Gadolinium-Loaded Viral Capsids as Magnetic Resonance Imaging Contrast Agents

Robert J. Usselman, Shefah Qazi, Priyanka Aggarwal, Sandra S. Eaton, Gareth R. Eaton, Stephen Russek, Trevor Douglas

Polymeric nanohybrid P22 virus capsids were used as templates for high density Gd(3+) loading to explore magnetic field-dependent (0.5-7.0 T) proton relaxivity. The field-dependence of relaxivity by the spatially constrained Gd(3+) in the capsids was similar when either the loading of the capsids or the concentration of capsids was varied. The ionic longitudinal relaxivity, r1, decreased from 25-32 mM(-1) s(-1) at 0.5 T to 6-10 mM(-1) s(-1) at 7 T. The ionic transverse relaxivity, r2, increased from 28-37 mM(-1) s(-1) at 0.5 T to 39-50 mM(-1) s(-1) at 7 T. The r2/r1 ratio increased linearly with increasing magnetic field from about 1 at 0.5 T, which is typical of T1 contrast agents, to 5-8 at 7 T, which is approaching the ratios for T2 contrast agents. Increases in electron paramagnetic resonance line widths at 80 and 150 K and higher microwave powers required for signal saturation indicate enhanced Gd(3+) electron spin relaxation rates for the Gd(3+)-loaded capsids than for low concentration Gd(3+). The largest r2/r1 at 7 T was for the highest cage loading, which suggests that Gd(3+)-Gd(3+) interactions within the capsid enhance r2 more than r1.