Radiopharmacology and molecular imaging of PD-L1 expression in cancer

Molecular Imaging and the PD-L1 Pathway: From Bench to Clinic

Programmed death-1 (PD-1) and programmed death ligand 1 (PD-L1) inhibitors target the important molecular interplay between PD-1 and PD-L1, a key pathway contributing to immune evasion in the tumor microenvironment (TME). Long-term clinical benefit has been observed in patients receiving PD-(L)1 inhibitors, alone and in combination with other treatments, across multiple tumor types. PD-L1 expression has been associated with response to immune checkpoint inhibitors, and treatment strategies are often guided by immunohistochemistry-based diagnostic tests assessing expression of PD-L1. However, challenges related to the implementation, interpretation, and clinical utility of PD-L1 diagnostic tests have led to an increasing number of preclinical and clinical studies exploring interrogation of the TME by real-time imaging of PD-(L)1 expression by positron emission tomography (PET). PET imaging utilizes radiolabeled molecules to non-invasively assess PD-(L)1 expression spatially and temporally. Several PD-(L)1 PET tracers have been tested in preclinical and clinical studies, with clinical trials in progress to assess their use in a number of cancer types. This review will showcase the development of PD-(L)1 PET tracers from preclinical studies through to clinical use, and will explore the opportunities in drug development and possible future clinical implementation.

Molecular imaging and immunotherapy

The utility of positron emission tomography (PET) for the evaluation of response to immunotherapy has been considered a hot topic, particularly in the last 2 to 3 years. Different experiences have been collected in clinical practice, with 18F-Fluorodeoxyglucose (FDG) PET/computed tomography (CT), particularly in patients affected by lymphoma, malignant melanoma, and lung cancer. It has been tested in different settings of disease, from the prediction to the prognosis relative to the response to immunotherapy. In the present mini-review, some evidence is reported about the role of FDG PET/CT in patient candidates to or treated with immunotherapy.

Response assessment of bone metastatic disease: seeing the forest for the trees RECIST, PERCIST, iRECIST, and PCWG-2

Tumor response is often used as a surrogate marker for survival practically in all clinical trials. Therefore, robust and valid response criteria during the course of trials are fundamental for the assessment of response to therapy. This aspect, however, becomes particularly challenging when it comes to bone metastases. In the era of targeted therapies and immune-checkpoint inhibitors (ICI), response assessment by morphologic-based criteria cannot detect the real tumor response and, consequently, fail to demonstrate the actual clinical benefit. This review will focus on some of the most common morphologic and metabolic response criteria and their application for bone lesions, highlighting relative strengths and weaknesses as well as potential future methods in the era of target therapies and immunotherapy with ICI.

Developing native peptide-based radiotracers for PD-L1 PET imaging and improving imaging contrast by pegylation

Herein, a PD-L1 targeted native peptide was developed for PET imaging. 18F and 64Cu were utilized to label the peptide. To improve the pharmacokinetics and biodistribution of the tracers, the peptide was further pegylated to form star-like tetramers. Consequently, four tracers were synthesized with acceptable radiochemical characteristics and their in vivo pharmacokinetics and PD-L1 imaging capability were systematically evaluated. This proof-of-principle study may provide new possibilities for PD-L1 PET imaging in cancers.