Glutathione depletion and cytotoxicity of buthionine sulphoximine and SR2508 in rodent and human cells

Ascorbate is a pro-oxidant in chromium-treated human lung cells

The human A549 lung cell line is used in this study as a model to evaluate chromium toxicity and mutagenesis since inhalation exposure of this metal gives rise to an epidemiology that indicates the lung as a target organ of chromium toxicity. Hexavalent chromium is considered the carcinogenic form of chromium, however it must be reductively activated following uptake into cells in order to react with intracellular constituents. We have previously established that the fluorescent dyes, dichlorofluorescein (DCF) and dihydrorhodamine, are effective indicators of the reductive activation of chromium and are sensitive measures of the formation of highly reactive chromium species (RCS) intracellularly. In order to examine the role of the two common intracellular reductants, glutathione and ascorbic acid (Vitamin C) in generating RCS intracellularly, we manipulated their intracellular levels through the use of buthionine sulfoximine (BSO) or by the addition of ascorbate into the culture media. We found that the high levels of glutathione in this cancer cell line lowered endogenous oxidation levels markedly, and that, by decreasing intracellular glutathione, BSO not only generated a higher background level of endogenous intracellular oxidation but the chromium-stimulated oxidation also increased markedly. Contrary to it appellation as an anti-oxidant, ascorbic acid stimulated a strong pro-oxidant response upon chromium treatment and this pro-oxidant response was evident regardless of the levels of glutathione in the cells. Based on these results, we conclude that ascorbic acid acts as a pro-oxidant in chromium-treated cells.

Evidences for adduct formation between intracellular non-protein thiols and nitroazoles possessing an alpha,beta-unsaturated carbonyl side chain and the effects on radiosensitization of hypoxic cells

Reactivity of a number of nitroazole derivatives bearing an alpha,beta-unsaturated carbonyl group on the side chain toward non-protein thiols (NPSH) was examined both in the phosphate buffer solution and in the biological system. These alpha,beta-unsaturated compounds reacted with NPSH, such as glutathione (GSH) and L-cysteine (Cys), in the buffer solution to afford the 1,4-addition products. The reaction gave a second-order rate constant. The adducts of methyl 4-(2'-nitroimidazol-1'-yl)crotonate (1) with GSH and Cys were isolated and characterized as two diastereomers (7a,b and 8a,b) in ca. 1:1 ratio, respectively. Similarly, exposure of EMT6/KU cells to 1 at 1.0 mM for 1 h resulted in depletion of the intracellular NPSH by more than 80%. Over 50% of the depleted NPSH was attributed to the formation of the conjugated diastereomeric adducts. On the other hand, incubation of EMT6/KU cells with 1 at 1.0 mM under hypoxic conditions before X-ray irradiation caused concurrently a sharp reduction of the shoulder of the dose-survival curves (reduced the extrapolation number (n) from 8.0 to ca. 1.0) and an increase in the slope (decreased the mean lethal dose (Do) to ca. 50% of the control level). The observed effects of 1 on the dose-survival curves were due to the NPSH depletion through the Michael addition occurred in the cellular system. A fairly linear relationship was obtained between the n value and the reduced intracellular NPSH level. It indicated that the shoulder effect of the dose-survival curves of hypoxic cells should be the result of the NPSH depletion by the alpha,beta-unsaturated carbonyl group attached to the nitroazoles.

Influence of sample preparation on cellular glutathione recovery from adherent cells in culture

During the last decade, the unbound glutathione content of cultured adherent cells has become a very important biological marker for many pharmacological and toxicological in vitro studies with regard to the protective role of the tripeptide in its reduced form (GSH). However, the literature does not provide extensive information on the influence of sample preparation on cellular GSH and thiol analyses. Using the fibroblast-like V79 cell line as model, we undertook a comparative study of the efficiency of different procedures reported in the literature with respect to GSH recovery. Depending on the preanalytical step, up to 10-fold discrepancies could be observed in the recovery of intracellular GSH. Different parameters that must be controlled in order to maximize GSH recovery are discussed. The optimal strategy consisted in rapid perchloric acid deproteinization performed directly in the dish, which was extremely valuable for preparing GSH samples from adherent cells, and especially from cells expressing elevated gamma-glutamyl transferase activity.

Electron-affinic radiosensitizers possessing NPSH-reactive side chains: cytotoxicity and radiosensitizing activity towards hypoxic EMT6 cells in vitro

A new class of dual-function nitroazole derivatives that are composed of electron-affinic nitroazole rings and a thiol-reactive alpha, beta-unsaturated carbonyl side chain were synthesized to evaluate their physico-chemical properties, reactivity with glutathione (GSH), and cytotoxicity and radiosensitizing activity towards EMT6/KU cells in vitro. Among this class of nitroazole compounds, 2-nitroimidazole-derivative (1), 3-nitro-1,2,4-triazole derivative (2), and 2-methyl-4-nitroimidazole derivative (3) with a common side-chain structure of trans CH2CH = CHCO2CH3 readily reacted with GSH in phosphate-buffer solution (pH 7.2, 310 K). These compounds showed higher cytotoxicities to both aerobic and hypoxic EMT6/KU cells than the corresponding alpha, beta-saturated counterparts (6-8) with a side-chain structure of CH2CH2CH2CO2CH2CH3. The hypoxic cytotoxicity of 1 and 2 with similar electron affinities to that of misonidazole (9) was potentiated by the combined effects of depletion of non-protein thiols (NPSH) by the alpha, beta-unsaturated carbonyl side chains and bioreduction of the nitroazole rings. The sensitizer enhancement ratios in vitro (SERvitro) of 1 (2.80 +/- 0.20) and 2 (2.63 +/- 0.27) at a dose of 1.0 mmol dm-3 are comparable with the oxygen enhancement ratio (OER = 2.90 +/- 0.10) and are significantly larger than those of their respective counterparts 6 (1.28 +/- 0.06) and 7 (1.22 +/- 0.09). A less electron-affinic compound, 3, also gave a large SERvitro = 1.80 +/- 0.18, whereas the counterpart 8 was not effective (1.10) in radiosensitizing hypoxic cells. Compounds 1-3 not only altered the slope, but also reduced the shoulder of the dose-survival curve. These 'dual-function' nitroazole radiosensitizers show much lower levels of in vitro radiosensitization, as measured by the C1-6, than the previously reported 'anomalous' radiosensitizers such as 5-substituted 4-nitroimidazole radiosensitizers.

Pharmacodynamics of prolonged treatment with L,S-buthionine sulfoximine

PURPOSE

To develop dosing criteria for the use of L-buthionine-S-sulfoximine (active diastereoisomer) as a glutathione depletor in the clinic, using a pharmacodynamic and pharmacokinetic in vitro-in vivo approach.

METHODS AND MATERIALS

In vitro: L-buthionine-S-sulfoximine uptake was determined in human glioblastoma cells (T98G) and NIH-3T3 cells using 35S-labeled drug. Dose response relationships were derived for inhibition of glutathione synthesis in CHO cells, and for depletion of glutathione in exponentially growing T98G and CHO cells, as a function of extracellular L-buthionine-S-sulfoximine concentration. Steady-state glutathione levels for CHO and NIH-3T3 cells were measured using an enzymatic assay, while glutathione synthesis rates in CHO cells were determined using a flow cytometric assay. In vivo: L-buthionine-S-sulfoximine biodistribution was determined in male nude mice carrying human glioblastomas (T98G) intracranially, using 35S-labeled drug infused subcutaneously by osmotic pump. Tissue glutathione levels were measured using an enzymatic assay.

RESULTS AND CONCLUSION

The observed cellular uptake t1/2 of approximately 55 min, coupled with a previously reported, rapid in vivo clearance of buthionine sulfoximine, suggest that continuous infusion would be preferable to bolus dosing. Effective concentrations of L-buthionine-S-sulfoximine (24 h exposure), required to lower cellular glutathione content to 50% of control (EC50), were under 1 mM for both cell lines. The amount of L-buthionine-S-sulfoximine in tissues (estimated from 35S drug disposition) reached steady state within 8 h and was proportional to the rate of infusion. Brain tumors were depleted to approximately 50% of control glutathione by a infusion rate of 0.25 mumoles/h (25 g mice). At lower infusion rates an increase in glutathione content was noted in certain nude mouse tissues including brain tumor xenografts.

Toxic and protective effects of L-dopa on mesencephalic cell cultures

The autoxidation of L-DOPA or dopamine (DA) and the metabolism of DA by monoamine oxidase generate a spectrum of toxic species, namely, hydrogen peroxide, oxy radicals, semiquinones, and quinones. When primary dissociated cultures of rat mesencephalon were incubated with L-DOPA (200 microM) for 48 h, the number of tyrosine hydroxylase-positive neurons (DA neurons) was reduced to 69.7% of control values, accompanied by a decrease in [3H]DA uptake to 42.3% of control values; the remaining DA neurons exhibited reduced neurite length and overall deterioration. Lack of simultaneous change in the number of neurons stained with neuron-specific enolase indicated that toxicity was relatively specific for DA neurons. At the same time, the level of GSH, a major cellular antioxidant, rose to 125.2% of control values. Thus, exposure of mesencephalic cultures to L-DOPA results in both damaging and antioxidant actions. Ascorbate (200 microM), an antioxidant, prevented the rise in GSH. The effect of ascorbate on GSH points to an oxidative signal to initiate the rise in GSH content. On the other hand, neither inhibition of monoamine oxidase with pargyline nor addition of superoxide dismutase or catalase to the culture medium prevented the rise in GSH level or the loss in [3H]DA uptake. The latter results tend to exclude the products of monoamine oxidase activity or the presence of hydrogen peroxide or superoxide in the medium as responsible agents for the rise in GSH or neuronal toxicity. In cultures treated with L-buthionine sulfoximine (L-BSO), an inhibitor of GSH synthesis, L-DOPA prevented cell death by L-BSO.

Hepatic glutathione concentrations and the release of pyrrolic metabolites of the pyrrolizidine alkaloid, monocrotaline, from the isolated perfused liver

We have examined the relationship between the metabolism of the pyrrolizidine alkaloid, monocrotaline, and glutathione concentration in the isolated, perfused rat liver. On perfusion of monocrotaline (300 microM) through the isolated liver, high concentrations (1.1 mM) of its metabolite glutathionyldehydroretronecine are released into bile, while much lower amounts (4.86 microM; 0.05 mumol/g liver) accumulate in the perfusate over a 1 hr perfusion period. Metabolite concentration in both the bile and perfusate increase when the level of monocrotaline perfused is increased to 900 microM. Metabolite release is also elevated in livers pretreated with phenobarbital. Monocrotaline perfusion lowered glutathione concentrations in the liver from 30 min onwards. Livers from animals treated with buthionine sulfoximine or chloroethanol showed much lower glutathione levels after 60 min perfusion. Livers from chloroethanol-treated (but not buthionine sulfoximine-treated) animals showed significantly lower release of pyrroles into the bile on perfusion with monocrotaline, but there is no effect on the rate of build-up of pyrrolic metabolites in the perfusate. We conclude that hepatic glutathione concentrations and the release of pyrrolic metabolites of monocrotaline mutually interact. Exposure of the liver to monocrotaline reduces glutathione concentrations, while marked depletion of liver glutathione concentration leads to a decrease in the release of monocrotaline metabolites.

Final report of the phase I trial of continuous infusion etanidazole (SR 2508): a Radiation Therapy Oncology Group study

Seventy-eight patients have been treated on a Phase I trial using continuous infusion etanidazole while undergoing brachytherapy for locally advanced tumors. There were two sequential schemata, the first treated 63 patients with doses ranging from 8-23 g/m2 over 48 hr and the second treated 15 patients with doses ranging from 20-23 g/m2 over 96 hr. The tumor sites were: brain (n = 42), cervix (n = 22), and breast (n = 14). Patients received a loading dose of etanidazole of 2 g/m2 followed by a continuous infusion for a total of 48 or 96 hr while radioactive implants were in place. Of the 63 patients in the 48-hr study, 52 were entered at doses of less than or equal to 21 g/m2 and there were no definite neuropathies but two patients with the cramping/arthralgia syndrome. Of the 11 patients entered at 22-23 g/m2, 1 patient had symptoms of peripheral neuropathy (Grade II) and 6 had the cramping/arthralgia syndrome. This is a new syndrome, distinct from the peripheral neuropathy, characterized by transient alterations in sensations consisting of cramping, arthralgias, or tingling that resolved completely at intervals varying from a few hours to about 1 week post-treatment. The cramping/arthralgia syndrome limited dose escalation; therefore, the maximum tolerated dose over 48 hr was determined to be 20-21 g/m2. The 96-hr infusion was limited to patients with recurrent gliomas undergoing stereotactic implantation. To date, 15 patients have been treated with doses of 20-23 g/m2. No toxicity was encountered at doses less than or equal to 22 g/m2. At 23 g/m2, one patient developed Grade III neuropathy and three patients had mild cramping/arthralgia syndrome, for whom the drug was discontinued. Therefore, it appears the maximum tolerated dose at 96 hr will be approximately 23 g/m2, which is 10-15% higher than for the 48-hr infusion.

Enhancement of SR 2508 (etanidazole) radiosensitization by buthionine sulphoximine at low-dose-rate irradiation

SR 2508 (etanidazole) (1 mM) or buthionine sulphoximine (BSO, 50 microM) or both drugs together did not radiosensitize oxic V79 Chinese hamster cells irradiated at either an acute dose rate (2.35 Gy/min) or at a low dose rate (0.117 Gy/min). BSO pretreatment (15 h at 37 degrees C) depleted cellular glutathione (GSH) to less than or equal to 1% of control level and radiosensitized hypoxic cells at both dose rates with an enhancement ratio (ER) of 1.2. SR 2508 alone radiosensitized hypoxic cells equally at both dose rates with an ER of 1.5. However, ER values of 2.2 and 2.5 were obtained with 1 mM SR 2508 in GSH-depleted cells at acute and low dose rate, respectively, with no significant difference between the two, i.e. there is no dose rate dependence for this potentiation. Since BSO increases SR 2508 radiosensitization and the combined BSO + SR 2508 treatment is extremely cytotoxic to hypoxic cells, our results suggest that combining BSO with SR 2508 will be useful in brachytherapy as well as external-beam therapy if the toxicity from both drugs in vivo is less than the gain in radiosensitization achieved.