Sarah Verhoeff

145 Summary, general discussion and future perspectives New directions Improving PD-L1 PET imaging To overcome limitations associated with antibody-based imaging to e.g., visualize PD-L1, lower molecular weight tracers have been developed for PET imaging. These smaller molecular peptides are cleared more easily by the kidneys, to allow for same-day imaging and the use of radiotracers with shorter half-lives including gallium-68 and florine-18. Examples of smaller protein-based radiotracers are mini-bodies, nanobodies, affibody molecules and adnectins15. Imaging targets beyond PD-L1 to predict ICI treatment response Although the initial focus has been on PD-L1 expression in the tumor, the expression of PD-L1 on macrophages, dendritic cells and PD-1 on T-cells have also been associated with response to ICI response19. This has already led to a different scoring of PD-L1 assessment in some clinical trials, reporting a combined positive score to include PD-L1 positive immune cells. The limitations to the assessment of PD-L1 remain, and ultimately it is unlikely that PD-L1 status will be useful as a sole biomarker to identify patients with long-term response to ICI treatment. Based on these observations, the role of the inflammatory state in the tumor microenvironment (TME) on the response to ICI treatment has been studied more intensively. Blockade of PD-L1 or other immunotherapies are believed to promote priming and infiltration of tumor-specific T cells within the TME and/or in the tumor draining lymph nodes20. Potentially, the number and intratumoral distribution of infiltrating T-cells (e.g., CD8+ T-cells) before or during treatment could predict or monitor response to single or combination ICI treatment response21.The visualization of these effector mechanisms would be of great value to increase our understanding. Currently, the visualization of whole tumor CD8+ T-cell infiltration is being performed using 89Zr-labeled anti CD8 mini-bodies ([89Zr]Zr-Df-crefmirlimab)22. Position of molecular PET imaging in drug development The increased interest in the tumor immune microenvironment, has also led to an increased interest in the role of [18F]FDG PET/CT in patients considered for ICI treatment. So far, the performance of a single pretreatment [18F]FDG PET/CT has not consistently been associated with ICI response23-25. However, it should be noted that most data were obtained retrospectively and included patients with different tumor types receiving different immunotherapy regimens. Furthermore, [18F]FDG PET/CT has been evaluated as a tool for ICI response monitoring and identification of patients with long-term progression free survival after discontinuation of ICI25-27. We believe that [18F]FDG PET/CT can contribute to the unmet need to identify tumor and patient heterogeneity to ultimately predict treatment response. Therefore, implementing [18F]FDG PET/ CT in future clinical trials could be a good first step. Since the procedure of [18F]FDG PET/CT is standardized and performed on a daily basis in the work-up of many cancer patients, implementing this should not be too difficult. However, the analyses and interpretation of this study data should be centralized/standardized to limit interobserver variability. 8