Thrombus-specific manganese-based “nanobialys” for MR molecular imaging of ruptured plaque

Summary

Mathematical modeling studies have suggested that nonspherical, disc-shaped nanoparticles may have optimal intravascular flow and homing characteristics. In this study, we report the development of a fibrin-specific high-relaxivity bialy-shaped polymeric nanoparticle using porphyrin-chelated manganese. We anticipate that this agent would be highly effective for molecular imaging of microthrombi in ruptured atherosclerotic plaques.

Background

Detection of microthrombi within fissures of vulnerable atherosclerotic plaques requires a sensitive molecular imaging contrast agent. Moreover, recent reports based on mathematical modeling suggest that nonspherical, disc-shaped nanoparticles could have improved intravascular flow characteristics, which may improve ligand-directed targeting. In light of the concern surrounding the use of gadolinium in patients with severe renal disease, the goal of this research was to develop a nonspherical fibrin-targeted manganese-based molecular imaging agent.

Methods

A new class of manganese (III)-labeled, a toroidal-shaped, vascularly-constrained nanoparticles, “nano-bialys (MnNBs)”, was designed, synthesized, physically characterized, and evaluated for MR properties. Single slice inversion recovery and multi-echo spin echo sequences were used to calculate the ionic (per metal) and particulate (per particle) relaxivities from 7 serial dilutions of nanobialys at 1.5T and 25°C. Fibrin clots supported on silk suture suspended in PBS were targeted with MnNB or control (non-paramagnetic) NB to the fibrin clots with avidin-biotin interactions and fibrin-specific antibodies (NIB5F3). Magnetic resonance images (3T) of the clots were acquired using T1-weighted gradient echo techniques.

Results

Mn-nano-bialys were 190nm ± 5nm with polydispersity of 0.26±0.01. (Fig 1) In the hydrated state, Manganese content was 25.6 ± 03 µg/mL by ICP OES, i.e., 165,000 Mn(III) per nanobialy. The particulate relaxivities of the MnNB were high, r1=612,307±7213 and r2= 866,989±10704 (s●mmol [nanobialy])-1 measured at 1.5T (25°C), with ionic r1 and r2 relaxivities of 3.7±1.1 and 5.2±1.1 (s●mmol [Mn])-1, respectively. MR imaging of MnNB targeted to fibrin clot phantoms showed clear contrast enhancement, while control clots had no (p<0.05) contrast change (Fig 2).

Figure 1

Cartoon depiction of NanoBialy based on hydrodynamic dynamic light scattering measurements. The particles appear biconcave or bialy-shaped, much like erythrocytes. The high surface-to-volume aspect of the particle is densely decorated with Mn-porphryn chelates.

Figure 2

MRI images of fibrin-targeted nanobialys (right) or control nanoparticles bound to cylindrical plasma clots measured at 3.0T. The ionic and particulate r1 relaxivities of serially diluted nanobialys at 3.0T were 3.1 ± 1.1 (s ● mmol [Mn])-1 and 512,863 ± 8408 (s ● mmol [nanobialy])-1 respectively.

Conclusions

Fibrin-specific MnNBs are a novel, high relaxivity, non-gadolinium, molecular imaging agent that offers a sensitive noninvasive MR imaging approach for diagnosis of ruptured atherosclerotic plaques.

Funding

NIH.