The data show that pharmacologic ascorbate concentrations produced Asc•− selectively in extracellular fluid compared with blood and that H2O2 formation occurred when Asc•− concentrations were >100 nM in extracellular fluid.
These data validate the hypothesis that ascorbate is a prodrug for selective delivery of reactive species to the extravascular space
pharmacologic ascorbate as a prooxidant drug for therapeutic use.
Recently we reported that pharmacologic ascorbic acid concentrations produced H2O2 concentrations of ≥25 μM, causing cancer cell death in vitro
We found that H2O2 concentrations generated in vivo were those that caused cancer cell death in vitro
When ascorbate was given parenterally, Asc•−, the product of a loss of one electron from ascorbate, was detected preferentially in extracellular fluid compared with blood
Asc•− generation in extracellular fluid depended on the ascorbate dose and the resulting concentrations
With i.v. administration of ascorbate, Asc•− concentrations were as much as 12-fold greater in extracellular fluid compared to blood and approached 250 nM
In blood, such Asc•− concentrations were never produced and were always <50 nM
These data are all consistent with the hypothesis that pharmacologic ascorbate concentrations in vivo serve as a prodrug for selective delivery of H2O2 to the extracellular space
After oral ingestion, control of intracellular and extracellular ascorbate concentrations is mediated by three mechanisms: intestinal absorption, tissue transport, and renal reabsorption
intestinal absorption, or bioavailability, declines at doses >200 mg
also at ≈60 μM, renal reabsorption approaches saturation, and excess ascorbate is excreted in urine
Parenteral administration bypasses tight control
When tight control is bypassed, H2O2 forms in the extracellular space
in vivo validation of ascorbate as a prodrug for selective H2O2 formation
Temporarily bypassing tight control with parenteral administration of ascorbate allows H2O2 to form in discrete time periods only, decreasing likelihood of harm, and provides a pharmacologic basis for therapeutic use of i.v. ascorbate
H2O2 formation results in selective cytotoxicity
Tumor cells are killed with exposure to H2O2 for ≤30 min
In vitro, killing is mediated by H2O2 rather than Asc•−
In addition to cancer treatment, another potential therapeutic use is for treatment of infections. H2O2 concentrations of 25–50 μM are bacteriostatic
Glutamine is the most abundant amino acid in blood
Rapidly proliferating healthy cells (GI epithelium, lymphocytes) or cells under physiologic stress have increased demand for glutamine
Glutamine is transported into cells by one of multiple amino acid transporters (e.g. ASCT2, BOAT2), several of which are thought to be upregulated in cancer cells
it is hydrolyzed to glutamate and ammonia by glutaminase (‘glutaminolysis’)
Glutamate, produced from glutamine by glutaminase and glutamine amidotransferase activities, may be further metabolized to alpha ketoglutarate and provide a carbon skeleton source for the mitochondrial tricarboxylic acid cycle (TCA cycle)
Glutamine-derived glutamate is also involved in the synthesis of the reducing equivalent glutathione, vital to maintaining cellular redox status
Many tumors become largely dependent on glutamine to provide carbon and nitrogen building blocks needed for proliferation
In cancer model systems, Eagle and colleagues first demonstrated tumor cells in culture require supplementation with exogenous glutamine for efficient proliferation
It was subsequently shown that when deprived of glutamine tumor cells undergo apoptosis
The most well-characterized oncogene to regulate glutamine metabolism is MYC (9), which enhances glutaminase expression, upregulates glutamine transporters, and enhances glutamine utilization in energy production and biosynthesis
Other pro-tumorigenic regulators such as KRAS and mTOR, as well as tumor suppressors (p53, VHL) have also been associated with alterations in glutamine metabolism
Tumor cells are highly adaptable and alter nutrient uptake and metabolic networks to resist single agent glutaminase inhibition
cells in the microenvironment of several tumor types upregulate glutamine production, thereby enabling tumor cells to escape glutaminase inhibition