PRUSSIAN BLUE (FERRIC
HEXACYANOFERRATE)
Ferric
hexacyanoferrate (insoluble Prussian blue) is a hydrated crystalline compound
in which Fe2+ and Fe3+atoms are coordi-nated
with cyanide groups in a cubic lattice structure. Although used as a dark blue
commercial pigment for nearly 300 years, it was only three decades ago that its
potential usefulness as a phar-maceutical chelator was recognized. Primarily by
ion exchange, and secondarily by mechanical trapping or adsorption, the
com-pound has high affinity for certain univalent cations, particularly cesium
and thallium. Used as an oral drug, insoluble Prussian blue undergoes minimal
gastrointestinal absorption (< 1%). Because the complexes it forms with cesium or thallium are
nonabsorb-able, oral administration of the chelator diminishes intestinal
absorption or interrupts enterohepatic and enteroenteric circula-tion of these
cations, thereby accelerating their elimination in the feces. In clinical case
series, the use of Prussian blue has been asso-ciated with a decline in the
biologic half-life (ie, in vivo retention) of radioactive cesium and thallium.
In 2003, the FDA approved
Prussian blue for the treatment of contamination with radioactive cesium (137Cs)
and intoxication with thallium salts. Approval was prompted by concern over
potential widespread human contamination with radioactive cesium caused by
terrorist use of a radioactive dispersal device (“dirty bomb”). The drug is
part of the Strategic National Stockpile of pharmaceuticals and medical
material maintained by the CDC (http://www.bt.cdc.gov/stockpile/#material). (Note: Although soluble
forms of Prussian blue, such as potassium ferric hexacyanoferrate, may have
better utility in thallium poi-soning, only the insoluble form is currently
available as a phar-maceutical.)
After exposure to 137Cs or thallium
salts, the approved adult dosage is 3 g orally three times a day; the
corresponding pediatric dosage (2–12 years of age) is 1 g orally three times a
day. Serial monitoring of urine and fecal radioactivity (137Cs) and
urinarythallium concentrations can guide the recommended duration of therapy.
Adjunctive supportive care for possible acute radiation illness (137Cs)
or systemic thallium toxicity should be instituted as needed.
Prussian blue has not been
associated with significant adverse effects. Constipation, which may occur in
some cases, should be treated with laxatives or increased dietary fiber.
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