not hypoxic cells in vitro. Another major conclusion is that the physico-chemical characteristics of drugs that have been shown to be important in enhancing radiosensitization or reducing toxicity (electron affinity, pKa, partition coefficient, DNA binding), have not been systematically optimized in any one compound. Systematic rational drug design is needed to achieve this. The search for new hypoxic cell radiosensitizers must not detract from the fact that a sensitizer of aerobic cells to low radiation doses is needed. A major challenge in achieving a useful drug for radiotherapy is the desirability for such a drug to be tumor specific. We present data which show that tumor hypoxia might be exploited to achieve this goal.

Radiosensitization by diamide analogs and arsenicals

Several analogs of the glutathione (GSH) oxidizing reagent diamide [diazenedicarboxylic acid bis(N,N'-diethylamide)] were tested as radiosensitizers of aerobic cells. In general, radiosensitization correlates with the rate of reaction with cellular reducing agents and occurs only when the reductive capacity of the cell is exceeded. SR-4077, [diazenedicarboxylic acid bis(N,N-piperidide)], is particularly suitable for mechanistic studies, because it is less cytotoxic than diamide, but is equally reactive toward cellular GSH. SR-4077 sensitizes CHO cells to X rays under aerobic conditions, even when the drug is added after irradiation. Radiosensitization is expressed both as a change in the exponential slope of the radiation cell survival curve and as a decrease in the shoulder of the survival curve. Phenylarsine oxide, a dithiol-binding reagent, sensitizes aerobic CHO cells to X rays by modification of the shoulder of the survival curve. The results are consistent with the hypothesis that the shoulder-modifying effect of GSH oxidants is caused by the loss of protein thiols, which might be involved in the repair of X ray-induced DNA damage.

The use of non-hypoxic cell sensitizers in radiobiology and radiotherapy

Many different non-hypoxic cell radiosensitizers have been identified over the years. Radiosensitization by these agents is mediated through a variety of mechanisms, including inhibition of repair (both enzymatic and chemical), modification in the DNA molecule, and perturbation and redistribution in the cell cycle. Recent clinical interest in the use of halogenated pyrimidines as radiosensitizers has prompted a number of questions requiring both laboratory and clinical research to maximize this therapeutic approach. Other radiosensitizers that alter cellular redox processes in different ways are discussed in the context of better understanding the cellular biochemical systems that are affected in aerobic radiosensitization. Advancement in the use of non-hypoxic cell sensitizers in radiation cancer treatment will most likely depend on a knowledge of the detailed biochemical mechanisms of these agents and how they might be used to exploit any subtle biochemical differences between tumor and normal tissue.

Multiple effects of presumed glutathione depletors on rabbit proximal tubules

The role of glutathione (GSH) in the protection of normal renal function has been investigated using rabbit proximal tubules. Compounds known to deplete GSH in various biological systems by alkylation (via GSH S-transferases, phorone; 2-cyclohexen-1-one, CHX; diethyl maleate, DEM) or by inhibiting GSH synthesis (buthionine sulfoximine, BSO) were added to suspensions of proximal tubules and incubated for 60 min. BSO (1 or 5 mM) did not decrease GSH concentrations, O2 consumption, or cause lactate dehydrogenase release (LDH). Concentrations of CHX (2 mM) and phorone (10 mM) that decreased GSH concentrations also inhibited O2 consumption and caused LDH release. DEM (10 mM) did not significantly decrease GSH concentrations but did inhibit oxygen consumption and cause slight LDH release. Time-course studies using CHX (3 mM) showed that GSH levels and O2 consumption decreased as early as 15 min while LDH release did not occur until 60 min. These results show that: there may be a relationship between O2 consumption and GSH levels; agents that have been used historically to reduce GSH concentrations have other cytotoxic effects; and rabbit renal proximal tubules appear to be resistant to GSH depletion.

Cysteamine, zinc, and thiols modify detectability of rat pituitary prolactin: a comparison with effects on bovine prolactin suggests differences in hormone storage

Little is known about the structure of prolactin (PRL) within secretory granules. Evidence from our previous studies in bovine tissue preparations suggests that control of secretion may reside, in part, in the conversion of storage hormone to releasable PRL. The conversion can be monitored by measuring changes in immunodetectability since the oligomeric, storage form is poorly recognized by antisera raised against monomeric PRL. Since many investigators use rats to study the secretory process and changes in detectability of rat pituitary PRL occur during lactation ("depletion-transformation"), we undertook the present immunodetectability studies to gain insight into the storage structure of rat (r) PRL. Cysteamine and zinc inhibited tissue PRL immunoassayability in male rat pituitary homogenates and also in partially purified secretory granules as they had inhibited bovine (b) PRL; however, zinc inhibited the rodent hormone less potently than the bovine. In vitro incubation of rat tissue samples without additions resulted in increases in rPRL detectability of up to 84% after 180 minutes; such incubation of bovine samples had no significant effect. A striking additional difference between the species was that exposure to reduced glutathione (GSH), cysteine, homocysteine, mercaptoethanol, and dithiothreitol inhibited rPRL by up to 44%. This compared to thiol stimulation of bPRL by as much as 450%. The inhibitory GSH effect on rPRL was abolished when 0.5% sodium dodecyl sulfate (SDS) was included; in contrast, the stimulatory GSH effect on bPRL did not change with added SDS. SDS alone had no effect on rat homogenate PRL, and only increased rat granule rPRL by 23% compared to its ability to increase bPRL assayability by 44%.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiosensitivity of normal and polyunsaturated fatty acid supplemented fibroblasts after depletion of glutathione

Mouse fibroblast LM cells have been modified with respect to their phospholipid composition in all subcellular fractions, including the nuclear membrane. The content of polyunsaturated fatty acids (PUFA) was significantly increased but no difference in cell survival after X-irradiation could be observed between the normal and PUFA enriched cells. It is concluded that the radiosensitive PUFAs in the membranes are well protected against radiation damage. This protection of the PUFA cells could not be caused by vitamin E, because this membrane protector was not present in these fibroblasts. The content of glutathione (GSH) was about the same in the normal and the modified cells. Reduction of the cellular GSH content to less than 5 per cent of that for non-treated cells did not alter cellular survival after radiation of either normal or PUFA enriched cells under oxic or anoxic conditions. The radiosensitive lipids present in the membranes of the PUFA enriched cells proved to be vulnerable to radiation-induced lipid peroxidation when extracted from the cells and reconstituted into liposomes, indicating that the fatty acids per se are peroxidizable. It is concluded that the lipids in the membranes of mammalian cells are not the principal target in radiation-induced reproductive death, and that no generalization is permitted with respect to glutathione, as being the major hydrogen donating species in mammalian cells responsible for the repair of those target molecules responsible for cell survival after radiation.

Radioprotection by WR-2721 in vitro at low oxygen tensions: implications for its mechanisms of action

Radioprotection of spheroids of Chinese hamster V79 cells by WR-2721 was found to be a function of spheroid size, with the greatest dose-modifying effect by the protector observed for spheroids almost large enough to contain radioresistant "anoxic" cells. The nature of the response suggested that most of the protective effect was due to the presence of an increased hypoxic fraction in the drug-treated spheroids. Similarly, when single-cell suspensions were irradiated at various oxygen tensions, one component of radioprotection by WR-2721 was found to be highly dependent upon the available oxygen. Two mechanisms of radioprotection of V79 cells by WR-2721 were thus demonstrated: a modest, oxygen-independent effect, presumably due to hydrogen donation, and an oxygen-depleting effect, which is of maximal significance for cells or tissues which would otherwise be partially sensitized by low levels of oxygen.

Abnormal radiosensitizing and cytotoxic properties of ortho-substituted nitroimidazoles

Various 5-substituted 4-nitroimidazoles have been shown to be much more efficient radiosensitizers and much more toxic than would have been predicted from their electron affinities, as measured by values of one-electron reduction potential, E17. Using Chinese hamster V79 cells in vitro, a comparison has been made with some isomeric 4-substituted 5-nitroimidazoles. These compounds have E17 values some 64mV greater than the 4-nitroimidazoles, yet show much lower sensitizing efficiency and also lower toxicity. Neither series of compounds shows the greater toxicity towards hypoxic cells usually associated with nitroaromatic and nitroheterocyclic compounds. The second-order rate constants, k2, for reaction of these isomeric nitroimidazoles with glutathione and dithiothreitol were determined. Within each series the value of k2 increased with increasing electron affinity, however, the 4-nitroimidazoles were always more reactive than their corresponding 5-nitro isomers. The sensitizing and toxic properties of these compounds may involve depletion of intracellular thiols; this possibility is discussed.

Radiosensitization of hypoxic tumor cells by depletion of intracellular glutathione

Depletion of glutathione in Chinese hamster ovary cells in vitro by diethyl maleate resulted in enhancement of the effect of x-rays on cell survival under hypoxic conditions but not under oxygenated conditions. Hypoxic EMT6 tumor cells were similarly sensitized in vivo. The action of diethyl maleate is synergistic with the effect of the electron-affinic radiosensitizer misonidazole, suggesting that the effectiveness of misonidazole in cancer radiotherapy may be improved by combining it with drugs that deplete intracellular glutathione.

125I-labelled insulin degradation by isolated rat hepatocytes: the roles of glutathione-insulin transhydrogenase and insulin-specific protease

Isolated rat hepatocytes degraded 125I-insulin with a Km of 150 nmol/l. Degradation was stimulated by the addition of glutathione and dithiothreitol. In cells incubated with diamide, glutathione was oxidised to the disulphide. Regeneration of reduced glutathione commenced after a further 30 min incubation at 37 degrees C. Diamide (1 mmol/l) significantly inhibited insulin degradation by hepatocytes (p less than 0.001). The 'apparent Vmax' for insulin degradation was decreased tenfold and the Km decreased to 25 nmol/l. The diamide-insensitive degrading activity was cell-associated and produced an intermediate of hormone degradation that was apparently of a higher molecular weight than insulin A chain. The biological activity of the intermediate was 0.03% of that of insulin. The diamide-insensitive activity was not due to release of protease into the medium by cell lysis. We conclude that there are at least two pathways capable of degrading insulin existing in rat hepatocytes.

The use of drugs which deplete intracellular glutathione in hypoxic cell radiosensitization

Diethylmaleate (DEM) is a thiol-binding reagent with specificity toward glutathione. Treatment of Chinese hamster ovary (CHO) cells in vitro with 2 x 10(-4) M DEM for one hour results in a decrease in glutathione content to less than 5% of control, without cytotoxicity. This treatment results in dose-modifying sensitization to radiation under hypoxic conditions, with no effect on the shoulder of the radiation survival curve. No effect on the radiation sensitivity of oxygenated cells was seen. DEM pretreatment enhances the radiosensitization of hypoxic cells by misonidazole, as well. Similar results were obtained in vivo with EMT6 tumors in BALB/c mice. Analysis of DNA damage by the alkaline elution assay indicates that DEM enhances radiation-induced single-strand breaks, but does not significantly affect repair, while diamide and N-ethylmaleimide inhibit repair, in addition to enhancing radiation-induced single-strand breaks.

Mechanisms of hypoxic cell radiosensitization and the development of new sensitizers

Some of the mechanisms by which drugs can potentiate the radiation response of tumors and cells in culture are discussed. Emphasis is placed on the action of nitroaromatic and heterocyclic compounds as hypoxic cell radiosensitizers, and some potential successors to misonidazole (MISO) are described. These include desmethylmisonidazole and SR 2508, selected because of their low toxicity in experimental systems. Groups of compounds, more efficient sensitizers than would be predicted from electron affinity correlations, have been examined and the use of Ro-03-8799 or RSU 1047 is proposed. Finally, ortho-substituted nitroimidazoles and electron-affinic compounds with alkylating groups are described. The latter group, in particular, holds promise for the development of compounds much superior to MISO.