Prussian blue, the first artificially synthesized pigment, was discovered unintentionally by Diesbach in 1704 while he was attempting to make cochineal red lake. Although it immediately became popular in art and later in printing, it took approximately 250 years to recognize that Prussian blue was able to attract monovalent alkali metals into its crystal lattice. Subsequently, in 1963, Nigrovic was the first investigator to demonstrate that Prussian blue enhanced cesium elimination from the gut of rats.33 In 2003, the US Food and Drug Administration (FDA) approved Prussian blue (Radiogardase®) for the treatment of thallium and radioactive cesium poisoning.
The literature associated with Prussian blue is complicated by many confusing chemical and physical terms. The product synthesized by Diesbach, Fe4[Fe(CN6)]3, commonly known as insoluble Prussian blue, is assigned the Chemical Abstracts Service (CAS) number 14038-43-8, and is the FDA-approved product Radiogardase® (Fig. A29–1). Synonyms for Prussian blue include Berlin blue, Hamburg blue, mineral blue, Paris blue, and Pigment blue 27, among others.51 These names are often used interchangeably to refer to both insoluble Prussian blue and a soluble (colloidal) Prussian blue that has the molecular formula either KFe[Fe(CN)6]3 or K3Fe[Fe(CN)6]3. Thus "Prussian blue" also carries two additional CAS numbers: 25869-98-1 and 12240-15-2.37 Compounds containing the same basic core structure, such as NH4Fe[Fe(CN)6]3 (ammonium ferric ferrocyanide or Chinese blue) and sodium ferric ferrocyanide, may have similar efficacy in binding monovalent cations, and are also sometimes incorrectly called Prussian blue. For the purpose of clarity, general statements that follow use the term "Prussian blue." In many instances the terms "insoluble" and "soluble" are chosen to highlight differences between the compounds. Unfortunately, because many studies do not specify which Prussian blue is used, some inherent ambiguity persists.
The chemical structure of insoluble Prussian blue. The Roman numerals II and III denote the valence state of iron. Although in the most current nomenclature this would be expressed as Fe2+ and Fe3+, the figure is drawn this way to be consistent with most available references, which use the older nomenclature.
Typically, the crystal lattice of Prussian blue takes up cationic potassium ions from the surrounding environment. However, because its affinity increases as the ionic radius of the monovalent cation increases, Prussian blue preferentially binds cesium (ionic radius: 0.169 nm) and thallium (ionic radius: 0.147 nm) over potassium (ionic radius: 0.133 nm).6,16 Additionally, binding for rubidium (ionic radius: 0.148 nm) has been demonstrated.44 Thus, when given orally, Prussian blue binds unabsorbed thallium or cesium in the gastrointestinal tract, preventing absorption as well as reversing the concentration gradient to enhance elimination through gut dialysis. In addition, Prussian blue can interfere with enterohepatic circulation, causing a further ...