RGD Peptide-Conjugated Multimodal NaGdF4:Yb3+/Er3+ Nanophosphors for Upconversion Luminescence, MR, and PET Imaging of Tumor Angiogenesis

Multimodal nanoparticles have been extensively studied for target-specific imaging and therapy of various diseases, including cancer. In this study, radiolabeled arginine-glycine-aspartic acid (RGD)-functionalized Er3+/Yb3+ co-doped NaGdF4 upconversion nanophosphors (UCNPs) were synthesized and evaluated as a multimodal PET/MR/optical probe with tumor angiogenesis-specific targeting properties. Methods: A dimeric cyclic RGDyk ((cRGDyk)(2)) peptide was conjugated to polyacrylic acid-coated NaGdF4:Yb3+/Er3+ UCNPs along with polyethylene glycol molecules and was consecutively radiolabeled with I-124. In vitro cytotoxicity testing was performed for 3 d. Upconversion luminescence imaging of (cRGDyk)(2)-UCNP was performed on U87MG cells with a laboratory-made confocal microscope. In vivo small-animal PET and clinical 3-T T1-weighted MR imaging of I-124-labeled RGD-functionalized UCNPs was acquired with or without blocking of cyclic RGD peptide in a U87MG tumor model. Inductively coupled plasma mass spectrometry and biologic transmission electron microscopy were done to evaluate gadolinium concentration and UCNP localization, respectively. Results: Polymer-coated UCNPs and dimeric RGD-conjugated UCNPs were monodispersely synthesized, and those of hydrodynamic size were 30 +/- 8 nm and 32 +/- 9 nm, respectively. (cRGDyk)(2)-UCNPs have a low cytotoxic effect on cells. Upconversion luminescence signals of (cRGDyk)(2)-UCNP were specifically localized on the surface of U87MG cells. I-124-c(RGDyk)(2)-UCNPs specifically accumulated in U87MG tumors (2.8 +/- 0.8 vs. 1.3 +/- 0.4 percentage injected dose per gram in the blocking experiment), and T1-weighted MR images showed significant positive contrast enhancement in U87MG tumors. Tumor localization of I-124-c(RGDyk)(2)-UCNPs was confirmed by inductively coupled plasma mass spectrometry and biologic transmission electron microscopy analysis. Conclusion: These results suggest that I-124-labeled RGD-functionalized UCNPs have high specificity for alpha(v)beta(3) integrin-expressing U87MG tumor cells and xenografted tumor models. Multimodal UCNPs can be used as a platform nanoparticle with multimodal imaging for cancer-specific diagnoses.