Evaluation of radiofluorinated carboximidamides as potential IDO-targeted PET tracers for cancer imaging

Radiopharmaceuticals and their applications in medicine

Radiopharmaceuticals involve the local delivery of radionuclides to targeted lesions for the diagnosis and treatment of multiple diseases. Radiopharmaceutical therapy, which directly causes systematic and irreparable damage to targeted cells, has attracted increasing attention in the treatment of refractory diseases that are not sensitive to current therapies. As the Food and Drug Administration (FDA) approvals of [177Lu]Lu-DOTA-TATE, [177Lu]Lu-PSMA-617 and their complementary diagnostic agents, namely, [68Ga]Ga-DOTA-TATE and [68Ga]Ga-PSMA-11, targeted radiopharmaceutical-based theranostics (radiotheranostics) are being increasingly implemented in clinical practice in oncology, which lead to a new era of radiopharmaceuticals. The new generation of radiopharmaceuticals utilizes a targeting vector to achieve the accurate delivery of radionuclides to lesions and avoid off-target deposition, making it possible to improve the efficiency and biosafety of tumour diagnosis and therapy. Numerous studies have focused on developing novel radiopharmaceuticals targeting a broader range of disease targets, demonstrating remarkable in vivo performance. These include high tumor uptake, prolonged retention time, and favorable pharmacokinetic properties that align with clinical standards. While radiotheranostics have been widely applied in tumor diagnosis and therapy, their applications are now expanding to neurodegenerative diseases, cardiovascular diseases, and inflammation. Furthermore, radiotheranostic-empowered precision medicine is revolutionizing the cancer treatment paradigm. Diagnostic radiopharmaceuticals play a pivotal role in patient stratification and treatment planning, leading to improved therapeutic outcomes in targeted radionuclide therapy. This review offers a comprehensive overview of the evolution of radiopharmaceuticals, including both FDA-approved and clinically investigated agents, and explores the mechanisms of cell death induced by radiopharmaceuticals. It emphasizes the significance and future prospects of theranostic-based radiopharmaceuticals in advancing precision medicine.

Tryptophan Kynurenine Pathway-Based Imaging Agents for Brain Disorders and Oncology-From Bench to Bedside

Tryptophan (Trp)-based radiotracers have excellent potential for imaging many different types of brain pathology because of their involvement with both the serotonergic and kynurenine (KYN) pathways. However, radiotracers specific to the kynurenine metabolism pathway are limited. In addition, historically Trp-based radiopharmaceuticals were synthesized with the short-lived isotope carbon-11. A newer generation of Trp-based imaging agents using the longer half-lived and commercially available isotopes, such as fluorine-18 and iodine-124, are being developed. The newly developed amino acid-based tracers have been demonstrated to have favorable radiochemical and imaging characteristics in pre-clinical studies. However, many barriers still exist in the clinical translation of KYN pathway-specific radiotracers.

Molecular imaging for cancer immunotherapy

Immunotherapy has changed the treatment landscape for many cancers; however, not all patients treated have a favorable response and others can develop immune-related adverse events. A method to predict the treatment response to immunotherapeutic agents could allow for improved selection of patients more likely to benefit from treatment while sparing those who would suffer serious complications. While this has been an active area of research and has resulted in significant insights, current proposed mechanisms do not fully explain responses to therapy. One problem is that our understanding relies mostly on tumor biopsy samples that do not account for the complex spatiotemporal heterogeneity of cancers and their microenvironment. Radiolabeled probes targeting immune biomarkers and imaged using positron emission tomography with computed tomography could provide in vivo, real-time and non-invasive imaging of these biomarkers. Here we review the current field of functional nuclear imaging agents in immuno-oncology including antibodies and small molecule tracers to image PD-1, PD-L1, CTLA-4, T-cell markers and other targets being studied for potential therapies.

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Treatment response of cancers to immunotherapy is difficult to predict.

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Positron emission tomography (PET) imaging may help predict treatment response.

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PET to evaluate immunotherapeutic targets or markers of immune activation shows promise.

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Antibodies and small molecules used for PET have different imaging characteristics.

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More studies are needed to better interpret and validate PET scans for this purpose.

Tracers for non-invasive radionuclide imaging of immune checkpoint expression in cancer

Abstract

Immunotherapy with checkpoint inhibitors demonstrates impressive improvements in the treatment of several types of cancer. Unfortunately, not all patients respond to therapy while severe immune-related adverse effects are prevalent. Currently, patient stratification is based on immunotherapy marker expression through immunohistochemical analysis on biopsied material. However, expression can be heterogeneous within and between tumor lesions, amplifying the sampling limitations of biopsies. Analysis of immunotherapy target expression by non-invasive quantitative molecular imaging with PET or SPECT may overcome this issue. In this review, an overview of tracers that have been developed for preclinical and clinical imaging of key immunotherapy targets, such as programmed cell death-1, programmed cell death ligand-1, IDO1 and cytotoxic T lymphocyte-associated antigen-4 is presented. We discuss important aspects to consider when developing such tracers and outline the future perspectives of molecular imaging of immunotherapy markers.

Graphical abstract

Current techniques in immune checkpoint imaging and its potential for future applications