Pet imaging of cytotoxic human T cells using an 89Zr-labeled anti-CD8 minibody

Positron Emission Tomography Probes for Imaging Cytotoxic Immune Cells

Non-invasive positron emission tomography (PET) imaging of immune cells is a powerful approach for monitoring the dynamics of immune cells in response to immunotherapy. Despite the clinical success of many immunotherapeutic agents, their clinical efficacy is limited to a subgroup of patients. Conventional imaging, as well as analysis of tissue biopsies and blood samples do not reflect the complex interaction between tumour and immune cells. Consequently, PET probes are being developed to capture the dynamics of such interactions, which may improve patient stratification and treatment evaluation. The clinical efficacy of cancer immunotherapy relies on both the infiltration and function of cytotoxic immune cells at the tumour site. Thus, various immune biomarkers have been investigated as potential targets for PET imaging of immune response. Herein, we provide an overview of the most recent developments in PET imaging of immune response, including the radiosynthesis approaches employed in their development.

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.

The Production, Quality Control, and Characterization of ZED8, a CD8-Specific 89Zr-Labeled Immuno-PET Clinical Imaging Agent

Immuno-PET is a molecular imaging technique utilizing positron emission tomography (PET) to measure the biodistribution of an antibody species labeled with a radioactive isotope. When applied as a clinical imaging technique, an immuno-PET imaging agent must be manufactured with quality standards appropriate for regulatory approval. This paper describes methods relevant to the chemistry, manufacturing, and controls component of an immuno-PET regulatory filing, such as an investigational new drug application. Namely, the production, quality control, and characterization of the immuno-PET clinical imaging agent, ZED8, an ⁸⁹Zr-labeled CD8-specific monovalent antibody as well as its desferrioxamine-conjugated precursor, CED8, is described and evaluated. PET imaging data in a human CD8-expressing tumor murine model is presented as a proof of concept that the imaging agent exhibits target specificity and comparable biodistribution across a range of desferrioxamine conjugate loads.

The Immunoimaging Toolbox

The recent clinical success of cancer immunotherapy has renewed interest in the development of tools to image the immune system. In general, immunotherapies attempt to enable the body's own immune cells to seek out and destroy malignant disease. Molecular imaging of the cells and molecules that regulate immunity could provide unique insight into the mechanisms of action, and failure, of immunotherapies. In this article, we will provide a comprehensive overview of the current state-of-the-art immunoimaging toolbox with a focus on imaging strategies and their applications toward immunotherapy.

Noninvasive PET Imaging of T cells

The rapidly evolving field of cancer immunotherapy recently saw the approval of several new therapeutic antibodies. Several cell therapies, for example, chimeric antigen receptor-expressing T cells (CAR-T), are currently in clinical trials for a variety of cancers and other diseases. However, approaches to monitor changes in the immune status of tumors or to predict therapeutic responses are limited. Monitoring lymphocytes from whole blood or biopsies does not provide dynamic and spatial information about T cells in heterogeneous tumors. Positron emission tomography (PET) imaging using probes specific for T cells can noninvasively monitor systemic and intratumoral immune alterations during experimental therapies and may have an important and expanding value in the clinic.