PROJECT SUMMARY/ABSTRACT Cyanide, a very toxic mitochondrial poison, has a wide spread usage in various industrial activities. It is present in many synthetic materials and can be released in gaseous form during a house fire. Since CN can be easily weaponized, it is considered as a serious threat for the civilian population. The treatment of acute CN intoxication relies on (1) specific antidotes, to decrease the concentration of CN in the blood and tissues, and (2) symptomatic treatments, essential at the early stage of exposure, consisting in an aggressive ventilatory and circulatory support, akin to the treatment of any conditions associated with cardio-respiratory depression. We have recently found that the phenothiazinium chromophore methylene blue (MB), which can counteract the toxicity of various mitochondrial poisons (hydrogen sulfide and Azure B), can also oppose CN- induced acute circulatory failure. The mechanisms of action of MB as well as the limits of its efficacy, following CN intoxication, remain to be determined. The objective of our proposal is first to validate our preliminary observations and to determine the interest of the use of MB during and following CN intoxication induced coma and cardiac failure in the rat. We will determine if MB can improve the immediate and long-term outcome of CN intoxication when used alone and more importantly as an adjuvant of antidotes already approved for the treatment of CN intoxication. Second, we will explore the potential mechanisms of action of MB, using isolated contracting cardiomyocytes intoxicated with CN. These studies will be performed in collaboration with the group of Dr. Joseph Cheung, Temple University. Since MB has been used for decades for treating methemoglobinemia (1-2 mg/kg iv) in humans, our goal is to reposition methylene blue, a drug already on the WHO's list of essential medications, as a treatment of CN intoxication.