Controlling the dissolution of MnO nanocrystals for time-dependent T1 MRI contrast agents

Introduction: Nanostructured inorganic nanoparticles and core-shell structures can be used as MRI contrast agents with the advantages of flexible surface modification characteristics [1]. Manganese based nanoparticles have potential as agents that can be "activated" when taken into cells. For example, Mn oxides or Mn carbonates are insoluble at pH 7 but dissolve to release Mn at the lower pH found in the endosome-lysosome pathway. The dissolution of Mn based particles in an acidic environment leads to large enhancement of the T1 relaxation rate [2]. In addition, Mn can leave the endosome-lysosome pathway to fill the entire cell leading to a much larger volume distribution of the contrast agent [2]. It would be advantageous to be able to control the rate of dissolution of Mn based nanoparticles to control T1 contrast signals, in vivo with time. To this end, five different coatings on MnO nanocrystals have been tested to study the release rate of the Mn ions and change in relaxivity at pH 7 compared to pH 5. Experiments: Highly monodisperse, single-phase, MnO nanocrystals (NCs), ~10 nm in diameter, were prepared by chemical routes and their magnetic properties were extensively characterized [3]. Mercaptosuccinic acid (MSA), poly(maleic anhydride-alt-1-octadecene) (PMAO), Pluronic F-127 (PF127), PMAO-PEG and SiO2 were then used, respectively, to transfer native hydrophobic particles to aqueous solutions for biocompatible applications. T1 relaxivities of these particles coated with five different molecules were determined by phantom experiments. In addition, particles were injected into the brain in the region of the thalamus [4], in order to test the rate of dissolution and subsequent neuronal tracing of the released Mn. Five rats received 100 nL of 16.8 mM MnCl2 solution into the left hemisphere and MnO@SiO2 solution into the right hemisphere. Images were acquired with an 11.7 T/31 cm horizontal bore magnet (Magnex Scientific Ltd., Abingdon, UK), which was interfaced to a Bruker Avance console (Bruker Biospin, Billerica, MA, USA). A Magnetization Prepared Rapid Gradient Echo (MP-RAGE) sequence was used. Sixteen coronal slices with FOV=2.56×2.56 cm, matrix 256×256, thickness=0.5 mm (TR=4000 ms, Echo TR/TE= 15/5 ms, TI=1000 ms, number of segments=4, Averages=8) were used to cover the area of interest at 100 μm in-plane resolution in 34 min. Results: MSA-, PMAO-, PF127, and PMAO-PEGcoated nanoparticles had relatively high relaxivities in PBS buffer solutions at pH 7 (1.8-2.5 smM) and dissolved very quickly at pH 5. For example, it took ~20 min to reach relaxivity of 6.94 for MSA coated particles at pH 5. SiO2 coated particles showed the smallest relaxivity (0.29 smM) at neutral pH, which was stable over time. The MnO@SiO2 nanoparticles had the best dynamic range for contrast change when the pH was lowered. Time dependent relaxivity measurements (Fig 1), at pH ~5.0 in acetate buffer solution of MnO@SiO2 nanoparticles showed values increasing to 2.44 smM by 53 min to 6.1 smM after 75 hours. This final relaxivity is equivalent to MnCl2 indicating that the particles had completely dissolved. The release rate of Mn ions was faster for the first 5 hrs, subsequently slowing down after 10 hrs. MP-RAGE images of the rat brain (Fig 2) showed that the signal intensity at the injection site of MnO@SiO2 particles (left sides in images) increased with time consistent with the slow dissolution rate measured in vitro. The signal at the site of MnCl2 injection (right sides of images) was elevated at the first image after injection and began to decrease slightly due to tracing of the Mn ions to different parts of the brain. Conclusions: Different coating for MnO nanoparticles can affect the T1 relaxivity at pH 7 and the rate of dissolution at pH 5. MnO@SiO2 particles had the lowest pH 7 relaxivity and the slowest dissolution rate at pH 5 in vitro. In vivo MRI of MnO@SiO2 particles injected into the brain showed time-dependent signal changes consistent with the in vitro rates. The MnO@SiO2 particles show the best potential for delaying the release of MRI contrast until specific biological processes have occurred, such as endocytosis. References: [1] Na et al, Adv. Mater. 21: 2133-2148 (2009). [2] Shapiro et al, Mag. Res. Med. 60: 265-269 (2008). [3] Lee et al, J Appl. Phys. Submitted (2009). [4] Tucciarone et al, NeuroImage 44: 923-931 (2009). Fig 1. (left) Time-dependent relaxivity of MnO@SiO2 nanopaticles soaking in pH 5.0 buffer solutions. Fig 2. (right) MP-RAGE images of injected MnCl2 and MnO@SiO2 solutions: (a) 1 hour, (b) 2 hours, (c) 4 hours, and (d) 6 hours post injection.