The USPIO GEH121333 as a dual R1 and R2 Contrast Agent for Imaging Response to Anti-angiogenic Therapy
Jana Cebulla1, Eugene Kim1, Dan E Meyer2, Karina Langseth3, Tone F Bathen1, Siver A Moestue1, and Else Marie Huuse1,4

1Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, Trondheim, Norway, 2Diagnostics, Imaging and Biomedical Technologies, Niskayuna, NY, United States, 3GE Healthcare AS, Oslo, Norway, 4Department of Medical Imaging, St. Olavs University Hospital, Trondheim, Norway

Synopsis

Preclinical-phase iron oxide particles (GEH121333), with a high r1/r2 ratio compared to other iron oxide nanoparticles, were used for monitoring vascular response to bevacizumab treatment in ovarian cancer xenografts. Susceptibility contrast MRI using T2 and T2* mapping revealed a treatment induced decrease in blood volume and vessel density, but not in vessel size. Additionally, DCE-MRI using gadodiamide detected a decrease in perfusion and/or permeability. In combination, these two methods provide a comprehensive assessment of anti-angiogenic treatment effects.
Lastly, GEH121333 particles induced a strong signal increase in T1w images, which shows promise for its use also as a positive contrast agent.

Purpose

GEH121333 are preclinical-phase ultrasmall superparamagnetic iron oxide (USPIO) particles (GE Global Research, Niskayuna, NY, USA, supplied through GE Healthcare AS, Oslo, Norway) with a high r1/r2 relaxivity ratio compared to other USPIOs [1], opening the possibility of both T1- and T2(*) contrast-enhanced MRI. The aim of this study was to investigate the use of the GEH121333 particles for susceptibility contrast MRI to detect changes in blood vessel morphology (blood volume, vessel size and vessel density) in xenograft tumors after bevacizumab treatment. The results were compared to DCE-MRI data. In addition, R1 maps were generated to explore dual mode use of the GEH121333 contrast agent.

Methods

All experimental procedures involving animals were approved by the institutional ethics committee and were in accordance with national, regional and institutional guidelines. TOV-21G ovarian cancer xenografts were grown on the hind leg of athymic mice (n=18). MRI was performed on a 7T Bruker Biospec with an 86mm volume resonator for RF transmission and a quadrature mouse brain surface coil for reception. Images of 4 sagittal slices with slice thickness=0.6mm, FOV=20×20mm2 and matrix=64×64 were acquired for each tumor using the following sequences:
Multi-echo Spin Echo (MSE): TE=10.5ms, 32 echoes with 10.5ms echo spacing, TR=3s, 2 averages;
Multi-echo Gradient Echo (MGE)
: TE=2.5ms, 30 echoes with 2.5ms echo spacing, flip angle=30°, 1 average;
Variable TR RARE (VTR)
: TEeff=13ms, TR=225/500/1500/3000/6000/12000ms, RARE factor=2, 1 average.
High resolution T2w images with matrix=256×256 were acquired for drawing tumor ROIs and 3D RARE images were acquired for tumor volume measurements.
The timeline for the experiment is shown in Fig. 1. GEH121333 was injected i.v. at a dose of 10mg Fe/kg. Immediately after imaging on day 0 and again on day 3, the mice received 5mg/kg bevacizumab (Avastin®, Roche AG, Basel, Switzerland) (n=9) or saline (vehicle control, n=9). R1, R2 and R2* maps were computed voxelwise from the VTR, MSE and MGE images, respectively. Pre-contrast R1, R2 and R2* maps were subtracted from the post-contrast maps to obtain ΔR1, ΔR2 and ΔR2* maps. ΔR2 and ΔR2* relaxation rates are measures for the microvascular and total blood volume fraction, respectively [2]. Estimates of blood vessel density (Q) and size (R) were calculated from ΔR2 and ΔR2* according to Q= ΔR2/ (ΔR2*)2/3 [3] and R= ΔR2*/ ΔR2 [4]. On day 6, DCE-MRI was performed using gadodiamide (Omniscan™, GE Healthcare, Oslo, Norway) with the same scan geometry as described above and analyzed using the extended Tofts model as described previously [5]. The area under the enhancement curve after 1 minute (AUC1min) and the volume transfer constant (Ktrans) were computed.
From the manually drawn tumor ROIs a three-voxel-wide (~1mm) tumor rim was automatically extracted and parameter medians were computed for these tumor rims for statistical analysis. Two-tailed independent sample t-tests were performed to compare changes in susceptibility contrast MRI parameters and DCE-MRI parameters in control versus treated tumors. In cases of failed contrast agent injection, the mice were excluded from statistical analyses (number of animals are indicated in the figures).

Results

Bevacizumab had no effect on tumor growth (p=0.68) during the treatment period, but caused a decrease in ΔR2 (p=0.015), ΔR2* (p=0.035) and Q (p=0.016), but not in R (p=0.312) (Fig. 2 and 3). Additionally, changes in vascular function after treatment were detected by DCE-MRI, which showed significantly lower AUC1min (p=0.04) and Ktrans (p=0.004) compared to the control group. GEH121333 injection resulted in a signal decrease in the tumor rim in T2w images, but also a signal increase in T1w images (Fig. 4), demonstrating the potential utility of these USPIOs for positive contrast-enhanced MRI. The decrease in ΔR1 after treatment as shown in Fig. 2 is very similar to the decrease in ΔR2. Also the longitudinal development of R1 and R2 (Fig. 5) was very similar. Already on day 1, the R1 and R2 were lower in treated compared to control tumors, which may reflect early treatment-induced differences in extravasation and accumulation of GEH121333. This trend was even more pronounced on day 6.

Discussion

Our data shows that the GEH121333 particles can be used for detecting changes in vascular morphology after anti-angiogenic treatment similar to other studies [6, 7]. The combination of DCE-MRI and USPIO-MRI allowed us to describe both changes in vascular function and morphology, thereby providing a more comprehensive assessment of anti-angiogenic treatment effects. The high r1/r2 relaxivity ratio of GEH121333 also shows promise for its use as a positive contrast agent for other applications such as combined susceptibility contrast MRI and DCE-MRI using only a single contrast injection.

Acknowledgements

Brian Bales2 and Bruce Hay2 and Ingrid Henriksen3 for the synthesis, formulation & characterization of GEH121333.

References

[1] Shi, et al., Contrast Media Mol Imaging, 2013, 8: 281-8. [2] Boxerman, et al., Magn Reson Med, 1995, 34: 555-66. [3] Jensen, et al., Magn Reson Med, 2000, 44: 224-30. [4] Dennie, et al., Magn Reson Med, 1998, 40: 793-9. [5] Cebulla, et al., Br J Cancer, 2015, 112: 504-13. [6] Sampath, et al., Neoplasia, 2013, 15: 694-711. [7] Persigehl, et al., Radiology, 2007, 244: 449-56.

Figures

Figure 1: Study design for MRI using the USPIO GEH121333 and Gd-based contrast agent Omniscan to detect treatment-induced vascular changes in ovarian cancer xenografts in mice. Mice were treated with bevacizumab (n=9) or saline (n=9).

Figure 2: Bar plots showing (mean±SD) of change in median USPIO-contrast MRI parameters from pre-treatment day 0 to post-treatment day 5, and of median DCE-MRI parameters on day 6. Independent sample t-test control versus treated group. * p<0.05, ** p<0.01

Figure 3: Anatomical images and vascular parametric maps of one representative control and treated tumor pre- (day 0) and post-treatment (day 5). The maps for the control tumor show stable parameters while the maps for the treated tumor show a decrease in all parameters except for the vessel size R.

Figure 4:
Left: High resolution T2w image of an axial slice through the leg and the tumor.
Middle: Relative signal change (rel ΔSI) in T1w image (TE=13ms, TR=500ms) after GEH121333 injection.
Right: Relative signal change (rel ΔSI) in T2w image (TE=73.5ms, TR=3000ms) after GEH121333 injection.

Figure 5: Longitudinal measurement of R1 and R2 (group mean±SD). Independent sample t-test of change between time points for treated vs control group: # p<0.05. Independent sample t-test of differences in R1 or R2 for treated vs control group: *p<0.05, **p<0.01.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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