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| Chemical name: | Eu-1,7-Bis(2-(methylene benzyloxy ether)-acetic acid) acetamide-4,10-bis(acetamidoacetic acid)-1,4,7,10- tetraazacyclododecane |  |
| Abbreviated name: | Eu-DOTA-OBZ2-Gly2, Eu-DOTAM-OBZ2-Gly2 | |
| Synonym: | ||
| Agent category: | Compound | |
| Target: | Non-targeted | |
| Target category: | Non-targeted | |
| Method of detection: | PARACEST magnetic resonance imaging (MRI) | |
| Source of signal\contrast: | Eu3+ | |
| Activation: | No | |
| Studies: |
| Click on the above structure for additional information in PubChem. |
[PubMed]
Chemical Exchange Saturation Transfer (CEST) is a novel magnetic resonance imaging (MRI) contrast mechanism (1) that is an attractive alternative to T1 and T2 contrast mechanisms, particularly at high magnetic fields (2). CEST agents possess a hydrogen proton with a moderate to slow exchange rate with water. Selective saturation of the MR frequency of this proton, followed by exchange with solvent water, reduces the MR signal of the water. PARACEST (PARAmagnetic CEST) agents include a paramagnetic lanthanide ion that shifts the MR frequencies of the exchangeable proton to unique values to facilitate selective detection (3, 4). Endogenous MR contrast may be continually monitored in the presence of PARACEST agents by neglecting to saturate the MR frequency of the exchangeable proton (and assuming that the T1 relaxation of the PARACEST agent is negligible).
The selective saturation is typically applied for two or more seconds to generate a steady-state of saturation (5), which greatly lengthens the time required for in vivo detection of PARACEST MRI contrast agents. However, computer simulations and studies with chemical solutions have shown that more rapid MRI acquisition schemes can be prepended with a selective saturation pulse to accelerate the detection of PARACEST MRI contrast agents (6). The choice of the MRI acquisition scheme is dependent on the T1 relaxation time of the endogenous tissue in the presence of the PARACEST agent. In addition, a second "control" MR image is typically acquired to account for the direct saturation of water by applying selective saturation at a MR frequency with an opposite sign relative to the saturation frequency of the first MR image (7). The need for a second MR image further lengthens the in vivo detection of PARACEST agents. However, a single control MR image can be acquired before injecting the PARACEST agent, which obviates the need to acquire control MR images after injection of the agent and accelerates the temporal detection of PARACEST agents after injection (6). Although dynamic changes in MR image contrast may be caused by faster T2* relaxation or magnetic susceptibilities caused by the agent, these effects are estimated to be much lower than the CEST effect, so that the dynamic changes in MR image contrast are attributed to CEST effect of the agent. These principles were used to detect the accumulation of a PARACEST agent, Eu-1,7-Bis(2-(methylene benzyloxy ether)-acetic acid) acetamide-4,10-bis(acetamidoacetic acid)-1,4,7,10-tetraazacyclododecane (Eu-DOTA-OBZ2-Gly2), within the liver tissue of a Balb/C mouse (6). The incorporation of two benzyloxyether moieties in the contrast agent promoted uptake of this agent in liver tissue, which also facilitated in the in vivo detection of the agent.
[PubMed]
DOTA-OBZ2-Gly2 was synthesized using previously published methods (8, 9). Two trans-amines of cyclen (1,4,7,10-tetraazacyclododecane) were selectively protected with benzylchloroformate, and the remaining amines were conjugated with bromo-tert-butoxyacetate followed by removal of the protecting groups with catalytic hydrogenation to form tBu-DO2A (1,7-(tert-butyloxyacetate)-1,4,7,10-tetraazacyclododecane) (6). 2-(2-Bromoacetamido)-3-(benzyloxy)propanoic acid was prepared from bromoacetyl bromide and 2-amino-3-(benzyloxy)propanoic acid. The acid moiety was subsequently protected by treatment with tert-butyl trichloroacetimidate to form tert-Butyl 2-(2-bromoacetamido)-3-benzyloxy)propanoate. This product was conjuated to tBu-DO2A, followed by deprotection of the tert-Butyl protecting groups to form DOTA-OBZ2-Gly2. NMR spectroscopy was used to identify the purified product.
To chelate Eu(III) with this product, an aqueous EuCl3 solution (0.21 M) was added to a solution of DOTA-OBZ2-Gly2 (0.13 mmol) in 50:50 water/methanol (15 mL, pH 6.0) (6). The resulting solution was stirred for 3 days at 60 Cand the pH was maintained between 6 and 7 throughout by addition of 1 N KOH. The absence of free Eu(III) ion was verified by colorimetric assay using xylenol orange (1 M AcONa/AcOH buffer, pH 5.3). The reaction mixture was allowed to cool to room temperature and the solvents were removed by freeze-drying. NMR spectroscopy was used to identify the purified product.
[PubMed]
A continuous series of CEST MR images were acquired with selective saturation applied at +54 ppm, which corresponded to the MR chemical shift of the amide protons of Eu-DOTA-OBZ2-Gly2 (6). The total acquisition time for one presat-Fast Low Angle Shot (FLASH) image was 76.8 s. A solution of 800 mM Eu-DOTA-OBZ2-Gly2 in 100 μL injection volume (3.2 mmol/kg) was infused within 30 s immediately after the third PARACEST MR image was acquired. After infusing the agent, the CEST MR images of the liver became increasingly darker relative to the CEST MR images of the liver prior to infusion. The CEST contrast was quantified at each time point after infusion by comparing the darker CEST MR signal at a particular time point with the brighter average MR signal amplitude during the first 3 CEST MR images acquired before infusion. This procedure avoided an interruption in continuous selective saturation at a single MR frequency. A single mouse was used for this preliminary study. The presat-FLASH method was successfully applied to this Dynamic Contrast Effect (DCE) CEST MRI study. The T1 relaxation time of the liver tissue was measured to be 1.27 s, which was not sufficiently long for a preset-Rapid Acquisition with Relaxation Enhancement (RARE) MRI acquisition sequence, which necessitated the use of the presat-FLASH sequence. To achieve a 99% probability that the contrast before and after injection of the agent was different, the contrast-to-noise (CNR) must reach 3√2 (10). The 99% CNR probability threshold was achieved 47.36 min after injection of the agent, when a 22.09% change in contrast was observed. This change in contrast was attributed to the dynamic increase in the concentration of the agent in the in vivo tumor tissue. Although this dynamic change in contrast may be partly due to a dynamic change in the T2 relaxation rate of water or a dynamic change in magnetic susceptibilities caused by the agent, these effects are estimated to be insufficient to account for this change in image contrast. Therefore the change in image contrast was attributed to the CEST effect of Eu-DOTA-OBZ2-Gly2.
R24 CA110943.
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