Available online at www.sciencedirect.com Journal of Pharmaceutical and Biomedical Analysis 47 (2008) 114–125 Quantitative determination of cesium binding to ferric hexacyanoferrate: Prussian blue  Patrick J. Faustino a,∗ , Yongsheng Yang a , Joseph J. Progar b , Charles R. Brownell a , Nakissa Sadrieh c , Joan C. May b , Eldon Leutzinger d , David A. Place d , Eric P. Duffy d , Florence Houn b , Sally A. Loewke e , Vincent J. Mecozzi a , Christopher D. Ellison a , Mansoor A. Khan a , Ajaz S. Hussain c , Robbe C. Lyon a a Division of Product Quality Research, Office of Testing and Research, Center for Drug Evaluation and Research, Food and Drug Administration, 10903 New Hampshire Avenue, Silver Spring, MD 20993, United States b Office of Vaccine Research and Review, Center for Biologics Evaluation and Research, Food and Drug Administration, 1401 Rockville Pike, Rockville, MD 20852, United States c Office of Pharmaceutical Science, Center for Drug Evaluation and Research, Food and Drug Administration, Building-21, 10903 New Hampshire Avenue, Silver Spring, MD 20993, United States d Office of New Drug Quality Assurance, Center for Drug Evaluation and Research, Food and Drug Administration, Building-22, 10903 New Hampshire Avenue, Silver Spring, MD 20993, United States e Office of New Drugs, Center for Drug Evaluation and Research, Food and Drug Administration, Building-21, 10903 New Hampshire Avenue, Silver Spring, MD 20993, United States Received 9 August 2007; received in revised form 25 November 2007; accepted 27 November 2007 Available online 14 December 2007 Abstract Ferric hexacyanoferrate (Fe 4 III [Fe II (CN) 6 ] 3 ), also known as insoluble Prussian blue (PB) is the active pharmaceutical ingredient (API) of the drug product, Radiogardase ® . Radiogardase ® is the first FDA approved medical countermeasure for the treatment of internal contamination with radioactive cesium (Cs) or thallium in the event of a major radiological incident such as a “dirty bomb”. A number of pre-clinical and clinical studies have evaluated the use of PB as an investigational decorporation agent to enhance the excretion of metal cations. There are few sources of published in vitro data that detail the binding capacity of cesium to insoluble PB under various chemical and physical conditions. The study objective was to determine the in vitro binding capacity of PB APIs and drug products by evaluating certain chemical and physical factors such as medium pH, particle size, and storage conditions (temperature). In vitro experimental conditions ranged from pH 1 to 9, to cover the range of pH levels that PB may encounter in the gastrointestinal (GI) tract in humans. Measurements of cesium binding were made between 1 and 24 h, to cover gastric and intestinal tract residence time using a validated atomic emission spectroscopy (AES) method. The results indicated that pH, exposure time, storage temperature (affecting moisture content) and particle size play significant roles in the cesium binding to both the PB API and the drug product. The lowest cesium binding was observed at gastric pH of 1 and 2, whereas the highest cesium binding was observed at physiological pH of 7.5. It was observed that dry storage conditions resulted in a loss of moisture from PB, which had a significant negative effect on the PB cesium binding capacity at time intervals consistent with gastric residence. Differences were also observed in the binding capacity of PB with different particle sizes. Significant batch to batch differences were also observed in the binding capacity of some PB API and drug products. Our results suggest that certain physiochemical properties affect the initial binding capacity and the overall binding capacity of PB APIs and drug products during conditions that simulated gastric and GI residence time. These physiochemical properties can be utilized as quality attributes to monitor and predict drug product quality under certain manufacturing and storage conditions and may be utilized to enhance the clinical efficacy of PB. Published by Elsevier B.V. Keywords: Prussian blue; Cesium binding; pH-profile; GI model; Particle size; Moisture; Atomic emission spectroscopy; Product quality  This scientific contribution is intended to support regulatory policy development. The views presented in this article have not been adopted as regulatory policies by the Food and Drug Administration at this time. ∗ Corresponding author. Tel.: +1 301 796 0021; fax: +1 301 796 9816. E-mail address: patrick.faustino@fda.hhs.gov (P.J. Faustino). 0731-7085/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jpba.2007.11.049