Polyphosphoric acid capping radioactive/upconverting NaLuF₄:Yb,Tm,¹⁵³Sm nanoparticles for blood pool imaging in vivo

Abstract

Nanoparticles that circulate in the bloodstream for a prolonged period of time have important biomedicine applications. However, no example of lanthanide-based nanoparticles having a long-term circulation bloodstream has been reported to date. Herein, we report on difunctional radioactive and upconversion nanoparticles (UCNP) coated with polyphosphoric acid ligand, that is ethylenediamine tetramethylenephosphonic acid (EDTMP), for an application in single-photon emission computed tomography (SPECT) blood pool imaging. The structure, size and zeta-potential of the EDTMP-coated nanoparticles (EDTMP-UCNP) are verified using transmission electron microscopy and dynamic light scattering. Injection of radioisotope samarium-153-labeled EDTMP-UCNP (EDTMP-UCNP:¹⁵³Sm) into mice reveal superior circulation time compared to control nanoparticles coated with citric acid (cit-UCNP:¹⁵³Sm) and ¹⁵³Sm complex of EDTMP (EDTMP-¹⁵³Sm). The mechanism for the extended circulation time may be attributed to the adhesion of EDTMP-UCNP on the membrane of red blood cells (RBCs). In vivo toxicity results show no toxicity of EDTMP-UCNP at the dose of 100 mg/kg, validating its safety as an agent for blood pool imaging. Our results provide a new strategy of nanoprobe for a long-term circulation bloodstream by introducing polyphosphoric acid as surface ligand.

Introduction

Lanthanide elements with unique 4f electron structures show rich optical and radioactive properties. Due to the similar ion radius, different lanthanide ions with physicochemical properties can be doped into one nanoparticle to fabricate a multifunctional system [1], [2], [3], [4], [5]. In particular, some nanoparticles doped with a sensitizer of Yb³⁺ and an activator of Er³⁺ (Tm³⁺, Ho³⁺, etc.) show a unique upconversion luminescence (UCL) process where low-energy light is converted to higher-energy one through sequent multiple photon absorptions or energy transfers [6], [7], [8], [9], [10], [11], [12], with special optical characteristics, such as sharp emission lines, large anti-Stokes shifts of several hundred nanometers, and the absence of autofluorescence of biosamples [13], [14], [15], [16]. As a result, lanthanide-based upconversion nanoparticles (UCNP) have been successfully used in bioimaging in vivo [17], [18], [19]. To overcome the shortcoming of UCNP as luminescent probes for in vivo imaging, that is, a deficiency of penetration depth in biological samples, UCNP have recently been developed into multifunctional nanoparticles with UCL, magnetic, and radioactive properties that could be used for multimodal imaging, including magnetic resonance imaging (MRI), X-ray computed tomography (CT) and nuclear imaging [20], [21].

Extending the circulation half-life of nanoparticles in blood is crucial for bio-applications, especially for drug delivery, tumor targeting and blood pool imaging [22], [23], [24], [25]. For UCNP, only two examples have been reported to display an extended circulation half-life in blood [26], [27]. Liu et, al. reported the Gd₂O₃:Yb³⁺, Er³⁺ upconversion nanoprobes as high-performance contrast agents for multi-modality imaging. Cao et, al. reported ¹⁵³Sm³⁺-doping NaYF₄:20%Yb, 1%Er upconversion nanoparticles with a polyethylene glycol (PEG) coating exhibited a prolonged blood retention time. It should be noted that the enhanced colloidal stability of UCNPs in the aforementioned two cases is all based on the surface modification of PEG chains, the sole strategy of introducing surface ligand containing PEG moiety limits the further application of UCNPs in blood pool imaging.

In the present study, we have developed ethylenediamine tetramethylenephosphonic acid (EDTMP) capped, Yb³⁺, Tm³⁺, and ¹⁵³Sm³⁺ co-doped NaLuF₄ nanoparticles to integrate UCL properties and radioactivity into one nanoparticle. Our design strategy is introducing a polyphosphonic acid as surface ligand to provide a large negative charge and doping radioactive isotope ¹⁵³Sm with a relatively long halftime of 1.99 days to achieve high sensitive nuclear imaging. Considering the relative slow metabolization process of nanoparticles, the long-term observation of radioactive nanoparticles within animals is beneficial. Therefore, ¹⁵³Sm-labeling becomes a good choice. Furthermore, ¹⁵³Sm³⁺ has a similar atomic radius and chemical character to the other lanthanide ions Ln³⁺, and hence it can be doped into NaLuF₄ nanocrystals easily. Meanwhile, polyphosphoric acid EDTMP [28] can also chelate with lanthanide cations [29], [30]. Moreover, the ¹⁵³Sm complex with an EDTMP ligand is a well-known commercial agent for single-photon emission computed tomography (SPECT) imaging. Therefore, it is reasoned that EDTMP can coordinate on the surface of UCNP and improve its water-solubility. Herein, the as-prepared EDTMP capped, ¹⁵³Sm-dopped UCNP EDTMP-NaLuF₄:Yb,Tm,¹⁵³Sm (denoted as EDTMP-UCNP:¹⁵³Sm) were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), dynamic light scattering (DLS) and spectroscopy techniques. Furthermore, the EDTMP-UCNP:¹⁵³Sm was developed for UCL and SPECT dual-modality blood pool application (Scheme 1). In addition, the toxicity of UCNP as blood pool imaging agent was also tested.