The Acidic Tumor Microenvironment Promotes the Reconversion of Nitrite into Nitric Oxide: Towards a New and Safe Radiosensitizing Strategy

Abstract

Purpose: The biological status of nitrite recently evolved from an inactive end product of nitric oxide catabolism to the largest intravascular and tissue storage of nitric oxide (NO). Although low partial O 2 pressure favors enzymatic reconversion of nitrite into NO, low pH supports a nonenzymatic pathway. Because hypoxia and acidity are characteristics of the tumor microenvironment, we examined whether nitrite injection could preferentially lead to NO production in tumors and influence response to treatments. Experimental Design: The effects of nitrite were evaluated on arteriole vasorelaxation, tumor cell respiration and tumor blood flow, oxygenation, and response to radiotherapy. Results: We first showed that a small drop in pH (-0.6 pH unit) favored the production of bioactive NO from nitrite by documenting a higher cyclic guanosine 3 ¶,5 ¶-monophosphated ependent arteriole vasorelaxation. We then documented that an i.v. bolus injection of nitrite to tumor-bearing mice led to a transient increase in partial O 2 pressure in tumor but not in healthy tissues. Blood flow measurements failed to reveal an effect of nitrite on tumor perfusion, but we found that O 2 consumption by nitrite-exposed tumor cells was decreased at acidic pH. Finally, we showed that low dose of nitrite could sensitize tumors to radiotherapy, leading to a significant growth delay and an increase in mouse survival (versus irradiation alone). Conclusions: This study identified low pH condition (encountered in many tumors) as an exquisite environment that favors tumor-selective production of NO in response to nitrite systemic injection.This work opens new perspectives for the use of nitrite as a safe and clinically applicable radiosensitizing modality.

Activated Macrophages as a Novel Determinant of Tumor Cell Radioresponse: The Role of Nitric Oxide–Mediated Inhibition of Cellular Respiration and Oxygen Sparing

“…By contrast, optimal iNOS activation was shown to induce apoptosis and to exert antitumor and antimetastatic effects. Besides, at micromolar concentrations, both NO and its oxidative product nitrite are known to downregulate respiratory mitochondrial functions, in a variety of cells including tumor cells, macrophages, astrocytes, hepatocytes and myocytes (5,10,22). The observed levels of NO/ nitrite production in both EMT-6 tumor cells and RAW 264.7 macrophages after exposure to LPS/IFN-g matched those concentrations.…”

Inorganic nitrite therapy: historical perspective and future directions

“…The characteristic of NO, by driving local vasodilation and increasing perfusion, suggests that it may have therapeutic potential in tumors to allow transient increase in the delivery and efficacy of anti-cancer drugs. Recently Frerart et al has evaluated the effects of nitrite anion therapy on tumor blood flow, oxygenation, and response to radiotherapy [262]. It was found that nitrite selectively induce a transient increase in tumor partial O 2 pressure (pO 2 ), which improved the efficacy of radiotherapy.…”

Targeting Tumor Perfusion and Oxygenation Modulates Hypoxia and Cancer Sensitivity to Radiotherapy and Systemic Therapies

“…We therefore tested whether the low pH of tumors (on average pH 6.7) could be exploited to generate NO from nitrites selectively in tumors. We observed ex vivo that nitrite-induced vasodilation was more pronounced at pH 6.7 compared to pH 7.4 (Frerart et al, 2008). We also found that the bioactivity of nitrites at low pH encompassed NO-mediated inhibition of tumor cell respiration, which indicates that the robust and transient increase in tumor pO 2 after nitrite delivery to mice is the result of the combination of vasoactive and metabolic responses.…”

“…We also found that the bioactivity of nitrites at low pH encompassed NO-mediated inhibition of tumor cell respiration, which indicates that the robust and transient increase in tumor pO 2 after nitrite delivery to mice is the result of the combination of vasoactive and metabolic responses. When administered to reach a plasma concentration of 100 µM in mice, nitrites sensitized tumors to radiotherapy (Frerart et al, 2008). Further clinical applications are however confronted to financial issues: clinical trials are now warranted whereas nitrites or their use in cancer therapy can not be patented.…”