RGD peptide-conjugated multimodal NaGdF₄:Yb³⁺/Er ³⁺ 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 Er ³⁺/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 ¹²⁴I. 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 ¹²⁴I-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. ¹²⁴I- 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 ¹²⁴I- c(RGDyk)₂- UCNPs was confirmed by inductively coupled plasma mass spectrometry and biologic transmission electron microscopy analysis. Conclusion: These results suggest that ¹²⁴I-labeled RGD-functionalized UCNPs have high specificity for α_vβ₃integrin-expressing U87MG tumor cells and xenografted tumor models. Multimodal UCNPs can be used as a platform nanoparticle with multimodal imaging for cancer-specific diagnoses. COPYRIGHT