Simplified synthesis and relaxometry of magnetoferritin for magnetic resonance imaging

Magnetoferritin nanoparticles have been developed as high‐relaxivity, functional contrast agents for MRI. Several previous techniques have relied on unloading native ferritin and re‐incorporation of iron into the core, often resulting in a polydisperse sample. Here, a simplified technique is developed using commercially available horse spleen apoferritin to create monodisperse magnetoferritin. Iron oxide atoms were incorporated into the protein core via a step‐wise Fe(II)Chloride addition to the protein solution under low O 2 conditions; subsequent filtration steps allow for separation of completely filled and superparamagnetic magnetoferritin from the partially filled ferritin. This method yields a monodisperse and homogenous solution of spherical particles with magnetic properties that can be used for molecular magnetic resonance imaging. With a transverse per‐iron and per‐particle relaxivity of 78 mM −1 sec −1 and 404,045 mM −1 sec −1 , respectively, it is possible to detect ∼10 nM nanoparticle concentrations in vivo. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.