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Characterization and labeling of exosomes. (A) Western blot analysis of common exosome markers (CD9, CD63, and Alix) and cell marker (calnexin); (B) The size distribution of the 4T1-derived exosome population according to labeling steps; (C,D) Representative transmission electron microscopy (TEM) images and zeta potential of exosomes according to labeling steps; (E) Fluorescence imaging of Cy7-labeled exosomes compared to the control exosomes; (F) Serum stability test of exosome-64 Cu until 36 h (n = 3).
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There has been considerable interest in the clinical use of exosomes as delivery vehicles for treatments as well as for promising diagnostic biomarkers, but the physiological distribution of exosomes must be further elucidated to validate their efficacy and safety. Here, we aimed to develop novel methods to monitor exosome biodistribution in vivo u...
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... proteins of exosomes such as CD9, CD63, and Alix were more highly expressed in purified exosomes than in cells, while cellular protein markers such as calnexin were more highly expressed in cells, as expected ( Figure 2A). We confirmed the presence of extracellular vesicles of the appropriate size (about 100 nm) using dynamic light scattering and exosome morphology through transmission electron microscopy (TEM) ( Figure 2B,C). ...
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... proteins of exosomes such as CD9, CD63, and Alix were more highly expressed in purified exosomes than in cells, while cellular protein markers such as calnexin were more highly expressed in cells, as expected ( Figure 2A). We confirmed the presence of extracellular vesicles of the appropriate size (about 100 nm) using dynamic light scattering and exosome morphology through transmission electron microscopy (TEM) ( Figure 2B,C). expressed in cells, as expected ( Figure 2A). ...
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... confirmed the presence of extracellular vesicles of the appropriate size (about 100 nm) using dynamic light scattering and exosome morphology through transmission electron microscopy (TEM) ( Figure 2B,C). expressed in cells, as expected ( Figure 2A). We confirmed the presence of extracellular vesicles of the appropriate size (about 100 nm) using dynamic light scattering and exosome morphology through transmission electron microscopy (TEM) ( Figure 2B As shown in Figure 1, our labeling strategy is based on the use of SCN-NOTA as a bifunctional chelator of radioisotopes such as 64 Cu and 68 Ga. ...
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... in cells, as expected ( Figure 2A). We confirmed the presence of extracellular vesicles of the appropriate size (about 100 nm) using dynamic light scattering and exosome morphology through transmission electron microscopy (TEM) ( Figure 2B As shown in Figure 1, our labeling strategy is based on the use of SCN-NOTA as a bifunctional chelator of radioisotopes such as 64 Cu and 68 Ga. For optical imaging, we also labeled with Cy7, a near- As shown in Figure 1, our labeling strategy is based on the use of SCN-NOTA as a bifunctional chelator of radioisotopes such as 64 Cu and 68 Ga. ...
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... optical imaging, we also labeled with Cy7, a near-infrared fluorescence dye. Cy7-labeled exosomes exhibited clear fluorescence signals ( Figure 2E). Thin layer chromatography, commonly used to confirm the serum stability of a radiolabeled tracer, demonstrated that after radioisotope labeling the 64 Cu/ 68 Ga-labeled exosomes (Exo-NOTA-64 Cu/ 68 Ga) demonstrated a labeling purity of approximately 98% after the removal of free radioisotopes by ExoQuick TM ( Figure S1). ...
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... size, polydispersion index (PDI), and zeta potential according to sequential synthesis steps are summarized in Table S1. The stability of radiolabeled exosomes was tested within serum; 64 Cu-labeled exosomes (Exo-NOTA-64 Cu) in serum were stable until 36 h after labeling ( Figure 2F). ...
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... images at 24 h after lymphatic injection of exosomes showed that radiolabeled exosomes (Exo-NOTA-64 Cu) showed greater uptake in lymph nodes than those of NOTA-64 Cu ( Figure 3A) or Free-64 Cu ( Figure S2A). In whole-body PET images, no significant uptake in other organs was observed ( Figure S2D,E). ...
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... images at 24 h after lymphatic injection of exosomes showed that radiolabeled exosomes (Exo-NOTA-64 Cu) showed greater uptake in lymph nodes than those of NOTA-64 Cu ( Figure 3A) or Free-64 Cu ( Figure S2A). In whole-body PET images, no significant uptake in other organs was observed ( Figure S2D,E). Optical images showed that Cy7 signals from exosomes were detected only in the brachial lymph node and at the injection site ( Figure 3A), whereas PET images could clearly visualize the localization of exosomes in the axillary as well as the brachial lymph nodes with higher sensitivity. ...
Citations
... The previously reported Pierce iodination method has been used and modified for radioiodination labeling [42][43][44]. Around 3 mCi of 131 I solution was added directly to the Iodogen-coated tube or four Iodogen beads in the test tube containing 0.1 mL of PBS. Then, the tube was gently swirled at room temperature for 5 min. ...
Mesenchymal stem/stromal cells (MSCs) are an extensively studied cell type in clinical trials due to their easy availability, substantial ex vivo proliferative capacity, and therapeutic efficacy in numerous pre-clinical animal models of disease. The prevailing understanding suggests that their therapeutic impact is mediated by the secretion of exosomes. Notably, MSC exosomes present several advantages over MSCs as therapeutic agents, due to their non-living nature and smaller size. However, despite their promising therapeutic potential, the clinical translation of MSC exosomes is hindered by an incomplete understanding of their biodistribution after administration. A primary obstacle to this lies in the lack of robust labels that are highly sensitive, capable of directly and easily tagging exosomes with minimal non-specific labeling artifacts, and sensitive traceability with minimal background noise. One potential candidate to address this issue is radioactive iodine. Protocols for iodinating exosomes and tracking radioactive iodine in live imaging are well-established, and their application in determining the biodistribution of exosomes has been reported. Nevertheless, the effects of iodination on the structural or functional activities of exosomes have never been thoroughly examined. In this study, we investigate these effects and report that these iodination methods abrogate CD73 enzymatic activity on MSC exosomes. Consequently, the biodistribution of iodinated exosomes may reflect the biodistribution of denatured exosomes rather than functionally intact ones.
... Salmonella enterica serotype typhi, a bacterium responsible for causing typhoid fever, infects humans when it is ingested in contaminated food and water and survives the acidic conditions in the stomach [105,106]. After entering the lymphoid tissue, infection occurs through lymphatic and hematogenous routes [107]. Typhoid emerges as a fever in the first week that progressively exacerbates to abdominal pain, constipation, and macular rashes in the second week, and to liver and spleen enlargement, ileocecal perforation, peritonitis, and septic shock in week 3. Death might result if the infection remains untreated [108,109]. ...
The clinical use of antibiotics has led to the emergence of multidrug-resistant (MDR) bacteria, leading to the current antibiotic resistance crisis. To address this issue, next-generation vaccines are being developed to prevent antimicrobial resistance caused by MDR bacteria. Traditional vaccine platforms, such as inactivated vaccines (IVs) and live attenuated vaccines (LAVs), were effective in preventing bacterial infections. However, they have shown reduced efficacy against emerging antibiotic-resistant bacteria, including MDR M. tuberculosis. Additionally, the large-scale production of LAVs and IVs requires the growth of live pathogenic microorganisms. A more promising approach for the accelerated development of vaccines against antibiotic-resistant bacteria involves the use of in silico immunoinformatics techniques and reverse vaccinology. The bioinformatics approach can identify highly conserved antigenic targets capable of providing broader protection against emerging drug-resistant bacteria. Multi-epitope vaccines, such as recombinant protein-, DNA-, or mRNA-based vaccines, which incorporate several antigenic targets, offer the potential for accelerated development timelines. This review evaluates the potential of next-generation vaccine development based on the reverse vaccinology approach and highlights the development of safe and immunogenic vaccines through relevant examples from successful preclinical and clinical studies.
... Given that near-infrared fluorescence (NIRF) imaging offers the advantages of realtime and high-resolution, multimodality SPECT-NIRF imaging provides more accurate spatial positioning and 3D information for detecting small lesions [181]. Thus, the modified EVs could be used as carriers for multimodal imaging, which could open up a new treatment avenue in precision medicine [176,182]. ...
Extracellular vesicles (EVs) have emerged as a promising platform for gene delivery owing to their natural properties and phenomenal functions, being able to circumvent the significant challenges associated with toxicity, problematic biocompatibility, and immunogenicity of the standard approaches. These features are of particularly interest for targeted delivery of the emerging clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) systems. However, the current efficiency of EV-meditated transport of CRISPR/Cas components remains insufficient due to numerous exogenous and endogenous barriers. Here, we comprehensively reviewed the current status of EV-based CRISPR/Cas delivery systems. In particular, we explored various strategies and methodologies available to potentially improve the loading capacity, safety, stability, targeting, and tracking for EV-based CRISPR/Cas system delivery. Additionally, we hypothesise the future avenues for the development of EV-based delivery systems that could pave the way for novel clinically valuable gene delivery approaches, and may potentially bridge the gap between gene editing technologies and the laboratory/clinical application of gene therapies.
Graphical Abstract
... Jung et al., developed a new multimodal method to monitor the biodistribution of EVs in vivo. EVs extracted from tumor cells were successfully labeled with SCN-NOTA-( 64 Cu or 68 Ga) and the Cy7 fluorescence dye and were visualized by PET and optical imaging [111]. Santos-Coquillat et al. designed a dual-sEV probe based on SPECT and fluorescent imaging to track goat milk EVs using 99m Tc(IV) and sulfo-cyanine5 NHS ester (SCy5) [112]. ...
... Examples of visualization of EVs in vivo using BLI, FLI, and SPECT/CT imaging. Representative images of subcutaneous CT26 tumor-bearing BALB/c mice at 24 h post-intravenous injection with the same dose of EVs, obtained from human embryonic kidney cells (Expi293F), engineered with CD63-NanoLuc Luciferase (Nluc) for bioluminescence imaging (A), labeled with DiR for fluorescence imaging (B), and radiolabeled with111 In-DTPA for SPECT/CT imaging (C). All panels are adapted and taken from[74]. ...
Extracellular vesicles (EVs) are a heterogeneous class of cell-derived membrane vesicles released by various cell types that serve as mediators of intercellular signaling. When released into circulation, EVs may convey their cargo and serve as intermediaries for intracellular communication, reaching nearby cells and possibly also distant organs. In cardiovascular biology, EVs released by activated or apoptotic endothelial cells (EC-EVs) disseminate biological information at short and long distances, contributing to the development and progression of cardiovascular disease and related disorders. The significance of EC-EVs as mediators of cell-cell communication has advanced, but a thorough knowledge of the role that intercommunication plays in healthy and vascular disease is still lacking. Most data on EVs derive from in vitro studies, but there are still little reliable data available on biodistribution and specific homing EVs in vivo tissues. Molecular imaging techniques for EVs are crucial to monitoring in vivo biodistribution and the homing of EVs and their communication networks both in basal and pathological circumstances. This narrative review provides an overview of EC-EVs, trying to highlight their role as messengers of cell-cell interaction in vascular homeostasis and disease, and describes emerging applications of various imaging modalities for EVs visualization in vivo.
... Engineered exosomes labeled with radioisotopes (including 99m Tc, 111 I, 125 I, 131 I, 64 Cu, 68 Ga) have achieved non-invasive PET or SPECT detection in animal models, and show promise for clinical translation (Morishita et al., 2015;Gangadaran et al., Frontiers in Pharmacology frontiersin.org 2018; Rashid et al., 2019;Jung et al., 2020;Lázaro-Ibáñez et al., 2021). Radio-labeled exosomes have excellent intra-tumoral homing ability and biosafety (Shi et al., 2019). ...
Warburg effect is characterized by excessive consumption of glucose by the tumor cells under both aerobic and hypoxic conditions. This metabolic reprogramming allows the tumor cells to adapt to the unique microenvironment and proliferate rapidly, and also promotes tumor metastasis and therapy resistance. Metabolic reprogramming of tumor cells is driven by the aberrant expression and activity of metabolic enzymes, which results in the accumulation of oncometabolites, and the hyperactivation of intracellular growth signals. Recent studies suggest that tumor-associated metabolic remodeling also depends on intercellular communication within the tumor microenvironment (TME). Small extracellular vesicles (sEVs), also known as exosomes, are smaller than 200 nm in diameter and are formed by the fusion of multivesicular bodies with the plasma membrane. The sEVs are instrumental in transporting cargoes such as proteins, nucleic acids or metabolites between the tumor, stromal and immune cells of the TME, and are thus involved in reprogramming the glucose metabolism of recipient cells. In this review, we have summarized the biogenesis and functions of sEVs and metabolic cargos, and the mechanisms through they drive the Warburg effect. Furthermore, the potential applications of targeting sEV-mediated metabolic pathways in tumor liquid biopsy, imaging diagnosis and drug development have also been discussed.
... In the covalent bounding strategy, a useful chelator for various radioisotopes (e.g. NOTA) can be conjugated with the amine group of the membrane proteins on EVs (Royo et al., 2019;Shi et al., 2019;Jung et al., 2020;Lázaro-Ibáñez et al., 2021). In the intraluminal radiolabeling strategy, a lipophilic radiotracer can easily penetrate to the EV lumen or ionophores allow radionuclides to be transported across the lipid membrane where they can be trapped as their loose lipophilicity (Son et al., 2020;Khan et al., 2022). ...
... The in vivo PET imaging of EVs is achievable by the gallium 68 Ga (t 1/2 = 68 min), cooper 64 Cu (t 1/2 = 12.7 h), zirconium 89 Zr (t 1/2 = 78.4 h) and iodine 124 I (t 1/2 = 100 h) isotopes, with the increasing half-life (Royo et al., 2019;Jung et al., 2020;Khan et al., 2022). The advent of high sensitivity total-body PET scanners opens possibility for an efficient in vivo tracking of EVs in the whole human body simultaneously (Stępień et al., 2021b;Vandenberghe et al., 2020) (Figure 2C). ...
This review introduce extracellular vesicles (EVs) to a molecular imaging field. The idea of modern analyses based on the use of omics studies, using high-throughput methods to characterize the molecular content of a single biological system, vesicolomics seems to be the new approach to collect molecular data about EV content, to find novel biomarkers or therapeutic targets. The use of various imaging techniques, including those based on radionuclides as positron emission tomography (PET) or single photon emission computed tomography (SPECT), combining molecular data on EVs , opens up the new space for radiovesicolomics—a new approach to be used in theranostics.
... Our data suggest that nanoparticle-bound NLuc (e.g., NLuc-E2) has better signal-to-noise ratios and is a better imaging tool than conventional fluorescently-labeled nanoparticles (e.g., AF-E2) for in vivo and ex vivo imaging and for imaging that needs a lower detection limit. This supports observations that conventional fluorescent labeling of nanoparticles for in vivo tracking does not provide enough signal and sensitivity, which remains problematic [62,76,77]. As the results of NLuc-E2 in vivo and ex vivo studies are supported by our previous biodistribution work done with flow cytometry [44], NLuc-E2 can be utilized as an alternative platform for in vivo non-invasive imaging and ex vivo biodistribution studies with great sensitivity and fidelity, both being vital characteristics for imaging probes in biodistribution studies [75,78]. ...
Bioluminescence imaging has advantages over fluorescence imaging, such as minimal photobleaching and autofluorescence, and greater signal-to-noise ratios in many complex environments. Although significant achievements have been made in luciferase engineering for generating bright and stable reporters, the full capability of luciferases for nanoparticle tracking has not been comprehensively examined. In biocatalysis, enhanced enzyme performance after immobilization on nanoparticles has been reported. Thus, we hypothesized that by assembling luciferases onto a nanoparticle, the resulting complex could lead to substantially improved imaging properties. Using a modular bioconjugation strategy, we attached NanoLuc (NLuc) or Akaluc bioluminescent proteins to a protein nanoparticle platform (E2), yielding nanoparticles NLuc-E2 and Akaluc-E2, both with diameters of ∼45 nm. Although no significant differences were observed between different conditions involving Akaluc and Akaluc-E2, free NLuc at pH 5.0 showed significantly lower emission values than free NLuc at pH 7.4. Interestingly, NLuc immobilization on E2 nanoparticles (NLuc-E2) emitted increased luminescence at pH 7.4, and at pH 5.0 showed over two orders of magnitude (>200-fold) higher luminescence (than free NLuc), expanding the potential for imaging detection using the nanoparticle even upon endocytic uptake. After uptake by macrophages, the resulting luminescence with NLuc-E2 nanoparticles was up to 7-fold higher than with free NLuc at 48 h. Cells incubated with NLuc-E2 could also be imaged using live bioluminescence microscopy. Finally, biodistribution of nanoparticles into lymph nodes was detected through imaging using NLuc-E2, but not with conventionally-labeled fluorescent E2. Our data demonstrate that NLuc-bound nanoparticles have advantageous properties that can be utilized in applications ranging from single-cell imaging to in vivo biodistribution.
... (B) EV membranes can be directly labeled by inserting agents into the membrane bilayers or by conjugating agents to the EV surface. EVs have been labeled with the lipophilic fluorescent dyes PKH26, DiI, DiD or DiR (Garofalo et al., 2019;Lassailly et al., 2010;Lehmann et al., 2016;Progatzky et al., 2013), phospholipids conjugated to Gd as T 1 -weighted MRI contrast agent (Abello et al., 2019;Rayamajhi et al., 2020) or the membrane proteins of EVs conjugated to fluorescent dyes (Bakirtzi et al., 2019;Hwang et al., 2019;Shi et al., 2019;Song et al., 2020;Zhang et al., 2020), the SPECT tracers 99m Tc (Varga et al., 2016) and 111 In (Faruqu et al., 2019;Lu et al., 2021), and the PET tracer 54 Cu (Banerjee et al., 2019;Jung et al., 2020;Shi et al., 2019). The EV core can also directly be loaded with gold nanoparticles (Cohen et al., 2021;Pan et al., 2020) or USPIOs or USPIOs coated with gold NPs (Jc Bose et al., 2018). ...
... Cartoons were created with Biorender.com (Abello et al., 2019;Lara et al., 2020;Lázaro-Ibáñez et al., 2021;Nishida-Aoki et al., 2020;Rayamajhi et al., 2020;Wan et al., 2020;Zhang et al., 2020), while the surface membrane proteins of EVs can be conjugated to probes (Banerjee et al., 2019;Faruqu et al., 2019;Hwang et al., 2015;Jung et al., 2020;Shi et al., 2019;Son et al., 2020;Varga et al., 2016). As in the case of magnetoelectroporation, the membranes of EVs can be temporarily opened, allowing nanoparticles to be entrapped inside the vesicle core. ...
... On the other hand, rapidly decaying PET radiotracers may be preferred if less exposure to the radioactive agent is desired (Tables 1-3). Regardless of the radionuclei used, no significant changes in EV size, size distribution, morphology, zeta potential and EV markers post-labeling were observed (Banerjee et al., 2019;Faruqu et al., 2019;Hwang et al., 2015;Jung et al., 2020;Shi et al., 2019;Varga et al., 2016). 99m Tc is the most commonly used SPECT tracer for liposome labeling and hence can be readily implemented for EV or NV tracking. ...
Extracellular vesicles (EVs) are lipid‐bilayer delimited vesicles released by nearly all cell types that serve as mediators of intercellular signalling. Recent evidence has shown that EVs play a key role in many normal as well as pathological cellular processes. EVs can be exploited as disease biomarkers and also as targeted, cell‐free therapeutic delivery and signalling vehicles for use in regenerative medicine and other clinical settings. Despite this potential, much remains unknown about the in vivo biodistribution and pharmacokinetic profiles of EVs after administration into living subjects. The ability to non‐invasively image exogeneous EVs, especially in larger animals, will allow a better understanding of their in vivo homing and retention patterns, blood and tissue half‐life, and excretion pathways, all of which are needed to advance clinical diagnostic and/or therapeutic applications of EVs. We present the current state‐of‐the‐art methods for labeling EVs with various diagnostic contrast agents and tracers and the respective imaging modalities that can be used for their in vivo visualization: magnetic resonance imaging (MRI), X‐ray computed tomography (CT) imaging, magnetic particle imaging (MPI), single‐photon emission computed tomography (SPECT), positron emission tomography (PET), and optical imaging (fluorescence and bioluminescence imaging). We review here the strengths and weaknesses of each of these EV imaging approaches, with special emphasis on clinical translation.
... In contrast to IC-Ls expressed at the cell surface, soluble IC-Ls can diffuse in various body compartments via the blood and lymphatic circulation and exert an inhibitory effects by interacting with cell surface receptors either in the local micro-environment or distally. Notably, in the past few years, several studies were performed to trace exosome biodistribution in vivo in mouse models [76][77][78]. To address this issue, radio or fluorescently labeled exosomes were injected into the tail vein [76], the lymphatic route [76], intradermally [77] or subcutaneously [78] and exosome anatomical distribution was assessed using positron emission tomography (PET) or in vivo imaging systems, respectively. ...
... Notably, in the past few years, several studies were performed to trace exosome biodistribution in vivo in mouse models [76][77][78]. To address this issue, radio or fluorescently labeled exosomes were injected into the tail vein [76], the lymphatic route [76], intradermally [77] or subcutaneously [78] and exosome anatomical distribution was assessed using positron emission tomography (PET) or in vivo imaging systems, respectively. These studies showed that labeled exosomes accumulated in the spleen, the kidneys and in draining LNs [76][77][78]. ...
... Notably, in the past few years, several studies were performed to trace exosome biodistribution in vivo in mouse models [76][77][78]. To address this issue, radio or fluorescently labeled exosomes were injected into the tail vein [76], the lymphatic route [76], intradermally [77] or subcutaneously [78] and exosome anatomical distribution was assessed using positron emission tomography (PET) or in vivo imaging systems, respectively. These studies showed that labeled exosomes accumulated in the spleen, the kidneys and in draining LNs [76][77][78]. ...
The limited development of broadly neutralizing antibodies (BnAbs) during HIV infection is classically attributed to an inadequate B-cell help brought by functionally impaired T follicular helper (Tfh) cells. However, the determinants of Tfh-cell functional impairment and the signals contributing to this condition remain elusive. In the present study, we showed that PD-L1 is incorporated within HIV virions through an active mechanism involving p17 HIV matrix protein. We subsequently showed that in vitro produced PD-L1high but not PD-L1low HIV virions, significantly reduced Tfh-cell proliferation and IL-21 production, ultimately leading to a decreased of IgG1 secretion from GC B cells. Interestingly, Tfh-cell functions were fully restored in presence of anti-PD-L1/2 blocking mAbs treatment, demonstrating that the incorporated PD-L1 proteins were functionally active. Taken together, the present study unveils an immunovirological mechanism by which HIV specifically exploits the regulatory potential of PD-L1 to suppress the immune system during the course of HIV infection.
... Morishita et al. analyzed the quantitative biodistribution of B16BL6-derived exosomes in mice by radiolabeling the surface of exosomes with iodine-125 ( 125 I) based on a streptavidin-biotin system, and they measured the time-dependent organ distribution using a gamma counter [47]. González et al. radiolabeled the surface of milk-derived exosomes using technetium-99m ( 99m Tc) for in vivo tracking in mice [48], and Jung et al. also labeled the surface of mouse breast cancer-derived exosomes with 64 Cu (or 68 Ga) and visualized in vivo-administered exosomes by PET imaging in mice [49]. While most studies radiolabel the surface of exosomes for in vivo tracking, some studies use the intraluminal labeling method. ...
... Although several studies showed accumulation of exosomes in the lungs of mice [47,49], we found minimal accumulation of exosomes in the lungs. This was consistent with our previous study which showed that significant amount of exosomes were accumulated in the lungs of mice only under septic conditions [23]. ...
For the successful clinical advancement of exosome therapeutics, the biodistribution and pharmacokinetic profile of exogenous exosomes in various animal models must be determined. Compared with fluorescence or bioluminescence imaging, radionuclide imaging confers multiple advantages for the in vivo tracking of biomolecular therapeutics because of its excellent sensitivity for deep tissue imaging and potential for quantitative measurement. Herein, we assessed the quantitative biodistribution and pharmacokinetics of good manufacturing practice-grade therapeutic exosomes labeled with zirconium-89 (89Zr) after systemic intravenous administration in mice and rats. Quantitative biodistribution analysis by positron emission tomography/computed tomography and gamma counting in mice and rats revealed that the total 89Zr signals in the organs were lower in rats than in mice, suggesting a higher excretion rate of exosomes in rats. A prolonged 89Zr signal for up to 7 days in most organs indicated that substantial amounts of exosomes were taken up by the parenchymal cells in those organs, highlighting the therapeutic potential of exosomes for the intracellular delivery of therapeutics. Exosomes were mainly distributed in the liver and to a lesser extent in the spleen, while a moderately distributed in the kidney, lung, stomach, intestine, urinary bladder, brain, and heart. Exosomes were rapidly cleared from the blood circulation, with a rate greater than that of free 89Zr, indicating that exosomes might be rapidly taken up by cells and tissues.
