Metal-Organic Polyhedron with Four Fe(III) Centers Producing Enhanced T 1 Magnetic Resonance Imaging Contrast in Tumors.

A metal-organic polyhedron (MOP) with four paramagnetic Fe(III) centers was studied as a magnetic resonance imaging (MRI) probe. The MOP was characterized in solution by using electron paramagnetic resonance (EPR), UV-visible (UV-vis) spectroscopies, Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry, and in the solid state with single-crystal X-ray diffraction. Water proton T ₁ relaxation properties were examined in solution and showed significant enhancement in the presence of human serum albumin (HSA). The r ₁ relaxivities in the absence and presence of HSA were 8.7 mM ^-1 s ^-1 and 21 mM ^-1 s ^-1 , respectively, per molecule (2.2 mM ^-1 s ^-1 and 5.3 mM ^-1 s ^-1 per Fe) at 4.7 T, 37 °C. In vivo studies of the iron MOP show strong contrast enhancement of the blood pool even at a low dose of 0.025 mmol/kg with prolonged residence in vasculature and clearance through the intestinal tract of mice. The MOP binds strongly to serum albumin and shows comparable accumulation in a murine tumor model as compared to a covalently linked Gd-HSA contrast agent.