Exceedingly small iron oxide nanoparticles as positive MRI contrast agents

Abstract

Medical imaging is routine in the diagnosis and staging of a wide range of medical conditions. In particular, magnetic resonance imaging (MRI) is critical for visualizing soft tissue and organs, with over 60 million MRI procedures performed each year worldwide. About one-third of these procedures are contrast-enhanced MRI, and gadolinium-based contrast agents (GBCAs) are the mainstream MRI contrast agents used in the clinic. GBCAs have shown efficacy and are safe to use with most patients; however, some GBCAs have a small risk of adverse effects, including nephrogenic systemic fibrosis (NSF), the untreatable condition recently linked to gadolinium (Gd) exposure during MRI with contrast. In addition, Gd deposition in the human brain has been reported following contrast, and this is now under investigation by the US Food and Drug Administration (FDA). To address a perceived need for a Gd-free contrast agent with pharmacokinetic and imaging properties comparable to GBCAs, we have designed and developed zwitterion-coated exceedingly small superparamagnetic iron oxide nanoparticles (ZES-SPIONs) consisting of ∼3-nm inorganic cores and ∼1-nm ultrathin hydrophilic shell. These ZES-SPIONs are free of Gd and show a high T₁ contrast power. We demonstrate the potential of ZES-SPIONs in preclinical MRI and magnetic resonance angiography.

Keywords: exceedingly small iron oxide nanoparticles; gadolinium-free positive MR contrast agent; preclinical magnetic resonance imaging; renal clearance.

Fig. 1.

(A) Rationally designed synthetic route of ZDS-coated SPIONs, (B) HR-TEM images of SPIONs with ∼3.0-nm inorganic core diameter, and (C) SQUID curves of 3-nm ES-SPIONs, ferumoxytol (Feraheme), as well as Gd-DTPA (Magnevist).

Fig. S1.

(A–D) TEM images of SPIONs with ∼7.0-, 5.5-, 3.0-, and 2.5-nm core diameters, respectively.

Fig. S2.

XRD of ferumoxytol.

Fig. S3.

(A) GFC curve of ZDS-coated SPIONs with 3-nm inorganic core diameters (ZES-SPIONs) and (B) the calibration curve of HD vs. retention time by using protein standards.

Fig. 2.

HD and T₁ contrast power of different contrast agents (ferumoxytol, Feraheme; Gd-DTPA, Magnevist; VSOP, very small iron oxide nanoparticles; ZES-SPIONs, zwitterion-coated exceedingly small iron oxide nanoparticles).

Fig. 3.

(A) Schematic of mouse biodistribution study using ⁵⁹Fe-labeled ZES-SPIONs, and (B) the percentages of ⁵⁹Fe-labeled ZES-SPIONs that stay in urine, blood, and organs, compared with the total amount of ⁵⁹Fe-labeled ZES-SPIONs injected (GIT, gastrointestinal tract). After 24 h following injection, 12.5 ± 0.7% of the injected ⁵⁹Fe-labeled ZES-SPIONs remained in the liver. The combination of blood, spleen, kidney, lung, gastrointestinal tract, heart, carcass, and tail had less than 25% of the injected ⁵⁹Fe-labeled ZES-SPIONs.

Fig. 4.

(A–J) T₁-weighted MR images of a mouse injected with ZDS-coated exceedingly small SPIONs (ZES-SPIONs) at 7 T. Time points underneath each image: the time after ZES-SPIONs injection: (A–E) one sagittal slice showing the heart (red arrow), the vena cava (green arrow), and the bladder (yellow arrow), and (F–J) another sagittal slice showing the kidney (blue arrow); (K) urine samples from mice taken at different time points after injection showing renal clearance of ZES-SPIONs in vivo. Before the injection of ZES-SPIONs, the heart (red arrow), the vena cava (green arrow), and the bladder (yellow arrow) do not show appreciable positive contrast. After injection, the heart and the vena cava display high positive contrast immediately after injection. At 8 min after injection, the bladder displays some positive contrast, indicating an excretion of urine containing ZES-SPIONs. With the increase of time postinjection, Fig. 4 D and E shows that the positive contrast region of the bladder increases, suggesting an accumulation of urine with ZES-SPIONs.

Fig. S4.

T₁-weighted MRI in vivo in (A) rat and (B) mouse (heart, blue oval; kidney, yellow oval; bladder, red oval) injected with ZES-SPIONs.

Fig. S5.

(A) The volume of urine containing contrast in the bladder of a mouse injected with ZES-SPIONs vs. time measured by MRI and (B) the decay of the log of relative MRI signal (T₂*) in mice blood after particles injection.

Fig. 5.

(A–C) T₁-weighted magnetic resonance angiography (MRA) of a mouse injected with ZDS-coated exceedingly small SPIONs (ZES-SPIONs) at 7 T. The time points beneath each image are the time after injection of the ZES-SPIONs. (D–H) Five different perspectives of the MRA, which are extracted from the 3D scan, at the 4-min mark. A clear positive contrast of the heart and blood vessels are seen, and this positive contrast fades in time while the signal from the bladder increases, consistent with a renal excretion pathway.