MAGNETOFERRITIN - CHARACTERIZATION OF A NOVEL SUPERPARAMAGNETIC MR CONTRAST AGENT

A protein-encaged superparamagnetic iron oxide has been developed and characterized hy using horse spleen apoferritin as a novel bioreactive environment. The roughly spherical magnetoferritin molecules, 120 ang strom in diameter, are composed of a monocrystalline maghemite or magn etite core 73 angstrom +/- 14 in diameter. Except for the additional p resence of iron-rich molecules of higher molecular weight, the appeara nce and molecular weight (450 kd) of magnetoferritin are identical to that of natural ferritin; the molecules are externally indistinguishab le from their precursor, with a pI (isoelectric point) in the range 4. 3-4.6. The measured magnetic moment of the superparamagnetic cores is 13,200 Bohr magnetons per molecule, with TI and T2 relaxivities (r1 an d r2) of 8 and 175 L.mmol-1 (Fe).sec-1, respectively, at body temperat ure and clinical field strengths. The unusually high r2/r1 ratio of 22 is thought to arise from ideal core composition, with no evidence of crystalline paramagnetic inclusions. T2 relaxation enhancement can be well correlated to the field-dependent molecular magnetization, as giv en by the Langevin magnetization function, raised to a power in the ra nge 1.4-1.6. With its nanodimensional biomimetic protein cage as a rig id, convenient matrix for complexing a plethora of bioactive substance s, magnetoferritin may provide a novel template for specific targeting of selected cellular sites.