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The hydroperoxyl radical () plays an important role in the chemistry of lipid peroxidation

The is a much stronger oxidant than superoxide anion-radical

Lipid peroxidation can be described generally as a process under which oxidants such as free radicals or nonradical species attack lipids containing carbon-carbon double bond(s), especially polyunsaturated fatty acids (PUFAs) that involve hydrogen abstraction from a carbon, with oxygen insertion resulting in lipid peroxyl radicals and hydroperoxides as described previously

under medium or high lipid peroxidation rates (toxic conditions) the extent of oxidative damage overwhelms repair capacity, and the cells induce apoptosis or necrosis programmed cell death

The overall process of lipid peroxidation consists of three steps: initiation, propagation, and termination

Once lipid peroxidation is initiated, a propagation of chain reactions will take place until termination products are produced.

The main primary products of lipid peroxidation are lipid hydroperoxides (LOOH)

Among the many different aldehydes which can be formed as secondary products during lipid peroxidation, malondialdehyde (MDA), propanal, hexanal, and 4-hydroxynonenal (4-HNE) have been extensively studied

MDA has been widely used for many years as a convenient biomarker for lipid peroxidation of omega-3 and omega-6 fatty acids because of its facile reaction with thiobarbituric acid (TBA)

MDA is one of the most popular and reliable markers that determine oxidative stress in clinical situations [53], and due to MDA’s high reactivity and toxicity underlying the fact that this molecule is very relevant to biomedical research community

4-HNE is considered as “second toxic messengers of free radicals,” and also as “one of the most physiologically active lipid peroxides,” “one of major generators of oxidative stress,” “a chemotactic aldehydic end-product of lipid peroxidation,” and a “major lipid peroxidation product”

MDA is an end-product generated by decomposition of arachidonic acid and larger PUFAs

Identifying in vivo MDA production and its role in biology is important as indicated by the extensive literature on the compound (over 15 800 articles in the PubMed database using the keyword “malondialdehyde lipid peroxidation” in December 2013)

MDA reactivity is pH-dependent

When pH decreases MDA exists as beta-hydroxyacrolein and its reactivity increases

MAA adducts are shown to be highly immunogenic [177–181]. MDA adducts are biologically important because they can participate in secondary deleterious reactions (e.g., crosslinking) by promoting intramolecular or intermolecular protein/DNA crosslinking that may induce profound alteration in the biochemical properties of biomolecules and accumulate during aging and in chronic diseases

MDA is an important contributor to DNA damage and mutation

This MDA-induced DNA alteration may contribute significantly to cancer and other genetic diseases.

Dietary intake of certain antioxidants such as vitamins was associated with reduced levels of markers of DNA oxidation (M1dG and 8-oxodG) measured in peripheral white blood cells of healthy subjects, which could contribute to the protective role of vitamins on cancer risk

4-HNE is an extraordinarily reactive compound

oxidative stress and ROS, produced in cancer-associated fibroblasts, has a “bystander effect” on adjacent cancer cells, leading to DNA damage, genomic instability and aneuploidy, which appears to be driving tumor-stroma co-evolution

tumor cells produce and secrete hydrogen peroxide, thereby “fertilizing” the tumor microenvironment and driving the “reverse Warburg effect.”

This type of stromal metabolism then produces high-energy nutrients (lactate, ketones and glutamine), as well as recycled chemical building blocks (nucleotides, amino acids, fatty acids), to literally “feed” cancer cells

loss of stromal caveolin (Cav-1) is sufficient to drive mitochondrial dysfunction with increased glucose uptake in fibroblasts, mimicking the glycolytic phenotype of cancer-associated fibroblasts.

oxidative stress initiated in tumor cells is transferred to cancer-associated fibroblasts.

Then, cancer-associated fibroblasts show quantitative reductions in mitochondrial activity and compensatory increases in glucose uptake, as well as high ROS production

These findings may explain the prognostic value of a loss of stromal Cav-1 as a marker of a “lethal” tumor microenvironment

Interruption of this process, by addition of catalase (an enzyme that detoxifies hydrogen peroxide) to the tissue culture media, blocks ROS activity in cancer cells and leads to apoptotic cell death in cancer cells

our results may also explain the “field effect” in cancer biology,5 as hydrogen peroxide secreted by cancer cells, and the propagation of ROS production, from cancer cells to fibroblasts, would create an increasing “mutagenic field” of ROS production, due to the resulting DNA damage

aerobic glycolysis takes place in cancer-associated fibroblasts, rather than in tumor cells, as previously suspected.

In this new paradigm, cancer cells induce oxidative stress in neighboring cancer-associated fibroblasts

cancer-associated fibroblasts have the largest increases in glucose uptake

cancer cells secrete hydrogen peroxide, which induces ROS production in cancer-associated fibroblasts

Then, oxidative stress in cancer-associated fibroblast leads to decreases in functional mitochondrial activity, and a corresponding increase in glucose uptake, to fuel aerobic glycolysis

cancer cells show significant increases in mitochondrial activity, and decreases in glucose uptake

fibroblasts and cancer cells in co-culture become metabolically coupled, resulting in the development of a “symbiotic” or “parasitic” relationship.

cancer-associated fibroblasts undergo aerobic glycolysis (producing lactate), while cancer cells use oxidative mitochondrial metabolism.

We have previously shown that oxidative stress in cancer-associated fibroblasts drives a loss of stromal Cav-1, due to its destruction via autophagy/lysosomal degradation

a loss of stromal Cav-1 is sufficient to induce further oxidative stress, DNA damage and autophagy, essentially mimicking pseudo-hypoxia and driving mitochondrial dysfunction

loss of stromal Cav-1 is a powerful biomarker for identifying breast cancer patients with early tumor recurrence, lymph-node metastasis, drug-resistance and poor clinical outcome

this type of metabolism (aerobic glycolysis and autophagy in the tumor stroma) is characteristic of a lethal tumor micro-environment, as it fuels anabolic growth in cancer cells, via the production of high-energy nutrients (such as lactate, ketones and glutamine) and other chemical building blocks

the upstream tumor-initiating event appears to be the secretion of hydrogen peroxide

one such enzymatically-active protein anti-oxidant that may be of therapeutic use is catalase, as it detoxifies hydrogen peroxide to water

numerous studies show that “catalase therapy” in pre-clinical animal models is indeed sufficient to almost completely block tumor recurrence and metastasis

by eliminating oxidative stress in cancer cells and the tumor microenvironment,55 we may be able to effectively cut off the tumor's fuel supply, by blocking stromal autophagy and aerobic glycolysis

breast cancer patients show systemic evidence of increased oxidative stress and a decreased anti-oxidant defense, which increases with aging and tumor progression.68–70 Chemotherapy and radiation therapy then promote further oxidative stress.69 Unfortunately, “sub-lethal” doses of oxidative stress during cancer therapy may contribute to tumor recurrence and metastasis, via the activation of myofibroblasts.

a loss of stromal Cav-1 is associated with the increased expression of gene profiles associated with normal aging, oxidative stress, DNA damage, HIF1/hypoxia, NFκB/inflammation, glycolysis and mitochondrial dysfunction

cancer-associated fibroblasts show the largest increases in glucose uptake, while cancer cells show corresponding decreases in glucose uptake, under identical co-culture conditions

Thus, increased PET glucose avidity may actually be a surrogate marker for a loss of stromal Cav-1 in human tumors, allowing the rapid detection of a lethal tumor microenvironment.

it appears that astrocytes are actually the cell type responsible for the glucose avidity.

In the brain, astrocytes are glycolytic and undergo aerobic glycolysis. Thus, astrocytes take up and metabolically process glucose to lactate.7

Then, lactate is secreted via a mono-carboxylate transporter, namely MCT4. As a consequence, neurons use lactate as their preferred energy substrate

both astrocytes and cancer-associated fibroblasts express MCT4 (which extrudes lactate) and MCT4 is upregulated by oxidative stress in stromal fibroblasts.34

In accordance with the idea that cancer-associated fibroblasts take up the bulk of glucose, PET glucose avidity is also now routinely used to measure the extent of fibrosis in a number of human diseases, including interstitial pulmonary fibrosis, postsurgical scars, keloids, arthritis and a variety of collagen-vascular diseases.

PET glucose avidity and elevated serum inflammatory markers both correlate with poor prognosis in breast cancers.

PET signal over-estimates the actual anatomical size of the tumor, consistent with the idea that PET glucose avidity is really measuring fibrosis and inflammation in the tumor microenvironment.

human breast and lung cancer patients can be positively identified by examining their exhaled breath for the presence of hydrogen peroxide.

tumor cell production of hydrogen peroxide drives NFκB-activation in adjacent normal cells in culture6 and during metastasis,103 directly implicating the use of antioxidants, NFκB-inhibitors and anti-inflammatory agents, in the treatment of aggressive human cancers.

Comparing mitochondrial metabolic activity revealed a difference between stroma and epithelial cells

metalloproteinases (MMP) such as MMP-2, MMP-3, and MMP-9 increase extracellular matrix turnover and are themselves activated by oxidative stress

Overexpression of NOX4 in normal breast epithelial cells results in cellular senescence, resistance to apoptosis, and tumorigenic transformation, as well as increased aggressiveness of breast cancer cells

Lowered expression of Cav-1 not only leads to myofibroblast conversion and inflammation but also seems to impact aerobic glycolysis, leading to secretion of high energy metabolites such as pyruvate and lactate that drive mitochondrial oxidative phosphorylation in cancer cells

Reverse Warburg Effect

secreted transforming growth factor β (TGFβ), insulin-like growth factor (IGF), platelet-derived growth factor (PDGF), fibroblast growth factor 2, and stromal-derived factor 1 (SDF1) are able to activate fibroblasts and increase cancer cell proliferation

oxidative stress has an important role in the initiation and preservation of breast cancer progression

cancer preventive role of healthy mitochondria

the cancer cells produce hydrogen peroxide and by driving the “Reverse Warburg Effect” initiate oxidative stress in fibroblasts. As a result of this process, fibroblasts exhibited reduced mitochondrial activity, increased glucose uptake, ROS, and metabolite production.

Oxidative stress results from an imbalance between unstable reactive species lacking one or more unpaired electrons (superoxide anion, hydrogen peroxide, hydroxyl radical, reactive nitrogen species) and antioxidants

cancer cells are able to induce drivers of oxidative stress, autophagy and mitophagy: HIF-1α and NFκB in surrounding stroma fibro-blasts

Studies show that loss of Cav-1 in adjacent breast cancer stroma fibroblasts can be prevented by treatment with N-acetyl cysteine, quercetin, or metformin

However, diets rich in antioxidants have fallen short in sufficiently preventing cancer

obstructing oxidative stress in the tumor microenvironment can lead to mitophagy and promote breast cancer shutdown is a promising discovery for the development of future therapeutic interventions.

It is widely held that HIF-1α function is dependent upon its location within the tumor microenvironment. It acts as a tumor promoter in CAFs and as a tumor suppressor in cancer cells

It was reported that overexpression of recombinant (SOD2) (Trimmer et al., 2011) or injection of SOD, catalase, or their pegylated counterparts can block recurrence and metastasis in mice

hydrogen peroxide is one of the main factors that can push fibroblasts and cancer cells into senescence

Recent studies show that in the breast cancer microenvironment, oxidative stress causes mitochondrial dysfunction

Females suffering from breast cancer had significantly decreased Superoxide dismutase (SOD) and reduced glutathione (GSH) levels in comparison to normal females

ADA seems to be a promising marker of inflammation in breast cancer thereby suggesting that it can be used as a diagnostic tool to detect the stage of breast cancer along with cytopathological studies

In conclusion, our study confirmed the role of oxidative stress in the pathogenesis of breast cancer.

Another potent antioxidant molecule is reduced glutathione. It acts as reductant which converts hydrogen peroxide into water and reduces lipid peroxidation products into their corresponding alcohols and thus mediates protective action.

In the present study, significantly low SOD activity has been observed in female patients suffering from carcinoma breast both pre as well as post operative in comparison to healthy females.

The compromised antioxidant defence system produces the oxidative stress which in turn creates the inflammatory response shown by concomitant increased adenosine deaminase (ADA) activity in female patients.

Increased ADA activity in breast cancer patients has also been reported

We observed significantly decreased SOD activity and GSH levels in patients belonging to clinical stage 4 as compared to those having stages 1, 2 or 3 of breast cancer.

increased oxidative stress gives rise to inflammation which could further aggravates the disease

Antioxidant status was highly depressed in advanced stages of breast cancer as compared to initial stage.

In the present study, significantly low GSH levels were observed in female patients of carcinoma breast as compared to normal females

Walia et al. (1995) reported increased ADA activity in breast cancer patients as compared to age matched normal subjects.

These free radicals are able to cause damage to membrane, mitochondria and macromolecules including proteins, lipids and DNA and actively take part in cell proliferation. This cascade in turn generates the inflammatory response and causes the progression of the disease.

Experimental and epidemiological evidences implicate the involvement of oxygen derived free radical in the pathogenesis of breast cancer.

Breast carcinoma involves a cascade of events that are highly inflammatory.

Marked oxidative stress in stage 4 of breast cancer indicated advancement of the disease, hence checking oxidative stress at initial stage could be helpful for controlling the progression of the disease.

They concluded that ADA is a better probable parameter for detection of breast cancer

Adenosine deaminase enzyme (ADA) catalyzes the conversion of adenosine to inosine which finally gets converted to uric acid

serum ADA activity tends to increase with advancing age,

Prevalence of oxidative stress gives rise to inflammation.

From these observations, it can be concluded that MDA levels play an important role in assessing the outcome of cancer

SOD and CAT are considered primary antioxidant enzymes, since they are involved in direct elimination of reactive oxygen metabolites. [13-16] They also act as anti-carcinogens and inhibitors at initiation and promotion/transformation stage in carcinogenesis

In our study, SOD and CAT levels were found to be low in all cancer patients as compared to controls

Fridovich and Tayarani have demonstrated in their respective studies that the reduction in SOD activity increases the toxic effects of O2 - and this might lead to severe cellular damage.

Mehrotra et al. in their study also observed high levels of MDA and low levels of SOD and CAT in patients of cancer cervix which is in sync with our observations.

strong evidence regarding the definitive role of free radicals in breast malignancy.

Vitamin C may serve as a pro-drug for hydrogen peroxide delivery to extravascular tissues, but without the presence of hydrogen peroxide in blood

not all cancer cells were killed by ascorbic acid in vitro

Intravascular hemolysis was reported after massive vitamin C administration in people with glucose-6-phosphate dehydrogenase deficiency

Administration of high-dose vitamin C to patients with systemic iron overload may increase iron absorption and represents a contraindication

Ascorbic acid is metabolized to oxalate, and 2 cases of acute oxalate nephropathy were reported in patients with pre-existing renal insufficiency given massive intravenous doses of vitamin C

Rare cases of acute tumour hemorrhage and necrosis were reported in patients with advanced cancer within a few days of starting high-dose intravenous vitamin C therapy, although this was not independently verified by pathologic review

related to intracellular hydrogen peroxide generation

only be obtained by intravenous administration of AA

Preferentially kills neoplastic cells

Is virtually non-toxic at any dosage

Does not suppress the immune system, unlike most chemotherapy agents

Increases animal and human resistance to infectious agents by enhancing lymphocyte blastogenesis, enhancing cellular immunity, strengthening the extracellular matrix, and enhancing bactericidal activity of neutrophils and modulation of complement protein

Strengthens the structural integrity of the extracellular matrix which is responsible for stromal resistance to malignant invasiveness

1969, researchers at the NCI reported AA was highly toxic to Ehrlich ascites cells in vitro

In 1977, Bram et al reported preferential AA toxicity for several malignant melanoma cell lines, including four human-derived lines

Noto et al reported that AA plus vitamin K3 had growth inhibiting action against three human tumor cell lines at non-toxic levels

Metabolites of AA have also shown antitumor activity in vitro

The AA begins to reduce cell proliferation in the tumor cell line at the lowest concentration, 1.76 mg/dl, and is completely cytotoxic to the cells at 7.04 mg/dl

the normal cells grew at an enhanced rate at the low dosages (1.76 and 3.52 mg/dl)

preferential toxicity of AA for tumor cells. >95% toxicity to human endometrial adenocarcinoma and pancreatic tumor cells (ATCC AN3-CA and MIA PaCa-2) occurred at 20 and 30 mg/dl, respectively.

No toxicity or inhibition was demonstrated in the normal, human skin fibroblasts (ATCC CCD 25SK) even at the highest concentration of 50 mg/dl.

the use of very high-dose intravenous AA for the treatment of cancer was proposed as early as 1971

Cameron and Pauling have published extensive suggestive evidence for prolonged life in terminal cancer patients orally supplemented (with and without initial intravenous AA therapy) with 10 g/day of AA

AA, plasma levels during infusion were not monitored,

the long-term, oral dosage used in those experiments (10 g/day), while substantial and capable of producing immunostimulatory and extracellular matrix modulation effects, was not high enough to achieve plasma concentrations that are generally cytotoxic to tumor cells in culture

This low cytotoxic level of AA is exceedingly rare

5 — 40 mg/dl of AA is required in vitro to kill 100% of tumor cells within 3 days. The 100% kill levels of 30 mg/dl for the endometrial carcinoma cells and 40 mg/dl for the pancreatic carcinoma cells in Figure 2 are typical

normal range (95% range) of 0.39-1.13 mg/dl

1 h after beginning his first 8-h infusion of 115 g AA (Merit Pharmaceuticals, Los Angeles, CA), the plasma AA was 3.7 mg/dl and at 5 h was 19 mg/dl. During his fourth 8-h infusion, 8 days later, the 1 h plasma level was 158 mg/dl and 5 h was 185 mg/dl

plasma levels of over 100 mg/dl have been maintained in 3 patients for more than 5 h using continuous intravenous infusion

In rare instances of patients with widely disseminated and rapidly proliferating tumors, intravenous AA administration (10 — 45 g/day) precipitated widespread tumor hemorrhage and necrosis, resulting in death

Although the outcomes were disastrous in these cases, they are similar to the description of tumor-necrosis-factor-induced hemorrhage and necrosis in mice (52) and seem to demonstrate the ability of AA to kill tumor cells in vivo.

toxic effects of AA on one normal cell line were observed at 58.36 mg/dl and the lack of side effects in patients maintaining >100 mg/dl plasma levels

Although it is very rare, tumor necrosis, hemorrhage, and subsequent death should be the highest priority concern for the safety of intravenous AA for cancer patients.

Klenner, who reported no ill effects of dosages as high as 150 g intravenously over a 24-h period

Cathcart (55) who describes no ill effects with doses of up to 200 g/d in patients with various pathological conditions

following circumstances: renal insufficiency, chronic hemodialysis patients, unusual forms of iron overload, and oxalate stone formers

Screening for red cell glucose-6-phosphate dehydrogenase deficiency, which can give rise to hemolysis of red blood cells under oxidative stress (57), should also be performed

any cancer therapy should be started at a low dosage to ensure that tumor hemorrhage does not occur.

patient is orally supplementing between infusions

a scorbutic rebound effect can be avoided with oral supplementation. Because of the possibility of a rebound effect, measurement of plasma levels during the periods between infusions should be performed to ensure that no such effect takes place

Every effort should be made to monitor plasma AA levels when a patient discontinues intravenous AA therapy.

Our data show that ascorbic acid selectively killed cancer but not normal cells, using concentrations that could only be achieved by i.v. administration

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

ascorbic acid administered i.v. in pharmacologic concentrations may serve as a pro-drug for H2O2 delivery to the extracellular milieu

H2O2 generated in blood is normally removed by catalase and glutathione peroxidase within red blood cells, with internal glutathione providing reducing equivalents

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.

Only recently has it been understood that the discordant clinical findings can be explained by previously unrecognized fundamental pharmacokinetics properties of ascorbate

Intracellular transport of ascorbate is tightly controlled in relation to extracellular concentration

Intravenous ascorbate infusion is expected to drastically change extracellular but not intracellular concentrations

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.

It is unknown why ascorbate, via H2O2, killed some cancer cells but not normal cells.

There was no correlation with ascorbate-induced cell death and glutathione, catalase activity, or glutathione peroxidase activity.

H2O2, as the product of pharmacologic ascorbate concentrations, has potential therapeutic uses in addition to cancer treatment, especially in infections

Neutrophils generate H2O2 from superoxide,

i.v. ascorbate is effective in some viral infections

H2O2 is toxic to hepatitis C

Use of ascorbate as an H2O2-delivery system against sensitive pathogens, viral or bacterial, has substantial clinical implications that deserve rapid exploration.

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

As much as a 70-fold difference in plasma concentrations is expected between oral and i.v. administration,

Complementary and alternative medicine practitioners worldwide currently use ascorbate i.v. in some patients, in part because there is no apparent harm

Human Burkitt's lymphoma cells

We first investigated whether ascorbate in pharmacologic concentrations selectively affected the survival of cancer cells by studying nine cancer cell lines

Clinical pharmacokinetics analyses show that pharmacologic concentrations of plasma ascorbate, from 0.3 to 15 mM, are achievable only from i.v. administration

plasma ascorbate concentrations from maximum possible oral doses cannot exceed 0.22 mM because of limited intestinal absorption

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

All tested normal cells were insensitive to 20 mM ascorbate.

Lymphoma cells were selected because of their sensitivity to ascorbate

As ascorbate concentration increased, the pattern of death changed from apoptosis to pyknosis/necrosis, a pattern suggestive of H2O2-mediated cell death

Apoptosis occurred by 6 h after exposure, and cell death by pyknosis was ≈90% at 14 h after exposure

In contrast to lymphoma cells, there was little or no killing of normal lymphocytes and monocytes by ascorbate

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

Whether or not intracellular ascorbate was preloaded, extracellular ascorbate induced the same amount and type of death.

extracellular ascorbate in pharmacologic concentrations mediates death of lymphoma cells by apoptosis and pyknosis/necrosis, independently of intracellular ascorbate.

H2O2 as the effector species mediating pharmacologic ascorbate-induced cell death

Superoxide dismutase was not protective

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

For both ascorbate and H2O2, death changed from apoptosis to pyknosis/necrosis as concentrations increased

Sensitivity to direct exposure to H2O2 was greater in lymphoma cells compared with normal lymphocytes and normal monocytes

There was no association between the EC50 for ascorbate-mediated cell death and intracellular glutathione concentrations, catalase activity, or glutathione peroxidase activity

H2O2 generation was dependent on time, ascorbate concentration, and the presence of trace amounts of serum in media

ascorbate radical is a surrogate marker for H2O2 formation.

whatever H2O2 is generated should be removed by glutathione peroxidase and catalase within red blood cells, because H2O2 is membrane permeable

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.

The occurrence of one predicted complication, oxalate kidney stones, is controversial

In patients with glucose-6-phosphate dehydrogenase deficiency, i.v. ascorbate is contraindicated because it causes intravascular hemolysis

ascorbate at pharmacologic concentrations in blood is a pro-drug for H2O2 delivery to tissues.

ascorbate, an electron-donor in such reactions, ironically initiates pro-oxidant chemistry and H2O2 formation

data here showed that ascorbate initiated H2O2 formation extracellularly, but H2O2 targets could be either intracellular or extracellular, because H2O2 is membrane permeant

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