Abstract
The multimodal strategy incorporating T1-weighted magnetic resonance imaging (MRI) and near-infrared (NIR) fluorescence imaging can complement their strengths to provide images with high sensitivity and spatial resolution for noninvasively and dynamically monitoring endothelial progenitor cells (EPCs) in potential EPC-dominated therapies. Here we report the development of a protein-based imaging probe, bCD-PLL-Cy5.5 Conjugate 1, in which the bacterial cytosine deaminase (bCD) protein was modified with poly-l-lysine (PLL) that is labeled with imaging reporters, including T1-weighted MRI contrast chelator and NIR fluorophore. Conjugate 1 showed low cytotoxicity in EPCs isolated from the rabbit peripheral blood. The normalized cell viability was maintained above 90% after incubation for 1 to 5 days. Fluorescence microscopy of live cells indicated rapid cellular uptake of Conjugate 1 into EPCs in 15 minutes, and flow cytometry studies demonstrated the time-dependent internalization of Conjugate 1 with maximum uptake 48 hours after the treatment. MRI of phantoms demonstrated significant reduction of the T1 value of the EPC pellet that was pretreated with 2 μM of Conjugate 1 for 24 hours. Our preliminary data suggest that as a multimodal imaging contrast medium, Conjugate 1 offers a promising imaging probe for tracking the delivery and therapeutic response of EPCs in vivo.
Highlights
W ITH ITS NONINVASIVE CHARACTERISTICS and exquisite spatial resolution, magnetic resonance imaging (MRI) is one of the most powerful techniques available for imaging diagnosis, and preclinical results can be relatively translated to clinical applications.[1]
After interventional procedures for stem cell delivery, the signal loss induced by iron oxide nanoparticle–based contrast agents may be difficult to distinguish from the susceptibility artifacts from air or postsurgical iron or hemosiderin deposition.[6]
Despite having lower detection sensitivity compared to iron oxide nanoparticle–based contrast agents, T1 contrast agents are advantageous for enabling a positive contrast on T1-weighted images, which is less likely to be confused with artifacts owing to metal- or air-induced postoperative local signal voids
Summary
W ITH ITS NONINVASIVE CHARACTERISTICS and exquisite spatial resolution, magnetic resonance imaging (MRI) is one of the most powerful techniques available for imaging diagnosis, and preclinical results can be relatively translated to clinical applications.[1]. Iron oxide nanoparticle–based MRI contrast agents effectively generate a decreased signal on T2-weighted images. A considerable disadvantage of this T2 effect is that magnetic field inhomogeneities can be mistaken for a contrast agent effect. These contrast agents are metabolized intracellularly, leading to a slowly decreasing contrast effect over time[4] and possible toxicity owing to accumulation of nonphysiologic intracellular iron.[5] Especially after interventional procedures for stem cell delivery, the signal loss induced by iron oxide nanoparticle–based contrast agents may be difficult to distinguish from the susceptibility artifacts from air or postsurgical iron or hemosiderin deposition.[6]. Gadolinium (Gd)-based contrast agents such as gadolinium-tetraazacyclododecanetetraacetic acid (GdDOTA) and gadophrin-2 have been successfully applied in previous cell labeling and in vivo cell-tracking studies.[7,8]
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