Supramolecular protamine/Gd-loaded liposomes adducts as relaxometric protease responsive probes.
ABSTRACT A new approach to enzyme-responsive MRI agents based on the use of liposomes loaded with a high number of paramagnetic metal complexes (Gd-HPDO3A) is presented. It relies on the disruption of low relaxivity aggregates formed by liposomes and a macromolecular substrate that is selectively cleaved by the enzyme of interest. The interaction of anionic liposomes composed of POPC:CHOL:DPGS and the cationic protein protamine yields a poorly soluble supramolecular assembly endowed with a low relaxivity. The action of the serine protease trypsin causes the digestion of protamine and the consequent de-assembly of the supramolecular adduct. The process is accompanied by an overall relaxation enhancement of solvent water protons as consequence of the dissolution of the aggregated liposomes. The observed increase of relaxivity is linearly dependent on the enzyme concentration. An illustrative example of the possible use of the herein presented responsive agent has been reported. It consists of the entrapment of the supramolecular assembly in alginate microcapsules that have often been used as envelopes for in vivo applications of stem cells and pancreatic islets. The change in the observed longitudinal relaxation rate R(1) (leading to an hyperintense signal in the corresponding MR images) may act as a sensor of the protease activity in the biological environment in which the capsules is located.
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ABSTRACT: One of the major challenges of MR imaging is the quantification of local concentrations of contrast agents. Cellular uptake strongly influences different parameters such as the water exchange rate and the pool of water protons, and results in alteration of the contrast agent's relaxivity, therefore making it difficult to determine contrast agent concentrations based on the MR signal only. Here, we propose a multimodal radiolabeled paramagnetic liposomal contrast agent that allows simultaneous imaging with SPECT and MRI. As SPECT-based quantification allows determination of the gadolinium concentration, the MRI signal can be deconvoluted to get an understanding of the cellular location of the contrast agent. The cell experiments indicated a reduction of the relaxivity from 2.7 ± 0.1 m m(-1) s(-1) to a net relaxivity of 1.7 ± 0.3 m m(-1) s(-1) upon cellular uptake for RGD targeted liposomes by means of the contrast agent concentration as determined by SPECT. This is not observed for nontargeted liposomes that serve as controls. We show that receptor targeted liposomes in comparison to nontargeted liposomes are taken up into cells faster and into subcellular structures of different sizes. We suggest that the presented multimodal contrast agent provides a functional readout of its response to the biological environment and is furthermore applicable in in vivo measurements. As this approach can be extended to several MRI-based contrast mechanisms, we foresee a broader use of multimodal SPECT/MRI nanoparticles to serve as in vivo sensors in biological or medical research.Contrast Media & Molecular Imaging 01/2012; 7(1):68-75. · 2.87 Impact Factor
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ABSTRACT: To develop a molecular probe for MRI detection of human tumor telomerase reverse transcriptase (hTERT) mRNA expression. Uniformly phosphorothioate-modified hTERT antisense oligonucleotide (ASON) homing hTERT mRNA was labeled with gadolinium (Gd) through the bifunctional chelator 1,4,7, 10-tetraazacyclododecane-N, N', N'', N'''-tetraacetic acid (DOTA) stirred within 45 minutes at 60 °C. The Gd labeled probes were characterized in vitro. The cellular uptake rate and biodistribution of (99m) Tc-DOTA-ASON was measured instead of that of Gd-DOTA-ASON. A549 lung adenocarcinoma model was established in BALB/c nude mice and Gd-DOTA-ASON was injected intraperitoneally and MR images were acquired using 7.0T Micro-MRI (Bruker Biospec, Ettlingen, Germany) at different time points. Immunohistochemical analysis of telomerase activity of each xenograft was operated two days after in vivo imaging. The binding efficiency of Gd-DOTA-ASON reached as high as 71.7 ± 4.5% (n = 6). Gd-DOTA-ASON displayed perfect stability in fresh human serum at 37 °C for 24 h. Compared with normal lung cells, A549 cells showed an obviously higher uptake of (99m) Tc-DOTA-ASON than that of lung cells (10.5 ± 2.7% vs. 4.8 ± 2.6%, P < 0.05). The signal intensity of A549 xenografts can be enhanced by Gd-DOTA-ASON and the signal to noise ratio (SNR) of tumor to muscle reached 2.37 and maintained a relatively high level within 6 h after injection. The activity of hTERT in A549 tumors can be suppressed by Gd-DOTA-ASON in pathological slices. The results of this study show that Gd-DOTA-ASON can be a promising intracellular MR contrast probe for targeting telomerase-positive carcinomas.Cancer Science 04/2012; 103(8):1434-9. · 3.48 Impact Factor