Quantitative determination of the biodistribution of nanoparticles: could radiolabeling be the answer?

Radiolabelling and in vivo radionuclide imaging tracking of emerging pollutants in environmental toxicology: A review

Emerging pollutants (EPs) have become a global concern, attracting tremendous attention because of serious threats to human and animal health. EP diversity emanates from their behaviour and ability to enter the body via multiple pathways and exhibit completely different distribution, transport, and excretion. To better understand the in vivo behaviour of EPs, we reviewed radiolabelling and in vivo radionuclide imaging tracking of various EPs, including micro- and nano-plastics, perfluoroalkyl substances, metal oxides, pharmaceutical and personal care products, and so on. Because this accurate and quantitative imaging approach requires the labelling of radionuclides onto EPs, the main strategies for radiolabelling were reviewed, such as synthesis with radioactive precursors, element exchange, proton beam activation, and modification. Spatial and temporal biodistribution of various EPs was summarised in a heat map, revealing that the absorption, transport, and excretion of EPs are markedly related to their type, size, and pathway into the body. These findings implicate the potential toxicity of diverse EPs in organs and tissues. Finally, we discussed the potential and challenges of radionuclide imaging tracking of EPs, which can be considered in future EPs studies.

Nanoparticles for Cancer Diagnosis, Radionuclide Therapy and Theranostics

Nanoparticles have unique properties that can be exploited for cancer diagnosis and therapy. Intravenously injected nanoparticles accumulate predominantly in organs of the mononuclear phagocytic system, in addition to localizing in tumors and at sites of inflammation and infection. Accumulation in the liver and spleen lowers nanoparticles' ability to target pathological sites and compromises their use for radionuclide therapy. As described by Lee et al. in this issue of ACS Nano, radionuclide retention in liver and spleen can be greatly reduced by using liposomes that are surface-modified with esterase-cleavable radionuclide anchors. Because esterase activity is high in healthy tissues and low in tumors, the authors found that liposome-associated radioactivity rapidly cleared from the body and remained high only in tumors. The resulting images had high contrast-to-background ratios and remarkable tumor delineation. In this Perspective, we discuss these advances from early detection, cancer diagnosis, radionuclide therapy, and theranostics points of view. We outline the current clinical landscape of radionuclide targeting, imaging and therapy, and reflect on the roles that nanoparticles can play in these applications. We highlight the potential of nanoparticles that are responsive to endogenous stimuli for intraoperative imaging and, particularly, for individualized and improved radionuclide treatment. Taking these advances into account, future studies exploring the robustness and the clinical feasibility of nanomedicine-based radiotheranostic probes are eagerly awaited.

Medical Imaging of Microrobots: Toward In Vivo Applications

Medical microrobots (MRs) have been demonstrated for a variety of non-invasive biomedical applications, such as tissue engineering, drug delivery, and assisted fertilization, among others. However, most of these demonstrations have been carried out in in vitro settings and under optical microscopy, being significantly different from the clinical practice. Thus, medical imaging techniques are required for localizing and tracking such tiny therapeutic machines when used in medical-relevant applications. This review aims at analyzing the state of the art of microrobots imaging by critically discussing the potentialities and limitations of the techniques employed in this field. Moreover, the physics and the working principle behind each analyzed imaging strategy, the spatiotemporal resolution, and the penetration depth are thoroughly discussed. The paper deals with the suitability of each imaging technique for tracking single or swarms of MRs and discusses the scenarios where contrast or imaging agent's inclusion is required, either to absorb, emit, or reflect a determined physical signal detected by an external system. Finally, the review highlights the existing challenges and perspective solutions which could be promising for future in vivo applications.

Multimodal Nanocarrier Probes Reveal Superior Biodistribution Quantification by Isotopic Analysis over Fluorescence

Absolute measurements of biodistribution are essential for understanding and optimizing the function of nanomaterials for in vivo diagnostic and therapeutic applications. Biodistribution analysis by optical imaging is desirable due to its low cost, wide accessibility, and high-throughput nature, but it is substantially less accurate than isotopic and chemical techniques. In this work, we developed multimodal probes for optical and nuclear imaging to analyze the quantitative limits of optical contrast in the red and near-infrared spectra for polysaccharide nanocarriers targeting macrophage cells. Probes incorporating three zwitterionic fluorophores together with radioactive copper distributed diffusely to optically dissimilar tissues that were either white (visceral adipose tissue) or dark red (liver and spleen) in obese rodents. We used in vivo positron emission tomography/computed tomography (PET/CT) imaging, in vivo hyperspectral tomographic fluorescence imaging, and ex vivo optical and isotopic analyses to determine correlations between optical and nuclear signals. PET imaging strongly correlated with standardized ex vivo methods for all tissue types, whereas no fluorescence signals exhibited substantial accuracy in quantification or localization in vivo. Optical imaging of resected tissues was most accurate in the 700 nm wavelength window, but only in white tissues. This work suggests that fluorescence can be used to measure diffuse probe distribution in white tissues over time or across animals, but not red tissues and not deep in the body. This work also highlights the importance of choosing validated experimental protocols and describes how optical measurements are impacted by fluorophore class and spectral properties, tissue properties, and imaging workflow.

Gold Nanoparticles as Boron Carriers for Boron Neutron Capture Therapy: Synthesis, Radiolabelling and In vivo Evaluation

Background: Boron Neutron Capture Therapy (BNCT) is a binary approach to cancer therapy that requires accumulation of boron atoms preferentially in tumour cells. This can be achieved by using nanoparticles as boron carriers and taking advantage of the enhanced permeability and retention (EPR) effect. Here, we present the preparation and characterization of size and shape-tuned gold NPs (AuNPs) stabilised with polyethylene glycol (PEG) and functionalized with the boron-rich anion cobalt bis(dicarbollide), commonly known as COSAN. The resulting NPs were radiolabelled with 124I both at the core and the shell, and were evaluated in vivo in a mouse model of human fibrosarcoma (HT1080 cells) using positron emission tomography (PET). Methods: The thiolated COSAN derivatives for subsequent attachment to the gold surface were synthesized by reaction of COSAN with tetrahydropyran (THP) followed by ring opening using potassium thioacetate (KSAc). Iodination on one of the boron atoms of the cluster was also carried out to enable subsequent radiolabelling of the boron cage. AuNPs grafted with mPEG-SH (5 Kda) and thiolated COSAN were prepared by ligand displacement. Radiolabelling was carried out both at the shell (isotopic exchange) and at the core (anionic absorption) of the NPs using 124I to enable PET imaging. Results: Stable gold nanoparticles simultaneously functionalised with PEG and COSAN (PEG-AuNPs@[4]-) with hydrodynamic diameter of 37.8 ± 0.5 nm, core diameter of 19.2 ± 1.4 nm and ξ-potential of -18.0 ± 0.7 mV were obtained. The presence of the COSAN on the surface of the NPs was confirmed by Raman Spectroscopy and UV-Vis spectrophotometry. PEG-AuNPs@[4]- could be efficiently labelled with 124I both at the core and the shell. Biodistribution studies in a xenograft mouse model of human fibrosarcoma showed major accumulation in liver, lungs and spleen, and poor accumulation in the tumour. The dual labelling approach confirmed the in vivo stability of the PEG-AuNPs@[4]-. Conclusions: PEG stabilized, COSAN-functionalised AuNPs could be synthesized, radiolabelled and evaluated in vivo using PET. The low tumour accumulation in the animal model assayed points to the need of tuning the size and geometry of the gold core for future studies.

Modification of the glycosylation of extracellular vesicles alters their biodistribution in mice

Extracellular vesicles (EVs) are considered sophisticated vehicles for cell-to-cell communication, thanks to the possibility of handling a variable cargo in a shell with multiple types of decoders. Surface glycosylation of EVs is a method that could be used to control their interaction with different cells and, consequently, the biodistribution of the vesicles in the body. Herein, we produced EVs derived from mouse liver proliferative cells, and we treated them with neuraminidase, an enzyme that digests the terminal sialic acid residues from glycoproteins. Afterwards, we labeled the EVs directly with [124I]Na and injected them in mice intravenously or into the hock. The amount of radioactivity in major organs was measured at different time points after administration both in vivo using positron emission tomography and ex vivo (after animal sacrifice) using dissection and gamma counting. The results showed that intravenous injection leads to the rapid accumulation of EVs in the liver. Moreover, after some hours the distribution led to the presence of EVs in different organs including the brain. Glycosidase treatment induced an accumulation in the lungs, compared with the intact EVs. Furthermore, when the EVs were injected through the hock, the neuraminidase-treated vesicles distributed better at the axillary lymph nodes than the untreated EVs. This result shows that modification of the glycosylated complexes on the EV surface can affect the distribution of these vesicles, and specifically removing the sialic acid residues allows more EVs to reach and accumulate at the lungs.

Medical Imaging for the Tracking of Micromotors

Micro/nanomotors are useful tools for several biomedical applications, including targeted drug delivery and minimally invasive microsurgeries. However, major challenges such as in vivo imaging need to be addressed before they can be safely applied on a living body. Here, we show that positron emission tomography (PET), a molecular imaging technique widely used in medical imaging, can also be used to track a large population of tubular Au/PEDOT/Pt micromotors. Chemisorption of an iodine isotope onto the micromotor's Au surface rendered them detectable by PET, and we could track their movements in a tubular phantom over time frames of up to 15 min. In a second set of experiments, micromotors and the bubbles released during self-propulsion were optically tracked by video imaging and bright-field microscopy. The results from direct optical tracking agreed with those from PET tracking, demonstrating that PET is a suitable technique for the imaging of large populations of active micromotors in opaque environments, thus opening opportunities for the use of this mature imaging technology for the in vivo localization of artificial swimmers.

Translational radionanomedicine: a clinical perspective

Many nanomaterials were developed for the anticipated in vivo theranostic use exploiting their unique characteristics as a multifunctional platform. Nevertheless, only a few nanomaterials are under investigation for human use, most of which have not entered clinical trials yet. Radionanomedicine, a convergent discipline of radiotracer technology and use of nanomaterials in vivo, can facilitate clinical nanomedicine because of its advantages of radionuclide imaging and internal radiation therapy. In this review, we focuse on how radionanomedicine would impact profoundly on clinical translation of nanomaterial theranostics. Up-to-date advances and future challenges are critically reviewed regarding the issues of how to radiolabel and engineer radionanomaterials, in vivo behavior tracing of radionanomaterials and then the desired clinical radiation dosimetry. Radiolabeled extracellular vesicles were further discussed as endogenous nanomaterials radiolabeled for possible clinical use.

7Be-recoil radiolabelling of industrially manufactured silica nanoparticles

Radiolabelling of industrially manufactured nanoparticles is useful for nanoparticle dosimetry in biodistribution or cellular uptake studies for hazard and risk assessment. Ideally for such purposes, any chemical processing post production should be avoided as it may change the physico-chemical characteristics of the industrially manufactured species. In many cases, proton irradiation of nanoparticles allows radiolabelling by transmutation of a tiny fraction of their constituent atoms into radionuclides. However, not all types of nanoparticles offer nuclear reactions leading to radionuclides with adequate radiotracer properties. We describe here a process whereby in such cases nanoparticles can be labelled with ⁷Be, which exhibits a physical half-life of 53.29 days and emits γ-rays of 478 keV energy, and is suitable for most radiotracer studies. ⁷Be is produced via the proton-induced nuclear reaction ⁷Li(p,n)⁷Be in a fine-grained lithium compound with which the nanoparticles are mixed. The high recoil energy of ⁷Be atoms gives them a range that allows the ⁷Be-recoils to be transferred from the lithium compound into the nanoparticles by recoil implantation. The nanoparticles can be recovered from the mixture by dissolving the lithium compound and subsequent filtration or centrifugation. The method has been applied to radiolabel industrially manufactured SiO₂ nanoparticles. The process can be controlled in such a way that no alterations of the ⁷Be-labelled nanoparticles are detectable by dynamic light scattering, X-ray diffraction and electron microscopy. Moreover, cyclotrons with maximum proton energies of 17-18 MeV that are available in most medical research centres could be used for this purpose.