1: Clin Cancer Res. 2008 May 1;14(9):2768-2774. The Acidic Tumor Microenvironment Promotes the Reconversion of Nitrite into Nitric Oxide: Towards a New and Safe Radiosensitizing Strategy. Frérart F, Sonveaux P, Rath G, Smoos A, Meqor A, Charlier N, Jordan BF, Saliez J, Noël A, Dessy C, Gallez B, Feron O. Authors' Affiliations: Unit of Pharmacology and Therapeutics, Université catholique de Louvain, UCL-FATH and Biomedical Magnetic Resonance, Université catholique de Louvain, UCL-REMA, Brussels, Belgium; and Laboratory of Tumor and Developmental Biology, University of Liège, Tour de Pathologie (B23), Sart-Tilman, Belgium. 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'-monophosphate-dependent 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. PMID: 18451244 [PubMed - as supplied by publisher] Related Links Insulin increases the sensitivity of tumors to irradiation: involvement of an increase in tumor oxygenation mediated by a nitric oxide-dependent decrease of the tumor cells oxygen consumption. [Cancer Res. 2002] PMID:12068004 Preferential action of arsenic trioxide in solid-tumor microenvironment enhances radiation therapy. [Int J Radiat Oncol Biol Phys. 2005] PMID:15817358 Nitric oxide-mediated increase in tumor blood flow and oxygenation of tumors implanted in muscles stimulated by electric pulses. [Int J Radiat Oncol Biol Phys. 2003] PMID:12605986 Irradiation-induced angiogenesis through the up-regulation of the nitric oxide pathway: implications for tumor radiotherapy. [Cancer Res. 2003] PMID:12615716 Modulation of the tumor vasculature functionality by ionizing radiation accounts for tumor radiosensitization and promotes gene delivery. [FASEB J. 2002] PMID:12397083