Synopsis

Small molecule conventional gadolinium (Gd) present some safety concerns including nephrogenic systemic fibrosis (NSF) and brain accumulation. Nanoparticle-based gadolinium (Gd) agents may alleviate these concerns by streamlining the clearance pathways of the agent.Liposomal Gd(III)DOTA-DSPE conjugate exhibit high relaxivity at clinically-relevant MR field strength, and enables molecular MR imaging mice. We report herein an investigation on the pharmacokinetics, bio-distribution and clearance of a liposomal-Gd agent in mice over a period of 28 days by T1-weighted MR imaging and ICP-MS analyses. Results show no significant skin or brain accumulation, suggesting NSF and brain accumulation concerns may be unlikely with such agent.

Purpose

Nanoparticle-based gadolinium (Gd) agents hold considerable promise for the development of molecular MR imaging agents. However, recent concerns regarding in vivo retention of conventional small molecule Gd chelates necessitates an understanding of the in vivo fate of novel Gd agents. The two big concerns with conventional Gd are nephrogenic systemic fibrosis (NSF) and brain accumulation. Nanoparticle formulations may streamline the biodistribution and clearance pathways, thereby alleviating some of these toxicity concerns. We recently demonstrated that a liposomal contrast agent based on the Gd(III)DOTA-DSPE conjugate (liposomal-Gd) exhibits high T1 relaxivity (2 × 105 mM−1s−1 on a particle basis) at clinically-relevant MR field strength, and enables molecular MR imaging of pathologies in mouse models of Alzheimer’s disease and atherosclerotic plaques1,2. In this work, we investigated the pharmacokinetics, bio-distribution and clearance of liposomal-Gd agent in mice.

Methods

Imaging studies in C57BL6 mice (3 males and 3 females per group) were performed on a 1T permanent magnet MR scanner. MRI was performed using a T1-weighted, 3D gradient recalled echo sequence (T1w-GRE). A longitudinal whole-body MRI study was performed for up to 28 days post-contrast to follow the biodistribution of liposomal-Gd. In addition, mice (3 males and 3 females per group) were intravenously administered liposomal-Gd contrast agent (0.1 mmol Gd/kg) and euthanized for organ harvesting at the following time points post-contrast injection: 2h, 4h, day 1, day 2, day 4, day 14 and day 28. Blood and target organs (liver, spleen, kidneys, heart, lungs, brain and skin) were harvested and digested for the quantification of Gd concentration by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).

Results

Images from the T1w-3D GRE scans (Figure 1a) showed progressive signal enhancement in the liver and spleen followed by a drop to prescan levels by day 28 post-injection. This was accompanied by a drop in signal intensity in the inferior vena cava (IVC). Maximum signal enhancement in the liver was observed at the 24 h time point and at the 4 h time point for the spleen. A plot of SNR against time for the IVC signal (Figure 1b), suggests a circulation clearance half-life of about 12 h. ICP-MS analysis to quantify Gd levels in the blood, liver and spleen tissue samples corroborate the general trends observed in the T1w-3D GRE images. However, unlike the image guided analysis, the of Gd levels in the liver and spleen peak at 7 days post-contrast at ~175 µg/g wet tissue and ~350 µg/g wet tissue respectively (Figure 2). It also shows some slight increases in Gd levels in the other organs including brain, lungs, kidneys and skin within the first 14 days post injection but these all return to baseline levels by day 28, while levels in the liver (~60 µg/g wet tissue) and spleen (~175 μg/g wet tissue) remain significant.

Conclusions

Longitudinal MRI demonstrated systemic clearance of Gd(III)DOTA-DSPE-based liposomal via the organs of reticulo-endothelial system i.e., liver and spleen. This is consistent with known clearance route of liposomal agents. MR signal intensity in the blood had returned to baseline level by day 14 and only liver and spleen signals noticeable by day 28 (Fig 2). The data shows no noticeable accumulation of the agent in the skin and brain suggesting that both NSF and brain accumulation are not likely. These results demonstrate some insights on the potential safety profile of Gd(III)DOTA-DSPE-based liposomal agents.

Acknowledgements

This work was supported by NIH (grant no. R21 EB020153 to E.A.T. and RO1 HD094347 to A.V.A.) and Alzeca Biosciences, Inc.

References

1. Tanifum, EA, Ghaghada, KB, Vollert, C, Head, E, Eriksen, JL, Annapragada, A. A novel liposomal nanoparticle for the imaging of amyloid plaques by MRI. J Alzheimers Dis 2016, 52(2), 731-745. 2. Woodside DG, Tanifum EA, Ghaghada KB, Biediger RJ, Caivano AR, Starosolski ZA, Khounlo S, Bhayana S, Abbasi S, Craft Jr JW, Maxwell DS, Patel C, Stupin IV, Bakthavatsalam D, Market RV, Willerson JT, Dixon RAF, Vanderslice P, and Annapragada AV. Magnetic resonance imaging of atherosclerotic plaque at clinically relevant field strengths (1T) by targeting the integrin α4β1. Scientific Reports 2018, 8(1):3733. doi: 10.1038/s41598-018-21893-x.