Quantitative measurement of cyanide released from Prussian Blue

Background. Prussian Blue (PB), ferric hexacyanoferrate is indicated for (oral) treatment of internal contamination with radioisotopes of cesium or thallium. Cyanide is 35-40% of PB's molecular composition, thus cyanide may be released during transit through the digestive tract under physiological pH. The U.S. Food and Drug Administration investigated the issue of cyanide release prior to drug approval to ensure the drug's benefits exceeded risks. Objectives. To determine cyanide released from PB under pH conditions that bracket human physiological exposure. Methods. PB was incubated in situ at pH 1.0-12, 37°C for 1-48 hours. Cyanide was measured using a validated colorimetric method by UV-VIS spectroscopy. Results. PB had the highest cyanide release at pH 1 (135 ug g) and lowest release at pH 5.0-7.0 from the highest daily dose of PB (17.5 g) (21 ug g). Considering the minimal lethal dose of cyanide is approximately 50 mg, the maximal cyanide released (1.6 mg) does not present a safety concern.