Pharmacologic ascorbic acid concentrations selectively kill cancer cells: Action as a p... - 0 views
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IV vitamin C cancer ascorbic acid vitamin C H2O2 GPX glutathione Perioxidase
shared by Nathan Goodyear on 09 Feb 11
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Taken together, these data indicate that ascorbate at concentrations achieved only by i.v. administration may be a pro-drug for formation of H2O2, and that blood can be a delivery system of the pro-drug to tissues.
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These findings give plausibility to i.v. ascorbic acid in cancer treatment, and have unexpected implications for treatment of infections where H2O2 may be beneficial
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pharmacologic concentrations of ascorbate killed cancer but not normal cells, that cell death was dependent only on extracellular but not intracellular ascorbate, and that killing was dependent on extracellular hydrogen peroxide (H2O2) formation with ascorbate radical as an intermediate
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Our data show that ascorbic acid selectively killed cancer but not normal cells, using concentrations that could only be achieved by i.v. administration
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Ascorbate-mediated cell death was due to protein-dependent extracellular H2O2 generation, via ascorbate radical formation from ascorbate as the electron donor. Like glucose, when ascorbate is infused i.v., the resulting pharmacologic concentrations should distribute rapidly in the extracellular water space (42). We showed that such pharmacologic ascorbate concentrations in media, as a surrogate for extracellular fluid, generated ascorbate radical and H2O2. In contrast, the same pharmacologic ascorbate concentrations in whole blood generated little detectable ascorbate radical and no detectable H2O2. These findings can be accounted for by efficient and redundant H2O2 catabolic pathways in whole blood (e.g., catalase and glutathione peroxidase) relative to those in media or extracellular fluid
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ascorbic acid administered i.v. in pharmacologic concentrations may serve as a pro-drug for H2O2 delivery to the extracellular milieu
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H2O2 generated in blood is normally removed by catalase and glutathione peroxidase within red blood cells, with internal glutathione providing reducing equivalents
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The electron source for glutathione is NADPH from the pentose shunt, via glucose-6-phosphate dehydrogenase. If activity of this enzyme is diminished, the predicted outcome is impaired H2O2 removal causing intravascular hemolysis, the observed clinical finding.
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The mechansism here is inadequate recycling of GSH due to lack of G6PD, build up of intracellular H2O2 and RBC lysis--hemolysis.
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Only recently has it been understood that the discordant clinical findings can be explained by previously unrecognized fundamental pharmacokinetics properties of ascorbate
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Intracellular transport of ascorbate is tightly controlled in relation to extracellular concentration
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Intravenous ascorbate infusion is expected to drastically change extracellular but not intracellular concentrations
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For i.v. ascorbate to be clinically useful in killing cancer cells, pharmacologic but not physiologic extracellular concentrations should be effective, independent of intracellular ascorbate concentrations.
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It is unknown why ascorbate, via H2O2, killed some cancer cells but not normal cells.
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There was no correlation with ascorbate-induced cell death and glutathione, catalase activity, or glutathione peroxidase activity.
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H2O2, as the product of pharmacologic ascorbate concentrations, has potential therapeutic uses in addition to cancer treatment, especially in infections
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Neutrophils generate H2O2 from superoxide,
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i.v. ascorbate is effective in some viral infections
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H2O2 is toxic to hepatitis C
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Use of ascorbate as an H2O2-delivery system against sensitive pathogens, viral or bacterial, has substantial clinical implications that deserve rapid exploration.
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Recent pharmacokinetics studies in men and women show that 10 g of ascorbate given i.v. is expected to produce plasma concentrations of nearly 6 mM, which are >25-fold higher than those concentrations from the same oral dose
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As much as a 70-fold difference in plasma concentrations is expected between oral and i.v. administration,
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Complementary and alternative medicine practitioners worldwide currently use ascorbate i.v. in some patients, in part because there is no apparent harm
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Human Burkitt's lymphoma cells
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We first investigated whether ascorbate in pharmacologic concentrations selectively affected the survival of cancer cells by studying nine cancer cell lines
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Clinical pharmacokinetics analyses show that pharmacologic concentrations of plasma ascorbate, from 0.3 to 15 mM, are achievable only from i.v. administration
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plasma ascorbate concentrations from maximum possible oral doses cannot exceed 0.22 mM because of limited intestinal absorption
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For five of the nine cancer cell lines, ascorbate concentrations causing a 50% decrease in cell survival (EC50 values) were less than 5 mM, a concentration easily achievable from i.v. infusion
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All tested normal cells were insensitive to 20 mM ascorbate.
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Lymphoma cells were selected because of their sensitivity to ascorbate
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As ascorbate concentration increased, the pattern of death changed from apoptosis to pyknosis/necrosis, a pattern suggestive of H2O2-mediated cell death
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Apoptosis occurred by 6 h after exposure, and cell death by pyknosis was ≈90% at 14 h after exposure
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In contrast to lymphoma cells, there was little or no killing of normal lymphocytes and monocytes by ascorbate
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Ascorbate is transported into cells as such by sodium-dependent transporters, whereas dehydroascorbic acid is transported into cells by glucose transporters and then immediately reduced internally to ascorbate
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Whether or not intracellular ascorbate was preloaded, extracellular ascorbate induced the same amount and type of death.
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extracellular ascorbate in pharmacologic concentrations mediates death of lymphoma cells by apoptosis and pyknosis/necrosis, independently of intracellular ascorbate.
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H2O2 as the effector species mediating pharmacologic ascorbate-induced cell death
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Superoxide dismutase was not protective
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Because these data implicated H2O2 in cell killing, we added H2O2 to lymphoma cells and studied death patterns using nuclear staining (19, 28). The death patterns found with exogenous H2O2 exposure were similar to those found with ascorbate
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For both ascorbate and H2O2, death changed from apoptosis to pyknosis/necrosis as concentrations increased
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Sensitivity to direct exposure to H2O2 was greater in lymphoma cells compared with normal lymphocytes and normal monocytes
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There was no association between the EC50 for ascorbate-mediated cell death and intracellular glutathione concentrations, catalase activity, or glutathione peroxidase activity
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H2O2 generation was dependent on time, ascorbate concentration, and the presence of trace amounts of serum in media
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ascorbate radical is a surrogate marker for H2O2 formation.
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whatever H2O2 is generated should be removed by glutathione peroxidase and catalase within red blood cells, because H2O2 is membrane permeable
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The data are consistent with the hypothesis that ascorbate in pharmacologic concentrations is a pro-drug for H2O2 generation in the extracellular milieu but not in blood.
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The occurrence of one predicted complication, oxalate kidney stones, is controversial
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In patients with glucose-6-phosphate dehydrogenase deficiency, i.v. ascorbate is contraindicated because it causes intravascular hemolysis
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ascorbate at pharmacologic concentrations in blood is a pro-drug for H2O2 delivery to tissues.
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ascorbate, an electron-donor in such reactions, ironically initiates pro-oxidant chemistry and H2O2 formation
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data here showed that ascorbate initiated H2O2 formation extracellularly, but H2O2 targets could be either intracellular or extracellular, because H2O2 is membrane permeant
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More than 100 patients have been described, presumably without glucose-6-phosphate dehydrogenase deficiency, who received 10 g or more of i.v. ascorbate with no reported adverse effects other than tumor lysis
