Composition-Tunable Ultrasmall Manganese Ferrite Nanoparticles: Insights into their In Vivo T1 Contrast Efficacy

The development of a highly efficient, low-toxicity, ultrasmall ferrite nanoparticle-based T 1 contrast agent for high-resolution magnetic resonance imaging (MRI) is highly desirable. However, the correlations between the chemical compositions, in vitro T 1 relaxivities, in vivo nano-bio interactions and toxicities remain unclear, which has been a challenge in optimizing the in vivo T 1 contrast efficacy. Methods : Ultrasmall (3 nm) manganese ferrite nanoparticles (Mn x Fe 3-x O 4 ) with different doping concentrations of the manganese ions (x = 0.32, 0.37, 0.75, 1, 1.23 and 1.57) were used as a model system to investigate the composition-dependence of the in vivo T 1 contrast efficacy. The efficacy of liver-specific contrast-enhanced MRI was assessed through systematic multiple factor analysis, which included the in vitro T 1 relaxivity, in vivo MRI contrast enhancement, pharmacokinetic profiles (blood half-life time, biodistribution) and biosafety evaluations ( in vitro cytotoxicity testing, in vivo blood routine examination, in vivo blood biochemistry testing and H&E staining to examine the liver). Results : With increasing Mn doping, the T 1 relaxivities initially increased to their highest value of 10.35 mM -1 s -1 , which was obtained for Mn 0.75 Fe 2.25 O 4 , and then the values decreased to 7.64 m M -1 s -1 , which was obtained for the Mn 1.57 Fe 1.43 O 4 nanoparticles. Nearly linear increases in the in vivo MRI signals (ΔSNR) and biodistributions (accumulation in the liver) of the Mn x Fe 3-x O 4 nanoparticles were observed for increasing levels of Mn doping. However, both the in vitro and in vivo biosafety evaluations suggested that Mn x Fe 3-x O 4 nanoparticles with high Mn-doping levels (x > 1) can induce significant toxicity. Conclusion : The systematic multiple factor assessment indicated that the Mn x Fe 3-x O 4 (x = 0.75-1) nanoparticles were the optimal T 1 contrast agents with higher in vivo efficacies for liver-specific MRI than those of the other compositions of the Mn x Fe 3-x O 4 nanoparticles. Our work provides insight into the optimization of ultrasmall ferrite nanoparticle-based T 1 contrast agents by tuning their compositions and promotes the translation of these ultrasmall ferrite nanoparticles for clinical use of high-performance contrast-enhanced MRI.