Small Extracellular Vesicles From Infarcted and Failing Heart Accelerate Tumor Growth.

BACKGROUND: Myocardial infarction (MI) and heart failure are associated with an increased incidence of cancer. However, the mechanism is complex and unclear. Here, we aimed to test our hypothesis that cardiac small extracellular vesicles (sEVs), particularly cardiac mesenchymal stromal cell-derived sEVs (cMSC-sEVs), contribute to the link between post-MI left ventricular dysfunction (LVD) and cancer. METHODS: We purified and characterized sEVs from post-MI hearts and cultured cMSCs. Then, we analyzed cMSC-EV cargo and proneoplastic effects on several lines of cancer cells, macrophages, and endothelial cells. Next, we modeled heterotopic and orthotopic lung and breast cancer tumors in mice with post-MI LVD. We transferred cMSC-sEVs to assess sEV biodistribution and its effect on tumor growth. Finally, we tested the effects of sEV depletion and spironolactone treatment on cMSC-EV release and tumor growth. RESULTS: Post-MI hearts, particularly cMSCs, produced more sEVs with proneoplastic cargo than nonfailing hearts did. Proteomic analysis revealed unique protein profiles and higher quantities of tumor-promoting cytokines, proteins, and microRNAs in cMSC-sEVs from post-MI hearts. The proneoplastic effects of cMSC-sEVs varied with different types of cancer, with lung and colon cancers being more affected than melanoma and breast cancer cell lines. Post-MI cMSC-sEVs also activated resting macrophages into proangiogenic and protumorigenic states in vitro. At 28-day follow-up, mice with post-MI LVD developed larger heterotopic and orthotopic lung tumors than did sham-MI mice. Adoptive transfer of cMSC-sEVs from post-MI hearts accelerated the growth of heterotopic and orthotopic lung tumors, and biodistribution analysis revealed accumulating cMSC-sEVs in tumor cells along with accelerated tumor cell proliferation. sEV depletion reduced the tumor-promoting effects of MI, and adoptive transfer of cMSC-sEVs from post-MI hearts partially restored these effects. Finally, spironolactone treatment reduced the number of cMSC-sEVs and suppressed tumor growth during post-MI LVD. CONCLUSIONS: Cardiac sEVs, specifically cMSC-sEVs from post-MI hearts, carry multiple protumorigenic factors. Uptake of cMSC-sEVs by cancer cells accelerates tumor growth. Treatment with spironolactone significantly reduces accelerated tumor growth after MI. Our results provide new insight into the mechanism connecting post-MI LVD to cancer and propose a translational option to mitigate this deadly association.

Multifunctional nanoprobe for real-time in vivo monitoring of T cell activation.

Genetically engineered T cells are a powerful new modality for cancer immunotherapy. However, their clinical application for solid tumors is challenging, and crucial knowledge on cell functionality in vivo is lacking. Here, we fabricated a nanoprobe composed of dendrimers incorporating a calcium sensor and gold nanoparticles, for dual-modal monitoring of engineered T cells within a solid tumor. T cells engineered to express a melanoma-specific T-cell receptor and loaded with the nanoprobe were longitudinally monitored within melanoma xenografts in mice. Fluorescent imaging of the nanoprobe's calcium sensor revealed increased intra-tumoral activation of the T cells over time, up to 24 h. Computed tomography imaging of the nanoprobe's gold nanoparticles revealed the cells' intra-tumoral distribution pattern. Quantitative analysis revealed the intra-tumoral T cell quantities. Thus, this nanoprobe reveals intra-tumoral persistence, penetration and functional status of genetically engineered T cells, which can advance T cell-based immunotherapy and promote next-generation live cell imaging.

CT and MRI Imaging of Theranostic Bimodal Fe3O4@Au NanoParticles in Tumor Bearing Mice.

Gold-containing nanoparticles are proven to be an effective radiosensitizer in the radiotherapy of tumors. Reliable imaging of nanoparticles in a tumor and surrounding normal tissues is crucial both for diagnostics and for nanoparticle application as radiosensitizers. The Fe3O4 core was introduced into gold nanoparticles to form a core/shell structure suitable for MRI imaging. The aim of this study was to assess the in vivo bimodal CT and MRI enhancement ability of novel core/shell Fe3O4@Au theranostic nanoparticles. Core/shell Fe3O4@Au nanoparticles were synthesized and coated with PEG and glucose. C57Bl/6 mice bearing Ca755 mammary adenocarcinoma tumors received intravenous injections of the nanoparticles. CT and MRI were performed at several timepoints between 5 and 102 min, and on day 17 post-injection. Core/shell Fe3O4@Au nanoparticles provided significant enhancement of the tumor and tumor blood vessels. Nanoparticles also accumulated in the liver and spleen and were retained in these organs for 17 days. Mice did not show any signs of toxicity over the study duration. These results indicate that theranostic bimodal Fe3O4@Au nanoparticles are non-toxic and serve as effective contrast agents both for CT and MRI diagnostics. These nanoparticles have potential for future biomedical applications in cancer diagnostics and beyond.

Nanomedicine to target multidrug resistant tumors.

Nanomedicine employs nanotechnologies to develop innovative applications, and more specifically nano-objects in the field of human health, through exploitation of the physical, chemical and biological properties of materials at the nanoscale. The use of nanovehicles capable of transporting and releasing the active therapeutic payload into target cells, particularly in the case of cancer or inflammatory diseases, can also enhance diagnosis. Therefore, nanomedicines improve the benefit/risk ratio of drugs by increasing their bioavailability, selectivity, and efficacy in the target tissue, while reducing the necessary doses and hence diminishing untoward toxicity to healthy tissues. Overcoming multidrug resistance (MDR) to antitumor agents is a central goal of cancer research and therapeutics, making it possible to treat these diseases more accurately and effectively. The adaptability of nanomedicines e.g. modulation of their components, surface functionalization, encapsulation of various active therapeutics as well as the possibility of combining several treatments using a single nanoparticle platform, are characteristics which are perfectly poised to address classical chemoresistance, a major obstacle towards curative cancer therapy. In this review, we discuss an assortment of nanomedicines along with those that should be developed in order to surmount cancer MDR; these include exosomes, natural compounds, lipid nanocapsules, prodrug self-assemblies, and gold nanoparticles.

Multifunctional Dendrimer-Entrapped Gold Nanoparticles for Labeling and Tracking T Cells Via Dual-Modal Computed Tomography and Fluorescence Imaging.

Nanosystems for monitoring and tracking T cells provide an important basis for evaluating the functionality and efficacy of T cell-based immunotherapy. To this end, we designed herein an efficient nanoprobe for T cell monitoring and tracking using poly(amidoamine) (PAMAM) dendrimer-entrapped gold nanoparticles (Au DENPs) conjugated with Fluo-4 for dual-mode computed tomography (CT) and fluorescence imaging. In this study, PAMAM dendrimers of generation 5 (G5) were modified with hydroxyl-terminated polyethylene glycol (PEG) and then used to entrap 2.0 nm Au NPs followed by acetylation of the excess amine groups on the dendrimer surface. Subsequently, the calcium ion probe was covalently attached to the dendrimer nanohybrids through the PEG hydroxyl end groups to gain the functional {(Au0)25-G5.NHAc-(PEG)14-(Fluo-4)2} nanoprobe. This nanoprobe had excellent water solubility, high X-ray attenuation coefficient, and good cytocompatibility in the given concentration range, as well as a high T cell labeling efficiency. Confocal microscopy and flow cytometry results demonstrated that the nanoprobe was able to fluorescently sense activated T cells. Moreover, the nanoprobe was able to realize both CT and fluorescence imaging of subcutaneously injected T cells in vivo. Thus, the developed novel dendrimer-based nanosystem may hold great promise for advancing and improving the clinical application of T cell-based immunotherapy.

Golden Exosomes Selectively Target Brain Pathologies in Neurodegenerative and Neurodevelopmental Disorders.

Exosomes, nanovesicles that are secreted by different cell types, enable intercellular communication at local or distant sites. Alhough they have been found to cross the blood brain barrier, their migration and homing abilities within the brain remain unstudied. We have recently developed a method for longitudinal and quantitative in vivo neuroimaging of exosomes based on the superior visualization abilities of classical X-ray computed tomography (CT), combined with gold nanoparticles as labeling agents. Here, we used this technique to track the migration and homing patterns of intranasally administrated exosomes derived from bone marrow mesenchymal stem cells (MSC-exo) in different brain pathologies, including stroke, autism, Parkinson's disease, and Alzheimer's disease. We found that MSC-exo specifically targeted and accumulated in pathologically relevant murine models brains regions up to 96 h post administration, while in healthy controls they showed a diffuse migration pattern and clearance by 24 h. The neuro-inflammatory signal in pathological brains was highly correlated with MSC-exo accumulation, suggesting that the homing mechanism is inflammatory-driven. In addition, MSC-exo were selectively uptaken by neuronal cells, but not glial cells, in the pathological regions. Taken together, these findings can significantly promote the application of exosomes for therapy and targeted drug delivery in various brain pathologies.

Molecular Imaging of Cancer Using X-ray Computed Tomography with Protease Targeted Iodinated Activity-Based Probes.

X-ray computed tomography (CT) is a robust, precise, fast, and reliable imaging method that enables excellent spatial resolution and quantification of contrast agents throughout the body. However, CT is largely inadequate for molecular imaging applications due mainly to its low contrast sensitivity that forces the use of large concentrations of contrast agents for detection. To overcome this limitation, we generated a new class of iodinated nanoscale activity-based probes (IN-ABPs) that sufficiently accumulates at the target site by covalently binding cysteine cathepsins that are exceptionally highly expressed in cancer. The IN-ABPs are comprised of a short targeting peptide selective to specific cathepsins, an electrophilic moiety that allows activity-dependent covalent binding, and tags containing dendrimers with up to 48 iodine atoms. IN-ABPs selectively bind and inhibit activity of recombinant and intracellular cathepsin B, L, and S. We compared the in vivo kinetics, biodistribution, and tumor accumulation of IN-ABPs bearing 18 and 48 iodine atoms each, and their control counterparts lacking the targeting moiety. Here we show that although both IN-ABPs bind specifically to cathepsins within the tumor and produce detectable CT contrast, the 48-iodine bearing IN-ABP was found to be optimal with signals over 2.1-fold higher than its nontargeted counterpart. In conclusion, this study shows the synthetic feasibility and potential utility of IN-ABPs as potent contrast agents that enable molecular imaging of tumors using CT.

CT Imaging of Enzymatic Activity in Cancer Using Covalent Probes Reveal a Size-Dependent Pattern.

X-ray CT instruments are among the most available, efficient, and cost-effective imaging modalities in hospitals. The field of CT molecular imaging is emerging which relies mainly on the detection of gold nanoparticles and iodine-containing compounds directed to tagging a variety of abundant biomolecules. Here for the first time we attempted to detect enzymatic activity, while the low sensitivity of CT scanners to contrast reagents made this a challenging task. Therefore, we developed a new class of nanosized cathepsin-targeted activity-based probes (ABPs) for functional CT imaging of cancer. ABPs are small molecules designed to covalently modify enzyme targets in an activity-dependent manner. Using a CT instrument, these novel probes enable detection of the elevated cathepsin activity within cancerous tissue, thus creating a direct link between biological processes and imaging signals. We present the generation and biochemical evaluation of a library of ABPs tagged with different sized gold nanoparticles (GNPs), with various ratios of cathepsin-targeting moiety and a combination of different polyethylene glycol (PEG) protective layers. The most potent and stable GNP-ABPs were applied for noninvasive cancer imaging in mice. Surprisingly, detection of CT contrast from the tumor had reverse correlation to GNP size and the amount of targeting moiety. Interestingly, TEM images of tumor sections show intercellular lysosomal subcellular localization of the GNP-ABPs. In conclusion, we demonstrate that the covalent linkage is key for detection using low sensitive imaging modalities and the utility of GNP-ABPs as a promising tool for enzymatic-based CT imaging.

Radiotherapy-Sensitized Tumor Photothermal Ablation Using γ-Polyglutamic Acid Nanogels Loaded with Polypyrrole.

Development of versatile nanoscale platforms for cancer diagnosis and therapy is of great importance for applications in translational medicine. In this work, we present the use of γ-polyglutamic acid (γ-PGA) nanogels (NGs) to load polypyrrole (PPy) for thermal/photoacoustic (PA) imaging and radiotherapy (RT)-sensitized tumor photothermal therapy (PTT). First, a double emulsion approach was used to prepare the cystamine dihydrochloride (Cys)-cross-linked γ-PGA NGs. Next, the cross-linked NGs served as a reactor to be filled with pyrrole monomers that were subjected to in situ oxidation polymerization in the existence of Fe(III) ions. The formed uniform PPy-loaded NGs having an average diameter of 38.9 ± 8.6 nm exhibited good water-dispersibility and colloid stability. The prominent near-infrared (NIR) absorbance feature due to the loaded PPy endowed the NGs with contrast enhancement in PA imaging. The hybrid NGs possessed excellent photothermal conversion efficiency (64.7%) and stability against laser irradiation, and could be adopted for PA imaging and PTT of cancerous cells and tumor xenografts. Importantly, we also explored the cooperative PTT and X-ray radiation-mediated RT for enhanced tumor therapy. We show that PTT of tumors can be more significantly sensitized by RT using the sequence of laser irradiation followed by X-ray radiation as compared to using the reverse sequence. Our study suggests a promising theranostic platform of hybrid NGs that may be potentially utilized for PA imaging and combination therapy of different types of tumors.

[CONCISE NANOMEDICINE REVIEW].

INTRODUCTION: Nanomedicine is a rapidly evolving medical domain utilizing 1-100nm nanoscale particles to achieve medical goals in either one or more medical aspects - diagnosis, imaging and therapy. Nanomedicine employs a combination of methods stemming from life and exact sciences. This review deals briefly with the principles behind the scenes guiding the design, manufacture and employment of these nanoparticles. Some representative examples of the various applications are provided from the abundance of existing nanoparticles. The main topics discussed are those related to composition, characteristics of nanoparticles, usage in cancer, drug delivery and various central nervous system applications. Possible toxicity and future teratogenicity research pertaining to nanoparticles are also elaborated upon.

Use of Nanoparticle Contrast Agents for Cell Tracking with Computed Tomography.

Efforts to develop novel cell-based therapies originated with the first bone marrow transplant on a leukemia patient in 1956. Preclinical and clinical examples of cell-based treatment strategies have shown promising results across many disciplines in medicine, with recent advances in immune cell therapies for cancer producing remarkable response rates, even in patients with multiple treatment failures. However, cell-based therapies suffer from inconsistent outcomes, motivating the search for tools that allow monitoring of cell delivery and behavior in vivo. Noninvasive cell imaging techniques, also known as cell tracking, have been developed to address this issue. These tools can allow real-time, quantitative, and long-term monitoring of transplanted cells in the recipient, providing insight on cell migration, distribution, viability, differentiation, and fate, all of which play crucial roles in treatment efficacy. Understanding these parameters allows the optimization of cell choice, delivery route, and dosage for therapy and advances cell-based therapy for specific clinical uses. To date, most cell tracking work has centered on imaging modalities such as MRI, radionuclide imaging, and optical imaging. However, X-ray computed tomography (CT) is an emerging method for cell tracking that has several strengths such as high spatial and temporal resolution, and excellent quantitative capabilities. The advantages of CT for cell tracking are enhanced by its wide availability and cost effectiveness, allowing CT to become one of the most popular clinical imaging modalities and a key asset in disease diagnosis. In this review, we will discuss recent advances in cell tracking methods using X-ray CT in various applications, in addition to predictions on how the field will progress.

Design principles for noninvasive, longitudinal and quantitative cell tracking with nanoparticle-based CT imaging.

Contradictory results in clinical trials are preventing the advancement and implementation of cell-based therapy. To explain such results, there is a need to uncover the mystery regarding the fate of the transplanted cells. To answer this need, we developed a technique for noninvasive in vivo cell tracking, which uses gold nanoparticles as contrast agents for CT imaging. Herein, we investigate the design principles of this technique for intramuscular transplantation of therapeutic cells. Longitudinal studies were performed, displaying the ability to track cells over long periods of time. As few as 500 cells could be detected and a way to quantify the number of cells visualized by CT was demonstrated. Moreover, monitoring of cell functionality was demonstrated on a mouse model of Duchenne muscular dystrophy. This cell-tracking technology has the potential to become an essential tool in pre-clinical as well as clinical trials and to advance the future of cell therapy.

The effect of nanoparticle size on the probability to cross the blood-brain barrier: an in-vitro endothelial cell model.

BACKGROUND: During the last decade nanoparticles have gained attention as promising drug delivery agents that can transport through the blood brain barrier. Recently, several studies have demonstrated that specifically targeted nanoparticles which carry a large payload of therapeutic agents can effectively enhance therapeutic agent delivery to the brain. However, it is difficult to draw definite design principles across these studies, owing to the differences in material, size, shape and targeting agents of the nanoparticles. Therefore, the main objective of this study is to develop general design principles that link the size of the nanoparticle with the probability to cross the blood brain barrier. Specifically, we investigate the effect of the nanoparticle size on the probability of barbiturate coated GNPs to cross the blood brain barrier by using bEnd.3 brain endothelial cells as an in vitro blood brain barrier model. RESULTS: The results show that GNPs of size 70 nm are optimal for the maximum amount of gold within the brain cells, and that 20 nm GNPs are the optimal size for maximum free surface area. CONCLUSIONS: These findings can help understand the effect of particle size on the ability to cross the blood brain barrier through the endothelial cell model, and design nanoparticles for brain imaging/therapy contrast agents.

Gold nanorods as absorption contrast agents for the noninvasive detection of arterial vascular disorders based on diffusion reflection measurements.

In this study we report the use of gold nanorods (GNRs) as absorption contrast agents in the diffusion reflection (DR) method for the in vivo detection of atherosclerotic injury. The early detection and characterization of atherosclerotic vascular disease is considered to be one of the greatest medical challenges today. We show that macrophage cells, which are major components of unstable active atherosclerotic plaques, uptake gold nanoparticles, resulting in a change in the optical properties of tissue-like phantoms and a unique DR profile. In vivo DR measurements of rats that underwent injury of the carotid artery showed a clear difference between the DR profiles of the injured compared with healthy arteries. The results suggest that DR measurements following GNRs administration represent a potential novel method for the early detection of atherosclerotic vascular disease.

Mechanisms and Barriers in Nanomedicine: Progress in the Field and Future Directions.

In recent years, steady progress has been made in synthesizing and characterizing engineered nanoparticles, resulting in several approved drugs and multiple promising candidates in clinical trials. Regulatory agencies such as the Food and Drug Administration and the European Medicines Agency released important guidance documents facilitating nanoparticle-based drug product development, particularly in the context of liposomes and lipid-based carriers. Even with the progress achieved, it is clear that many barriers must still be overcome to accelerate translation into the clinic. At the recent conference workshop "Mechanisms and Barriers in Nanomedicine" in May 2023 in Colorado, U.S.A., leading experts discussed the formulation, physiological, immunological, regulatory, clinical, and educational barriers. This position paper invites open, unrestricted, nonproprietary discussion among senior faculty, young investigators, and students to trigger ideas and concepts to move the field forward.

Phosphate-Trapping Liposomes for Long-Term Management of Hyperphosphatemia.

Hyperphosphatemia is a typical complication of end-stage renal disease, characterized by elevated and life-threatening serum phosphate levels. Hemodialysis does not enable sufficient clearance of phosphate, due to slow cell-to-plasma kinetics of phosphate ions; moreover, dietary restrictions and conventional treatment with oral phosphate binders have low success rates, together with adverse effects. Here, we developed a new concept of phosphate-trapping liposomes, to improve and prolong the control over serum phosphate levels. We designed liposomes modified with polyethylene glycol and encapsulated with the phosphate binder ferric citrate (FC liposomes). These liposomes were found to trap phosphate ions in their inner core, and thereby lower free phosphate ion concentrations in solution and in serum. The FC liposomes showed higher phosphate binding ability as phosphate concentrations increased. Moreover, these liposomes showed a time-dependent increase in uptake of phosphate, up to 25 h in serum. Thus, our findings demonstrate effective long-term phosphate trapping by FC liposomes, indicating their potential to reduce serum phosphate toxicity and improve current management of hyperphosphatemia.

Antibody Delivery into the Brain by Radiosensitizer Nanoparticles for Targeted Glioblastoma Therapy.

Background: Glioblastoma is the most lethal primary brain malignancy in adults. Standard of care treatment, consisting of temozolomide (TMZ) and adjuvant radiotherapy (RT), mostly does not prevent local recurrence. The inability of drugs to enter the brain, in particular antibody-based drugs and radiosensitizers, is a crucial limitation to effective glioblastoma therapy. Methods: Here, we developed a combined strategy using radiosensitizer gold nanoparticles coated with insulin to cross the blood-brain barrier and shuttle tumor-targeting antibodies (cetuximab) into the brain. Results: Following intravenous injection to an orthotopic glioblastoma mouse model, the nanoparticles specifically accumulated within the tumor. Combining targeted nanoparticle injection with TMZ and RT standard of care significantly inhibited tumor growth and extended survival, as compared to standard of care alone. Histological analysis of tumors showed that the combined treatment eradicated tumor cells, and decreased tumor vascularization, proliferation, and repair. Conclusions: Our findings demonstrate radiosensitizer nanoparticles that effectively deliver antibodies into the brain, target the tumor, and effectively improve standard of care treatment outcome in glioblastoma.

Photothermal Therapy with HER2-Targeted Silver Nanoparticles Leading to Cancer Remission.

Nanoparticles exhibiting the localized surface plasmon resonance (LSPR) phenomenon are promising tools for diagnostics and cancer treatment. Among widely used metal nanoparticles, silver nanoparticles (Ag NPs) possess the strongest light scattering and surface plasmon strength. However, the therapeutic potential of Ag NPs has until now been underestimated. Here we show targeted photothermal therapy of solid tumors with 35 nm HER2-targeted Ag NPs, which were produced by the green synthesis using an aqueous extract of Lavandula angustifolia Mill. Light irradiation tests demonstrated effective hyperthermic properties of these NPs, namely heating by 10 °C in 10 min. To mediate targeted cancer therapy, Ag NPs were conjugated to the scaffold polypeptide, affibody ZHER2:342, which recognizes a clinically relevant oncomarker HER2. The conjugation was mediated by the PEG linker to obtain Ag-PEG-HER2 nanoparticles. Flow cytometry tests showed that Ag-PEG-HER2 particles successfully bind to HER2-overexpressing cells with a specificity comparable to that of full-size anti-HER2 IgGs. A confocal microscopy study showed efficient internalization of Ag-PEG-HER2 into cells in less than 2 h of incubation. Cytotoxicity assays demonstrated effective cell death upon exposure to Ag-PEG-HER2 and irradiation, caused by the production of reactive oxygen species. Xenograft tumor therapy with Ag-PEG-HER2 particles in vivo resulted in full primary tumor regression and the prevention of metastatic spread. Thus, for the first time, we have shown that HER2-directed plasmonic Ag nanoparticles are effective sensitizers for targeted photothermal oncotherapy.

Placenta-Derived Mesenchymal-like Adherent Stromal Cells as an Effective Cell Therapy for Cocaine Addiction in a Rat Model.

Recent research points to mesenchymal stem cells' potential for treating neurological disorders, especially drug addiction. We examined the longitudinal effect of placenta-derived mesenchymal stromal-like cells (PLX-PAD) in a rat model for cocaine addiction. Sprague-Dawley male rats were trained to self-administer cocaine or saline daily until stable maintenance. Before the extinction phase, PLX-PAD cells were administered by intracerebroventricular or intranasal routes. Neurogenesis was evaluated, as was behavioral monitoring for craving. We labeled the PLX-PAD cells with gold nanoparticles and followed their longitudinal migration in the brain parallel to their infiltration of essential peripheral organs both by micro-CT and by inductively coupled plasma-optical emission spectrometry. Cell locations in the brain were confirmed by immunohistochemistry. We found that PLX-PAD cells attenuated cocaine-seeking behavior through their capacity to migrate to specific mesolimbic regions, homed on the parenchyma in the dentate gyrus of the hippocampus, and restored neurogenesis. We believe that intranasal cell therapy is a safe and effective approach to treating addiction and may offer a novel and efficient approach to rehabilitation.

Noninvasive Tracking of Natural Killer Cells Using Gold Nanoparticles.

Natural killer (NK)-cell-based immunotherapy is emerging as an attractive approach for cancer treatment. However, to facilitate and expedite clinical implementation, important questions must be answered regarding the in vivo functionality and trafficking patterns of the transferred cells. We have recently developed a noninvasive cell-tracking technique, based on gold nanoparticles (GNPs) as cell-labeling and contrast agents for whole-body computed tomography (CT) imaging. Herein, we report the implementation of this technique for longitudinal and quantitative tracking of NK cell kinetics, the migration and biodistribution in tumor-bearing mice. NK cells were successfully labeled with GNPs, without impairing their biological function, as assessed both in vitro, by cytokine release and cytotoxicity assays, and in vivo, using a xenograft model of human tumors. Using CT, we longitudinally tracked the migration of intravenously injected NK cells and observed an accumulation of effector cell clusters at the tumor site, up to 72 h. Fluorescence imaging of the cells over time correlated with ex vivo quantitative analysis of gold content in the tumor, validating the accuracy and reliability of our technique. Our cell-tracking approach thus offers a valuable tool for preclinical studies, as well as for clinical applications, to elucidate the fate of NK cells and promote the implementation of NK-cell-based immunotherapy.