18F-AlF-NOTA-PCP1 PET/CT Imaging Visualize Radiation-Induced PD-L1 Expression in Glioblastoma Subcutaneous Xenograft Mouse Models

Abstract

<h3>Purpose/Objective(s)</h3> Remarkable clinical benefits from radiotherapy combined with PD-1/PD-L1 immune checkpoint blockades were reported across several cancer patients. However, the optimal time window of combining treatment remains undefined, which may be correlated with the timing of upregulation of PD-L1 expression. PD-L1 antibody-based PET/CT imaging was difficult to translate into clinical settings secondary to the long interval before the scan. We therefore generated a peptide-based PET radiotracer to monitor radiation-induced PD-L1 expression in a non-invasive manner, which could be used to assess the optimal time window of combining radio- and immunotherapy. <h3>Materials/Methods</h3> Peptide targeting PD-L1 (PCP1) was conjugated with NOTA and radiolabeled with 18F ([18F]-AlF-NOTA-PCP1). The Binding assay was performed in U87MG and GL261 glioblastoma cell lines with different levels of PD-L1 expression. [18F]-AlF-NOTA-PCP1 PET/CT imaging was prepared in U87MG and GL261 xenografts and syngeneic tumor models. <i>In vivo</i> imaging data was confirmed by <i>ex vivo</i> biodistribution and immunohistochemistry (IHC). To evaluate the radiation-induced PD-L1 upregulation, [18F]-AlF-NOTA-PCP1 PET/CT scan was prepared 3 days after U87MG tumor on mice subjecting to 20 Gy × 1F external radiation therapy. <h3>Results</h3> [18F]-AlF-NOTA-PCP1 was developed with high radiochemical purity and stability. The Binding assay showed more than 20 folds [18F]-AlF-NOTA-PCP1 binding to U87MG compared with GL261 cell line (p = 0.0495). The U87MG and GL261 tumors and excretory organs were identified by [18F]-AlF-NOTA-PCP1 PET/CT imaging. [18F]-AlF-NOTA-PCP1 PET/CT data showed different %ID/cc (percentage of injected dose per cubic centimeter of tissue) values of ROIs (regions of interest) among tumor models as evaluated by ex vivo methods. Preliminary results demonstrated the increase in PD-L1 expression of irradiated tumors on [18F]-AlF-NOTA-PCP1 PET/CT, showing a %ID/cc value of nearly 4 folds development post-radiation. <h3>Conclusion</h3> PET imaging with [18F]-AlF-NOTA-PCP1 is an effective approach for specific and non-invasive visualization of tumor PD-L1 level. PD-L1 expression can be induced by radiotherapy treatments and [18F]-AlF-NOTA-PCP1 PET can quantify these changes in glioblastoma subcutaneous xenograft mouse models.