Dual-Function Probe for PET and Near-Infrared Fluorescence Imaging of Tumor Vasculature

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Abstract
To date, the in vivo imaging of quantum dots (QDs) has been mostly qualitative or semiquantitative. The development of a dual-function PET/near-infrared fluorescence (NIRF) probe can allow for accurate assessment of the pharmacokinetics and tumor-targeting efficacy of QDs. Methods: A QD with an amine-functionalized surface was modified with RGD peptides and 1,4,7,10-tetraazacyclodocecane-N,N′,N″,N‴-tetraacetic acid (DOTA) chelators for integrin αvβ3–targeted PET/NIRF imaging. A cell-binding assay and fluorescence cell staining were performed with U87MG human glioblastoma cells (integrin αvβ3–positive). PET/NIRF imaging, tissue homogenate fluorescence measurement, and immunofluorescence staining were performed with U87MG tumor–bearing mice to quantify the probe uptake in the tumor and major organs. Results: There are about 90 RGD peptides per QD particle, and DOTA–QD–RGD exhibited integrin αvβ3–specific binding in cell cultures. The U87MG tumor uptake of 64Cu-labeled DOTA–QD was less than 1 percentage injected dose per gram (%ID/g), significantly lower than that of 64Cu-labeled DOTA–QD–RGD (2.2 ± 0.3 [mean ± SD] and 4.0 ± 1.0 %ID/g at 5 and 18 h after injection, respectively; n = 3). Taking into account all measurements, the liver-, spleen-, and kidney-to-muscle ratios for 64Cu-labeled DOTA–QD–RGD were about 100:1, 40:1, and 1:1, respectively. On the basis of the PET results, the U87MG tumor-to-muscle ratios for DOTA–QD–RGD and DOTA–QD were about 4:1 and 1:1, respectively. Excellent linear correlation was obtained between the results measured by in vivo PET imaging and those measured by ex vivo NIRF imaging and tissue homogenate fluorescence (r2 = 0.93). Histologic examination revealed that DOTA–QD–RGD targets primarily the tumor vasculature through an RGD–integrin αvβ3 interaction, with little extravasation. Conclusion: We quantitatively evaluated the tumor-targeting efficacy of a dual-function QD-based probe with PET and NIRF imaging. This dual-function probe has significantly reduced potential toxicity and overcomes the tissue penetration limitation of optical imaging, allowing for quantitative targeted imaging in deep tissue.

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