Trimodal molecular imaging using a broad-spectrum metal attaching ultra-small quantum dots

Objectives: Development of nanomaterial-based imaging probes is challenging due to the vast non-specific accumulation in the mononuclear phagocyte system, such as liver and spleen. Ultra-small nanoparticles (USNPs) could possess small molecule-like in vivo pharmacokinetic profiles while maintaining the integrated functions capacity. To incorporate radioisotopes for nanoparticle labeling, metal chelators have been commonly utilized. However, the chelators would influence the surface properties and in vivo behaviors as well as attaching efficiency of other peptides/drugs. Herein, we report a novel chelator-free and post-synthetic strategy for broad-spectrum metal ion attaching, without involving complicated re-synthesis or relying on the active property of the metal ions. Materials and Methods: The ultra-small Ag₂Se@Mn quantum dots (QDs, < 3 nm) were prepared by quasi-biosynthesis with biocompatible surface. The Ag₂Se@Mn QDs were conjugated with octreotate (TATE) peptide to specifically target to the somatostatin receptor SSTR2-expressing neuroendocrine tumors (NETs). Fluorescence optical and MR imaging were performed both in vitro with NET cells (AR42J cells) and in vivo with NET bearing mice (AR42J mice). For radioisotopes labeling, the ⁶⁴Cu²⁺ or ⁸⁶Y³⁺ solution was added dropwise, and the mixture was slowly heated to 60 °C and kept for 60 min to ensure the completion of the reaction. Purification was performed via centrifugation with 10-kD centrifugal filter. In vivo PET imaging was performed with both AR42J (SSTR2⁺) and HCT116 (SSTR2^-) tumor xenografts.

Results: The developed ultra-small Ag₂Se QDs were covered with an active oxygen layer (O^2-) which may act as inherent oxygen donors. This oxygen coat allowed for facile incorporation of various metals, not only for active metal such as copper and manganese, but also for relatively inert metal such as yttrium. The Ag₂Se@Mn QDs showed high fluorescence quantum yield (14.0 %, using ICG as the reference) and superior longitudinal relaxivity (12.87 mM⁻¹s⁻¹), almost four times higher than that of the commercial contrast agent Gd-DTPA) for a good performance in both fluorescence and MR imaging. After conjugating with a SSTR2 targeting peptide, octreotate, the probe yielded a decent tumor to tissue ratio as high as ~9. Importantly, the Ag₂Se@Mn@Cu QDs could be excreted from body mainly through renal-urinary system within 12 h (Figure).

Conclusions: We reported a new chelator-free post-synthetic strategy to construct an ultra-small magnetically/radioactive engineered NIR Ag₂Se@Mn-Cu QDs. The sub 3 nm Ag₂Se QDs showed small-molecule in vivo pharmacokinetic profiles and it could be used as the host for the surface engineering with Mn²⁺ and Cu²⁺/Y³⁺ for the NIR, MR, and PET trimodal imaging.

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