Metal-Oxo Polymer Nanobeads as Potential Multi-Modal Contrast Agents for Magnetic Resonance Imaging

Abstract

Magnetic Resonance Imaging (MRI) has become one of the most powerful non-invasive imaging modalities in clinical diagnostics and research, being able to distinguish soft tissues at a very high spatial resolution. The use of contrast agents is of great assistance when the inherent contrast in the human body is insufficient to distinguish between diseased and healthy tissues. Hence most MRI examinations involve the use of exogenously administered paramagnetic, superparamagnetic and sometimes even ferromagnetic agents to enhance detection by locally decreasing the longitudinal (T1) and transverse (T2) relaxation times of water protons in biological systems. This translates into increased brightening and/or darkening of resulting MRI images, allowing better interpretation of pathological conditions.

Gadolinium chelates are the most widely used T1 ¬agents in the clinical community, while superparamagnetic iron oxide nanoparticles are used as T2 agents. There is however a growing need for non-gadolinium T1 agents particularly for patients with compromised renal function who can develop nephrogenic systemic fibrosis, a potentially fatal disorder. Gadolinium chelates have also found to accumulate in the brain of healthy patients. Although iron oxide nanoparticles are more biogenic, the darkening effect that is generated can lead to ambiguous and false positive results when identifying pathologies. Metal-oxo clusters have been gaining interest recently as potential MRI contrast agents, as these ensembles bridge the gap between the constrained structure, magnetic properties of gadolinium chelates and the superparamagnetic behavior of iron oxide nanoparticles to produce a synergistic effect.

The biogenic metal-oxo clusters that we have previously identified were herein exploited by encapsulating in “nanocarriers” to prevent dissociation, interface with the biological environment and more importantly to increase relaxivity - the parameter for contrast enhancement. The dodecanuclear cluster, Mn8Fe4O12(O2CCH3)16·4H2O or Mn8Fe4 was modified by ligand exchange and co-polymerized into mono dispersed polystyrene nanobeads of 72.74 ± 18.29 nm, showing T1-T2 dual modal behavior. During the miniemulsion synthesis, the polymer matrix was made porous for increased water accessibility while an optical probe was introduced for multi modal imaging. The trinuclear cluster Mn3(O2CCH3)6(Bpy)2 or Mn3Bpy was also utilized to synthesize monodispersed, hydrophilic polyacrylamide nanobeads of 124 ± 35 nm. Excellent T1-T2 dual modal behavior was observed surpassing commercially used Gd-DTPA chelates. The surface was functionalized using a co-monomer to conjugate methotrexate, a therapeutic targeting ligand as well as Cyanine 7, a near IR optical probe.

The stability of these nanobeads in simulated biological conditions was extensively studied for potential use as contrast agents. Metal leaching, hydrodynamic diameter and relaxation were monitored over time. Hemo-compatibility, cellular uptake and viability in vitro was also investigated along with blood clearance and biodistribution in vivo. Finally the feasibility of these materials for MRI contrast enhancement and their integrity after opsonization was tested using live mouse models.