Imaging of cell-based therapy using 89Zr-oxine ex vivo cell labeling for positron emission tomography

Biodistribution and persistence of human umbilical cord-derived mesenchymal stem cells in NCG mice: a comparative study

INTRODUCTION

This study aims to investigate the biodistribution and persistence of human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) in NCG mice post-intravenous injection, utilizing 89Zr-PET/CT, bioluminescence imaging, multiplex immunohistochemistry (mIHC), and quantitative polymerase chain reaction (qPCR).

METHODS

hUC-MSCs were labeled with 89Zr-oxine (89Zr-MSCs) or transduced with luciferase gene (Luc-MSCs). Real-time tracking of 89Zr-MSCs lasted for 14-days followed by mIHC staining of hCD73. Real-time tracking of Luc-MSCs lasted for 7-days, followed by mIHC staining of hCD73 and human Alu-based qPCR. All methods adhered to ICH and other regulatory guidelines for development of cell-based drugs.

RESULTS

A biodistribution and persistence pattern was observed in the order of lung > liver > kidney > >spleen, although discrepancies were noted for the liver and kidney.

CONCLUSION

Each method exhibited strengths and weaknesses: 89Zr-PET/CT enabled long-term tracking but encountered issues with 89Zr shedding and dead cells; bioluminescence provided specific detection but was hampered by a rapid decline in signal; mIHC identified cells but relied on antigen abundance; qPCR detected minimal cell quantities but was unable to differentiate between live and dead cells. These limitations may obscure the true fate of cells in vivo, highlighting the need for more accurate and reliable assessment techniques.

Intracellular Protein Binding of Zr-89 Oxine Cell Labeling for PET Cell Tracking Studies

Background/Objectives: 89Zr-oxine is an ex vivo cell labeling agent that enables cells to be tracked in vivo by positron emission tomography (PET) over a period of up to two weeks. To better understand where 89Zr-oxine binds within cellular components, factors affecting labeling and intracellular distribution of 89Zr were examined. Methods: Mouse primary T cells, natural killer cells, dendritic cells, and monocytes, and cell lines EL4 (mouse lymphoma), DC2.4 (mouse dendritic cell), Kit225K6 (human T cell leukemia) and MC38 (mouse colon adenocarcinoma) were labeled with 89Zr-oxine or 111In-oxine and protein binding within the cellular compartments, the labeling thresholds, and radioactivity retention were subsequently determined. Results: Cell incorporation of 89Zr-oxine (27.8-71.8 kBq/106 cells) positively correlated with cellular size and protein mass. Most (>97%) 89Zr was protein-bound and primarily localized in the cytoplasm, membrane, and nuclear fractions (>81%) with distribution patterns varying by cell type. By contrast, 111In-oxine showed lower protein-binding activity of approximately 59-65%, with 62-65% of 111In localized in the cytoplasm. Autoradiography of electrophoresed subcellular fractionated cell samples indicated stable binding by 89Zr-oxine to proteins in all subcellular fractions but unstable protein binding by 111In. Saturation studies showed that 89Zr-oxine labeling was saturable, and further labeling reduced cellular retention. Biodistribution of dendritic cells labeled with either 89Zr-oxine or 111In-oxine indicated greater retention of 89Zr in the labeled cells in vivo than 111In. Conclusions: 89Zr-oxine stably binds many intracellular proteins and shows much higher and more stable protein binding than 111In-oxine. Intracellular protein binding of 89Zr accounts for the ability of 89Zr-oxine labeling to successfully track cells in vivo long-term on PET.

Mesenchymal stromal cells and CAR-T cells in regenerative medicine: The homing procedure and their effective parameters

Mesenchymal stromal cells (MSCs) and chimeric antigen receptor (CAR)-T cells are two core elements in cell therapy procedures. MSCs have significant immunomodulatory effects that alleviate inflammation in the tissue regeneration process, while administration of specific chemokines and adhesive molecules would primarily facilitate CAR-T cell trafficking into solid tumors. Multiple parameters affect cell homing, including the recipient's age, the number of cell passages, proper cell culture, and the delivery method. In addition, several chemokines are involved in the tumor microenvironment, affecting the homing procedure. This review discusses parameters that improve the efficiency of cell homing and significant cell therapy challenges. Emerging comprehensive mechanistic strategies such as non-systemic and systemic homing that revealed a significant role in cell therapy remodeling were also reviewed. Finally, the primary implications for the development of combination therapies that incorporate both MSCs and CAR-T cells for cancer treatment were discussed.

89Zr-leukocyte labelling for cell trafficking: in vitro and preclinical investigations

BACKGROUND

The non-invasive imaging of leukocyte trafficking to assess inflammatory areas and monitor immunotherapy is currently generating great interest. There is a need to develop more robust cell labelling and imaging approaches to track living cells. Positron emission tomography (PET), a highly sensitive molecular imaging technique, allows precise signals to be produced from radiolabelled moieties. Here, we developed a novel leukocyte labelling approach with the PET radioisotope zirconium-89 (89Zr, half-life of 78.4 h). Experiments were carried out using human leukocytes, freshly isolated from whole human blood.

RESULTS

The 89Zr-leukocyte labelling efficiency ranged from 46 to 87% after 30-60 min. Radioactivity concentrations of labelled cells were up to 0.28 MBq/1 million cells. Systemically administered 89Zr-labelled leukocytes produced high-contrast murine PET images at 1 h-5 days post injection. Murine biodistribution data showed that cells primarily distributed to the lung, liver, and spleen at 1 h post injection, and are then gradually trafficked to liver and spleen over 5 days. Histological analysis demonstrated that exogenously 89Zr-labelled human leukocytes were present in the lung, liver, and spleen at 1 h post injection. However, intravenously injected free [89Zr]Zr4+ ion showed retention only in the bone with no radioactivity in the lung at 5 days post injection, which implied good stability of radiolabelled leukocytes in vivo.

CONCLUSIONS

Our study presents a stable and generic radiolabelling technique to track leukocytes with PET imaging and shows great potential for further applications in inflammatory cell and other types of cell trafficking studies.

Comparative in vivo biodistribution of cells labelled with [89Zr]Zr-(oxinate)4 or [89Zr]Zr-DFO-NCS using PET

BACKGROUND

In vivo monitoring of cell biodistribution using positron emission tomography (PET) provides a quantitative non-invasive method to further optimize cell therapies and related new developments in the field. Our group has earlier optimized and evaluated the in vitro properties of two radiotracers,[89Zr]Zr-(oxinate)4 and [89Zr]Zr-DFO-NCS, for the radiolabelling of different cell types. Here, we performed a microPET study to assess the in vivo biodistribution of cells in rats using these two radiotracers. Human decidual stromal cells (hDSC) and rat macrophages (rMac) were radiolabelled with [89Zr]Zr-(oxinate)4 or [89Zr]Zr-DFO-NCS. Rats were intravenously injected with radiolabelled cells, and the in vivo biodistribution was monitored with microPET/CT imaging for up to day 7. Organ uptake was evaluated and presented as a percentage of injected activity per gram tissue (%IA/g) and total absorbed organ doses (mSv/MBq).

RESULTS

The biodistribution in vivo showed an immediate uptake in the lungs. Thereafter, [89Zr]Zr-(oxinate)4 labelled cells migrated to the liver, while the signal from [89Zr]Zr-DFO-NCS labelled cells lingered in the lungs. The differences in the in vivo behaviour for the same cell type appeared related to the radiotracer labelling. After 24 h, [89Zr]Zr-(oxinate)4 labelled cells had over 70% higher liver uptake for both hDSC and rMac compared to [89Zr]Zr-DFO-NCS labelled cells, whereas [89Zr]Zr-DFO-NCS labelled cells showed over 60% higher uptake in the lungs compared to [89Zr]Zr-(oxinate)4 labelled cells. This difference in both lung and liver uptake continued until day 7. Dosimetry calculations showed a higher effective dose (mSv/MBq) for [89Zr]Zr-DFO-NCS compared to [89Zr]Zr-(oxinate)4, for both cell types. Although the bone uptake was higher for [89Zr]Zr-(oxinate)4 labelled cells, the prolonged uptake in the lungs contributed to a significant crossfire to bone marrow resulting in a higher bone dose.

CONCLUSION

The [89Zr]Zr-DFO-NCS labelled cells suggest a prolonged accumulation in the lungs, while [89Zr]Zr-(oxinate)4 suggests quicker clearance of the lungs followed by accumulation in the liver. Accumulation of radiolabelled cells in the liver corresponds to other cell-tracking methods. Further studies are required to determine the actual location of the [89Zr]Zr-DFO-NCS labelled cell.

Characterization of chimeric antigen receptor modified T cells expressing scFv-IL-13Rα2 after radiolabeling with 89Zirconium oxine for PET imaging

BACKGROUND

Chimeric antigen receptor (CAR) T cell therapy is an exciting cell-based cancer immunotherapy. Unfortunately, CAR-T cell therapy is associated with serious toxicities such as cytokine release syndrome (CRS) and neurotoxicity. The mechanism of these serious adverse events (SAEs) and how homing, distribution and retention of CAR-T cells contribute to toxicities is not fully understood. Enabling in vitro methods to allow meaningful, sensitive in vivo biodistribution studies is needed to better understand CAR-T cell disposition and its relationship to both effectiveness and safety of these products.

METHODS

To determine if radiolabelling of CAR-T cells could support positron emission tomography (PET)-based biodistribution studies, we labeled IL-13Rα2 targeting scFv-IL-13Rα2-CAR-T cells (CAR-T cells) with 89Zirconium-oxine (89Zr-oxine) and characterized and compared their product attributes with non-labeled CAR-T cells. The 89Zr-oxine labeling conditions were optimized for incubation time, temperature, and use of serum for labeling. In addition, T cell subtype characterization and product attributes of radiolabeled CAR-T cells were studied to assess their overall quality including cell viability, proliferation, phenotype markers of T-cell activation and exhaustion, cytolytic activity and release of interferon-γ upon co-culture with IL-13Rα2 expressing glioma cells.

RESULTS

We observed that radiolabeling of CAR-T cells with 89Zr-oxine is quick, efficient, and radioactivity is retained in the cells for at least 8 days with minimal loss. Also, viability of radiolabeled CAR-T cells and subtypes such as CD4 + , CD8 + and scFV-IL-13Rα2 transgene positive T cell population were characterized and found similar to that of unlabeled cells as determined by TUNEL assay, caspase 3/7 enzyme and granzyme B activity assay. Moreover, there were no significant changes in T cell activation (CD24, CD44, CD69 and IFN-γ) or T cell exhaustion (PD-1, LAG-3 and TIM3) markers expression between radiolabeled and unlabeled CAR-T cells. In chemotaxis assays, migratory capability of radiolabeled CAR-T cells to IL-13Rα2Fc was similar to that of non-labeled cells.

CONCLUSIONS

Importantly, radiolabeling has minimal impact on biological product attributes including potency of CAR-T cells towards IL-13Rα2 positive tumor cells but not IL-13Rα2 negative cells as measured by cytolytic activity and release of IFN-γ. Thus, IL-13Rα2 targeting CAR-T cells radiolabeled with 89Zr-oxine retain critical product attributes and suggest 89Zr-oxine radiolabeling of CAR-T cells may facilitate biodistribution and tissue trafficking studies in vivo using PET.

Detection of iron oxide nanoparticle (IONP)-labeled stem cells using quantitative ultrashort echo time imaging: a feasibility study

Background

In this study, we investigated the feasibility of quantitative ultrashort echo time (qUTE) magnetic resonance (MR) imaging techniques in the detection and quantification of iron oxide nanoparticle (IONP)-labeled stem cells.

Methods

A stem cell phantom containing multiple layers of unlabeled or labeled stem cells with different densities was prepared. The phantom was imaged with quantitative UTE (qUTE) MR techniques [i.e., UTE-T1 mapping, UTE-T2* mapping, and UTE-based quantitative susceptibility mapping (UTE-QSM)] as well as with a clinical T2 mapping sequence on a 3T clinical MR system. For T1 mapping, a variable flip angle (VFA) method based on actual flip angle imaging (AFI) technique was utilized. For T2* mapping and UTE-QSM, multiple images with variable, interleaved echo times including UTE images and gradient recalled echo (GRE) images were used. For UTE-QSM, the phase information from the multi-echo images was utilized and processed using a QSM framework based on the morphology-enabled dipole inversion (MEDI) algorithm. The qUTE techniques were also evaluated in an ex vivo experiment with a mouse injected with IONP-labeled stem cells.

Results

In the phantom experiment, the parameters estimated with qUTE techniques showed high linearity with respect to the density of IONP-labeled stem cells (R2>0.99), while the clinical T2 parameter showed impaired linearity (R2=0.87). In the ex vivo mouse experiment, UTE-T2* mapping and UTE-QSM showed feasibility in the detection of injected stem cells with high contrast, whereas UTE-T1 and UTE-T2* showed limited detection. Overall, UTE-QSM demonstrated the best contrast of all, with other methods being subjected more to a confounding factor due to different magnetic susceptibilities of various types of neighboring tissues, which creates inhomogeneous contrast that behaves similar to IONP.

Conclusions

In this study, we evaluated the feasibility of a series of qUTE imaging techniques as well as conventional T2 mapping for the detection of IONP-labeled stem cells in vitro and ex vivo. UTE-QSM performed superior amongst other qUTE techniques as well as conventional T2 mapping in detecting stem cells with high contrast.

M. de Rosales R. Direct Cell Radiolabeling for in Vivo Cell Tracking with PET and SPECT Imaging

The arrival of cell-based therapies is a revolution in medicine. However, its safe clinical application in a rational manner depends on reliable, clinically applicable methods for determining the fate and trafficking of therapeutic cells in vivo using medical imaging techniques─known as in vivo cell tracking. Radionuclide imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) has several advantages over other imaging modalities for cell tracking because of its high sensitivity (requiring low amounts of probe per cell for imaging) and whole-body quantitative imaging capability using clinically available scanners. For cell tracking with radionuclides, ex vivo direct cell radiolabeling, that is, radiolabeling cells before their administration, is the simplest and most robust method, allowing labeling of any cell type without the need for genetic modification. This Review covers the development and application of direct cell radiolabeling probes utilizing a variety of chemical approaches: organic and inorganic/coordination (radio)chemistry, nanomaterials, and biochemistry. We describe the key early developments and the most recent advances in the field, identifying advantages and disadvantages of the different approaches and informing future development and choice of methods for clinical and preclinical application.

Near-Infrared Bioluminescence Imaging of Macrophage Sensors for Cancer Detection In Vivo

Melanoma is an aggressive type of skin cancer with a poor prognosis after it gets metastasized. The early detection of malignant melanoma is critical for effective therapy. Because melanoma often resembles moles, routine skin check-up may help for timely identification of suspicious areas. Recently, it has been shown that the interplay of melanoma cells with the immune system can help develop efficient therapeutic strategies. Here, we leveraged engineered macrophages (BMC2) as cell-based sensors for metastatic melanoma. To perform dual-color bioluminescence imaging (BLI) in vivo, macrophages were engineered to express a green click beetle luciferase (CBG2) and a near-infrared fluorescent dye (DiR), and B16F10 melanoma cells were instead engineered to express a near-infrared click beetle luciferase (CBR2). Using real-time in vivo dual-color BLI and near-infrared fluorescence (FL) imaging, we could demonstrate that macrophages were able to sense and substantially accumulate in subcutaneous and metastatic melanoma tissues at 72 h after systemic injections. Together, we showed the potentiality to use optical imaging technologies to track circulating macrophages for the non-invasive detection of metastatic melanoma.

Radionuclide Imaging of Cytotoxic Immune Cell Responses to Anti-Cancer Immunotherapy

Cancer immunotherapy is an evolving and promising cancer treatment that takes advantage of the body’s immune system to yield effective tumor elimination. Importantly, immunotherapy has changed the treatment landscape for many cancers, resulting in remarkable tumor responses and improvements in patient survival. However, despite impressive tumor effects and extended patient survival, only a small proportion of patients respond, and others can develop immune-related adverse events associated with these therapies, which are associated with considerable costs. Therefore, strategies to increase the proportion of patients gaining a benefit from these treatments and/or increasing the durability of immune-mediated tumor response are still urgently needed. Currently, measurement of blood or tissue biomarkers has demonstrated sampling limitations, due to intrinsic tumor heterogeneity and the latter being invasive. In addition, the unique response patterns of these therapies are not adequately captured by conventional imaging modalities. Consequently, non-invasive, sensitive, and quantitative molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using specific radiotracers, have been increasingly used for longitudinal whole-body monitoring of immune responses. Immunotherapies rely on the effector function of CD8⁺ T cells and natural killer cells (NK) at tumor lesions; therefore, the monitoring of these cytotoxic immune cells is of value for therapy response assessment. Different immune cell targets have been investigated as surrogate markers of response to immunotherapy, which motivated the development of multiple imaging agents. In this review, the targets and radiotracers being investigated for monitoring the functional status of immune effector cells are summarized, and their use for imaging of immune-related responses are reviewed along their limitations and pitfalls, of which multiple have already been translated to the clinic. Finally, emerging effector immune cell imaging strategies and future directions are provided.

A bioluminescence reporter mouse model for visualizing and quantifying CD8+ T cells in vivo

Cytotoxic CD8+ T cells are the primary effector cells mediating anti-tumor responses. In vivo monitoring of CD8+ T cells has broad implications for the development of novel cancer therapies. Here we describe the development of a genetically engineered mouse model (GEMM) in which CD8+ T cells are labeled with an optical reporter, enabling in vivo, longitudinal monitoring using bioluminescence imaging (BLI). Firefly luciferase (Luc2), human diphtheria toxin receptor (DTR), and enhanced green fluorescence protein (eGFP) cDNAs are engineered under the CD8α promoter to generate a transgenic mouse line. Luciferase mRNA and CD8α mRNA were generally correlated in various tissues from these mice. Sorted splenic CD8+ T cells, CD4+ T cells and CD3- non-T cells verified that the luciferase signal is specific to CD8+ T cells. In vivo imaging showed that luciferase signal was detected in various immune organs, such as lymph nodes, thymus, and spleen, and the detection was confirmed by ex vivo examination. Administration of diphtheria toxin markedly reduced luciferase signal systemically, confirming the function of the DTR. In the MC38 mouse syngeneic model, we observed significant increases in CD8+ T cells with mDX400 treatment, an anti PD-1 mouse monoclonal antibody that correlated with tumor growth inhibition. This novel reporter GEMM is a valuable drug discovery tool for profiling compounds and understanding mechanisms of action in immunotherapy of cancer.

Tracking of NK Cells by Positron Emission Tomography Using 89Zr-Oxine Ex Vivo Cell Labeling

A ⁸⁹Zr-oxine ex vivo cell labeling method for tracking various cells by positron emission tomography (PET) imaging has recently been developed. ⁸⁹Zr-oxine is synthesized from oxine and ⁸⁹Zr-chloride, which was converted from ⁸⁹Zr-oxalate, with neutralization. To track migration of natural killer (NK) cells in vivo in real time by PET imaging, NK cells are labeled with ⁸⁹Zr-oxine ex vivo and infused to a recipient. The labeling is performed by mixing ⁸⁹Zr-oxine solution to NK cell suspension at room temperature, followed by washing. Care should be taken to label the cells at optimal radioactivity doses that maintain their viability and functionality. ⁸⁹Zr-oxine labeled NK cells can be tracked for their migration and distribution by PET/computed tomography imaging for at least 7 days. Of note, this protocol is applicable to other types of cells.

Toward comprehensive imaging of oncolytic viroimmunotherapy

Oncolytic viruses infect, replicate in, and kill cancer cells, leaving normal cells unharmed; they also recruit and activate immune cells against tumor cells. While clinical indications for viroimmunotherapy are growing, barriers to widespread treatment remain. Ensuring real-time tracking of viral replication and resulting anti-tumor immune responses will overcome some of these barriers and is thus a top priority. Clinically optimizing trackability of viral replication will promote safe dose increases, guide serial dosing, and enhance treatment effects. However, viral delivery is only half the story. Oncolytic viruses are known to upregulate immune checkpoint expression, thereby priming otherwise immunodeficient tumor immune microenvironments for treatment with checkpoint inhibitors. Novel modalities to track virus-induced changes in tumor microenvironments include non-invasive measurements of immune cell populations and responses to viroimmunotherapy such as (1) in situ use of radiotracers to track checkpoint protein expression or immune cell traffic, and (2) ex vivo labeling of immune cells followed by nuclear medicine imaging. Herein, we review clinical progress toward accurate imaging of oncolytic virus replication, and we further review the current status of functional imaging of immune responses to viroimmunotherapy.

Translational Development of a Zr-89-Labeled Inhibitor of Prostate-specific Membrane Antigen for PET Imaging in Prostate Cancer

Purpose

We present here a Zr-89-labeled inhibitor of prostate-specific membrane antigen (PSMA) as a complement to the already established F-18- or Ga-68-ligands.

Procedures

The precursor PSMA-DFO (ABX) was used for Zr-89-labeling. This is not an antibody, but a peptide analogue of the precursor for the production of [¹⁷⁷Lu]Lu-PSMA-617. The ligand [⁸⁹Zr]Zr-PSMA-DFO was compared with [⁶⁸Ga]Ga-PSMA-11 and [¹⁸F]F-JK-PSMA-7 in vitro by determination of the K_d value, cellular uptake, internalization in LNCaP cells, biodistribution studies with LNCaP prostate tumor xenografts in mice, and in vivo by small-animal PET imaging in LNCaP tumor mouse models. A first-in-human PET was performed with [⁸⁹Zr]Zr-PSMA-DFO on a patient presenting with a biochemical recurrence after brachytherapy and an ambiguous intraprostatic finding with [¹⁸F]F-JK-PSMA-7 but histologically benign cells in a prostate biopsy 7 months previously.

Results

[⁸⁹Zr]Zr-PSMA-DFO was prepared with a radiochemical purity ≥ 99.9% and a very high in vitro stability for up to 7 days at 37 °C. All radiotracers showed similar specific cellular binding and internalization, in vitro and comparable tumor uptake in biodistribution experiments during the first 5 h. The [⁸⁹Zr]Zr-PSMA-DFO achieved significantly higher tumor/background ratios in LNCaP tumor xenografts (tumor/blood: 309 ± 89, tumor/muscle: 450 ± 38) after 24 h than [⁶⁸Ga]Ga-PSMA-11 (tumor/blood: 112 ± 57, tumor/muscle: 58 ± 36) or [¹⁸F]F-JK-PSMA-7 (tumor/blood: 175 ± 30, tumor/muscle: 114 ± 14) after 4 h (p < 0.01). Small-animal PET imaging demonstrated in vivo that tumor visualization with [⁸⁹Zr]Zr-PSMA-DFO is comparable to [⁶⁸Ga]Ga-PSMA-11 or [¹⁸F]F-JK-PSMA-7 at early time points (1 h p.i.) and that PET scans up to 48 h p.i. clearly visualized the tumor at late time points. A late [⁸⁹Zr]Zr-PSMA-DFO PET scan on a patient with biochemical recurrence (BCR) had demonstrated intensive tracer accumulation in the right (SUV_max 13.25, 48 h p.i.) and in the left prostate lobe (SUV max 9.47), a repeat biopsy revealed cancer cells on both sides.

Conclusion

[⁸⁹Zr]Zr-PSMA-DFO is a promising PSMA PET tracer for detection of tumor areas with lower PSMA expression and thus warrants further clinical evaluation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-021-01632-x.

The Future of Cancer Diagnosis, Treatment and Surveillance: A Systemic Review on Immunotherapy and Immuno-PET Radiotracers

Immunotherapy is an effective therapeutic option for several cancers. In the last years, the introduction of checkpoint inhibitors (ICIs) has shifted the therapeutic landscape in oncology and improved patient prognosis in a variety of neoplastic diseases. However, to date, the selection of the best patients eligible for these therapies, as well as the response assessment is still challenging. Patients are mainly stratified using an immunohistochemical analysis of the expression of antigens on biopsy specimens, such as PD-L1 and PD-1, on tumor cells, on peritumoral immune cells and/or in the tumor microenvironment (TME). Recently, the use and development of imaging biomarkers able to assess in-vivo cancer-related processes are becoming more important. Today, positron emission tomography (PET) with 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) is used routinely to evaluate tumor metabolism, and also to predict and monitor response to immunotherapy. Although highly sensitive, FDG-PET in general is rather unspecific. Novel radiopharmaceuticals (immuno-PET radiotracers), able to identify specific immune system targets, are under investigation in pre-clinical and clinical settings to better highlight all the mechanisms involved in immunotherapy. In this review, we will provide an overview of the main new immuno-PET radiotracers in development. We will also review the main players (immune cells, tumor cells and molecular targets) involved in immunotherapy. Furthermore, we report current applications and the evidence of using [18F]FDG PET in immunotherapy, including the use of artificial intelligence (AI).