Detection of U-87 Tumor Cells by RGD-Functionalized/Gd-Containing Giant Unilamellar Vesicles in Magnetization Transfer Contrast Magnetic Resonance Images

Abstract

Objectives: The targeting of tumor cells and their visualization with MRI is an important task in biomedicine. The low sensitivity of this technique is a significant drawback, and one that may hamper the detection of the imaging reporters used.To overcome this sensitivity issue, this work explores the synergy between two strategies: i) RGDfunctionalized Giant Unilamellar Vesicles (GUVs) loaded with Gd-complexes to accumulate large amounts of MRI contrast agent at the targeting site; and ii) the use of Magnetization Transfer Contrast (MTC), which is a sensitive MRI technique for the detection of Gd-complexes in the tumor region.Materials and Methods: GUVs were prepared using the gentle swelling method, and the cyclic RGD targeting moiety was introduced onto the external membrane. Paramagnetic Gd-containing complexes were both part of the vesicle membranes and were the payload within the inner aqueous cavity together with the fluorescent probe, rhodamine. GUVs that were loaded with the imaging reporters, but devoid of the RGD targeting moiety, were used as controls. U-87 MG human glioblastoma cells, which are known to overexpress the targets for RGD moieties, were used. In the in-vivo experiments, U-87 MG cells were subcutaneously injected into nu/nu mice and the generated tumors were imaged using MRI, 15 days after cell administration. MRI was carried out at 7 T, and T2W, T1W and MTC/Z-spectra were acquired. Confocal microscopy images and ICP-MS were used for result validation.Results: In-vitro results show that RGD GUVs specifically bind to U-87 MG cells. Microscopy demonstrates that: i) RGD GUVs were anchored onto the external surface of the tumor cells without any internalization; ii) a low number of GUVs per cell were clustered at specific regions; iii) there is no evidence for macrophage uptake or cell toxicity. The MRI of cell pellets after incubation with RGD GUVs and untargeted ctrl-GUVs was performed. No difference in T1 signal was detected, whereas a 15% difference in MT contrast is present between the RGD-GUV-treated cells and the ctrl-GUV-treated cells.MR images of tumor-bearing mice were acquired before and after (t=0, 4 h and 24 h) the administration of RGD GUVs and ctrl-GUVs. A roughly 16% MTC difference between the two groups was observed after 4 h. Immunofluorescence analyses and ICP-MS analyses (for Gddetection) of the explanted tumors confirmed the specific accumulation of RGD GUVs in the tumor region.Conclusions: RGD GUVs appear to be interesting carriers that can facilitate the specific accumulation of MRI contrast agents at the tumor region. However, the concentration achieved is still below the threshold needed for T1w-MRI visualization. Conversely, MTC proved to be sufficiently sensitive for the visualization of detectable contrast between pre-and post-targeting images.

Reliable Assessment of Swine Renal Fibrosis Using Quantitative Magnetization Transfer Imaging

“…Magnetization transfer imaging (MTI) is a powerful molecular MRI technique that takes advantage of this tissue characteristic to assess tissue macromolecule content 20,21 . By selective saturation of the bound pool, the measured MR signal from the free water pool is also decreased due to the MT process between these 2 pools.…”

“…Magnetization transfer imaging (MTI) is a powerful molecular MRI technique that takes advantage of this tissue characteristic to assess tissue macromolecule content. 20,21 By selective saturation of the bound pool, the measured MR signal from the free water pool is also decreased due to the MT process between these 2 pools. Thus, MTI has been carried out in 2 manners, that is, semiquantitatively for measurement of the MT ratio (MTR), or measurably by applying quantitative MT (qMT).…”

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Evaluation of Physicochemical Properties, Pharmacokinetics, Biodistribution, Toxicity, and Contrast-Enhanced Cancer MRI of a Cancer-Targeting Contrast Agent, MT218

“…There have been continuous efforts to design and develop targeted MRI contrast agents since the clinical application of MRI in the early 1980s. [5][6][7][8][9] Antibody targeted GBCAs were explored to target the biomarkers on cancer cell surfaces. Because of the low concentration of the cell-surface tumor markers (typically nM) and poor sensitivity of MRI for molecular imaging GBCA (typically μM), these targeted contrast agents have been unable to generate tumor-specific MRI detectable signal enhancement.…”

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