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Pharmacological Ascorbate Radiosensitizes Pancreatic Cancer
Abstract and Figures
The toxicity of pharmacological ascorbate is mediated by the generation of H2O2 via the oxidation of ascorbate. Since pancreatic cancer cells are sensitive to H2O2 generated by ascorbate they would also be expected to become sensitized to agents that increase oxidative damage such as ionizing radiation. The current study demonstrates that pharmacological ascorbate enhances the cytotoxic effects of ionizing radiation as seen by decreased cell viability and clonogenic survival in all pancreatic cancer cell lines examined, but not in non-tumorigenic pancreatic ductal epithelial cells. Ascorbate radiosensitization was associated with an increase in oxidative stress-induced DNA damage, which was reversed by catalase. In mice with established heterotopic and orthotopic pancreatic tumor xenografts, pharmacological ascorbate combined with ionizing radiation decreased tumor growth and increased survival, without damaging the gastrointestinal tract or increasing systemic changes in parameters indicative of oxidative stress. Our results demonstrate the potential clinical utility of pharmacological ascorbate as a radiosensitizer in the treatment of pancreatic cancer.
Copyright © 2015, American Association for Cancer Research.
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... Our in vitro, in vivo [12][13][14], and human studies [11], combined with those of others [15], provide a solid foundation for using P-AscH − as a radiosensitizer in PDAC therapy. P-AscH − has been shown to enhance the cytotoxic effects of chemotherapies [16] and radiation in all the PDAC cell lines examined but not in non-tumorigenic pancreatic ductal epithelial cells [17,18]. In mice with established PDAC xenografts, P-AscH − combined with radiation decreased tumor growth and increased survival [18]. ...
... P-AscH − has been shown to enhance the cytotoxic effects of chemotherapies [16] and radiation in all the PDAC cell lines examined but not in non-tumorigenic pancreatic ductal epithelial cells [17,18]. In mice with established PDAC xenografts, P-AscH − combined with radiation decreased tumor growth and increased survival [18]. Radiosensitization by P-AscH − was associated with an increase in oxidative stress-induced DNA damage, which was reversed by catalase [18]. ...
... In mice with established PDAC xenografts, P-AscH − combined with radiation decreased tumor growth and increased survival [18]. Radiosensitization by P-AscH − was associated with an increase in oxidative stress-induced DNA damage, which was reversed by catalase [18]. Also, P-AscH − reversed radiation-induced damage to the jejunum and did not increase systemic changes in parameters indicative of oxidative stress [17]. ...
The toxicity of ionizing radiation limits its effectiveness in the treatment of pancreatic ductal adenocarcinoma. Pharmacologic ascorbate (P-AscH−) has been shown to radiosensitize pancreatic cancer cells while simultaneously radioprotecting normal cells. We hypothesize that P-AscH− protects the small intestine while radiosensitizing pancreatic cancer cells partially through an oxidative stress mechanism. Duodenal samples from pancreaticoduodenectomy specimens of patients who underwent radio-chemotherapy ± P-AscH− and mouse tumor and jejunal samples treated with radiation ± P-AscH− were evaluated. Pancreatic cancer and non-tumorigenic cells were treated with radiation ± P-AscH− to assess lipid peroxidation. To determine the mechanism, pancreatic cancer cells were treated with selenomethionine or RSL3, an inhibitor of glutathione peroxidase 4 (GPx4). Radiation-induced decreases in villi length and increases in 4-HNE immunofluorescence were reversed with P-AscH− in human duodenum. In vivo, radiation-induced decreases in villi length and increased collagen deposition were reversed in P-AscH−-treated jejunal samples. P-AscH− and radiation increased BODIPY oxidation in pancreatic cancer cells but not in non-tumorigenic cells. Selenomethionine increased GPx4 protein and activity in pancreatic cancer and reversed P-AscH−-induced toxicity and lipid peroxidation. RSL3 treatment inhibited GPx4 activity and increased lipid peroxidation. Differences in oxidative stress may play a role in radioprotecting normal cells while radiosensitizing pancreatic cancer cells when treated with P-AscH−.
... Therefore, the impact of P-AscH − on the induction of DNA damage was evaluated within this model system. Previous research showed that P-AscH − induces single-strand DNA breaks through site-specific oxidations via hydroxyl radicals [23][24][25]. Thus, it was hypothesized that the induction of single-stranded DNA damage by P-AscH − might occur in a subtype-specific pattern, similar to the in vitro enhancement of SOC. ...
... Thus, classical GBM cells were the most sensitive to DNA damage induced by P-AscH − . Although P-AscH − is primarily believed to affect DNA through the induction of single-strand breaks [23][24][25], it is conceivable that double-strand breaks are also created due to spontaneous generation or impaired DNA repair processes or close proximity of single-strand breaks, suggesting that classical GBMs may possess defective DNA repair machinery that promotes the persistence of single-strand breaks into double-strand breaks [32,33]. Interestingly, these findings reveal that P-AscH − responses do not consistently align with the known prognostic outcomes of these subtypes, as proneural tumor cells often exhibit increased therapeutic sensitivity to SOC [21,22]. ...
Glioblastoma (GBM), a highly lethal and aggressive central nervous system malignancy, presents a critical need for targeted therapeutic approaches to improve patient outcomes in conjunction with standard-of-care (SOC) treatment. Molecular subtyping based on genetic profiles and metabolic characteristics has advanced our understanding of GBM to better predict its evolution, mechanisms, and treatment regimens. Pharmacological ascorbate (P-AscH−) has emerged as a promising supplementary cancer therapy, leveraging its pro-oxidant properties to selectively kill malignant cells when combined with SOC. Given the clinical challenges posed by the heterogeneity and resistance of various GBM subtypes to conventional SOC, our study assessed the response of classical, mesenchymal, and proneural GBM to P-AscH−. P-AscH− (20 pmol/cell) combined with SOC (5 µM temozolomide and 4 Gy of radiation) enhanced clonogenic cell killing in classical and mesenchymal GBM subtypes, with limited effects in the proneural subtype. Similarly, following exposure to P-AscH− (20 pmol/cell), single-strand DNA damage significantly increased in classical and mesenchymal but not proneural GBM. Moreover, proneural GBM exhibited increased hydrogen peroxide removal rates, along with increased catalase and glutathione peroxidase activities compared to mesenchymal and classical GBM, demonstrating an altered H2O2 metabolism that potentially drives differential P-AscH− toxicity. Taken together, these data suggest that P-AscH− may hold promise as an approach to improve SOC responsiveness in mesenchymal GBMs that are known for their resistance to SOC.
... In the oxidative tumor environment, P-AscHis auto-oxidized to the highly reactive ascorbate radical via a redox active metal-dependent mechanism which ultimately induces lethal feed-forward hydrogen peroxide (H 2 O 2 ) fluxes. Previously, studies have consistently shown that P-AscH -induced peroxide fluxes induce radiosensitization in vitro and in vivo in diverse tumor models including pancreatic adenocarcinoma, non-small cell lung cancer, and glioblastoma (GBM) (12,(17)(18)(19). A completed phase 1 trial showed that concurrent P-AscHis well tolerated and offers encouraging survival with temozolomide and definitive radiation treatment for adults with GBM. ...
Diffuse intrinsic pontine gliomas (DIPG) are an aggressive type of pediatric brain tumor with a very high mortality rate. Surgery has a limited role given the tumor’s location. Palliative radiation therapy alleviates symptoms and prolongs survival, but median survival remains less than 1 year. There is no clear role for chemotherapy in DIPGs as trials adding chemotherapy to palliative radiation therapy have failed to improve survival compared to radiation alone. Thus, there is a critical need to identify tissue-specific radiosensitizers to improve clinical outcomes for patients with DIPGs. Pharmacologic (high dose) ascorbate (P-AscH–) is a promising anticancer therapy that sensitizes human tumors, including adult high-grade gliomas, to radiation by acting selectively as a generator of hydrogen peroxide (H2O2) in cancer cells. In this study we demonstrate that in contrast to adult glioma models, P-AscH– does not radiosensitize DIPG. DIPG cells were sensitive to bolus of H2O2 but have faster H2O2 removal rates than GBM models which are radiosensitized by P-AscH–. These data support the hypothesis that P-AscH– does not enhance DIPG radiosensitivity, likely due to a robust capacity to detoxify and remove hydroperoxides.
... The prognosis of patients diagnosed with pancreatic cancer remains dismal, with a less than 3% survival at 5 years [37]. At supraphysiologic levels, ascorbate (vitamin C) acts as a pro-oxidant, donating electrons to form H 2 O 2 and, thereby, causing selective cytotoxicity and oxidative stress in pancreatic cancer cells [38,39]. Thus, pharmacological ascorbate has potential as a radiosensitizer at the same time that several studies evidence its radioprotective potential in normal tissues [40]. ...
Ionizing radiation (IR) is the energy released by atoms in the form of electromagnetic waves (e [...]
Hypoxic tumor microenvironments pose a significant challenge in cancer treatment. Hypoxia-activated prodrugs like evofosfamide aim to specifically target and eliminate these resistant cells. However, their effectiveness is often limited by reoxygenation after cell death. We hypothesized that ascorbate’s pro-oxidant properties could be harnessed to induce transient hypoxia, enhancing the efficacy of evofosfamide by overcoming reoxygenation.
To test this hypothesis, we investigated the sensitivity of MIA Paca-2 and A549 cancer cells to ascorbate in vitro and in vivo. Ascorbate induced a cytotoxic effect at 5 mM that could be alleviated by endogenous administration of catalase, suggesting a role for hydrogen peroxide in its cytotoxic mechanism. In vitro, Seahorse experiments indicated generation of hydrogen peroxide consumes oxygen, which is offset at later time points by a reduction in oxygen consumption due to hydrogen peroxide’s cytotoxic effect.
In vivo, photoacoustic imaging showed ascorbate treatment at sublethal levels triggered a complex, multi-phasic response in tumor oxygenation across both cell lines. Initially, ascorbate generated transient hypoxia within minutes through hydrogen peroxide production, via reactions that consume oxygen. This initial hypoxic phase peaked at around 150 seconds and then gradually subsided. However, at longer time scales (approximately 300 seconds) a vasodilation effect triggered by ascorbate resulted in increased blood flow and subsequent reoxygenation. Combining sublethal levels of ascorbate with evofosfamide significantly prolonged tumor doubling time in MIA Paca-2 and A549b xenografts compared to either treatment alone. This improvement, however, was only observed in a subpopulation of tumors, highlighting the complexity of the oxygenation response.
Objective: Cold atmospheric plasma (CAP) is a novel therapeutic approach for cancer treatment. It can be used to treat liquids - plasma-activated media (PAM) - which are then transferred to the target as an exogenous source of reactive oxygen and nitrogen species (RONS). The present study aimed at chemically characterizing different PAM and assessing their in vitro selectivity against head and neck cancer cell lines (HNC).
Materials and methods: PAM were obtained by exposing 2 and 5 mL of medium to CAP for 5, 10 and 20 minutes at a 6 mm working distance. Anions kinetics was evaluated by ion chromatography. In addition, inhibition of cell proliferation by MTS assay, apoptosis occurrence and cell cycle modifications by flow cytometry were assessed on primary human gingival fibroblasts (hGF) and the HNC cell lines HSC2, HSC4 and A253.
Results: All the 2 mL conditions showed a significant reduction in cell proliferation whereas for the 5 mL the effect was milder, but the time-dependence was more evident. In addition, hGF were unaffected by the 5 mL PAM, indicating a selectivity for cancer cells.
Conclusions: The media chemical composition modified by CAP exposure influenced cell proliferation by modulating cell cycle and inducing apoptosis in cancer cells, without affecting normal cells.
Clinical Relevance: The present investigation represents a starting point to favour the clinical translation of CAP as a precision medicine tool by proposing an innovative method, namely ion chromatography, to standardize the quantification of plasma-derived RONS and proving its selectivity in inactivating tumor cells over non-malignant cells. These strategies could be applied to identify the optimal parameter configuration to achieve the desired treatment/therapeutic outcome and to aid the definition of clinical protocols.
Pharmacological ascorbate (P-AscH–, high dose, intravenous vitamin C) preferentially sensitizes human pancreas ductal adenocarcinoma (PDAC) cells to radiation-induced toxicity compared to non-tumorigenic epithelial cells. Radiation-induced G2-checkpoint activation contributes to the resistance of cancer cells to DNA damage induced toxicity. We hypothesized that P-AscH– induced radio-sensitization of PDAC cells is mediated by perturbations in the radiation induced activation of the G2-checkpoint pathway. Both non-tumorigenic pancreatic ductal epithelial and PDAC cells display decreased clonogenic survival and increased doubling times after radiation treatment. In contrast, the addition of P-AscH– to radiation increases clonogenic survival and decreases the doubling time of non-tumorigenic epithelial cells but decreasing clonogenic survival and increasing the doubling time of PDAC cells. Results from the mitotic index and propidium iodide assays showed that while the P-AscH– treatments did not affect radiation-induced G2-checkpoint activation, it enhanced G2-accumulation. The addition of catalase reverses the increases in G2-accumulation, indicating a peroxide-mediated mechanism. In addition, P-AscH– treatment of PDAC cells suppresses radiation-induced accumulation of cyclin B1 protein levels. Both translational and post-translational pathways appear to regulate cyclin B1 protein levels after the combination treatment of PDAC cells with P-AscH– and radiation. The protein changes seen are reversed by the addition of catalase suggesting that hydrogen peroxide mediates P-AscH– induced radiation sensitization of PDAC cells by enhancing G2-accumulation and reducing cyclin B1 protein levels.
Non-small cell lung cancer (NSCLC) poses a significant global health burden with unsatisfactory survival rates, despite advancements in diagnostic and therapeutic modalities. Novel therapeutic approaches are urgently required to improve patient outcomes. Pharmacological ascorbate (P-AscH−; ascorbate at millimolar concentration in plasma) emerged as a potential candidate for cancer therapy for recent decades. In this present study, we explore the anti-cancer effects of P-AscH− on NSCLC and elucidate its underlying mechanisms. P-AscH− treatment induces formation of cellular oxidative distress; disrupts cellular bioenergetics; and leads to induction of apoptotic cell death and ultimately reduction in clonogenic survival. Remarkably, DNA and DNA damage response machineries are identified as vulnerable targets for P-AscH− in NSCLC therapy. Treatments with P-AscH− increase the formation of DNA damage and replication stress markers while inducing mislocalization of DNA repair machineries. The cytotoxic and genotoxic effects of P-AscH− on NSCLC were reversed by co-treatment with catalase, highlighting the roles of extracellular hydrogen peroxide in anti-cancer activities of P-AscH−. The data from this current research advance our understanding of P-AscH− in cancer treatment and support its potential clinical use as a therapeutic option for NSCLC therapy.
Pancreatic ductal adenocarcinoma is an unsolved health problem with nearly 75% of patients diagnosed with advanced disease and an overall 5-year survival rate near 5%. Despite the strong link between mortality and malignancy, the mechanisms behind pancreatic cancer dissemination and metastasis are poorly understood. Correlative pathological and cell culture analyses suggest the chemokine receptor CXCR4 plays a biological role in pancreatic cancer progression. In vivo roles for the CXCR4 ligand CXCL12 in pancreatic cancer malignancy were investigated. CXCR4 and CXCR7 were consistently expressed in normal and cancerous pancreatic ductal epithelium, established cell lines, and patient-derived primary cancer cells. Relative to healthy exocrine ducts, CXCL12 expression was pathologically repressed in pancreatic cancer tissue specimens and patient-derived cell lines. To test the functional consequences of CXCL12 silencing, pancreatic cancer cell lines stably expressingthe chemokine were engineered. Consistent with a role for CXCL12 as a tumor suppressor, cells producing the chemokine wereincreasingly adherent and migration deficient in vitro and poorly metastatic in vivo, compared to control cells. Further, CXCL12 reintroduction significantly reduced tumor growth in vitro, with significantly smaller tumors in vivo, leading to a pronounced survival advantage in a preclinical model. Together, these data demonstrate a functional tumor suppressive role for the normal expression of CXCL12 in pancreatic ducts, regulating both tumor growth andcellulardissemination to metastatic sites.
Ascorbate (vitamin C) was an early, unorthodox therapy for cancer, with an outstanding safety profile and anecdotal clinical benefit. Because oral ascorbate was ineffective in two cancer clinical trials, ascorbate was abandoned by conventional oncology but continued to be used in complementary and alternative medicine. Recent studies provide rationale for reexamining ascorbate treatment. Because of marked pharmacokinetic differences, intravenous, but not oral, ascorbate produces millimolar concentrations both in blood and in tissues, killing cancer cells without harming normal tissues. In the interstitial fluid surrounding tumor cells, millimolar concentrations of ascorbate exert local pro-oxidant effects by mediating hydrogen peroxide (H2O2) formation, which kills cancer cells. We investigated downstream mechanisms of ascorbate-induced cell death. Data show that millimolar ascorbate, acting as a pro-oxidant, induced DNA damage and depleted cellular adenosine triphosphate (ATP), activated the ataxia telangiectasia mutated (ATM)/adenosine monophosphate-activated protein kinase (AMPK) pathway, and resulted in mammalian target of rapamycin (mTOR) inhibition and death in ovarian cancer cells. The combination of parenteral ascorbate with the conventional chemotherapeutic agents carboplatin and paclitaxel synergistically inhibited ovarian cancer in mouse models and reduced chemotherapy-associated toxicity in patients with ovarian cancer. On the basis of its potential benefit and minimal toxicity, examination of intravenous ascorbate in combination with standard chemotherapy is justified in larger clinical trials.
Labile iron, i.e. iron that is weakly bound and is relatively unrestricted in its redox activity, has been implicated in both the pathogenesis as well as treatment of cancer. Two cancer treatments where labile iron may contribute to their mechanism of action are pharmacological ascorbate and ionizing radiation (IR). Pharmacological ascorbate has been shown to have tumor-specific toxic effects due to the formation of hydrogen peroxide. By catalyzing the oxidation of ascorbate, labile iron can enhance the rate of formation of hydrogen peroxide; labile iron can also react with hydrogen peroxide. Here we have investigated the magnitude of the labile iron pool in tumor and normal tissue. We also examined the ability of pharmacological ascorbate and IR to change the size of the labile iron pool. Although a significant amount of labile iron was seen in tumors (Mia PaCa-2 cells in athymic nude mice), higher levels were seen in murine tissues that were not susceptible to pharmacological ascorbate. Pharmacological ascorbate and irradiation were shown to increase the labile iron in tumor homogenates from this murine model of pancreatic cancer. As both IR and pharmacological ascorbate may rely on labile iron for their effects on tumor tissues, our data suggest that pharmacological ascorbate could be used as a radio-sensitizing agent for some radio-resistant tumors.
The development of an effective therapy for radiation-induced gastrointestinal damage is important, because it is currently a major complication of treatment and there are few effective therapies available. Although we have recently demonstrated that pretreatment with ascorbic acid attenuates lethal gastrointestinal damage in irradiated mice, more than half of mice eventually died, thus indicating that better approach was needed. We then investigated a more effective therapy for radiation-induced gastrointestinal damage. Mice receiving abdominal radiation at 13 Gy were orally administered ascorbic acid (250 mg/kg/day) for three days before radiation (pretreatment), one shot of engulfment (250 mg/kg) at 8 h before radiation, or were administered the agent for seven days after radiation (post-treatment). None of the control mice survived the abdominal radiation at 13 Gy due to severe gastrointestinal damage (without bone marrow damage). Neither pretreatment with ascorbic acid (20% survival), engulfment (20%), nor post-treatment (0%) was effective in irradiated mice. However, combination therapy using ascorbic acid, including pretreatment, engulfment and post-treatment, rescued all of the mice from lethal abdominal radiation, and was accompanied by remarkable improvements in the gastrointestinal damage (100% survival). Omitting post-treatment from the combination therapy with ascorbic acid markedly reduced the mouse survival (20% survival), suggesting the importance of post-treatment with ascorbic acid. Combination therapy with ascorbic acid may be a potent therapeutic tool for radiation-induced gastrointestinal damage.
ABSTRACT Renewed interest in using pharmacological ascorbate (AscH-) to treat cancer has prompted interest in leveraging its cytotoxic mechanism of action. A central feature of AscH- action in cancer cells is its ability to act as an electron donor to O2 for generating H2O2. We hypothesized that catalytic manganoporphyrins (MnPs) would increase AscH- oxidation rates, thereby increasing H2O2 fluxes and cytotoxicity. Three different MnPs were tested (MnTBAP, MnT2EPyP, and MnT4MPyP) exhibiting a range of physicochemical and thermodynamic properties. Of the MnPs tested, MnT4MPyP exerted the greatest effect on increasing the rate of AscH- oxidation as determined by the concentration of ascorbate radical [Asc•-] and the rate of oxygen consumption. At concentrations that had minimal effects alone, combining MnPs and AscH- synergized to decrease clonogenic survival in human pancreatic cancer cells. This cytotoxic effect was reversed by catalase, but not superoxide dismutase, consistent with a mechanism mediated by H2O2. MnPs increased steady-state concentrations of Asc•- upon ex vivo addition to whole blood obtained either from mice infused with AscH- or patients treated with pharmacologic AscH-. Lastly, tumor growth in vivo was inhibited more effectively by combining MnT4MPyP with AscH-. We concluded that MnPs increase the rate of oxidation of AscH- to leverage H2O2 flux and ascorbate-induced cytotoxicity.
Purpose:
Ketogenic diets are high in fat and low in carbohydrates as well as protein which forces cells to rely on lipid oxidation and mitochondrial respiration rather than glycolysis for energy metabolism. Cancer cells (relative to normal cells) are believed to exist in a state of chronic oxidative stress mediated by mitochondrial metabolism. The current study tests the hypothesis that ketogenic diets enhance radio-chemo-therapy responses in lung cancer xenografts by enhancing oxidative stress.
Experimental design:
Mice bearing NCI-H292 and A549 lung cancer xenografts were fed a ketogenic diet (KetoCal 4:1 fats: proteins+carbohydrates) and treated with either conventionally fractionated (1.8-2 Gy) or hypofractionated (6 Gy) radiation as well as conventionally fractionated radiation combined with carboplatin. Mice weights and tumor size were monitored. Tumors were assessed for immunoreactive 4-hydroxy-2-nonenal-(4HNE)-modified proteins as a marker of oxidative stress as well as proliferating cell nuclear antigen (PCNA) and γH2AX as indices of proliferation and DNA damage, respectively.
Results:
The ketogenic diets combined with radiation resulted in slower tumor growth in both NCI-H292 and A549 xenografts (P < 0.05), relative to radiation alone. The ketogenic diet also slowed tumor growth when combined with carboplatin and radiation, relative to control. Tumors from animals fed a ketogenic diet in combination with radiation showed increases in oxidative damage mediated by lipid peroxidation as determined by 4HNE-modified proteins as well as decreased proliferation as assessed by decreased immunoreactive PCNA.
Conclusions:
These results show that a ketogenic diet enhances radio-chemo-therapy responses in lung cancer xenografts by a mechanism that may involve increased oxidative stress.
Background:
Treatment for pancreatic cancer with pharmacological ascorbate (ascorbic acid, vitamin C) decreases tumor progression in preclinical models. A phase I clinical trial was performed to establish safety and tolerability of pharmacological ascorbate combined with gemcitabine in patients with biopsy-proven stage IV pancreatic adenocarcinoma.
Design:
Nine subjects received twice-weekly intravenous ascorbate (15-125 g) employing Simon's accelerated titration design to achieve a targeted post-infusion plasma level of ≥350 mg/dL (≥20 mM). Subjects received concurrent gemcitabine. Disease burden, weight, performance status, hematologic and metabolic laboratories, time to progression and overall survival were monitored.
Results:
Mean plasma ascorbate trough levels were significantly higher than baseline (1.46 ± 0.02 vs. 0.78 ± 0.09 mg/dL, i.e., 83 vs. 44 μM, p < 0.001). Adverse events attributable to the drug combination were rare and included diarrhea (n = 4) and dry mouth (n = 6). Dose-limiting criteria were not met for this study. Mean survival of subjects completing at least two cycles (8 weeks) of therapy was 13 ± 2 months.
Conclusions:
Data suggest pharmacologic ascorbate administered concurrently with gemcitabine is well tolerated. Initial data from this small sampling suggest some efficacy. Further studies powered to determine efficacy should be conducted.
ABSTRACT Pharmacological ascorbate, via its oxidation, has been proposed as a pro-drug for the delivery of H(2)O(2) to tumors. Pharmacological ascorbate decreases clonogenic survival of pancreatic cancer cells, which can be reversed by treatment with scavengers of H(2)O(2). The goal of this study was to determine if inhibitors of intracellular hydroperoxide detoxification could enhance the cytotoxic effects of ascorbate. Human pancreatic cancer cells were treated with ascorbate alone or in combination with inhibitors of hydroperoxide removal including the glutathione disulfide reductase inhibitor 1,3 bis (2-chloroethyl)-1-nitrosurea (BCNU), siRNA targeted to glutathione disulfide reductase (siGR), and 2-deoxy-D-glucose (2DG), which inhibits glucose metabolism. Changes in the intracellular concentration of H(2)O(2) were determined by analysis of the rate of aminotriazole-mediated inactivation of endogenous catalase activity. Pharmacological ascorbate increased intracellular H(2)O(2) and depleted intracellular glutathione. When inhibitors of H(2)O(2) metabolism were combined with pharmacological ascorbate the increase in intracellular H(2)O(2) was amplified and cytotoxicity was enhanced. We conclude that inclusion of agents that inhibit cellular peroxide removal produced by pharmacological ascorbate leads to changes in the intracellular redox state resulting in enhanced cytotoxicity.
Since the discovery of vitamin C, the number of its known biological functions is continually expanding. Both the names ascorbic acid and vitamin C reflect its antiscorbutic properties due to its role in the synthesis of collagen in connective tissues. Ascorbate acts as an electron-donor keeping iron in the ferrous state thereby maintaining the full activity of collagen hydroxylases; parallel reactions with a variety of dioxygenases affect the expression of a wide array of genes, for example via the HIF system, as well as via the epigenetic landscape of cells and tissues. In fact, all known physiological and biochemical functions of ascorbate are due to its action as an electron donor. The ability to donate one or two electrons makes AscH(-) an excellent reducing agent and antioxidant. Ascorbate readily undergoes pH-dependent autoxidation producing hydrogen peroxide (H(2)O(2)). In the presence of catalytic metals this oxidation is accelerated. In this review, we show that the chemical and biochemical nature of ascorbate contribute to its antioxidant as well as its prooxidant properties. Recent pharmacokinetic data indicate that intravenous (i.v.) administration of ascorbate bypasses the tight control of the gut producing highly elevated plasma levels; ascorbate at very high levels can act as prodrug to deliver a significant flux of H(2)O(2) to tumors. This new knowledge has rekindled interest and spurred new research into the clinical potential of pharmacological ascorbate. Knowledge and understanding of the mechanisms of action of pharmacological ascorbate bring a rationale to its use to treat disease especially the use of i.v. delivery of pharmacological ascorbate as an adjuvant in the treatment of cancer.