Comparative Analysis of Human Nucleoside Kinase-Based Reporter Systems for PET Imaging

PET Reporter Probes for Brain Imaging of Transduced Gene and Cell Expression: Status and Challenges

Gene therapy and cell transduction are gaining interest as therapeutic strategies for neurological and psychiatric disorders. Positron emission tomography (PET) has been established as a uniquely powerful modality for brain molecular imaging in vivo. The utility of PET depends on the development and application of suitably specific radiotracers and/or reporter probes. PET probes are potentially useful to confirm the success of gene therapy or cell transduction without the need for brain biopsy or necroscopy. Probes are needed to target proteins expressed by specific exogenous transgenes or cells and could play a crucial role in elucidating neurobiological mechanisms and in longitudinal tracking of expression for therapeutic applications. This perspective article describes the current status and ongoing challenges for the design and development of PET reporter probes for verifying the expression of reporter genes and cells in the brain. Radiochemical aspects, applications, and translational challenges for diagnostic and therapeutic interventions are highlighted.

A Genetically Engineered Reporter System Designed for 2H-MRI Allows Quantitative In Vivo Mapping of Transgene Expression

The ability to obtain quantitative spatial information on subcellular processes of deep tissues in vivo has been a long-standing challenge for molecular magnetic resonance imaging (MRI) approaches. This challenge remains even more so for quantifying readouts of genetically engineered MRI reporters. Here, we set to overcome this challenge with a molecular system designed to obtain quantitative 2H-MRI maps of a gene reporter. To this end, we synthesized deuterated thymidine, d3-thy, with three magnetically equivalent deuterons at its methyl group (-CD3), showing a singlet peak with a characteristic 2H-NMR frequency (δ = 1.7 ppm). The upfield 3.0 ppm offset from the chemical shift of the HDO signal (δ = 4.7 ppm) allows for spectrally resolving the two 2H NMR signals and quantifying the concentration of d3-thy based on the known concentration of a tissue's HDO. Following systemic administration of d3-thy, its accumulation as d3-thy monophosphate in cells expressing the human thymidine kinase 1 (hTK1) transgene was mapped with 2H-MRI. The data obtained in vivo show the ability to use the d3-thy/hTK1 pair as a reporter probe/reporter gene system to quantitatively map transgene expression with MRI. Relying on a structurally unmodified reporter probe (d3-thy) to image the expression of unmutated human protein (hTK1) shows the potential of molecular imaging with 2H-MRI to monitor gene reporters and other relevant biological targets.

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.

Pathophysiology and Molecular Imaging of Diabetic Foot Infections

Diabetic foot infection is the leading cause of non-traumatic lower limb amputations worldwide. In addition, diabetes mellitus and sequela of the disease are increasing in prevalence. In 2017, 9.4% of Americans were diagnosed with diabetes mellitus (DM). The growing pervasiveness and financial implications of diabetic foot infection (DFI) indicate an acute need for improved clinical assessment and treatment. Complex pathophysiology and suboptimal specificity of current non-invasive imaging modalities have made diagnosis and treatment response challenging. Current anatomical and molecular clinical imaging strategies have mainly targeted the host's immune responses rather than the unique metabolism of the invading microorganism. Advances in imaging have the potential to reduce the impact of these problems and improve the assessment of DFI, particularly in distinguishing infection of soft tissue alone from osteomyelitis (OM). This review presents a summary of the known pathophysiology of DFI, the molecular basis of current and emerging diagnostic imaging techniques, and the mechanistic links of these imaging techniques to the pathophysiology of diabetic foot infections.

Pathophysiology and Molecular Imaging of Diabetic Foot Infections

Diabetic foot infection is the leading cause of non-traumatic lower limb amputations worldwide. In addition, diabetes mellitus and sequela of the disease are increasing in prevalence. In 2017, 9.4% of Americans were diagnosed with diabetes mellitus (DM). The growing pervasiveness and financial implications of diabetic foot infection (DFI) indicate an acute need for improved clinical assessment and treatment. Complex pathophysiology and suboptimal specificity of current non-invasive imaging modalities have made diagnosis and treatment response challenging. Current anatomical and molecular clinical imaging strategies have mainly targeted the host's immune responses rather than the unique metabolism of the invading microorganism. Advances in imaging have the potential to reduce the impact of these problems and improve the assessment of DFI, particularly in distinguishing infection of soft tissue alone from osteomyelitis (OM). This review presents a summary of the known pathophysiology of DFI, the molecular basis of current and emerging diagnostic imaging techniques, and the mechanistic links of these imaging techniques to the pathophysiology of diabetic foot infections.

Imaging CAR T-cell kinetics in solid tumors: Translational implications

Success in solid tumor chimeric antigen receptor (CAR) T-cell therapy requires overcoming several barriers, including lung sequestration, inefficient accumulation within the tumor, and target-antigen heterogeneity. Understanding CAR T-cell kinetics can assist in the interpretation of therapy response and limitations and thereby facilitate developing successful strategies to treat solid tumors. As T-cell therapy response varies across metastatic sites, the assessment of CAR T-cell kinetics by peripheral blood analysis or a single-site tumor biopsy is inadequate for interpretation of therapy response. The use of tumor imaging alone has also proven to be insufficient to interpret response to therapy. To address these limitations, we conducted dual tumor and T-cell imaging by use of a bioluminescent reporter and positron emission tomography in clinically relevant mouse models of pleural mesothelioma and non-small cell lung cancer. We observed that the mode of delivery of T cells (systemic versus regional), T-cell activation status (presence or absence of antigen-expressing tumor), and tumor-antigen expression heterogeneity influence T-cell kinetics. The observations from our study underscore the need to identify and develop a T-cell reporter—in addition to standard parameters of tumor imaging and antitumor efficacy—that can be used for repeat imaging without compromising the efficacy of CAR T cells in vivo.

A brief review of reporter gene imaging in oncolytic virotherapy and gene therapy

Reporter gene imaging (RGI) can accelerate development timelines for gene and viral therapies by facilitating rapid and noninvasive in vivo studies to determine the biodistribution, magnitude, and durability of viral gene expression and/or virus infection. Functional molecular imaging systems used for this purpose can be divided broadly into deep-tissue and optical modalities. Deep-tissue modalities, which can be used in animals of any size as well as in human subjects, encompass single photon emission computed tomography (SPECT), positron emission tomography (PET), and functional/molecular magnetic resonance imaging (f/mMRI). Optical modalities encompass fluorescence, bioluminescence, Cerenkov luminescence, and photoacoustic imaging and are suitable only for small animal imaging. Here we discuss the mechanisms of action and relative merits of currently available reporter gene systems, highlighting the strengths and weaknesses of deep tissue versus optical imaging systems and the hardware/reagents that are used for data capture and processing. In light of recent technological advances, falling costs of imaging instruments, better availability of novel radioactive and optical tracers, and a growing realization that RGI can give invaluable insights across the entire in vivo translational spectrum, the approach is becoming increasingly essential to facilitate the competitive development of new virus- and gene-based drugs.

Imaging of T-cell Responses in the Context of Cancer Immunotherapy

Immunotherapy, which promotes the induction of cytotoxic T lymphocytes and enhances their infiltration into and function within tumors, is a rapidly expanding and evolving approach to treating cancer. However, many of the critical denominators for inducing effective anticancer immune responses remain unknown. Efforts are underway to develop comprehensive ex vivo assessments of the immune landscape of patients prior to and during response to immunotherapy. An important complementary approach to these efforts involves the development of noninvasive imaging approaches to detect immune targets, assess delivery of immune-based therapeutics, and evaluate responses to immunotherapy. Herein, we review the merits and limitations of various noninvasive imaging modalities (MRI, PET, and single-photon emission tomography) and discuss candidate targets for cellular and molecular imaging for visualization of T-cell responses at various stages along the cancer-immunity cycle in the context of immunotherapy. We also discuss the potential use of these imaging strategies in monitoring treatment responses and predicting prognosis for patients treated with immunotherapy.

Non-invasive Reporter Gene Imaging of Cell Therapies, including T Cells and Stem Cells

Cell therapies represent a rapidly emerging class of new therapeutics. They are intended and developed for the treatment of some of the most prevalent human diseases, including cancer, diabetes, and for regenerative medicine. Currently, they are largely developed without precise assessment of their in vivo distribution, efficacy, or survival either clinically or preclinically. However, it would be highly beneficial for both preclinical cell therapy development and subsequent clinical use to assess these parameters in situ to enable enhancements in efficacy, applicability, and safety. Molecular imaging can be exploited to track cells non-invasively on the whole-body level and can enable monitoring for prolonged periods in a manner compatible with rapidly expanding cell types. In this review, we explain how in vivo imaging can aid the development and clinical translation of cell-based therapeutics. We describe the underlying principles governing non-invasive in vivo long-term cell tracking in the preclinical and clinical settings, including available imaging technologies, reporter genes, and imaging agents as well as pitfalls related to experimental design. Our emphasis is on adoptively transferred T cell and stem cell therapies.

How Non-invasive in vivo Cell Tracking Supports the Development and Translation of Cancer Immunotherapies

Immunotherapy is a relatively new treatment regimen for cancer, and it is based on the modulation of the immune system to battle cancer. Immunotherapies can be classified as either molecular or cell-based immunotherapies, and both types have demonstrated promising results in a growing number of cancers. Indeed, several immunotherapies representing both classes are already approved for clinical use in oncology. While spectacular treatment successes have been reported, particularly for so-called immune checkpoint inhibitors and certain cell-based immunotherapies, they have also been accompanied by a variety of severe, sometimes life-threatening side effects. Furthermore, not all patients respond to immunotherapy. Hence, there is the need for more research to render these promising therapeutics more efficacious, more widely applicable, and safer to use. Whole-body in vivo imaging technologies that can interrogate cancers and/or immunotherapies are highly beneficial tools for immunotherapy development and translation to the clinic. In this review, we explain how in vivo imaging can aid the development of molecular and cell-based anti-cancer immunotherapies. We describe the principles of imaging host T-cells and adoptively transferred therapeutic T-cells as well as the value of traceable cancer cell models in immunotherapy development. Our emphasis is on in vivo cell tracking methodology, including important aspects and caveats specific to immunotherapies. We discuss a variety of associated experimental design aspects including parameters such as cell type, observation times/intervals, and detection sensitivity. The focus is on non-invasive 3D cell tracking on the whole-body level including aspects relevant for both preclinical experimentation and clinical translatability of the underlying methodologies.

Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?

The first reporter systems were developed in the early 1980s and were based on measuring the activity of an enzyme-as a surrogate measure of promoter-driven transcriptional activity-which is now known as a reporter gene system. The initial objective and application of reporter techniques was to analyze the activity of a specific promoter (namely, the expression of a gene that is under the regulation of the specific promoter that is linked to the reporter gene). This system allows visualization of specific promoter activity with great sensitivity. In general, there are 2 classes of reporter systems: constitutively expressed (always-on) reporter constructs used for cell tracking, and inducible reporter systems sensitive to endogenous signaling molecules and transcription factors that characterize specific tissues, tumors, or signaling pathways.This review traces the development of different reporter systems, using fluorescent and bioluminescent proteins as well as radionuclide-based reporter systems. The development and application of radionuclide-based reporter systems is the focus of this review. The question at the end of the review is whether the "promise" of reporter gene imaging has been realized. What is required for moving forward with radionuclide-based reporter systems, and what is required for successful translation to clinical applications?

Imaging of T-cells and their responses during anti-cancer immunotherapy

Immunotherapy has proven to be an effective approach in a growing number of cancers. Despite durable clinical responses achieved with antibodies targeting immune checkpoint molecules, many patients do not respond. The common denominator for immunotherapies that have successfully been introduced in the clinic is their potential to induce or enhance infiltration of cytotoxic T-cells into the tumour. However, in clinical research the molecules, cells and processes involved in effective responses during immunotherapy remain largely obscure. Therefore, in vivo imaging technologies that interrogate T-cell responses in patients represent a powerful tool to boost further development of immunotherapy. This review comprises a comprehensive analysis of the in vivo imaging technologies that allow the characterisation of T-cell responses induced by anti-cancer immunotherapy, with emphasis on technologies that are clinically available or have high translational potential. Throughout we discuss their respective strengths and weaknesses, providing arguments for selecting the optimal imaging options for future research and patient management.

CAR T cells for brain tumors: Lessons learned and road ahead

Malignant brain tumors, including glioblastoma, represent some of the most difficult to treat of solid tumors. Nevertheless, recent progress in immunotherapy, across a broad range of tumor types, provides hope that immunological approaches will have the potential to improve outcomes for patients with brain tumors. Chimeric antigen receptors (CAR) T cells, a promising immunotherapeutic modality, utilizes the tumor targeting specificity of any antibody or receptor ligand to redirect the cytolytic potency of T cells. The remarkable clinical response rates of CD19-targeted CAR T cells and early clinical experiences in glioblastoma demonstrating safety and evidence for disease modifying activity support the potential of further advancements ultimately providing clinical benefit for patients. The brain, however, is an immune specialized organ presenting unique and specific challenges to immune-based therapies. Remaining barriers to be overcome for achieving effective CAR T cell therapy in the central nervous system (CNS) include tumor antigenic heterogeneity, an immune-suppressive microenvironment, unique properties of the CNS that limit T cell entry, and risks of immune-based toxicities in this highly sensitive organ. This review will summarize preclinical and clinical data for CAR T cell immunotherapy in glioblastoma and other malignant brain tumors, including present obstacles to advancement.

Cousins at work: How combining medical with optical imaging enhances in vivo cell tracking

Microscopy and medical imaging are related in their exploitation of electromagnetic waves, but were developed to satisfy differing needs, namely to observe small objects or to look inside subjects/objects, respectively. Together, these techniques can help elucidate complex biological processes and better understand health and disease. A current major challenge is to delineate mechanisms governing cell migration and tissue invasion in organismal development, the immune system and in human diseases such as cancer where the spatiotemporal tracking of small cell numbers in live animal models is extremely challenging. Multi-modal multi-scale in vivo cell tracking integrates medical and optical imaging. Fuelled by basic research in cancer biology and cell-based therapeutics, it has been enabled by technological advances providing enhanced resolution, sensitivity and multiplexing capabilities. Here, we review which imaging modalities have been successfully used for in vivo cell tracking and how this challenging task has benefitted from combining macroscopic with microscopic techniques.

Multimodality reporter gene imaging: Construction strategies and application

Molecular imaging has played an important role in the noninvasive exploration of multiple biological processes. Reporter gene imaging is a key part of molecular imaging. By combining with a reporter probe, a reporter protein can induce the accumulation of specific signals that are detectable by an imaging device to provide indirect information of reporter gene expression in living subjects. There are many types of reporter genes and each corresponding imaging technique has its own advantages and drawbacks. Fused reporter genes or single reporter genes with products detectable by multiple imaging modalities can compensate for the disadvantages and potentiate the advantages of each modality. Reporter gene multimodality imaging could be applied to trace implanted cells, monitor gene therapy, assess endogenous molecular events, screen drugs, etc. Although several types of multimodality imaging apparatus and multimodality reporter genes are available, more sophisticated detectors and multimodality reporter gene systems are needed.

Advancing Immune and Cell-Based Therapies Through Imaging

Immunotherapies include various approaches, ranging from stimulating effector mechanisms to counteracting inhibitory and suppressive mechanisms, and creating a forum for discussing the most effective means of advancing these therapies through imaging is the focus of the newly formed Imaging in Cellular and Immune Therapies (ICIT) interest group within the World Molecular Imaging Society. Efforts are being made in the identification and validation of predictive biomarkers for a number of immunotherapies. Without predictive biomarkers, a considerable number of patients may receive treatments that have no chance of offering a benefit. This will reflect poorly on the field of immunotherapy and will yield false hopes in patients while at the same time contributing to significant cost to the healthcare system. This review summarizes the main strategies in cancer immune and cell-based therapies and discusses recent advances in imaging strategies aimed to improve cancer immunotherapy outcomes.